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Coordination Chemistry Reviews 255 (2011) 151815 40

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Coordination Chemistry Reviews

j ou rna l h om epag e : www.e l sev i e r. com/ loca t e / cc r

Review

Recent advances in oxidation catalysis using ionic liquids as solvents

Daniel Betz a , Philipp Altmann b , Mirza Cokoja a , Wolfgang A. Herrmann a , Fritz E. Khn a , b , a Chair of Inorganic Chemistry, Catalysis Research Center, Technische Universitt Mnchen, Lichtenbergstrae 4, D-85747 Garching, Germanyb Molecular Catalysis, Catalysis Research Center, Technische Universitt Mnchen, Lichtenbergstrae 4, D-85747 Garching, Germany

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15182. Oxidation of suldes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15203. Oxidation of alcohols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15224. Oxidation of oximes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1525

5. Oxidation of olens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15266. The BaeyerVilliger reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15307. Special oxidations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1532

7.1. Oxidation of nitrotoluene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15327.2. Carbonylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15337.3. Oxidation of cysteine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15357.4. Oxidation of cyclohexane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15357.5. Oxidation of halides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15387.6. - To s y l o x i l a t i o n o f k e t o n e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15387.7. Synthesis of thiazoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1538

8. Conclusion and perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1539Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1539R e f e r e n c e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1539

a r t i c l e i n f o

Article history:Received 2 September 2010Accepted 8 December 2010Available online 16 December 2010

Keywords:Ionic liquidsTwo-phase catalysisTransition metal catalystsOxidation

a b s t r a c t

Ionic liquids arean interesting alternativeto classicalsolvents presentingseveraladvantages. A variety of catalysts showgood or even enhanced activities when applied in ionic liquids. Oxidation catalysis repre-sents a large segment of industrial chemistry, providing both bulk chemicals and intermediates for highvalue added special products and pharmaceuticals. Particularly for the latter products organometallicand inorganic catalysts are being developed that can be applied in systems consisting of or containingionic liquids. This work provides an overview on recent developments in this eld.

2011 Published by Elsevier B.V.

1. Introduction

Ionic liquids (ILs) have been used in a variety of catalyticreactions during the last decade [13] . They attracted consider-able attention due to their physical properties, such as thermalstability, low volatility, low ash point, and high polarity. Addi-

Corresponding author at: Chair of Inorganic Chemistry, Catalysis ResearchCenter, Technische Universitt Mnchen, Lichtenbergstrae 4, D-85747 Garching,Germany. Tel.: +49 89 289 13080; fax: +49 89 289 13143.

E-mail address: [email protected] (F.E. Khn).

tionally, properties such as temperature-depending miscibilitywith water make them attractive alternatives to organic sol-vents [46] . Organometallic complexes, which are immiscible withhydrocarbons,areoftensolubleinILs.Thereforetheyprovideanon-aqueous alternative for two-phase catalysis, in which the catalystis immobilised in the ionic liquid phase and can easily be sepa-rated from the product. ILs have been used for several types of reactions, such as hydrogenation, hydrosilylation and oligomeri-sation of olens. Regarding oxidation reactions, Song reportedthe rst manganese(III) (salen) complex, capable of catalysingan asymmetric epoxidation in an ionic liquid less than a decadeago [7]. Since then, ILs have been successfully applied in olen

0010-8545/$ see front matter 2011 Published by Elsevier B.V.

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D. Betz et al. / Coordination Chemistry Reviews 255 (2011) 15181540 1519

Table 1Oxidation of suldes with 5mol% of Ti 4 [(OCH2 )3 CMe]2 (i-OPr) in [bmim].BF 4 at r.t.

Entry Educt Product Time (h) Yield (%)

1

S S

O

O

10 min 95

2

S SO

O

2.5 93

3S

S

O

O 1 93

4

S S

O

O

2 98

5

S S

O

O

2 91

6 SS

O O 2 89

7

S Ph S Ph

O

O

2 90

8

S

O

S

O

OO

1.5 90

9

S OCH 3

O

S

O

O

OCH 3

O2 91

10

SOH

S

O

OOH

3 91

11

S S

O

O

2.5 87

12

S S

O

O

2 91

13

SCN

SCN

O

O

2 91

14

S S

O

O

1 94

epoxidations, e.g. utilising manganese(III) porphyrins as catalysts[811] .

The ionic liquids described in this review are used either as sol-vents or as extractant. In some cases they are even catalytically

active themselves and do not require an additional organometallic

complex as catalyst. As a result of extensive studies on oxidationcatalysis in ionic liquids, several reviews have been published dur-ing the past decade [1214] . Therefore, this review highlights themost recent results. We also excluded the very well-known oxida-

tion catalyst methyltrioxorhenium from this work since Saladino


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1520 D. Betz et al. / Coordination Chemistry Reviews 255 (2011) 15181540

Scheme 1. The general process of the oxidative desulfurisation in ILs.

Table 2Oxidative desulfurisation of DBT in different ionic liquids at r.t.

Entry IL Yield (%)

1 [bmim].Cl/FeCl 3 992 [omim].Cl/FeCl 3 873 Et3 NHCl/FeCl3 37

Reaction conditions: m(DBT)/m(IL)= 3:1, t =10min.

et al. described its activity in non-conventional solvents in 2010[15] .

2. Oxidation of suldes

All systems described here the aim on the removal of sulfur-containing compounds in diesel fuel. The removal of sulfurcontaining compounds is an important process in the fuel indus-try.Hence, research focuses on costefcient liquid phase processes,such as the oxidation of suldes, to remove compounds from fuels,which are corrosive for car engines and potentially problematicfor the environment. Several research teams have focused on thesulde oxidation in ionic liquids. The involved ionic liquids were

used both as reaction media and as extractants, which dissolve theformed sulfones. Scheme 1 shows the reaction principle.

The extraction of sulfur-containing compounds from diesel oilby ionic liquids could be an attractive alternative to commondesulfurisation by hydrotreating. The efciency of the extrac-tion increases if the S-species are previously oxidised to thecorresponding sulfoxides and sulfones [16] . Reddy and Verkadedescribed the oxidation of organic suldes into sulfones usingTi4 [(OCH2 )3 CMe]2 (i-OPr) 10 . The reaction was investigated bothin methanol (MeOH) and in three different RTILs ([emim].BF 4 ,[bmim].BF 4 and [bmim].PF 6 ) as solvents. H 2 O2 was used as oxidantand the reaction was performed at room temperature. Under theseconditions, only sulfones but no sulfoxides were found. The activ-ities in the different RTILs are very similar and the authors found

for some substrates an acceleration of about 30% compared withMeOH as a solvent. Additionally, the catalyst could be recycled bysimply extracting the productusingdiethylether. The catalyticsys-tem could be reused for six cycles without a loss of activity. Table 1shows the product yields in [bmim].BF 4 . The velocity of the oxida-tion reaction is in good accordance with the steric hindrance of thesubstituents at the sulfur atom [17] .

Li et al. investigated three different ionic liquids based on ironchloride in the catalytic oxidation/desulfurisation (ODS) systemsfor removal of benzothiophene (BT), dibenzothiophene (DBT) and4,6-dimethyldibenzothiophene (4,6-DMDBT). The authors statedthat the system [bmim].Cl/FeCl 3 /H2 O2 is able to remove 99% of DBT under mild reaction conditions. As can be seen in Table 2 ,[bmim].Cl/FeCl 3 and [omim].Cl/FeCl 3 ionic liquids show higher

ability to remove sulfur than Et 3 NHCl/FeCl3 [18,19] .

Table 3Different investigated systems with their corresponding yields (%) in the desulfuri-sation of DBT.

Entry Type of IL IL IL +H2 O2 IL+Na2 MoO 4 + H2 O2

1 [bmim].BF 4 16 32 992 [omim].BF 4 21 35 683 [bmim].PF 6 15 39 704 [omim].PF 6 20 45 785 [bmim].TA 15 31 496 [omim].TA 21 32 37

Reaction conditions: T = 70 C, t = 3 h, 5mol% catalyst.

Table 4The reactivity of different Mo catalysts for desulfurisation.

Entry Catalyst Yield (%)

1 Na 2 MoO 4 992 H2 MoO 4 943 (NH 4 )6 Mo 7 O24 984 H3 PMo 12 O40 935 (NH 4 )3 PMo 12 O40 986 Na 3 PMo 12 O40 99

Reaction conditions: T = 70 C, t = 3 h, 5 mol% catalyst in [bmim].BF 4 .

In addition, the authors studied the recycling of the[bmim].Cl/FeCl 3 system. They observed only a small drop from 99to 91% after six cycles. However, if the IL phase was extracted withCCl4 after each run, no loss in activity could be found after ninecycles [18,19] .

The same group investigated the desulfurisation with a cat-alytic system containing Na 2 MoO4 2H2 O, H2 O2 , and [bmim].BF 4 .When using DBT as a model compound, a sulfur removal of 99%was reached. The same reaction without the ionic liquid leads to adesulfurisationof 4%.The authorsstated thatwithout theIL,most of H2 O2 decomposed at the applied temperature. Hence,the IL acts asan extractant, as reaction medium and as stabilising agent. Table 3shows the different investigated systems with their correspondingoxidative desulfurisation yields [20,21] .

Table 4 displays the oxidative desulfurisation with different Mocatalysts. Li et al. found that the reaction including molybdenumsalts as catalysts was more efcient in polar ILs than in acidic ones,since they exhibit a lower electrolyte strength [20] .

Leachingexperimentswere also performed and it was observedthat in case of Na 2 MoO4 1.2 mg (0.9%) leach in 1L of the substratephase. This amount could be completely removed by extractionwith water (2 10mL).

Li et al. investigated the catalytic activity of V 2 O5 for the oxida-tive desulfurisation of fuels [22] . The best results were obtainedwhen a combination of H 2 O2 and V 2 O5 in [bmim].BF 4 was used.In this case, the removal of sulfur was about 99%. The oxidation of methylphenylsulde was performed at 35 C using TBHP or UHPas an oxidising agent and 2.5mol% of the catalyst. The authors

stated that during the reaction, V 2 O5 was oxidised by H 2 O2 to the


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Table 5Different investigated systems with their corresponding yields (%) in the desulfuri-sation of DBT with phosphotungstic acid.

Entry Type of IL IL IL + H2 O2 IL+H3 PW12 O40 + H2 O2

1 [bmim].BF 4 14 26 982 [omim].BF 4 18 27 653 [bmim].PF 6 12 27 984 [omim].PF 6 18 35 64

Reaction conditions: T = 30 C, t = 1 h, 1mol% catalyst.

Table 6Different decatungstate catalysts with their corresponding DBT desulfurisationyields (%).

Entry Catalyst Yield (%)

1 [(C4 H9 )4 N]4 W 10 O32 982 [(CH3 )4 N]4 W10 O32 973 [(C2 H5 )3 NC7 H7 ]4 W 10 O32 664 Na 4 W10 O32 95

Reaction conditions: T = 60 C, t = 0.5h, 1 mol% catalyst in [bmim].PF 6 .

peroxovanadiumcompound which oxidises the suldes to the cor-responding sulfones. Without any ionic liquid the sulfur removalwas lower than 3%.

Thesame group studied the oxidative desulfurisationof fuels byphosphotungstic acid (H 3 PW12 O40 ) and different decatungstates,respectively [2325] . The reaction with phosphotungstic acid wasperformed in different ILs and H 2 O2 as an oxidant. The results aresummarised in Tables 5 and 6 , clearly pointing out the advantageof the [bmim].-type ILs in combination with the used catalysts.

Both investigated catalytic systems lead to almost quantita-tive yields at a catalyst concentration 1 mol%. By the same group[WO(O 2 )2 Phenanthroline H2 O], [MoO(O 2 )2 Phenanthroline] andPeroxophosphomolybdate catalysts were dissolved in ILs, e.g.[bmim].BF 4 , [omim].BF 4 , [bmim].PF 6 , and [omim].PF 6 to oxidiseDBT with hydrogen peroxide under moderate conditions. The sul-fur removal of DBT-containing model oil reached 99% at 70 C. The

catalytic oxidation system containing WO(O 2 )2 Phen H2 O, H2 O2 ,and [bmim].BF 4 can be recycled four times without a signicantdecrease in activity [26,27] .

Li et al. further investigated the desulfurisation of dibenzothio-phene by a combination of both chemical oxidation and solventextraction [28] . Benzyltrimethylammonium chloride 2ZnCl2 is alow-cost ionic liquid and was used as an extractant for oxidativedesulfurisation of DBT in n-octane. DBT was oxidised to the corre-sponding sulfone by peraceticacid,in situ prepared from H 2 O2 andacetic acid. With this system, the desulfurisation yield of DBT was94%at 30min and 99%after 50min at room temperature.Accordingto the authors, the metal-containing ionic liquid could be recycledsix times without a signicant decrease in activity.

In addition to thedescribed systems there areseveral reports on

the investigation of Brnsted acidic ionic liquids which are them-selves acting as oxidative desulfurisation catalysts. Zhao et al. usedthe Brnsted acidic ionic liquid N-methyl-pyrrolidonium tetrauo-roborate ([hnmp].BF 4 ) as a catalystfor theoxidative desulfurisationof DBT in the presence of H 2 O2 as an oxidant [29,30] . It was found

Table 7Results of the recyclability.

Cycle Yield (%) Cycle Yield (%)

1 100 7 1002 100 8 983 99 9 954 99 10 975 100 11 956 100 12 93

Reaction conditions: T = 60 C, t =2h, V oil /V IL = 1:1.

that a coordination compound was generated between hydrogenperoxide and the cation of the ionic liquid, which results in theformation of hydroxyl radicals. The sulfur-containing compoundswere rst dissolved in the IL and then oxidised by the radicals. Dueto the high polarity the formed sulfones could only be detectedin the IL phase. Table 7 shows the recyclability of the investigatedsystem.

The same group studied the desulfurisation of thio-phene with a non-uorinated and environmentally benign IL (C4 H9 )4 NBr 2C6 H11 NO as an active catalyst [31,32] . A combinationof hydrogen peroxide and acetic acid acts as an oxygen source forthe reaction. After the oxidation the formed sulfoxide, sulfone andsulfate are more polar and could be extracted by the ionic liquid.A desulfurisation level of up to 99% was obtained after 30 min and40 C. An advanced oxidation process was studied by Zhao andcoworkers as well [33] . In this case they used a combination of ozone and hydrogen peroxide leading to the formation of hydroxylradicals. In contrast to applying ozone alone (64% yield), thiscombination led to an increased oxidation of DBT of 99% DBT at50 C after a reaction time of 150min [bmim].BF 4 was investigatedas a reaction medium and extractant. The authors stated that thereaction became faster with increasing temperature but a higherconcentration of ozone was also required.

The same reaction was investigated by the group of Gao. In this

casethey usedtheBrnstedacidic ionicliquid[hmim].BF 4 asacata-lyst forthe oxidativedesulfurisation of DBTin the presence of H 2 O2as an oxidant [34] .

The deep oxidative desulfurisation in the presence of H 2 O2 andUV irradiation withoutanycatalystat roomtemperature andatmo-spheric pressure was investigatedby Zhao et al. The sulfur removalreached up to 99.5% within 8 h. Without UV irradiation the yieldwas around 52% after 12h [bmim].PF 6 was used as the extractionmedia [35] . Guiet al.studied some task-specicionicliquids whichcontain carboxyl groups in their cations. Fig. 1 shows the structureof the ionic liquids. In the following case the ILs act as both catalystand extractant.

The oxidation potential of DBT with H 2 O2 as an oxidantdecreases in the following order: E>F>C>A>B. The maximum

yield was 97% with compound E as catalyst [36] .The group of Halligudi investigated a Tibinol complex, sup-ported on an ionic liquid-phase for enantioselective suldeoxidation [37] . The principle of synthesising the SILP-type cat-alyst (SILP= supported ionic liquid-phase) is shown in Scheme 2 .

NN NN NNCH 2COOH CH 2CH 2COOH CH 2CH 2COOH

A: X = HSO 4-

B: X = H2PO 4-

C: X = Cl E: X = HSO 4-

F: X = H2PO 4-

Fig. 1. Structure of the task-specic ionic liquids.


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Scheme 4. Different peroxotungstates investigated by Xian-Ying and Jun-Fa.

Table 9Recyclability of theaerobicoxidation of benzylalcohol under thetwo differentcon-ditions.

PhCH 2 PhCOOH 2 HPhCHOBA

X Y .

Cycle Time (h) Condition Yield (%)

X Y

1st 8 A 9012 B 89

2nd 8 A 8412 B 88

3rd 10 A 83

15 B 76Reaction condition A: 2 mmol benzyl alcohol, 2 mol% VO(acac) 2 , 6mol% DABCO,1bar O 2 , 0.3 g of [bmim].PF 6 , 95 C; reaction condition B: 2 mmol benzyl alcohol,2 mol% VO(acac) 2 , 2 mol% Cu(II) 2-ethylhexanoate, 6 mol% DABCO, 1bar O 2 , 0.3g of [hmim].OTf, 95 C for the specic time.

were achieved with imidazolium-type ionic liquids especiallywith [bmim].PF 6 . Subsequently, different substrates have beeninvestigated in this RTIL ( Table 10 ).

To prove the recyclability of the catalytic system, the oxidationof 1-phenylpropan-1-ol to the corresponding acetophenone wastested in ve subsequent runs. The yield decreased from 91% inthe rst cycle to 84% in the fth cycle. Later, Han and coworkersemployedthesame substratesfor theoxidation witha novel copper

Schiff-base complex (see Fig. 2) [45,46] .The authors found that in this case, the BF 4 -type RTIL leads to

the highest conversion. In addition, the selectivity of the formationof thecorrespondingacidreachesup to 98%. Theauthorsalso statedthat with TBHP the reaction works best, instead of using H 2 O2 orNaClO as an oxidising agent. Finally, the reaction of the aromaticalcohols was faster than the aliphatic alcohols.

The catalytic oxidation with Ni(II)-Schiff-base catalysts in an[emim].-based IL and NaOCl as an oxidising agent was publishedby Bhat and coworkers [47] . They investigated different substratesand reached yields of >61% after 15min at room temperature. Thesame group published analogous Co(II)-complexes ( Scheme 5 ). Adifference in the catalytic activity between the Ni(II) and the Co(II)catalysts could not be observed. The authors further stated that the

Fig. 2. Structure of the Cu-Schiff-base catalyst.

activity is strongly inuenced by the bulkiness of the substituent R in the ligand [47,48] .

A. Shaabani et al. investigated the oxidation of alkyl arenes andalcohols to the correspondingcarbonyl compounds in ionic liquids.A variety of metallo-phthalocyanines and ionic liquids were usedand the best results were obtained using Co(II) phthalocyanine,[bmim].Br and an oxygen pressure of 0.1 atm. For the oxidation of alkyl arenes at 100 C, yields between 74% and 93% were obtained,between 80% and 92% for the oxidation of alcohols, respectively[49] . The same catalyst was used in a tetrasulfonated type for theoxidative deprotection of trimethylsilyl ether to the correspond-ing carbonyl compound. In [bmim].Cl yields of up to 80% could beobtained [50] . The groups of Hajipour et al.worked on this reactionin the presence of a catalytic amount of [bmim].Br (10mol%). Theyfound an efcientmethod to obtain the carbonylcompounds under

Table 10Oxidation of secondary alcohols with 3mol% Cu(acac) 2 as a catalyst.

Entry Educt Product Time (h) Yield (%)

1

OH O

5 91

2

OH O

5 91

3 Cl

OH

Cl

O

5 93

4

Cl OH Cl O

5 41

5

OH

O

O

O5 94

6

OH O

5 93

7

OH O

5 65

8

OH O

15 66

9

OH O

15 58


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CoCl

Cl

Ph 3P

PPh 3+

R

OH

N

HN

N

R

ON

HNN

CHCl 3, Co

PPh 3

Cl

R = H, Cl, Br, NO 2, OCH 3

Scheme 5. Synthesis of the Co(II) catalysts.

Fig. 3. Structure of a TEMPO-IL.

solvent-free conditions using potassium persulfate as an oxidant.After a reaction time of 15min they got a maximum yield of 90%[51] . Fadini and coworkers studied the manganese(III) catalysedcleavage of vicinal diols. When applying ILs in this reaction, theyields increased between 10% and 60% compared to conventionalsolvents. With a concentration of [Mn(salen)(Py)].(OAc) of 5 mol%

at a temperature of 60%, a quantitative oxidative CC bond cleav-age of 1,1,2,2-tetraphenyl-1,2-ethanediol could be obtained after2 h and with oxygen as oxidant [52] .

In 2008, Liu et al. studied the oxidation activity of differenttransition-metal salts by dissolving an equimolar amount of itin a so-called TEMPO-IL (TEMPO = 2,2,6,6-tetramethylpiperidine N-oxyl; Fig. 3) [53,54] .

While Co(OAc) 2 , CoCl2 , FeCl3 , Mn(OAc) 2 and NiCl 2 show noactivities as catalysts of the oxidation of benzyl alcohol, CuClexhibits a high catalytic activity, yielding benzaldehyde in 94%.Table 11 shows all the investigated substrates.

After the reaction, the IL phase was distilled and reused for vecycles without a loss of activity. In 2008, Liu et al. used the samecatalytic system and found out that the addition of molecular sieve

Table 11TEMPO-IL/CuCl catalysed oxidation of alcohols.

Entry Substrate Temp. ( C) Time (h) Conversion(%)

Yield (%)

1 Benzyl alcohol 40 12 67 552 Benzyl alcohol 65 12 99 943 Cinnamyl alcohol 65 30 99 844 4-Nitrobenzyl alcohol 100 24 96 845 4-Chlorbenzyl alcohol 75 19 99 886 4-Methoxybenzyl alcohol 65 21 99 917 Diphenyl carbinol 65 21 94 858 2-Phenylethanol 65 30 54 309 Furfuryl alcohol 65 50 40

10 Cyclohexanol 65 40 11 Lauryl alcohol 65 40 23

MS3A results in a remarkable faster reaction rate [55] . The authorsstated that the acceleration results from the property of the MS3Ato actas a Brnsted base andit is independenton thewatercontentof the reaction mixture.

Lei and coworkers studied a highly chemoselective oxida-tion of benzylic alcohols in the presence of aliphatic alco-hols to the corresponding hydroxyl benzyl aldehydes andketones in a [bmim].PF 6 H2 O-mixture. The reaction is an effec-

tive catalytic oxidation system, which leads to high yieldsusing N-chlorosuccinimide (NCS)/NaBr/TEMPO-IL. The [bmim].PF 6 ,together with the catalyst TEMPO-IL could be recycled for ten sub-sequent runs without any loss of activity neither in terms of yieldnor selectivity of the product [56] . Another effective system witha TEMPO functionalised imidazolium salt, a carboxylic acid sub-stituted imidazolium salt and NaNO 2 for the aerobic oxidation of alcohols was established by He and coworkers [57] .

Ogawa et al. compared the catalytic activity of tetranuclearvanadium(IV) complex bearing 3-hydroxypicolic acid (hpic) as lig-and ( Fig. 4) in organic solvents and [bmim].BF 4 [58] .

The authors stated that commercially available vanadium com-plexes are not active in the oxidation of benzyl alcohol. However,with 0.5mol% [(VO) 4 (hpic) 4 ]. in acetonitrile (MeCN) a yield of 62%

was reached with a selectivity of 100% to the corresponding alde-hyde. In protic solvents (EtOH) the yields are even higher (up to71%)but theselectivity is decreasedbecause thealdehydeis furtheroxidised to the corresponding acid. The oxidation of benzhydrol to

N

N

N

NO

O

O

O

O

O

O O

O

O

OOV V

V V

O O

OO

(VO) 4(hpic) 4

Fig. 4. Structure of the vanadium(IV) catalyst.


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Table 12Oxidation of 1-(naphthylen-2-yl) ethanol under variable conditions.

Parameter Yield (%)

Pressure (atm)2 602.7 933.4 99

Temp. ( C)80 79

90 92100 99Catalyst loading (%)

20 2630 5840 9950 9960 91

benzophenone was also investigated under an atmosphere of oxy-gen and [bmim].BF 4 as solvent. Benzophenone was formed with ayield of 64%.

In 2008 Halligudi and coworkers investigated the selective oxi-dation of alcohols by a heteropoly molybdovanadophosphoric acid(H5 PMo 10 V2 O40 ) supported ionic liquid-phase catalyst [59] . Thecompound was immobilised on a mesoporous silica SBA-15. Theauthors obtained high activity in both primary and secondaryalco-hols to the correspondingaldehydes and ketones, respectively. Thecatalyst showed no activity with respect to the oxidation of theketone and the aldehyde to the carboxylic acid. The authors per-formed the catalytic reactions in an autoclave under air pressure.Table 12 shows the results of the oxidation of 1-(naphthylen-2-yl)ethanol under different conditions.

The experiments were performed by dissolving the catalyst inMeCN and adding a radical initiator (AIBN or TBHP). At an oxy-gen pressure of 1 atm the yield was only 40% (increasing at higherpressure). With regard to the temperature the authors stated thatno conversion was obtained at 50 C. Under the best conditions theauthors investigated the activity of different substrates which aredepicted in the following Table 13 .

The authors were able to nd the conditions ( T =100 C, pres-sure: 3.4atm and c = 0.02 mol%) which led to high yields of 8399%in all investigated substrates.

Li and Xia studied the biphasic oxidative cyclocarbonylationof -aminoalcohols and 2-aminophenol to their corresponding 2-

Table 13Variation of the substrates.

Entry Substrate Time (h) Yield (%)

1 1-(Naphthylen-2-yl) ethanol 7 992 Diphenylmethanol 7 993 Cyclohexanol 6 994 Phenylethanol 6 995 2-Hexanol 5 986 2-Phenylpropanol 7 937 4-Methoxy phenylethanol 6 988 4-Methyl phenylethanol 6 969 4-Chloro phenylethanol 6 98

10 4-Bromo phenylethanol 6 9811 4-Nitro phenylethanol 6 9412 Benzoin 7 9513 Menthol 6 9614 [1,7,7]Trimethylbicyclo [2,2,1]heptan-2-ol 8 9515 3,5,5-Trimethylcyclohex-2-enol 7 9416 Benzyl alcohol 12 9817 1,3-Butanediol 8 8318 Geraniol 10 9719 Cinnamyl alcohol 13 9820 Pyridin-2-methanol 11 96

Reaction conditions: T =100 C; air pressure: 3.4 atm; catalyst concentration:

0.02mol%.

Scheme 6. Oxidative cyclocarbonylation of ethanolamine.

Scheme 7. The oxidation of various oximes with KMnO 4 .

oxazolidinones. Pd(phen)Cl 2 acts as the catalyst which is stabilisedby [bmim].I. Scheme 6 shows the oxidation reaction.

The best results were obtained using [bmim].I as an ionic liquidwith a TOF of 3288 h 1 and a conversion of 94% [60] . The selectiveoxidation of alcohols in high conversion and selectivity using 12-tungstophosphoricacid(H 3 PW12 O40 )/MCM-41 in ionic liquids wasstudied by Shen et al. The best performance of the photocatalyticsystem was obtained with a catalyst loading of 30%, [omim].BF 4 assolvent and oxygen as oxidant. The yields were between 90% and99%. The immobilisation resulted in an amorphous phase with a

BET surface of 632 m 2 /g, a pore volume of 0.53cm 2 /g with a porediameter of 29.7 A [61] .

The immobilisation of perruthenate (RuO 4 ) on 1-vinyl-3-butylimidazolium chloride leads to an active catalyst for the aerobicoxidation of benzyl alcohol to benzyl aldehyde. The catalytic reac-tion wasdone in supercritical CO 2 , toluene anddichloromethane at80 C. Hanet al.coulddemonstrate that the catalystwas very activeand highlyselective. Thereactionrate in CO 2 depended stronglyonpressure and reached a maximum at about 14MPa [62] .

4. Oxidation of oximes

Compounds such as aldoximes andketoximes are derivatives of carbonyl compounds and are, for example, used for the characteri-

sation and protection of carbonyl compounds. The regeneration of thecarbonylspecies is achievedby theoxidationof thecorrespond-ing oximes [63,64] . Safaei-Ghomi and Hajipour investigated theoxidation of oximes with KMnO 4 as an oxidising agent ( Scheme 7 )[65] .

The best results could be obtained with a 1:0.7:0.4 ratio of oxime:IL:KMnO 4 at room temperature. The authors stated thatthe RTIL inhibits the further oxidation of the carbonyl compoundto the respective carboxylic acid. In all the examined substrates

Table 14Oxidation of oximes with KMnO 4 and [bmim].Br.

Entry R 1 R 2 Time (min) Yield (%)

1 C6H

5 H 40 95

2 4-O 2 NC6 H4 H 9 >983 3-O 2 NC6 H4 H 18 954 4-MeOC 6 H4 H 53 915 3-MeOC 6 H4 H 42 946 2,4-(MeO) 2 NC6 H3 H 73 897 4-ClC6 H4 H 12 948 4-BrC 6 H4 H 17 979 2,6-Cl 2 NC6 H3 H 23 91

10 2,4-Cl 2 NC6 H3 H 10 >9811 4-MeC 6 H4 H 50 9312 2-MeC 6 H4 H 41 9513 3-MeC 6 H4 H 60 9114 C6 H5 C6 H5 55 8115 C6 H5 CH3 52 9216 4-ClC 6 H4 CH3 32 9317 4-BrC 6 H4 CH2 Br 17 9318 2-HOC 6 H4 CH3 28 90


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yields >81% are obtained. The results for various oximes are listedin Table 14 .

In 2009, Shaabani and Farhangi investigated the aerobic cleav-age of oximes in imidazole-based ionic liquids with phthalocyanin(Pc) catalysts [66] . The best results are obtained with Co-Pc cata-lysts and [bmim].Br as solvent. Table 15 gives an overview of theexamined substrates.

In addition, it was shown that the catalyst can be reused afterthe extraction of the product with just a minor activity loss (seeTable 15 , Entry 1). Another advantage of using CoPc/RTIL in con-trast to conventional methods is the performance under neutralconditions. Application of RTILs is also suitable for acid-sensitivefunctional groups.

5. Oxidation of olens

In 2008 Welton et al. used a number of ionic liquidsas co-solvents for the catalytic epoxidation of alkenes withOxone (KHSO5 )and N -alkyl-3,4-dihydroisoquinolinium salts [67] .Because of the possible oxidation of imidazolium based ILs,pyridinium cations were preferred. Welton et al. found that epox-idations carried out in water soluble ILs are not more efcientthan those performed in MeCN. The results of the epoxidationof different substrates by 2-methyl-3,4-dihydroisoquinoliniumtetrauoroborate [mdhqm].NTf 2 catalysts are depicted in Table 16 .

The 2-methyl-3,4-dihydroisoquinolinium cation is a compoundwhichis able to catalyse theolenepoxidation without aninvolvedmetal. The mechanism is shown in Scheme 8 .

The authors tried a range of ILs as a co-solvent for the oxida-tion of 1-phenyl-cyclohexene at room temperature. Interestingly,in water immiscible ILs the epoxidation does not take place. With

Fig. 5. The investigated dioxomolybdenum(VI) complex.

water miscible ILs the authors described the conversions of 63%in case of [bmim].OTf and 53% with [bmim].BF 4 to the corre-sponding epoxide. The authors explained this phenomenon withphase-transfer problems of the HSO 5 in the case of the biphasicsystem. In recent years, there were some publications dealing withthe catalytic epoxidation of olens with different molybdenumcompounds as catalysts. Valente et al. described dioxomolybde-num(VI) complex bearing an anionic N,O oxazoline ligand ( Fig. 5)[68] .

Fig. 6. The investigated Mo complexes.

NR

NR

NR

KHSO 5

OOSO 3-

-SO 42-

O

R 3

R1

R2R 3

R1

R2

O

Scheme 8. Olen epoxidation without an involved metal-containing catalyst.


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Table 15Oxidation of oximes with Co-Pc/[bmim].Br at 70 C.

Entry Educt Product Time (min) Yield (%)

1 Ph

HON O

Ph Ph

O O

Ph 60 90, 88, 87

2 Ph

HON OH

Ph Ph

O OH

Ph 60 91

3

Me

NOH

H

Me

O

H 30 89

4

H 3 CO

NOH

H

H 3 CO

O

H 50 92

5

Cl

NOH

H

Cl

O

H 40 93

6

Br

NOH

H

Br

O

H 40 92

7

O 2 N

NOH

H

O 2N

O

H 30 80

8

NOH

H

O 2 N

O

H

O 2N 30 88

9

O

NOH

HPh

O

O

HPh 40 91

10H

NOH

H

O

50 86

11

NOH

H

Cl

Cl

O

H

Cl

Cl 70 83

12

NOH

Me

O

Me 80 82

13

Cl

NOH

Me

Cl

O

Me 60 87

14

NOH

Me

Br

O

Me

Br

40 85

15

NOH O

150 85

16

S

NOH

HS

O

H 200 80

17NOH O

40 88


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Mo

SCN

SCN

NCS

NCSO

O(PPh 4)2 Mo

SCN

SCN

N

N

O

OMoNCS O

N N

O OMo SCN

NN

OO

54 6

Fig. 8. Mo complexes used for the epoxidation of olens by Pillinger et al.

Table 18Epoxidation of methyl oleate and methyl linoleate catalysed by MoO(O 2 )2 2QOH.

Entry Solvent Conversion (%) Selectivity (%) TON (TOF)

Methyl oleate Methyl linoleate

1 No solvent 55 31 90 3690 (1845)2 [bmim].BF 4 92 78 93 7812 (3906)3 [bmim].PF 6 75 44 94 5358 (2679)4 [Hydemim].BF 4 96 89 95 8740 (4370)5 CH3 CN/30% CH3 CN 85 63 92 6624 (3312)6 70% [Hydemim].BF 4 94 84 95 8360 (4180)7 C2 H5 OH/30% C2 H5 OH 81 45 93 5580 (2790)8 70% [Hydemim].BF 4 90 74 95 7695 (3848)

Reaction conditions: T = 30 C, t = 2 h, oxidant: H 2 O2 , 0.01 mol% catalyst, co-catalyst: NaHCO 3 .

For the investigated substrate mixture, especially [bmim].BF 4and [Hydemim].BF 4 showed high activities. Table 18 shows theconversions, selectivities and TOFs of the investigated ionic liquids.

The authors obtained the best results when using ILs withBF4 counterions. The polar character of these species may play a

key role in the epoxidation reaction. The positive effect concern-ing the activity of the catalyst was conrmed by the addition of [Hydemim].BF 4 to an organic solvent. Both the conversion and theselectivity to the correspondingepoxide increased. In addition, thecatalyticsystem could be recycled for at least ve runs (by washingwith diethyl ether and drying) without any loss of selectivity and just a minor drop in conversion of methyl oleate (87%) and methyllinoleate (82%).

In 2010, Goncalves et al. described different cationic molybde-num(VI) dioxo complexes containing weakly coordinating anionsfor the cyclooctene epoxidation ( Fig. 10 ) [75] .

The catalysts were used at 55 C in different solvents (DCE,[bmim].BF 4 , [bmPy].BF 4 , [bmim].PF 6 and [bmPy].PF 6 ). The perfor-mance with DCEas a solvent results in yields between 61%and 98%

after 24h with a selectivity of 100%. In all solvents complex 9 is lesssoluble than 7 and 8, resulting in the lowest conversions.

Fig. 9. Oxo-bisperoxo Mo catalyst.

In contrast to compound 9 , 7 could be completely dissolved inall ILs. Compound 8 was completely dissolved in the BF 4 -type ILs,but was poorly soluble in PF 6 -type ILs. The different solubility of the catalysts is obviously the most importantfactor concerningtheactivity. Table 19 shows the results of the catalytic reactions.

The authors stated that the reaction proceeds as a heteroge-neous process,because theorganicphase remainedcolorless, whilethe IL phase is yellow, because of the active species being formed.Theauthorsadditionallycomparedthe results whenusingdifferentoxidants in the oxidation of cyclooctene ( Table 20 ).

In the case of water-free oxidants (TBHP in decane, UHP) thereactions are signicantly faster, most presumably because of thecoordinating properties of water which nally leads to a less active

MoClO

O

N N

N

N

NN

CH[Y]-

Y = Cl (7 ), BF 4 (8), PF 6 (9)

Fig. 10. Cationic Mo-catalyst.


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Table 19Epoxidation of cyclooctene catalysed by 79 .

Solvent Catalyst Conversion (%) TOF (h 1 )

DCE 7 96 2018 98 1689 61 69

[bmim].BF 4 7 78 768 75 919 40 15

[bmPy].BF 4 7 80 648 81 1029 40 15

[bmim].PF 6 7 91 1428 45 679 42 22

[bmPy].PF 6 7 94 1638 51 739 40 19

Table 20Catalytic epoxidation of cyclooctene with 7 in [bmim].PF 6 using different oxidants.

Oxidant Conversion (%) TOF (h 1 )

TBHP (decane) 91 142TBHP (aq) 50 18

H2 O2 (aq) 38 UHP 92 16

catalytic species. Interestingly, even when applying aqueous TBHPor H 2 O2 the authors did not observe any diol formation.

The catalytic system 7/[bmim].PF 6 /TBHP was nally used toinvestigate the epoxidation of different olens. Cyclooctene gavethe best results under the applied conditions (91%). It is followedby norbornene (55%), cyclohexene (37%) and styrene -pinene(15%).

Tsang et al. described the palladium-catalysed oxidation of styrene using differentmulticarboxylicacid appendedimidazoliumILs (Fig. 11 ) as reaction medium [76] .

Thetreatment of the ionic liquids with the precatalyst PdCl 2 ,ledto the formation of a species containing the PdCl 4 2 or PdCl 2 Br2 2anion, which was shown to be an active catalyst of the selectivecatalytic oxidation of styrene to acetophenone with hydrogen per-oxideas oxygen source. Comparedwithneat PdCl 2 , the investigatedsystem requires less PdCl 2 and is more active. The turnover fre-quency reached a maximum of 146h 1 , a conversion of 100% witha selectivity of 93%. The same reaction without an IL led to a TOFof 21 h 1 with a conversion of 25% and a selectivity of 86% to ace-tophenone. Table 21 shows the results of the investigated systems.

After investigating all the possible combinations the authorsfound that the reaction temperature was the most important fac-tor for the rate of the oxidation, independent of the nature of thecation and anions. A great advantage of the multicarboxylic acidcontaining ILs compared to [bmim].BF 4 is the possibility of reusing

the catalytic system. With [bmim].BF 4 , the selectivity decreased to


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NN COOH

COOH

HOOCCl -

NN COOH

COOH

Br -NNHOOC

Cl -COOH

10 11 12

Fig. 11. Multicarboxylic acid containing ILs applied by Tsang et al.

that the rate of BTSP-decomposition is slower than the rate of theoxidation reaction only if a co-catalyst is present. The comparisonof different catalysts (BF 3 OEt2 , AlCl3 , SnCl3 ) in [bmim].NTf 2 anddichloromethane leads to similar results for the ionic liquid, whilethedifferences between thecatalystsin dichloromethane are muchmore pronounced ( Table 22 ).

The only catalyst free oxidation of cyclic ketones could be car-ried out in [bmim].OTf ( Table 23 ). The reason for that is, due to theauthors, the inuence of the OTf anion. To proof their statementcatalysis was successfully performed in dichloromethane togetherwith NaOTf ( Table 23 ). The proposed reaction mechanism is shownin Scheme 11 .

In further studies, Chrobok studied the BaeyerVilliger reac-tion with molecular oxygen as an oxygen source in the presenceof benzaldehyde [83] , as this combination showed good resultsin previous examinations [84] . During this reaction benzaldehydeis converted to benzoic acid. The addition of a radial initiatorsuch as 1,1 -azobis(cyclohexanecarbonitrile) (ACHN) increases thereaction rate of the lactonisation by a factor of four. The optimalconcentration of ACHN was found to be 0.033mol/L and the bestsubstrate to benzaldehyde ratio was 1:2. The addition of Fe 2 O3 asa co-catalyst does not accelerate the reaction. The catalytic per-formance in different ionic liquids and with different substrates issummarised in Table 24 .

Table 23Oxidation of ketones in [bmim].OTf.

Entry Ketone Lactone Time (h) Temp. ( C) Yield (%)

1

O O

O1 25 98

2O O

O

2 25 95

3

O O

O

2 25 94

4

OO

O

O

O

2 25 96

5

O

O

O

8 40 94

6

OO

O

10 40 41

7 O O O 24 40 (87)

8 O

O

O

OO

15 40 (89)

9 O

O

O 8 25 99

Reaction conditions: ketone (0.5 mmol), BTSP (1mmol), [bmim].OTf (2 mL); yield determined by GC; isolated yields in parenthesis.


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Table 24Lactonisation of various ketones with oxygen.

Entry Ketone Lactone Solvent Time (h) Conversion (%) Yield lactone (%)

1

O O

O

[bmp].NTf 2 2.5 95 892 : : [tmba].NTf 2 2.5 95 883 : : [bmim].BF 4 2.5 90 854 : : [emim].OSO 3 Me 2.5 85 765 : : [bmim].CF 3 COO 2.5 80 726 : : [bmim].OTf 2.5 62 557 : : [bmim].NTf 2 2.5 96 90

8O O

O

[bmim].NTf 2 2 99 85

9

OO

O

O

O

[bmim].NTf 2 2 99 95

10 O O O [bmim].NTf 2 4 100 96

11 O

O

O [bmim].NTf 2 4 100 94

12

OO

O [bmim].NTf 2 10 49 40

Reaction conditions: ketone (3mmol); benzaldehyde (6 mmol); ACHN (0.033mol/L); solvent (2 mL); 90 C; yields determined by GC.

The ionic liquids could be recovered via extraction methodsafter the reaction, this is possible, whether they are hydrophilicor hydrophobic. The same ionic liquids could be used for four runs

without activity loss.Another method to reuse ionic liquids is the heterogenisation of [pmim].HSO 4 on a silica support ( Scheme 12 ) [85] . The tetheringof the catalyst (HSO 4 ) is obtained via cation-anion interaction. TheBaeyerVilliger reaction was performed with H 2 O2 (68%) at 50 Cin dichloromethane as solvent, the data are given in Table 25 .

Recycling experiments showedno loss of activityand 90% of thecatalyst could be recovered after each of the four runs.

7. Special oxidations

7.1. Oxidation of nitrotoluene

The oxidation of nitrotoluene and derivatives to their corre-spondingnitrobenzoicacids withmolecular oxygen in ionic liquids

Scheme 11. Proposed reaction mechanism of the BaeyerVilliger oxidation of ketones by BTSP in the presence of [bmim].OTf.


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Scheme 12. Heterogenisation of [pmim].HSO 4 on a silica support.

was rst reported in 2009 [4]. Shan et al. performed the oxida-tion in a biphasic system containing an aqueous sodium hydroxidesolution and different ionic liquids. As catalyst for the oxidationof para -nitrotoluene(PNT), different metallo-phthalocyaninecom-plexes were tested ( Table 26 ). The special behaviour of the chosenionic liquid ([omim].BF 4 ), to be miscible with water at tempera-tureshigher70 C andimmisciblebelow, leads toaneasy separationof product and catalyst. The reaction was performed at 90 C inhomogeneous phase. After cooling down the reaction mixture, thewater and the ionic liquid phases separate, whereas the productis soluble only in the aqueous phase and catalyst and substratein the ionic liquid. The catalyst was recycled by simple extractionmethods and was reused for at least ve more runs without loss of activity ( Table 26 , Entry 11: cat. used for 6 times). Other substratesused were ortho- (ONT), meta- (MNT), di-nitrotoluene (DNT) andtoluene.Asitcanbeseenfromthedatain Table26 , MNTandtolueneshow no activity due to their relatively low deprotonation abilityin alkali aqueous solution which plays a major role in the reactionpathway towards benzoic acid [86] . The major role of NaOH can beseen in a decrease of the yield with decreasing NaOH concentration(compare Table 26 , Entries 1 and 5).

7.2. Carbonylation

In 2008, ionic liquids were used as solvents for copper catal-ysed carbonylation of methanol to dimethyl carbonate (DMC) by

Liu and coworkers [87] . Besides DMC, the main product, threeother by-products were detected: dimethoxymethane (DMM),dimethylether (DME) and methylformate. Amongst these byprod-ucts, DME reaches normally the highest yields. The different ionicliquids and catalysts as well as the space-time yield (STY), conver-sionand selectivitiesare listed in Table27 . The catalysiswas usuallyperformed with 1mmol catalyst, 4g MeOH, 2g ionic liquid and2.4MPa CO and O 2 (ratio 2:1) at room temperature. A model sys-tem containing 1 mmol CuCl, 4 g MeOH, 2 g [bPy].BF 4 and 2.4MPaCO/O2 (ratio 2:1) at 120 C was chosen to study different reactionparameters. After 4h the conversion of MeOH stopped and no dif-ferences regarding selectivity were observed. Higher gas pressures(from2.4MPato6.0MPa)ofCOandO 2 leadto higher conversions of MeOH (from 19.8% to 37.1%)while theselectivityof DMCremainedstable at nearly 100%. Withtemperatureshigherthan 120 C, higherconversions could be reached, however, the selectivity of DMC alsodecreases.An increase of the ratio ofCO toO 2 hampers the reactionmore instead of increasing the selectivity of DMC.

In 2010, Stricker et al. inserted the copper catalyst directly intothe ionic liquid [88] . They prepared three different types of cata-lysts, tetrakis(1-dodecylimidazole)copper(I)hexauorophosphate[Cu(Im 12 )4 ][PF6 ], bis(1-dodecylimidazole)cuproniumdihalogeno-cuprate [Cu(Im 12 )2 ][CuX2 ] and[dmim]. n [CuX2n ] for the carbonyla-tionof methanol. Withthe applied conditions, the catalysisshowedbetter conversions and similar selectivities compared to the resultsof Liu and coworkers ( Table 28 ).


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Table 25BaeyerVilliger reaction with different ketones and a heterogenised ionic liquid.

Entry Ketone Lactone H 2 O2 (mol/mol ketone) Catalyst (g) Time (h) Conversion (%) Yield (%)

1O O

O

3 8 31 62 : : 3 0.4 (silica support) 8 30 53 : : 2 0.4 (pmimHSO 4SiO2 ) 8 86 604 : : 3 0.4 (pmimHSO 4SiO2 ) 5 75 555 : : 3 0.4 (pmimHSO 4SiO2 ) 8 98 756 : : 3 0.2 (pmimHSO 4SiO2 ) 8 60 457 : : 3 0.6 (pmimHSO 4SiO2 ) 8 98 748 : : 4 0.4 (pmimHSO 4SiO2 ) 8 98 659 : : 3 0.4 (pmimHSO 4SiO2 ) 8 30 6

10 : : 3 0.8 (tbapHSO 4SiO2 ) 8 95 72

11

O O

O3 0.4 (pmimHSO 4SiO2 ) 5 100 96

12

O O

O

3 0.4 (pmimHSO 4SiO2 ) 15 86 64

13 O O O 3 0.4 (pmimHSO 4SiO2 ) 10 95 (89)

14 O

O

O 3 0.4 (pmimHSO 4SiO2 ) 10 95 (88)

15

OO

O

3 0.4 (pmimHSO 4SiO2 ) 15 81 (75)

16

OO

O

3 0.4 (pmimHSO 4SiO2 ) 20 78 (74)

17 O

O

O

OO

3 0.4 (pmimHSO 4SiO2 ) 20 64 60

Reaction conditions: ketone (1 mmol); H 2 O2 (68%); dry DCM (2mL); 50 C; yield determined by GC; isolated yields in parentheses.

Table 26Oxidation of nitrotoluenes by molecular oxygen.

Entry Substrate Ionic liquid Catalyst (mg) NaOH (g) P(O 2 ) (MPa) Yield (%)

1 PNT [omim].BF 4 FeIIPc; 10 1.5 2 922 PNT [omim].BF 4 FeIIIPc; 10 1.5 2 89

3 PNT [omim].BF 4 CuII

Pc; 10 1.5 2 704 PNT [omim].BF 4 FeIIPc; 5 1.5 2 855 PNT [omim].BF 4 FeIIPc; 10 1 2 706 PNT [omim].BF 4 FeIIPc; 10 1.5 2.5 937 PNT [omim].BF 4 FeIIPc; 10 1.5 1.5 848 PNT [omim].Tf 2 N FeIIPc; 10 1.5 2 139 PNT [dmim].BF 4 FeIIPc; 10 1.5 2 72

10 PNT Fe IIPc; 10 1.5 2 11 PNT [omim].BF 4 FeIIPc; 10 1.5 2 9212 ONT [omim].BF 4 FeIIPc; 10 1.5 2 9313 MNT [omim].BF 4 FeIIPc; 10 1.5 2 14 DNT [omim].BF 4 FeIIPc; 10 1.5 2 9315 Toluene [omim].BF 4 FeIIPc; 10 1.5 2

Reaction conditions: 0.2 mmol substrate; 10mL ionic liquid; 5 mL water; 90 C; 12h.


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Scheme 13. Oxidation of cysteine to cystine.

Table 31

Oxidation of halides in ionic liquids.Entry Ionic liquid Product Time (h) Yield (%)

1 1b 4 422 [bmim][FeCl 4 ] 1b 2 753 [hmim][FeCl 4 ] 1b 2 784 [omim][FeCl 4 ] 1b 2 835 [dmim][FeCl 4 ] 1b 2 916 [C12 mim][FeCl 4 ] 1b 2 947 [C12 mim][FeCl 4 ] 2b 1.5 938 [C12 mim][FeCl 4 ] 3b 1.5 939 [C12 mim][FeCl 4 ] 4b 1.5 95

10 [C12 mim][FeCl 4 ] 5b 1.5 9611 [C12 mim][FeCl 4 ] 6b 1.5 9812 [C12 mim][FeCl 4 ] 7b 1.5 9813 [C12 mim][FeCl 4 ] 8b 1.5 9514 [C12 mim][FeCl 4 ] 9b 2 9215 [C

12mim][FeCl

4] 10b 2.5 91

16 [C12 mim][FeCl 4 ] 11b 1.5 9717 [C12 mim][FeCl 4 ] 12b 2 9618 [C12 mim][FeCl 4 ] 13b 2 9319 [C12 mim][FeCl 4 ] 14b 2.5 9520 [C12 mim][FeCl 4 ] 15b 2.5 9021 [C12 mim][FeCl 4 ] 16b 2 9422 [C12 mim][FeCl 4 ] 17b 3 8823 [C12 mim][FeCl 4 ] 18b 3 90

Reaction conditions: organic halide (10mmol); H 5 IO6 (11 mmol); 30 C; ionic liquid (0.4 mmol).

Table 32Oxidation of organic halides with H 5 IO6 /V2 O5 .

Entry Catalyst Ionic liquid Product Time (h) Yield (%)

1 V2 O5 2b 10 532 [bmpy].PF 6 2b 6 483 V2 O5 [bmim].BF 4 2b 7 744 V2 O5 [bmim].Cl 2b 8 685 V2 O5 [hmim].OTf 2b 6 766 V2 O5 [hmim].PF 6 2b 6 837 V2 O5 [bpy].PF 6 2b 3 898 V2 O5 [bmpy].PF 6 2b 3 979 V2 O5 [bmpy].PF 6 1b 3 92

10 V2 O5 [bmpy].PF 6 3b 5 9311 V 2 O5 [bmpy].PF 6 4b 3 9812 V2 O5 [bmpy].PF 6 5b 3 9613 V2 O5 [bmpy].PF 6 6b 3 9814 V2 O5 [bmpy].PF 6 7b 3 9915 V2 O5 [bmpy].PF 6 8b 3 9616 V2 O5 [bmpy].PF 6 9b 6 8917 V

2O

5 [bmpy].PF

6 10b 8 87

18 V2 O5 [bmpy].PF 6 11b 3 9619 V2 O5 [bmpy].PF 6 12b 3 9320 V2 O5 [bmpy].PF 6 13b 6 9121 V2 O5 [bmpy].PF 6 14b 6 9422 V2 O5 [bmpy].PF 6 15b 10 8723 V2 O5 [bmpy].PF 6 16b 3 9224 V2 O5 [bmpy].PF 6 17b 24 6325 V2 O5 [bmpy].PF 6 18b 24 7526 V2 O5 [bmpy].PF 6 19b 3 9027 V2 O5 [bmpy].PF 6 20b 3 9528 V2 O5 [bmpy].PF 6 21b 6 89

Reaction conditions: organic halide (10mmol); V 2 O5 (0.3 mmol); H 5 IO6 (12 mmol); ionic liquid (5 mL); 50 C.


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I O

N N

Br

O N N

Br

O

I

N N

Br

I

A

B

C

D

I O N N

Br

Fig. 12. IL-supported PhI A D.

mixture of cyclohexanol, cyclohexanone and with ZSM-5 cata-lysts cyclohexyl hydroperoxide (CHHP). Temperature screeningsrevealed 90 C as the optimal temperature and the best ratio of substrate and oxidant was found to be 1:2. Iron turned out to bethe most active metal for the oxidation of cyclohexane and recy-cling of the catalyst by decantation lead to no decrease in activityafter four runs.

7.5. Oxidation of halides

The oxidation of halides to ketones and aldehydes with H 5 IO6proceededbetter if ionic liquids were doted to the reactionmixture(Table 31 ) [92] . Further studies dealt with V 2 O5 as catalyst in ionicliquids ( Table 32 ) [93] . The differentoxidised species for both waysare shown in Scheme 14 .

7.6. -Tosyloxilation of ketones

The oxidation of various ketones was carried out in [emim].OTsas solvent as well as small amounts of IL-supported PhI A D(Fig. 12 ) together with m -chloroperbenzoic acid (MCPBA) and p-toluenesulfonic acid (PTSA H2 O) [94] .

The different products and yields are summarised in Table 33 .After extraction of theproductsand oxidants the ionic liquids werereused twice with small losses in activity.

Table 33Tosyloxidation of ketones.

Entry Product Yields (%)

A B C D

1 OTs

O

82 83 80 70

2 OTs

O

60 58 39 48

3OTs

O

Cl 77 76 67 67

4

OTs

O

O 2 N 65 70 72 71

5a OTs

O

83 75 63 70

6a OTs

O

81 75 52 56

7

O

OTs 57 64 61 69

8

O

OTs 57 66 62 53

Reactionconditions:substrate(1 mmol); MCPBA (1.3equiv.);PTSA H2 O (1.1equiv.);A, B, C or D (0.1equiv.); [emim].OTs (2 mL); 50 C; 5h.

a PTSA H2 O (0.5 equiv.).

7.7. Synthesis of thiazoles

Additionally to tosyloxylation the condensation of acetophe-none and thioamides to thiazoles was studied by Akiike et al.Applying the same conditions as used in the tosyloxilation, yieldsbetween 37% and 72% were reached [94] .

Wanget al. studied theoxidativereactionof 2-aminothiophenoland aldehydes to thiazoles with air as oxidant ( Scheme 15 ) [95] .

The experimental data is summarised in Table 34 . Extraction of theionic liquidwithdiethyl ether andreuseof the IL forthreetimeslead only toa slight decrease ofyield. Moreover,the authors appliedtheir catalytic procedure to synthesise 5-heteroaryl-substituted-2 -deoxyuridines.

Scheme 15. Oxidative condensation towards thiazoles.


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Table 34Thiazole formation.

Entry R 1 R 2 Solvent Catalyst (0.05 equiv.) Time (h) Temp. ( C) Yield (%)

1 C6 H5 H THF RuCl3 3 Reux 682 C6 H5 H CH3 CN RuCl3 3 Reux 623 C6 H5 H Toluene RuCl 3 3 80 504 C6 H5 H dichloromethane RuCl 3 3 Reux 615 C6 H5 H [bmim].PF 6 3 80 Trace6 C6 H5 H : InCl3 2 80 Trace

7 C6 H5 H : CeCl3 2 80 Trace8 C6 H5 H [bmim].BF 4 RuCl3 0.5 80 759 C6 H5 H [bmmim].PF 6 : : 80 63

10 C6 H5 H [bmim].PF 6 : : 60 7811 C6 H5 H : : : 80 8312 4-BrC 6 H4 H : : : : 8513 4-CH 3 OC6 H4 H : : : : 8014 4-CNC 6 H4 H : : : : 8515 2-BrC 6 H4 H : : 1 : 8116 2-NO 2 C6 H4 H : : 0.5 : 8217 2-ClC 6 H4 H : : 1 : 8118 3-NO 2 C6 H4 H : : 0.5 : 8619 3-BrC 6 H4 H : : : : 8420 3-CH 3 C6 H4 H : : : : 7921 C6 H5 4-Cl : : : : 8322 4-NO 2 C6 H4 4-Cl : : : : 8823 4-BrC 6 H4 4-Cl : : : : 8324 4-CH 3 OC6 H4 4-Cl : : : : 8225 3-ClC 6 H4 4-Cl : : : : 8326 3-BrC 6 H4 4-Cl : : : : 8027 2-NO 2 C6 H4 4-Cl : : : : 8028 2-ClC 6 H4 4-Cl : : 1 : 8029 2-BrC 6 H4 4-Cl : : 1 : 7930 N-propyl H : : 2 : 7531 N-propyl 4-Cl : : 2 : 7632 C6 H5 3-Cl : : 4 : 6233 4-NO 2 C6 H4 3-Cl : : 3 : 7534 3-CH 3 C6 H4 3-Cl : : 4 : 6035 2-NO 2 C6 H4 3-Cl : : 6 : 4336 4-BrC 6 H4 4-CH3 : : 2 : 8037 3-CH 3 C6 H4 4-CH3 : : 2 : 78

Reaction conditions: starting materials (1 mmol); solvent (1mL).

8. Conclusion and perspective

Clearly, ionic liquids attract more and more attention in manyelds of catalytic applications. This broadening attractivity rangesfrom well examined reactions, like the oxidative desulfurisation of organic compounds, to more uncommon reactions, e.g. the oxida-tion of oximes. In anycase,ionicliquids are usually able to improvethe performance of the catalytic reaction. They could thereforecer-tainly act as substitutes for conventional organic solvents, in somecases even as catalysts themselves or as extraction media for theseparation of products. The transfer of ionic liquids from lab-scaleapplication to industrial processes would be desirable especiallyfrom the green chemical point of view and will probably happenin the future due to their signicant advantages as well. Particu-larly the recycling of the catalyticsystem might play a keyrole withregard to sustainability, provided IL are environmentally neutral orbenign.

Acknowledgements

D.B. thanks the Bayerische Forschungsstiftung for a Ph.D. grant.P.A. thanks Sdchemie AG for nancial support.

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