Fine tuning of the structure of phosphine–phosphoramidites: application for rhodium-catalyzed asymmetric hydrogenations

Tetrahedron: Asymmetry 24 (2013) 66–74

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Tetrahedron: Asymmetry

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Fine tuning of the structure of phosphine–phosphoramidites: applicationfor rhodium-catalyzed asymmetric hydrogenations

Szabolcs Balogh a,�, Gergely Farkas a, Áron Szöll}osy b, Ferenc Darvas c, László Ürge c, József Bakos a,⇑a Department of Organic Chemistry, University of Pannonia, 8200 Veszprém, Egyetem u. 10, Hungaryb Department of Inorganic and Analytical Chemistry, University of Technology and Economics, 1111 Budapest, M}uegyetem rkp. 3-9, Hungaryc ThalesNano Nanotechnology Inc., 1031 Budapest, Záhony u. 7, Graphisoft Park, Hungary

a r t i c l e i n f o

Article history:Received 22 October 2012Accepted 16 November 2012

0957-4166/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.tetasy.2012.11.013

⇑ Corresponding author. Tel.: +36 88624355; fax: +E-mail address: [email protected] (J. Bakos).

� Current address: MR laboratory, University of Pan8200 Veszprém, Egyetem u. 10, Hungary.

a b s t r a c t

Diastereomers of (4-(diphenylphosphino)pentan-2-yl)-N-isopropyl-{dinaphtho[2,1-d:10 ,20-f][1,3,2]dioxa-phosphepin-2-yl}-4-amine, (S)-(2S,4S)-1, and (S)-(2R,4R)-3; the octahydro derivative 4 of (S)-(2S,4S)-1,and derivative 2 of (S)-(2S,4S)-1 containing a 1,3-propanediyl backbone, have been synthesized and usedfor rhodium-catalyzed asymmetric hydrogenations of prochiral olefins in order to study the role of thestereogenic elements in the backbone and in the terminal moiety. The central chirality in the bridgehas been found to determine the configuration of the product with a cooperative effect from the terminalgroups. Excellent ee’s (up to 99.9%) were obtained in the hydrogenation of methyl (Z)-a-acetamidocinna-mate using a rhodium complex with the matched arrangement (S)-(2S,4S)-1. The hydrogenation isaccomplished in a highly enantioselective manner by using green solvents such as propylene carbonate.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Asymmetric hydrogenation of olefins is an appropriate methodin terms of selectivity, activity, and economic viability for prepar-ing optically active compounds.1 Chiral bidentate ligands withtwo phosphorus donor atoms are the most widely used com-pounds in asymmetric hydrogenation reactions.2 It is well knownthat hetero-bidentate ligands provide overall excellent selectivi-ties.3 Among them, phosphine–phosphoramidites4 have emergedover the last decade as a new class of ligands suitable for asymmet-ric catalytic syntheses including asymmetric hydrogenations,5 Cu-catalyzed asymmetric additions of diethylzinc to enones,6 andasymmetric hydroformylations.7 They form five to seven-mem-bered chelate rings with the metal and some of these catalysts pro-vide high selectivity in asymmetric transformations.

While in the review of Crévisy, phosphine–phosphoramiditesare divided into four classes based on their amine building blocks:cyclic amines, ferrocene, benzyl/aryl-amines, and chiral-pooldiphenylphosphino-amines,8 Reek et al. have introduced an alter-native classification based on the linker unit between the phos-phine and phosphoramidite. The linker can be a rigid cyclicamine (class 1), a rigid (bi)cyclic moiety (class 2), or a flexible chaincontaining at least one non-cyclic sp3-hybridized carbon atom(class 3).9 All of the phosphine–phosphoramidites have the same

ll rights reserved.

36 88624469.

nonia, Institute of Chemistry,

terminal moieties, predominantly diphenylphosphine and diox-aphosphepine (Fig. 1).

Leitner et al. reported the first example of a phosphine–phospho-ramidite ligand, QUINAPHOS, and demonstrated that this ligand ishighly enantioselective for the Rh-catalyzed asymmetric hydroge-nation of benchmark substrates such as dimethyl itaconate(ee = 99%) and methyl 2-acetamidoacrylate (ee = 98%) or in the Ru-catalyzed hydrogenation of ketones with ee’s of up to 94%. (Ra,Rc)-and (Ra,Sc)-QUINAPHOS yield 4.8% (S) and 74% (S) ee’s in the asym-metric hydroformylation of styrene, respectively.4 A pronouncedmatched/mismatched effect was observed for the configuration ofthe two stereocenters. Zheng et al. showed, that (Sc,Sa)- and (Sc,Ra)-PEAPhos give 99.1% (R) and 48.5% (R) in the asymmetric hydrogena-tion of methyl (Z)-a-acetamidocinnamate, and 99.5% (R) and 73.9%(R) in the asymmetric hydrogenation of N-(1-phenylvinyl)acetam-ide, respectively.10 These results indicate that the absolute configu-ration of the product is controlled by the chirality of the bridge.

The same research group introduced the ferrocene based PPFA-Phos ligand containing three chiral elements.11This ligand pro-vided ee’s of 99.6% (S) and 82.6% (R) with (Sc,Sp,Ra) and (Sc,Sp,Sa)configurations, respectively, in the asymmetric hydrogenation ofN-(1-phenylvinyl)acetamide. The results clearly indicate that theconfiguration of the product and the selectivity of the hydrogena-tion are mainly determined by the axial chirality of the BINOLbackbone. These examples show that the properly chosen chiralbridge associated with the matching terminal chiral moieties canlead to viable ligands in asymmetric transformations.

A review on the application of organic carbonates as solvents incatalysis has recently been published.12 Until now, organic cyclic

N

P(R1)2

PO

O

R2

R2

N

n-Bu

PPh2

PO

O

(Ra,Sc)-nBu-QUINAPHOS

PPh2

NP

O

O

(Sc,Sa)-PEAPHOS

FePPh2

N PO

O

(Sc,Sp,Sa)-PPFAPhos

PPh2

NR

PO

O

(S)-AnilaPhosderivatives

HN

PPh2

PO

O

(R,R)-THNAPhos (S)-HY-Phos

HN

PPh2

PO

O

(S)-indolphosderivatives

N PO

O

Crévisy's ligandwith one stereogenic center

PPh2

Figure 1. Selected phosphine–phosphoramidite ligands.

S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74 67

carbonates have not played a major role as solvents for asymmetrichydrogenations.13 The exceptions in homogeneous catalysis in-clude the platinum-catalyzed hydrosilylations of unsaturated fattyacids investigated by Behr et al.;14 regioselective, rhodium-cata-lyzed hydroformylations;15 iridium- and rhodium-catalyzed asym-metric hydrogenations and palladium-catalyzed asymmetric allylicalkylations by Börner et al.;16 palladium-catalyzed Heck reactionby Reetz et al.,17 and rhodium-catalyzed intermolecular alkynehydroacylations by Willis et al.18

Recently, we reported a new phosphine–phosphoramidite li-gand containing an aliphatic chiral linker, which provided excel-lent selectivity and activity in the asymmetric hydrogenation ofchallenging substrates.19 These promising catalytic properties in-spired us to study the role of the stereogenic elements in thebridge. Herein our aim was the development of effective chiral li-gands that could be easily prepared and successfully applied inasymmetric synthesis using alkylene carbonates as green solvents.

2. Results and discussion

2.1. Ligand synthesis methods

Chiral aminoalkylphosphines can be conveniently preparedfrom cyclic sulfate esters, which are synthesized according to the

OS

O

O O

i-Pr-NH2

+H2N

O

Scheme 1. Synthesis of aminoalkylp

Sharpless’ method.20 The ring opening of a cyclic sulfate with anamine takes place smoothly, with complete inversion at the stere-ogenic center to give the sulfated amine (Scheme 1). Subsequentreaction with lithium-diphenylphosphide gave the desired amin-oalkylphosphine with complete inversion at the stereogenic centerand with good yield.

These valuable intermediate aminoalkylphosphines A(Scheme 2) can be combined with chlorophosphites to obtainphosphoramidite products. Despite the relatively strong basicityof aminoalkylphosphines, the reaction only takes place if the aminecontains the 1,3-propanediyl backbone (S)-2 (Fig. 2). In the case ofaminoalkylphosphines with a 2,4-pentanediyl backbone 1, 3, and4, the reaction did not take place, presumably due to steric hin-drance around nitrogen.

In the next set of experiments, the aminoalkyl-phosphine wastreated with PCl3 in the presence of triethylamine, which led toamino-dichlorophosphine B. After the addition of the correspond-ing diol in the presence of triethylamine, the desired compound Dwas obtained with up to 32% yield. The diastereomers 1 and 3 werereadily prepared by the reaction of (S)-1,10-bi-2-naphthol with(2S,4S)- and (2R,4R)-amino-dichlorophosphine B, respectively.

Next we attempted to increase the yield of the product by theactivation of the amine. Lithium amide C of the correspondingamine was formed with Li metal in the presence of styrene.21

SO3-

LiPPh2HN

PPh2

hosphines from cyclic sulfates.

N

NEt3, THF

PCl3

NEt3, THF

15-32 %

NEt3, THF

Li / styreneBuLi

20 %

NEt3, THF

RR

PPh2

Cl2P HN

RR

PPh2

N

RR

PPh2

Li

N

RR

PPh2

PO

O

P ClO

O

OH

OH

P ClO

O

AB C

D

DMAPNEt3, THF

44 %

Scheme 2. Strategies for the synthesis of phosphine–phosphoramidite ligands.

O

OP N

PPh2

O

OP N

PPh2

O

OP N

PPh2

O

OP N

PPh2

(S)-(2S,4S)-1

(S)-(2R,4R)-2 (S)-(2S,4S)-4

(S)-2

Figure 2. Phosphine–phosphoramidite ligands with an aliphatic bridge.

68 S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74

The lithiated amine readily reacts with the chloro compound butside reactions take place, which make the work-up and purificationof the ligand difficult, resulting in the product being obtained inlow yield (20%).

In order to improve the yield of product D, a new methodologywas developed using 4-dimethylaminopyridine (DMAP) as the cat-alyst.22 The role of DMAP is to activate the chlorophosphite by theformation of a labile ion pair between the chloride and the phos-phityl-pyridinium cation. The addition of 10 mol % of DMAP as cat-alyst and triethylamine as the proton acceptor to the reactionmixture improved the yield of the desired compound to 44%.

2.2. Stereoelectronic characterization of the ligands

The new ligands 2–4 (Fig. 2) are considerably air- and moisture-stable white solids and have been characterized by 1H-, 13C- and31P NMR spectroscopy. Their 31P{1H} NMR spectra were particu-larly informative with regards to the purity (Table 1): in each case,two singlets in the expected range were observed, one for thephosphorus in the phosphoramidite unit (PN) and one for the phos-

phorus in the phosphine moiety (PC). From the d(PN) data it can beconcluded that ligand 4 possessing an H8-biaryl moiety has a phos-phoramidite group with a stronger r-donor ability compared totheir H0-analogues.

The coordination chemistry of the rhodium complexes of li-gands 1–4 was investigated because of its relevance to the asym-metric catalysis described below. The products formed upontreatment of the chiral ligands with 1 equiv of [Rh(COD)2]BF4 wereevaluated by 31P{1H} NMR spectroscopy. At a 1:1 ligand:metal ra-tio, two double doublets were achieved indicating the formation ofa seven-membered chelate ring in the [Rh(COD)(PP0)]BF4 com-plexes. The coordination chemical shifts (Dd = d(com-plex) � d(ligand)) were found to be negative for the PN andpositive for PC moieties, as expected for a good p-acceptor and agood d-donor moiety, respectively.

The r-donor ability of the phosphorus atoms in phosphine–phosphoramidites 1–4 has been evaluated by measuring the mag-nitude of the 1J(77Se–31P) coupling constant in the 77Se isotopomerof the corresponding seleno-phosphate-amide and phosphine–sel-enide. This method is one of the simplest and most reliable ones for

Table 131P NMR data of ligands 1–4 and the corresponding [Rh(COD)(PP0)]BF4 complexesa

Ligand d (PN) (ppm) 1J(RhPN) (Hz) d (PC) (ppm) 1J(RhPC) (Hz) 1J(PNPC) (Hz) dlig(PN) (ppm) dlig(PC) (ppm) Dd (PN) (ppm) Dd (PC) (ppm)

1 137.3 242.8 19.2 140.5 40.1 153.5 3.0 �16.2 16.22 139.2 (broad) 234.8 7.9 141.2 45.7 151.1 �16.6 �11.9 24.53 148.8 235.5 18.1 135.7 34.1 152.9 0.0 �4.1 18.14 132.9 240.5 19.4 140.9 40.0 146.0 1.8 �13.1 17.6

a All spectra measured in CDCl3 at 20 �C. PN is related to the phosphoramidite group and PC is related to the phosphino group. The coordination chemical shift wascalculated as follows: Dd = d(complex) � d(ligand).

S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74 69

assessing the donating ability of phosphorus donors. The magni-tude of lJ(Se–P) is very much dependent upon the nature of the or-ganic groups bonded to the phosphorus: electron-withdrawinggroups cause the coupling constant to increase whereas electron-donating and bulky groups cause it to decrease.23

The synthesis of diselenides was performed by the in situ NMRtube reaction of the corresponding ligand with elemental sele-nium. According to the preparation method, the reaction proceedsin a stepwise manner. The phosphine site readily reacts with sele-nium powder after some minutes while the formation of disele-nides takes only one hour in toluene at room temperature(Table 2).

Table 231P–77Se coupling constants in diselenides of ligands 1–7a

SePh2P N

P

O

O Ph2P NP

O

OSe Se

Ligand 1J(77Se–31P)A (Hz) 1J(77Se–31P)B (Hz)

(S)-(2S,4S)-1 959.8 732.8(S)-2 962.5 722.8(S)-5 1038.3b 750.5(S)-(2R,4R)-3 955.0 742.5(S)-(2S,4S)-4 947.1 728.2(S)-6 971.124 —(2S,4S)-7 — 72225

a All spectra measured in toluene, at 20 �C. 1J(77Se–31P)A is the coupling constantin the phosphoramidite group and 1J(77Se–31P)B in the phosphino group.

b 1J(77Se–31P)A is the coupling constant in the phosphite group.

From the data of Table 2, it can be seen that the electronic effectof a minor change in the backbone or in the dioxaphosphepinemoiety is effectively transmitted to the P-donors. The partiallyhydrogenated ligand 4 is slightly more basic then the binaphthylderivatives 1–3. Thus, the coupling constants of the diselenide de-rived from partially saturated binaphthol derivative 4 are 12.7 and4.6 Hz smaller than those derived from the related binaphthol 1.

It is also important to note that the electronic changes of thephosphorus functionalities of the same type in the diastereomers1 and 3 show opposite trends. The 1J(77Se–31P) of an analogousphosphine–phosphite 526 (Fig. 3) 1038.3 Hz and 750.5 Hz reflects

O

OP O

PPh2

(S)-5 (S)-MonoP

Figure

a significantly larger difference (DJ = 287.8 Hz) in the electronicproperties of the phosphine and phosphite than in the case of thephosphine–phosphoramidite (DJ = 239.7 Hz). Our ligands de-scribed herein are more electron-donating than the phosphites be-cause the electronegativity of nitrogen (3.04) is less than that ofoxygen (3.44); the donor character is further increased by thehyperconjugation effect of the isopropyl group connected to nitro-gen. It is interesting to note that the diselenide of 2 (Fig. 4) is char-acterized by the largest, and at the same time with the lowestcoupling constant among the compounds investigated herein, indi-cating the lowest and the highest r-donor capability of phospho-ramidite and phosphine. For the sake of comparison, a typicalphosphoramidite such as MonoPhos 6, is much more p-acidic thanany of our ligands has a 1J(77Se–31P) value as high as 971.1 Hz. Itshould be noted that the electron density at the P-donor of adiphosphine, such as the BDPP ligand 7, is very similar(1J(77Se–31P) = 722 Hz) to that obtained for the most basic phos-phine site of 2 (1J(77Se–31P)B = 722.8 Hz).

2.3. Catalytic experiments

Recently, we demonstrated the superior performance of ligand 1in Rh-catalyzed asymmetric hydrogenations of a wide range of ole-fin substrates.15 The free ligand and the catalyst was not just stablein green solvents such as alkylene carbonates or protic solvents suchas methanol, even the selectivity and activity were better in thesesolvents compared to dichloromethane. The catalyst provided excel-lent ee’s of up to 99.0% in propylene carbonate for the asymmetrichydrogenation of the L-DOPA precursor (3-methoxy-4-acetoxy-(Z)-a-acetamidocinnamic acid). In addition, similarly high ee’s wereachieved in methanol solution for acetamidocinnamic ester and itsortho- and para-methoxy derivatives.

Our structure offers flexible opportunities for derivatization formore precise control over the positioning of the steric bulk. Hereinwe have tested ligands 1–4 in asymmetric hydrogenation (Table 1).All reactions were run until full conversion. Ligand 2, without ste-reogenic elements in the bridge showed low selectivities for allsubstrates (entries 3, 4, 10, and 14). In addition, ligand 2 gavethe weakest performance among the ligands. Crévisy et al. reportedon a similar ligand, which does not contain a stereogenic center inthe linker; however, no application of this ligand in catalysis hasbeen reported on. Another phosphine–phosphoramidite with onestereogenic center in the bridge (Fig. 1) introduced by Crévisy

O

OP N

P P

hos 6 (2S,4S)-BDPP 7

3.

Figure 4. 31P{1H} NMR spectrum (161 MHz, CDCl3) of diselenide-2.

70 S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74

and Mauduit was evaluated in the copper-catalyzed conjugateaddition of dietylzinc to enones.6 The best ee’s of up to 53% wereobtained when cyclohex-2-enone was used as the substrate. Thiscatalyst was not tested in the catalytic asymmetric hydrogenationreaction. To the best of our knowledge, our results are the firstexamples that give valuable information on the role of the stereo-genic center in the bridge when the linker is an aliphatic chain.

One of the key principles of green chemistry is the eliminationof solvents in chemical processes or the replacement of hazardoussolvents with environmentally friendly ones. Alkylene carbonateshave recently emerged as green solvents. The JEFFSOL� carbonatescan be used as safe and environmentally friendly solvents and re-place the commonly used methylene chloride, acetone, aromaticsolvents, and other highly volatile and hazardous solvents. Theyalso have a low vapor pressure which allows their easy applicationat higher temperature (See Table 3).

Table 3Asymmetric hydrogenation with Rh(I) catalysts of chiral phosphine–phosphoramiditeligandsa

Entry Substrate Ligand Reactiontimeb (h)

ee (%)

1 COOMe

NHAc

(S)-(2S,4S)-1 2.8 99.2 (R)2c 2.0 99.9 (R)3 (S)-2 2.3 44.5 (R)4c 0.5 44.8 (R)5 (S)-(2R,4R)-3 4.1 55.8 (S)6c 2.2 77.2 (S)7 (S)-(2S,4S)-4 2.0 98.8 (R)8c 1.5 99.7 (R)9 COOMe

NHAc

(S)-(2S,4S)-1 2.2 97.7 (S)10 (S)-2 1.8 24.2 (S)11 (S)-(2R,4R)-3 2.5 60.4 (R)12 (S)-(2S,4S)-4 2.5 95.1 (S)13 (S)-(2S,4S)-1 3.0 99.5 (R)14 (S)-2 3.4 77.5 (R)15

MeOOC COOMe

(S)-(2R,4R)-3 2.5 49.0 (S)16 (S)-(2S,4S)-4 2.3 99.6 (R)17d (S)-(2S,4S)-1 1.0 99.5 (R)

a S/Rh = 125, P/Rh = 1.1, Solvent: 5 mL propylene carbonate, Pressure: 5 bar H2,RT.

b Time for 100% conversion.c Reaction carried out in MeOH.d S/Rh = 2500, P/Rh = 1.1, no solvent, Pressure: 50 bar H2, 50 �C.

We found that the catalyst generated by mixing [Rh(COD)2]BF4

with ligand (S)-(2S,4S)-1 in propylene carbonate or methanol un-der a hydrogen pressure of 5 bar led to full conversion of the ace-tamidocinnamic ester with ee values of 99.2% and 99.9%,respectively. In contrast, ligand (S)-(2R,4R)-3 with the (2R,4R)-pen-tanediyl fragment displayed low enantioselectivity and favored ahydrogenation product with the opposite configuration. Enantio-meric excesses of 55.8% (S) and 77.2% (S) were measured in propyl-ene carbonate and in methanol, respectively, for methyl (Z)-a-acetamidocinnamate (entries 5 and 6 compared to entries 1 and2). The same change could be observed for methyl 2-acetamidoac-rylate (compare entries 9 and 11) and dimethyl itaconate (compareentries 13 and 15). It is known that the bulky chiral BINOL andoctahydro-BINOL moieties are generally good chiral informationtransmitters but in this case, the 2,4-pentanediyl moiety has moreinfluence in determining the absolute configuration of the predom-inant hydrogenation product.

The properly selected (2S,4S)-configurations in the backbonelead to enhanced enantioselectivities (1, matched arrangements)in contrast to the results displayed by the mismatched arrange-ments (S)-(2R,4R)-3.

The high selectivity obtained is the result of a combined actionbetween the chiral BINOL phosphite moiety and the ligand back-bone chirality. It is most likely that in the matched arrangementthe methyl groups dispose the terminal phenyl rings and the naph-thyl moieties into a chiral arrangement (alternating edge-face and/or equatorial-axial). By replacing the H0-BINOL moiety with its par-tially hydrogenated and bulkier H8 analogue, no significant selec-tivity change was observed when compared to ligand 1.

A highly desirable goal of green chemistry is to replace organicsolvents in chemical reactions with water. An alternative is to carryout the reaction without any solvent. In our case we were able tocarry out the hydrogenation of dimethyl itaconate without any sol-vent. The hydrogenated product was obtained at 50 �C under50 bar of hydrogen pressure with 99.5% enantioselectivity at a sub-strate/catalyst molar ratio of 2500 (entry 17). The excellent cata-lytic activity was also demonstrated by the use of a low catalystloading. The reduction of the catalyst loading, and the elevatedtemperature and pressure did not affect the conversion andenantioselectivity.

The catalytic application mentioned above might confirm thatupdating this synthetic approach could provide access to useful li-gands. We are currently following this approach and are compiling

S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74 71

a pool of hybrid phosphine–phosphoramidites of a fine scale ofelectronic and steric nature. This set of ligands should supportour efforts to develop efficient and environmentally benign cata-lytic systems.

3. Conclusion

In conclusion, we have synthesized a series of phosphine–phosphoramidite ligands to investigate the role of the linkingbridge between the two phosphorus atoms. Our phosphine–phosphoramidite ligands represent a new class of ligands: thelinker unit between the phosphine and phosphoramidite con-tains three sp3-hybridized carbon atoms. Ligand 2, which doesnot have a stereogenic element in the 1,3-propanediyl bridge,showed much lower ee’s than ligand 1. Moreover, the selectivityand the configuration of the product changed from 99.9% (R) to77.2% (S) when ligand 3 was used in the asymmetric hydrogena-tion of methyl (Z)-a-acetamidocinnamate in methanol. These re-sults indicate that despite the excellent chiral informationtransmission ability of the BINOL moiety, the high selectivityachieved is the result of a combined action between the chiralBINOL phosphite moiety and the ligand backbone chirality. Weassume that the methyl groups dispose the chelate conforma-tion, terminal phenyl rings, and the naphthyl moieties into a chi-ral arrangement. To the best of our knowledge, our results arethe first example that gives valuable information on the role ofthe stereogenic center in the bridge when the linker is an ali-phatic chain.

4. Experimental

4.1. General

All reactions and manipulations were carried out under argonatmosphere using standard Schlenk techniques. Methanol, THF,and diethyl ether were distilled and dried before use. JEFSOL� Pro-pylene carbonate was provided by Huntsman Corporation andused without drying or degassing (H2O content: 0.05 m/m%). Thesubstrates methyl (Z)-a-acetamidocinnamate and methyl 2-acet-amidoacrylate used for the asymmetric hydrogenations were syn-thesized according to the literature.27 All other chemicals wereobtained commercially. 31P{1H}-, 13C{1H}- and 1H NMR spectrawere recorded on a Bruker Avance 400 spectrometer in the NMRLaboratory at the University of Pannonia in Veszprém, or on a VAR-IAN UNITY 300 or Bruker DRX-500 spectrometer at the Universityof Technology and Economics in Budapest.

4.2. GC analyses

4.2.1. Methyl (Z)-acetamidocinnamateCHIRASIL-L-VAL column (25 m � 0.25 mm, df = 0.12 lm). Tem-

perature program: 2 min at 140 �C; 2 �C/min from 140 �C to 180 �C;40 min at 180 �C. Retention times were 12.3 min for (R), 13.2 minfor (S)-product, and 22.1 min for (Z)-a-acetamidocinnamic acidmethyl ester.

4.2.2. Methyl 2-acetamidoacrylateb-Dex 255 column (30 m � 0.25 mm, df = 0.25 lm). Retention

times at 140 �C isotherm were 6.7 min for (S), 7.4 min for (R) -prod-uct, and 5.9 min for acetamidoacrylic acid methyl ester.

4.2.3. Dimethyl itaconateb-Dex 255 column (30 m � 0.25 mm, df = 0.25 lm). Retention

times at 85 �C isotherm were 18.8 min for (R), 20.0 min for (S) -product, and 27.6 min for dimethyl itaconate.

4.3. Synthesis

4.3.1. (11bS)-N-((2S,4S)-4-(Diphenylphosphino)pentan-2-yl)-N-isopropyl-{dinaphtho[2,1-d:10,20-f][1,3,2]dioxaphosphepin-2-yl}-4-amine 1

A typical synthesis using DMAP follows: (2S,4S)-2-diphenyl-phosphino-4-i-propylamino-pentane (1.25 g, 4.0 mmol) and tri-ethylamine (670 lL, 5.0 mmol) were dissolved in 30 mL of THF.4-Dimethylaminopyridine (36 mg, 0.3 mmol) was added to thesolution and then cooled to approximately �30 �C. At this temper-ature (S)-chloro-binaphtho[2,1-d:10,20-f][1,3,2]dioxaphosphepine(1.5 g, 4.5 mmol) in 20 mL of THF was added over 30 min. The for-mation of a white precipitate was observed. The mixture was stir-red at RT for 2 h. The reaction was followed by silicagel TLC platewith chloroform/methanol = 4/1 eluent. Rf for (2S,4S)-2-diphenyl-phosphino-4-i-propylamino-pentane is 0.5. After consumption ofthe substrate, the suspension was filtered through a short pad ofAl2O3. After evaporation of the volatiles, 1.1 g of a white foam(yield: 44.1%) was obtained.

4.3.2. Diselenide of 1A mixture of 1 (40 mg, 0.064 mmol) and elemental selenium

(15 mg, 0.19 mmol) in toluene (2.5 mL) was stirred for 24 h. Theunreacted selenium was left to settle and the required quantityof the solution was added into an NMR tube. 31P{1H} NMR(121 MHz, toluene): d = 84.0 (d, 1J(Se–P) = 959.8 Hz), 49.1 ppm (d,1J(Se–P) = 732.8 Hz).

4.3.3. 1-i-Propylamino-3-sulfato-propane2,2-Dioxide-1,3,2-dioxathian (5.35 g, 38.7 mmol) and i-propyl-

amine (3.17 mL, 38.7 mmol) were stirred in 40 mL of ether for 24 hat RT. The reaction mixture was filtered and the solid was washedthree times with ether. The remaining volatiles were evaporated togive 5.7 g of white powder (Yield: 74.6%). Mp: 181–183 �C. Anal.Calcd for C6H15NO4S: C, 36.53%; H, 7.66%; N, 7.10%; S, 16.26%. Found:C, 36.83%; H, 7.76%; N, 7.56%; S, 15.86%. 1H NMR (400 MHz, DMSO):d = 8.15 (s, 2H, NH2+), 3.84 (t, J = 6.06 Hz, 2H, CH2), 3.28 (m,J = 6.51 Hz, 1H, CH), 2.96 (t, J = 7.38 Hz, 2H, CH2), 1.84 (m,J = 6.73 Hz, 2H, CH2), 1.19 (d, J = 6.52 Hz, 6H, CH3), 13C {1H} NMR(75 MHz, DMSO): d = 63.04 (s), 49.49 (s), 41.73 (s), 26.16 (s), 18.66(s).

4.3.4. 1-Diphenylphosphino-3-isopropylamino-propaneAt first, 1-i-propylamino-3-sulfato-propane (3.7 g, 19.8 mmol)

was added dropwise to a solution of LiPPh2⁄1,4-dioxane (10.6 g,

37.8 mmol) in 60 mL of abs. THF under argon and stirred for2 h. The original color of the reaction mixture turned brownand at the end of the reaction green. After evaporation of the sol-vent, 80 mL of distilled oxygen-free water and 60 mL of etherwere added to the residue and the mixture was stirred untilthe two phases became clear solutions. The pH was then set to1 with 10% solution of oxygen free HCl. The phases were thenseparated and the water phase was washed three times with30 mL portions of ether. The pH was then set to around 9–10by the dropwise addition of Na2CO3. A white precipitationformed which was filtered, dissolved in dichloromethane, anddried over MgSO4. The solution was evaporated to give 2.25 gof a white solid (yield: 63.6%). Mp: 181 �C. Anal. Calcd forC18H24NP: C, 75.76%; H, 8.48%; N, 4.91%; P, 10.85%. Found: C,75.49%; H, 8.31%; N, 5.00%. 1H NMR (300 MHz, CDCl3): d = 9.52–8.14 (s, 1H, NH), 7.46–7.37 (m, 4H, aromatic), 7.36–7.27 (m,6H, aromatic), 3.23 (q, J = 6.31 Hz, 1H, CH), 2.93 (t, J = 7.16 Hz,2H, CH2), 2.20–2.09 (m, 2H, CH2), 2.09–1.93 (m, 2H, CH2), 1.37–1.30 (d, J = 6.3 Hz, 6H, CH3), 31P{1H} NMR (121 MHz, CDCl3)d = �17.30 (s). 13C{1H} NMR (75 MHz, CDCl3): d = 138.04 (d,J = 12.7 Hz), 132.76 (d, J = 18.7 Hz), 128.80 (s), 128.60 (d,

72 S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74

J = 6.7 Hz), 49.94 (s), 45.19 (d, J = 14.3 Hz), 25.51 (d, J = 13.1 Hz),22.82 (d, J = 18.2 Hz), 19.22 (s).

4.3.5. (11bS)-N-(3-(Diphenylphosphino)propyl)-N-isopropyl-{dinaphtho[2,1-d:10,20-f][1,3,2]dioxaphosphepin-2-yl}-4-amine2

1-Diphenylphosphino-3-isopropylamino-propane (2.25 g,7.89 mmol) and triethylamine (1.7 mL, 11.83 mmol) were dis-solved in 40 mL of abs. THF under argon. A solution of (S)-2-chloro-binaphtho[2,1-d:10,20-f][1,3,2]dioxaphosphepine (3.46 g,9.86 mmol) in 20 mL of abs. THF was then added to the reactionmixture via canulla under vigorous stirring at 0 �C. After beingstirred at RT for one hour the reaction mixture was filteredthrough a short pad of Al2O3 and then the solvent was evapo-rated. The remaining white foam was further purified with flashchromatography over silicagel using diisopropyl-ether/petrole-ther = 1/1 eluent to give 1.95 g of white powder (yield: 41.2%).½a�20

D ¼ þ336:4 (c 0.99, CH2Cl2), mp: 60–62 �C. Anal. Calcd forC38H35NO2P2: 76.11; H, 5.88; N, 2.34. Found: C, 76.27; H, 5.58;N, 2.27. 1H NMR (500 MHz, CDCl3): d = 8.03–7.56 (m, 4H, aro-matic), 7.53–7.26 (m, 18H, aromatic), 3.57–3.44 (m, 1H, CH),2.96–2.73 (m, 2H, CH2), 1.91–1.44 (m, 4H, CH2), 1.20 (d,J = 6.60 Hz,3H, CH3), 1.15 (d, J = 6.60 Hz, 3H, CH3). 31P{1H} NMR(121 MHz, CDCl3): d = 151.10 (s), �16.63 (s). 13C {1H} NMR(75 MHz, CDCl3): d = 150.30 (d, J = 5.2 Hz), 149.78 (s), 138.77(dd, J = 26.9 Hz, J = 13.0 Hz), 132.86 (m), 132.55 (s), 131.39 (s),130.64 (s), 130.22 (s), 129.78 (s), 128.41 (m), 127.07 (d,J = 6.5 Hz, 126.03 (d, J = 2.8 Hz), 124.60 (d, J = 23.2 Hz), 124.09(d, J = 5.1 Hz), 122.25 (d, J = 1.7 Hz), 122.05 (s), 47.80 (d,J = 23.3 Hz), 43.89 (dd, J = 14.5 Hz, J = 13.5 Hz), 28.42 (dd,J = 15.8 Hz, J = 2.8 Hz), 25.46 (d, J = 11.4 Hz), 23.28 (dd,J = 13.3 Hz, J = 7.1 Hz).

4.3.6. Diselenide of 2This compound was prepared by following the procedure de-

scribed for the preparation of diselenide of 1. 31P{1H} NMR(121 MHz, toluene): d = 88.2 (d, 1J(Se–P) = 962.5 Hz), 33.8 ppm (d,1J(Se–P) = 722.8 Hz).

4.3.7. (2R,4S)-2-i-Propylamino-4-sulfato-pentaneAt first, (4S,6S)-4,6-dimethyl-2,2-dioxide-1,3,2-dioxathiane

(4 g, 24.07 mmol) and i-propylamine (6.2 mL, 72.2 mmol) werestirred in 80 mL of THF for 48 h during which time a white precip-itate was formed. The solvent was then evaporated after which60 mL of ether was added to the residue. The suspension was stir-red for 30 min and filtered. The solid was washed two times withether, and dried with azeothropic distillation of toluene. The sol-vent was evaporated in vacuo to give 4.80 g of a white powder(Yield: 88.5%). Mp: 248 �C. Anal. Calcd for C8H19NO4S: C, 42.65;H, 8.50; N, 6.22; S, 14.23. Found: C, 42.69; H, 8.31; N, 6.28; S,14.58. 1H NMR (500 MHz, DMSO): d = 8.00 (s, 2H, NH2

+), 4.26–4.09 (m, 1H, CH), 3.40–3.21 (m, 2H, CH), 1.73–1.59 (m, 1H, CH2),1.51–1.38 (m, 1H, CH2), 1.24–0.90 (m, 12H, CH3). 13C {1H} NMR(75 MHz, DMSO): d = 70.95 (s), 49.23 (s), 46.64 (s), 22.56 (s),19.84 (s), 18.59 (s), 16.81 (s).

4.3.8. (2R,4R)-2-Diphenylphosphino-4-i-propylamino-pentaneAt first, LiPPh2�1,4-dioxane (20.9 g, 74.6 mmol) was dissolved

in 40 mL of abs. THF under an argon atmosphere and then cooledto about �30 �C. Next, (2R,4S)-2-i-propylamino-4-sulfato-pentane(4.8 g, 21.3 mmol) was added drop wise to the red solution forover 10 minutes. The cooling bath was removed, the reactionmixture was allowed to warm up to RT, and stirred for one hour.The color of the reaction mixture remains red. After evaporation

of the solvent 80 mL of distilled oxygen free water and 60 mLof ether were added to the residue and the mixture was stirreduntil the two phases became clear solutions. The pH was thenset to 1 with a 10% solution of oxygen-free HCl. The phases werethen separated and the water phase was washed three times with30 mL portions of ether. The pH was then set to around 9–10 bythe dropwise addition of Na2CO3. The product was then extractedfour times with 30 mL portions of ether. After drying over MgSO4

the mixture was filtered and evaporated to give 4.95 g of a clearor pale yellow oil (Yield: 74.2%). ½a�20

D ¼ þ74:3 (c 1, CH2Cl2). Anal.Calcd for C8H28NP C, 76.64; H, 9.00; N, 4.47. Found: C, 76.62; H,8.99; N, 4.47. 1H NMR (500 MHz, CDCl3): d = 7.44–7.32 (m, 4H,aromatic), 7.25–7.16 (m, 6H, aromatic), 2.86–2.71 (m, 2H, CH),2.35–2.24 (m, 1H, CH), 1.44–1.33 (m, 1H, CH2), 1.26–1.15 (m,1H, CH2), 0.98–0.85 (m, 12H, CH3). 31P{1H} NMR (121 MHz,CDCl3): d = �0.03 (s). 13C {1H} NMR (75 MHz, CDCl3): d = 137.06(dd, J = 14.4 Hz, J = 7.5 Hz), 133.72 (dd, J = 19.1 Hz, J = 5.8 Hz),128.75 (s), 128.34 (t, J = 6.6 Hz), 47.69 (d, J = 12.7 Hz), 45.27 (s),40.86 (d, J = 17.0 Hz), 27.31 (d, J = 9.7 Hz), 23.25 (d, J = 24.8 Hz),20.20 (s), 16.30 (d, J = 15.5 Hz).

4.3.9. (11bS)-N-((2R,4R)-4-(Diphenylphosphino)pentan-2-yl)-N-isopropyl-{dinaphtho[2,1-d:10,20-f][1,3,2]dioxaphosphepin-2-yl}-4-amine 3

Compound 3 was synthesized with the lithiation method(Scheme 2). (2R,4R)-2-Diphenylphosphino-4-i-propylamino-pen-tane (2.5 g, 7.9 mmol), Li metal (111.7 mg, 15.9 mmol), THF(550 lL, 6.8 mmol) and styrene (457 lL, 4.0 mmol) were stirredvigorously in 40 mL of hexane under argon for 3 h at RT andfor 1 h at 45 �C. A yellow solution formed with Li metal particlesin the solvent. After filtration of the excess Li metal, the yellowfiltrate was added to a (S)-2-chloro-binaphtho[2,1-d:1́,2́-f][1,3,2]dioxaphosphepine (4.2 g, 12.0 mmol) in 50 mL of ethersolution and cooled to about �10 �C for 30 min under argon.The cooling bath was removed and the reaction mixture wasstirred for 2 h on RT. The white precipitates were filtered andwashed with ether. After evaporation of the filtrate 4.5 g ofwhite foam was obtained. Further purification was performedwith flash chromatography on silica gel with the toluene/n-hex-ane = 1/1 (Rf. 0.7) to give the pure product as a white powder(1.0 g, yield: 20.0%). ½a�20

D ¼ þ299:5 (c 1, CH2Cl2), mp: 63–64 �C.Anal. Calcd for C40H39NO2P2: C, 76.54; H, 6.26; N, 2.23. Found:C, 76.55; H, 6.21; N, 2.17. 1H NMR (500 MHz, CDCl3): d = 8.00–7.87 (m, 4H, aromatic), 7.50–7.14 (m, 18H, aromatic), 3.44–3.19 (m, 2H, CH), 2.22–2.05 (m, 1H, CH), 1.76–1.51 (m, 2H,CH2), 1.34–1.08 (m, 9H, CH3), 0.69–0.52 (m, 3H, CH3). 31P{1H}NMR (121 MHz, CDCl3): d = 152.90 (s), 0.00 (s), 13C {1H} NMR(75 MHz, CDCl3): d = 150.42 (d, J = 6.82 Hz), 150.10 (s), 137.22(d, J = 14.27 Hz), 136.91 (d, J = 14.89 Hz), 133.85 (d,J = 19.23 Hz), 133.34 (d, J = 18.61 Hz), 132.84 (s), 130.89 (d,J = 62.03 Hz), 129.75 (d, J = 62.03 Hz), 129.09 (s), 128.65 (d,J = 11.79 Hz), 128.26 (d, J = 6.82 Hz), 128.08 (s), 126.48 (d,J = 84.98 Hz), 124.48 (d, J = 19.85 Hz), 123.90 (d, J = 4.96 Hz),122.35 (s), 121.95 (d, J = 2.48 Hz), 47.29 (d, J = 10.54 Hz), 47.12(d, J = 9.30 Hz), 45.19 (d, J = 14.27 Hz), 27.43 (d, J = 10.54 Hz),24.85 (d, J = 13.65 Hz), 21.20 (d, J = 15.61 Hz), 15.25 (d,J = 14.89 Hz),. 15.3 (d, J = 14.9 Hz).

4.3.10. Diselenide of 3This compound was prepared by following the procedure de-

scribed for the preparation of the diselenide of 1. 31P{1H} NMR(121 MHz, toluene): d = 84.5 (d, 1J(Se–P) = 955 Hz), 49.4 ppm (d,1J(Se–P) = 742.5 Hz).

S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74 73

4.3.11. Preparation of (11bS)-N-((2S,4S)-4-(diphenylphosphino)-pentan-2-yl)-N-isopropyl-{8,9,10,11,12,13,14,15-octahydrodina-phtho[2,1-d:10,20-f][1,3,2]dioxaphosphepin-2-yl}-4-amine 4

(2S,4S)-4-(Diphenylphosphino)-N-isopropyl-pentan-2-yl-amine(2.1 g, 6.7 mmol) in 60 mL of abs. THF was added dropwise to astirred solution of PCl3 (2.45 mL, 26.8 mmol) and triethylamine(3.71 mL, 26.8 mmol) in 15 mL THF over 20 min at –20 �C temper-ature. The mixture was then left to warm to room temperature andwas stirred over 1 h. The solvent was evaporated at reduced pres-sure and the remaining white solid was suspended in 80 mL ofether. After filtering of triethylamine-hydrochloride and the excessamount of triethylamine-trichlorophosphine adduct, the organicphase was evaporated to give a clear oil. This was then dissolvedin 60 mL of abs. THF. Next, triethylamine (1.80 mL, 13.0 mmol)and a solution of (S)-5,50,6,60,7,70,8,80-octahydro-1,10-binaphtha-lene-2,20-diol (1.43 g, 4.86 mmol) in 10 mL of abs. THF was addedto the reaction mixture over 10 min at �20 �C. The reaction mix-ture was then evaporated at reduced pressure. The remainingwhite solid was filtered on a short pad of activated Al2O3 with tol-uene or chloroform. The product precipitates in ether, is partiallysoluble in toluene, and very soluble in chloroform or dichlorometh-ane. Final purification can be performed with flash chromatogra-phy under ambient conditions with silicagel as the stationaryphase and commercial chloroform as the eluent (Rf: 0.9) to give1.37 g of a white powder (Yield: 32.3%). ½a�20

D ¼ 60:1 (c 1, CH2Cl2),mp: 155 �C. Anal. Calcd for C40H47NO2P2: C, 75.57; H, 7.45; N,2.20. Found: C, 75.84; H, 7.72; N, 2.22. 1H NMR (500 MHz, DMSO):d = 7.49–7.41 (m, 4H, aromatic), 7.36–7.27 (m, 6H, aromatic), 6.97(dd, 2H, J = 28.21 Hz, J = 8.20 Hz, aromatic), 6.75 (dd, 2H,J = 33.21 Hz, J = 7.92 Hz, aromatic), 3.44–3.30 (m, 1H, CH), 3.21–3.09 (m, 1H, CH), 2.85-2.71 (m, 3H, CH, CH2), 2.71–2.50 (m, 3H,CH, CH2), 2.31–2.10 (m, 3H, CH, CH2), 1.81–1.64 (m, 7H, CH2 (6Hin H8 binaphthalene, 1H in pentane bridge), 1.54–1.40 (m, 3H,CH2 (2H in H8 binaphthalene, 1H in pentane bridge), 1.17 (d, 3H,J = 6.60 Hz, CH3), 1.09 (d, 3H, J = 6.88 Hz, CH3), 1.01 (d, 3H,J = 7.16 Hz, CH3), 0.77 (dd, 3H, J = 14.68 Hz J = 6.60 Hz, CH3).31P{1H} NMR (121 MHz, CDCl3): d = 146.03 (s), 1.81 (s). 13C {1H}NMR (75 MHz, CDCl3): d = 149.49 (s), 148.26 (d, J = 4.96 Hz),137.95 (d, J = 1.86 Hz), 137.32 (s), 137. 05 (d, J = 4.97 Hz), 136.86(d, J = 5.58 Hz), 133.86 (d, J = 19.23 Hz), 133.87 (d, J = 1.86 Hz),133.45 (d, J = 17.98 Hz), 132.53 (s), 129.36 (d, J = 4.34 Hz), 129.16(d, J = 13.65 Hz), 128.69 (d, J = 8.68 Hz), 128.39 (d, J = 3.09 Hz),128.30 (d, J = 3.10 Hz), 127.59 (d, J = 1.86 Hz), 118.87 (d,J = 2.48 Hz), 118.62 (s), 46.35 (d, J = 13.03 Hz), 46.18 (d,J = 13.03 Hz), 45.12 (d, J = 10.55 Hz), 29.15 (d, J = 11.79 Hz), 27.77(d, J = 18.61 Hz), 27.24 (d, J = 10.55 Hz), 24.30 (d, J = 8.69 Hz),22.70 (d, J = 19.23 Hz), 22.76 (d, J = 1.86 Hz), 21.27 (d, J = 8.07 Hz),14.98 (d, J = 13.64 Hz).

4.3.12. Diselenide of 4This compound was prepared by following the procedure de-

scribed for the preparation of diselenide of 1. 31P{1H} NMR(121 MHz, toluene): d = 82.4 (d, 1J(Se–P) = 947.1 Hz), 48.9 ppm (d,1J(Se–P) = 728.2 Hz).

4.4. Typical procedure for the asymmetric hydrogenation

In a Schlenk tube, a mixture of [Rh(COD)2]BF4 (4.1 mg,0.01 mmol) and ligand 3 (6.9 mg, 0.011 mmol) was dissolved in pro-pylene-carbonate under argon and stirred for 20 min. Methyl (Z)-acetamidocinnamate (275 mg, 1.25 mmol) was added and the mix-ture was transferred under argon into the stainless steel autoclave(100 mL internal volume) equipped with a glass test tube. The mix-ture was pressurized to 5 bar with H2 and stirred at 600 RPM untilfull conversion, which was determined by gas consumption.

4.5. Typical procedure to obtain [Rh(COD)(1-4)]BF4 NMRsamples

Compound 2 (17.99 mg, 0.03 mmol) in CH2Cl2 (5 mL) was addeddropwise to a solution of [Rh(COD)2]BF4 (12.18 mg, 0.03 mmol) inCH2Cl2 (5 mL). The resulting orange solution was stirred for20 min, concentrated, and dissolved in CDCl3 (0.6 mL).

Acknowledgments

The authors thank the National Office for Research and Technol-ogy (KMOP 1.1.4) for financial support as well as the HuntsmanCorporation for providing samples of their green cyclic carbonatesolvents. This article was partly made under the project TÁMOP-4.2.1/B-09/1/KONV-2010-0003 and TÁMOP-4.2.2/B-10/1-2010-0025. These projects are supported by the European Union andco-financed by the European Social Fund. Special thanks are dueto Béla Édes for the professional technical assistance.

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