Asymmetric Synthesis Asymmetric Epoxidation Using Hydrogen Peroxide as Oxidant Chuan Wang [a] and Hisashi Yamamoto* [a, b] Chem. Asian J. 2015, 10, 2056 – 2068 # 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2056 Focus Review DOI: 10.1002/asia.201500293
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Asymmetric Synthesis
Asymmetric Epoxidation Using Hydrogen Peroxide as Oxidant
Chuan Wang[a] and Hisashi Yamamoto*[a, b]
Chem. Asian J. 2015, 10, 2056 – 2068 Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2056
Abstract: Asymmetric epoxidation is one of the most impor-
tant transformations in organic synthesis. Although tremen-dous progress was achieved in this field in the 1980s and
1990s, it is still desirable from both economical and ecologi-cal views to develop environmentally friendly catalytic epoxi-dation with a broad substrate scope. Hydrogen peroxide isa safe and cheap oxidant, which is easy to handle and gen-
erates water as the sole byproduct. Therefore, asymmetric
epoxidation of olefins using hydrogen peroxide as oxidanthas been a very active research field and has been investi-
gated by many research groups in recent years. In thisreview, the exciting very recent developments of this rapidlygrowing area are surveyed and organized according to thecatalyst systems.
1. Introduction
Asymmetric epoxidation is one of the most important organictransformations, since it provides direct access to various opti-
cally active epoxides, which can be further utilized as precur-
sors for the synthesis of natural products and synthetic ana-logues with biological activities.[1] The milestone discoveries in
this area were accomplished by Sharpless et al. in 1980, whoreported the first titanium-catalyzed asymmetric epoxidation
of allylic alcohols.[2] Being complementary to the Sharplessmethod, the manganese–salen (salen = N,N’-ethylenebis(salicy-
limine)) catalytic system, which was independently developed
in the 1990s by Jacobsen and Katsuki, turned out to be a pow-erful catalyst for highly enantioselective epoxidation of unfunc-
tionalized olefins.[3, 4] Furthermore, Shi et al. reported in 1996the first organocatalyzed asymmetric epoxidation of olefins
using a fructose derivative as catalyst.[5]
These classic methods mentioned above found broad appli-
cations in the synthesis of complex molecules and also evoked
enormous interest from numerous researchers to further devel-op the asymmetric epoxidation reaction. On the one side,
great efforts have been made to design novel catalytic sys-tems, through which the substrate scope of the asymmetric
epoxidation could be significantly expanded. For instance, ourgroup successfully applied bis-hydroxamic acids as ligands in
the enantioselective epoxidation of primary, secondary, and
tertiary allylic and homoallylic alcohols as well as alkenyl sulfo-namides.[6] On the other side, since the Sharpless, Jacobsen–
Katsuki, Shi, and many other successful epoxidation reactionsusually require the use of toxic, costly, or low atom-efficient ox-
idants, such as alkyl peroxide, oxone, sodium hypochlorite, andhypervalent iodine compounds, it is highly desirable from both
an ecological and economical point of view to develop newcatalytic asymmetric epoxidation reactions by employing hy-drogen peroxide as oxidant, which is safe, cheap, easy to
handle, and generates water as the sole byproduct. Therefore,
asymmetric epoxidation of olefins using hydrogen peroxide asoxidant has been a very active research field and has been in-
vestigated by many research groups in recent years. In thisreview the most recent progress, which is not described in the
previous reviews, is surveyed and organized according to the
catalyst systems.[7]
2. Metal-Catalyzed Asymmetric EpoxidationUsing Hydrogen Peroxide as Oxidant
2.1. Titanium-Catalyzed Asymmetric Epoxidation
In 2005, Katsuki et al. reported the first titanium-catalyzed
asymmetric epoxidation of both conjugated and non-conjugat-ed unfunctionalized olefins by employing novel salalen as
ligand (tetradentate [ONNO]-type ligands comprised of an
amine-phenolate and an imine-phenolate subunit).[8] His pio-neering research work evoked further studies in this area. For
instance, Berkessel et al. developed a new structurally simplecis-salalen ligand, which could be readily prepared in two
steps and easily form the active catalyst complex in situ withTi(OiPr)4.[9, 10] Remarkably, this simplified catalyst demonstrates
excellent capability to promote highly enantioselective epoxi-
dation of non-conjugated olefins, while its trans analogue pre-viously developed by the same group was found to be an effi-
cient catalyst for the epoxidation of conjugated olefins.[11]
Under catalysis with these novel Ti--cis-salalen complexes (1–
4), various terminal non-conjugated olefins, which are notori-ously difficult substrates, were successfully used as precursors
for asymmetric epoxidation reactions using aqueous hydrogenperoxide as oxidant (Scheme 1).[9, 10] Generally, the products
were obtained in high enantioselectivities. Furthermore, thefirst titanium–salalen Ti–peroxo complexes were isolated andcharacterized by X-ray crystallography. Interestingly, the peroxo
complexes alone did not affect olefin epoxidation, whereas inthe presence of aqueous hydrogen peroxide the peroxo com-
plexes can promote the epoxidation reaction smoothly.Sun et al. developed a series of biaryl-bridged salalen–titani-
um complexes 5, which served as efficient catalysts for the
asymmetric epoxidation of a variety of aromatic epoxides fur-nishing the products with high enantioselectivities
(Scheme 2).[12] Notably, in the cases of terminal and cis olefinsas substrates, the reactions employing the biaryl-bridged cata-
lysts afforded the products with higher enantiomeric excessesthan the non-bridged titanium complex. However, relatively
[a] Dr. C. Wang, Prof. Dr. H. YamamotoDepartment of ChemistryThe University of Chicago5735 South Ellis Avenue, Chicago, IL 60637 (USA)E-mail : [email protected]
[b] Prof. Dr. H. YamamotoMolecular Catalyst Research CenterChubu University1200 Matsumoto, Kasugai, Aichi 487-8501 (Japan)E-mail : [email protected]
Chem. Asian J. 2015, 10, 2056 – 2068 www.chemasianj.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2057
low asymmetric induction was observed in the case of non-conjugated olefins.
Katsuki and co-workers successfully applied the titanium–sal-
alen complex 6 as catalyst for the enantioselective epoxidationof cis-aldehyde enol esters furnishing the products in excellentenantioselectivities (Scheme 3).[13] The authors also demonstrat-
ed that one reaction product was readily converted into thecorresponding 1,2-diol through LiBH4-mediated reduction with-
out erosion of the high enantiomeric excess.Alongside titanium–salalen complexes, more synthetically
accessible titanium—salan complexes have also been investi-
gated as catalysts for asymmetric epoxidation with hydrogenperoxide.[14] Bryliakov and co-workers systematically studied
the influence of the ligand structure on the catalytic activityand selectivity by surveying the performance of a series of
chiral salan ligands with varying steric and electronic proper-ties in the asymmetric epoxidation of aromatic epoxides
(Scheme 4).[15] The results revealed that the electronic effectcontrols the catalytic activity, whereas the steric effect ac-
counts for the facial selectivity of the epoxidation. Generally,the best results with respect to the enantioselectivities were
obtained in the case of Ti complex 7.Recently, Falck and co-workers reported that Ti–salan com-
plex 7 is able to catalyze distal selective epoxidation of conju-gated dienes even in the presence of other olefins and adja-
cent stereocenters (Scheme 5).[16] In this context, a variety of
well-established methods for asymmetric epoxidation weresurveyed for the regioselective epoxidation of conjugateddienes. The reaction using Ti–salan complex 7 or its enantio-mer as catalyst furnished the products with complete regiose-lectivities in favor of the Z- or trisubstituted olefins over E ole-fins. Notably, these reactions also proceeded with high diaster-
eo- or enantioselectivities.
2.2. Scandium-Catalyzed Asymmetric Epoxidation
Feng et al. discovered that the chiral N,N’-dioxide–scandium
complex 8 was able to catalyze the nucleophilic epoxidation ofelectronically deficient olefins, such as a,b-unsaturated ketonesand a,b-unsaturated amides (Scheme 6).[17] The corresponding
products were furnished in high yields and excellent enantio-selectivities under mild reaction conditions. Notably, under the
Chuan Wang was born in Xuzhou (P.R. China)and studied chemistry at the University ofGçttingen (Germany) from 2002 to 2007where he received his Masters degree underthe guidance of Prof. H. Laatsch. Subsequent-ly, he joined the group of Prof. D. Enders atthe RWTH Aachen University (Germany) andin 2010 he obtained his Ph.D. degree. In 2011he undertook postdoctoral research at theJìlich Research Centre (Germany) with Prof. J.Pietruszka. In 2012 he moved to the Universityof Chicago (USA) as a Humboldt postdoctoralfellow with Prof. H. Yamamoto.
Hisashi Yamamoto received his Bachelordegree from Kyoto University (Japan) and hisPh.D. from Harvard University. He became as-sistant professor at Kyoto University and in1977 was appointed as associated professorat the University of Hawaii (USA). In 1980 hemoved to Nagoya University (Japan) as a fullprofessor. In 2002, he moved to the UnitedStates as a professor at the University of Chi-cago. In 2012, he was appointed as professorand director of the Homogeneous Catalyst Re-search Center at Chubu University (Japan).During his research career of over four de-cades, his research group has developeda wide range of synthetic methods, which laid the foundation for modernLewis acid catalysis, Brønsted acid catalysis, and catalytic asymmetric oxida-tion. He has been awarded numerous academic awards and honors includingthe IBM Science Award (1988), the Chemical Society of Japan Award (1995), theNational Prize of Purple Medal (2002), the Yamada Prize (2004), the HumboldtResearch Award (2007), and the Noyori Prize (2012), among others.
Scheme 1. Asymmetric epoxidation of terminal non-conjugated olefins cata-lyzed by Ti–cis-salalen complexes.
In 2012, Simonneux and his co-workers developed a series of
chiral Halterman iron–porphyrins 9, which are able to promotethe asymmetric epoxidation using hydrogen peroxide as oxi-
dant (Scheme 8).[20, 21] Notably, the sulfonated Halterman iron–porphyrin is water soluble and thus under its catalysis the ep-
oxidation can be conducted under aqueous conditions. How-
ever, the substrate scope of these reactions is narrow and in-cludes only styrene derivatives and 1,2-dihydronaphthalene.
Furthermore, the products are afforded mostly with moderate-ly good enantioselectivities. Therefore, further investigations
are desired in this area to enhance the asymmetric inductionto a synthetically valuable level.
Tetradentate nitrogen-containing ligands, which have elec-
tronic properties that resemble those of porphyrin, alsoproved to be efficient catalysts for the Fe-catalyzed asymmetric
epoxidation of olefins. For instance, Bryliakov and Talsi et al.developed chiral Fe–aminopyridine catalyst 10, which is capa-
ble of promoting highly enantioselective epoxidation of di-verse olefins with aqueous hydrogen peroxide as oxidant
(Scheme 9).[22] Furthermore, the authors also discovered thatthe enantioselectivity of this process increased with growing
steric demand of acidic additives. A plausible mechanism wasproposed by the authors (Scheme 10). Initially, the FeII complexis oxidized to an FeIII intermediate by H2O2. Subsequently, anactive species FeIV complex is formed through a carboxylic acid
assisted heterocyclic cleavage of the O¢O bond. In the nextstep the olefin is epoxidized by the FeIV species to afford theproduct and the FeII intermediate for the next cycle.
Costas and co-workers improved the catalytic behavior of
the Fe–aminopyridine complex significantly by introducing a di-methylamino moiety to the pyridine rings (Scheme 11).[23] The
authors also employed a catalytic amount of carboxylic acid as
cocatalyst, which could improve both yields and enantioselec-tivities of the epoxidation reaction through synergistical coop-
eration with the iron complex leading to more efficient O¢Obond cleavage and the formation of epoxidizing species. After
careful screening of a variety of carboxylic acids, Ibuprofenturned out to be the best coligand. This new epoxidation
Scheme 7. Asymmetric epoxidation of 2-arylindene-1,3-diketones catalyzedby N,N’-dioxide–ScIII complexes.
Scheme 8. Asymmetric epoxidation of unfunctionalized olefins catalyzed byFe–porphyrin complexes.
Scheme 9. Asymmetric epoxidation of olefins catalyzed by Fe–aminopyridinecomplexes.
Scheme 10. Proposed mechanism for the non-porphyrin Fe-catalyzed asym-metric epoxidation of olefins with acidic additives.
Chem. Asian J. 2015, 10, 2056 – 2068 www.chemasianj.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2060
deficient olefins, thereby yielding the product in moderatelygood enantioselectivities (Scheme 15).[27] Furthermore, the au-
thors also observed that the use of sterically demanding car-boxylic acid instead of acetic acid as additive could improve
the enantioselectivities of this Mn-catalyzed epoxidation reac-tion.
In 2012, Costas et al. also reported a chiral bis-pyrrolidine-
based ligand bearing modified pyridine rings as ligand armsfor the Mn-catalyzed asymmetric epoxidation using hydrogen
peroxide as oxidant (Scheme 16). In this case the productswere provided in high yields albeit with low to moderate
enantioselectivities.[28]
In 2012, Sun et al. described a Mn-catalyzed asym-metric epoxidation by employing a tetradentate N-
ligand containing chiral bis-pyrrolidine and benzimi-dazole moieties (Scheme 17). Under the optimum re-action conditions, high to excellent enantioselectivi-ties were achieved when using chalcones and cis-dis-
ubstituted olefins as substrates.[29] In contrast, rela-tively poor facial control was observed in the cases
of terminal, cis-disubstituted olefins, and a,b-unsatu-rated esters.
Costas et al. and Bryliakov et al. discovered inde-pendently that introduction of electron-donatingsubstituents to the pyridine rings of the aminopyri-
dine ligands led to the enhancement of both the effi-ciency and enantioselectivities of the Mn-catalyzed
epoxidation with hydrogen peroxide as terminal oxi-dant (Schemes 18 and 19).[30, 31] In the cases of a,b-un-saturated carbonyl compounds and chromenes as
substrates the products were yielded with high enan-tioselectivities. Furthermore, Costas et al. studied the
use of the Mn complex for the diastereoselective ep-oxidation of unsaturated steroids and the results re-
vealed that the formation of ß-epoxides was favored whenusing the Mn catalysts.
In 2013, Gao et al. developed a porphyrin-inspired chiralMn–amino-oxazoline complex and applied it successfully in the
asymmetric epoxidation of a variety of olefins using aqueous
hydrogen peroxide as the terminal oxidant (Scheme 20).[32, 33] Inthe cases of trans-stilbene and cyclic olefins such as indene, di-
hydronaphthalenes, as well as chromenes as precursors, theproducts were furnished in good to excellent enantiomeric ex-
Scheme 14. Asymmetric epoxidation of unfunctionalized olefins catalyzedby Mn–porphyrin complexes.
Scheme 15. Asymmetric epoxidation of olefins catalyzed by Mn–aminopyri-dine complexes.
Scheme 16. Asymmetric epoxidation of olefins catalyzed by an Mn–aminopyridine com-plex.
Chem. Asian J. 2015, 10, 2056 – 2068 www.chemasianj.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2062
cesses. Moreover, the authors also observed an in-crease in enantioselectivity when the reactions were
conducted in the presence of an acidic additive.
In 2015, Abdi et al. designed chiral Mn complex 22with an N4-ligand based on the diaminocyclohexane
scaffold, which turned out to be an efficient catalystfor the enantioselective epoxidation of chalcones,
indene, and chromenes to give the products in goodenantioselectivities (Scheme 21).[34] The stereochemis-
try of the diaminocyclohexane ligand was not deter-
mined by the authors.
2.5. Tungsten-Catalyzed Asymmetric Epoxidation
Over the last few decades, the use of peroxotung-
states as catalysts for the epoxidation with H2O2 hasattracted much attention as a result of their high ca-
pability for oxygen transfers and low activity for dis-proportion of H2O2. Recently, our group developedthe first tungsten-catalyzed asymmetric epoxidationof allylic and homoallylic alcohols.[35] Under the catal-
ysis of our W–bishydroxamic acid (BHA) catalyticsystem both primary, secondary, and tertiary allylic as
well as homoallylic alcohols were successfully em-ployed as precursors of the asymmetric epoxidation reactionto furnish the products generally with excellent enantioselec-tivities (Scheme 22). Notably, the reactions were performedunder air and in most cases at room temperature, and requir-
ing no anhydrous solvent or preparation of the metal-complexcatalyst prior to the catalytic process.
Moreover, this method demonstrates good chemoselectivityfor primary alcohols over secondary and tertiary alcohols. Forinstance, two farnesol derivatives bearing three olefins and
two alcohol moieties were reacted as precursors of the epoxi-dation reaction; they provided the corresponding products in
almost complete regioselectivities in favor of the oxidation ofthe C=C double bond, which is closer to the primary alcohol
(Scheme 23). These results promised the use of this method inthe late stage of the synthesis of complex molecules.
3. Organocatalyzed Asymmetric Epoxidationusing Hydrogen Peroxide as Oxidant
3.1. Amine-Catalyzed Asymmetric Epoxidation
The reaction of hydrogen peroxide with an a,ß-unsaturated
ketone or aldehyde to form an epoxide was described alreadyin 1921 by Weitz and Scheffer.[36] In 2004, Jørgensen et al. de-
veloped the first secondary-amine-catalyzed asymmetric epoxi-dation of a,b-unsaturated aldehydes with aqueous hydrogen
Scheme 17. Asymmetric epoxidation of olefins catalyzed by an Mn–amino-benzimidazole complex.
Scheme 18. Asymmetric epoxidation of diverse olefins catalyzed by an Mn–aminopyridine complex.
Scheme 19. Asymmetric epoxidation of diverse olefins catalyzed by Mn–aminopyridinecomplexes.
Chem. Asian J. 2015, 10, 2056 – 2068 www.chemasianj.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2063
peroxide as oxidant.[37] This method was shown toprovide good yields and high enantioselectivities.Later, the research group of List reported that less
sterically demanding primary amines were to able topromote enantioselective epoxidation of a,b-unsatu-rated ketones.[38, 39] According to the generally accept-
ed mechanism for this type of epoxidation, the reac-tion consists of a cascade of an iminium-mediated
oxa-Michael reaction with H2O2 as nucleophile and anenamine-mediated intramolecular a-oxygenation of
the carbonyl group (Scheme 24).
Recent mechanistic studies on this reaction report-ed by Jørgensen et al. indicate that the hydrate or
peroxyhydrate of the product serves as a phase-transfer catalyst, which can increase the reaction rate
through its autoinductive effect.[40] Relying on thisobservation, the authors discovered that the use of
chloral hydrate as additive accelerated the reactionsignificantly (Scheme 25). Thus, this organocatalyticepoxidation method was greatly improved in termsof the efficiency and applicability in industry.
The seminal work of List et al. demonstrated thatcinchona alkaloid-derived primary amines or their
salts are powerful catalysts for the asymmetric epoxi-dation of a broad substrate scope including a-
branched a,b-unsaturated aldehydes, aliphatic lineara,b-unsaturated ketones, and small, medium andlarge-ring-sized cyclic a,b-unsaturated ketones, which
provide the products generally in high enantioselec-tivities (Scheme 26).[41, 42] Furthermore, this primary-
amine-catalyzed epoxidation was found to havea complete stereoconvergence, since the use of the E
or Z isomer of the olefin as the substrate resulted in
the same enantiomer. These results indicate that E/Zmixtures can be employed as precursors without on-
erous separation. Notably, in the cases of challenging sub-strates such as cyclopentenones and a-branched enals, the use
chiral acids as additives was required to achieve high enantio-selectivities. Limitations of this method are observed in the
cases of aromatic enones, terminal enones, and tri- and tetra-
substituted aliphatic a,b-unsaturated ketones, which failed togive the products or provided the products in low yields.
3.2. Peptide-Catalyzed Asymmetric Epoxidation
Generally, peptide-catalyzed asymmetric epoxidation can beclassified into two categories : 1) Juli�–Colona epoxidation em-
Scheme 20. Asymmetric epoxidation of diverse olefins catalyzed by an Mn–amino-oxazo-line complex.
Scheme 21. Asymmetric epoxidation of diverse olefins catalyzedby an Mn–diaminocyclohexane complex.
Scheme 22. W–BHA-catalyzed asymmetric epoxidation of allylic and homoallylic alcohols.
Chem. Asian J. 2015, 10, 2056 – 2068 www.chemasianj.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2064
ploying oligopeptides to control the reactivity of hydrogen
peroxide through a nucleophilic epoxidation mechanism; and2) electrophilic epoxidation using peptide-embedded chiral
carboxylic acids or ketones as catalysts, which can be convert-ed into the corresponding peracids or dioxiranes in situ, re-
spectively (Scheme 27).
In the 1980s, Juli� and Colona introduced poly-l-leucine asa catalyst for the enantioselective epoxidation of chalcone and
its derivatives.[43] Recent studies on this type of reaction basedon NMR spectroscopy and modeling by Berkessel et al. re-
vealed that high enantioselectivity can be achieved with atleast five l-leucine residues, whereas the reaction rate reaches
a plateau at approximately 12 residues.[44] In 2011, Demizu andKurihara et al. synthesized a series of l-leucine-rich heptapepti-
des containing l-serine, d-serine, and l-homoserine derivativesat their third and seventh positions (Scheme 28).[45] These sta-
pled helical heptapeptides turned out to be successful cata-lysts for the asymmetric epoxidation of a,b-unsaturated ke-
tones. Under the optimum reaction conditions, the productswere obtained in good to excellent enantiomeric excesses.
Miller et al. developed a series of peptide-based catalysts
containing aspartic acid for regio- and enantioselective epoxi-dation of allylic alcohols (Scheme 29).[46–48] By using this ap-proach, farnesol and its analogues were selectively epoxidizedat a certain position, which was controlled by the catalysts em-
ployed. Hexapeptide 30 activated the allylic alcohols througha hydroxyl-directing mechanism to provide 2,3-epoxy alcohols
as the major products. In contrast, under the catalysis of pen-
tapeptide 31 the epoxidation proceeded favoring the internalolefin of farnesol giving the 6,7-epoxy alcohol with good regio-
selectivity, albeit with moderate enantioselectivity. Further-more, this method was also applicable to the asymmetric ep-
oxidation of simple allylic alcohols. In the case of cis olefins theproducts were obtained in high yields and enantiomeric ex-
cesses. In the catalytic cycle the terminal carboxylic acid of as-
partic acid is converted into a peracid under the activation ofa stoichiometrical amount of diisopropylcarobodiimide with 1-
hydroxybenzotriazole and N,N-dimethylaminopyridine as cata-lysts. Subsequently, the oxygen atom is transferred from the
peracid to the olefin yielding the epoxide as a product and re-leasing the catalyst for the next catalytic cycle.
The same research group also developed peptide-based cat-
alyst 32, which has a terminal trifluoromethyl ketone and canbe converted into a transient dioxirane with hydrogen perox-
ide as terminal oxidant (Scheme 30).[49] The active species diox-irane turned out to be capable of transferring the oxygen
atom to an olefin in an enantioselective manner. Generally, theproducts were afforded only in moderately good enantiomeric
Chiral phase transfers are also known to be able to catalyze
the asymmetric epoxidation of electron-deficient olefins usinghydrogen peroxide as oxidant. For instance, in 2013 Chen et al.reported an efficient H2O2-mediated epoxidation of ß-trifluoro-methyl ß,ß-disubstituted enones catalyzed by quinidine-de-rived quaternary ammonium salt 33 (Scheme 31).[50] This
method afforded ß-trifluoromethyl a,ß-epoxy ketones bearinga quaternary stereocenter in high yields and excellent stereose-
lectivities. Notably, the catalyst employed could be simply recy-cled through filtration and reused with retained catalytic activi-
ty and selectivity. Furthermore, this reaction was up-scaled to
a gram scale and the product was still obtained with excellentresults. With respect to the mechanism, the authors proposed
a plausible transition state, in which a hydrogen peroxideanion is captured by the catalyst as countercation and the
enone is activated through a hydrogen bond together withp,p stacking.
Scheme 23. W–BHA-catalyzed regioselective epoxidation of the farnesol de-rivatives.
Scheme 25. Secondary-amine-catalyzed asymmetric epoxidation of a,b-unsa-turated aldehydes with chloral hydrate as additive.
Scheme 24. General mechanism for the amine-catalyzed asymmetric epoxi-dation of a,b-unsaturated aldehydes and ketones.
Chem. Asian J. 2015, 10, 2056 – 2068 www.chemasianj.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2065
Chiral oxaziridinium salts are highly active catalysts
for asymmetric epoxidation of unfunctionalized ole-fins. Generally, this reaction requires oxone as the
stoichiometric oxidant and converts the precatalystiminium salt into an active species, oxaziridinium salt,
which is then able to transfer the oxygen atom to
the olefins. In 2013 Page and co-workers reportedthe first use of hydrogen peroxide as terminal oxi-
dant with diphenyl diselenide as a crucial additive inthe asymmetric epoxidation of a trisubstituted alkene
to provide the product in good yield and enantiose-lectivity (Scheme 32).[51] In the plausible catalytic
cycle benzeneperseleninic acid is generated through
the oxidation of diphenyl diselenide by hydrogenperoxide. Subsequently, the oxaziridinium required
for the enantioselective epoxidation is formed by oxi-dizing the iminium precatalyst with benzenepersele-
ninic acid as oxidant. In the next step the oxygenatom is transferred from the oxaziridinium to the
olefin in a stereoselective manner.
4. Conclusion and Outlook
In this review the most recent advances in the fieldof asymmetric epoxidation using hydrogen peroxide
as oxidant are covered. In recent years, tremendousprogress has been achieved in this area by employ-
ing both transition metals and organocatalysts. Theasymmetric epoxidation using hydrogen peroxide asoxidant provides a simple, direct, and highly stereo-
selective way to access versatile epoxides under mildand environmentally benign reaction conditions. No-
tably, the substrate scope now includes various types
Scheme 26. Primary-amine-catalyzed asymmetric epoxidation of a,b-unsaturated ketonesand aldehydes.
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Scheme 30. Peptide-catalyzed asymmetric epoxidation of unfunctionalized olefins.
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Manuscript received: March 26, 2015
Accepted Article published: June 9, 2015
Final Article published: July 14, 2015
Scheme 31. Asymmetric epoxidation of ß-trifluoromethyl ß,ß-disubstituted enones cata-lyzed by chiral phase transfer.
Scheme 32. Asymmetric epoxidation catalyzed by a chiral oxazirindiniumsalt.
Chem. Asian J. 2015, 10, 2056 – 2068 www.chemasianj.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2068