Catalytic Ylide-mediated Epoxidation

Furnkawa et al. introdnced the catalytic ylide mediated epoxidation by using a reaction system that is composed of alkyl sulfide, alkyl halide and aldehyde in the... 

Of course, the key limitation of the ylide-mediated methods discussed so far is the use of stoichiometric amounts of the chiral reagent. Building on their success with catalytic asymmetric ylide-mediated epoxidation (see Section 1.2.1.2), Aggarwal and co-workers have reported an aza version that provides a highly efficient catalytic asymmetric synthesis of trans-aziridines from imines and diazo compounds or the corresponding tosylhydrazone salts (Scheme 1.43) [68-70]. 

The sulfur ylide-mediated epoxidation of aldehydes has been thoroughly investigated [70, 71]. The chiral sulfur ylides reported by Aggarwal have been most broadly applicable, and a catalytic, asymmetric process yielding aromatic transepoxides has been developed [72]. In this process, the sulfur ylides are produced in situ from diazo compounds, generated in turn from tosylhydrazone salts (Scheme 9.15) [73],... 

Aggarwal VK, Winn CL. Catalytic, asymmetric sulfur ylide-mediated epoxidation of carbonyl compounds scope, selectivity, and applications in synthesis. Acc. Chem. Res. 2004 37 611-620. Li A-H, Dai L-X, Aggarwal VK. Asymmetric ylide reactions epoxidation, cyclopropanation, aziridination, olefination, and rearrangement. Chem. Rev. 1997 97 2341-2372. 

Scope, selectivity, and applications in the synthesis of catalytic, asymmetric sulfur ylide-mediated epoxidation of carbonyl compounds 04ACR611. 

It should be mentioned at this stage that in addition to the catalytic version, numerous sulfur-ylide mediated asymmetric diastereoselec-tive epoxidations which require the use of stoichiometric amounts of the sulfur-ylide were reported. In these cases, the sulfonium salts had to be prepared in a first step, and were used in a subsequent diastereoselective epoxidation step. For some selected references for this type of stoichiometric asymmetric two-step epoxidation, see (a) T. Durst, L. Breau, R. N. 

Catalytic asymmetric epoxidation and aziridination mediated by sulfur ylides 98SL329. 

AggarwaU, V. K. Catalytic Asymmetric Epoxidation and Aziridination Mediated by Sulfur Ylides. Evolution of a Project, Synlett 1998, 329-336. 

The ylide, of general structure (4.132), is formed by reaction of the corresponding sulphide (4.133) with a metallocarbenoid resulting from transition metal salt-mediated decomposition of a diazo compound (4.134) or N-tosyl hydrazone salt (4.135). Reaction of the sulfonium ylide (4.132) with aldehyde (4.136) yields the trans-epoxide (4.137) as the major product and sulfide (4.133) which is then returned to the catalytic cycle (Figure 4.10). 

In catalytic epoxidation reactions an alternative to the ylide generation method via alkylation/deprotonation is the transition metal-mediated carbene transfer from diazo compounds to sulfide catalysts. In 1994, Aggarwal and coworkers employed this method in the enantioselective catalytic epoxidation of aldehydes [25]. Using 20mol% of non-racemic sulfide 17 and lmol% of Rh2(OAc)4 together with the slow addition of PhCHN2, a 58% yield and 11% ee were obtained in the epoxidation of benzaldehyde (Scheme 20.10). The enantioselectivity was similar to the results obtained by Breau and Durst using preformed sulfonium salts [26]. 

Scheme 7.42 Catalytic epoxide (X=0), aziridine (X=iV-Ts or iV-SES) and cyclopropane (X=CH-COY) formation mediated by chalcogen-ylides generated from diazo compounds via metal carbenes (start at upper right corner)...

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