Iminium Salt-Catalyzed Epoxidation

Aggarwal subsequently reported the use of a chiral binaphthyl iminium-salt 21, giving 31% ee for the epoxidation of ( )-stilbene and 71% ee for the epoxidation of 1-phenylcyclohexene [82]. Several alternative iminium catalysts have been tested 

The same study also highlighted problems with the use of tetraphenylborate as a counterion for iminium catalysts. It was noted that under oxidative conditions decomposition of the tetraphenylborate occurred, with biphenyl being produced as a by-product. This affects both the solubility and partition ability of the iminium and oxaziridinium species and as such can have significant effects on the enanti-oselectivity of the reaction. 

NMR studies of the conformation of two diastereoisomers of azepine catalyst 32 have provided an explanation for the observation that both exocyclic-diastere-omers give the same absolute configuration of epoxide product, as well as for the lower reactivity of compared to R, S)-32 [99]. The explanation Ues 

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Abstract Organo-catalyzed asymmetric epoxidation has received much attention in the past 30 years and significant progress has been made for various types of olefins. This review will cover the advancement made in the field of chiral ketone and chiral iminium salt-catalyzed epoxidations. 

Scheme 1 Ketone/iminium salt-catalyzed epoxidations...

Scheme 12.9 Synthesis of levcromakalim 21 via iminium salt-catalyzed epoxidation. (For experimental details see Chapter 14.13.7).

Iminium Salt-Catalyzed Epoxidation Under Non-Aqueous Conditions [56] (p. 411)... 

Scheme 12.1 Ketone- and iminium salt-catalyzed alkene epoxidation.

Asymmetric epoxidation of olefins is an effective approach for the synthesis of enan-tiomerically enriched epoxides. A variety of efficient methods have been developed [1, 2], including Sharpless epoxidation of allylic alcohols [3, 4], metal-catalyzed epoxidation of unfunctionalized olefins [5-10], and nucleophilic epoxidation of electron-deficient olefins [11-14], Dioxiranes and oxazirdinium salts have been proven to be effective oxidation reagents [15-21], Chiral dioxiranes [22-28] and oxaziridinium salts [19] generated in situ with Oxone from ketones and iminium salts, respectively, have been extensively investigated in numerous laboratories and have been shown to be useful toward the asymmetric epoxidation of alkenes. In these epoxidation reactions, only a catalytic amount of ketone or iminium salt is required since they are regenerated upon epoxidation of alkenes (Scheme 1). 

In 1996, Aggarwal and coworkers synthesized binaphthyl-based iminium salt 76 via oxidation and methylation from binaphthylamine (Scheme 15) [147], Catalyst loading of 5 mol% is sufficient to catalyze the epoxidation of a number of olefins in good yield. Up to 71% ee can be obtained for 1-phenylcyclohexene oxide using this catalytic system (Table 7, entry 8). 

The epoxidation of olefins catalyzed by iminium salts and amines (or ammonium salts) is emerging as a new technique for the functionalization of simple aUcenes. These catalysts have relatively simple structures and hence are easily produced at scale they offer potential as green oxidation catalysts. These organic salts are effective oxygen transfer reagents towards electron-rich unfunctionalized olefins. For the iminium salt systems oxone oxidizes an iminium salt to the oxaziridi-nium intermediate, which then transfers oxygen to the olefin and as oxone reacts readily with iminium ions to regenerate the oxaziridinium species catalyti-cally, efficient oxidation is possible. 

Recently, oxaziridinium salts derived in situ from chiral iminium salts (1 and 2) and Oxone were found to catalyze epoxidation with moderate-to-good enantioselectivity (up to 73% ee) (Scheme 6B.4) [6], Although the substrates are limited to conjugated olefins, this reaction has an advantage in being catalytic with respect to chiral iminium salts. 

Advances in the chemistry of ring-fused oxiranes during the period under review (1995-2007) principally involve new or expanded methods of asymmetric synthesis including metallosalen-catalyzed, and chiral dioxirane- and iminium salt-mediated processes. Developments in the reactivity of such species include extensive work in the area of epoxide ring opening and advances in the chemistry of lithiated epoxides. 

M-K. Wong, L-M. Ho, Y-S. Zheng, C-Y. Ho, D. Yang, Asymmetric epoxidation of olefins catalyzed by chiral iminium salts generated in situ from amines and aldehydes, Org. Lett. 3 (2001) 2587. 

Organocatalytic Oxidation. Asymmetric Epoxidation of Olefins Catalyzed by Chiral Ketones and Iminium Salts Wong, O.A. Shi, Y. Chem. Rev. 2008,108,3958. 

Oxone and its derivatives have also been used with chiral iminium salts and amines to formenantiomerically enriched epoxides. The scope and enantioselectivity of epoxidation with chiral iminium salts with Oxone andPluPHSOs have made progress during recent years. Chiral iminium salts (7 and 8) have been particularly successful for various olefins (eqs 40 and 41). Amine-catalyzed epoxidation is a relatively new area, and the active species is thought to be an ammonium peroxymonosulfate salt which acts as a phase transfer catalyst and undergoes electrophilic attack by an olefin.Use of chiral amines has given rise to enantiomerically enriched epoxides (eq 42). 

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