Epoxidations sulfur ylide catalysis

A major improvement addressing the issue of practicability and safety by avoidance of the direct use of (potentially) explosive diazo compounds was recently reported by Aggarwal and co-workers [82, 83], The direct addition of diazo compounds was replaced by use of suitable precursors which form the desired diazo compound in situ. The Aggarwal group developed this concept for the corresponding sulfur ylide type epoxidation (see Section 6.8) [82], and successfully extended it to aziridination [83]. Starting from the tosylhydrazone salt 66 the diazo compound is formed in situ under conditions (phase-transfer-catalysis at 40 °C) which were found to be compatible with the sulfur ylide type aziridination [82, 83], The concept of this improved method, for which sulfide 67 (Scheme 5.41) is the most efficient catalyst, is shown in Scheme 5.40. 

Titanosilicate catalysis has been examined by DFT studies <2001PCB3493>. The reaction of vinylcarbenoids and aromatic aldehydes has also been studied <2001JOC5395>. A DFT study of a model system for epoxide formation using a sulfur ylide has been published by Aggarwal et al. <2002JA5747>. 

The addition of a diazocarbonyl compound to an alkene with metal catalysis is an effective method for the formation of cyclopropanes, as discussed above. However, direct addition to aldehydes, ketones or imines is normally poor. Epoxide or aziridine formation can be promoted by trapping the carbene with a sulfide to give an intermediate sulfur ylide, which then adds to the aldehyde or imine. For example, addition of tetrahydrothiophene to the rhodium carbenoid generated from phenyldiazomethane gave the ylide 131, which adds to benzaldehyde to give the trans epoxide 132 in high yield (4.104). On formation of the epoxide, the sulfide is released and hence the sulfide (and the rhodium complex) can be used in substoichiometric amounts. 

Highly enantioselective ylide-type covalent catalysis has been achieved with sulfides, phosphines, tertiary amines, selenides, and teUurides, and the reported reaction types include epoxidation, aziridination, cyclopropanation, and other cyclization reactions. So far, the sulfur ylide-mediated reactions are the best... 

Aggarwal, V.K., Richardson, J. The complexity of catalysis origins of enantio-and diasteieocontrol in sulfur ylide mediated epoxidation reactions. Chem. Com-mun. 2003, 2644-2651. 

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