Epoxidation early mechanistic studies

VI. THE EPOXIDATION OF ALKENES WITH PERACIDS A. Early Mechanistic Studies... 

On the basis of theoretical studies by Bach and co-workers,17 it was found that the nucleophilic 71-bond of the alkene attacks the 0-0 cr-bond in an Sn2 fashion with displacement of a neutral carboxylic acid. There are, however, some mechanistic anomalies. For example, a protonated peracid should be a much more effective oxygen transfer agent over its neutral counterpart, but experiments have shown only modest rate enhancements for acid catalysed epoxidation. Early attempts to effect acid catalysis in alkene epoxidation where relatively weak acids such as benzoic acid were employed proved unsuccessful.18 The picture is further complicated by contradictory data concerning the influence of addition of acids on epoxidation rates.19 Trichloroacetic acid catalyses the rate of epoxidation of stilbene with perbenzoic acid, but retards the rate of a double bond containing an ester constituent such as ethyl crotonate.20 Recent work has shown that a seven-fold increase in the rate of epoxidation of Z-cyclooctene with m-chloroperbenzoic acid is observed upon addition of the catalyst trifluoroacetic acid.21 Kinetic and theoretical studies suggest that the rate increase is due to complexation of the peroxy acid with the undissociated acid catalyst (HA) rather than protonation of the peroxy acid. Ab initio calculations have shown that the free energy of ethylene with peroxy-formic acid is lowered by about 3 kcal mol-1 upon complexation with the catalyst.21... 

Mechanistic studies of 33-curing of epoxies have been undertaken in the past and concluded that curing takes place initially at both N atoms, but propagation occurs from only one center (Eqs. 24-26). These early studies did not account for several apparent anomalies, however, such as ketone formation during polymerization, the initial molecular weight distribution, and the wide differences in the activity of different imidazole derivatives. A recent mechanistic study, however, has addressed these anomalies and shown that several reactions are indeed taking place at once. It has been shown that the imidazolium ions formed early in the polymerization of epoxides are susceptible to attack by alkoxide anions and Hofmann eliminations. The first reaction, Eq. (27), caused by the... 

The mechanistic picture obtained from the ESMS studies fully explain the solution-phase observations. It was early recognized by Kochi et al. that (salen)Mn complexes with electron-donating substituents such as the 5,5 -dimethoxy derivative effect only poor yields of epoxide, whereas the catalyst with 5,5 -dinitro substituents gave the best product yields [102]. While the reactivity differences seen with differently substituted achiral salens led to the conclusion that the more electron-deficient ligand will give the more effective catalyst, the interplay between epoxidation efficiency and selectivity is much more subtle and less predictable for asymmetric epoxidation. In 1991, Jacobsen reported the dramatic... 

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