Chromium salen catalysed asymmetric alkene epoxidation. Influence of substituents at the 3,3′-positions on the salen rings

Substituent electronic effects in chiral ligands for asymmetric catalysis

“…Gilheany and co-workers have carried out extensive investigations into the asymmetric epoxidation of trans-bmethylstyrene using oxo-chromium(V)-salen complexes 2, Scheme 2 [3,4].…”

Salen ligands derived from trans-1,2-dimethyl-1,2-cyclohexanediamine: preparation and application in oxo-chromium salen mediated asymmetric epoxidation of alkenes

“…Iodosylbenzene was used to generate oxo-12 and oxo-13 and the reactions were carried out both in the presence and absence of our standard donor ligand additive, triphenylphosphine oxide [8,12,15]. In all cases the yields did not exceed 20%, which is not unusual for these stoichiometric reactions [10].…”

“…Initially, we were not unduly worried by this because it is very common in the preparation of chromium salen complexes [29,30]. It is often caused by the presence of an impurity arising from the CrCl 2 insertion process [31], which we have previously shown [12,32] is inactive in the oxidation process. Another reason for the discrepancy could be the number of solvent molecules that coordinate to the transition metal centre.…”

“…In recent years our group has been working on the development of epoxidation catalysts based on a chromium salen template [6,[8][9][10][11][12][13][14][15][16]. These show a contrasting selectivity pattern to their manganese analogues in that they allow the asymmetric epoxidation, with iodosylbenzene as stoichiometric oxidant, of E-1,2-disubstituted with high enantioselectivity (>90% ee with complex 2) [8,12].…”

“…These show a contrasting selectivity pattern to their manganese analogues in that they allow the asymmetric epoxidation, with iodosylbenzene as stoichiometric oxidant, of E-1,2-disubstituted with high enantioselectivity (>90% ee with complex 2) [8,12]. A major advantage of this system is that reactions can be studied stoichiometrically as well as the catalytic level thus allowing study of stereoselectivity separated from catalytic cycle issues.…”