Science of Synthesis

Adolfsson, H., Science of Synthesis, (2008) 37, 241.

Asymmetric chromium-mediated epoxidations can be performed using complex ‌16‌.[‌55‌,‌56‌] In contrast to the corresponding manganese-based epoxidation systems (see Section ‌37.2.1.4.1‌), oxochromium complexes such as ‌16‌ are isolable and have therefore been studied in great detail in both stoichiometric and catalytic epoxidations. These chiral chromium(V) catalysts give good enantioselectivity in the epoxidation of E-disubstituted alkenes. This can be exemplified by the epoxidation of (1E)-prop-1-enylbenzene using a stoichiometric quantity of ‌15‌ (X = NO3; L = Ph3PO) and 1.2 equivalents of iodosylbenzene, where the corresponding epoxide is obtained in 45% yield and 92% ee. Changing the terminal oxidant to hydrogen peroxide results in a poor yield (2%) and no enantiomeric excess. Under catalytic conditions (10 mol% of ‌16‌) in the presence of iodosylbenzene, the epoxide is formed in higher yield (71%) but in slightly lower enantiomeric excess (82% ee).[‌55‌]

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