Rearrangement of Fischer Carbene Complexes to Ketones:  Stereochemistry and Mechanism

Abstract

Fischer carbene complexes of general structure (CO)5MC(Ar)(OCHRAr‘), where M = Cr or W and R = H or Me, rearrange to ketones, ArC(O)CHRAr‘Cr(CO)3 or ArC(O)CHRAr‘ (for M = W), on heating. For M = W and Ar‘ = 2-furyl, the rearrangement is the most facile. A probable mechanism is proposed. Rearrangement also occurs with (CO)5WC(4-Me-C6H4)(OCH2CHCMe2) and (CO)5WC(Ph)(OCH2CHCHPh); the ketones do not result from a [3,3]-sigmatropic shift alone. Excellent diastereoselectivity is observed with (CO)5CrC(Ph){OCH(Me)(C6H4-2-OMe)}; that the product obtained is a result of kinetic control is established by an equilibration experiment. The structure of this product was determined by X-ray crystallography.