REARRANGEMENT OF FISCHER CARBENE COMPLEXES TO KETONES - STEREOCHEMISTRY AND MECHANISM

Fischer carbene complexes of general structure (CO)(5)M=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)(5)W=C(4-MeC(6)H(4))(O CH2CH=CMe(2)) and (CO)(5)W=C(Ph)(OCH2CH=CHPh); the ketones do not resu lt from a [3,3]-sigmatropic shift alone. Excellent diastereoselectivit y is observed with (CO)(5)Cr=C(Ph){OCH(Me)(C6H4-2-OMe)}; that the prod uct obtained is a result of kinetic control is established by an equil ibration experiment. The structure of this product was determined by X -ray crystallography.