New Lewis-acidic molybdenum(II) and tungsten(II) catalysts for intramolecular carbonyl ene and prins reactions. Reversal of the stereoselectivity of cyclization of citronellal

New Mo(II) complexes BnEt3N+[Mo(CO)(4)ClBr2](-) (A) and Mo(CO)(5)(OTf)(2) (B) and their W(II) congeners D and E have been developed as catalysts for the title reactions. Unlike other Lewis acids, the latter catalysts exhibit cis-stereoselectivity in the cyclization of citronellal (1 --> 3 with A and 1 --> 5 with B). Isotopic labeling allowed formulation of the reaction mechanism, according to which these complexes act as bulky Lewis acids, eta(1)-coordinated to the carbonyl oxygen. The stereochemistry appears to be controlled by the protruding ligand L-p, which dictates the boatlike transition state III. The kinetically formed cis-alkenol 3 can be equilibrated by [MO(CO)(4)Br-2](2) (C) or ZnCl2 to its trans-epimer 2 via a retro-ene reaction.