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 BnEt(3)N(+)[Mo(CO)(4)ClBr(2)](-) (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 ZnCl(2) to its trans-epimer 2 via a retro-ene reaction.