Molybdenum(II)- and Tungsten(II)-Catalyzed Allylic Substitution

The molybdenum(II) complexes Mo(CO)(5)(OTf)(2) (7a), [Mo(CO)(4)Br(2)](2) (8a), their tungsten(II) congeners 7b and 8b, and bimetallic complex Mo(CO)(3)(MeCN)(2)(SnCl(3))Cl (9a) have been found to catalyze the C-C bond-forming allylic substitution with silyl enol ethers derived from beta-dicarbonyls (e.g., 16 + 30 --> 46) or from simple ketones (e.g., 16 + 32 --> 50), aldehydes, and esters as nucleophiles under mild conditions (room temperature, 1-2 h). Methanol, as a prototype oxygen nucleophile, reacts in a similar fashion (e.g., 16 + MeOH --> 43). Mechanistic and stereochemical experiments are indicative of Lewis-acid catalysis rather than a metal template-controlled process.