The stereochemistry of organometallic compounds. Part X. Further evidence as to the mechanism of borohydride reduction of organomercurials from a study of photochemical and organotin hydride reductions

Reductions of 3-exo-acetoxynorborn-5-en-2-exo-ylmercury(II) chloride (1), 5-exo-acetoxy-3-exo-nortricyclyl-mercury(II) chloride (2), and 7-anti-acetoxy-2-exo-bromonorborn-5-ene with tributyltin hydride, triphenyltin hydride, or photochemically in methanol give similar isomeric mixtures of 2-exo-acetoxynorborn-5-ene (3), 7-anti-acetoxynorborn-2-ene (4), and 3-acetoxynortricyclene (5) as do reductions of the mercurials with sodium borohydride in aqueous tetrahydrofuran. Reductions of 3-acetoxynorborn-5-en-2- ylmercury(II) chloride and 7-anti-acetoxy-2-exo-bromonorbornene with tributyltin deuteride gives deuterio-substituted acetates, (3), (4), and (5), and the stereochemistry of deuterium incorporation is identical with that obtained by reduction of the mercurials with sodium borodeuteride. The results indicate that a radical mechanism is operating in the reduction of organomercurials with borohydride.