ORIGIN OF THE BETA-OXYGEN EFFECT IN THE BARTON DEOXYGENATION REACTION

Photolysis of O-neopentyl S-tributylstannyl dithiocarbonate with hexaphenyl distannane, and 4-methoxyacetophenone as sensitizer, results in crossover of the stannyl groups. The reaction of O-octyl O'-(2-butoxyethyl) thiocarbonate with tributyltin deuteride or tris(trimethylsilyl)silane and a radical initiator shows no significant preference for the cleavage of either C-O bond. Intermolecular competitions between O-octyl O'-phenyl thiocarbonate and O-(2-butoxyethyl) O'-phenyl thiocarbonate for a deficiency of stannane or silane also indicated no significant preference for reaction of the beta-oxygen-substituted substrate, leading to the conclusion that in conformationally unrestricted systems there is no significant beta-oxygen effect in the Barton deoxygenation reaction. Competition experiments between the cis- and trans-O-(4-phenylcyclohexyl) S-methyl dithiocarbonates and the cis- and trans-O-(2-phenyl-1,3-dioxan-5-yl) S-methyl dithiocarbonates for reaction with tributylstannane reveal that in every case the heterocyclic system is more reactive. The cis-isomers of 4-phenylcyclohexyl S-methyl dithiocarbonate and O-(2-phenyl-1,3-dioxan-5-yl) S-methyl dithiocarbonate, with their axial xanthates, are more reactive than the corresponding trans-isomers. Molecular mechanics calculations suggest that the greater reactivity of the cis-series with respect to the tl-ans is due to the greater relief of strain on fragmentation.