D. Crich et al., ORIGIN OF THE BETA-OXYGEN EFFECT IN THE BARTON DEOXYGENATION REACTION, Journal of the American Chemical Society, 117(34), 1995, pp. 8757-8768
Photolysis of O-neopentyl S-tributylstannyl dithiocarbonate with hexap
henyl distannane, and 4-methoxyacetophenone as sensitizer, results in
crossover of the stannyl groups. The reaction of O-octyl O'-(2-butoxye
thyl) 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-oc
tyl O'-phenyl thiocarbonate and O-(2-butoxyethyl) O'-phenyl thiocarbon
ate for a deficiency of stannane or silane also indicated no significa
nt preference for reaction of the beta-oxygen-substituted substrate, l
eading to the conclusion that in conformationally unrestricted systems
there is no significant beta-oxygen effect in the Barton deoxygenatio
n reaction. Competition experiments between the cis- and trans-O-(4-ph
enylcyclohexyl) S-methyl dithiocarbonates and the cis- and trans-O-(2-
phenyl-1,3-dioxan-5-yl) S-methyl dithiocarbonates for reaction with tr
ibutylstannane reveal that in every case the heterocyclic system is mo
re reactive. The cis-isomers of 4-phenylcyclohexyl S-methyl dithiocarb
onate 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 rea
ctivity of the cis-series with respect to the tl-ans is due to the gre
ater relief of strain on fragmentation.