Competitive C-O Cleavage, S-O Cleavage, and Electron Transfer in the LiAlH₄ Reduction of Hindered Disulfonates

The reaction of heterogeneous slurries of LiAlH₄with several alkanesulfonate substrates is reported. The reducible functional group(s) in each of the substrates is attached to the 1 and/or 4 position of a bicyclo[2.2.2]octyl skeleton. The ratio of S-O to C-O bond cleavage reflects a strong transannular electronic effect and is shown to vary with changes in solvent. A new reaction leading to carbon-carbon bond cleavage is also demonstrated, and the competition among these three pathways is shown to be related to both solvent composition and to the heterogeneity of the reaction. The powerful effect of even small amounts of HMPA in THF is delineated and a possible electron-transfer mechanism is presented to account for the carbon-carbon bond cleavage product.