THE PHOTOCHEMICAL GENERATION OF ALPHA-SULFUR-SUBSTITUTED CYCLOPROPYLCARBINYL RADICALS AND THEIR UTILIZATION AS A MECHANISTIC PROBE FOR ELECTRON-TRANSFER IN THE PEROXIDE SULFIDE REACTION

The photolytically induced n-Bu3SnH reduction of alpha-chloro-substituted sulfides 4 and photochemical addition of BrCCl3 to thiones 7 afford alpha-sulfur-substituted cyclopropylcarbinyl radicals, which rearrange efficiently to afford thioenol ethers as ring-opened products. It is shown that alpha-sulfur substitution does not significantly influence the propensity for ring opening of these hitherto unknown cyclopropylcarbinyl radical probes. Since in the reaction of cyclopropyl sulfides 1 and benzoyl peroxide, except for sulfoxides 2 (oxygen transfer products), only intact alpha-benzoyloxy-substituted sulfides 3 (insertion products) are formed, the lack of ring opening is construed as evidence against alpha-sulfur-substituted cyclopropylcarbinyl radicals in the peroxide-sulfide reaction. Instead of electron transfer between the sulfide and benzoyl peroxide, an S(N)2 mechanism is proposed to afford a sulfonium-benzoate ion pair, which serves as a common precursor to the sulfoxide and insertion product (Pummerer rearrangement). The novel photochemical addition of BrCCl3 to thiones 7 constitutes a convenient and efficient synthesis for the previously unknown trichloromethyl-substituted thioenol ethers 8.