A PHOTOCHEMICAL PROBE FOR SINGLE-ELECTRON TRANSFER IN NUCLEOPHILIC ALIPHATIC SUBSTITUTION - EVIDENCE FOR GEMINATE RADICAL COUPLING IN THE SOLVENT CAGE

A major effort to establish single electron transfer (SET) as an important pathway in nucleophilic aliphatic subsitution reactions has involved the use of cyclizable probes, e.g., 6-iodo-5,5-dimethyl-1-hexene. In order to examine the partition between cyclization and direct radical-radical recombination, we have investigated the ground-state and excited-state chemistry of the 9-phenylfluorenyl anion (9PF(-)) with neopentyl-type iodides. It has been shown that 9PF(-) does not react with Me(3)CCH(2)I, but it does undergo efficient reaction upon irradiation (Phi = 1.0) to yield nucleophilic aliphatic substitution products, With the sterically analogous cyclizable probe 6-iodo-5,5-dimethyl-1-hexene, no ground-state reaction is observed, However, both cyclized and uncyclized products of substitution, 9PFR(c) and 9PFR(u), are produced upon irradiation. Thus photoproducts clearly involving electron-transfer-induced radical intermediates can result without acommpanying free-radical cyclization. These results suggest that, although the observation of cyclized products in the reaction of a cyclizable radical probe with a nucleophile is evidence of a radical intermediate, the absence of such cyclized products does not require the absence of radical intermediates.