RADICAL-ION PROBES .7. BEHAVIOR OF A HYPERSENSITIVE PROBE FOR SINGLE-ELECTRON TRANSFER IN REACTIONS NOT INVOLVING ELECTRON-TRANSFER

hyl-5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one (8) and hyl-5,7-di- tert-butylspiro[2.5]octa-4,7-dien-6-one (1) react with thiophenoxide i on to produce cyclopropane ring-opened products. Thermodynamic conside rations effectively rule out any possibility that single electron tran sfer is involved in theses reactions; the process PhS- + substrate --> PhS. + substrate(.-) is endothermic by over 50 kcal/mol! Nucleophilic attack occurs both at the least- and most-hindered carbons of the cyc lopropyl group, and the product ratio (R(1 degrees/2 degrees) from 8 a nd R(1 degrees/3 degrees) from 1, where 1 degrees, 2 degrees, and 3 de grees refer to the regioisomeric phenyl sulfides formed from these sub strates) is found to vary with solvent. In dipolar, aprotic solvents, nucleophilic attack occurs preferentially at the least-hindered carbon of the cyclopropyl group (R(1 degrees/2 degrees) and R(1 degrees/3 de grees) approximate to 4-5), consistent with an S(N)2 mechanism. In pro tic solvents, products arising from nucleophilic attack at the more-su bstituted carbon of the cyclopropyl group become increasingly importan t, consistent with the onset of a carbocationic (S(N)2(C+)) pathway. T he strengths and weaknesses of 1 and 8 as probes for single electron t ransfer are discussed in the context of these results.