Radical Ion Probes. 7. Behavior of a “Hypersensitive” Probe for Single Electron Transfer in Reactions Not Involving Electron Transfer

Abstract

1-Methyl-5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one (8) and 1,1-dimethyl-5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one (1) react with thiophenoxide ion to produce cyclopropane ring-opened products. Thermodynamic considerations effectively rule out any possibility that single electron transfer 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 cyclopropyl group, and the product ratio (R(1°/2°) from 8 and R(1°/3°) from 1, where 1°, 2°, and 3° refer to the regioisomeric phenyl sulfides formed from these substrates) 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°/2°) and R(1°/3°) ≈ 4−5), consistent with an SN2 mechanism. In protic solvents, products arising from nucleophilic attack at the more-substituted carbon of the cyclopropyl group become increasingly important, consistent with the onset of a carbocationic (SN2(C+)) pathway. The strengths and weaknesses of 1 and 8 as probes for single electron transfer are discussed in the context of these results.

Radical Ion Probes. 8. Direct and Indirect Electrochemistry of 5,7-Di-tert-butylspiro[2.5]octa-4,7-dien-6-one and Derivatives

“…Our interest in this system is related to the fact that 1c has proven to be a highly effective “probe” for distinguishing between SET and conventional polar pathways in reactions of nucleophiles with carbonyl compounds based upon the observed regiochemistry of the products (Scheme ). The observed behavior of this substrate in reactions with nucleophiles which do and do not react with carbonyl compounds via SET has confirmed the key elements of this hypothesis.

1

…”