Electron-transfer nucleophilic substitution reactions on neopentyl- and phenyl-substituted alkyl chlorides. Effect of the bridge length on the intramolecular electron-transfer catalysis

The nucleophilic substitution reaction of the chlorides RMe2CCH2Cl (R = Me, 4; Ph, 5a; PhCH2, 5b) and their relative reactivities toward diphenyl phos phide ions were studied under irradiation in liquid ammonia. The relative r eactivities determined were k(5a)/k(4) congruent to 9 and k(5b)/k(4) congru ent to 0.85. These reactions are proposed to occur through the S(RN)1 mecha nism. The higher reactivity of 5a is explained on the basis of its higher e lectron affinity due to the phenyl substitution and the efficient intramole cular electron transfer from this group to the C-Cl sigma* bond (intra-ET c atalysis). Although 5b also has a phenyl ring, its lower reactivity is ascr ibed to a decrease in the rate of the intra-ET by elongation of the bridge in one methylene unit. The relative reactivity of 5a versus 5b (k(5a)/k(5b) congruent to 6.4) is proposed to indicate the ratio of the intra-ET rates of the radical anions of both compounds. AM1 calculations performed on the system are in agreement with the experimental results.