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 phosphide ions were studied under irradiation in liquid ammonia. The relative reactivities determined were k(5a)/k(4) congruent to 9 and k(5b)/k(4) congruent to 0.85. These reactions are proposed to occur through the S(RN)1 mechanism. The higher reactivity of 5a is explained on the basis of its higher electron affinity due to the phenyl substitution and the efficient intramolecular electron transfer from this group to the C-Cl sigma* bond (intra-ET catalysis). Although 5b also has a phenyl ring, its lower reactivity is ascribed 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.