Theoretical studies of the effects of ortho‐methyl substitution on SN1 and SN2 processes at primary and secondary benzylic carbons

Abstract

AbstractThe effects of o‐methyl substitution on the transition‐state (TS) structure and reactivity of SN1 and SN2 processes were investigated for primary and secondary benzyl (α‐tert‐butyl) derivatives MO theoretically using the AM1 method. The o‐methyl groups have both stabilizing (by electron donation) and destabilizing (steric) effects on the SN2 TS; the former prevails for the primary whereas the latter effect becomes dominant for the secondary compounds. In contrast, complex interplay of the two effects are operative in the SN1 TS of both primary and secondary derivatives leading to the reactivity order which is consistent with that observed by solvolytic experiments, o,o′‐dimethyl < H < o‐methyl. The bond length–reactivity rule applies only for the primary benzyl SN2 series for which steric effects are insignificant. For reactions of benzyl derivatives is general the coplanarity between the carbenium sp2 centre and the benzene ring, π–σ, is of the utmost importance in stabilizing TS structure for which other structural requirements may be relaxed.