The kinetics and mechanism of the aminolysis of aryl dithiobenzoates (RC( S)SC 6 H 4 Z; R = Ph) with pyridines (XC 5 H 4 N) in acetonitrile at 60.0 • C have been studied. A biphasic Brönsted plot is obtained with a change in slope from a large value (β X ∼ = 0.7-0.8) to a small value (β X ∼ = 0.2) at pK a • = 5.2, which is interpreted to indicate a change of the ratedetermining step from breakdown to formation of the zwitterionic tetrahedral intermediate, T ± , at pK a = 5.2 as the pyridine basicity is increased. Rates are compared with those corresponding values for aryl dithioacetates (R = Me). The faster rates for R = Me than for R = Ph, for the rate-limiting formation of T ± , can be attributed to the hyperconjugative charge transfer effect of the Me group, pseudo-π Me → π * C=S . Clear-cut change in the cross-interaction constants, ρ XZ , from +1.47 to −0.20 supports the proposed mechanistic change. The breakpoint at pK a • = 5.2 for R = Ph in the present work is in agreement with those for the pyridinolysis of R = Me and 2-furyl, and attests to the insignificant effect of acyl group, R, on the breakpoint. C
The addition reaction of benzylamine (XC6H4CH2NH2) to benzylidene-3,5-heptadione (BHD; YC6H(4-)CH=C(COEt)(2)) in acetonitrile is investigated. The rate is slower than the corresponding rate for benzylidenediethylmalonate (YC6H4CH=C(OOEt)(2)) as the result of a greater steric hindrance in the planar dicarbonyl transition state. The kinetic isotope effects (k(H)/k(D)) involving deuterated amine nucleophiles (XC6H4CH2ND2) are greater than 1 (1.37-2.04), indicating N-H bond stretching with concurrent N-C(alpha) and H-C(beta) bond formation in the TS. The trend of change in k(H)/k(D) with variation of substituent X in the nucleophile conforms to the Bell-Evans-Polanyi principle. It has been stressed that the dicarbonyl group activated olefins exhibit insignificant charge imbalance in the TS for the benzylamine additions in acetonitrile as a result of the two strong n(c) --> pi*(C=O) vicinal charge-transfer interactions.
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