Kinetics and mechanism of the addition of benzylamines to benzylidene-3,5-heptadione in acetonitrile

The addition reaction of benzylamine (XC6H4CH2NH2) to benzylidene-3,5-heptadione (BHD; YC6H4-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 kH/kD 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.