Kinetics and Mechanism of the Addition of Benzylamines to Benzylidene Meldrum's Acids in Acetonitrile

Abstract

Nucleophilic addition reactions of benzylamines <TEX>$(XC_6H_4CH_2NH_2)$</TEX> to benzylidene Meldrum's acids (BMA; <TEX>$YC_6H_4CH=C(COO)_2C(CH_3)_2$</TEX>) have been investigated in acetonitrile at 20.0 ℃. The rates of addition are greatly enhanced due to the abnormally high acidity of Meldrum's acid. The magnitudes of the Hammett <TEX>$({\rho}_X\;and\;{\rho}_Y)$</TEX> and Bronsted <TEX>$({\rho}_X$)$</TEX> coefficients are rather small suggesting an early transition state. The sign and magnitude of the cross-interaction constant, <TEX>${\rho}_{XY}$</TEX> (= -0.33), and kinetic isotope effects <TEX>$(k_H/k_D\;{\stackrel}{~}{=}\;1.5-1.7)$</TEX> involving deuterated benzylamine nucleophilies <TEX>$(XC_6H_4CH_2ND_2)$</TEX> are indicative of hydrogen-bonded cyclic transition state. The activation parameters, <TEX>${\Delta}H^{\neq}\;{\stackrel}{~}{=}\;4\;kcal\;mol^{-1}\;and\;{\Delta}S^{\neq}\;{\stackrel}{~}{=}\;-37\;e.u.$</TEX>, are also in line with the proposed mechanism.