Journal of Organic Chemistry p. 3042 - 3050 (1987)
Update date:2022-08-11
Topics:
Bernasconi, Claude F.
Panda, Markandeswar
Rate (k1) and equilibrium constants (K1) for piperidine and morpholine addition to benzylidene Meldrum's acid (BMA) and substituted BMA's (Z=4-NO2, 3-Cl, 4-CN, 4-OMe, 4-NMe2, 4-NEt2) were determined in water and in 50percent, 70percent, and 90percent aqueous Me2SO.The equilibrium for addition is highly favorable, with K1 values (piperidine) as high as 7.8*107M-1, which is the highest value measured in a series of olefins of the type PhCH=CXY.The rates are also quite high (k1 up to 2.1*106M-1s-1), indicating a relatively high intrinsic rate constant (k0=k1 for K1=1) which ranks BMA second among seven PhCH=CXY-type olefins with respect to kinetic reactivity.This ranking is "reasonable" based on a correlation between k0 for nucleophilic addition to PhCH=CXY and k0 for deprotonation of carbon acids of the type CH2XY.βnucn (d log k1/ d log K1, variation of amine) is very amall, particularly in aqueous solution.This result appears to be part of a trend toward lower βnucn values with increasing thermodynamic stability of the adducts of PhCH=CXY. αnucn (d log k1/ d log K1, variation of Z) is significantly larger than βnucn, implying a substantial imbalance in these reactions.However, after correction of αnucn for the effect of the developing positive charge on the amine nitrogen the remaining "true" imbalance is quite small.The small imbalance as well as the high k0 value are consistent with the Meldrum's acid anion deriving most of its exceptional stability from its bislactone structure rather than from resonance.Strong ?-donor substituents (4-NMe2, 4-NEt2) have a strong stabilizing effect on the olefin, leading to a large reduction in K1.Contrary to expectations based on the principle of nonperfect synchronization (PNS), this resonance effect does not lead to a strong reduction of the intrinsic rate constant, probably because the polarization in the olefin (Me2N+=C6H4+CHC(COO)2-C(CH3)2) helps in partially offsetting the PNS effect caused by delayed development of resonance on the carbanionic side of the adduct
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