Kinetic analysis of the divergence of reaction pathways in the chiral Lewis base promoted aldol additions of trichlorosilyl enol ethers: A Rapid-injection NMR study

The aldol addition reaction of trichlorosilyl enol ethers and aldehydes with and without chiral Lewis base catalysts has been kinetically analyzed. The uncatalyzed reactions display classic first-order dependence on each component. The reactions catalyzed by bulky chiral phosphoramide 5 were examined by React1R and exhibited first-order dependence on the catalyst. To examine the kinetic behavior of the reaction catalyzed by phosphoramide 4, a Rapid-Injection (RI) NMR apparatus was constructed and employed to allow rapid in-situ mixing and monitoring of the reaction course. The aldol addition catalyzed by 4 displayed second-order dependence on phosphoramide, thus providing direct evidence that two catalyzed pathways (with complimentary stereochemical consequences) exist that depend on phosphoramide structure and concentration. Arrhenius activation parameters for all three reactions showed striking characteristics of negligible enthalpies and extremely high entropies of activation. Comparison of these values was precluded by the existence of complex preequilibria in the catalyzed process.