An experimental-theoretical approach to the kinetics and mechanism of Michael type addition:: α,β-Unsaturated tungsten Fischer carbene complex as the substrate

Through variable-temperature H-1 and C-13 NMR experiments and density functional calculations, the kinetics and mechanism of Michael type addition were investigated using alkynyl carbene A as the substrate. The two conformers of substrate A were distinguished from the H-1 and C-13 NMR spectra, and the calculated results showed that the syn-conformer is more stable than the anti- by 6.5 kJ mol(-1) with the activation barrier between these two conformers as 62.5 kJ mol(-1). The full reaction mechanism of Michael type addition was first presented to us, which differs from the previous solely based on the kinetic studies. It contains three elementary steps (see the scheme): (1) Formation of C-8-N-2 bond via transition state TS1. (2) Conformation conversion from In-1 to In-2, which is very important but ignored before. (3) Intramolecular proton transfer via transition state TS2 forming the product. The first step is rate determining with an activation barrier of 73.0 kJ mol(-1), very close to the experimental value of 89.6 kJ mol(-1). The product P is dominant over P' in population contrary to the situation of tautomer B' over B, which is caused by larger activation barriers to P' and the less stabilities of structures related to B' from the first transition state to the product. [GRAPHICS] (c) 2006 Elsevier B.V. All rights reserved.