SOLVENT AND SUBSTITUENT EFFECTS IN THE PERISELECTIVITY OF THE STAUDINGER REACTION BETWEEN KETENES AND ALPHA,BETA-UNSATURATED IMINES - A THEORETICAL AND EXPERIMENTAL-STUDY

The outcome of the cycloaddition between activated ketenes and alpha,b eta-unsaturated imines has been investigated both experimentally and t heoretically. Our results indicate that activated monosubstituted kete nes yield exclusively [2 + 2]cycloadducts. Disubstituted activated ket enes yield [2 + 2] and/or [4 + 2] cycloadducts. In one case, an unexpe cted piperidin-2-one has been obtained, although its relative abundanc e with respect to the corresponding [2 + 2] or [4 + 2] cycloadducts ca n be minimized by the proper choice of experimental conditions. The ab ility of different ab initio and semiempirical methods to account for these results has been tested. The best agreement between theory and e xperiment is achieved at the MP2/6-31G level of theory, with solvent effects taken into account. The semiempirical hamiltonian AM1, at the RHF level, tends to overestimate the stability of the transition struc tures leading to six-membered cycloadducts, whereas 3 x 3CI-HE/AM1 and CASSCF(2,2)/6-31G methods tend to overestimate the stability and the biradical character of the transition structures leading to [2 + 2] c ycloadducts.