A dynamic density functional study of the stepwise migratory insertion of isocyanides into zirconium-carbon bonds anchored to a calix[4]arene moiety

The stepwise migratory insertion of methyl isocyanide into the zirconium-carbon bonds in [calix[4](OMe)2(O)2-ZrMe2] has been investigated by means of both static and dynamic density functional calculations. Dynamics simulations have shown that methyl isocyanide insertion takes place via the initial formation of an ?1-iminoacyl species that is suddenly converted into the more stable ?2-isomer. The energy profiles for the two pathways branching from the initially formed ?2-iminoacyl, i.e., (i) the insertion of a second isocyanide molecule into the residual alkyl group leading to a bis-?2-iminoacyl and (ii) the insertion of the residual alkyl group into the iminoacyl moiety leading to an ?2-bound imine, have been characterized. Formation of the bis-?2-iminoacyl species was found to be thermodynamically favored at low temperature (?G =7.2 vs 6.4 kcal mol-1, ?E = -38.5 vs -12.2 kcal mol-1). However, the large entropic contribution to the barrier for this intermolecular process kinetically favors the intramolecular imine formation at room temperature (?G =11.5 vs 6.4 kcal mol-1), providing a rationale for the experimentally characterized temperature selectivity of the overall reaction.