Theoretical and experimental studies on the structure and isomerization of isocyano and cyano cyclopolyenes

Quantum-chemical calculations in terms of the density functional theory showed that cyclopolyenyl isocyanides RNC are considerably less stable than the corresponding cyanides and that they are capable of undergoing RNC -> RCN isomerization according to both 1,2-shift mechanism (cyclopropenyl and cyclopentadienyl isocyanides; Delta E double dagger = 35.0 and 37.5 kcal/mol, respectively) and previously unknown 2,5-sigmatropic shift mechanism (cycloheptatrienyl isocyanide, Delta E double dagger = 26.4 kcal/mol). Migration of cyano group in the cyclopentadiene and cycloheptatriene systems follows the 1,5-sigmatropic shift pattern. The activation barrier to 1,5-shift of cyano group around the cycloheptatriene ring was estimated by dynamic NMR in deuterated nitrobenzene (Delta G double dagger(190C) = 26.5 kcal/mol).