1,2-Carbon to nitrogen migrations. Part 2. Ab initio study on the rearrangement of (α-methylazo)alkyl isocyanates
Ab initio molecular orbital calculations have been employed to study the rearrangement of (α-methylazo)alkyl isocyanates to 1,2,5-trisubstituted 1,2-dihydro-1,2,4-triazol-3-ones. The migration of three different substituents (methyl, ethyl and isopropyl) has been investigated. Geometries of stationary points on the potential energy hypersurface were optimized at the HF/6-31G* level of theory. Second order Moeller Plesset perturbation theory with the 6-31G* basis set was applied in order to correct for correlation effects. Selected geometries were reoptimized at the MP2/6-31G* level. HF/6-31G*, MP2/6-31G*//HF/6-31G* and MP2/6-31G* energies predict migratory aptitudes in the order isopropyl > ethyl > methyl. In the transition states, partial charges obtained from natural population analysis indicate strong electron deficiency at the nitrogen atom being the target for migration. The transition states show partial carbocation character of the migrating group with respect to charge distribution and geometry. An alternative reaction pathway, namely 1,2-shift of a methyl group to the adjacent nitrogen of the isocyanato function leading to formation of 2,4,5-trimethyl-2,4-dihydro-1,2,4-triazol-3-one, has been investigated, but can be excluded due to the much higher activation energy required.