Isomerization of indole. Quantum chemical calculations and kinetic modeling

Density functional theory calculations were carried out to investigate the pathways of the unimolecular isomerizations of indole. Equilibrium and transition-state structures were optimized by the Lee--Yang--Parr correlation functional approximation (B3LYP) using the Dunning correlation-consistent polarized double-xi basis set. Energy values were calculated at the CCSD(T) level of theory. Rate parameters for all the steps on the surfaces of the indole isomerizations were evaluated using B3LYP frequencies and CCSD(T)//B3LYP energy values. 3H-Indole, which is a stable tautomer of indole, is a precursor in the formation of o-tolyl isocyanide that by -NC --> -CN flip forms o-tolunitrile. The latter is one of the isomers of indole. A second isomer, phenylacetonitrile, is obtained directly from indole. A kinetics scheme containing all the elementary steps on the surfaces was constructed, apparent rate constants for product formation were calculated, and comparison with experimentally available formation rates was made. The agreement for phenylacetonitrile is excellent. For o-tolunitrile the calculations somewhat underestimate the rate. Despite many efforts, a reaction path for the formation of m-tolunitrile based on indole as a starting point could not be calculated with activation energies compatible with the experimental findings. On the other hand, a reaction path for the isomerization starting from indole radical, with low-lying transition states and intermediates, was calculated. It is therefore suggested that indole radicals that are formed by various abstraction reactions are involved in the isomerization of indole to m-tolunitrile.