A DFT investigation of the retro-ene reactions of β-hydroxyacetylenes: concerted or stepwise mechanism

The retro-ene reactions of β-hydroxyacetylene and its 10 derivatives have been investigated theoretically at the B3LYP/6-31+G(d) level. The reactions are found to proceed through asynchronous concerted transition states. The thermodynamic data reveal that the reactions are exergonic with increase in the standard entropy. However, due to high values of the activation Gibbs free energy (∆G # ) and negative values of the activation entropy (∆S # ), reactions occur at elevated temperatures. The substituent groups in general do not have pronounced effect on the activation enthalpies, except the phenyl group in which case, it is lowered. The concerted versus stepwise mechanism has been investigated by complete active space-self consistent field (CASSCF) calculations also.