Predicting reactivity and stereoselectivity in the Nazarov reaction: a combined computational and experimental study

The Nazarov reaction of pentadienyl cations generated by protonation of either dienones or alkoxytrienes has been examined in detail both experimentally and by DFT calculations. In particular, calculations at the B3LYP/6-311G** level of theory accurately predicted, and accounted for, the outcome of the Brønsted acid catalyzed electrocyclization of 4pi-electron systems in which one of the double bonds involved in the process was embedded in N- and S-heterocyclic rings. Calculations showed that both heteroatoms are capable of accelerating the ring closure by stabilizing the partial positive charge which develops at C-6 (C-2) in the transition state, with S-heterocyclic derivatives being more reactive than the corresponding N-containing compounds. In general, pentadienyl cations generated by protonation of alkoxytrienes were expected to react faster than those obtained by protonation of the corresponding dienones, as the latter were stabilized by a hydrogen bond. The presence of a substituent on the heterocyclic ring significantly affects the stereoselectivity (torquoselectivity) only in the case of the N-heterocyclic derivatives, in which a 2-alkyl group is axially oriented, providing the cis-2,5-disubstituted isomer only. Instead, with substituted S-heterocyclic compounds, the anticipated torquoselectivity was very low and, in fact, a 3:1 diastereomeric mixture between the trans and cis products was experimentally found after ring closure. For this study, the synthesis of the appropriate N- and S-containing dienones and alkoxytrienes was realized to evaluate the predictivity power of the DFT computations, which was very good in all of the cases examined, both in terms of reactivity and stereoselectivity. The consistency observed between computational and experimental results, therefore, shows the usefulness of DFT calculations at the B3LYP/6-311G** level of theory as a robust instrument for the prediction of reactivity and stereoselectivity in the Nazarov electrocyclic reaction.