Solvent effect as the result of frontier molecular-orbital interaction. Part 3. Hetero Diels–Alder reaction with inverse electron demand between 4-arylidenepyrazol-5-ones and isobutyl vinyl ether

Abstract

The rate of the hetero Diels–Alder reaction between 4-arylidenepyrazol-5-ones (1a,b) and isobutyl vinyl ether (2) has been determined in different solvents and a correlation between kinetic data and the solvent acceptor number has been found. This is interpreted in terms of FMO interactions between the solvent and the heterodiene which determine the LUMO energy of the solvated pyrazolones. Primary and secondary aliphatic alcohols, the high acceptor numbers of which result from their acidity due to hydrogen bonding, are also involved in significant co-ordination to the vinyl ether. This results in a negative contribution to the rate. The stereoselectivity of the reaction between (1a) and (2) is found to be a function of the solvent and these data, and of those taken from the literature for the reaction of cyclopentadiene and methyl acrylate derivatives. This in turn suggests that in Diels–Alder reactions the solvent behaves as an electrophile which co-ordinates to either diene or dienophile, or both, depending on their MO parameters.