Dynamic Solvent Effects on Thermal Cyclization Reaction of Chromenes

Abstract

3,3-Disubstituted 3H-naphtho[2,1-b]pyrans generate thermodynamically unstable open chain species under UV irradiation and they thermally go back to the original pyrans. In order to get information on the relationship between microscopic friction and steric bulk of substituents, viscosity dependence of this thermal cyclization rate was studied for 3,3-dimethyl- and 3,3-diphenyl-3H-naphtho[2,1-b]pyran in highly viscous solvents such as 2-methylpentane-2,4-diol (MPD) at various pressrues. The results clearly demonstrated that the friction felt by the reactant in the activation step was larger in a sterically bulkier diphenyl derivative. This observation suggests that dynamic solvent effects may be used to identify a moving molecular moiety in a solvation shell.