A kinetic investigation of the addition of diazomethane to styrenes. Role of dipolar aprotic solvents in predicting direction of dipole orientation

Abstract

AbstractThe addition of diazomethane to a series of substituted styrenes has been kinetically investigated with emphasis on polar, solvent, and solvation effects. Although the reaction is generally insensitive to solvent polarity, a sizeable substituent effect (ρ = +0.90) has been observed. Also, a sizeable increase in rate was generally observed in protic solvents such as water, and a considerable rate enhancement occurred in the case of nitrostyrene in the dipolar aprotic solvent, dimethylformamide. The results are discussed in terms of a concerted mechanism involving a partially‐charged transition state II, with negative charge formation on the α‐carbon of the styrene.The methoxystyrene offers an exception to the Hammett equation and the unexpectedly high rate constant for this compound could be discussed in terms of a possible participation of the highly nucleophilic resonance form of p‐methoxystyrene and an α‐addition via the transition state III. However, solvation effects in dimethylformamide (failure to obtain the anticipated rate increase) and the formation of a 3‐substituted pyrazoline adduct from the cycloaddition reaction indicate steric control to predominate over electronic factors and β‐addition to prevail unequivocally.It is proposed that solvation effects in protic solvents can be utilized to advantage in cycloaddition reactions leading to 1‐pyrazolines, for which no other routes are available at the present time.