TRIAZOLINES .4. SOLVATION EFFECTS AND ROLE OF PROTIC-DIPOLAR APROTIC SOLVENTS IN 1,3-CYCLOADDITION REACTIONS

Though kinetic investigations of the bimolecular addition reaction of diazomethane to aromatic Schiff bases reveal that the reaction follows essentially a concerted process and is not generally dependent on solvent polarity, a sizeable increase in rate is generally noticed in the presence of protic solvents such as water and a considerable rate enhancement occurs in the case of certain anils but not others in dipolar aprotic solvents such as DMF. The results are discussed in terms of better solvation of the negatively charged transition state II, relative to the polar reactant molecules in the protic or dipolar aprotic solvent media. Solvation by dipolar aprotic solvents is greatest for the transition state resulting from the reaction of benzal p-nitroaniline with maximum charge dissipation and least for the one corresponding to benzalaniline with minimum charge distribution. Solvation by protic solvents follows a reverse order. A concerted mechanism involving a charge imbalance in the transition state is characteristic of 1,3-cycloaddition reactions in general; consequently, the solvation effects observed here, might well apply to other cycloaddition reactions. Investigation indicates that this is indeed the case. The many isolated reports on the catalytic effect of water, the alcohols and DMF on cycloaddition reactions can now be unified and explained as a direct consequence of solvation effects in these solvents. Solvation effects enhance the utility and versatility of cycloaddition reactions and can be used to advantage in performing many cycloadditions with synthetical applications. A number of examples are presented including the synthesis of 3,4-diphenyl pyrazoline for the first time by utilizing the catalytic effect of water on the cycloaddition of diazomethane to stilbene. © 1969.