1,3-dipolar cycloadditions of benzonitrile oxide with various dipolarophiles in aqueous solutions. A kinetic study

The second-order rate constants for the 1,3-dipolar cycloaddition of benzon itrile oxide (1) with various dipolarophiles (2a-e) were determined in aque ous media and in organic solvents to gain more insight into the influence o f an aqueous medium on pericyclic reactions. 1,3-Dipolar cycloadditions wit h electron-poor dipolarophiles are accelerated in water and protic solvents , whereas an aqueous medium has no special effect when electron-poor dipola rophiles are involved. These observations can be explained using frontier m olecular orbital theory. In all cases, enforced hydrophobic interactions pr omote the reaction. These results are supported by kinetic experiments in w ater-ethanol mixtures. Sodium dodecyl sulfate micelles accelerate the cyclo addition of 1 with both electron-rich and electron-poor dipolarophiles. Mor e than for other systems, the present results show that hydrogen bonding an d hydrophobic interactions can either cooperate or counteract each other in determining the kinetic medium effects in aqueous solutions.