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 benzonitrile oxide (1) with various dipolarophiles (2a-e) were determined in aqueous media and in organic solvents to gain more insight into the influence of an aqueous medium on pericyclic reactions. 1,3-Dipolar cycloadditions with electron-poor dipolarophiles are accelerated in water and protic solvents, whereas an aqueous medium has no special effect when electron-poor dipolarophiles are involved. These observations can be explained using frontier molecular orbital theory. In all cases, enforced hydrophobic interactions promote the reaction. These results are supported by kinetic experiments in water-ethanol mixtures. Sodium dodecyl sulfate micelles accelerate the cycloaddition of 1 with both electron-rich and electron-poor dipolarophiles. More than for other systems, the present results show that hydrogen bonding and hydrophobic interactions can either cooperate or counteract each other in determining the kinetic medium effects in aqueous solutions.