Kinetic and synthetic influences of water and solvent-free conditions on 1,3-dipolar cycloaddition reactions: the phthalazinium and pyridazinium dicyanomethanide 1,3-dipoles: surprisingly successful synthetic methods

The influence of water on the kinetic and synthetic 1,3-dipolar cycloaddition reactions of phthalazinium2-dicyanomethanide 1, and pyridazinium dicyanomethanide 2, with a wide range of dipolarophiles is reported. Water enhanced the rates of all reactions. The dipolarophiles were classified into two groups, water-normal and water-super. The former displayed rate enhancements of <20 times and the latter gave rate enhancements of >45 times, but more often some hundred times, on changing the solvent from acetonitrile to water. A ketone C=O conjugated to an alkene or alkyne constitutes a water-super dipolarophile. Esters, ethers, sulfones, nitriles and aryl rings conjugated to an alkene are water-normal dipolarophiles. The causes of these water effects are explored experimentally and with high level DFT. Hydrophobic effects and special hydrogen bonding interactions are the main factors involved. Synthetic implications are examined. Despite insolubility of the reactants in water successful high-yield reactions were achieved in water and under solvent-free conditions.