A HETERO RETRO-DIELS-ALDER REACTION IN AQUEOUS-SOLUTION - A DRAMATIC WATER-INDUCED INCREASE OF THE EQUILIBRIUM-CONSTANT AND INHIBITION OF CYCLOREVERSION

The adduct of the Diels-Alder reaction of nitrosobenzene with cyclopen tadiene is not stable in solution. The equilibrium constant for the re action depends strongly on the medium and water induces a spectacular shift to the adduct. Comparison with the bimolecular addition of nitro sobenzene to 1,3-cyclohexadiene enables separation of the effect of th e aqueous medium on the rate constants for the forward and reverse rea ction. In water, the former reaction is accelerated and the latter is retarded. The deceleration of the retro reaction in water is due to hy drogen-bond stabilization of the adduct. The influence of cosolvents a nd micelles on the cycloreversion is discussed.