SUBSTITUENT EFFECTS ON AN INVERSE ELECTRON DEMAND HETERO-DIELS-ALDER REACTION IN AQUEOUS-SOLUTION AND ORGANIC-SOLVENTS - CYCLOADDITION OF SUBSTITUTED STYRENES TO DI(2-PYRIDYL)-1,2,4,5-TETRAZINE

The kinetics of the Diels-Alder reactions of di(2-pyridyl)-1,2,4,5-tet razine (1) with substituted styrenes 2 was investigated in aqueous med ia and in organic solvents. The second-order rate constants ofthis rea ction increase dramatically in water-rich media. A decrease in pH acce lerates the aqueous Diels-Alder reaction even more. The Hammett p-valu es and also the electronic demand eta-values of the reaction are solve nt sensitive. In protic solvents the dipolar character of the activate d complex is increased, but simultaneously hydrogen-bond interactions stabilize the activated complex. These effects are most pronounced in 2,2,2-trifluoroethanol, which shows that the aqueous accelerations can not be solely attributed to solvent-induced changes of the reaction me chanism.