Solvent-dependent changes in the triazolinedione-alkene ene reaction mechanism

The influence of the solvent on the triazolinedione-alkene ene reaction mechanism has been investigated. Both inter- and intramolecular kinetic isotope effects with tetramethylethylenes and 2,2,2-(trideuterio)methyl-7-methyl-2,6-octadiene-[D3]-1,1,1 provide, for the first time, strong evidence for changes in the mechanism of the reaction on going from non-protic to polar protic solvents. In non-protic polar or apolar solvents, an aziridinium imide that equilibrates to an insignificant extent with an open intermediate (a dipolar or a polarized biradical) is formed irreversibly in the first, rate-determining step of the reaction, which is followed by fast hydrogen abstraction. On the contrary, in polar protic solvents, hydrogen abstraction is rate limiting, allowing the main dipolar intermediate to equilibrate with its open intermediate(s) as well as with the starting reagents.