[2++4] Cycloadditions of Iminium Ions − Concerted or Stepwise Mechanism of Aza Diels−Alder Reactions?

The N,N-dimethylmethyleneammonium ion 1 reacts regio- and stereoselectively with the 1,3-dienes 2a−2f to yield the 1,2,5,6-tetrahydropyridinium ions 3. The kinetics of these hetero Diels−Alder reactions, which have been followed by dilatometry and 1H-NMR spectroscopy, obey second-order rate laws. Since the observed rate constants are only 30−300 times larger than those calculated for the stepwise process by the linear free-enthalpy relationship lg k = s (E + N), it is concluded that these cycloadditions proceed in a stepwise manner or through pericyclic transition states that are not significantly stabilized by the concerted formation of two new σ-bonds. In contrast, the reaction of the iminium ion 1 with cyclopentadiene (2g) yields 2,2-dimethyl-2-azoniabicyclo[2.2.1]hept-5-ene (3g) 2×104 times more rapidly than predicted for the stepwise cycloaddition process, indicating a free enthalpy of concert of 27 kJ mol−1.