[2(+)+4] cycloadditions of iminium tons - 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 follo wed by dilatometry and H-1-NMR spectroscopy obey second-order rate laws. Si nce the observed rate constants are only 30-300 times larger than those cal culated for the stepwise process by the linear free-enthalpy relationship I g k = s (E + N), it is concluded that these cycloadditions proceed in a ste pwise manner or through pericyclic transition states that are not significa ntly stabilized by the concerted formation of two new a-bonds. In contrast, the reaction of the iminium ion 1 with cyclopentadiene (2g) yields 2,2-dim ethyl-2-azoniabicyclo[2.2.1]hept-5-ene (3g) 2X10(4) times more rapidly than predicted for the stepwise cycloaddition process, indicating a free enthal py of concert of 27 kJ mol(-1).