Kinetics of the [2++4]-cycloaddition reactions of 1,3-dithian-2-ylium Ions with 1,3-dienes

The kinetics of the [2(+) + 4] cycloadditions of 1,3-dithian-2-ylium ions (1) with 1,3-dienes was investigated photometrically in dichloromethane. The second-order rate constants determined for the reactions of 1 with 2,3-dimethyl-1,3-butadiene (2a) and isoprene (2b) were identical to those calculated for the first step of a stepwise cycloaddition pathway by the correlation Ig k = s (E + N). Though a concerted cycloaddition pathway is not excluded by this finding, it is obvious that the transition states of these reactions are not noticeably stabilized by the simultaneous formation of two new sigma bonds.