KINETICS OF THE [2(-CYCLOADDITION REACTIONS OF 1,3-DITHIAN-2-YLIUM IONS WITH 1,3-DIENES()+4])

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