The mechanism of the prototype cation radical cycloaddition reaction: the cyclodimerization of N-vinylcarbazole

The mechanism of the first cation radical cycloaddition reaction to be disc overed, the cyclodimerization of N-vinylcarbazole, has now been studied by means of a stereochemical criterion. The cyclodimerization of N-(cis-2-deut eriovinyl)carbazole is observed to be non-stereospecific, yielding a cyclod imer in which approximately 80% of the deuterium atoms are cis to the carba zolyl group and 20% are trans to the carbazolyl group. This result stands i n contrast to the stereospecific cyclodimerization of trans- and cis-anetho le and other stereospecific cation radical cyclobutanation reactions which have been studied more recently. The cyclodimerization of N-vinylcarbazole must therefore occur via a stepwise path, involving an intermediate distoni c cation radical, as originally proposed by the Ledwith group. Further, the gauche conformation of the distonic cation radical is implied to be prefer red over the anti form.