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 discovered, the cyclodimerization of N-vinylcarbazole, has now been studied by means of a stereochemical criterion. The cyclodimerization of N-(cis-2-deuteriovinyl)carbazole is observed to be non-stereospecific, yielding a cyclodimer in which approximately 80% of the deuterium atoms are cis to the carbazolyl group and 20% are trans to the carbazolyl group. This result stands in contrast to the stereospecific cyclodimerization of trans- and cis-anethole 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 distonic cation radical, as originally proposed by the Ledwith group. Further, the gauche conformation of the distonic cation radical is implied to be preferred over the anti form.