Our studies of cation radical pericyclic chemistry are continuing and are focussing upon the substrate ionization step, in which an aminium salt cation radical accepts an electron from a substrate molecule. Studies involve the Diels-Alder reaction, cyclobutanation, and cyclopropanation.
The stereochemistry of cation radical cyclobutanation and Diels-Alder reactions is also of major interest. We have recently established via deuterium labelling studies that the propotype cation radical cyclobutanation reaction is stepwise as opposed to concerted. The stereochemistry of several other cyclobutanation and Diels-Alder reactions has also been studied.
We have recently found that the use of a two phase methylene chloride/water solvent system for aminium salt catalyzed cycloadditions if effective in minimizing acid catalyzed competing reactions. As a consequence of this discovery we have been able to establish much better yields in a variety of cation radical cycloadditions. As a case in point, N-(trans-1-propenyl)carbazole has been found to add to a variety of dienes in 80-96% yields.
The Diels-Alder adducts of vinyl and propenylcarbazole with cyclopentadiene have been found to be suitable monomers for ROMP polymerization, thus affording another novel type of carbazole-containing polymer.
Anion Radical Cycloadditions
In collaboration with the Mike Krische group, we have found that anion radicals of tethered (bis)enones undergo intramolecular cyclobutanation reactions. The anion radicals have been generated by both cathodic reduction (i.e., electrochemically) and by electron transfer from persistent aromatic anion radicals, such as sodium naphthalene. Novel examples of anion radical oxa Diels-Alder cycloadditions have also been established. The electrochemically initiated reactions are substantially electrocatalytic. Intermolecular anion radical cyclobutanations and cross cyclobutanations have also been realized and are currently under active investigation.