Cycloadditions in Syntheses. LII. Stereochemical Pathways of 1-Isoquinolone-Chloroethylene Photo [2 + 2] cycloaddition: Determination of Regio- and Stereostructures of the Products and Explanation for Their Formation

The cycloaddition of isoquinolone and its N-methyl derivative to all the chlorinated ethylenes has been studied. The structures of all of the photoadducts were determined on the basis of X-ray crystallographic analysis as well as nuclear magnetic resonance spectroscopy and it was found that the cyclobutane rings in the adducts took puckered conformation. The details of the two-step closure process via biradical intermediates are discussed and it is concluded that σ-bond rotation prior to spin relaxation in the biradical intermediates takes a primary role in determination of the stereochemical outcome. The interesting fact that more trans products are formed from cis-dichloroethylene and more cis products from trans-dichloroethylene can also be explained in terms of the present proposal.