ON THE REGIO- AND STEREOSELECTIVITY OF PYRIDINYL RADICAL DIMERIZATION

The present work was carried out in order to elucidate the combined e ects of the electron spin density on the ring carbons and the steric hindrance of the ring substituents upon the regio- and stereoselectivity of the dimerization of 3-carbamoyl- and 3-cyanopyridinyl radicals. To this purpose the composition of mixtures of diastereoisomeric dimers arising from the one-electron electrochemical reduction of several 3-carbamoyl and 3-cyano substituted pyridinium salts has been studied by 1H NMR spectroscopy. In some cases, single diastereoisomers have been isolated from mixtures by preparative HPLC. The results show that : (a) hindering steric e ect of substituents at coupling sites prevails over electron spin density on coupling carbons in governing regioselectivity of 3-carbamoylpyridinyl radical dimerization ; (b) large bulky N-ring substituents produce a signiÐcant shielding e ect on the adjacent dimerization site ; (c) the relative amounts of diastereoisomers in the mixtures of 4,4'- and 6,6'-linked dimers indicate that the dimerization process is largely stereoselective ; (d) otherwise, nearly equal amounts of 4,4'- and 4,6'-linked dimers, and relative diastereoisomers as well, arise from the reduction of 3-cyano substituted pyridinium salts. This Ðnding indicates that the presence of the carbamoyl substituent at the 3 position is a primary factor in inducing the regio- and stereoselectivity of pyridinyl radical dimerization.