ON THE REGIOSELECTIVITY AND STEREOSELECTIVITY OF PYRIDINYL RADICAL DIMERIZATION

The present work was carried out in order to elucidate the combined ef fects of the electron spin density on the ring carbons and the steric hindrance of the ring substituents upon the regio- and stereoselectivi ty of the dimerization of 3-carbamoyl- and 3-cyanopyridinyl radicals. To this purpose the composition of mixtures of diastereoisomeric dimer s arising from the one-electron electrochemical reduction of several 3 -carbamoyl and 3-cyano substituted pyridinium salts has been studied b y H-1 NMR spectroscopy. In some cases, single diastereoisomers have be en isolated from mixtures by preparative HPLC. The results show that: (a) hindering steric effect of substituents at coupling sites prevails over electron spin density on coupling carbons in governing regiosele ctivity of 3-carbamoylpyridinyl radical dimerization; (b) large bulky N-ring substituents produce a significant shielding effect on the adja cent dimerization site; (c) the relative amounts of diastereoisomers i n the mixtures of 4,4'- and 6,6'-linked dimers indicate that the dimer ization process is largely stereoselective; (6) otherwise, nearly equa l amounts of 4,4'- and 4,6'-linked dimers, and relative diastereoisome rs as well, arise from the reduction of 3-cyano substituted pyridinium salts. This finding indicates that the presence of the carbamoyl subs tituent at the 3 position is a primary factor in inducing the regio- a nd stereoselectivity of pyridinyl radical dimerization.