INFLUENCE OF STERIC HINDRANCE ON THE REGIO- AND STEROSELECTIVITY OF PYRIDINYL RADICAL DIMERIZATION

One-electron electrochemical reduction of 4-methyl- and 6-methyl-substituted nicotinamide salts 1 and 4, produces free radicals which irreversibly dimerize. A diastereomeric pair of 6,61-linked dimers (2a and 2b) were the main products obtained from salt 1; this represents the first example of a symmetrical dimerization of 3-carbamoyl-substituted pyridinyl radicals implicating the 6 positions. In contrast, salt 4 yielded a diastereomeric pair of 4,4′-linked dimers (5a and 5b) as the main products. Minor amounts of 4,6′-linked dimers were also obtained from both 1 and 4. These results show that, in the present case, steric hindrance is the prevalent factor controlling the regioselectivity of the pyridinyl radical dimerization, but that the electron spin density on the coupling positions is still a relevant element. Furthermore, the studied dimers exhibited anomalous UV spectral properties with respect to both the absorption bands and molar absorbances.