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H/D exchange between arenium ions bearing various substituents (Y = Et, Me2CH, CF3, CF3CO, NO2, Me3Si) on a perdeuterated ring and bases having at least one H atom at the active site are described. A radiolytic technique operating at atmospheric pressure and allowing the recovery of neutral end products was used. The exchange depends on features of both the base (structure, proton affinity) and the arenium ion. The elementary step for back proton transfer from the protonated base to the neutral arene has been studied by NMR analysis of the H distribution within YC6D4H. The protonation was found to involve the most basic ring positions:  ortho/para for alkyl substituents (Y = Et, Me2CH) and ortho/meta for electron-withdrawing substituents (Y = CF3, CF3CO, NO2).

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.

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