Catalytic deuterium exchange reactions with organics. XLII. The picolines, lutidines, thiophen, and furan on unsupported Group VIII transition metals

Abstract

The picolines (methylpyridines), lutidines (dimethylpyridines), thiophen, and furan have been exchanged with heavy water on a variety of Group VIII transition metals. Platinum is the most reactive general labelling catalyst for the picolines and lutidines, whereas cobalt catalyses exchange exclusively in the positions α to the nitrogen atom. The labelling pattern in the picolines and lutidines is similar to that of the alkylbenzenes in that (i)positions ortho to a methyl group are severely deactivated and (ii)positions flanked by two methyl groups are completely deactivated. Compensation effects are observed involving steric hindrance to adsorption from the alkyl groups and enhanced strength of adsorption from the nitrogen lone pair. The compensation effect in this charge-transfer adsorption slightly favours the nitrogen lone pair. Thiophen and furan generally do not exchange appreciably with pre-reduced Group VIII transition metal catalysts. Thiophen exchanges rapidly on self-activated platinum oxide whereas self-activated iridium oxide is the most useful for furan. With nickel chloride, furan exchanges exclusively in the or positions. A theory is proposed to account for the novel exchange in both thiophen and furan.