Formal transfers of hydride from carbon–hydrogen bonds. Attempted generation of H2 by intramolecular protonolyses of the activated carbon–hydrogen bonds of dihydrobenzimidazoles

Abstract

Protonolyses of carbon–hydrogen bonds can occur under suitable conditions to produce carbocations and H2. In an effort to accelerate these fundamental reactions, we have attempted to make them intramolecular by devising compounds in which carbon–hydrogen bonds designed to be particularly good formal donors of hydride are held in close proximity to acidic sites. Dihydrobenzimidazoles 4 and 11 are compounds of this type, since the carbon–hydrogen bonds at C2 are activated as formal donors of hydride by adjacent lone pairs in a dihydroaromatic ring, and acidic anilinium and carboxylic acid groups are held nearby. Unfortunately, this proximity does not lead to the formation of H2 by protonolysis; instead, other reactions intervene when compounds 4 and 11 are subjected to pyrolysis. Key words: intramolecular protonolysis of carbon–hydrogen bonds, formal donors of hydride, generation of H2, dihydrobenzimidazoles.