Formal transfers of hydride from carbon-hydrogen bonds. Generation of H-2 from orthoformamides designed to undergo intramolecular protonolyses of activated carbon-hydrogen bonds

Protonolysis of the central carbon-hydrogen bond of tricyclic orthoformamide 1 occurs readily to liberate H-2 and give the corresponding guanidinium ion 15 under mild conditions. To accelerate this process, we have attempted to make the reaction intramolecular by constructing molecules in which carbon-hydrogen bonds similarly activated as formal donors of hydride are held close to acidic sites. Spectroscopic and structural studies have indicated that orthoformamide 25 contains a central carbon-hydrogen bond activated as a formal donor of hydride by three antiperiplanar lone pairs on nitrogen, as well as an acidic ethylammonium group. As expected, pyrolysis of compound 25 produced the corresponding guanidinium ion 28 in high yield, anti H-2 was liberated and could be trapped in 39% yield. However, analogous bimolecular reactions of butylammonium chloride with simple orthoformamides 1 and 29, which do not contain intramolecular acidic sites, occurred at similar rates. This suggests that protonolyses of such structures may occur by collinear attack on the activated central carbon-hydrogen bond or that the observed liberation of Hz does not involve direct protonolysis.