Evidence for direct hydride delivery from formic acid in transfer hydrogenation

The regioselectivity of hydrometalation in transfer hydrogenation was investigated by tracing the fate of the deuterium label in both the formyl and carboxyl positions of formic acid during the isomerization of a cis enol ether. The deuterium from the formyl position is incorporated exclusively into the electron-deficient center of the double bond; this indicates that hydride attack occurs during transfer hydrogenation. The fact that the formyl hydrogen is more reactive than the carboxyl hydrogen suggests that the identities of the two hydrogens are not scrambled during transfer hydrogenation. We propose that the delivery of the formyl hydrogen atom is concerted with decarboxylation rather than proceeding through the formation of a metal dihydrido species.