Proof of a Reversible, Pairwise Hydrogen Transfer during the Homogeneously Rhodium( I )‐Catalyzed Hydrogenation of α,β‐Unsaturated Carbonic Acid Derivatives with In Situ NMR Spectroscopy and Parahydrogen

Pairwise exchange of geminal protons in unsaturated carbonic acid derivatives of the type H2CC(R)COOR′ during the rhodium(I)-catalyzed hydrogenation [Eq. (a)] has been proven conclusively for the first time by using in situ NMR spectroscopy and parahydrogen (PHIP method). The most important conclusion is that the formation of the hydridoalkyl intermediates must be a reversible reaction.