Real-Time Detection of Intermediates in Rhodium-Catalyzed Hydrogenation of Alkynes and Alkenes by Dissolution DNP

The hydrogenation of alkynes and alkenes using Shrock-Osborn catalysts was followed in situ with dissolution dynamic nuclear polarization (dDNP) NMR. Natural abundance and C-13-labeled dimethyl acetylenedicarboxylate was hyperpolarized prior to hydrogenation using (1,4-bis{diphenylphosphino}butane)-(2,5-norbornadiene) rhodium (I) perchlorate, [Rh(NBD)(DPPB)]-ClO4. The increased signal-to-noise ratio of dDNP compared to conventional C-13 NMR allowed real-time detection of substrates and products as well as the modeling of the hydrogenation kinetics. The build-up of an intermediate was observed during interruption in hydrogen flow, substantiating the current view of the reaction mechanism. Selective inversion of the carbonyl NMR signal of the substrate was applied to demonstrate unequivocally that the new peak appearing in the spectrum originates from a reaction intermediate. The scope of the dDNP method for following reaction dynamics in real time was further demonstrated by substrate competition experiments.