Rhodium-Catalyzed Selective Partial Hydrogenation of Alkynes

The cationic rhodium complex [Rh­(P^<i>c</i>Pr₃)₂(η⁶-PhF)]⁺­[B­{3,5-(CF₃)₂C₆H₃}₄]⁻ (P^<i>c</i>Pr₃ = triscyclopropylphosphine, PhF = fluorobenzene) was used as a catalyst for the hydrogenation of the charge-tagged alkyne [Ph₃P­(CH₂)₄C₂H]⁺[PF₆]⁻. Pressurized sample infusion electrospray ionization mass spectrometry (PSI-ESI-MS) was used to monitor reaction progress. Experiments revealed that the reaction is first order in catalyst and first order in hydrogen, so under conditions of excess hydrogen the reaction is pseudo-zero order. Alkyne hydrogenation was 40 times faster than alkene hydrogenation. The turnover-limiting step is proposed to be oxidative addition of hydrogen to the alkyne (or alkene)-bound complex. Addition of triethylamine caused a dramatic reduction in rate, suggesting a deprotonation pathway was not operative

Simultaneous Orthogonal Methods for the Real-Time Analysis of Catalytic Reactions

Continuous monitoring of catalyzed reactions using infrared spectroscopy (IR) measures the transformation of reactant into product, whereas mass spectrometry delineates the dynamics of the catalytically relevant species present at much lower concentrations, a holistic approach that provides mechanistic insight into the reaction components whose abundance spans 5 orders of magnitude. Probing reactions to this depth reveals entities that include precatalysts, resting states, intermediates, and also catalyst impurities and decomposition products. Simple temporal profiles that arise from this analysis aid discrimination between the different types of species, and a hydroacylation reaction catalyzed by a cationic rhodium complex is studied in detail to provide a test case for the utility of this methodology