H addition to C=O bonds is ubiquitous in carbonyl reduction reactions. Here, we probe with kinetic and isotopic methods and describe the H addition mechanism, specifically, (1) hydride transfer during vapor phase transfer hydrogenation of linear aldehydes catalyzed by solid Brønsted acid catalysts (H-MFI, H-FAU, and polyoxometalate clusters) and (2) hydrogen adatom addition during vapor phase hydrogenation of aldehydes and ketones catalyzed by Group VIII metal clusters (Ni, Co, Fe, Ru, Pt, Pd). We will also compare and contrast these hydride and hydrogen transfer mechanisms to the proton transfer mechanism, which occurs during condensed phase hydrogenation of these reactants in the presence of polar-protic solvents on transition metal clusters. Transfer hydrogenation of linear aldehydes occurs via a kinetically-relevant hydride transfer step, during which a hydride donor donates a hydride ion to a protonated aldehyde; subsequent kinetically-irrelevant dehydration leads to an alkene. A larger and more negative hydride ion affinity difference between the carbenium ions of the hydride donor and the hydride acceptor pair leads to a commensurately higher hydride transfer rate. In a separate case, the hydrogenation of these aldehydes (and ketones) at metal cluster surfaces proceeds via a different route through successive insertion of chemisorbed H atoms, during which the second H addition step remains as the kinetically-relevant step. In contrast, the hydrogenation reaction occurs via a separate and more effective route in polar-protic solvents via an initial transfer of protons assisted by the solvent, which couples together with the transfer of electrons through the metal bulk. In this pathway, the second hydrogen addition becomes the kinetically-relevant step. These distinct pathways that involve the different reactive hydrogen species lead to their different dependences on the chemical properties (e.g., hydride ion affinities, proton affinities) of the reactants and the thermochemical properties of the catalysts.    <br/>

Mechanistic requirements of hydrogen addition to carbonyl compounds

253rd National Meeting (2017)

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