Site-Selective Hydrogenation of Electron-Poor Alkenes and Dienes Enabled by a Rh-Catalyzed Hydride Addition/Protonolysis Mechanism

The transition metal catalyzed hydrogenation of alkenes is a well-developed technology used on a lab scale as well as on large scales in the chemical industry. Site- and chemoselective mono-hydrogenations of polarized conjugated dienes remain challenging. Instead, stoichiometric main-group hydrides are used rather than H2. As part of an effort to develop a scalable route to prepare geranylacetone, we discovered that Rh(CO)2acac/xantphos based catalysts enable the selective monohydrogenation of electron-poor 1,3-dienes, enones, and other polyunsaturated substrates. D-labeling and DFT studies support a mechanism where a nucleophilic Rh(I)-hydride selectively adds to electron-poor alkenes and the resulting Rh-enolate undergoes subsequent inner-sphere protonation by alcohol solvent. The finding that (Ln)Rh(H)(CO) type catalysts can enable selective mono-hydrogenation of electron-poor (poly)enes provides a valuable tool in the design of related chemoselective reduction processes of unsaturated substrates.