Site‐Selective Hydrogenation of Electron‐Poor Alkenes and Dienes Enabled by a Rhodium‐Catalyzed Hydride Addition/Protonolysis Mechanism**

Abstract The transition metal catalyzed hydrogenation of alkenes is a well‐developed technology used on 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 H 2 . As part of an effort to develop a scalable route to prepare geranylacetone, we discovered that Rh(CO) 2 acac/xantphos based catalysts enable the selective mono‐hydrogenation 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 (L n )Rh(H)(CO) type catalysts can enable selective mono‐hydrogenation of electron‐poor 1,3‐dienes provides a valuable tool in the design of related chemoselective hydrogenation processes.