Highly Selective Hydrogenation of C═C Bonds Catalyzed by a Rhodium Hydride

Abstract

Under mild conditions (room temperature, 80 psi of H2) Cp*Rh­(2-(2-pyridyl)­phenyl)H catalyzes the selective hydrogenation of the CC bond in α,β-unsaturated carbonyl compounds, including natural product precursors with bulky substituents in the β position and substrates possessing an array of additional functional groups. It also catalyzes the hydrogenation of many isolated double bonds. Mechanistic studies reveal that no radical intermediates are involved, and the catalyst appears to be homogeneous, thereby affording important complementarity to existing protocols for similar hydrogenation processes.

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

“…This change in mechanism explains the lower observed selectivity and tendency for over-hydrogenation when using non-alcohol solvents in the hydrogenation of 1 and related substrates. Norton and co-workers recently suggested a similar mechanism for hydrogenations occurring in MeOH using Cp*Rh(PhPy)H. [4] Here, labeling studies showed only partial D-incorporation when using D2 or d4-MeOD (~80%), which indicates multiple mechanistic pathways for reduction and could explain the relatively poor performance in the hydrogenation of 1. At this stage Rh-B can either oxidatively add H2 to form the Rh(III)-dihydrido complex Rh-C with a calculated reaction energy of DE = -24.9 kJ/mol, or it can coordinate methanol to form complex Rh-D with DE = -30.9 kJ/mol.…”

“…At lower catalyst loadings (0.02 mol% [Rh]), xantphos was far superior, giving 94% yield at full conversion vs 21% yield at 58% conversion with P(OPh)3 (Fig 2C , inset). Cp*Rh(PhPy)H has been recently reported to be an effective catalyst for the hydrogenation of enones; [4] however its use for the reduction of pseudoionone led to poor conversion with multiple products generated (34% conv., 17% yield). The optimized conditions allow for high yield because undesirable hydrogenation of the remaining alkenes in geranylacetone (2) is avoided.…”

“…[1] Catalytic hydrogenations using H2, however, are preferred for large-scale industrial production processes. [2] Known homogeneous systems that can promote the chemoselective (and enantioselective) alkene hydrogenation of conjugated enones include Pd-, Ru-, or Rh-bisphosphine based systems, [3] Cp*Rh(ppy)H, [4] and Irbased catalysts of the Crabtree/Pfaltz-type with P,N-ligands. [5] The site-selective mono-hydrogenation of polarized conjugated dienes is more challenging as compared to alkenes or simple enones and remains largely unaddressed for acyclic substrates.…”

Methanol as a hydrogen source: room-temperature highly-selective transfer hydrogenation of α,β-unsaturated ketones

“…Transition-metal catalyzed reduction using H 2 gas as a reductant is simpler and more user-friendly. 19–25 However, the need for a specialized reactor to handle hazardous pressurized hydrogen gas is unavoidable Scheme 1.…”

“…Transition-metal catalyzed reduction using H 2 gas as a reductant is simpler and more user-friendly. [19][20][21][22][23][24][25] However, the need for a specialized reactor to handle hazardous pressurized hydrogen gas is unavoidable Scheme 1. Alternatively, transition-metal catalyzed transfer hydrogenation in the presence of cheaper and greener hydrogen donors has proven to be the most successful and safe strategy over traditional dissolving metal reductions or direct hydrogenation using molecular hydrogen under high pressure.…”