Jorge A. Garduño

Homogeneous hydrogenations and hydrofunctionalizations of carbon–oxygen and carbon–carbon bonds with copper catalysts are reviewed herein. The title literature revision covers mostly contributions made over the last decade on hydrogenation of carbonyl compounds, alkenes, alkynes, and CO₂, as well as hydrosilylation, hydroboration, hydroamination, hydroallylation, hydroalkylation, hydrocarboxylation, or hydrocarbonylation of either alkenes, alkynes, allenes, carbonyl compounds, or CO₂. This contribution features well‐defined copper catalytic precursors as well as mixtures of various copper sources, either Cu(I) or Cu(II) salts, and ligands for in situ formation of the catalytic systems. Asymmetric copper catalysis is also highlighted in several kinds of transformations throughout this review.

The use of the nickel(II) complex [(TEEDA)NiCl₂] (1; TEEDA= N,N,N′,N′-tetraethyl-ethylendiamine) and nickel(0) complex [Ni(COD)₂] (5) as pre-catalysts in the additive-free catalytic hydrogenation of benzonitrile (BN) is reported. In the presence of 1 (1 mol%), BN was hydrogenated under relatively mild reaction conditions (100 °C, 120 psi H₂, 72 h) to the corresponding secondary imine, N-benzylidenebenzylamine (BBA), in very good yield (83%). As a counterpart, 5 (1 mol%) selectively hydrogenated BN to benzylamine (BA) in excellent yield (96%) under similar reaction conditions (80 °C, 120 psi H₂, 24 h). In both cases, nickel nanoparticles (Ni-NPs) were identified as the catalytically active species. These Ni-NPs were formed in situ from 1 and 5 without external additives or additional stabilizers. The use of complex 5 was extended to the hydrogenation of different (hetero) aromatic and aliphatic nitriles.

The first example of a homogeneous hydration of aromatic nitriles catalyzed by manganese molecular compounds is reported. The Mn(I) organometallics fac-[(CO)₃Mn(dippe)(Z)]¹⁻ⁿX_1−n (n = 0, 1; Z = Br, OTf, PhCN; X = OTf) were synthesized and characterized, and their reactivity was studied. The species fac-[(CO)₃Mn(dippe)(OTf)] (2) was used as a catalyst precursor for the selective hydration of benchmark benzonitrile (2 mol% 2, THF/H₂O 1 : 2 v/v, 18 h, 100 °C) to produce benzamide in 90% isolated yield. A series of (hetero)aromatic nitriles were hydrated to synthesize the corresponding amides in very good to excellent yields (88–94%). Isotopic labeling studies accounted for a proton transfer as the rate-determining step.

The homogeneous transfer hydrogenation of benzonitrile with 2-propanol or 1,4-butanediol produced N-benzylidene benzylamine (BBA, 85% yield) using 5 mol % [Ni(COD)₂] as a catalytic precursor and a mixture of Cy₂P(CH₂)₂PCy₂ and Cy₂P(CH₂)₂P(O)Cy₂ as ancillary ligands, under mild reaction conditions (120 °C, 96 h, tetrahydrofuran). 1,4-Butanediol performed better than 2-propanol as a hydrogen donor and yielded γ-butyrolactone as the product of transfer dehydrogenation. Selectivity toward dibenzylamine (DBA, 62% yield) was achieved by varying the amount of 1,4-butanediol in the catalytic system. A reaction mechanism was proposed, involving a ligand-assisted O–H bond activation, end-on substrate coordination, and a key dihydrido-Ni(II) intermediate, leading to the in situ formation of primary imines and amines to ultimately yield the secondary imines observed.

The use of nickel compounds in low oxidation states allowed a variety of useful transformations of interest for academia, industry and in the solution of environmental issues.