Catalytic Acceptorless Dehydrogenation (CAD) of Secondary Benzylic Alcohols into Value-Added Ketones Using Pd(II)-NHC Complexes

For the creation of adaptable carbonyl compounds in organic synthesis, the oxidation of alcohols is a crucial step. As a sustainable alternative to the harmful traditional oxidation processes, transition-metal catalysts have recently attracted a lot of interest in acceptorless dehydrogenation reactions of alcohols. Here, using well-defined, air-stable palladium(II)-NHC catalysts (A-F), we demonstrate an effective method for the catalytic acceptorless dehydrogenation (CAD) reaction of secondary benzylic alcohols to produce the corresponding ketones and molecular hydrogen (H2). Catalytic acceptorless dehydrogenation (CAD) has been successfully used to convert a variety of alcohols, including electron-rich/electron-poor aromatic secondary alcohols, heteroaromatic secondary alcohols, and aliphatic cyclic alcohols, into their corresponding value-added ketones while only releasing molecular hydrogen as a byproduct.

Activation of H-H, HO-H, C(sp2)-H, C(sp3)-H, and RO-H Bonds by Transition-Metal Frustrated Lewis Pairs Based on M/N (M = Rh, Ir) Couples

Reaction of the dimers [(Cp*MCl)₂(μ-Cl)₂] (Cp* = η⁵-C₅Me₅) with Ph₂PCH₂CH₂NC(NH(p-Tolyl))₂ (H₂L) in the presence of NaSbF₆ affords the chlorido complexes [Cp*MCl(κ²N,P-H₂L)][SbF₆] (M = Rh, 1; Ir, 2). Upon treatment with aqueous NaOH, solutions of 1 and 2 yield the corresponding complexes [Cp*M(κ³N,N′,P-HL)][SbF₆] (M = Rh, 3; Ir, 4) in which the ligand HL presents a fac κ³N,N′,P coordination mode. Treatment of THF solutions of complexes 3 and 4 with hydrogen gas, at room temperature, results in the formation of the metal hydrido-complexes [Cp*MH(κ²N,P-H₂L)][SbF₆] (M = Rh, 5; Ir, 6) in which the N(p-Tolyl) group has been protonated. Complexes 3 and 4 react with deuterated water in a reversible fashion resulting in the gradual deuteration of the Cp* group. Heating at 383 K THF/H₂O solutions of the complexes 3 and 4 affords the orthometalated complexes [Cp*M(κ³C,N,P-H₂L_-H)][SbF₆] [M = Rh, 7; Ir, 8, H₂L_-H = Ph₂PCH₂CH₂NC(NH(p-Tolyl))(NH(4-C₆H₃Me))], respectively. At 333 K, complexes 3 and 4 react in THF with methanol, primary alcohols, or 2-propanol giving the metal-hydrido complexes 5 and 6, respectively. The reaction involves the acceptorless dehydrogenation of the alcohols at a relatively low temperature, without the assistance of an external base. The new complexes have been characterized by the usual analytical and spectroscopic methods including the X-ray diffraction determination of the crystal structures of complexes 1–5, 7, and 8. Notably, the chlorido complexes 1 and 2 crystallize both as enantiopure conglomerates and as racemates. Reaction mechanisms are proposed based on stoichiometric reactions, nuclear magnetic resonance studies, and X-ray crystallography as well as density functional theory calculations.

In solution, masked transition-metal frustrated Lewis pairs (TMFLPs) give rise to the corresponding TMFLP species which activate dihydrogen, water, and alcohols following FLP reaction pathways. When D₂O or alcohols with deuterated OH groups were employed, H/D exchange at the Cp* ligand (involving C(sp³)−H activation) was observed. C(sp²)−H bond activation involving orthometalation of the p-Tolyl ring was also observed.

The catalytic dehydrogenation of ethanol by heterogeneous catalysts

In this review, recent advances in the catalytic dehydrogenation of ethanol to acetaldehytde with the release of hydrogen catalyzed by a heterogeneous catalyst aresummerized and discussed.

Reactivity of the parent amido complexes of iridium with olefins: C-NH2 bond formation versus C-H activation

Herein we report on the different chemical reactivity displayed by two mononuclear terminal amido compounds depending on the nature of the coordinated diene. Hence, treatment of amido-bridged iridium complexes [{Ir(μ-NH₂)(tfbb)}₃] (1; tfbb = tetrafluorobenzobarrelene) with dppp (dppp = bis(diphenylphosphane)propane) leads to the rupture of the amido bridges forming the mononuclear terminal amido compound [Ir(NH₂)(dppp)(tfbb)] (3) in the first stage. On changing the reaction conditions, the formation of a C-NH₂ bond between the amido moiety and the coordinated diene is observed and a new dinuclear complex [{Ir(1,2-η²-4-κ-C₁₂H₈F₄N)(dppp)}₂(μ-dppp)] (4) has been isolated. On the contrary, the diiridium amido-bridged complex [{Ir(μ-NH₂)(cod)}₂] (2; cod = 1,5-cyclooctadiene) in the presence of dppb (dppb = bis(diphenylphosphane)butane) allows the isolation of a mononuclear complex [Ir(1,2,3-η³-6-κ-C₈H₁₀)H(dppb)] (5), as a consequence of the extrusion of ammonia. The monitoring of the reaction of 2 with dppb (and dppp) allowed us to detect terminal amido complexes [Ir(NH₂)(P-P)(cod)] (P-P = dppb (6), dppp (7)) in solution, as confirmed by an X-ray analysis of 7. Complex 7 was observed to evolve into hydrido species 5 at room temperature. DFT studies showed that C-H bond activation occurs through the deprotonation of one methylene fragment of the cod ligand by the highly basic terminal amido moiety instead of C-H oxidative addition to the Ir(i) center.

Acceptorless dehydrogenation and dehydrogenative coupling of alcohols catalysed by protic NHC ruthenium complexes

A new family of protic NHC Ru complexes can serve as efficient catalysts for acceptorless dehydrogenative transformations of alcohols.

NH3-promoted ligand lability in eleven-vertex rhodathiaboranes

The reaction of the 11-vertex rhodathiaborane, [8,8-(PPh3)2-nido-8,7-RhSB9H10] (1), with NH3 affords inmediately the adduct, [8,8,8-(NH3)(PPh3)2-nido-8,7-RhSB9H10] (4). The NH3-Rh interaction induces the labilization of the PPh3 ligands leading to the dissociation product, [8,8-(NH3)(PPh3)-nido-8,7-RhSB9H10] (5), which can then react with another molecule of NH3 to give [8,8,8-(NH3)2(PPh3)-nido-8,7-RhSB9H10] (6). These clusters have been characterized in situ by multielement NMR spectroscopy at different temeperatures. The variable temperature behavior of the system demonstrates that the intermediates 4-6 are in equilibrium, involving ligand exchange processes. On the basis of low intensity signals present in the (1)H NMR spectra of the reaction mixture, some species are tentatively proposed to be the bis- and tris-NH3 ligated clusters, [8,8-(NH3)2-nido-8,7-RhSB9H10] (7) and [8,8,8-(NH3)3-nido-8,7-RhSB9H10] (8). After evaporation of the solvent and the excess of NH3, the system containing species 4-8 regenerates the starting reactant, 1, thus closing a stoichiometric cycle of ammonia addition and loss. After 40 h at room temperature, the reaction of 1 with NH3 gives the hydridorhodathiaborane, [8,8,8-(H)(PPh3)2-nido-8,7-RhSB9H9] (2), as a single product. The reported rhodathiaboranes show reversible H3N-promoted ligand lability, which implies weak Rh-N interactions, leading to a rare case of metal complexes that circumvent "classical" Werner chemistry.