A Highly Active Manganese Catalyst for Enantioselective Ketone and Ester Hydrogenation

Preparation and Study of Reusable Polymerized Catalysts for Ester Hydrogenation

A cross-linked catalyst organic framework was prepared by an alternating ring-opening olefin metathesis polymerization between dichloro{N,N'-bis({(2-diphenylphosphino)phenyl}methylidene)bicyclo[2.2.1]-hept-5-ene-2,3-diamine}ruthenium, 1,2-N-di(cis-5-norbornene-2,3-endo-dicarboximido)-ethane, and cis-cyclooctene catalyzed by RuCl2(=CHPh)(PCy3)2 in the presence of a BaSO4 support. The heterogenized catalyst hydrogenated methyl benzoate at a similar rate to the homogeneous catalyst (0.0025 mol % catalyst, 10 mol % KO t Bu, 80 °C, 50 atm, tetrahydrofuran, 21 h, ∼15 000 turnovers during the first 1 h). The catalyst was used five times for a total of 121 680 turnovers. A study on the reusability of this catalyst showed that ester hydrogenations with bifunctional catalysts slow as the reaction proceeds. This inhibition is removed by isolating and reusing the catalyst, suggesting that future catalyst design should emphasize avoiding product inhibition.

Synthesis of Quinolines Through Acceptorless Dehydrogenative Coupling Catalyzed by Rhenium PN(H)P Complexes

A practical and sustainable synthesis of substituted quinolines was achieved through the annulation of 2-aminobenzyl alcohol with various secondary alcohols, ketones, aldehydes, or nitriles, under hydrogen-borrowing conditions. Under the catalysis of well-defined rhenium complexes bearing tridentate diphosphinoamino ligands, the reaction proceeded efficiently (31 examples were isolated with yields up to 96 %) affording a variety of quinoline derivatives.

Selective Hydroboration of Carboxylic Acids with a Homogeneous Manganese Catalyst

Catalytic reduction of carboxylic acid to the corresponding alcohol is a challenging task of great importance for the production of a variety of value-added chemicals. Herein, a manganese-catalyzed chemoselective hydroboration of carboxylic acids has been developed with a high turnover number (>99 000) and turnover frequency (>2000 h^-1) at 25 °C. This method displayed tolerance of electronically and sterically differentiated substrates with high chemoselectivity. Importantly, aliphatic long-chain fatty acids, including biomass-derived compounds, can efficiently be reduced. Mechanistic studies revealed that the reaction occurs through the formation of active manganese-hydride species via an insertion and bond metathesis type mechanism.

Transfer-dehydrogenation of secondary alcohols catalyzed by manganese NNN-pincer complexes

Novel catalytic systems based on pentacarbonylmanganese bromide and stable NNN-pincer ligands are presented for the transfer-dehydrogenation of secondary alcohols to give the corresponding ketones in good to excellent isolated yields.

Hydrosilylation of carbonyl and carboxyl groups catalysed by Mn(i) complexes bearing triazole ligands

Manganese(i) complexes bearing readily accessible triazole ligands are effective catalysts for the hydrosilylation of carbonyl and carboxyl compounds.

Towards practical earth abundant reduction catalysis: design of improved catalysts for manganese catalysed hydrogenation

Rational design using kinetic studies has led to a 3-fold-increase in the reaction-rates compared to an already-promising lead catalyst for the reduction of ketones and esters.

Facile and reversible double dearomatization of pyridines in non-phosphine MnI complexes with N,S-donor pyridinophane ligand

Single and double dearomatization of pyridine rings was observed in Mn^I complexes with an N2S2 pyridinophane ligand via deprotonation of one or two CH₂ arms, respectively.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

Hydrogenation of Nitriles and Ketones Catalyzed by an Air-Stable Bisphosphine Mn(I) Complex

Efficient hydrogenations of nitriles and ketones with molecular hydrogen catalyzed by a well-defined bench-stable bisphosphine Mn(I) complex are described. These reactions are environmentally benign and atomically economic, implementing an inexpensive, earth-abundant nonprecious metal catalyst. A range of aromatic and aliphatic nitriles and ketones were efficiently converted into primary amines and alcohols, respectively, in good to excellent yields. The hydrogenation of nitriles proceeds at 100 °C with catalyst loading of 2 mol % and 20 mol % base ( t-BuOK), while the hydrogenation of ketones takes place already at 50 °C, with a catalyst loading of 1 mol % and 5 mol % of base. In both cases, a hydrogen pressure of 50 bar was applied.

Manganese-Catalyzed α-Alkylation of Ketones, Esters, and Amides Using Alcohols

Herein we report the manganese-catalyzed C–C bond-forming reactions via α-alkylation of ketones, amides, and esters, using primary alcohols. β-Alkylation of secondary alcohols by primary alcohols to obtain α-alkylated ketones is also reported. The reactions are catalyzed by a (ⁱPr-PNP)Mn(H)(CO)₂ pincer complex under mild conditions in the presence of (catalytic) base liberating water (and H₂ in the case of secondary alcohol alkylation) as the sole byproduct.

Aminotriazole Mn(I) Complexes as Effective Catalysts for Transfer Hydrogenation of Ketones

A catalytic system based on complexes comprising abundant and cheap manganese together with readily available aminotriazole ligands is reported. The new Mn(I) complexes are catalytically competent in transfer hydrogenation of ketones with 2-propanol as hydrogen source. The reaction proceeds under mild conditions at 80 °C for 20 h with 3 % of catalyst loading using either KO t Bu or NaOH as base. Good to excellent yields were obtained for a wide substrate scope with broad functional group tolerance. The obtained results by varying the substitution pattern of the ligand are consistent with an out-sphere mechanism for the H-transfer.

Highly Selective, Efficient Deoxygenative Hydrogenation of Amides Catalyzed by a Manganese Pincer Complex via Metal-Ligand Cooperation

Deoxygenative hydrogenation of amides to amines homogeneously catalyzed by a complex of an Earth-abundant metal is presented. This manganese-catalyzed reaction features high efficiency and selectivity. A plausible reaction mechanism, involving metal–ligand cooperation of the manganese pincer complex, is proposed based on NMR studies and relevant stoichiometric reactions.

Hydrogenation of CO2 -Derived Carbonates and Polycarbonates to Methanol and Diols by Metal-Ligand Cooperative Manganese Catalysis

The first base-metal-catalysed hydrogenation of CO₂ -derived carbonates to alcohols is presented. The reaction proceeds under mild conditions in the presence of a well-defined manganese complex with a loading as low as 0.25 mol %. The non-precious-metal homogenous catalytic system provides an indirect route for the conversion of CO₂ into methanol with the co-production of value-added (vicinal) diols in yields of up to 99 %. Experimental and computational studies indicate a metal-ligand cooperative catalysis mechanism.

Manganese-Catalyzed Hydrosilylation Reactions

Over the past few decades, manganese-catalyzed hydrosilylation of C=O or C=C/C≡C unsaturated bonds have undergone enormous developments. In this focus review, the hydrosilylation reactions of alkenes, alkynes, and carbonyl-containing substrates catalyzed by manganese complexes are summarized. Moreover, the mechanisms of the manganese-catalyzed hydrosilylation are briefly discussed.

Ultrafast Iron-Catalyzed Reduction of Functionalized Ketones: Highly Enantioselective Synthesis of Halohydrines, Oxaheterocycles, and Aminoalcohols

A molecularly defined chiral boxmi iron alkyl complex catalyzes the hydroboration of various functionalized ketones and provides the corresponding chiral halohydrines, oxaheterocycles (oxiranes, oxetanes, tetrahydrofurans, and dioxanes) and amino alcohols with excellent enantioselectivities (up to >99 %ee) and conversion efficiencies at low catalyst loadings (as low as 0.5 mol %). Turnover frequencies of greater than 40000 h^-1 at -30 °C highlight the activity of this earth-abundant metal catalyst which tolerates a large number of functional groups.

One-Pot Sequential Kinetic Profiling of a Highly Reactive Manganese Catalyst for Ketone Hydroboration: Leveraging σ-Bond Metathesis via Alkoxide Exchange Steps

A comprehensive experimental and computational mechanistic study of the highly enantioselective hydroboration of ketones catalyzed by a manganese(II) alkyl boxmi pincer complex is reported. The catalyst operates at low catalyst loadings (down to 0.01 mol %) under very mild conditions (typically -40 °C) and facilitates the reduction of both aryl alkyl and dialkyl ketones with excellent selectivity (up to >95%ee). Catalyst activation pathways were investigated, demonstrating that a manganese(II) hydride and a manganese(II) alkoxide species are part of the catalytic cycle and can be generated via σ-bond metathesis of the alkyl precursor with the borane or by alcoholysis. Extensive kinetic experiments based on a "one-pot sequential kinetic profiling" approach under various conditions in combination with kinetic simulations reveal that two catalytic cycles are effective with this earth-abundant base metal catalyst: (i) a minor MnH/borane-mediated insertion cycle, in which the subsequent, product-releasing metathesis step is rate determining ( k _m = 0.076 s^-1), giving a background reaction, which is zeroth order in substrate concentrations, and (ii) a major MnOR/borane-based alkoxide exchange process, leveraging the high-barrier metathesis via the affiliation to an insertion step. The latter features non-integer reaction orders in both reagents due to a combination of an adduct formation step ( k _a = 2.12 M^-1 s^-1, k _-a = 0.49 s^-1) and a substrate insertion step of comparable rates ( k _ai = 3.74 M^-1 s^-1). The kinetic findings are underpinned by high-level density functional theory calculations of the mechanism, control experiments, and kinetic isotope effect/Hammett/Eyring analysis in different concentration regimes. The study highlights the role of a rigorous mechanistic understanding of homogeneous catalytic processes in 3d metals for rational catalyst discovery and optimization.