Reduction of sulfoxides catalyzed by the commercially available manganese complex MnBr(CO)5

A new methodology for the reduction of a wide variety of aliphatic and aromatic sulfoxides catalyzed by the air-stable, cheap and commercially available manganese catalyst MnBr(CO)5 with excellent yields is reported in this work. The catalytic system MnBr(CO)5/PhSiH3 is highly chemoselective, allowing the effective reduction of the SO bond in the presence of different functional groups.

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (Mδ+⋯M'δ-) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M+) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac-[(CO)3(bis(NHC))MnBr] and NaBPh4 shows high activity for H2 production from several substrates (Me2NHBH3, tBuNH2BH3, MeNH2BH3, NH3BH3) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me2NHBH3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.

Bis(N-Heterocyclic Carbene) Manganese(I) Complexes in Catalytic N-Formylation/N-Methylation of Amines Using Carbon Dioxide and Phenylsilane

A series of six Mn(I) complexes with general formula [MnBr(bisNHC)(CO)3 ], having a bidentate bis(N-heterocyclic carbene) ligand (bisNHC), has been developed by varying the bridging group between the NHC donors, the nitrogen wingtip substituents and the heterocyclic ring. The synthesis of the complexes has been accomplished by in situ transmetalation of the bisNHC from the corresponding silver(I) complexes. Removal of the bromide anion affords the corresponding solvento complexes [Mn(bisNHC)(CO)3 (CH3 CN)](BF4 ). The influence of the bisNHC structure on its electron donor ability has been evaluated by FTIR and 13 C NMR spectroscopy, both in the neutral and cationic complexes. Finally, the isolated Mn(I)-bisNHC complexes have been employed as homogeneous catalysts in the reductive N-formylation and N-methylation of amines with CO2 as C1 source and phenylsilane as reducing agent, showing a high selectivity for the N-methylated product. Preliminary mechanistic investigations suggest that, in the adopted reaction conditions, the formylated product can be formed via different reaction pathways, either metal-catalyzed or not, while the methylation reaction requires the use of the Mn(I) catalyst.

Bis(N-Heterocyclic Carbene) Manganese(I) Complexes: Efficient and Simple Hydrogenation Catalysts

The use of bis(NHC) manganese(I) complexes 3 as catalysts for the hydrogenation of esters was investigated. For that purpose, a series of complexes has been synthesized via an improved two step procedure utilizing bis(NHC)-BEt3 adducts. By applying complexes 3 with KHBEt3 as additive, various aromatic and aliphatic esters were hydrogenated successfully at mild temperatures and low catalyst loadings, highlighting the efficiency of the novel catalytic system. The versatility of the developed catalytic system was further demonstrated by the hydrogenation of other substrate classes like ketones, nitriles, N-heteroarenes and alkenes. Mechanistic experiments and DFT calculations indicate an inner sphere mechanism with the loss of one CO ligand and reveal the role of BEt3 as cocatalyst.

t BuOLi-Promoted Hydroboration of Esters and Epoxides

Commercially available and inexpensive lithium tert-butoxide ( ^t BuOLi) acts as a good precatalyst for the hydroboration of esters, lactones, and epoxides using pinacolborane as a borylation agent. Functional groups such as cyano-, nitro-, amino-, vinyl, and alkynyl are unaffected under the presented hydroboration process, representing high chemoselectivity. This transformation has also been effectively applied to the synthesis of key intermediates of Erlotinib and Cinacalcet. Preliminary investigations of the mechanism show that the hydroboration proceeds through the in situ formed BH₃ species.

Manganese-catalyzed hydrogenation, dehydrogenation, and hydroelementation reactions

The emerging field of organometallic catalysis has shifted towards research on Earth-abundant transition metals due to their ready availability, economic advantage, and novel properties. In this case, manganese, the third most abundant transition-metal in the Earth's crust, has emerged as one of the leading competitors. Accordingly, a large number of molecularly-defined Mn-complexes has been synthesized and employed for hydrogenation, dehydrogenation, and hydroelementation reactions. In this regard, catalyst design is based on three pillars, namely, metal-ligand bifunctionality, ligand hemilability, and redox activity. Indeed, the developed catalysts not only differ in the number of chelating atoms they possess but also their working principles, thereby leading to different turnover numbers for product molecules. Hence, the critical assessment of molecularly defined manganese catalysts in terms of chelating atoms, reaction conditions, mechanistic pathway, and product turnover number is significant. Herein, we analyze manganese complexes for their catalytic activity, versatility to allow multiple transformations and their routes to convert substrates to target molecules. This article will also be helpful to get significant insight into ligand design, thereby aiding catalysis design.

Redox-active ligand based Mn(I)-catalyst for hydrosilylative ester reduction

Herein a Mn(I) catalyst bearing a redox-active phenalenyl (PLY) based ligand is reported for the efficient hydrosilylation of esters to alcohols using the inexpensive silane source polymethylhydrosiloxane (PMHS) under mild conditions. Mechanistic investigations suggest a strong ligand-metal cooperation where a ligand-based single electron transfer (SET) process initiates the reaction through Si-H bond activation.

Click-Derived Triazoles and Triazolylidenes of Manganese for Electrocatalytic Reduction of CO2

A series of new fac-[Mn(L)(CO)₃Br] complexes where L is a bidentate chelating ligand containing mixed mesoionic triazolylidene-pyridine (MIC^py, 1), triazolylidene-triazole (MIC^trz, 2), and triazole-pyridine (trz^py, 3) ligands have been prepared and fully characterized, including the single crystal X-ray diffraction studies of 1 and 2. The abilities of 1–3 and complex fac-[Mn(MIC^MIC)(CO)₃Br] (4) to catalyze the electroreduction of CO₂ has been assessed for the first time. It was found that all complexes displayed a current increase under CO₂ atmosphere, being 3 and 4 the most active complexes. Complex 3, bearing a N^N-based ligand exhibited a good efficiency and an excellent selectivity for reducing CO₂ to CO in the presence of 1.0 M of water, at low overpotential. Interestingly, complex 4 containing the strongly electron donating di-imidazolylidene ligand exhibited comparable activity to 3, when the experiments were performed in neat acetonitrile at slightly higher overpotential (−1.86 vs. −2.14 V).

Manganese-Catalyzed Hydroborations with Broad Scope

Reductive transformations of easily available oxidized matter are at the heart of synthetic manipulation and chemical valorization. The applications of catalytic hydrofunctionalization benefit from the use of liquid reducing agents and operationally facile setups. Metal‐catalyzed hydroborations provide a highly prolific platform for reductive valorizations of stable C=X electrophiles. Here, we report an especially facile, broad‐scope reduction of various functions including carbonyls, carboxylates, pyridines, carbodiimides, and carbonates under very mild conditions with the inexpensive pre‐catalyst Mn(hmds)₂. The reaction could be successfully applied to depolymerizations.

Reduction of Carboxylic Acids to Alcohols via Manganese(I) Catalyzed Hydrosilylation

The reduction of carboxylic acids to the respective alcohols, in mild conditions, was achieved using [MnBr(CO)₅] as the catalyst and bench stable PhSiH₃ as the reducing agent. It was shown that the reaction with the earth-abundant metal catalyst could be performed either with a catalyst loading as low as 0.5 mol %, rare with the use of [MnBr(CO)₅], or on a gram scale employing only 1.5 equiv of PhSiH₃, the lowest amount of silane reported to date for this transformation. Kinetic data and control experiments have provided initial insight into the mechanism of the catalytic process, suggesting that it proceeds via the formation of silyl ester intermediates and ligand dissociation to generate a coordinatively unsaturated Mn(I) complex as the active species.