Ruthenium(II)‐Catalyzed β ‐Methylation of Alcohols using Methanol as C 1 Source

β-Methylation of Primary Alcohols with Methanol Catalyzed by a Metal-Ligand Bifunctional Iridium Catalyst

The development of efficient methods for the direct introduction of a methyl group into molecules is becoming increasingly important. Herein, the β-methylation of primary alcohols with methanol has been accomplished under environmentally benign conditions using [Cp*Ir(2,2'-bpyO)(H2O)] as a catalyst. It was found that functional groups in the ligand are crucially important for the activity of the iridium complex. Furthermore, the mechanistic research and application potential of our catalytic system are also presented.

Ruthenium Complexes with NNN-Pincer Ligands for N-Methylation of Amines Using Methanol

A series of ruthenium complexes (Ru1-Ru4) bearing new NNN-pincer ligands were synthesized in 58-78% yields. All of the complexes are air and moisture stable and were characterized by IR, NMR, and high-resolution mass spectra (HRMS). In addition, the structures of Ru1-Ru3 were confirmed by X-ray crystallographic analysis. These Ru(II) complexes exhibited high catalytic efficiency and broad functional group tolerance in the N-methylation reaction of amines using CH3OH as both the C1 source and solvent. Experimental results indicated that the electronic effect of the substituents on the ligands considerably affects the catalytic reactivity of the complexes in which Ru3 bearing an electron-donating OMe group showed the highest activity. Deuterium labeling and control experiments suggested that the dehydrogenation of methanol to generate ruthenium hydride species was the rate-determining step in the reaction. Furthermore, this protocol also provided a ready approach to versatile trideuterated N-methylamines under mild conditions using CD3OD as a deuterated methylating agent.

Mechanistic Insight into Acceptorless Dehydrogenation of Methanol to Syngas Catalyzed by MACHO-Type Ruthenium and Manganese Complexes: A DFT Study

The acceptorless dehydrogenation of methanol to produce carbon monoxide (CO) and dihydrogen (H2) mediated by MACHO-type 1-Ru and 1-Mn complexes was theoretically investigated via density functional theory calculations. The 1-Ru-catalyzed process involves the formation of active species 4-Ru through a methanol-bridged H2 release pathway. Methanol dehydrogenation by 4-Ru yields formaldehyde and 1-Ru, followed by H2 release to regenerate 4-Ru (rate-determining step, ΔG‡ = 32.5 kcal/mol). Formaldehyde further reacts with methanol via nucleophilic attack of the MeO- ligand in the Ru complex (ΔG‡ = 9.6 kcal/mol), which is more favorable than the traditional methanol-to-formaldehyde nucleophilic attack (ΔG‡ = 33.8 kcal/mol) due to the higher nucleophilicity of MeO-. CO is ultimately produced through the methyl formate decarbonylation reaction. Accelerated H2 release in the early reaction stage compared to CO results from the initial methanol dehydrogenation and condensation of formaldehyde with methanol. In contrast, CO generation occurs later via methyl formate decarbonylation. The 1-Mn-catalyzed reaction has reduced efficiency compared to 1-Ru for the higher Gibbs energy barrier (ΔG‡ = 34.1 kcal/mol) of the rate-determining step. Excess NaOtBu promotes the reaction of CO and methanol, forming methyl formate, significantly reducing the CO/H2 ratio as the catalyst amount decreases. These findings deepen our understanding of the methanol-to-syngas transformation and can drive progress in this field.

Borrowing Hydrogen β-Phosphinomethylation of Alcohols Using Methanol as C1 Source by Pincer Manganese Complex

Herein, we report a manganese-catalyzed three-component coupling of β-H containing alcohols, methanol, and phosphines for the synthesis of γ-hydroxy phosphines via a borrowing hydrogen strategy. In this development, methanol serves as a sustainable C1 source. A variety of aromatic and aliphatic substituted alcohols and phosphines could undergo the dehydrogenative cross-coupling process efficiently and deliver the corresponding β-phosphinomethylated alcohol products in moderate to good yields. Mechanistic studies suggest that this transformation proceeds in a sequential manner including catalytic dehydrogenation, aldol condensation, Michael addition, and catalytic hydrogenation.

Production of carbon monoxide and hydrogen from methanol using a ruthenium pincer complex: a DFT study

To understand the mechanism of the dehydrogenation of methanol to CO and H2 catalyzed by a ruthenium pincer complex, a density functional theory (DFT) study has been conducted on two different cycles which differ in the substances entering the cycle (methanol (cycle 1) versus methoxymethanol (cycle 2)). Our calculated results show that both cycles consist of three stages: dehydrogenation of alcohol to aldehyde (stage I); hydrogen formation (stage II); and decarbonylation with the regeneration of the catalyst (stage III). The energy barriers of the rate-determining steps for cycles 1 and 2 are 49.6 and 28.5 kcal mol-1, respectively. Thus cycle 2 is more energetically feasible. For stage III of cycle 2, our results did not support the mechanism proposed in the experiment (CO release occurs prior to decarbonylation). Instead, we suggested and examined an alternative pathway, that is, decarbonylation occurs prior to CO release. The mechanistic insights gained in the present paper could be beneficial for further designing of these kinds of reactions.

Manganese-Catalyzed Chemoselective Coupling of Secondary Alcohols, Primary Alcohols and Methanol

Herein, we report a manganese-catalyzed three-component coupling of secondary alcohols, primary alcohols and methanol for the synthesis of β,β-methylated/alkylated secondary alcohols. Using our method, a series of 1-arylethanol, benzyl alcohol derivatives, and methanol undergo sequential coupling efficiently to construct assembled alcohols with high chemoselectivity in moderate to good yields. Mechanistic studies suggest that the reaction proceeds via methylation of a benzylated secondary alcohol intermediate to generate the final product.

Recent advances in C/N-alkylation with alcohols through hydride transfer strategies

This review highlights the most recent reports in three powerful and ever-growing fields of borrowing hydrogen, acceptorless dehydrogenative coupling, and base-mediated hydride transfer strategies; which pave the way for generating reactive intermediates via shuttling hydrogen (or hydride) between starting materials without any need for an external hydrogen source to easily construct more complex structures. There is a thorough focus on diversifying the utility of alcohols for C/N-alkylation leading to the synthesis of branched ketones, alcohols, amines, indols, and 6-membered nitrogen-containing heterocycles such as pyridines and pyrimidines, various transformations with the focus on C-C and C-N bond-forming reactions via metal-based catalysis or metal-free approaches in this context to give a global overview in this area.

Guerbet-type β-alkylation of secondary alcohols catalyzed by chromium chloride and its corresponding NNN pincer complex

β-Alkylation of alcohols has been efficiently accomplished using readily available 3d metal Cr under microwave conditions in air. Well-defined molecular Cr is involved with a KIE of 7.33 and insertion of α-alkylated ketone into Cr–H bond as the RDS.

Cyclometalated (NNC)Ru(II) complex catalyzed β-methylation of alcohols using methanol

Indolyl fragment containing phenanthroline based new ligands and their corresponding Ru(II) complexes were synthesized and fully characterized by various spectroscopic techniques. Catalytic activity of these newly synthesized cyclometalated (NNC)Ru(II) complexes...

Ligand-free Guerbet-type reactions in air catalyzed by in situ formed complexes of base metal salt cobaltous chloride

Inexpensive, earth-abundant & environmentally benign CoCl2 efficiently catalyses the β-alkylation of alcohol in unprecedented yields (89%) & turnovers (8900). Mechanistic studies are indicative of in situ generated homogeneous molecular Co catalysts.

Acceptorless Dehydrogenation of Methanol to Carbon Monoxide and Hydrogen using Molecular Catalysts

The acceptorless dehydrogenation of methanol to carbon monoxide and hydrogen was investigated using homogeneous molecular complexes. Complexes of ruthenium and manganese comprising the MACHO ligand framework showed promising activities for this reaction. The molecular ruthenium complex [RuH(CO)(BH4 )(HN(C2 H4 PPh2 )2 )] (Ru-MACHO-BH) achieved up to 3150 turnovers for carbon monoxide and 9230 turnovers for hydrogen formation at 150 °C reaching pressures up to 12 bar when the decomposition was carried out in a closed vessel. Control experiments affirmed that the metal complex mediates the initial fast dehydrogenation of methanol to formaldehyde and methyl formate followed by subsequent slow decarbonylation. Depending on the catalyst and reaction conditions, the CO/H2 ratio in the gas mixture thus varies over a broad range from almost pure hydrogen to the stoichiometric limit of 1:2.

Bulky PNP ligands blocking metal-ligand cooperation allow for isolation of Ru(0), and lead to catalytically active Ru complexes in acceptorless alcohol dehydrogenation

We synthesized two 4Me-PNP ligands which block metal-ligand cooperation (MLC) with the Ru center and compared their Ru complex chemistry to their two traditional analogues used in acceptorless alcohol dehydrogenation catalysis. The corresponding 4Me-PNP complexes, which do not undergo dearomatization upon addition of base, allowed us to obtain rare, albeit unstable, 16 electron mono CO Ru(0) complexes. Reactivity with CO and H 2 allows for stabilization and extensive characterization of bis CO Ru(0) 18 electron and Ru(II) cis and trans dihydride species that were also shown to be capable of C(sp2)-H activation. Reactivity and catalysis are contrasted to non-methylated Ru(II) species, showing that an MLC pathway is not necessary, with dramatic differences in outcomes during catalysis between i Pr and t Bu PNP complexes within each of the 4Me and non-methylated backbone PNP series being observed. Unusual intermediates are characterized in one of the new and one of the traditional complexes, and a common catalysis deactivation pathway was identified.

Ru-Catalyzed Selective Catalytic Methylation and Methylenation Reaction Employing Methanol as the C1 Source

Methanol can be employed as a green and sustainable methylating agent to form C-C and C-N bonds via borrowing hydrogen (BH) methodology. Herein we explored the activity of the acridine-derived SNS-Ru pincer for the activation of methanol to apply it as a C1 building block in different reactions. Our catalytic system shows great success toward the β-C(sp³)-methylation reaction of 2-phenylethanols to provide good to excellent yields of the methylated products. We investigated the mechanistic details, kinetic progress, and temperature-dependent product distribution, which revealed the slow and steady generation of in situ formed aldehyde, is the key factor to get the higher yield of the β-methylated product. To establish the environmental benefit of this reaction, green chemistry metrics are calculated. Furthermore, dimerization of 2-naphthol via methylene linkage and formation of N-methylation of amine are also described in this study, which offers a wide range of substrate scope with a good to excellent yield.

Recent advances in sustainable organic transformations using methanol: expanding the scope of hydrogen-borrowing catalysis

Recent progress relating to sustainable approaches using methanol as a C1-alkylating agent for C–Me and N–Me bond formation is discussed.

Carbon monoxide and hydrogen (syngas) as a C1-building block for selective catalytic methylation

A catalytic reaction using syngas (CO/H₂) as feedstock for the selective β-methylation of alcohols was developed whereby carbon monoxide acts as a C1 source and hydrogen gas as a reducing agent. The overall transformation occurs through an intricate network of metal-catalyzed and base-mediated reactions. The molecular complex [Mn(CO)₂Br[HN(C₂H₄PⁱPr₂)₂]] 1 comprising earth-abundant manganese acts as the metal component in the catalytic system enabling the generation of formaldehyde from syngas in a synthetically useful reaction. This new syngas conversion opens pathways to install methyl branches at sp³ carbon centers utilizing renewable feedstocks and energy for the synthesis of biologically active compounds, fine chemicals, and advanced biofuels.

A broadly applicable catalytic process for the selective β-methylation of alcohols is presented using syngas (CO/H₂) directly as a C1 building block and the shown manganese complex in the presence of a base as the catalytic system.

Iron-Catalyzed α-Alkylation of Ketones with Secondary Alcohols: Access to β-Disubstituted Carbonyl Compounds

An iron-catalyzed borrowing hydrogen strategy has been applied in the synthesis of β-branched carbonyl compounds. Various secondary benzylic and aliphatic alcohols have been used as alkylating reagents under mild reaction conditions. The ketones have been isolated in good to excellent yield. Deuterium labeling experiments provide evidence that the alcohol is the hydride source in this reaction and that no reversible step or hydrogen/deuterium scrambling takes place during the process.

Manganese-Catalyzed β-Methylation of Alcohols by Methanol

We report an earth-abundant-metal-catalyzed double and single methylation of alcohols. A manganese catalyst, which operates at low catalyst loadings and short reaction times, mediates these reactions efficiently. A broad scope of primary and secondary alcohols, including purely aliphatic examples, and 1,2-aminoalcohols can be methylated. Furthermore, alcohol methylation for the synthesis of pharmaceuticals has been demonstrated. The catalyst system tolerates many functional groups among them hydrogenation-sensitive examples and upscaling is easily achieved. Mechanistic investigations are indicative of a borrowing hydrogen or hydrogen autotransfer mechanism involving a bimetallic K-Mn catalyst. The catalyst accepts hydrogen as a proton and a hydride from alcohols efficiently and reacts with a chalcone via hydride transfer.

Iron-Catalyzed β-Alkylation of Alcohols

β-Branched alkylated alcohols have been prepared in good yields using a double-hydrogen autotransfer strategy in the presence of our diaminocyclopentadienone iron tricarbonyl complex Fe1. The alkylation of some 2-arylethanol derivatives was successfully addressed with benzylic alcohols and methanol as alkylating reagents under mild conditions. Deuterium labeling experiments suggested that both alcohols (2-arylethanol and either methanol or benzyl alcohol) served as hydrogen donors in this cascade process.

Iron-Catalyzed Borrowing Hydrogen β-C(sp3)-Methylation of Alcohols

Herein we report the iron-catalyzed β-C(sp3)-methylation of primary alcohols using methanol as a C1 building block. This borrowing hydrogen approach employs a well-defined bench-stable (cyclopentadienone)iron(0) carbonyl complex as precatalyst (5 mol %) and enables a diverse selection of substituted 2-arylethanols to undergo β-C(sp3)-methylation in good isolated yields (24 examples, 65% average yield).