Scope and Mechanistic Analysis for Chemoselective Hydrogenolysis of Carbonyl Compounds Catalyzed by a Cationic Ruthenium Hydride Complex with a Tunable Phenol Ligand

TiCl4-mediated deoxygenative reduction of aromatic ketones to alkylarenes with ammonia borane

A rapid and mild protocol for the exhaustive deoxygenation of various aromatic ketones to corresponding alkanes was described, which was mediated by TiCl4 and used ammonia borane (AB) as the reductant. This reduction protocol applies to a wide range of substrates in moderate to excellent yields at room temperature. The gram-scale reaction and syntheses of some key building blocks for SGLT2 inhibitors demonstrated the practicability of this methodology. Preliminary mechanistic studies revealed that the ketone is first converted into an alcohol, which then undergoes a carbocation to give the alkane via hydrogenolysis.

Synthesis and Application of New Salan Titanium Complexes in the Catalytic Reduction of Aldehydes

Complexes of formula [(H2N2O2)TiCl2] and [(H2N2O2)Ti(OiPr)2] (H2N2O2H2 = HOPh’CH2NH(CH2)2NHCH2Ph’OH, where Ph’ = 2,4-(CMe2Ph)C6H2) were synthesized by the reaction of the salan ligand precursor H2N2O2H2 with TiCl4 and Ti(OiPr)4, respectively, in high yields. The dichlorido complex [(H2N2O2)TiCl2] revealed to be an efficient catalyst for the reduction of benzaldehyde in toluene. Full conversion was observed after 24 h at 55 °C in THF. The same catalyst also converted phenylacetaldehyde and hydrocinnamaldehyde into the corresponding alkanes quantitatively.

Highly Effective and Chemoselective Hydrodeoxygenation of Aromatic Alcohols

Effective hydrodeoxygenation (HDO) of aromatic alcohols is very attractive in both conventional organic synthesis and upgrading of biomass-derived molecules, but the selectivity of this reaction is usually low because of...

Ru(dppbsa)-catalyzed hydrodeoxygenation and reductive etherification of ketones and aldehydes

Ru(dppbsa)-catalyzed hydrodeoxygenation and reductive etherification of ketones and aldehydes were developed. The carbonyl substrates without β-CH functionality follow the hydrogenation-hydrogenolysis path, wherein the hydrogenolysis of the alkanol intermediates presents as...

Experimental and Computational Studies on the Ruthenium-Catalyzed Dehydrative C-H Coupling of Phenols with Aldehydes for the Synthesis of 2-Alkylphenol, Benzofuran, and Xanthene Derivatives

The cationic Ru-H complex [(C₆H₆)(PCy₃)(CO)RuH]⁺BF₄^- (1) was found to be an effective catalyst for the dehydrative C-H coupling reaction of phenols and aldehydes to form 2-alkylphenol products. The coupling reaction of phenols with branched aldehydes selectively formed 1,1-disubstituted benzofurans, while the coupling reaction with salicylaldehydes yielded xanthene derivatives. A normal deuterium isotope effect was observed from the coupling reaction of 3-methoxyphenol with benzaldehyde and 2-propanol/2-propanol-d₈ (k_H/k_D = 2.3 ± 0.3). The carbon isotope effect was observed on the benzylic carbon of the alkylation product from the coupling reaction of 3-methoxyphenol with 4-methoxybenzaldehyde (C(3) 1.021(3)) and on both benzylic and ortho-arene carbons from the coupling reaction with 4-trifluorobenzaldehdye (C(2) 1.017(3), C(3) 1.011(2)). The Hammett plot from the coupling reaction of 3-methoxyphenol with para-substituted benzaldehydes p-X-C₆H₄CHO (X = OMe, Me, H, F, Cl, CF₃) displayed a V-shaped linear slope. Catalytically relevant Ru-H complexes were observed by NMR from a stoichiometric reaction mixture of 1, 3-methoxyphenol, benzaldehyde, and 2-propanol in CD₂Cl₂. The DFT calculations provided a detailed catalysis mechanism featuring an electrophilic aromatic substitution of the aldehyde followed by the hydrogenolysis of the hydroxy group. The calculations also revealed a mechanistic rationale for the strong electronic effect of the benzaldehdye substrates p-X-C₆H₄CHO (X = OMe, CF₃) in controlling the turnover-limiting step. The catalytic C-H coupling method provides an efficient synthetic protocol for 2-alkylphenols, 1,1-disubstituted benzofurans, and xanthene derivatives without employing any reactive reagents or forming wasteful byproducts.

Metal-Organic Framework-Confined Single-Site Base-Metal Catalyst for Chemoselective Hydrodeoxygenation of Carbonyls and Alcohols

Chemoselective deoxygenation of carbonyls and alcohols using hydrogen by heterogeneous base-metal catalysts is crucial for the sustainable production of fine chemicals and biofuels. We report an aluminum metal-organic framework (DUT-5) node support cobalt(II) hydride, which is a highly chemoselective and recyclable heterogeneous catalyst for deoxygenation of a range of aromatic and aliphatic ketones, aldehydes, and primary and secondary alcohols, including biomass-derived substrates under 1 bar H₂. The single-site cobalt catalyst (DUT-5-CoH) was easily prepared by postsynthetic metalation of the secondary building units (SBUs) of DUT-5 with CoCl₂ followed by the reaction of NaEt₃BH. X-ray photoelectron spectroscopy and X-ray absorption near-edge spectroscopy (XANES) indicated the presence of Co^II and Al^III centers in DUT-5-CoH and DUT-5-Co after catalysis. The coordination environment of the cobalt center of DUT-5-Co before and after catalysis was established by extended X-ray fine structure spectroscopy (EXAFS) and density functional theory. The kinetic and computational data suggest reversible carbonyl coordination to cobalt preceding the turnover-limiting step, which involves 1,2-insertion of the coordinated carbonyl into the cobalt-hydride bond. The unique coordination environment of the cobalt ion ligated by oxo-nodes within the porous framework and the rate independency on the pressure of H₂ allow the deoxygenation reactions chemoselectively under ambient hydrogen pressure.

Chemical-Reductant-Free Electrochemical Deuteration Reaction using Deuterium Oxide

We report a method for the electrochemical deuteration of α,β-unsaturated carbonyl compounds under catalyst- and external-reductant-free conditions, with deuteration rates as high as 99 % and yields up to 91 % in 2 h. The use of graphite felt for both the cathode and the anode was key to ensuring chemoselectivity and high deuterium incorporation under neutral conditions without the need for an external reductant. This method has a number of advantages over previously reported deuteration reactions that use stoichiometric metallic reductants. Mechanistic experiments showed that O₂ evolution at the anode not only eliminates the need for an external reductant but also regulates the pH of the reaction mixture, keeping it approximately neutral.

General and selective deoxygenation by hydrogen using a reusable earth-abundant metal catalyst

Chemoselective deoxygenation by hydrogen is particularly challenging but crucial for an efficient late-stage modification of functionality-laden fine chemicals, natural products, or pharmaceuticals and the economic upgrading of biomass-derived molecules into fuels and chemicals. We report here on a reusable earth-abundant metal catalyst that permits highly chemoselective deoxygenation using inexpensive hydrogen gas. Primary, secondary, and tertiary alcohols as well as alkyl and aryl ketones and aldehydes can be selectively deoxygenated, even when part of complex natural products, pharmaceuticals, or biomass-derived platform molecules. The catalyst tolerates many functional groups including hydrogenation-sensitive examples. It is efficient, easy to handle, and conveniently synthesized from a specific bimetallic coordination compound and commercially available charcoal. Selective, sustainable, and cost-efficient deoxygenation under industrially viable conditions seems feasible.

Dehalogenative Deuteration of Unactivated Alkyl Halides Using D2O as the Deuterium Source

The general dehalogenation of alkyl halides with zinc using D₂O or H₂O as a deuterium or hydrogen donor has been developed. The method provides an efficient and economic protocol for deuterium-labeled derivatives with a wide substrate scope under mild reaction conditions. Mechanistic studies indicated that a radical process is involved for the formation of organozinc intermediates. The facile hydrolysis of the organozinc intermediates provides the driving force for this transformation.

Nickel-Catalyzed Decarbonylative Alkylation of Aroyl Fluorides Assisted by Lewis-Acidic Organoboranes

Herein, nickel-catalyzed decarbonylative C-F bond alkylation of aroyl fluorides with organoboron reagents is reported. Aroyl fluorides are more chemically stable than the corresponding aroyl chlorides and can be readily synthesized from the corresponding carboxylic acids. The fluoronickel intermediate formed via oxidative addition interacts with Lewis-acidic trialkylboranes, and the subsequent decarbonylative alkylation proceeds. This new synthetic methodology allows 1,2-bifunctionalization of aromatic carboxylic acids via palladium-catalyzed ortho-C-H arylation. In addition, an unprecedented 1,4-nickel migration on ortho-arylated aroyl fluorides was observed. As a demonstration of the synthetic utility of the present reaction, the sequential 1-alkyl-2-arylation of 3-hydroxy-2-naphthoic acid was accomplished via chemoselective alkylation at a fluorocarbonyl moiety and the subsequent C-O bond arylation at an acetoxy group.

Teaching an old carbocation new tricks: Intermolecular C-H insertion reactions of vinyl cations

Vinyl carbocations have been the subject of extensive experimental and theoretical studies over the past five decades. Despite this long history in chemistry, the utility of vinyl cations in chemical synthesis has been limited, with most reactivity studies focusing on solvolysis reactions or intramolecular processes. Here we report synthetic and mechanistic studies of vinyl cations generated through silylium-weakly coordinating anion catalysis. We find that these reactive intermediates undergo mild intermolecular carbon-carbon bond-forming reactions, including carbon-hydrogen (C-H) insertion into unactivated sp³ C-H bonds and reductive Friedel-Crafts reactions with arenes. Moreover, we conducted computational studies of these alkane C-H functionalization reactions and discovered that they proceed through nonclassical, ambimodal transition structures. This reaction manifold provides a framework for the catalytic functionalization of hydrocarbons using simple ketone derivatives.

Benzoate Cyclometalation Enables Oxidative Addition of Haloarenes at a Ru(II) Center

The first Ru(II)-catalyzed arylation of substrates without a directing group was recently developed. Remarkably, this process only worked in the presence of a benzoate additive, found to be crucial for the oxidative addition step at Ru(II). However, the exact mode of action of the benzoate was unknown. Herein, we disclose a mechanistic study that elucidates the key role of the benzoate salt in the C–H arylation of fluoroarenes with aryl halides. Through a combination of rationally designed stoichiometric experiments and DFT studies, we demonstrate that the aryl–Ru(II) species arising from initial C–H activation of the fluoroarene undergoes cyclometalation with the benzoate to generate an anionic Ru(II) intermediate. The enhanced lability of this intermediate, coupled with the electron-rich anionic Ru(II) metal center renders the oxidative addition of the aryl halide accessible. The role of an additional (NMe₄)OC(CF₃)₃ additive in facilitating the overall arylation process is also shown to be linked to a shift in the C–H pre-equilibrium associated with benzoate cyclometalation.

Room Temperature Chemoselective Deoxygenation of Aromatic Ketones and Aldehydes Promoted by a Tandem Pd/TiO2 + FeCl3 Catalyst

A rapid and practical protocol for the chemoselective deoxygenation of various aromatic ketones and aldehydes was described, which used a tandem catalyst composed of heterogeneous Pd/TiO₂ + homogeneous FeCl₃ with the green hydrogen source, polymethylhydrosiloxane (PMHS). The developed catalytic system was robust and scalable, as exemplified by the deoxygenation of acetophenone, which was performed on a gram scale in an atmospheric environment utilizing only 0.4 mol % Pd/TiO₂ + 10 mol % FeCl₃ catalyst to give the corresponding ethylbenzene in 96% yield within 10 min at room temperature. Furthermore, the Pd/TiO₂ catalyst was shown to be recyclable up to three times without an observable decrease in efficiency and it exhibited low metal leaching under the reaction conditions. Insights toward the reaction mechanism of Pd-catalyzed reductive deoxygenation for aromatic ketones and aldehydes were investigated through operando IR, NMR, and GC-MS techniques.

Synthesis of Symmetric and Unsymmetric Secondary Amines from the Ligand-Promoted Ruthenium-Catalyzed Deaminative Coupling Reaction of Primary Amines

The catalytic system generated in situ from the tetranuclear Ru-H complex with a catechol ligand (1/L1) was found to be effective for the direct deaminative coupling of two primary amines to form secondary amines. The catalyst 1/L1 was highly chemoselective for promoting the coupling of two different primary amines to afford unsymmetric secondary amines. The analogous coupling of aniline with primary amines formed aryl-substituted secondary amines. The treatment of aniline- d₇ with 4-methoxybenzylamine led to the coupling product with significant deuterium incorporation on CH₂ (18% D). The most pronounced carbon isotope effect was observed on the α-carbon of the product isolated from the coupling reaction of 4-methoxybenzylamine (C(1) = 1.015(2)). A Hammett plot was constructed from measuring the rates of the coupling reaction of 4-methoxyaniline with a series of para-substituted benzylamines 4-X-C₆H₄CH₂NH₂ (X = OMe, Me, H, F, CF₃) (ρ = -0.79 ± 0.1). A plausible mechanistic scheme has been proposed for the coupling reaction on the basis of these results. The catalytic coupling method provides an operationally simple and chemoselective synthesis of secondary amine products without using any reactive reagents or forming wasteful byproducts.

Structural Determination of Ruthenium Complexes Containing Bi-Dentate Pyrrole-Ketone Ligands

A series of ruthenium compounds containing a pyrrole-ketone bidentate ligand, 2-(2′-methoxybenzoyl)pyrrole (1), have been synthesized and characterized. Reacting 1 with [(η⁶-cymene)RuCl₂]₂ and RuHCl(CO)(PPh₃)₃ generated Ru(η⁶-cymene)[C₄H₃N-2-(CO-C₆H₄-2-OMe)]Cl (2) and {RuCl(CO)(PPh₃)₂[C₄H₃N-2-(COC₆H₄-2-OMe)]} (3), respectively, in moderate yields. Successively reacting 2 with sodium cyanate and sodium azide gave {Ru(η⁶-cymene)[C₄H₃N-2-(CO-C₆H₄-2-OMe)]X} (4, X=OCN; 5, X=N₃) with the elimination of sodium chloride. Compounds 2–5 were all characterized by ¹H and ¹³C-NMR spectra and their structures were also determined by X-ray single crystallography.

DFT methods applied to answer the question: how accurate is the ligand acidity constant method for estimating the pKa of transition metal hydride complexes MHXL4 when X is varied?

Additive ligand acidity constants A_L of anionic ligands are calculated for neutral hydrides of iron(ii), ruthenium(ii) and osmium(ii) with phosphine and carbonyl co-ligands; constant A_L in green, more variable A_L in red.

Transfer hydrogenation and hydration of aromatic aldehydes and nitriles using heterogeneous NiO nanofibers as a catalyst

NiO NFs as an efficient heterogeneous catalyst for the synthesis of aromatic alcohols and amides.

Auto-Tandem Catalysis with Frustrated Lewis Pairs for Reductive Etherification of Aldehydes and Ketones

Herein we report that a single frustrated Lewis pair (FLP) catalyst can promote the reductive etherification of aldehydes and ketones. The reaction does not require an exogenous acid catalyst, but the combined action of FLP on H₂ , R-OH or H₂ O generates the required Brønsted acid in a reversible, "turn on" manner. The method is not only a complementary metal-free reductive etherification, but also a niche procedure for ethers that would be either synthetically inconvenient or even intractable to access by alternative synthetic protocols.

Hierarchically porous materials: synthesis strategies and structure design

This review addresses recent advances in synthesis strategies of hierarchically porous materials and their structural design from micro-, meso- to macro-length scale.

Determination of 2H KIEs from Competition Experiments: Increased Accuracy via Isotopic Enrichment

Methods for the determination of ²H KIEs at natural abundance of deuterium have not been widely used due to the requirement for very long NMR times or large reaction scale. We previously reported a simple methodology for reducing these restrictions by the addition of a small amount of deuterated substrate. Herein, we evaluate the deuterium loadings that give the lowest errors in the determination of ²H KIEs. Our simulations indicate that our approach leads to a 4000-fold reduction in NMR time over natural abundance methods.

Deuterodechlorination of Aryl/Heteroaryl Chlorides Catalyzed by a Palladium/Unsymmetrical NHC System

The catalytic deuterodechlorination of aryl/heteroaryl chlorides was developed with a palladium/unsymmetrical NHC system, and the precisely controlled introduction of deuterium into a variety of aryl/heteroaryl compounds was achieved with a high level of efficiency, selectivity, and deuteration degree. This method was also successfully applied to the transformation of bioactive agents even in a gram-scale synthesis. The crystal structure analysis of Pd-NHC complexes led to the observation of Pd-arene interaction.

Catalytic Synthesis of Substituted Indoles and Quinolines from the Dehydrative C-H Coupling of Arylamines with 1,2- and 1,3-Diols

The cationic ruthenium-hydride complex catalyzes dehydrative C-H coupling reaction of arylamines with 1,2-diols to form the indole products. The analogous coupling of arylamines with 1,3-diols afforded the substituted quinolines. The catalytic method directly forms these coupling products in a highly regioselective manner without generating any toxic byproducts.