Molybdenum-Catalyzed Dehydrogenative Synthesis of Imines from Alcohols and Amines

Advances in Group VI Metal-Catalyzed Homogeneous Hydrogenation and Dehydrogenation Reactions

Transition metal-catalyzed homogeneous hydrogenation and dehydrogenation reactions for attaining plethora of organic scaffolds have evolved as a key domain of research in academia and industry. These protocols are atom-economic, greener, in line with the goal of sustainability, eventually pave the way for numerous novel environmentally benign methodologies. Appealing progress has been achieved in the realm of homogeneous catalysis utilizing noble metals. Owing to their high cost, less abundance along with toxicity issues led the scientific community to search for sustainable alternatives. In this context, earth- abundant base metals have gained substantial attention culminating enormous progress in recent years, predominantly with pincer-type complexes of nickel, cobalt, iron, and manganese. In this regard, group VI chromium, molybdenum and tungsten complexes have been overlooked and remain underdeveloped despite their earth-abundance and bio-compatibility. This review delineates a comprehensive overview in the arena of homogeneously catalysed (de)hydrogenation reactions using group VI base metals chromium, molybdenum, and tungsten till date. Various reactions have been described; hydrogenation, transfer hydrogenation, dehydrogenation, acceptorless dehydrogenative coupling, hydrogen auto transfer, along with their scope and brief mechanistic insights.

Microwave-Assisted Synthesis of E-Aldimines, N-Heterocycles, and H2 by Dehydrogenative Coupling of Benzyl Alcohol and Aniline Derivatives Using CoCl2 as a Catalyst

The acceptorless dehydrogenative coupling (ADC) between alcohols and amines to produce imines has been achieved mostly by employing precious-metal-based complexes or complexes of earth-abundant metal ions with sensitive and complicated ligand systems as catalysts mostly under harsh reaction conditions. Methodologies using readily available earth-abundant metal salts as catalysts without the requirement of ligand, oxidant, or any external additives are not explored. We report an unprecedented microwave-assisted CoCl₂-catalyzed acceptorless dehydrogenative coupling of benzyl alcohol and amine for the synthesis of E-aldimines, N-heterocycles, and H₂ under mild condition, without any complicated exogenous ligand template, oxidant, or other additives. This environmentally benign methodology exhibits broad substrate scope (43 including 7 new products) with fair functional-group tolerance on the aniline ring. Detection of metal-associated intermediate by gas chromatography (GC) and HRMS, H₂ detection by GC, and kinetic isotope effect reveal the mechanism of this CoCl_2-catalyzed reaction to be via ADC. Furthermore, kinetic experiments and Hammett analysis with variation in the nature of substituents over the aniline ring reveal the insight into the reaction mechanism with different substituents.

Correlation between the Properties of Surface Lattice Oxygen on NiO and Its Reactivity and Selectivity towards the Oxidative Dehydrogenation of Propane

Modified NiO catalysts with controllable vacancies and dopants are promising for alkene production from oxidative dehydrogenation (ODH) of light alkanes, and a molecular understanding of the modification on elementary reaction steps would facilitate the design of highly efficient catalysts and catalytic processes. In this study, density functional theory (DFT) calculations was used to map out the complete reaction pathways of propane ODH on the NiO (100) surfaces with different modifiers. The results demonstrated that the presence of vacancies (O and Ni) and dopants (Li and Al) alters the electrophilicity of surface oxygen species, which in turn affects the reactivity towards C-H bond activation and the overall catalytic activity and selectivity. The strongly electrophilic O favors a radical mechanism for the first C-H activation on O followed by the second C-H activation on O-O site, whereas weak electrophilic O favors concerted C-H bond breaking over Ni-O site. The C-H bond activation proceeds through a late transition state, characterized by the almost completion of the O-H bond formation. Consequently, the adsorption energy of H adatom on O rather than p-band center or Bader charge of O has been identified to be an accurate descriptor to predict the activation barrier for C-H breaking (activity) as well as the difference between the activation barriers of propene and CH₃ CCH₃ (selectivity) of ODH.

Ni(ii) immobilized on poly(guanidine-triazine-sulfonamide) (PGTSA/Ni): a mesoporous nanocatalyst for synthesis of imines

Mesoporous materials have been the subject of intense research regarding their unique structural and textural properties and successful applications in various fields. This study reports a novel approach for synthesizing a novel porous polymer stabilizer through condensation polymerization in which Fe₃O₄ magnetic nanoparticles (Fe₃O₄ MNPs) are used as hard templates. Using this method allowed the facile and fast removal of the template and mesopores formation following the Fe₃O₄ MNPs. Different techniques were performed to characterize the structure of the polymer. Based on the obtained results, the obtained mesoporous polymeric network was multi-layered and consisted of repeating units of sulfonamide, triazine, and guanidine as a novel heterogeneous multifunctional support. Afterward, the new nickel organometallic complex was supported on its inner surface using the porous poly sulfonamide triazine guanidine (PGTSA/Ni). In this process, the obtained PGTSA/Ni nanocomposite was used as a heterogeneous catalyst in the synthesis of imines from amines. Since this reaction has an acceptorless dehydrogenation pathway, the hydrogen gas is released as its by-product. The synthesized nanocatalyst was structurally confirmed using different characterization modalities, including FT-IR, SEM, XRD, EDX, TEM, elemental mapping, ICP-AES, BET, and TGA. In addition, all products were obtained in high turnover frequency (TOF) and turnover number (TON). The corresponding results revealed the high selectivity and activity of the prepared catalyst through these coupling reactions. Overall, the synthesized nanocatalyst is useable for eight cycles with no considerable catalytic efficiency loss.

Efficient transfer hydrogenation of ketones by molybdenum complexes through comprehensively verifying auxiliary ligands

Molybdenum complexes, ligated with N1,N1-dialkyl-N2-(5,6,7,8-tetrahydroquinolin-8-yl)ethane-1,2-diamines along with auxiliary ligands, provide various structural features as [NNH/NNHN]Mo(CO)4/3 (Mo1 – Mo3), [NNHN]Mo(CO)2Br] (Mo4 – Mo5), [NNH]Mo(CO)(η3-C3H5)Br](Mo6) and [NNHN/S] Mo(CO)(PPh3)2] (Mo7 – Mo8). All...

Highly regio- and stereoselective phosphinylphosphination of terminal alkynes with tetraphenyldiphosphine monoxide under radical conditions

The homolytic cleavage of the P^V(O)–P^III bond in tetraphenyldiphosphine monoxide simultaneously provides both pentavalent and trivalent phosphorus-centered radicals with different reactivities. The method using V-40 as an initiator is successfully investigated for the regio- and stereoselective phosphinylphosphination of terminal alkynes giving the corresponding trans-isomers of 1-diphenylphosphinyl-2-diphenylthiophosphinyl-1-alkenes in good yields. The protocol can be applied to a wide variety of terminal alkynes including both alkyl- and arylalkynes.

Radical condensation between benzylic alcohols and acetamides to form 3-arylpropanamides

A new radical condensation reaction is developed where benzylic alcohols and acetamides are coupled to generate 3-arylpropanamides with water as the only byproduct. The transformation is performed with potassium tert-butoxide as the only additive and gives rise to a variety of 3-arylpropanamides in good yields. The mechanism has been investigated experimentally with labelled substrates, trapping experiments and spectroscopic measurements. The findings indicate a radical pathway where potassium tert-butoxide is believed to serve a dual role as both base and radical initiator. The radical anion of the benzylic alcohol is proposed as the key intermediate, which undergoes coupling with the enolate of the amide to form the new C–C bond. Subsequent elimination to the corresponding cinnamamide and olefin reduction then affords the 3-arylpropanamides.

Benzylic alcohols and acetamides are coupled into 3-arylpropanamides by a new radical condensation through the radical anion of the alcohol.

Zinc Oxide-Catalyzed Dehydrogenation of Primary Alcohols into Carboxylic Acids

Zinc oxide has been developed as a catalyst for the dehydrogenation of primary alcohols into carboxylic acids and hydrogen gas. The reaction is performed in mesitylene solution in the presence of potassium hydroxide, followed by workup with hydrochloric acid. The transformation can be applied to both benzylic and aliphatic primary alcohols and the catalytically active species was shown to be a homogeneous compound by a hot filtration test. Dialkylzinc and strongly basic zinc salts also catalyze the dehydrogenation with similar results. The mechanism is believed to involve the formation of a zinc alkoxide which degrades into the aldehyde and a zinc hydride. The latter reacts with the alcohol to form hydrogen gas and regenerate the zinc alkoxide. The degradation of a zinc alkoxide into the aldehyde upon heating was confirmed experimentally. The aldehyde can then undergo a Cannizzaro reaction or a Tishchenko reaction, which in the presence of hydroxide leads to the carboxylic acid.