An Efficient Homogenized Ruthenium(II) Pincer Complex for N -Monoalkylation of Amines with Alcohols

Iron(II) complexes of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine and related ligands with annelated distal heterocyclic donors

Following a published synthesis of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine (L1), treatment of α,α'-dibromo-2,6-diacetylpyridine with 2 equiv. 2-aminopyrimidine or 2-aminoquinoline in refluxing acetonitrile respectively gives 2,6-bis(imidazo[1,2-a]pyrimidin-2-yl)pyridine (L2) and 2,6-bis(imidazo[1,2-a]quinolin-2-yl)pyridine (L3). Solvated crystals of [Fe(L1)2][BF4]2 (1[BF4]2) and [Fe(L2)2][BF4]2 (2[BF4]2) are mostly high-spin, although one solvate of 1[BF4]2 undergoes thermal spin-crossover on cooling. The iron coordination geometry is consistently distorted in crystals of 2[BF4]2 which may reflect the influence of intramolecular, inter-ligand N⋯π interactions on the molecular conformation. Only 1 : 1 Fe : L3 complexes were observed in solution, or isolated in the solid state; a crystal structure of [FeBr(py)2L3]Br·0.5H2O (py = pyridine) is presented. A solvate crystal structure of high-spin [Fe(L4)2][BF4]2 (L4 = 2,6-di{quinolin-2-yl}pyridine; 4[BF4]2) is also described, which exhibits a highly distorted six-coordinate geometry with a helical ligand conformation. The iron(II) complexes are high-spin in solution at room temperature, but 1[BF4]2 and 2[BF4]2 undergo thermal spin-crossover equilibria on cooling. All the compounds exhibit a ligand-based emission in solution at room temperature. Gas phase DFT calculations mostly reproduce the spin state properties of the complexes, but show small anomalies attributed to intramolecular, inter-ligand dispersion interactions in the sterically crowded molecules.

N-Alkylation and N-Methylation of Amines with Alcohols Catalyzed by Nitrile-Substituted NHC-Ir(III) and NHC-Ru(II) Complexes

A series of nitrile-modified N-heterocyclic carbene (NHC) complexes of Ir(III) (2a-e) and Ru(II) (3a-d) have been prepared by transmetallation of [IrCp*Cl2]2 and [RuCl2(p-cymene)]2 forming an in situ NHC-Ag complex. The structures of all complexes were characterized by 1H NMR, 13C NMR, and Fourier transform infrared (FT-IR) spectroscopies. And the structures were clearly elucidated by performing X-ray diffraction studies on 2b, 3a, and 3c single crystals. The complexes of NHC-Ir(III) (2a-e) and NHC-Ru(II) (3a-d) were investigated in the N-alkylation reaction of aniline derivatives with benzyl alcohols to form N-benzyl amines and in the N-methylation reaction of aniline derivatives with methanol. Both reactions were performed in solvent-free media. The Ir(III) complexes (2a-e) were found to perform essentially better than similar Ru(II) complexes (3a-d) in the N-alkylation and N-methylation reactions. Among the Ir(III) complexes (2a-e), the best results were obtained with 2b. The catalytic mechanisms of both reactions were revealed by 1H NMR study. Formation of Ir-hydride species was observed for both reactions. This new report provides useful information to evaluate the activity of complexes and the differences in sensitivity between the NHCs.

N-Heterocyclic Carbene-Supported Nickel-Catalyzed Selective (Un)Symmetrical N-Alkylation of Aromatic Diamines with Alcohols

The "borrowing hydrogen" (BH) approach for the N-alkylation of phenylenediamines using alcohols as coupling partners is highly challenging due to the selectivity issue of the generated products. Furthermore, the development of base-metal systems that can potentially substitute precious metals with competitive activity is a major challenge in BH catalysis. We present herein an efficient protocol for the N,N'-di-alkylation of aromatic diamines using an in situ-generated Ni-NHC complex from NiCl₂ and the ligand L1, which gave access to a wide range of N,N'-di-alkylated orthophenylene diamines (rather than the generally observed benzimidazole derivatives), meta- and para-phenylene diamines along with 2,6-diamino pyridine derivatives in good to excellent yields. Moreover, the catalyst system was also successful in the derivatization of a clinically important drug molecule, Dapsone. Notably, the present protocol could be applied effectively to synthesize unsymmetrically substituted N,N'-di-alkylated diamines via sequential alkylation and is the first report in the base-metal system to the best of our knowledge. Diverse control experiments including the deuterium incorporation studies suggest that the present protocol proceeds via a BH sequence.

Solvent/metal-free benzimidazolium-based carboxyl-functionalized porphyrin photocatalysts for the room-temperature alkylation of amines under the irradiation of visible light

The improvement of novel sustainable catalytic methods for green chemical production is an emergent area in chemical science.

Coordinatively Unsaturated Hf-MOF-808 Prepared via Hydrothermal Synthesis as a Bifunctional Catalyst for the Tandem N-Alkylation of Amines with Benzyl Alcohol

The modulated hydrothermal (MHT) synthesis of an active and selective Hf-MOF-808 material for the N-alkylation reaction of aniline with benzyl alcohol under base-free mild reaction conditions is reported. Through kinetic experiments and isotopically labeled NMR spectroscopy studies, we have demonstrated that the reaction mechanism occurs via borrowing hydrogen (BH) pathway, in which the alcohol dehydrogenation is the limiting step. The high concentration of defective -OH groups generated on the metallic nodes through MHT synthesis enhances the alcohol activation, while the unsaturated Hf⁴⁺, which acts as a Lewis acid site, is able to borrow the hydrogen from the methylene position of benzyl alcohol. This fact makes this material at least 14 times more active for the N-alkylation reaction than the material obtained via solvothermal synthesis. The methodology described in this work could be applied to a wide range of aniline and benzyl alcohol derivates, showing in all cases high selectivity toward the corresponding N-benzylaniline product. Finally, Hf-MOF-808, which acts as a true heterogeneous catalyst, can be reused in at least four consecutive runs without any activity loss.

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.

Designed pincer ligand supported Co(II)-based catalysts for dehydrogenative activation of alcohols: Studies on N-alkylation of amines, α-alkylation of ketones and synthesis of quinolines

Base-metal catalysts Co1, Co2 and Co3 were synthesized from designed pincer ligands L1, L2 and L3 having NNN donor atoms respectively. Co1, Co2 and Co3 were characterized by IR, UV-Vis. and ESI-MS spectroscopic studies. Single crystal X-ray diffraction studies were investigated to authenticate the molecular structures of Co1 and Co3. Catalysts Co1, Co2 and Co3 were utilized to study the dehydrogenative activation of alcohols for N-alkylation of amines, α-alkylation of ketones and synthesis of quinolines. Under optimized reaction conditions, a broad range of substrates including alcohols, anilines and ketones were exploited. A series of control experiments for N-alkylation of amines, α-alkylation of ketones and synthesis of quinolines were examined to understand the reaction pathway. ESI-MS spectral studies were investigated to characterize cobalt-alkoxide and cobalt-hydride intermediates. Reduction of styrene by evolved hydrogen gas during the reaction was investigated to authenticate the dehydrogenative nature of the catalysts. Probable reaction pathways were proposed for N-alkylation of amines, α-alkylation of ketones and synthesis of quinolines on the basis of control experiments and detection of reaction intermediates.

NaOH-Mediated Direct Synthesis of Quinoxalines from o-Nitroanilines and Alcohols via a Hydrogen-Transfer Strategy

A NaOH-mediated sustainable synthesis of functionalized quinoxalines is disclosed via redox condensation of o-nitroamines with diols and α-hydroxy ketones. Under optimized conditions, various o-nitroamines and alcohols are well tolerated to generate the desired products in 44-99% yields without transition metals and external redox additives.

Biorenewable carbon-supported Ru catalyst for N-alkylation of amines with alcohols and selective hydrogenation of nitroarenes

A renewable carbon-supported Ru catalyst (Ru/PNC-700) facilely prepared via simple impregnation followed by the pyrolysis process for N-alkylation of anilines with benzyl alcohol and chemoselective hydrogenation of nitroarenes.

Empowering alcohols as carbonyl surrogates for Grignard-type reactions

The Grignard reaction is a fundamental tool for constructing C-C bonds. Although it is widely used in synthetic chemistry, it is normally applied in early stage functionalizations owing to poor functional group tolerance and less availability of carbonyls at late stages of molecular modifications. Herein, we report a Grignard-type reaction with alcohols as carbonyl surrogates by using a ruthenium(II) PNP-pincer complex as catalyst. This transformation proceeds via a carbonyl intermediate generated in situ from the dehydrogenation of alcohols, which is followed by a Grignard-type reaction with a hydrazone carbanion to form a C-C bond. The reaction conditions are mild and can tolerate a broad range of substrates. Moreover, no oxidant is involved during the entire transformation, with only H₂ and N₂ being generated as byproducts. This reaction opens up a new avenue for Grignard-type reactions by enabling the use of naturally abundant alcohols as starting materials without the need for pre-synthesizing carbonyls.

Alcohols are more naturally abundant than carbonyl compounds, which in turn are well known for their reactivity in Grignard reactions. Here, the authors showcase a distinct Grignard-like reactivity by using alcohols as coupling partners with hydrazones and synthesize more complex alcohols under ruthenium catalysis.

A Microwave-Assisted SmI2-Catalyzed Direct N-Alkylation of Anilines with Alcohols

A new protocol for the alkylation of aromatic amines has been described using alcohols in the presence of SmI₂ as a catalyst with the generation of water as the sole byproduct. The reaction proceeds under MW conditions and selectively generates monoalkylated amines. This protocol features a broad substrate scope and good functional-group tolerance with moderate to high yields.

Well-Defined Phosphine-Free Iron-Catalyzed N-Ethylation and N-Methylation of Amines with Ethanol and Methanol

An iron(0) complex bearing a cyclopentadienone ligand catalyzed N-methylation and N-ethylation of aryl and aliphatic amines with methanol or ethanol in mild and basic conditions through a hydrogen autotransfer borrowing process is reported. A broad range of aromatic and aliphatic amines underwent mono- or dimethylation in high yields. DFT calculations suggest molecular hydrogen acts not only as a reducing agent but also as an additive to displace thermodynamic equilibria.

Mechanistic studies on the N-alkylation of amines with alcohols catalysed by iridium(i) complexes with functionalised N-heterocyclic carbene ligands

Experimental and theoretical studies give support for an iridium-catalyzed C–N bond formation.

An efficient and atom-economical route to N-aryl amino alcohols from primary amines

In this paper we reported a novel method for generation of N-aryl amino alcohols from N,N-disubstituted picolinamides through reduction/ring-opening reaction with NaBH₄. The N,N-disubstituted picolinamides can be easily obtained from primary amines after convenient condensation with picolinic acid and coupling with cyclic ethers. The whole route proceeded under simple and mild conditions with high efficiency. Picolinic acid can be recovered in the form of piconol after reaction. It indicated an efficient and atom-economical route for the preparation of N-aryl amino alcohols from primary amines.

A novel method for generation of N-aryl amino alcohols from N,N-disubstituted picolinamides through reduction/ring-opening reaction with NaBH₄ was developed.

Bimetallic Cobalt-Rhodium Nanoparticle-Catalyzed Reductive Amination of Aldehydes with Nitroarenes Under Atmospheric Hydrogen

A cobalt-rhodium heterobimetallic nanoparticle (Co₂Rh₂/C)-catalyzed tandem reductive amination of aldehydes with nitroaromatics to sec-amines has been developed. The tandem reaction proceeds without any additives under mild conditions (1 atm H₂ and 25 °C). This procedure can be scaled up to the gram scale, and the catalyst can be reused more than six times without loss of activity.