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Relevant academic research and scientific papers

Convenient and Reusable Manganese-Based Nanocatalyst for Amination of Alcohols

The development of new sustainable nanocatalytic systems for green chemical synthesis is a growing area in chemical science. Herein, a reusable heterogeneous N-doped graphene-based manganese nanocatalyst (Mn@NrGO) for selective N-alkylation of amines with alcohols is described. Mechanistic studies illustrate that the catalytic reaction follows a domino dehydrogenation-condensation-hydrogenation sequence of alcohols and amines with the formation of water as the sole by-product. The scope of the reaction is extended to the synthesis of pharmaceutically important N-alkylated amine intermediates. The heterogeneous nature of the catalyst made it easy to separate for long-term performance, and the recycling study revealed that the catalyst was robust and retained its activity after several recycling experiments.

Mechanistic Studies of Hydride Transfer to Imines from a Highly Active and Chemoselective Manganate Catalyst

We introduce a highly active and chemoselective manganese catalyst for the hydrogenation of imines. The catalyst has a large scope, can reduce aldimines and ketimines, and tolerates a variety of functional groups, among them hydrogenation sensitive examples such as an olefin, a ketone, nitriles, nitro groups, and an aryl iodo substituent or a benzyl ether. We could investigate the transfer step between imines and the hydride complex in detail. We found that double deprotonation of the ligand is essential and excess base does not lead to a higher rate in the transfer step. We identified the actual hydrogenation catalyst as a K-Mn-bimetallic species and could obtain a structure of the K-Mn complex formed after hydride transfer by X-ray analysis. NMR experiments indicate that the hydride transfer is a well-defined reaction, which is first order in imine, first order in the bimetallic (K-Mn) hydride, and independent in rate from the concentration of the potassium base. We propose an outer-sphere mechanism in which protons do not seem to be involved in the rate-determining step, leading to a transiently negatively charged nitrogen atom in the substrate which reacts rapidly with HOtBu (2-methylpropan-2-ol) to produce the amine. This is based on several observations, such as no dependency of the reaction rate on the HOtBu concentration, no observable manganese amide complex, and a high reaction constant in a conducted Hammett study. Furthermore, hydrogen transfer of the catalytic cycle was experimentally probed and monitored by NMR with subsequent quantitative regeneration of the catalyst by H2.

Manganese catalyzed N-alkylation of anilines with alcohols: Ligand enabled selectivity

Ligand enabled Earth-abundant manganese catalyzed N-alkylation of amines with alcohols via a hydrogen auto-transfer strategy is reported. The choice of the ligand plays a significant role in the alcohol reactivity (aliphatic or aromatic) toward N-alkylation reactions.

Bifunctional Br?nsted Base Catalyzed Mannich Reaction of β-Alkoxy α-Keto Amides: Stereocontrolled Entry to Functionalized Amino Diols

The potential of β-alkoxy α-keto amides as pronucleophiles in the enantioselective Mannich type reaction with p-nosyl imines is presented. The proper combination of β-alkoxy α-keto amides and a squaramide-based Br?nsted base catalyst produced highly enantioenriched Mannich adducts, which may be transformed into functionalized amino diols.

B(C6F5)3-Catalyzed Reductive Amination using Hydrosilanes

In contrast to the established dogma that B(C6F5)3 is irreversibly poisoned by excess H2O/amine (or imine) bases, B(C6F5)3 is actually a water-tolerant catalyst for the reductive amination of primary and secondary arylamines with aldehydes and ketones in "wet solvents" at raised temperatures and using only 1.2 equiv of Me2PhSiH as reductant. Arylamines/N-arylimines do not result in the irreversible deprotonation of H2O-B(C6F5)3, allowing sufficient B(C6F5)3 to be evolved at raised temperatures to effect catalytic reductions. Stronger Br?nsted basic amines such as tBuNH2 (and derived imines) result in irreversible formation of [HO-B(C6F5)3]- from H2O-B(C6F5)3, precluding the formation of B(C6F5)3 at raised temperatures and thus preventing any imine reduction. A substrate scope exploration using 1 mol % nonpurified B(C6F5)3 and "wet solvents" demonstrates that this is an operationally simple and effective methodology for the production of secondary and tertiary arylamines in high yield, with imine reduction proceeding in preference to other possible reactions catalyzed by B(C6F5)3, including the dehydrosilylation of H2O and the reduction of carbonyl moieties (e.g., esters).

Synthesis and catalytic activity of μ-oxo ruthenium(IV) porphyrin species to promote amination reactions

This work describes the synthesis of ruthenium(IV) m-oxo porphyrin complexes of general formula [RuIV(TPP)(X)]2O which have been applied as catalysts in nitrene transfer reactions using aryl azides (ArN3) as nitrene sources. Collected data indicated that the catalytic efficiency of [RuIV(TPP)(OCH3)]2O was comparable to that of RuII(TPP)CO because of their analogous reactivity towards aryl azides to give the same catalytically active bis-imido species RuVI(TPP)(ArN)2. The reaction of [RuIV(TPP)(OCH3)]2O with Ph3CN3 or (CH3)3SiN3 afforded [RuIV(TPP)(N3)]2O which was fully characterised, its molecular structure was also determined by single crystal X-ray analysis.

Glycoporphyrin Catalysts for Efficient C-H Bond Aminations by Organic Azides

We report herein the synthesis of new glycoporphyrin ligands which bear a glucopyranoside derivative on each meso-aryl moiety of the porphyrin skeleton. The saccharide unit is directly conjugated to the porphyrin or a triazole spacer is placed between the carbohydrate and porphyrin ring. The obtained glycoporphyrin ligands were employed to synthesize cobalt(II), ruthenium(II), and iron(III) complexes which were tested as catalysts of C-H bond aminations by organic azides. Two of the synthesized complexes were very efficient in promoting catalytic reactions, and the results achieved indicated that ruthenium and iron complexes show an interesting complementary catalytic activity in several amination reactions. The eco-friendly iron catalyst displayed very good chemical stability in catalyzing the amination reaction for three consecutive runs without losing catalytic activity. (Chemical Equation Presented).

Comparative Study of the Catalytic Amination of Benzylic C-H Bonds Promoted by Ru(TPP)(py)2 and Ru(TPP)(CO)

A combined experimental and DFT-based theoretical analysis elucidated the influence of the axial ligand L on the catalytic activity of Ru(porphyrin)L complexes in promoting the amination of benzylic C-H bonds by organic azides (RN3). Experimental data indicated that the catalytic activity of Ru(TPP)(CO) (1) (TPP = dianion of tetraphenylporphyrin) is comparable to that of Ru(TPP)(py)2 (2) (py = pyridine). DFT modelling disclosed that 2 can be regarded as a precatalyst that becomes active after the endergonic loss of one pyridine ligand to give the unsaturated species [Ru](py) (11) {[Ru] = Ru(porphine)}. This complex would react with RN3 to give the mono-imido singlet complex [Ru](py)(NR)S (6S), which can be easily transformed into the triplet isomer 6T having diradical character at the imido N atom. The subsequent formation of the benzylic amine PhCH2NHR occurs through a radical homolytic activation of one C-H bond of the toluene substrate (PhCH3). Conversely, by staying on the singlet potential-energy surface, 6S can undergo dissociation of the pyridine ligand to form [Ru](NR). This complex can activate another RN3 molecule to form the bis-imido compound [Ru](NR)2, which is also catalytically active. At this point, the mechanism becomes independent of the nature of the original ligand L coordinated to [Ru]. The catalytic activity of Ru(TPP)(py)2 (2) (TPP = dianion of tetraphenylporphyrin, py = pyridine) in the amination of C-H bonds by organic azides (RN3) was experimentally compared to that of Ru(TPP)(CO) (1). A computational analysis corroborates similar radical mechanisms with a triplet imido intermediate, in spite of the electronic differences of the apical ligands.

Direct reductive amination using triethylsilane and catalytic bismuth(III) chloride

Direct reductive amination (DRA) using triethylsilane (TESH) and catalytic bismuth(III) chloride (BiCl3) is described for the first time. The use of TESH and BiCl3 provides easy handling, low cost, non-toxicity, and a mild Lewis acid activity, thereby meeting the demand for green and sustainable chemistry. The developed DRA is highly chemoselective and applicable to less-basic amines. The experimental results of this study revealed that the developed DRA could be catalyzed by BiCl3, which was gradually reduced to Bi(0) or bismuth with a low valency by TESH, but TESCl, Bi(0), and Bi(0) with TESCl catalyzed the DRA to some extent.

Impregnated palladium on magnetite as catalyst for multicomponent reductive amination reactions and other related reducing processes

The impregnated palladium on magnetite catalyst is a versatile system for different reduction processes using inexpensive polymehtylhydrosiloxane, including multicomponent reductive amination reactions, and aldehyde, imine, sulfinimide and sulfoxide reductions. This catalyst avoids the use of any type of expensive and quite expensive organic ligand, showing excellent yields, under mild reaction conditions. The catalyst is easily removed from the reaction medium, just by using a magnet. The catalytic system is very selective permitting the discrimination between ketones and aldehydes in the reductive amination process.