Reductive Amination of Aldehydes and Ketones with Sodium Triacetoxyborohydride. Studies on Direct and Indirect Reductive Amination Procedures(1)

Sodium triacetoxyborohydride is presented as a general reducing agent for the reductive amination of aldehydes and ketones. Procedures for using this mild and selective reagent have been developed for a wide variety of substrates. The scope of the reaction includes aliphatic acyclic and cyclic ketones, aliphatic and aromatic aldehydes, and primary and secondary amines including a variety of weakly basic and nonbasic amines. Limitations include reactions with aromatic and unsaturated ketones and some sterically hindered ketones and amines. 1,2-Dichloroethane (DCE) is the preferred reaction solvent, but reactions can also be carried out in tetrahydrofuran (THF) and occasionally in acetonitrile. Acetic acid may be used as catalyst with ketone reactions, but it is generally not needed with aldehydes. The procedure is carried out effectively in the presence of acid sensitive functional groups such as acetals and ketals; it can also be carried out in the presence of reducible functional groups such as C-C multiple bonds and cyano and nitro groups. Reactions are generally faster in DCE than in THF, and in both solvents, reactions are faster in the presence of AcOH. In comparison with other reductive amination procedures such as NaBH(3)CN/MeOH, borane-pyridine, and catalytic hydrogenation, NaBH(OAc)(3) gave consistently higher yields and fewer side products. In the reductive amination of some aldehydes with primary amines where dialkylation is a problem we adopted a stepwise procedure involving imine formation in MeOH followed by reduction with NaBH(4).     

Building addressable libraries: spatially isolated, chip-based reductive amination reactions

A Pd(II) reagent has been generated at preselected sites on an electrochemically addressable chip and used to effect the oxidation of the neighboring alcohols on the polymer coating the chip's surface. The resulting carbonyls were then used to accomplish site-selective reductive amination reactions on the chips. The work demonstrates that the confinement strategy developed for spatially isolated Wacker oxidations to specific sites on the chips is general and can be used for other Pd(II)-based reactions.     

N-(2-formyl-1-methylimidazol-4-yl)-2,2-dimethylpropanamide: a versatile reagent for preparing imidazole-amine ligands with variable second-coordination spheres

Two synthetic pathways to N-(2-formyl-1-methylimidazol-4-yl)-2,2-dimethylpropanamide from 1-methyl-2-carboxaldehyde are described. The reagent serves as a useful synthon for reductive amination reactions with primary and secondary amines in the presence of sodium cyanoborohydride to yield a series of ligands with second coordination sphere functional groups. Protocols for the syntheses of related imidazole synthons functionalized in the 4-position with amino acids, Schiff bases, and other amides are also reported.     

Reductive amination agents: comparison of Na(CN)BH3 and Si-CBH

Reductive amination is a chemical reaction commonly employed by organic chemists in academics and the pharmaceutical industry. In this reaction a carbonyl group is converted to an amine via an imine intermediate, the formation of which is rate limiting. A major reagent necessary for the completion of this reaction is a hydride source, commonly sodium cyanoborohydride (Na(CN)BH₃). The objective of this research was to compare the efficacy of Na(CN)BH₃ with silica-bound cyanoborohydride (Si-CBH) as hydride sources in reductive amination reactions. Work has shown that reactions employing Si-CBH as a hydride source showed significant improvement, exhibiting an average percent conversion 25% greater than reactions using Na(CN)BH₃.     

Reductive Amination by Photoredox Catalysis and Polarity‐Matched Hydrogen Atom Transfer

The excitation of a Ru^II photosensitizer in the presence of ascorbic acid leads to the reduction of iminium ions to electron‐rich α‐aminoalkyl radical intermediates, which are rapidly converted into reductive amination products by thiol‐mediated hydrogen atom transfer (HAT). As a result, the reductive amination of carbonyl compounds with amines by photoredox catalysis proceeds in good to excellent yields and with broad substrate scope and good functional group tolerance. The three key features of this work are 1) the rapid interception of electron‐rich α‐aminoalkyl radical intermediates by polarity‐matched HAT in a photoredox reaction, 2) the method of reductive amination by photoredox catalysis itself, and 3) the application of this new method for temporally and spatially controlled reactions on a solid support, as demonstrated by the attachment of a fluorescent dye on an activated cellulose support by photoredox‐catalyzed reductive amination.     

A highly stereoselective reductive amination of 3-ketosteroid with amines: an improved synthesis of 3α-aminosteroid

A highly stereoselective reductive amination method was developed based on in situ generated sodium acyloxyborohydride and was successfully applied to a steroidal skeleton with various amine sources. High yield (96%) and a de of up to 95% were achieved by modifying the reducing reagent. The effect of this reagent will be discussed.     

N-Alkylation of Amino Acid Esters Using Sodium Triacetoxyborohydride

N-Alkylation of amino acid esters by reductive amination using sodium cyanoborohydride may often be problematic. Sodium triacetoxyborohydride was found to be a convenient reagent for this transformation. This methodology was employed to alkylate various amino acid methyl esters.     

Direct Reductive Amination of Aldehydes and Ketones with 2,4-Ionene-Based Borohydride Exchange Resin as a Novel Polymer-Supported Reducing Agent

Direct reductive amination of aldehydes and ketones were accomplished efficiently using high-capacity, ionene-based, polymer-supported borohydride reagent in isopropyl alcohol at reflux under neutral conditions. The reagent is easily prepared by mixing aqueous solution of 2,4-ionene bromide with an alkaline solution of sodium borohydride at room temperature. The generality of reaction was established using both aliphatic and aromatic aldehydes, ketones, and amines.     

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.     

ZrCl4/Hantzsch 1,4-dihydropyridine as a new and efficient reagent combination for the direct reductive amination of aldehydes and ketones with weakly basic amines

ZrCl4/Hantzsch 1,4-dihydropyridine is a mild and highly efficient reagent combination for the direct reductive amination.Weakly basic amines such as anilines substituted by electron-withdrawing group and heteroaromatic amines can be reductivelyalkylated with electron rich aldehydes and ketones under mild conditions to form the secondary amines in excellent yields.     

Direct reductive amination of carbonyl compounds using bis(triphenylphosphine) copper(I) tetrahydroborate

A direct reductive amination protocol for aldehydes/ketones using bis(triphenylphosphine) copper(I) tetrahydroborate as a novel reducing agent in the presence of sulfamic acid has been developed. The reagent chemoselectively reduces the imine moiety and does not affect other reducible functionalities such as chloro, nitro, cyano and methoxy.     

A stereoselective route to hydroxyethylamine dipeptide isosteres

An efficient synthesis of stereodefined hydroxyethylamine dipeptide isosteres has been developed, utilizing a syn-selective Grignard addition and reductive amination as the key reactions.     

Ruthenium-Catalyzed Direct Reductive Amination of Carbonyl Compounds for the Synthesis of Amines: An Overview

Reductive amination is a valuable method for amine synthesis that has been the topic of a century‘s worth of in-depth study in both academia and industry. Amines and their derivatives serve as incredibly adaptable building blocks for a broad array of organic substrates and are significant precursors for a myriad of advanced chemicals, physiologically active compounds, agrochemicals, biomolecules, pharmaceuticals, and polymers. The creation of innovative catalytic processes for the long-term and selective synthesis of amines from readily accessible and environmentally benign reagents remains a top priority in chemical research. Both heterogeneous and homogeneous catalysts have been designed with success to enable these reactions to explore new amines. Ruthenium catalysts are employed in reductive amination owing to their stability, selectivity, versatility, low toxicity, and high efficiency. This review comprehensively overviews the Ru-catalyzed reductive amination processes and includes the literature from 2009 to 2022.     

A sort synthesis of polyhydroxylated pyrrolizidines via sequential 1,3-dipolar cycloaddition and reductive amination

Polyhydroxylated pyrrolizidines bearing a methyl group at C-5 have been synthesized by 1,3-dipolar cycloaddition of five membered cyclic nitrones with methyl vinyl ketone followed by a N–O reductive cleavage and in situ intramolecular reductive amination. The stereochemistry of the obtained compounds is examined in relation to the reactions mechanism.     

Biomimetic reductive amination under the continuous-flow reaction conditions

This study present a full account of continuous-flow reaction conditions for biomimetic reductive amination of fluorinated carbonyl compounds to corresponding amines and amino acids of biomedical importance. We demonstrate that simple silica-adsorbed DBU can be used as efficient catalysts for on-column 1,3-proton shift reaction, a key transformation in the biomimetic reductive amination process. This new on-column process features operationally convenient conditions, higher chemical yields, enantioselectivity and purity of the corresponding products as compared with traditional in-flask reactions. Moreover the removal of base-catalyst, the most delicate problem of the in-flask reactions, is not an issue in the on-column process, as the silica-adsorbed DBU or polymer-bound guanidine remains on the column and can be reused. This feature renders the overall process substantially more economical and synthetically efficient, in particular, for large-scale synthesis of the corresponding fluorinated amines and amino acids target.     

An air-stable palladium/N-heterocyclic carbene complex and its reactivity in aryl amination

[reaction: see text] The synthesis and characterization of [Pd(IPr)Cl(2)](2) (1), an air- and moisture-stable complex, is reported. The utilization of 1 as a catalyst for amination of aryl chlorides and bromides with a variety of amine coupling partners under mild conditions is described. The amination reactions with 1 show a remarkable insensitivity to oxygen and water, and thus the amination reactions could be performed in air on the benchtop with undried reagent grade solvents and substrates with small effects on reaction times and conversions.     

Fast Reductive Amination by Transfer Hydrogenation “on Water”

Reductive amination of various ketones and aldehydes by transfer hydrogenation under aqueous conditions has been developed, by using cyclometallated iridium complexes as catalysts and formate as hydrogen source. The pH value of the solution is shown to be critical for a high catalytic chemoselectivity and activity, with the best pH value being 4.8. In comparison with that in organic solvents, the reductive amination in an aqueous phase is faster, and the molar ratio of the substrate to the catalyst (S/C) can be set as high as 1×10⁵, the highest S/C value ever reported in reductive amination reactions. The catalyst is easy to access and the reaction is operationally simple, allowing a wide range of ketones and aldehydes to react with various amines in high yields. The protocol provides a practical and environmental friendly new method for the synthesis of amine compounds.     

Enantioselective organocatalytic reductive amination

The first enantioselective organocatalytic reductive amination reaction has been accomplished. The development of a new chiral phosphoric acid catalyst has provided a convenient strategy for the enantioselective construction of protected primary amines and provided a highly stereoselective method for the reductive amination of heterocyclic amines. A diverse spectrum of ketone and amine substrates can be accommodated in high yield and excellent enantioselectivity. This new protocol realizes a key benefit of reductive amination versus imine reduction, in that ketimines derived from alkyl-alkyl ketones are unstable to isolation, a fundamental limitation that is comprehensively bypassed using this direct organocatalytic reductive amination.     

Enzymatic asymmetric synthesis of the silodosin amine intermediate

Various enantiocomplementary ω-transaminases (ωTAs) were investigated in kinetic resolution and asymmetric reductive amination reactions to prepare silodosin amine. Whilst the enzymatic kinetic resolution gave moderate to good results with respect to the yield and enantioselectivity, the asymmetric reductive amination proved to be superior. The best results were obtained with the ωTA originating from (R)-Arthrobacter sp. which afforded the desired bioactive (R)-enantiomer in enantiomerically pure form (ee >97%) at excellent conversion (conv. >97%) under mild and benign reaction conditions.     

TiCl(OPr)3 and NaBH(OAc)3: an efficient reagent combination for the reductive amination of aldehydes by electron-deficient amines

Sodium triacetoxyborohydride, NaBH(OAc)₃ with tri-isopropoxytitanium chloride, TiCl(OⁱPr)₃ is a useful reagent combination for reductive amination. Electron-deficient amines and heteroaromatic amines such as 2-aminopyrimidine and 2-aminothiazole can be reductively alkylated at room temperature to afford the corresponding secondary amines in good yields.