Asymmetric Reductive Amination with Pinacolborane Catalyzed by Chiral SPINOL Borophosphates

The catalytic asymmetric reductive amination of ketones with pinacolborane employing chiral SPINOL-derived borophosphates as catalysts has been realized. A series of chiral amine derivatives bearing multiple functional groups were obtained in good to excellent yields and enantioselectivities (up to 97% yield, 98% ee) under mild reaction conditions. Moreover, the synthetic applicability of the established method has been demonstrated by the asymmetric synthesis of (R)-Fendiline.

Reductive Amination Revisited: Reduction of Aldimines with Trichlorosilane Catalyzed by Dimethylformamide─Functional Group Tolerance, Scope, and Limitations

Aldimines, generated in situ from aliphatic, aromatic, and heteroaromatic aldehydes and aliphatic, aromatic, and heteroaromatic primary or secondary amines, can be reduced with trichlorosilane in the presence of dimethylformamide (DMF) as an organocatalyst (≤10 mol %) in toluene or CH₂Cl₂ at room temperature. The reduction tolerates ketone carbonyls, esters, amides, nitriles, sulfones, sulfonamides, NO₂, SF₅, and CF₃ groups, boronic esters, azides, phosphine oxides, C═C and C≡C bonds, and ferrocenyl nucleus, but sulfoxides and N-oxides are reduced. α,β-Unsaturated aldimines undergo 1,2-reduction only, leaving the C═C bond intact. N-Monoalkylation of primary amines is attained with a 1:1 aldehyde to amine ratio, whereas excess of the aldehyde (≥2:1) allows second alkylation, giving rise to tertiary amines. Reductive N-alkylation of α-amino acids proceeds without racemization; the resulting products, containing a C≡C bond or N₃ group, are suitable for click chemistry. This reaction thus offers advantages over the traditional methods (borohydride reduction or catalytic hydrogenation) in terms of efficiency and chemoselectivity. Solubility of some of the reacting partners appears to be the only limitation. The byproducts generated by the workup with aqueous NaHCO₃ (i.e., NaCl and silica) are environmentally benign. As a greener alternative, DMA can be employed as a catalyst instead of DMF.

Highly Enantioselective Direct Asymmetric Reductive Amination of 2-Acetyl-6-Substituted Pyridines

A highly direct asymmetric reductive amination of a variety of ketone substrates, including 2-acetyl-6-substituted pyridines, β-keto esters, β-keto amides, and 1-(6-methylpyridin-2-yl)propan-2-one, has been disclosed for the first time (94.6% to >99.9% ee). With ammonium trifluoroacetate as the nitrogen source, various chiral corresponding primary amines were prepared in excellent enantioselectivity and conversion in the presence of a commercially available and inexpensive chiral catalyst, Ru(OAc)₂{(S)-binap}, under 0.8 MPa of hydrogen gas pressure.

A novel route to unsymmetrical disubstituted ureas and thioureas by HMPA catalyzed reductive alkylation with trichlorosilane

A HMPA catalyzed reductive alkylation of ureas and thioureas with trichlorosilane under mild reaction conditions has been developed.

Chiral Brønsted Acid-Catalyzed Metal-Free Asymmetric Direct Reductive Amination Using 1-Hydrosilatrane

The asymmetric direct reductive amination of prochiral ketones with aryl amines using 1-hydrosilatrane with a chiral Brønsted acid catalyst is reported. This is the first known example of chiral Brønsted acid-catalyzed asymmetric reductive amination using a silane as the hydride source. The reaction features a highly practical reducing reagent and proceeds efficiently at room temperature without a specialized reaction setup or equipment to exclude air or moisture. This method provides high conversion and enantiomeric excess up to 84% of the desired chiral secondary amines with minimal side products.

Synthesis of N-Substituted 3-Amino-4-halopyridines: A Sequential Boc-Removal/Reductive Amination Mediated by Brønsted and Lewis Acids

N-Substituted 3-amino-4-halopyridines are valuable synthetic intermediates, as they readily provide access to imidazopyridines and similar heterocyclic systems. The direct synthesis of N-substituted 3-amino-4-halopyridines is problematic, as reductive aminations and base-promoted alkylations are difficult in these systems. A high yielding deprotection/alkylation protocol mediated by trifluoroacetic acid and trimethylsilyl trifluoromethanesulfonate is described, providing access to a wide scope of N-substituted 3-amino-4-halopyridines. This protocol furnishes many reaction products in high purity without chromatography. Similar reductive amination conditions were also established for deactivated anilines.

Mechanistic investigations of the asymmetric hydrosilylation of ketimines with trichlorosilane reveals a dual activation model and an organocatalyst with enhanced efficiency

Mechanistic investigations of a catalyst used for the asymmetric hydrosilylation have revealed a ‘dual activation’ model and catalyst that maintains its performance at 0.01 mol% loading.

l-Valine derived chiral N-sulfinamides as effective organocatalysts for the asymmetric hydrosilylation of N-alkyl and N-aryl protected ketimines

l-Valine derived N-sulfinamides have been developed as efficient enantioselective Lewis basic organocatalysts for the asymmetric reduction of N-aryl and N-alkyl ketimines with trichlorosilane.

Reductive amination using a combination of CaH2 and noble metal

Amines were prepared by a reductive amination reaction in the presence of calcium hydride and Pt/C.

Novel and high affinity fluorescent ligands for the serotonin transporter based on (s)-citalopram

Novel rhodamine-labeled ligands, based on (S)-citalopram, were synthesized and evaluated for uptake inhibition at the human serotonin, dopamine, and norepinephrine transporters (hSERT, hDAT, and hNET, respectively) and for binding at SERT, in transiently transfected COS7 cells. Compound 14 demonstrated high affinity binding and selectivity for SERT (K i = 3 nM). Visualization of SERT, using confocal laser scanning microscopy, validated compound 14 as a novel tool for studying SERT expression and distribution in living cells.

Ethyl acetate as an acyl donor in the continuous flow kinetic resolution of (±)-1-phenylethylamine catalyzed by lipases

The synthesis of chiral amines is still a challenge for organic synthesis since optically pure amines are of great importance for the pharmaceutical and agrochemical industries. Among all the methodologies developed until now, chemoenzymatic dynamic kinetic resolution has proven to be useful for the preparation of enantioenriched primary chiral amines. In our continuous efforts toward the development of a continuous flow process, herein we report our results on the continuous flow kinetic resolution of (±)-1-phenylethylamine leading to the desired products with high enantiomeric ratios (>200) and short residence times (40 minutes) using ethyl acetate as the acyl donor.

Ammonium formate as a green hydrogen source for clean semi-continuous enzymatic dynamic kinetic resolution of (+/−)-α-methylbenzylamine

The chemoenzymatic dynamic kinetic resolution of (+/−)-α-methylbenzylamine under continuous flow conditions in the presence of Pd/BaSO₄as racemization catalyst and ammonium formate as reductant is described.

Enantioselective reduction of ketoimines promoted by easily available (S)-proline derivatives

The behavior of readily synthesized and even commercially available (S)-proline derivatives, was studied in the trichlorosilane-mediated reduction of ketoimines. A small library of structurally and electronically modified chiral Lewis bases was considered; such compounds were shown to promote the enantioselective reduction of different substrates in good chemical yields. In the HSiCl₃ addition to the model substrate N-phenylacetophenone imine, the organocatalyst of choice led to the formation of the corresponding amine with good stereoselectivity, up to 75% ee. Theoretical studies were also performed in order to elucidate the origin of the stereoselection.

Highly functionalized 1,2-diamino compounds through reductive amination of amino acid-derived β-keto esters

1,2-Diamine derivatives are valuable building blocks to heterocyclic compounds and important precursors of biologically relevant compounds. In this respect, amino acid-derived β–keto esters are a suitable starting point for the synthesis of β,γ–diamino ester derivatives through a two-step reductive amination procedure with either simple amines or α–amino esters. AcOH and NaBH₃CN are the additive and reducing agents of choice. The stereoselectivity of the reaction is still an issue, due to the slow imine-enamine equilibria through which the reaction occurs, affording mixtures of diastereoisomers that can be chromatographically separated. Transformation of the β,γ–diamino esters into pyrrolidinone derivatives allows the configuration assignment of the linear compounds, and constitutes an example of their potential application in the generation of molecular diversity.

Asymmetric reductive amination

Asymmetric reductive amination (ARA) affords synthetically valuable chiral amines straightforwardly. This chapter reviews the recent advances made in the area, focusing on ARA by hydrogenation, transfer hydrogenation, organocatalytic reduction, and biocatalytic reduction.