First Highly Enantioselective Synthesis of Benzodiazepinones by Catalytic Hydrogenation

N-Triflylphosphoramides: highly acidic catalysts for asymmetric transformations

N-Triflylphosphoramides (NTPA), have become increasingly popular catalysts in the development of enantioselective transformations as they are stronger Brønsted acids than the corresponding phosphoric acids (PA). Their highly acidic, asymmetric active site can activate difficult, unreactive substrates. In this review, we present an account of asymmetric transformations using this type of catalyst that have been reported in the past ten years and we classify these reactions using the enantio-determining step as the key criterion. This compendium of NTPA-catalysed reactions is organised into the following categories: (1) cycloadditions, (2) electrocyclisations, polyene and related cyclisations, (3) addition reactions to imines, (4) electrophilic aromatic substitutions, (5) addition reactions to carbocations, (6) aldol and related reactions, (7) addition reactions to double bonds, and (8) rearrangements and desymmetrisations. We highlight the use of NTPA in total synthesis and suggest mnemonics which account for their enantioselectivity.

Synthesis of Benzodiazepines Through Ring Opening/Ring Closure of Benzimidazole Salts

Pyrido-benzodiazepine derivatives are undoubtedly one of the most important structural motifs in the marketed drugs and the drug candidates. Commonly synthetic methods for construction of the benzodiazepine ring derivatives are based on the condensation reactions of two highly functionalized synthons. The development of synthesis for these compounds, however, is hampered by the regioselectivity and atom economy. In this work, a one-step synthesis of pyrido-benzodiazepine backbones and its analogues is achieved through continuous ring-opening hydrolysis of benzimidazole salts and intramolecular C-H bond activation. The reaction mechanism is explored by control experiments and density functional theory (DFT) calculations.

Direct and Enantioselective Synthesis of N-H-Free 1,5-Benzodiazepin-2-ones by an N-Heterocyclic Carbene Catalyzed [3+4] Annulation Reaction

An NHC-catalyzed formal [3+4] annulation of α,β-unsaturated acylazoliums with protecting-group-free aryl 1,2-diamines was developed for a direct and highly enantioselective synthesis of 4-aryl N-H-free 1,5-benzodiazepin-2-ones. This methodology offers an efficient and rapid access to a wide range of enantioenriched target compounds from easily accessible starting materials. The protocol is also scalable and the desired products can easily undergo subsequent N-functionalization to afford diverse N-substituted derivatives. Additionally, a mechanism was proposed to explain the high enantioselectivity in this process.

Recent advances in organocatalytic enantioselective transfer hydrogenation

Robust, environmentally friendly reductants enable highly enantioselective reactions in the presence of chiral catalysts.

Ru-Catalyzed asymmetric transfer hydrogenation of substituted dibenzo[b,f][1,4]oxazepines in water

A phosphine free, additive free catalytic ATH protocol of dibenzo[b,f][1,4]oxazepine compounds using an (R,R)-Ru-Ts-DPEN complex was developed for the first time using HCOOH–HCOONa as a green hydrogen source.

Metal-free oxidative ring contraction of benzodiazepinones: an entry to quinoxalinones

Herein is reported a practical method for the construction of 3-benzoylquinoxalinones from benzodiazepinones in the presence of DMSO which serves both as a solvent and an oxidant.

Magnetic nanoparticle-supported phosphine gold(i) complex: a highly efficient and recyclable catalyst for the direct reductive amination of aldehydes and ketones

The direct reductive amination of aldehydes and ketones has been achieved with excellent yields using recyclable magnetic nanoparticle-supported gold(i) catalyst.

Enantioselective transfer hydrogenation, a key step for the synthesis of 3-aminotetrahydroquinolines

An enantioselective transfer hydrogenation has been successfully achieved to furnish 3-aminotetrahydroquinolines. The reaction was conducted in the presence of Hantzsch dihydropyridine and a catalytic amount of chiral phosphoric acid under mild conditions.

Mechanism and selectivity of N-triflylphosphoramide catalyzed (3(+) + 2) cycloaddition between hydrazones and alkenes

Brønsted acid catalyzed (3(+) + 2) cycloadditions between hydrazones and alkenes provide a general approach to pyrazolidines. The acidity of the Brønsted acid is crucial for the catalytic efficiency: the less acidic phosphoric acids are ineffective, while highly acidic chiral N-triflylphosphoramides are very efficient and can promote highly enantioselective cycloadditions. The mechanism and origins of catalytic efficiencies and selectivities of these reactions have been explored with density functional theory (M06-2X) calculations. Protonation of hydrazones by N-triflylphosphoramide produces hydrazonium-phosphoramide anion complexes. These ion-pair complexes are very reactive in (3(+) + 2) cycloadditions with alkenes, producing pyrazolidine products. Alternative 1,3-dipolar (3 + 2) cycloadditions with the analogous azomethine imines are much less favorable due to the endergonic isomerization of hydrazone to azomethine imine. With N-triflylphosphoramide catalyst, only a small distortion of the ion-pair complex is required to achieve its geometry in the (3(+) + 2) cycloaddition transition state. In contrast, the weak phosphoric acid does not protonate the hydrazone, and only a hydrogen-bonded complex is formed. Larger distortion energy is required for the hydrogen-bonded complex to achieve the "ion-pair" geometry in the cycloaddition transition state, and a significant barrier is found. On the basis of this mechanism, we have explained the origins of enantioselectivities when a chiral N-triflylphosphoramide catalyst is employed. We also report the experimental studies that extend the substrate scope of alkenes to ethyl vinyl ethers and thioethers.

Asymmetric Brønsted acid-catalyzed aza-Diels-Alder reaction of cyclic C-acylimines with cyclopentadiene

A new chiral Brønsted acid-catalyzed aza-Diels-Alder reaction of cyclic C-acylimines with cyclopentadiene has been developed. The reaction provides optically active aza-tetracycles in good yields with high diastereo- and enantioselectivities under mild reaction conditions.

Iridium catalyzed asymmetric hydrogenation of cyclic imines of benzodiazepinones and benzodiazepines

Highly enantioselective Ir-catalyzed hydrogenation of seven-membered cyclic imines of benzodiazepinones and benzodiazepines was achieved with up to 96% ee. This method provides a direct access to synthesize a range of chiral cyclic amines existing in numerous important natural products and clinical drugs.

Continuous-flow catalytic asymmetric hydrogenations: Reaction optimization using FTIR inline analysis

The asymmetric organocatalytic hydrogenation of benzoxazines, quinolines, quinoxalines and 3H-indoles in continuous-flow microreactors has been developed. Reaction monitoring was achieved by using an inline ReactIR flow cell, which allows fast and convenient optimization of reaction parameters. The reductions proceeded well, and the desired products were isolated in high yields and with excellent enantioselectivities.

Transfer hydrogenation with Hantzsch esters and related organic hydride donors

In recent years, Hantzsch esters and their related organic hydride donors have been widely utilized in biomimetic approaches of asymmetric transfer hydrogenation (ATH) reactions. Various compounds containing C=C, C=N and C=O unsaturated functionalities could be reduced in the presence of organocatalysts or transition metal complexes, affording versatile chiral building blocks in high yields and excellent enantioselectivities under mild conditions. In this critical review, recent advances in this area are summarized and classified according to unsaturated functional groups being reduced and catalytic systems employed (91 references).

Dihydrophenanthridine: a new and easily regenerable NAD(P)H model for biomimetic asymmetric hydrogenation

A new and easily regenerable NAD(P)H model 9,10-dihydrophenanthridine (DHPD) has been designed for biomimetic asymmetric hydrogenation of imines and aromatic compounds. This reaction features the use of hydrogen gas as terminal reductant for the regeneration of the DHPD under the mild condition. Therefore, the substrate scope is not limited in benzoxazinones; the biomimetic asymmetric hydrogenation of benzoxazines, quinoxalines, and quinolines also gives excellent activities and enantioselectivities. Meanwhile, an unexpected reversal of enantioselectivity was observed between the reactions promoted by the different NAD(P)H models, which is ascribed to the different hydride transfer pathway.

Enantioselective synthesis of 4-substituted 4,5-dihydro-1H-[1,5]benzodiazepin-2(3H)-ones by the Lewis base-catalyzed hydrosilylation

Enantioselective synthesis of 4-substituted 4,5-dihydro-1H-[1,5]benzodiazepin-2(3H)-ones has been accomplished through chiral Lewis base-catalyzed hydrosilylation. The corresponding products were obtained in excellent yields (up to 99%) and enantioselectivities (up to 98%). The absolute configuration of product 3n has been determined as S by X-ray crystallographic analysis.