Oxazoline-Based Organocatalyst for Enantioselective Strecker Reactions: A Protocol for the Synthesis of Levamisole

Organocatalytic Synthesis of α-Aminonitriles: A Review

α-Aminonitriles, which have anticancer, antibacterial, antiviral, and antifungal properties, have played an important role in pharmacology. Furthermore, they can also be used to synthesize natural and unnatural amino acids. The main bottleneck in the commercialization of these products is their large-scale production with controlled chirality. A variety of methods have been used to synthesize α-aminonitriles. Among other reported methods for preparing α-aminonitriles, the Strecker reaction is considered appropriate. Recent developments, however, have enabled the α-cyanation of tertiary and secondary amines by functionalizing the carbon–hydrogen (C–H) bond as an attractive alternative procedure for the preparation of α-aminonitriles in the presence of an oxidant and a cyanide source. In most cases, these reactions are catalyzed by transition metal catalysts, such as Fe, Cu, Rh, V, Au, Ru, Mo, Pt, Re, and Co, or by photocatalysts. As an alternative, organocatalysts can also be used to produce aminonitriles. Although there have been numerous reviews on the preparation of α-aminonitriles, no such reviews have been published specifically on the organocatalyzed synthesis of α-aminonitriles. Organocatalysis plays a significant role in synthesizing α-aminonitriles via Strecker-type reactions and cross dehydrogenative coupling reactions (CDC). In this mini review, we discuss the organocatalyzed synthesis of these molecules. A review of new organocatalysts for the synthesis of aminonitriles is expected to provide insight into the development of new industrial catalysts.

Chiral diene-promoted room temperature conjugate arylation: highly enantioselective synthesis of substituted chiral phenylalanine derivatives and α,α-di(arylmethyl)acetates

A highly enantiocontrolled room temperature rhodium-catalyzed conjugate arylation process was developed. The reaction proceeds through 1,4-addition of α-substituted acrylates followed by enantioselective protonation using a C1-symmetric chiral bicyclo[2,2,2] diene as the ligand and water as the proton source. This exceptionally simple protocol provides a reliable and practical access to structurally important phenylalanine derivatives and α,α-di(arylmethyl)acetates in high yields (up to 99%) with good to excellent ee values (up to 99%).

Structural Dynamism-Actuated Reversible CO2 Adsorption Switch and Postmetalation-Induced Visible Light Cα-H Photocyanation with Rare Size Selectivity in N-Functionalized 3D Covalent Organic Framework

The impact of dimensionality and flexibility on anticipated properties has prompted major research focus to three-dimensional covalent organic frameworks (3D COFs), where astute functionalization of porous channels for dynamic CO₂ adsorption as well as size-exclusive C-H activation under eco-friendly condition are the most intriguing advanced applications. Herein, we report an imine-based, diamondoid COF that embraces one-dimensional porous channels in spite of ninefold interpenetration. A combination of intrinsic microporosity and pore wall decoration with accessible N atoms from linear strut renders this 3D COF display reasonable CO₂ affinity with decent selectivity (CO₂/N₂: 64.2; CO₂/CH₄: 10.5) alongside worthy multicyclic CO₂ uptake-release recurrence. Interestingly, the COF undergoes solvent-assisted alteration to a pore-stretched structure via -C═N- "pedal" motion with a concomitant enhancement in CO₂ uptake, where steady reversibility of such structural dynamism instigates unprecedented CO₂ adsorption switch up to seven consecutive cycles. Integration of 2,2'-bipyridyl units benefits anchoring of homogeneous catalyst to device first-ever Ru(Bpy)₂²⁺ hooked diamondoid COF (Ru-COF), which performs visible-light-triggered oxidative cyanation of tertiary amines at room temperature, using molecular oxygen as a selective oxidant in green solvent H₂O. The photocatalyst-engineered COF manifests excellent recyclability and comparable activity to that of homogeneous catalyst. To the best of Ru-COF, atom-economic photocyanation is realized via in situ generated iminium ion, wherein larger-sized substrates exhibit insignificant conversion of α-aminonitriles and validate rarest size selectivity in oxidative Strecker reaction. This study not only demonstrates potential of 3D COF as next-generation dynamic CO₂ adsorbent but also sheds light on tailor-made fabrication of smart functional material for promising catalytic applications through an environmentally benign route.

Enantioselective Strecker and Allylation Reactions with Aldimines Catalyzed by Chiral Oxazaborolidinium Ions

Chiral oxazaborolidinium ion (COBI)-catalyzed enantioselective nucleophilic addition reactions of aldimines using tributyltin cyanide and allyltributylstannane have been developed. Various α-aminonitriles and homoallylic amines were synthesized in high yield (up to 98%) with high to excellent enantioselectivity (up to 99% ee). A rational mechanistic model for the complex of COBI and aldimine is provided to account for these enantioselective reactions.

Three helical chain-based 3D coordination polymers: solvent-induced syntheses, tunable structures and catalytic properties for the Strecker reaction

Three 3D Sc-CPs with different helical chains have been synthesized from isophthalic acid derivatives by solvent induced methods. Among them, 3 is optically active. Furthermore, 1 exhibits good catalytic activity for the Strecker reactions.

Biocatalytic Approach to Chiral β-Nitroalcohols by Enantioselective Alcohol Dehydrogenase-Mediated Reduction of α-Nitroketones

Chiral β-nitroalcohols are important building blocks in organic chemistry. The synthetic approach that is based on the enzyme-mediated reduction of α-nitroketones has been scarcely considered. In this work, the use of commercial alcohol dehydrogenases (ADHs) for the reduction of aromatic and aliphatic nitroketones is investigated. High conversions and enantioselectivities can be achieved with two specific ADHs, affording either the (S) or (R)-enantiomer of the corresponding nitroalcohols. The reaction conditions are carefully tuned to preserve the stability of the reduced product, and to avoid the hydrolytic degradation of the starting substrate. The further manipulation of the enantioenriched nitroalcohols into Boc-protected amminoalcohols is also described.

Enantioselective synthesis of pyrazolone α-aminonitrile derivatives via an organocatalytic Strecker reaction

A highly enantioselective organocatalytic Strecker reaction of pyrazolone-derived ketimines with TMSCN has been developed providing an efficient entry to pyrazolone α-aminonitrile derivatives with a tetra-substituted stereocenter.

Guanidine-catalyzed asymmetric Strecker reaction: modes of activation and origin of stereoselectivity

Density functional theory calculations were employed to study the catalytic mechanism, modes of activation, and origin of enantioselectivity of guanidine-catalyzed asymmetric Strecker reaction of N-benzhydryl imine with hydrogen cyanide. Two types of bifunctional activation mode were identified, namely conventional bifunctional Brønsted acid activation and unconventional bifunctional Brønsted–Lewis acid activation. The lowest-energy transition states correspond to the conventional bifunctional mode of activation. The calculated enantiomeric excess, based on eight lowest-energy C–C bond forming transition states, is in good accord with observed enantioselectivity. NCI (noncovalent interaction) analysis of the key transition states reveals extensive noncovalent interactions, including aromatic interactions and hydrogen bonds, between the guanidinium catalyst and substrates. Multiple aryl–aryl interactions between the phenyl groups of guanidine catalyst and the phenyl rings of N-benzhydryl imine are the key stabilizations in the most stable (R)-inducing transition state. Differential attractive aryl–aryl stabilization is the major factor for stereoinduction.

Natural low-molecular mass organic compounds with oxidase activity as organocatalysts

Organocatalysts, low-molecular mass organic compounds composed of nonmetallic elements, are often used in organic synthesis, but there have been no reports of organocatalysts of biological origin that function in vivo. Here, we report that actinorhodin (ACT), a natural product derived from Streptomyces coelicolor A3(2), acts as a biocatalyst. We purified ACT and assayed its catalytic activity in the oxidation of L-ascorbic acid and L-cysteine as substrates by analytical methods for enzymes. Our findings were as follows: (i) oxidation reactions producing H2O2 proceeded upon addition of ACT to the reaction mixture; (ii) ACT was not consumed during the reactions; and (iii) a small amount (catalytic amount) of ACT consumed an excess amount of the substrates. Even at room temperature, atmospheric pressure, and neutral pH, ACT showed catalytic activity in aqueous solution, and ACT exhibited substrate specificity in the oxidation reactions. These findings reveal ACT to be an organocatalyst. ACT is known to show antibiotic activity, but its mechanism of action remains unknown. On the basis of our results, we propose that ACT kills bacteria by catalyzing the production of toxic levels of H2O2. We also screened various other natural products of bacterial, plant, and animal origins and found that several of the compounds exhibited catalytic activity, suggesting that living organisms produce and use these compounds as biocatalysts in nature.