Azaborabutadienes: Synthesis by Metal‐Free Carboboration of Nitriles and Utility as Building Blocks for B,N‐Heterocycles

Metal‐free regioselective carboboration of arylnitriles with L₂PhB: ( 1 : L=oxazol‐2‐ylidene) catalyzed by Et₃B afforded the unprecedented acyclic 2‐aza‐4‐borabutadienes 2 , thus demonstrating a new strategy to construct a B,C,N‐mixed π‐system involving B=C and C=N bonds. Thermal isomerization of 2 gave C‐borylimines ( 3 ), and diverse reactivity of 2 a towards several substrates, such as H⁺, F⁺, O₂, S, Se, and isonitriles, allowed construction of boron‐containing heterocycles with various ring sizes, thus illustrating the utility of 2 as a synthetic building block.     

Phosphine‐Catalyzed Enantioselective Synthesis of Oxygen Heterocycles

Chiral phosphepine 1 catalyzes the transformation of an array of hydroxy‐2‐alkynoates into saturated oxygen heterocycles with good enantioselectivity. Phenols are also shown to participate in such phosphine‐catalyzed cyclizations, including an asymmetric variant. This method provides a new approach to the enantioselective synthesis of tetrahydrofurans, tetrahydropyrans, and dihydrobenzopyrans.

     

Enantioselective Formal [4+2] Cycloadditions to 3‐Nitroindoles by Trienamine Catalysis: Synthesis of Chiral Dihydrocarbazoles

The first enantioselective formal [4+2] cycloadditions of 3‐nitroindoles are presented. By using 3‐nitroindoles in combination with an organocatalyst, chiral dihydrocarbazole scaffolds are formed in moderate to good yields (up to 87 %) and enantioselectivities (up to 97 % ee). The reaction was extended to include enantioselective [4+2] cycloadditions of 3‐nitrobenzothiophene. The reaction proceeds through a [4+2] cycloaddition/elimination cascade under mild reaction conditions. Furthermore, a diastereoselective reduction of an enantioenriched cycloadduct is presented. The mechanism of the reaction is discussed based on experimental and computational studies.     

Catalytic Asymmetric Synthesis of Enantioenriched Heterocycles Bearing a CCF3 Stereogenic Center

Given the important agricultural and medicinal application of optically pure heterocycles bearing a trifluoromethyl group at the stereogenic carbon center in the heterocyclic framework, the exploration of efficient and practical synthetic strategies to such types of molecules remains highly desirable. Catalytic enantioselective synthesis has one clear advantage that it is more cost‐effective than other synthetic methods, but remains limited by challenges in achieving excellent yield and stereoselectivities with a low catalyst loading. Thus far, numerous models of organo‐ and organometal‐catalyzed asymmetric reactions have been exploited to achieve this elusive goal over the past decade. This review article describes recent progress on this research topic, and focuses on an understanding of the catalytic asymmetric protocols exemplified in the catalytic enantioselective synthesis of a wide range of complex enantioenriched trifluoromethylated heterocycles.     

Yb(OTf)3 Anchored on Crosslinked Chitosan Microsphere: A Green Heterogenized Catalyst for the Synthesis of Bispiro-Fused Heterocycles

Using renewable biomass chitosan as a raw material, crosslinked chitosan microsphere (CCM) was prepared by inverse suspension, which was then immersed in the ethanol solution of Yb(OTf)₃ to fabricate CCM-supported Yb(OTf)₃ (Yb(OTf)₃@CCM). The three-component [3+2] cycloaddition of isatins, tetrahydroisoquinolines and 5-alkenyl rhodanines was developed with Yb(OTf)₃@CCM as a green heterogeneous catalyst, which gave access to a suite of architecturally complex bispiro-fused heterocycles merging four pharmacophores with moderate to excellent yields (50–96 %) and diastereoselectivities (up to 99 : 1 dr). The reaction mechanism was preliminarily clarified by controlled experiments and dynamic high-resolution mass spectrometry studies. The synthetic potential of this protocol was proved by a gram-scale reaction which furnished the product with comparable results (85 % yield and 72 : 28 dr). Moreover, the catalyst could be recovered just through filtering, washing and drying process, and reused five times with the catalytic activity remaining in an acceptable range (from 93 % to 73 % yield).     

Gold‐Catalyzed Cascade Reactions for Synthesis of Carbo‐ and Heterocycles: Selectivity and Diversity

This account describes our recent efforts devoted to gold chemistry since 2009. Based on furyl–Au 1,3‐dipole analogues and related gold carbene intermediates, a rich variety of gold‐catalyzed cascade reactions have been developed, which provide facile access to a diverse range of novel carbo‐ and heterocycles. In these reactions, the selectivity can be well controlled by the catalyst (ligand and metal), substrate or reagent. In addition, we have also developed the corresponding enantioselective variants, which are guided by bis(phosphinegold) complexes derived from axially chiral scaffolds and asymmetric gold/chiral Brønsted acid relay catalysis.     

Sonochemistry: Synthesis of Bioactive Heterocycles

In recent years, green chemistry using environmentally benign reagents and conditions has become one of the most fascinating developments in the synthesis of widely used organic compounds. This review describes the theory and importance of ultrasound as an efficient tool in the synthesis of heterocyclic compounds.     

Utilization of CO2 Feedstock for Organic Synthesis by Visible-Light Photoredox Catalysis

CO₂ is a highly abundant, green, and sustainable carbon feedstock. Despite its kinetic inertness and thermodynamic stability, the development of various catalytic techniques has enabled the conversion of CO₂ to value-added products such as carboxylic acids, amino acids, and heterocyclic compounds, where visible-light photocatalysis has emerged to be an efficient promoter of these processes. This Minireview covers the progress in the areas of CO₂ incorporation onto organic matters based on the combined venture of renewable resources of CO₂ and light energy with significant emphasis on the last three years’ developments.     

Synthesis of Heterocycles through Transition‐Metal‐Catalyzed Isomerization Reactions

Metal‐catalyzed isomerization of N‐ and O‐allylic systems is emerging as an effective method to form synthetically useful iminium and oxocarbenium intermediates. In the presence of tethered nucleophiles, several recent examples illuminate this approach as a powerful strategy for the synthesis of structurally complex and diverse heterocycles. In this Concept article, we attempt to cover this area of research through a selection of recent versatile examples.     

Noncovalent Substrate‐Directed Enantioselective Heck Reactions: Synthesis of S‐ and P‐Stereogenic Heterocycles

S‐ and P‐Stereogenic heterocycles were synthesized by a remarkably simple enantioselective Heck desymmetrization reaction based on the unprecedented noncovalent directing effect of S=O and P=O functionalities. Selected prochiral symmetric substrates were efficiently arylated using the recently disclosed chiral PyraBOx ligand under mild and open‐flask reaction conditions. Several five‐membered aryl‐ sulfones, sulfoxides, and phosphine oxides were synthesized in good to excellent yields, in good to high diastereoselectivity, and enantiomeric ratios up to 98:2. Theoretical calculations also support the noncovalent directing effect of the S=O and P=O functionalities during the arylation process.     

Borane-Catalyzed Chemoselective and Enantioselective Reduction of 2-Vinyl-Substituted Pyridines

Herein, we report that highly chemoselective and enantioselective reduction of 2-vinyl-substituted pyridines has been achieved for the first time. The reaction, which uses chiral spiro-bicyclic bisboranes as catalysts and HBpin and an acidic amide as reducing reagents, proceeds through a cascade process involving 1,4-hydroboration followed by transfer hydrogenation of a dihydropyridine intermediate. The retained double bond in the reduction products permits their conversion to natural products and other useful heterocyclic compounds by simple transformations.     

2H-吡咯的简易合成方法:金催化与路易斯酸催化的组合应用

报道了利用金催化与铜催化相结合合成5-氨基-2H-吡咯的方法. 首先通过均相金催化异恶唑与炔酰胺反应生成α-亚胺金卡宾中间体, 再经[3+2]环加成反应合成并分离出5-氨基-3H-吡咯, 然后通过铜催化的去酰基化和基团迁移反应来实现3H-吡咯向2H-吡咯的转化. 该方法不但操作简单、反应条件温和、官能团兼容性良好, 而且还能进行克量级放大. 这项研究为合成具有重要生物活性的5-氨基-2H-吡咯化合物提供了广谱实用的合成方法.     

Recent Developments in the Synthesis of Five‐ and Six‐Membered Heterocycles Using Molecular Iodine

Heterocyclic scaffolds represent the key structural subunits of many biologically active compounds. Over the last few years iodine‐mediated reactions have been extensively studied due to their low cost and eco‐friendliness. This Review covers advances in the field of iodine‐mediated synthesis of heterocyclic compounds since 2006, especially with an emphasis on mechanisms of ring formation. In this article, syntheses of different heterocycles are classified based on the manipulation of functional groups.     

Trio Catalysis Merging Enamine,Brønsted Acid,and Metal Lewis Acid Catalysis: Asymmetric Three‐Component Aza‐Diels–Alder Reaction of Substituted Cinnamaldehydes,Cyclic Ketones,and Arylamines

A trio catalyst system, composed of arylamine, BINOL‐derived phosphoric acid, and Y(OTf)₃, enables the combination of enamine catalysis with both hard metal Lewis acid catalysis and Brønsted acid catalysis for the first time. Using this catalyst system, a three‐component aza‐Diels–Alder reaction of substituted cinnamaldehyde, cyclic ketone, and arylamine is carried out with high chemo‐ and enantioselectivity, affording a series of optically active 1,4‐dihydropyridine (DHP) derivatives are obtained in 91–99 % ee and 59–84 % yield. DHPs bearing a chiral quaternary carbon center are also obtained with good enantioselectivity and moderate yield (three examples). Preliminary mechanistic investigations have also been conducted.