Cyclic (Alkyl)(amino)carbene Ligand-Promoted Nitro Deoxygenative Hydroboration with Chromium Catalysis: Scope, Mechanism, and Applications

(CAAC)CuCl: A Competent Precatalyst for Carbonyl and Ester Hydrosilylation

Cu-catalyzed carbonyl hydrosilylation involves a ligated "[(L)CuH]" as the active catalyst, where the ligand L has a crucial role toward the stability, stereoselectivity, and enhancement of the hydridicity. Strongly σ-donating N-heterocyclic carbenes (NHCs), their ring-expanded form, and an abnormal NHC as ligands have yielded robust and efficient Cu catalysts. However, cyclic(alkyl)(amino)carbenes (CAACs), despite being stronger σ-donors than NHCs and already having a salient Cu(I) chemistry, are yet to be reported as a similar ligand platform for this purpose. We establish here the familiar [(Me2CAAC)CuCl] as a powerful precatalyst in this regard. Additionally, it also catalyzes the more challenging ester hydrosilylation, which is a rare feat for a Cu catalyst. Apart from the stronger σ-donating ability, the more steric "openness" of CAACs than bulky NHCs also seems to be advantageous. To corroborate, three new (CAAC)CuCl complexes [(ArCH2,MeCAAC)CuCl] (Ar = Ph, 1-naphthyl, and 1-prenyl) are devised, where the effective steric around the copper is practically unaltered from the case of [(Me2CAAC)CuCl]. All three are equally active in carbonyl and ester hydrosilylation as [(Me2CAAC)CuCl]. Computation suggests the carbonyl insertion into a "(CAAC)Cu-H" as the rate-limiting step. To elucidate the involvement of a "(CAAC)CuH", "(PhCH2,MeCAAC)CuH" is generated in situ and is trapped as its BH3 adduct (PhCH2,MeCAAC)CuBH4.

Metal-carbene-guided twofold cross-coupling of ethers with chromium catalysis

Coupling by metal-carbene transfer enables the formation of several different bonds at the carbenoid site, enabling prochiral Csp3 centers that are fundamental three-dimensional substructures for medicines to be forged with increased efficiency. However, strategies using bulk chemicals are rare because of the challenge of breaking two unactivated geminal bonds. Herein, we report the reactivity of ethers to form metal-carbene intermediate by cleavage of α-Csp3-H/Csp3-O bonds, which achieve selective coupling with arylmagnesium bromides and chlorosilanes. These couplings are catalysed by cyclic (alkyl)(amino)carbene-chromium complex and enable the one-step formation of 1,n-arylsilyl alcohols and α-arylated silanes. Mechanistic studies indicate that the in-situ formed low-valent Cr might react with iodobenzene to form phenyl radical species, which abstracts the α-H atom of ether in giving α-oxy radical. The latter combines with Cr by breaking α-Csp3-O bond to afford metal-carbene intermediate, which couples with aryl Grignard and chlorosilane to form two σ-bonds.

Cr-catalyzed borylation of C(aryl)-F bonds using a terpyridine ligand

The defluoroborylation of fluoroarenes by chromium-catalyzed cleavage of unactivated C-F bonds is described. The reaction uses HBpin as the boron source, low-cost and commercially available chromium salt as the precatalyst, and terpyridine as a crucial ligand, providing a protocol with atom-efficient benefits and a wide range of applicable substrates for the functionalization of aryl C-F bonds. Preliminary mechanistic studies indicate that an unprecedented Cr-catalyzed magnesiation of the unactivated C-F bond occurred. The generated arylmagnesium intermediates then participated in the subsequent borylation reaction. The application of the strategy in the preparation of valuable derivatives is demonstrated by the late-stage functionalization of boronate ester groups.

Wingtip-Flexible N-Heterocyclic Carbenes: Unsymmetrical Connection between IMes and IPr

IMes (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and IPr (IPr=1,3- bis(2,6-diisopropylphenyl)imidazol-2-ylidene) represent by far the most frequently used N-heterocyclic carbene ligands in homogeneous catalysis, however, despite numerous advantages, these ligands are limited by the lack of steric flexibility of catalytic pockets. We report a new class of unique unsymmetrical N-heterocyclic carbene ligands that are characterized by freely-rotatable N-aromatic wingtips in the imidazol-2-ylidene architecture. The combination of rotatable N-CH2 Ar bond with conformationally-fixed N-Ar linkage results in a highly modular ligand topology, entering the range of geometries inaccessible to IMes and IPr. These ligands are highly reactive in Cu(I)-catalyzed β-hydroboration, an archetypal borylcupration process that has had a transformative impact on the synthesis of boron-containing compounds. The most reactive Cu(I)-NHC in this class has been commercialized in collaboration with MilliporeSigma to enable broad access of the synthetic chemistry community. The ligands gradually cover %Vbur geometries ranging from 37.3 % to 52.7 %, with the latter representing the largest %Vbur described for an IPr analogue, while retaining full flexibility of N-wingtip. Considering the modular access to novel geometrical space in N-heterocyclic carbene catalysis, we anticipate that this concept will enable new opportunities in organic synthesis, drug discovery and stabilization of reactive metal centers.

An un-forgotten classic: the nitro-Mannich reaction between nitrones and silyl nitronates catalysed by B(C6F5)3

Herein we report the B(C6F5)3-catalysed nitro-Mannich reaction between nitrones and silyl nitronates, affording silyl-protected α-nitro hydroxylamines with yields up to 99% and diastereoselectivities up to 99 : 1. Crucially, the obtained products can be converted into 1,2-diamines under simple reductive conditions. This work provides a new orthogonal method to the existing routes for the instalment of a nitro moiety under Lewis acid catalysed conditions, and expands the state-of-the-art substrate scope with respect to the silyl nitronates.

Engineering organic polymers as emerging sustainable materials for powerful electrocatalysts

Cost-effective and high-efficiency catalysts play a central role in various sustainable electrochemical energy conversion technologies that are being developed to generate clean energy while reducing carbon emissions, such as fuel cells, metal-air batteries, water electrolyzers, and carbon dioxide conversion. In this context, a recent climax in the exploitation of advanced earth-abundant catalysts has been witnessed for diverse electrochemical reactions involved in the above mentioned sustainable pathways. In particular, polymer catalysts have garnered considerable interest and achieved substantial progress very recently, mainly owing to their pyrolysis-free synthesis, highly tunable molecular composition and microarchitecture, readily adjustable electrical conductivity, and high stability. In this review, we present a timely and comprehensive overview of the latest advances in organic polymers as emerging materials for powerful electrocatalysts. First, we present the general principles for the design of polymer catalysts in terms of catalytic activity, electrical conductivity, mass transfer, and stability. Then, the state-of-the-art engineering strategies to tailor the polymer catalysts at both molecular (i.e., heteroatom and metal atom engineering) and macromolecular (i.e., chain, topology, and composition engineering) levels are introduced. Particular attention is paid to the insightful understanding of structure-performance correlations and electrocatalytic mechanisms. The fundamentals behind these critical electrochemical reactions, including the oxygen reduction reaction, hydrogen evolution reaction, CO2 reduction reaction, oxygen evolution reaction, and hydrogen oxidation reaction, as well as breakthroughs in polymer catalysts, are outlined as well. Finally, we further discuss the current challenges and suggest new opportunities for the rational design of advanced polymer catalysts. By presenting the progress, engineering strategies, insightful understandings, challenges, and perspectives, we hope this review can provide valuable guidelines for the future development of polymer catalysts.

Chromium-Catalyzed Cross-Coupling of Methyl Ketones with Cyclic Ketones toward the Selective Synthesis of β-Branched β,γ-Unsaturated Ketones

Chromium-catalyzed cross-coupling of methyl ketones with cyclic ketones to β-branched β,γ-unsaturated ketones are reported. Interestingly, single-crossed aldol condensation products are formed, even in reactions in which a mixture of products is possible. The reaction is highly chemoselective and regioselective. This catalytic route gives a unique opportunity to integrate the chemistry of the synthetic challenge cross-coupling reaction of ketones and the alkene migration reaction into a reaction pot.

Deoxygenative Hydroboration of Aromatic Nitro Compounds Catalyzed by Tetra(diisopropylamido) Rare-Earth Metal-Lithium Bimetallic Complexes

The first example of the reduction of a nitro compound with HBPin catalyzed by tetra(diisopropylamido) rare-earth metal-lithium bimetallic complexes LiRE(NiPr2)4(THF) (RE = La, Nd, Sm, Gd, and Y) was disclosed. A series of aromatic nitro compounds were reduced to N-borylamines in high yields (up to 99%). The derivatives of N-borylamines─amides and carbamates─were obtained in a sequential one-pot manner. Furthermore, kinetic studies of the deoxygenative hydroboration of nitro compounds were carried out.

Visible-light-induced iron-catalyzed reduction of nitroarenes to anilines

An efficient visible-light-induced iron-catalyzed reduction of nitroarenes to anilines by using N-ethylmorpholine (NEM) as a reductant under mild conditions has been developed. The reaction proceeds with photosensitizer-free conditions and features good to excellent yields and broad functional group tolerance. Preliminary mechanistic investigations showed that this reaction was conducted via ligand-to-metal (NEM to Fe3+) charge transfer and nitro triplet biradical-induced hydrogen atom transfer processes.

Divergent Synthesis of Chelating Aziridines and Cyclic(Alkyl)(Amino)Carbenes (CAACs) from Pyridyl-Tethered Robust Azomethine Ylides

Azomethine ylides are typically in situ generated synthons for making N-heterocycles through cycloaddition reactions. But an offbeat aspect about them is the isomeric nature of aldiminium-based azomethine ylides and (alkyl/aryl)(amino)carbenes, interconvertible by a formal 1,3-H+ transfer. Herein, two thermally robust azomethine ylides with a N-appended picolyl sidearm are isolated, which cyclize to py aziridines at 80 °C but unprecedentedly result N-pico CAAC-CuCl (CAAC=cyclic(alkyl)(amino)carbene) complexes when heated with CuCl at merely 60 °C. The pendant Npy , as revealed by computational analysis, plays a crucial role in this unusual 1,3-H+ shift using a deprotonation-protonation sequence, as well as in placing the CuCl at the carbenic site in tandem. The softer nature of Cu(I) is also critical. Chelating CAACs are rare and one with a N-tethered additional donor is priorly unknown. Both N-pico CAAC and py aziridine are bidentate chelators giving highly active cationic Rh(I) catalysts for hydrosilylating unactivated olefins by Et3 SiH. Notably, the py aziridine-Rh(I) is superior than the N-pico CAAC-Rh(I) catalyst.

Chemoselective Three-Component Geminal Cross Couplings of Dihaloalkanes with Cr Catalysis: Rapid Access to Tertiary and Quaternary Alkanes via a Metal-Carbene Intermediate

Geminal cross couplings using multiple components enable the formation of several different bonds at one site in the building of tertiary and quaternary alkanes. Nevertheless, there are remaining issues of concern-cleavage of two geminal bonds and control of selectivity present challenges. We report here the geminal cross couplings of three components by reactions of dihaloalkanes with organomagnesium and chlorosilanes or alkyl tosylates by Cr catalysis, affording the formation of geminal C-C/C-Si or C-C/C-C bonds in the creation of tertiary and quaternary alkanes. The geminal couplings are catalyzed by low-cost CrCl2 , enabling the sluggishness of competitive Kumada-type side couplings and homocouplings of Grignard reagents, in achieving high chemoselectivity. Experimental and theoretical studies indicate that two geminal C-halide bonds are continuously cleaved by Cr to afford a metal carbene intermediate, which couples with a Grignard reagent, followed by silylation, in the formation of geminal C-C and C-Si bonds via a novel inner-sphere radical coupling mechanism. These three-component geminal cross couplings are value-addition to the synthesis of commercial drugs and bioactive molecules in medicinal chemistry.

Heterogeneous V2O5/TiO2-Mediated Photocatalytic Reduction of Nitro Compounds to the Corresponding Amines under Visible Light

The hydrogenation of nitro compounds to their corresponding amines is developed using a heterogeneous and recyclable catalyst (V₂O₅/TiO₂) under irradiation of blue LED (9 W) at ambient temperature. Hydrazine hydrate is used as a reductant and ethanol is used as a solvent, facilitating green, sustainable, low-cost production. The synthesis of 32 (hetero)arylamines and their pharmaceutically relevant molecules (five) are described. Significant features of the protocol include catalyst recyclability, green solvent, ambient temperature, and gram-scale reactions. Among the other aspects studied are ¹H-NMR-assisted reaction progress monitoring, control experiments for mechanistic studies, protocol applications, and recyclability studies. Furthermore, the developed protocol enabled wide functional group tolerance, chemo-selectivity, high yield, and low-cost, sustainable, and environmentally benign synthesis.

A Donor-Acceptor Cyclopropane by Intramolecular C(sp3)-H Activation at a Cyclic(alkyl)(amino)carbene Center and Its Fascinating Ring-Opening Chemistry

Virtually irreversible intramolecular C-H activations are deleterious for aza-carbenes. A picolyl-tethered cyclic(alkyl)(amino)carbene (CAAC) isomerizes into a donor-acceptor cyclopropane in this manner but restores the CAAC status by retro-C-H activation in the presence of trapping agents like Se or CuCl. The same DA cyclopropane is readily hydrolyzed to a pyrrolidin-2-ol that acts as another ^picoCAAC precursor by undergoing 1,1-dehydration in the presence of Se or CuCl. The chemistry is distinct from the N-heterocyclic carbene analogue throughout.

Chromium-catalyzed stereodivergent E- and Z-selective alkyne hydrogenation controlled by cyclic (alkyl)(amino)carbene ligands

The hydrogenation of alkynes allows the synthesis of olefins, which are important feedstock for the materials, pharmaceutical, and petrochemical industry. Thus, methods that enable this transformation via low-cost metal catalysis are desirable. However, achieving stereochemical control in this reaction is a long-standing challenge. Here, we report on the chromium-catalyzed E- and Z-selective olefin synthesis via hydrogenation of alkynes, controlled by two carbene ligands. A cyclic (alkyl)(amino)carbene ligand that contains a phosphino anchor enables the hydrogenation of alkynes in a trans-addition manner, selectively forming E-olefins. With an imino anchor-incorporated carbene ligand, the stereoselectivity can be switched, giving mainly Z-isomers. This ligand-enabled geometrical stereoinversion strategy by one metal catalysis overrides common methods in control of the E- and Z-selectivity with two different metal catalysis, allowing for highly efficient and on-demand access to both E- and Z-olefins in a stereo-complementary fashion. Mechanistic studies indicate that the different steric effect between these two carbene ligands may mainly dominate the selective forming E- or Z-olefins in control of the stereochemistry.

Methanol as a Potential Hydrogen Source for Reduction Reactions Enabled by a Commercial Pt/C Catalyst

Catalytic reduction reactions using methanol as a transfer hydrogenating agent is gaining significant attention because this simple alcohol is inexpensive and produced on a bulk scale. Herein, we report the catalytic utilization of methanol as a hydrogen source for the reduction of different functional organic compounds such as nitroarenes, olefins, and carbonyl compounds. The key to the success of this transformation is the use of a commercially available Pt/C catalyst, which enabled the transfer hydrogenation of a series of simple and functionalized nitroarenes-to-anilines, alkenes-to-alkanes, and aldehydes-to-alcohols using methanol as both the solvent and hydrogen donor. The practicability of this Pt-based protocol is showcased by demonstrating catalyst recycling and reusability as well as reaction upscaling. In addition, the Pt/C catalytic system was also adaptable for the N-methylation and N-alkylation of anilines via the borrowing hydrogen process. This work provides a simple and flexible approach to prepare a variety of value-added products from readily available methanol, Pt/C, and other starting materials.

Aluminium-Catalyzed Selective Hydroboration of Esters and Epoxides to Alcohols: C-O Bond Activation

In this work, the molecular aluminium dihydride complex bearing an N, N'-chelated conjugated bis-guanidinate (CBG) ligand is used as a catalyst for reducing a wide range of aryl and alkyl esters with good tolerance of alkene (C=C), alkyne (C≡C), halides (Cl, Br, I and F), nitrile (C≡N), and nitro (NO₂ ) functionalities. Further, we investigated the catalytic application of aluminium dihydride in the C-O bond cleavage of alkyl and aryl epoxides into corresponding branched Markovnikov ring-opening products. In addition, the chemoselective intermolecular reduction of esters over other reducible functional groups, such as amides and alkenes, has been established. Intermediates are isolated and characterized by NMR and HRMS studies, which confirm the probable catalytic cycles for the hydroboration of esters and epoxides.

EDA mediated S-N bond coupling of nitroarenes and sodium sulfinate salts

Despite their long-known photochemical properties and their industrial value, the use of nitroarenes as a productive photochemical handle in organic synthesis has remained relatively unexplored. More specifically, the photochemical formation of nitrogen-sulfur bonds from nitroarenes remains to be demonstrated. Herein, we report the design and application of a sulfinate-nitroarene electron donor-acceptor (EDA) complex and its subsequent use in the first light mediated catalyst-free synthesis of N-hydroxy-sulfonamides. The presence of the EDA was assessed spectroscopically and studied via DFT and TD-DFT calculations. A total of 32 examples including both electron withdrawing and electron donating groups were synthesized under our oxygen- and water-tolerant conditions.

Sustainable production of active pharmaceutical ingredients from lignin-based benzoic acid derivatives via “demand orientation”

Catalytic production of several representative active pharmaceutical ingredients (APIs) from lignin.

CAAC-IPr*: easily accessible, highly sterically-hindered cyclic (alkyl)(amino)carbenes

IPr* (IPr* = 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) has emerged as a powerful highly hindered and sterically-flexible ligand platform for transition-metal catalysis. CAACs (CAAC = cyclic (al-kyl)(amino)carbenes) have gained major attention as strongly electron-rich carbon analogues of NHCs (NHC = N-heterocyclic carbene) with broad applications in both industry and academia. Herein, we report a merger of CAAC ligands with highly-hindered IPr*. The efficient synthesis, electronic characterization and application in model Cu-catalyzed hydroboration of alkynes is described. The ligands are strongly electron-rich, bulky and flexible around the N-Ar wingtip. The availability of various IPr* and CAAC templates offers a significant potential to expand the existing arsenal of NHC ligands to electron-rich bulky architectures with critical applications in metal stabilization and catalysis.

Zn-Catalyzed Regioselective and Chemoselective Reduction of Aldehydes, Ketones and Imines

An operationally convenient Zn-catalyzed synthesis of alcohols by the reduction of aldehydes, ketones, and α,β-unsaturated aldehydes/ketones is reported. It is a rare example of using mild and sustainable HBpin as a reductant for catalytic reduction of carbonyl compounds in the absence of acid or base as hydrolysis reagent. The reaction is upscalable and proceeds in high selectivity without the formation of boronate ester by-products, and tolerates sensitive functionalities, such as iodo, bromo, chloro, fluoro, nitro, trifluoromethyl, aminomethyl, alkynyl, and amide. The Zn(OAc)₂/HBpin combination has been also proved to be chemoselective for the C=N reduction of imine analogs.

Halogen-atom and group transfer reactivity enabled by hydrogen tunneling

The generation of carbon radicals by halogen-atom and group transfer reactions is generally achieved using tin and silicon reagents that maximize the interplay of enthalpic (thermodynamic) and polar (kinetic) effects. In this work, we demonstrate a distinct reactivity mode enabled by quantum mechanical tunneling that uses the cyclohexadiene derivative γ-terpinene as the abstractor under mild photochemical conditions. This protocol activates alkyl and aryl halides as well as several alcohol and thiol derivatives. Experimental and computational studies unveiled a noncanonical pathway whereby a cyclohexadienyl radical undergoes concerted aromatization and halogen-atom or group abstraction through the reactivity of an effective H atom. This activation mechanism is seemingly thermodynamically and kinetically unfavorable but is rendered feasible through quantum tunneling.

A Reusable FeCl3∙6H2O/Cationic 2,2′-Bipyridyl Catalytic System for Reduction of Nitroarenes in Water

The association of a commercially-available iron (III) chloride hexahydrate (FeCl3∙6H2O) with cationic 2,2′-bipyridyl in water was proven to be an operationally simple and reusable catalytic system for the highly-selective reduction of nitroarenes to anilines. This procedure was conducted under air using 1–2 mol% of catalyst in the presence of nitroarenes and 4 equiv of hydrazine monohydrate (H2NNH2∙H2O) in neat water at 100 °C for 12 h, and provided high to excellent yields of aniline derivatives. After separation of the aqueous catalytic system from the organic product, the residual aqueous solution could be applied for subsequent reuse, without any catalyst retreatment or regeneration, for several runs with only a slight decrease in activity, proving this process eco-friendly.

Chromium-catalyzed couplings of C(aryl)-SMe bonds for accessing arylated and alkylated benzaldehyde derivatives

Described here is the chromium-catalyzed cleavage of C(aryl)-SMe bonds leading to coupling with organomagnesium to give functionalized benzaldehydes under mild conditions. This reaction was promoted specifically by a low-cost and simple CrCl₂ salt used as a precatalyst, enabling synchronous activations of ortho-C(aryl)-SMe and ortho'-C(aryl)-H bonds to achieve difunctionalization of benzaldimines. This work provided a strategy for accessing arylated, alkylated, and diarylated benzaldehyde derivatives as a result of the couplings of C(aryl)-SMe and C(aryl)-SMe/C(aryl)-H bonds promoted with cost-effective Cr catalysis.

Recent Advances in the Domain of Cyclic (Alkyl)(Amino) Carbenes

Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.

The transition metal-catalysed hydroboration reaction

This review covers the development of the transition metal-catalysed hydroboration reaction, from its beginnings in the 1980s to more recent developments including earth-abundant catalysts and an ever-expanding array of substrates.

KB3H8: an environment-friendly reagent for the selective reduction of aldehydes and ketones to alcohols

Selective reduction of aldehydes and ketones to their corresponding alcohols with KB₃H₈, an air- and moisture-stable, nontoxic, and easy-to-handle reagent, in water and THF has been explored under an air atmosphere for the first time. Control experiments illustrated the good selectivity of KB₃H₈ over NaBH₄ for the reduction of 4-acetylbenzaldehyde and aromatic keto esters.

Deoxygenation of Nitrous Oxide and Nitro Compounds Using Bis(N-Heterocyclic Silylene)Amido Iron Complexes as Catalysts

Herein, we report the efficient degradation of N₂ O with a well-defined bis(silylene)amido iron complex as catalyst. The deoxygenation of N₂ O using the iron silanone complex 4 as a catalyst and pinacolborane (HBpin) as a sacrificial reagent proceeds smoothly at 50 °C to form N₂ , H₂ , and (pinB)₂ O. Mechanistic studies suggest that the iron-silicon cooperativity is the key to this catalytic transformation, which involves N₂ O activation, H atom transfer, H₂ release and oxygenation of the boron sites. This approach has been further developed to enable catalytic reductions of nitro compounds, producing amino-boranes with good functional-group tolerance and excellent chemoselectivity.

Multiwavelets applied to metal-ligand interactions: Energies free from basis set errors

Transition metal-catalyzed reactions invariably include steps where ligands associate or dissociate. In order to obtain reliable energies for such reactions, sufficiently large basis sets need to be employed. In this paper, we have used high-precision multiwavelet calculations to compute the metal-ligand association energies for 27 transition metal complexes with common ligands, such as H₂, CO, olefins, and solvent molecules. By comparing our multiwavelet results to a variety of frequently used Gaussian-type basis sets, we show that counterpoise corrections, which are widely employed to correct for basis set superposition errors, often lead to underbinding. Additionally, counterpoise corrections are difficult to employ when the association step also involves a chemical transformation. Multiwavelets, which can be conveniently applied to all types of reactions, provide a promising alternative for computing electronic interaction energies free from any basis set errors.

Mechanistic Diversity of Low-Valent Chromium Catalysis: Cross-Coupling and Hydrofunctionalization

ConspectusTransition-metal catalysis has traditionally been dominated by precious metals because of their high reactivity toward chemical transformations. As a cost-effective alternative, catalysis by earth-abundant group 6 metal chromium is underdeveloped, and its reactivity remains largely unexplored, although the industrially important Phillips catalyst, which is composed of Cr as the active metal, is currently used to supply almost 40% of the total world demand for high-density polyethylene. Cr has traditionally served in organoreagents with high-valent states (≥2+), which are typified by reactions involving Nozaki-Hiyama-Kishi (NHK) and Takai-Utimoto one-electron transfer processes. Given that low-valent metals usually facilitate the process of oxidative addition (OA), studying the catalysis of Cr in the low-valent state provides the opportunity to develop new transformations. However, probably because of the low stability of reactive low-valent Cr or the lack of catalytic activity of structurally stable complexes, there has been limited success with respect to developing catalysis promoted by low-valent Cr. In recent years, our group has probed the reactivity of low-valent Cr in catalysis by adopting a strategy of forming reactive Cr in situ. In this Account, we detail our efforts to study the catalytic behavior and mechanism of low-valent Cr in challenging transformations, such as the cleavage of chemically inert bonds for the cross-coupling and hydrofunctionalization of arenes and nitro motifs, by developing strategies to address the prominent selectivity issues. We highlight the finding that low-valent Cr, being formed in situ, possesses the intriguing ability to promote the catalytic cleavage of unactivated C-O, C-N, and C-H bonds to achieve the Kumada couplings and even to enable challenging cross-coupling between two unactivated C(aryl)-O/C(aryl)-N bonds. During these catalytic processes, Cr usually adopts a high-spin state to interact with chemicals, allowing for insertion into unactivated σ-bonds. The OA catalytic model involving a two-electron process for the cleavage of unactivated bonds has rarely been considered for Cr. We highlight the finding that Cr allows for the breakage of two chemically inert bonds in one catalytic cycle. This ability is intriguing because most transition metals are suitable only for the cleavage of one unactivated bond in catalysis. Mechanisms involving two-electron OA for Cr are unusual, with processes involving one-electron transfer more often proposed, as exemplified in the NHK reactions. These reactions provide efficient strategies for forming functionalized benzaldehydes, amides, anilines, and amines, usually with high levels of selectivity. We hope that this account will extend the scope of cognition to Cr catalysis.

Chemoselective hydroborative reduction of nitro motifs using a transition-metal-free catalyst

The first transition-metal-free catalysts for deoxygenative reduction of nitroarenes, nitro heteroarenes and even notoriously challenging nitroalkanes using pinacolborane have been reported.