Origins of unique gold-catalysed chemo- and site-selective C-H functionalization of phenols with diazo compounds

Mechanism and stereoselectivity in metal and enzyme catalyzed carbene insertion into X-H and C(sp2)-H bonds

Constructing highly proficient C-X (X = O, N, S, etc.) and C-C bonds by leveraging TMs (transition metals) (Fe, Cu, Pd, Rh, Au, etc.) and enzymes to catalyze carbene insertion into X-H/C(sp2)-H is a highly versatile strategy. This is primarily achieved through the in situ generation of metal carbenes from the interaction of TMs with diazo compounds. Over the last few decades, significant advancements have been made, encompassing a wide array of X-H bond insertions using various TMs. These reactions typically favor a stepwise ionic pathway where the nucleophilic attack on the metal carbene leads to the generation of a metal ylide species. This intermediate marks a critical juncture in the reaction cascade, presenting multiple avenues for proton transfer to yield the X-H inserted product. The mechanism of C(sp2)-H insertion reactions closely resembles those of X-H insertion reactions and thus have been included here. A major development in carbene insertion reactions has been the use of engineered enzymes as catalysts. Since the seminal report of a non-natural "carbene transferase" by Arnold in 2013, "P411", several heme-based enzymes have been reported in the literature to catalyze various abiological carbene insertion reactions into C(sp2)-H, N-H and S-H bonds. These enzymes possess an extraordinary ability to regulate the orientation and conformations of reactive intermediates, facilitating stereoselective carbene transfers. However, the absence of a suitable stereochemical model has impeded the development of asymmetric reactions employing a lone chiral catalyst, including enzymes. There is a pressing need to investigate alternative mechanisms and models to enhance our comprehension of stereoselectivity in these processes, which will be crucial for advancing the fields of asymmetric synthesis and biocatalysis. The current review aims to provide details on the mechanistic aspects of the asymmetric X-H and C(sp2)-H insertion reactions catalyzed by Fe, Cu, Pd, Rh, Au, and enzymes, focusing on the detailed mechanism and stereochemical model. The review is divided into sections focusing on a specific X-H/C(sp2)-H bond type catalyzed by different TMs and enzymes.

Modular Synthesis of Tetrasubstituted Vinyl Sulfides via One-Pot Sequential Carbene Transfer Reaction from Thiols with α-Diazo Carbonyl Compounds

We present a one-pot reaction that offers an efficient approach to synthesizing tetrasubstituted vinyl sulfides with high stereoselectivity. This method involves the sequential Wolff rearrangement, ylide formation, and [1,4]-aryl transfer by utilizing aryl and alkyl thiols and α-diazo carbonyl compounds as substrates. Notably, this reaction features commercially available materials, straightforward operation, atom economy, and broad substrate scope. Moreover, the primary photophysical properties (aggregation-induced emission effect) of the products were also investigated, which might be useful in functional materials via structural modification.

Is Enol Always the Culprit? The Curious Case of High Enantioselectivity in a Chiral Rh(II) Complex Catalyzed Carbene Insertion Reaction

The mechanism of Rh2 (S-NTTL)4 catalyzed carbene insertion into C(3)-H of indole is investigated using DFT methods. Since the commonly accepted enol mechanism cannot account for enantioinduction, a concerted oxocarbenium pathway was proposed in an earlier work using a model catalyst. However, after considering the full catalytic system, this study finds that akin to other reactions, here, too, the enol pathway is of lower energy, which now naturally raises a conundrum regarding the mode of chiral induction. Herein, a new water promoted mechanistic pathway involving a metal-associated enol intermediate hydrogen bonding and stereochemical model are proposed to solve this puzzle. It is shown how the catalyst bowl-shaped structure along with substrate-catalyst binding is crucial for achieving high levels of enantioselectivity. A stereodetermining water-assisted proton transfer is proposed and confirmed through deuterium-labeling experiments. The water molecules are held together by H-bonding interactions with the carboxylate ligands that is reminiscent of enzyme catalysis. Although several previous studies have aimed at understanding the mechanism of metal catalyzed carbene insertion reactions, the origin of high stereoinduction especially with chiral metal complexes remains unclear, and till date there is no transition state model that can explain the high enantioselectivity with such chiral Rh complexes. The metal-associated enol pathway is currently underrepresented in catalytic cycles and may play a crucial role in catalyst design. Since the enol pathway is commonly adopted in other metal-catalyzed X-H insertion reactions involving a diazoester, the presented results are not specific to the current reaction. Therefore, this study could provide the direction for achieving high levels of enantioselectivity which is otherwise difficult to achieve with a single metal catalyst.

Borane catalyzed transesterification of tert-butyl esters using α-aryl α-diazoesters

The B(C6F5)3-catalyzed transesterification of a series of 3-alkenyl-oxindoles and other unsaturated tert-butyl esters with aryl-diazo esters is reported. This protocol is facile and generally high yielding proceeding under mild conditions and is remarkably chemoselective leaving the CC bonds intact.

Palladium-Catalyzed Fluorinative Bifunctionalization of Aziridines and Azetidines with gem-Difluorocyclopropanes

An unprecedented Pd-catalyzed fluorinative bifunctionalization of aziridines and azetidines was successfully developed via regioselective C-C and C-F bond cleavage of gem-difluorocyclopropanes, leading to various β,β'-bisfluorinated amines and β,γ-bisfluorinated amines. This reaction was achieved by incorporating a 2-fluorinated allyl group and a fluorine atom scissored from gem-difluorocyclopropane in 100 % atom economy for the first time. The mechanistic investigations indicated that the reaction underwent amine attacking 2-fluorinated allyl palladium complex to generate η2 -coordinated N-allyl aziridine followed by fluoride ligand transfer affording the final β- and γ-fluorinated amines.

Theoretical Study on the Copper-Catalyzed ortho-Selective C-H Functionalization of Naphthols with α-Phenyl-α-Diazoesters

The aromatic C(sp2)-H functionalization of unprotected naphthols with α-phenyl-α-diazoesters under mild conditions catalyzed by CuCl and CuCl2 exhibits high efficiency and unique ortho-selectivity. In this study, the combination of density functional theory (DFT) calculations and experiments is employed to investigate the mechanism of C-H functionalization, which reveals the fundamental origin of the site-selectivity. It explains that CuCl-catalyzed ortho-selective C-H functionlization is due to the bimetallic carbene, which differs from the reaction catalyzed by CuCl2 via monometallic carbene. The results demonstrate the function of favourable H-bond interactions on the site- and chemo-selectivity of reaction through stabilizing the rate-determining transition states in proton (1,3)-migration.

B(C6F5)3-catalyzed cyclopropanation of 3-alkenyl-oxindoles with diazomethanes

Spirocyclopropane-oxindoles are key motifs in biologically active compounds and are versatile synthetic intermediates. Herein, we report a metal-free, B(C₆F₅)₃ catalyzed cyclopropanation of 3-alkenyl-oxindoles with diazomethanes. This provides 25 variants of spirocyclopropane-oxindole derivatives. These spirocyclopropane-oxindole products were obtained in good to excellent yields (up to 99%) and high diastereoselectivities (up to 20 : 1 d.r.) under mild reaction conditions and could be performed on a gram scale.

A rhodium-catalyzed ylide formation/Smiles rearrangement reaction of chalcogenide ether and triazoles

A Rh(ii)-catalyzed highly stereoselective chalcogenide ylide formation/Smiles rearrangement reaction of diaryl thioether/selenoethers and triazoles was successfully developed.

Chemo- and Regioselective Multiple C(sp2 )-H Insertions of Malonate Metal Carbenes for Late-Stage Functionalizations of Azahelicenes

Chiral quinacridines react up to four times, step-by-step, with α-diazomalonates under RuII and RhII catalysis. By selecting the catalyst, [CpRu(CH3 CN)3 ][PF6 ] (Cp=cyclopentadienyl) or Rh2 (oct)4 , chemo and regioselective insertions of derived metal carbenes are achieved in favor of mono- or bis-functionalized malonate derivatives, respectively, (r.r.>49 : 1, up to 77 % yield, 12 examples). This multi-introduction of malonate groups is particularly useful to tune optical and chemical properties such as absorption, emission or Brønsted acidity but also cellular bioimaging. Density-functional theory further elucidates the origin of the carbene insertion selectivity and also showcases the importance of conformations in the optical response.

Highly Site-Selective Direct C-H Bond Functionalization of Unactivated Arenes with Propargyl α-Aryl-α-diazoacetates via Scandium Catalysis

Highly chemo- and regio-selective C-H bond functionalization of unactivated arenes with propargyl α-aryl-α-diazoacetates has been developed using scandium catalysis. A variety of unactivated, mildly deactivated, and electronically activated arenes have been functionalized using this protocol. The synergistic combination of scandium triflate as a catalyst and propargyl α-aryl-α-diazoacetate as a reagent played a pivotal role in the effective C-H bond functionalization of arenes without the assistance of any directing group or ligand. The practicality of the protocol has been demonstrated by the gram-scale synthesis of very useful α,α-diarylacetates including antispasmodic drug-adiphenine. Based on the experimental observations, labeling experiment, and density functional theory calculations, a plausible reaction mechanism has been outlined.

Theoretical study on the mechanism, chemo- and enantioselectivity of the Ag- vs. Rh-catalyzed intramolecular carbene transfer reaction of diazoacetamides

To explore the mechanism of silver and rhodium catalysis and reveal the origin of the chemo- and enantioselectivity of the reaction, density functional theory calculations were performed on the first silver-catalyzed highly enantioselective carbene transfer reaction. The calculation results reveal that when silver is used as a catalyst, due to the participation of the phosphate anion in the transition state, the enhanced nucleophilicity of the α-diazoacetamide unit promotes smooth dearomatization before generation of the silver carbene. Because the generated rhodium carbene has stronger electrophilicity, typical carbene reactions (C-H insertion and the Büchner reaction) are favored. In addition, in the process of silver catalyzed dearomatization, the formation of an R-type transition state is determined by the small torsion energy and strong interaction energy.

Overcoming O-H Insertion to Para-Selective C-H Functionalization of Free Phenols: Rh(II)/Xantphos Catalyzed Geminal Difunctionalization of Diazo Compounds

Para-selective C-H functionalization of free phenols by metal carbenoids is rather challenging due to the generally more favorable competing O-H insertion. Herein, with the use of the combination of Rh(II) and a Xantphos ligand as the catalyst, a novel multicomponent reaction of free phenols, diazoesters, and allylic carbonates was successfully developed, affording a wide variety of phenol derivatives, bearing an all-carbon quaternary center and a synthetically useful allylic unit. This reaction is likely to occur through a tandem process of carbene-induced para-selective C-H functionalization, followed by Rh(II)/Xantphos-catalyzed allylation. The distinctive reactivity of para-selective C-H rather than O-H insertion for the carbenoid intermediate, combined with features of excellent functional group compatibility, high atom and step economy, and ease in further diversification of the products, might render this protocol highly attractive in facile functionalization of unprotected phenols.

Selective Functionalization of Arene C(sp2)–H Bonds by Gold Catalysis: The Role of Carbene Substituents

The complete regioselective incorporation of carbene units to nonactivated arene rings has been achieved employing gold(I) catalysts bearing alkoxydiaminophosphine ligands, with readily available, nonelaborated ethyl 2-phenyldiazoacetate as the carbene source. These results are in contrast with the scarce precedents which required highly elaborated diazo substrates. Density functional theory (DFT) calculations have revealed the important role of the R group in the C(R)CO₂Et fragment, which dramatically affects the energy profile of this transformation.

Asymmetric Arylation of Diazoesters with Anisoles Enabled by Cooperative Gold and Phosphoric Acid Catalysis

An enantioselective insertion of a carbene into the Csp²-H bond of anisole derivatives has been accomplished using an achiral gold complex and a chiral phosphoric acid as the catalytic system, providing a novel protocol for the synthesis of chiral α,α-diaryl acetates. Density functional theory calculations reveal the reactivity and the origin of the enantioselectivity of this reaction.

Recent advances in transition-metal-catalyzed carbene insertion to C-H bonds

C-H functionalization has been emerging as a powerful method to establish carbon-carbon and carbon-heteroatom bonds. Many efforts have been devoted to transition-metal-catalyzed direct transformations of C-H bonds. Metal carbenes generated in situ from transition-metal compounds and diazo or its equivalents are usually applied as the transient reactive intermediates to furnish a catalytic cycle for new C-C and C-X bond formation. Using this strategy compounds from unactivated simple alkanes to complex molecules can be further functionalized or transformed to multi-functionalized compounds. In this area, transition-metal-catalyzed carbene insertion to C-H bonds has been paid continuous attention. Diverse catalyst design strategies, synthetic methods, and potential applications have been developed. This critical review will summarize the advance in transition-metal-catalyzed carbene insertion to C-H bonds dated up to July 2021, by the categories of C-H bonds from aliphatic C(sp³)-H, aryl (aromatic) C(sp²)-H, heteroaryl (heteroaromatic) C(sp²)-H bonds, alkenyl C(sp²)-H, and alkynyl C(sp)-H, as well as asymmetric carbene insertion to C-H bonds, and more coverage will be given to the recent work. Due to the rapid development of the C-H functionalization area, future directions in this topic are also discussed. This review will give the authors an overview of carbene insertion chemistry in C-H functionalization with focus on the catalytic systems and synthetic applications in C-C bond formation.

Highly Stereoselective Synthesis of Tetrasubstituted Vinyl Selenides via Rhodium-Catalyzed [1,4]-Acyl Migration of Selenoesters and Diazo Compounds

Herein, we disclose a highly stereoselective Rh(II)-catalyzed 1,4-acyl rearrangement of selenium esters and α-diazo carbonyl compounds, which provides an efficient method for synthesizing tetrasubstituted vinyl selenides. Furthermore, this reaction also offers a synthetic tool for medium and large ring compounds.

Rhodium-Catalyzed C-H Methylation and Alkylation Reactions by Carbene-Transfer Reactions

In this combined computational and experimental study, the C-H functionalization of 2-phenyl pyridine with diazoalkanes was investigated. Initial evaluation by computational methods allowed the evaluation of different metal catalysts and diazoalkanes and their compatibility in this C-H functionalization reaction. With these findings, suitable reaction conditions for the C-H methylation reactions were quickly identified by using highly reactive TMS diazomethane and C-H alkylation reactions with donor/acceptor diazoalkanes, which is applied to a broad scope on alkylation reactions of 2-aryl pyridines with TMS diazomethane and donor/acceptor diazoalkane (51 examples, up to 98 % yield).

Nickel/Brønsted acid dual-catalyzed regioselective C–H bond allylation of phenols with 1,3-dienes

A nickel/Brønsted acid dual-catalyzed C-H bond ortho-allylation of phenols with 1,3-dienes has been developed. This methodology is readily applicable to the modification of complex pharmaceutical molecules.

Catalytic Gold Chemistry: From Simple Salts to Complexes for Regioselective C-H Bond Functionalization

Gold coordinated to neutral phosphines (R₃ P), N-heterocyclic carbenes (NHCs) or anionic ligands is catalytically active in functionalizing various C-H bonds with high selectivity. The sterics/electronic nature of the studied C-H bond, oxidation state of gold and stereoelectronic capacity of the coordinated auxiliary ligand are some of the associated selectivity factors in gold-catalyzed C-H bond functionalization reactions. Hence, in this review a comprehensive update about the action of different types of gold catalysts, from simple to sophisticated ones, on C-H bond reactions and their regiochemical outcome is disclosed. This review also highlights the catalytic applications of Au(I)- and Au(III)-species in creating new opportunities for the regio- and site-selective activation of challenging C-H bonds. Finally, it also intends to stress the potential applications in selective C-H bond activation associated with a variety of heterocycles recently described in the literature.

Recent Advances in Theoretical Studies on Transition-Metal-Catalyzed Carbene Transformations

Metal carbene plays a vital role in modern organic synthesis. The neutral divalent carbon of metal carbene renders it an active intermediate throughout a range of reactions. In experiments, diverse metal carbene-related transformation reactions have been established, including transition-metal-catalyzed cross-coupling reactions using N-heterocyclic carbenes as ligands, metal carbene insertion into σ bonds, cyclopropanations, ylide formation, and so forth. The remarkable progress achieved in synthetic chemistry, in turn, has increased the demand for mechanistic studies of carbene chemistry. A thorough understanding of reaction mechanisms can extend the application scope of metal carbene compounds and inspire the rational design of new carbene transformation reactions.Density functional theory (DFT) calculations have been performed in our group to gain more mechanistic insights into metal carbene-related reactions. This account focuses on computational studies of transition-metal-catalyzed carbene transformation reactions with nucleophiles. The generation of metal carbene or metal-ligated free carbene and subsequent carbene transformation pathways is discussed. According to our mechanistic studies of carbene transformation with nucleophiles, three generalized reaction models are summarized, including the intramolecular migratory insertion of metal carbene, intermolecular nucleophilic addition toward metal carbene, and outer-sphere nucleophilic addition to the metal-ligated free carbene.In general, the intermolecular nucleophilic addition mechanism is commonly proposed since metal carbene has an electrophilic carbene carbon. From a mechanistic point of view, the intramolecular migratory insertion mechanism is also widely used because metal carbene insertion into σ bonds formally occurs through this mechanism. An outer-sphere nucleophilic addition mechanism is proposed for reactions that form a metal-ligated free carbene complex instead of the commonly proposed metal carbene. The metal-ligated free carbene complex contains a naked carbene carbon that is not coordinated with the metal center. In this case, a transition-metal catalyst is used only as a Lewis acid, and nucleophilic addition occurs directly at the free carbene carbon. Our computational results suggested that outer-sphere nucleophilic addition is a facile step because metal ligation could stabilize the transition state as well as the generated intermediate. The intramolecular migratory insertion mechanism also has a low energy barrier due to the lack of an entropy penalty. Carbene formation from carbene precursors is usually the rate-determining step, except in intermolecular nucleophilic addition, and the reactivity of nucleophiles has a significant influence on the overall reaction rate. We can also envision that the weak nucleophilicity of nucleophiles would suppress outer-sphere nucleophilic addition. These computational studies showcase the characteristics of three carbene transformation models, and we hope that it will spur the development of mechanistic studies of carbene chemistry.

Transition-metal-catalyzed ortho-selective C-H functionalization reactions of free phenols

Phenols are important readily available chemical feedstocks and versatile synthetic building blocks for diverse synthetic transformations. Their motifs are prevalent in a diverse array of natural products, pharmaceuticals, functional materials, and privileged chiral ligands. Consequently, the development of facile and direct site-selective C-H bond functionalization of free phenols is of great importance and considerable interest to both industry and academic research. Over the past decades, transition-metal-catalyzed C-H bond functionalization has become as a powerful synthetic tool in organic synthesis. In this review, we provide a brief overview of recent progress in the transition-metal-catalyzed direct ortho-selective C-H functionalization of free phenols.

Multi C-H Functionalization Reactions of Carbazole Heterocycles via Gold-Catalyzed Carbene Transfer Reactions

Herein we describe a multiple C-H functionalization reaction of carbazole heterocycles with diazoalkanes. We show that gold catalysts play a distinct role in enabling a multiple C-H functionalization reaction to introduce up to six carbene fragments onto molecules containing multiple carbazole units or to link multiple carbazole units into a single molecule. A one-pot stepwise approach enables the introduction of two different carbene fragments to allow orthogonal deprotection and straightforward derivatization.

Iron-catalysed chemo- and ortho-selective C–H bond functionalization of phenols with α-aryl-α-diazoacetates

We present a novel chemo- and ortho-selective C–H bond functionalization of phenols with α-aryl-α-diazoacetates catalysed by a new iron porphyrin.

Catalyst-free synthesis of α,α-disubstituted carboxylic acid derivatives under ambient conditions via a Wolff rearrangement reaction

Herein, a hexafluoroisopropanol (HFIP)-promoted Wolff rearrangement reaction was developed, delivering various α,α-disubstituted carboxylic acid derivatives in good to excellent yields.

Brønsted acid catalysed chemo- and ortho-selective aminomethylation of phenol

We have developed a Brønsted acid catalysed highly ortho-selective functionalization of free phenols with readily available N,O-acetals under mild conditions, furnishing various corresponding aminomethylated phenol products in moderate to excellent yields. The salient features of this transformation include mild conditions, good substrate scope, excellent ortho-selectivity, high efficiency, and ease of further transformation.

Rationalization of the mechanism and chemoselectivity of versatile Au-catalyzed reactions of diazoesters with allyl-functionalized sulfides, selenides, amines, or ethers by DFT

The origin of chemoselectivity and the mechanism of the title reactions were fully rationalized by density functional theory (DFT).

How CuCl and CuCl2 Insert into C-N Bonds of Diazo Compounds: An Electronic Structure and Mechanistic Study

The transition-metal Cu catalysts CuCl and CuCl₂ have been widely employed to catalyze a series of chemical reactions with diazo compounds because of their high efficiency and selectivity. However, how to yield the active Cu carbene species from the Cu catalysts and diazo compounds still remains unclear. In this work, we performed a comprehensive theoretical investigation on the electronic structures of CuCl and CuCl₂ in solution. The results indicate that the most stable structures for CuCl and CuCl₂ are dimer and monomer, respectively. The C-N bond insertion of aryldiazoacetate by CuCl yields a stable bimetallic carbene species, which differs from the monometallic carbene generated from CuCl₂.

K2S2O8-HFIP synergistically promoted para-selective sp3 C-H bond diarylation of glycine esters

A metal-free K₂S₂O₈-HFIP synergistically promoted double Friedel-Crafts alkylation between a glycine derivative and N-substituted aniline was developed to efficiently synthesize diarylmethane derivatives with high para-selectivity. The reaction proceeded smoothly in the absence of any metal and ligand, and exhibited a good tolerance of functional groups.

Gold-catalyzed intermolecular [4+1] spiroannulation via site-selective aromatic C(sp2)–H functionalization and dearomatization of phenol derivatives

A novel gold(i)-catalyzed chemo- and site-selective cascade C–H functionalization/dearomatization of naphthols or phenols with o-alkynylaryl-α-diazoesters has been developed.

Selective functionalisation of aromatic alcohols with supramolecularly regulated gold(i) catalysts

Phosphite-based Au(i) catalytic systems containing an array of structurally diverse polyether-based regulation sites were designed and synthesised.

Copper-catalysed ortho-selective C–H bond functionalization of phenols and naphthols with α-aryl-α-diazoesters

An unprecedented CuCl₂-catalysed chemo- and ortho-selective C–H bond functionalization of phenols and naphthols with diazoesters has been developed.