Metal‐Free Cross‐Dehydrogenative Coupling (CDC): Molecular Iodine as a Versatile Catalyst/Reagent for CDC Reactions

The development of ecofriendly methods for carbon–carbon (C?C) and carbon–heteroatom (C?Het) bond formation is of great significance in modern‐day research. Metal‐free cross‐dehydrogenative coupling (CDC) has emerged as an important tool for organic and medicinal chemists as a means to form C?C and C?Het bonds, as it is atom economical and more efficient and greener than transition‐metal catalyzed CDC reactions. Molecular iodine (I₂) is recognized as an inexpensive, environmentally benign, and easy‐to‐handle catalyst or reagent to pursue CDCs under mild reaction conditions, with good regioselectivities and broad substrate compatibility. This review presents the recent developments of I₂‐catalyzed C?C, C?N, C?O, and C?S/C?Se bond‐forming reactions for the synthesis of various important organic molecules by cross‐dehydrogenative coupling.     

One‐Pot Tandem Photoredox and Cross‐Coupling Catalysis with a Single Palladium Carbodicarbene Complex

The combination of conventional transition‐metal‐catalyzed coupling (2 e^? process) and photoredox catalysis (1 e^? process) has emerged as a powerful approach to catalyze difficult cross‐coupling reactions under mild reaction conditions. Reported is a palladium carbodicarbene (CDC) complex that mediates both a Suzuki–Miyaura coupling and photoredox catalysis for C?N bond formation upon visible‐light irradiation. These two catalytic pathways can be combined to promote both conventional transition‐metal‐catalyzed coupling and photoredox catalysis to mediate C?H arylation under ambient conditions with a single catalyst in an efficient one‐pot process.     

Synthesis of Phenolic Compounds via Palladium Catalyzed C−H Functionalization of Arenes

Phenol and its derivatives are extremely useful compounds in organic synthesis, medicinal chemistry and material sciences. The synthesis of phenols involving selective construction of the C?O bond at a C?H bond of arenes using transition‐metal catalysis represents the most appealing strategy. Indeed, active research is currently going on for the synthesis of valuable phenolic compounds using a transition‐metal‐catalyzed C?H functionalization strategy. This short review summarizes recent advances on palladium‐catalyzed C?O bond forming reactions that enable direct access to phenolic compounds. These catalytic reactions proceed either via C?H esterification with trifluoroacetic acid/trifluoroacetic anhydride followed by in situ hydrolysis of the ester or via direct C?H hydroxylation. A brief analysis of substrate scope and limitation, reaction mechanism as well as synthetic utility of these reactions has been included.     

Gold‐Catalyzed Fluorination–Hydration: Synthesis of α‐Fluorobenzofuranones from 2‐Alkynylphenol Derivatives

The Au^I‐catalyzed fluorination–hydration of 2‐alkynylphenol derivatives in the presence of Selectfluor [1‐chloromethyl‐4‐fluoro‐1,4‐diazoniabicyclo‐[2.2.2]octane bis(tetrafluoroborate)] has been developed. This method provides straightforward access to α‐fluorobenzofuranones with the construction of C?O, C=O, and C?F bonds in a single step on the basis of an Au^I/Au^III redox catalytic cycle. Several control experiments, including the asymmetric variant of this reaction, were also conducted to gain insight into the reaction mechanism.     

Scalable Rhodium(III)‐Catalyzed Aryl C−H Phosphorylation Enabled by Anodic Oxidation Induced Reductive Elimination

Transition metal catalyzed C?H phosphorylation remains an unsolved challenge. Reported methods are generally limited in scope and require stoichiometric silver salts as oxidants. Reported here is an electrochemically driven Rh^III‐catalyzed aryl C?H phosphorylation reaction that proceeds through H₂ evolution, obviating the need for stoichiometric metal oxidants. The method is compatible with a variety of aryl C?H and P?H coupling partners and particularly useful for synthesizing triarylphosphine oxides from diarylphosphine oxides, which are often difficult coupling partners for transition metal catalyzed C?H phosphorylation reactions. Experimental results suggest that the mechanism responsible for the C?P bond formation involves an oxidation‐induced reductive elimination process.     

Silver‐Catalyzed Cross‐Coupling of Isocyanides and Active Methylene Compounds by a Radical Process

Isocyanides are versatile building blocks, and have been extensively exploited in C? H functionalization reactions. However, transition‐metal‐catalyzed direct C? H functionalization reactions with isocyanides suffer from over‐insertion of isocyanides. Reported herein is a radical coupling/isomerization strategy for the cross‐coupling of isocyanides with active methylene compounds through silver‐catalysis. The method solves the over‐insertion issue and affords a variety of otherwise difficult to synthesize β‐aminoenones and tricarbonylmethanes under base‐ and ligand‐free conditions. This report presents a new fundamental C? C bond‐forming reaction of two basic chemicals.     

Acyl Fluorides in Late‐Transition‐Metal Catalysis

In this Review, we summarize the current state of the art in late‐transition‐metal‐catalyzed reactions of acyl fluorides, covering both their synthesis and further transformations. In organic reactions, the relationship between stability and reactivity of the starting substrates is usually characterized by a trade‐off. Yet, acyl fluorides display a very good balance between these properties, which is mostly due to their moderate electrophilicity. Thus, acyl fluorides (RCOF) can be used as versatile building blocks in transition‐metal‐catalyzed reactions, for example, as an “RCO” source in acyl coupling reactions, as an “R” source in decarbonylative coupling reactions, and as an “F” source in fluorination reactions. Starting from the cleavage of the acyl C?F bond in acyl fluorides, various transformations are accessible, including C?C, C?H, C?B, and C?F bond‐forming reactions that are catalyzed by transition‐metal catalysts that contain the Group 9–11 metals Co, Rh, Ir, Ni, Pd, or Cu.     

Tandem Catalytic C(sp3)H Amination/Sila‐Sonogashira–Hagihara Coupling Reactions with Iodine Reagents

A new tandem C? N and C? C bond‐forming reaction has been achieved through Rh^II/Pd⁰ catalysis. The sequence first involves an iodine(III) oxidant, then the in situ generated iodine(I) by‐product is used as a coupling partner. The overall process demonstrates the synthetic value of iodoarenes produced in trivalent iodine reagent mediated oxidations.     

An Original L‐shape,Tunable N‐Heterocyclic Carbene Platform for Efficient Gold(I) Catalysis

The synthesis and characterization of original NHC ligands based on an imidazo[1,5‐a]pyridin‐3‐ylidene (IPy) scaffold functionalized with a flanking barbituric heterocycle is described as well as their use as tunable ligands for efficient gold‐catalyzed C?N, C?O, and C?C bond formations. High activity, regio‐, chemo‐, and stereoselectivities are obtained for hydroelementation and domino processes, underlining the excellent performance (TONs and TOFs) of these IPy‐based ligands in gold catalysis. The gold‐catalyzed domino reactions of 1,6‐enynes give rise to functionalized heterocycles in excellent isolated yields under mild conditions. The efficiency of the NHC gold 5^Me complex is remarkable and mostly arises from a combination of steric protection and stabilization of the cationic Au^I active species by ligand 1^Me .     

An Original L‐shape,Tunable N‐Heterocyclic Carbene Platform for Efficient Gold(I) Catalysis

The synthesis and characterization of original NHC ligands based on an imidazo[1,5‐a]pyridin‐3‐ylidene (IPy) scaffold functionalized with a flanking barbituric heterocycle is described as well as their use as tunable ligands for efficient gold‐catalyzed C?N, C?O, and C?C bond formations. High activity, regio‐, chemo‐, and stereoselectivities are obtained for hydroelementation and domino processes, underlining the excellent performance (TONs and TOFs) of these IPy‐based ligands in gold catalysis. The gold‐catalyzed domino reactions of 1,6‐enynes give rise to functionalized heterocycles in excellent isolated yields under mild conditions. The efficiency of the NHC gold 5^Me complex is remarkable and mostly arises from a combination of steric protection and stabilization of the cationic Au^I active species by ligand 1^Me .     

Organophosphine syntheses via activation of the phosphorus‐silicon bond of silylphosphines

This paper describes the recent advances in the syntheses of organophosphines by forming several types of phosphorus‐carbon bonds via activation of the phosphorus‐silicon bond of silylphosphines. In this account, the activation methods are classified into three types: nucleophile‐induced activations, reactions with Lewis acid‐activated electrophiles, and transition metal catalyzed reactions. Nucleophile‐induced activations of silylphosphines, especially by a fluoride, generated a reactive phosphide equivalent for selective formation of a P‐C bond. Silylphosphines also reacted selectively with Lewis acid‐activated electrophiles. The Lewis acid mediated/catalyzed additions and substitutions, to form sp³‐carbon‐phosphorus bonds including an asymmetric reaction, are described. Several important types of transition metal catalyzed reactions of silylphosphines are also mentioned in this account. Selective formation of a P‐C bond is achieved through these activations to produce a variety of functional organophosphines including optically active ones. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 236–245; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200900011     

B(C6F5)3‐Catalyzed sp3 C—Si Bond Forming Consecutive Reactions

Transition metal‐catalyzed hydrosilylation is one of the most widely utilized reduction methods as an alternative to hydrogenation in academia and industry. One feature distinct from hydrogenation would be able to install sp³ C—Si bond(s) onto substrates skeleton via hydrosilylation of alkenes. Recently, B(C₆F₅)₃ with hydrosilanes has been demonstrated to be an efficient, metal‐free catalyst system for the consecutive transformation of heteroatom‐containing substrates accompanied by the formation of sp³ C—Si bond(s), which has not been realized thus far under the transition metal‐catalyzed hydrosilylative conditions. In this review, I outline the B(C₆F₅)₃‐mediated consecutive hydrosilylations of heteroarenes containing quinolines, pyridines, and furans, and of conjugated nitriles/imines to provide a new family of compounds having sp³ C—Si bond(s) with high chemo‐, regio‐ and/or stereoselectivities. The silylative cascade conversion of unactivated N‐aryl piperidines to sila‐N‐heterocycles catalyzed by B(C₆F₅)₃ involving consecutive dehydrogenation, hydrosilylation, and intramolecular C(sp²)—H silylation, is presented in another section. Chemical selectivity and mechanism of the boron catalysis focused on the sp³ C—Si bond formation are highlighted.     

Highly Stereoselective Cobalt(III)‐Catalyzed Three‐Component C−H Bond Addition Cascade

A highly stereoselective three‐component C(sp²)?H bond addition across alkene and polarized π‐bonds is reported for which Co^III catalysis was shown to be much more effective than Rh^III. The reaction proceeds at ambient temperature with both aryl and alkyl enones employed as efficient coupling partners. Moreover, the reaction exhibits extremely broad scope with respect to the aldehyde input; electron rich and poor aromatic, alkenyl, and branched and unbranched alkyl aldehydes all couple in good yield and with high diastereoselectivity. Multiple directing groups participate in this transformation, including pyrazole, pyridine, and imine functional groups. Both aromatic and alkenyl C(sp²)?H bonds undergo the three‐component addition cascade, and the alkenyl addition product can readily be converted into diastereomerically pure five‐membered lactones. Additionally, the first asymmetric reactions with Co^III‐catalyzed C?H functionalization are demonstrated with three‐component C?H bond addition cascades employing N‐tert‐butanesulfinyl imines. These examples represent the first transition metal catalyzed C?H bond additions to N‐tert‐butanesulfinyl imines, which are versatile and extensively used intermediates for the asymmetric synthesis of amines.     

CO Oxidation Promoted by the Gold Dimer in Au2VO3− and Au2VO4− Clusters

Investigations on the reactivity of atomic clusters have led to the identification of the elementary steps involved in catalytic CO oxidation, a prototypical reaction in heterogeneous catalysis. The atomic oxygen species O^.? and O^2? bonded to early‐transition‐metal oxide clusters have been shown to oxidize CO. This study reports that when an Au₂ dimer is incorporated within the cluster, the molecular oxygen species O₂^2? bonded to vanadium can be activated to oxidize CO under thermal collision conditions. The gold dimer was doped into Au₂VO₄^? cluster ions which then reacted with CO in an ion‐trap reactor to produce Au₂VO₃^? and then Au₂VO₂^?. The dynamic nature of gold in terms of electron storage and release promotes CO oxidation and O? O bond reduction. The oxidation of CO by atomic clusters in this study parallels similar behavior reported for the oxidation of CO by supported gold catalysts.     

Light‐Induced Mechanistic Divergence in Gold(I) Catalysis: Revisiting the Reactivity of Diazonium Salts

In a systematic study of the Au‐catalyzed reaction of o‐alkynylphenols with aryldiazonium salts, we find that essentially the same reaction conditions lead to a change in mechanism when a light source is applied. If the reaction is carried out at room temperature using a Au^I catalyst, the diazonium salt undergoes electrophilic deauration of a vinyl Au^I intermediate and provides access to substituted azobenzofurans. If the reaction mixture is irradiated with blue LED light, C?C bond formation due to N₂‐extrusion from the diazonium salt is realized selectively, using the same starting materials without the need for an additional photo(redox) catalyst under aerobic conditions. We report a series of experiments demonstrating that the same vinyl Au^I intermediate is capable of producing the observed products under photolytic and thermal conditions. The finding that a vinyl Au^I complex can directly, without the need for an additional photo(redox) catalyst, result in C?C bond formation under photolytic conditions is contrary to the proposed mechanistic pathways suggested in the literature till date and highlights that the role of oxidation state changes in photoredox catalysis involving Au is thus far only poorly understood and may hold surprises for the future. Computational results indicate that photochemical activation can occur directly from a donor–acceptor complex formed between the vinyl Au^I intermediate and the diazonium salt.     

Rhodium(III)‐Catalyzed Alkenylation–Annulation of closo‐Dodecaborate Anions through Double B−H Activation at Room Temperature

1,2,3‐Trisubstituted closo‐dodecaborates with B?O, B?N, and B?C bonds as well as a fused borane oxazole ring have been synthesized by rhodium‐catalyzed direct cage B?H alkenylation and annulation of ureido boranes in the first reported example of regioselective B?H bond functionalization of the [B₁₂H₁₂]^2? cage by transition‐metal catalysis. This reaction proceeded at room temperature under ambient conditions and exhibited excellent selectivity for efficient monoalkenylation with good functional‐group tolerance. The urea moiety enabled B?H activation by acting as a directing group, was incorporated in the oxazole ring in situ, and also avoided multiple alkenylation. A possible mechanism is proposed on the basis of the isolation of a rhodium agostic intermediate and control experiments.     

1,3‐O‐Transposition or Trisubstituted Z‐Enol Ester? A Comparative Study of Reactions of Ynones

Ynones are useful substrates for transition‐metal‐mediated synthesis. The Au^I‐catalyzed 1,3‐O‐transposition is an important reaction of ynones. Recently, an efficient Cu^I‐catalyzed synthesis of trisubstituted Z‐enol esters via interrupting the traditional 1,3‐O‐transposition reaction of ynones was reported by Zhu's group. Herein, density functional theory studies disclosed that the hydrogen bond formed by carboxylic acid plays an important role for the reactivity and selectivity in this novel reaction. A qualitative rule was also found to explain the substituent effect in the ynone substrate, and this is consistent with experiments. The Au^I‐catalyst and Cu^I‐catalyst were further compared to interpret the essential cause of why the Au^I‐catalyst prefers the 1,3‐O‐transpostion reaction. These conclusions might be helpful for the rational design of reactions of ynones.     

Transmetalation in the Suzuki–Miyaura Coupling: The Fork in the Trail

The Suzuki–Miyaura coupling is one of the few transition‐metal‐catalyzed C? C bond‐forming reactions that have been used in applications ranging from discovery chemistry to manufacturing processes. Although coupling proceeds through the generic three‐stage ‘oxidative addition, transmetalation, reductive elimination’ sequence, there are a number of features that differentiate the Suzuki–Miyaura process from other transition‐metal‐catalyzed cross‐couplings. Most of these features are centered around, or are a consequence of, activation of the boron reagent for transmetalation through one or both of two distinct pathways. This review focuses on the evidence that has been presented for this ‘fork in the trail′, and the potential to apply such mechanistic insight to the design of reaction conditions.     

Enantioselective Trichloromethylation of MBH‐Fluorides with Chloroform Based on Silicon‐assisted C−F Activation and Carbanion Exchange Induced by a Ruppert–Prakash Reagent

Enantioselective trichloromethylation of Morita–Baylis–Hillman (MBH)‐type allylic fluorides with chloroform (HCCl₃) under organocatalysis was achieved with high to excellent enantioselectivities. Silicon‐assisted C?F bond activation by a Ruppert–Prakash reagent and direct activation of HCCl₃ by a carbanion exchange process with trifluoromethyl (CF₃) carbanion generated in situ from the Ruppert‐Prakash reagent realized the direct asymmetric trichloromethylation at a stereogenic allylic positon, without any help from transition metal catalysis, and under very mild conditions. Pre‐activation of HCCl₃ was not required. This method was extended to the direct enantioselective introduction of other C?H compounds such as alkyne, arene, indene, and FBSM without any pre‐activation under a metal‐free system.