Electrocatalytic Access to Azetidines via Intramolecular Allylic Hydroamination: Scrutinizing Key Oxidation Steps through Electrochemical Kinetic Analysis.

Azetidines are prominent structural scaffolds in bioactive molecules, medicinal chemistry, and ligand design for transition metals. However, state-of-the-art methods cannot be applied to intramolecular hydroamination of allylic amine derivatives despite their underlying potential as one of the most prevalent synthetic precursors to azetidines. Herein, we report an electrocatalytic method for intramolecular hydroamination of allylic sulfonamides to access azetidines for the first time. The merger of cobalt catalysis and electricity enables the regioselective generation of key carbocationic intermediates, which could directly undergo intramolecular C-N bond formation. The mechanistic investigations including electrochemical kinetic analysis suggest that either the catalyst regeneration by nucleophilic cyclization or the second electrochemical oxidation to access the carbocationic intermediate is involved in the rate-determining step (RDS) of our electrochemical protocol and highlight the ability of electrochemistry in providing ideal means to mediate catalyst oxidation.

Copper Hydride-Catalyzed Enantioselective Olefin Hydromethylation.

The enantioselective installation of a methyl group onto a small molecule can result in the significant modification of its biological properties. While hydroalkylation of olefins represents an attractive approach to introduce alkyl substituents, asymmetric hydromethylation protocols are often hampered by the incompatibility of highly reactive methylating reagents and a lack of general applicability. Herein, we report an asymmetric olefin hydromethylation protocol enabled by CuH catalysis. This approach leverages methyl tosylate as a methyl source compatible with the reducing base-containing reaction environment, while a catalytic amount of iodide ion transforms the methyl tosylate in situ into the active reactant, methyl iodide, to promote the hydromethylation. This method tolerates a wide range of functional groups, heterocycles, and pharmaceutically relevant frameworks. Density functional theory studies suggest that after the stereoselective hydrocupration, the methylation step is stereoretentive, taking place through an SN2-type oxidative addition mechanism with methyl iodide followed by a reductive elimination.

Oxidatively Induced Reductive Elimination: Exploring the Scope and Catalyst Systems with Ir, Rh, and Ru Complexes.

Direct conversion of C-H bonds into C-C bonds is a promising alternative to the conventional cross-coupling reactions, thus giving rise to a wide range of efficient catalytic C-H functionalization reactions. Among the elementary stages in the catalytic C-C bond formation, reductive elimination constitutes a key step of the catalytic cycle, and, therefore, extensive studies have been made to facilitate this process. In this regard, oxidation on the metal center of a post-transmetalation intermediate would be an appealing approach. Herein, we have explored the substrate scope, catalyst systems, and oxidation tools to prove that the oxidatively induced reductive elimination ( ORE) plays a critical role in the product-releasing C-C bond formation. Notably, we have demonstrated that ORE broadly operates with a series of half-sandwich d6 Ir(III)-, Rh(III)-, and Ru(II)-aryl complexes. We have described that the metal center oxidation of the isolable post-transmetalation intermediates by means of chemical- or electro-oxidation can readily deliver the desired arylated products upon reductive elimination even at ambient temperature. Computational studies delineated the thermodynamics of the reductive elimination, where the activation barriers are shown to be significantly reduced upon increasing the oxidation states of the intermediates. We were also successful in corroborating this ORE in the corresponding Rh- methyl complex. In addition, catalytic conditions were optimized to incorporate this mechanistic understanding into the Ir-, Rh-, and Ru-catalyzed C-C bond formations under mild conditions.

Engaging Aldehydes in CuH-Catalyzed Reductive Coupling Reactions: Stereoselective Allylation with Unactivated 1,3-Diene Pronucleophiles.

Recently, CuH-catalyzed reductive coupling processes involving carbonyl compounds and imines have become attractive alternatives to traditional methods for stereoselective addition because of their ability to use readily accessible and stable olefins as surrogates for organometallic nucleophiles. However, the inability to use aldehydes, which usually reduce too rapidly in the presence of copper hydride complexes to be viable substrates, has been a major limitation. Shown here is that by exploiting relative concentration effects through kinetic control, this intrinsic reactivity can be inverted and the reductive coupling of 1,3-dienes with aldehydes achieved. Using this method, both aromatic and aliphatic aldehydes can be transformed into synthetically valuable homoallylic alcohols with high levels of diastereo- and enantioselectivities, and in the presence of many useful functional groups. Furthermore, using a combination of theoretical (DFT) and experimental methods, important mechanistic features of this reaction related to stereo- and chemoselectivities were uncovered.

Transition-metal-catalyzed C-N bond forming reactions using organic azides as the nitrogen source: a journey for the mild and versatile C-H amination.

Owing to the prevalence of nitrogen-containing compounds in functional materials, natural products and important pharmaceutical agents, chemists have actively searched for the development of efficient and selective methodologies allowing for the facile construction of carbon-nitrogen bonds. While metal-catalyzed C-N cross-coupling reactions have been established as one of the most general protocols for C-N bond formation, these methods require starting materials equipped with functional groups such as (hetero)aryl halides or their equivalents, thus generating stoichiometric amounts of halide salts as byproducts. To address this aspect, a transition-metal-catalyzed direct C-H amination approach has emerged as a step- and atom-economical alternative to the conventional C-N cross-coupling reactions. However, despite the significant recent advances in metal-mediated direct C-H amination reactions, most available procedures need harsh conditions requiring stoichiometric external oxidants. In this context, we were curious to see whether a transition-metal-catalyzed mild C-H amination protocol could be achieved using organic azides as the amino source. We envisaged that a dual role of organic azides as an environmentally benign amino source and also as an internal oxidant via N-N2 bond cleavage would be key to develop efficient C-H amination reactions employing azides. An additional advantage of this approach was anticipated: that a sole byproduct is molecular nitrogen (N2) under the perspective catalytic conditions. This Account mainly describes our research efforts on the development of rhodium- and iridium-catalyzed direct C-H amination reactions with organic azides. Under our initially optimized Rh(III)-catalyzed amination conditions, not only sulfonyl azides but also aryl- and alkyl azides could be utilized as facile amino sources in reaction with various types of C(sp(2))-H bonds bearing such directing groups as pyridine, amide, or ketoxime. More recently, a new catalyst system using Ir(III) species was developed for the direct C-H amidation of arenes and alkenes with acyl azides under exceptionally mild conditions. As a natural extension, amidation of primary C(sp(3))-H bonds could also be realized on the basis of the superior activity of the Cp*Ir(III) catalyst. Mechanistic investigations revealed that a catalytic cycle is operated mainly in three stages: (i) chelation-assisted metallacycle formation via C-H bond cleavage; (ii) C-N bond formation through the in situ generation of a metal-nitrenoid intermediate followed by the insertion of an imido moiety to the metal carbon bond; (iii) product release via protodemetalation with the concomitant catalyst regeneration. In addition, this Account also summarizes the recent advances in the ruthenium- and cobalt-catalyzed amination reactions using organic azides, developed by our own and other groups. Comparative studies on the relative performance of those catalytic systems are briefly described.

Cp*Ir(III)-Catalyzed Mild and Broad C-H Arylation of Arenes and Alkenes with Aryldiazonium Salts Leading to the External Oxidant-Free Approach.

Reported herein is the development of Cp*Ir(III)-catalyzed direct C-H arylation of arenes and alkenes using aryldiazonium tetrafluoroborates, the use of which as an aryl precursor and also as an oxidant via C-N2 bond cleavage was a key to success in achieving a mild and external oxidant-free procedure. Mechanistic experiments and DFT calculations revealed the turnover-limiting step to be closely related to the formation of an Ir(V)-aryl intermediate rather than the presupposed C-H cleavage. Under the developed mild arylation conditions, a wide range of benzamides were smoothly arylated. In addition, synthetic utility of the current C-H arylation procedure was also demonstrated successfully for the (Z)-selective arylation of enamides and C8-selective reaction of quinoline N-oxides.

Iridium-catalyzed C-H amination with anilines at room temperature: compatibility of iridacycles with external oxidants.

Described herein is the development of an iridium-catalyzed direct C-H amination of benzamides with anilines at room temperature, representing a unique example of an Ir catalyst system that is compatible with external oxidants. Mechanistic details, such as the isolation and characterization of key iridacycle intermediates, are also discussed.

Mechanistic studies of the rhodium-catalyzed direct C-H amination reaction using azides as the nitrogen source.

Direct C-H amination of arenes offers a straightforward route to aniline compounds without necessitating aryl (pseudo)halides as the starting materials. The recent development in this area, in particular in the metal-mediated transformations, is significant with regard to substrate scope and reaction conditions. Described herein are the mechanistic details on the Rh-catalyzed direct C-H amination reaction using organic azides as the amino source. The most important two stages were investigated especially in detail: (i) the formation of metal nitrenoid species and its subsequent insertion into a rhodacycle intermediate, and (ii) the regeneration of catalyst with concomitant release of products. It was revealed that a stepwise pathway involving a key Rh(V)-nitrenoid species that subsequently undergoes amido insertion is favored over a concerted C-N bond formation pathway. DFT calculations and kinetic studies suggest that the rate-limiting step in the current C-H amination reaction is more closely related to the formation of Rh-nitrenoid intermediate rather than the presupposed C-H activation process. The present study provides mechanistic details of the direct C-H amination reaction, which bears both aspects of the inner- and outer-sphere paths within a catalytic cycle.

Iridium(III)-catalyzed direct C-7 amination of indolines with organic azides.

Iridium-catalyzed regioselective C-7 amination of indolines has been achieved with organic azides as a facile nitrogen source. The developed procedure is convenient to perform even at room temperature and applicable to a wide range of substrates with high catalytic activity. Various types of organic azides (sulfonyl, aryl, and alkyl derivatives) were all successfully reacted under the present conditions as the viable reactant. Furthermore, indoline substrates bearing easily removable N-protecting groups such as N-Boc or N-Cbz could readily be employed, highlighting the synthetic utility of this methodology.

Ir(III)-catalyzed mild C-H amidation of arenes and alkenes: an efficient usage of acyl azides as the nitrogen source.

Reported herein is the development of the Ir(III)-catalyzed direct C-H amidation of arenes and alkenes using acyl azides as the nitrogen source. This procedure utilizes an in situ generated cationic half-sandwich iridium complex as a catalyst. The reaction takes place under very mild conditions, and a broad range of sp(2) C-H bonds of chelate group-containing arenes and olefins are smoothly amidated with acyl azides without the intervention of the Curtius rearrangement. Significantly, a wide range of reactants of aryl-, aliphatic-, and olefinic acyl azides were all efficiently amidated with high functional group tolerance. Using the developed approach, Z-enamides were readily accessed with a complete control of regio- and stereoselectivity. The developed direct amidation proceeds in the absence of external oxidants and releases molecular nitrogen as a single byproduct, thus offering an environmentally benign process with wide potential applications in organic synthesis and medicinal chemistry.

Palladium-Catalyzed Electrooxidative Hydrofluorination of Aryl-Substituted Alkenes with a Nucleophilic Fluorine Source.

Herein, we report an electrocatalytic hydrofluorination of aryl-substituted alkenes with a nucleophilic fluorine source. The merger of palladium catalysis with electrooxidation enables the transformation of various substrates ranging from styrenes to more challenging α,ß-unsaturated carbonyl derivatives to the corresponding benzylic fluorides. This method can also be applied to the late-stage modification of pharmaceutical derivatives. Mechanistic studies suggest that the generation of a high-valent palladium intermediate via anodic oxidation is the crucial step in this electrocatalytic hydrofluorination.

Copper-mediated sequential cyanation of aryl C-B and arene C-H bonds using ammonium iodide and DMF.

The cyanation of aromatic boronic acids, boronate esters, and borate salts was developed under copper-mediated oxidative conditions using ammonium iodide and DMF as the source of nitrogen and carbon atom of the cyano unit, respectively. The procedure was successfully extended to the cyanation of electron-rich benzenes, and regioselective introduction of a cyano group at the arene C-H bonds was also achieved. The observation that the reaction proceeds via a two-step process, initial iodination and then cyanation, led us to propose that ammonium iodide plays a dual role to provide iodide and nitrogen atom of the cyano moiety.

Rhodium-catalyzed direct C-H amination of benzamides with aryl azides: a synthetic route to diarylamines.

No muss, no fuss: A rhodium-catalyzed direct intermolecular C-H amination of benzamides and ketoximes using aryl azides as the amine source has been developed. The reaction exhibits a broad substrate scope with excellent functional-group tolerance, requires no external oxidants, releases N(2) as the only by-product, and produces diarylamines in high yields.

Exogenous Ligand-Free NiH-Catalyzed Hydroacylation of Aryl Alkenes with Aroyl Fluorides.

Acyl fluorides have emerged as efficient acyl group donors, but these attractive reagents have rarely been utilized in transition-metal-catalyzed hydroacylation. Herein we report a nickel hydride-catalyzed hydroacylation of aryl alkenes using aroyl fluorides. The reaction proceeds without recourse to an exogenous ligand under mild conditions. The synthetic utility of the present method is demonstrated by the glovebox-free, gram-scale reaction and the late-stage derivatization of complex molecules containing pharmaceutical frameworks.

Iridium-catalysed arylation of C-H bonds enabled by oxidatively induced reductive elimination.

Direct arylation of C-H bonds is in principle a powerful way of preparing value-added molecules that contain carbon-aryl fragments. Unfortunately, currently available synthetic methods are not sufficiently effective to be practical alternatives to conventional cross-coupling reactions. We propose that the main problem lies in the late portion of the catalytic cycle where reductive elimination gives the desired carbon-aryl bond. Accordingly, we have developed a strategy where the Ir(III) centre of the key intermediate is first oxidized to Ir(IV). Density functional theory calculations indicate that the barrier to reductive elimination is reduced by nearly 19 kcal mol-1 for this oxidized complex compared with that of its Ir(III) counterpart. Various experiments confirm this prediction, affording a new methodology capable of directly arylating C-H bonds at room temperature with a broad substrate scope and in good yields. This work highlights how the oxidation states of intermediates can be targeted deliberately to catalyse an otherwise impossible reaction.

Orthogonal reactivity of acyl azides in C-H activation: dichotomy between C-C and C-N amidations based on catalyst systems.

The dual reactivity of acyl azides was utilized successfully in C-H activation by the choice of catalyst systems: while selective C-C amidation was achieved under thermal Rh catalysis, a Ru catalyst was found to mediate direct C-N amidation also highly selectively. Investigations of the mechanistic dichotomy between two catalytic systems are also presented.

Chelation-assisted hydroesterification of alkenes: new ruthenium catalyst systems and ligand effects.

New types of ruthenium catalysts were developed for the chelation-assisted intermolecular olefin hydroesterification that employs 2-pyridylmethyl formate as an ester source. Two classes of ligands, NHCs and phosphines, were found to facilitate the reaction delivering isomeric ester products (linear versus branched) with different ratios, thus allowing access to ligand-guided selective hydroesterification.