Converting gem-dimethyl groups into cyclopropanes via Pd-catalyzed sequential C-H activation and radical cyclization

Sulfur-Mediated Formal Allylic C-H Cyclopropanation of α-Methylstyrenes

Allylic C-H cyclopropanation of α-methylstyrene and its derivatives was realized through a one-pot two-step sequence, formally converting two aliphatic C-H bonds to C-C bonds with a good yield and high diastereoselectivity, thus providing a quick entry to the synthetically useful vinyl cyclopropane structures.

Palladium-Catalyzed Enantioselective β-C(sp3)-H Activation Reactions of Aliphatic Acids: A Retrosynthetic Surrogate for Enolate Alkylation and Conjugate Addition

Enolate alkylation and conjugate addition into an α,β-unsaturated system have served as long-standing strategic disconnections for the installation of α- or β-substituents on carbonyl-containing compounds. At the onset of our efforts to develop C-H activation reactions for organic synthesis, we set our eye toward developing asymmetric β-C-H activation reactions of aliphatic acids with the perspective that this bond-forming event could serve as a more flexible retrosynthetic surrogate for both canonical carbonyl-related asymmetric transformations.In this Account, we describe our early efforts using strongly coordinating chiral oxazolines to probe reaction mechanism and the stereochemical nature of the C-H cleavage transition state. The characterization of key reactive intermediates through X-ray crystallography and computational studies suggested a transition state with C-H and Pd-OAc bonds being approximately coplanar for optimum interaction. We then moved forward to develop more practical, weakly coordinating monodentate amide directing groups, a necessary advance toward achieving the β-C-H activation of weakly coordinating native carboxylic acids. Throughout this journey, gradual deconvolution between a substrate's directing effect and its intimate interplay with ligand properties has culminated in the design of new ligand classes that ultimately allowed the competency of native carboxylic acids in β-C-H activation. These efforts established the importance of ligand acceleration in Pd-catalyzed C-H activation, where the substrate's weak coordination is responsible for positioning the catalyst for C-H cleavage, while the direct participation from the bifunctional ligand is responsible for enthalpically stabilizing the C-H cleavage transition state.Building upon these principles, we developed five classes of chiral ligands (MPAA, MPAQ, MPAO, MPAThio, MPAAM) to enable enantioselective β-C-H activation reactions, including carbon-carbon and carbon-heteroatom bond formation. The accumulated data from our developed enantioselective C-H activation reactions indicate that ligands possessing point chirality are most effective for imparting stereoinduction in the C-H activation step, the application of which enabled the desymmetrization and subsequent C-H functionalization of enantiotopic carbon and protons across a range of weakly coordinating arylamides and, more recently, free carboxylic acids. Progress in ligand design, in conjunction with the enabling nature of alkali metal countercations, led to the realization of a suite of β-methyl and now methylene C(sp3)-H activation reactions. These advancements also enabled the use of economical oxidants, such as peroxides and molecular oxygen, to facilitate catalyst turnover. In the future, continued progress in designing more efficient bifunctional chiral ligands is likely to provide a myriad of enantioselective β-C-H activation reactions of readily available native substrates.

Transition-Metal-Catalyzed, Coordination-Assisted Functionalization of Nonactivated C(sp3)-H Bonds

Transition-metal-catalyzed, coordination-assisted C(sp³)-H functionalization has revolutionized synthetic planning over the past few decades as the use of these directing groups has allowed for increased access to many strategic positions in organic molecules. Nonetheless, several challenges remain preeminent, such as the requirement for high temperatures, the difficulty in removing or converting directing groups, and, although many metals provide some reactivity, the difficulty in employing metals outside of palladium. This review aims to give a comprehensive overview of coordination-assisted, transition-metal-catalyzed, direct functionalization of nonactivated C(sp³)-H bonds by covering the literature since 2004 in order to demonstrate the current state-of-the-art methods as well as the current limitations. For clarity, this review has been divided into nine sections by the transition metal catalyst with subdivisions by the type of bond formation. Synthetic applications and reaction mechanism are discussed where appropriate.

Manganese/bipyridine-catalyzed non-directed C(sp3)-H bromination using NBS and TMSN3

A Mn(II)/bipyridine-catalyzed bromination reaction of unactivated aliphatic C(sp³)−H bonds has been developed using N-bromosuccinimide (NBS) as the brominating reagent. The reaction proceeded in moderate-to-good yield, even on a gram scale. The introduced bromine atom can be converted into fluorine and allyl groups.

β-Diazocarbonyl Compounds: Synthesis and their Rh(II)-Catalyzed 1,3 C-H Insertions

Herein, we describe the first electrophilic diazomethylation of ketone silyl enol ethers with diazomethyl-substituted hypervalent iodine reagents that gives access to unusual β-diazocarbonyl compounds. The potential of this unexplored class of diazo compounds for the development of new reactions was demonstrated by the discovery of a rare Rh-catalyzed intramolecular 1,3 C-H carbene insertion that led to complex cyclopropanes with excellent stereocontrol.

Vicinal, Double C-H Functionalization of Alcohols via an Imidate Radical-Polar Crossover Cascade

A double functionalization of vicinal sp3 C-H bonds has been developed, wherein a β amine and γ iodide are incorporated onto an aliphatic alcohol in a single operation. This approach is enabled by an imidate radical chaperone, which selectively affords a transient β alkene that is amino-iodinated in situ. Overall, the radical-polar-crossover cascade entails the following key steps: (i) β C-H iodination via 1,5-hydrogen atom transfer (HAT), (ii) desaturation via I2 complexation, and (iii) vicinal amino-iodination of an in situ generated allyl imidate. The synthetic utility of this double C-H functionalization is illustrated by conversion of aliphatic alcohols to a diverse collection of α,β,γ substituted products bearing heteroatoms on three adjacent carbons. The radical-polar crossover mechanism is supported by various experimental probes, including isotopic labeling, intermediate validation, and kinetic studies.

Photoexcited Pd(ii) auxiliaries enable light-induced control in C(sp3)-H bond functionalisation

Herein we report the photophysical and photochemical properties of palladacycle complexes derived from 8-aminoquinoline ligands, commonly used auxiliaries in C–H activation. Spectroscopic, electrochemical and computational studies reveal that visible light irradiation induces a mixed LLCT/MLCT charge transfer providing access to synthetically relevant Pd(iii)/Pd(iv) redox couples. The Pd(ii) complex undergoes photoinduced electron transfer with alkyl halides generating C(sp³)–H halogenation products rather than C–C bond adducts. Online photochemical ESI-MS analysis implicates participation of a mononuclear Pd(iii) species which promotes C–X bond formation via a distinct Pd(iii)/Pd(iv) pathway. To demonstrate the synthetic utility, we developed a general method for inert C(sp³)–H bond bromination, chlorination and iodination with alkyl halides. This new strategy in auxiliary-directed C–H activation provides predictable and controllable access to distinct reactivity pathways proceeding via Pd(iii)/Pd(iv) redox couples induced by visible light irradiation.

Visible light irradiation of 8-aminoquinoline Pd(ii) complexes initiates photoinduced electron transfer with alkyl halides, affording C–H halogenation over C–C bond adducts. A method for inert C(sp³)–H bond halogenation (Br, Cl and I) is reported.

β C-H di-halogenation via iterative hydrogen atom transfer

A radical relay strategy for mono- and di-halogenation (iodination, bromination, and chlorination) of sp3 C-H bonds has been developed. This first example of β C-H di-halogenation is achieved through sequential C-H abstraction by iterative, hydrogen atom transfer (HAT). A double C-H functionalization is enabled by in situ generated imidate radicals, which facilitate selective N˙ to C˙ radical translocation and tunable C-X termination. The versatile, geminal di-iodide products are further elaborated to β ketones and vinyl iodides. Mechanistic experiments explain the unique di-functionalization selectivity of this iterative HAT pathway, wherein the second C-H iodination is twice as fast as the first.

Highly Versatile β-C(sp3)-H Iodination of Ketones Using a Practical Auxiliary

The first example of palladium(II)-catalyzed β-C(sp3)-H iodination of a wide range of ketones using a commercially available aminooxyacetic acid auxiliary has been achieved. This L, X-type directing group overcomes the limitations of the transient directing group approach for C(sp3)-H functionalization of ketones. Practical advantages of this method include simple installation of the auxiliary without chromatography, exceptional tolerance of α-functional groups, as well as alkenes and alkynes, and rapid access to diverse sterically hindered quaternary centers.

Ligand-Enabled Pd(II)-Catalyzed Bromination and Iodination of C(sp3)-H Bonds

We herein report the palladium(II)-catalyzed bromination and iodination of a variety of α-hydrogen-containing carboxylic acid and amino acid-derived amides. These reactions are exclusively enabled by quinoline-type ligands. The halogenated products obtained in this reaction are highly versatile and rapidly undergo further diversification. Further, we report the first example of a free carboxylic acid-directed Pd(II)-catalyzed C(sp³)-H bromination, enabled by quinoline ligands.

Pd-catalysed ligand-enabled carboxylate-directed highly regioselective arylation of aliphatic acids

α-amino acids bearing aromatic side chains are important synthetic units in the synthesis of peptides and natural products. Although various β-C-H arylation methodologies for amino acid derivatives involving the assistance of directing groups have been extensively developed, syntheses that directly employ N-protected amino acids as starting materials remain rare. Herein, we report an N-acetylglycine-enabled Pd-catalysed carboxylate-directed β-C(sp3)-H arylation of aliphatic acids. In this way, various non-natural amino acids can be directly prepared from phthaloylalanine in one step in good to excellent yields. Furthermore, a series of aliphatic acids have been shown to be amenable to this transformation, affording β-arylated propionic acid derivatives in moderate to good yields. More importantly, this ligand-enabled direct β-C(sp3)-H arylation could be easily scaled-up to 10 g under reflux conditions, highlighting the potential utility of this synthetic method.

Silver-Catalyzed C(sp3)-H Chlorination

A silver-catalyzed chlorination of benzylic, tertiary, and secondary C(sp³)-H bonds was developed. The reaction proceeded with as low as 0.2 mol % catalyst loading at room temperature under air atmosphere with synthetically useful functional group compatibility. The regioselectivity and reactivity tendencies suggest that the chlorination proceeded through a radical pathway, but an intermediate alkylsilver species cannot be ruled out.

Palladium-Catalyzed Transformations of Alkyl C-H Bonds

This Review summarizes the advancements in Pd-catalyzed C(sp³)-H activation via various redox manifolds, including Pd(0)/Pd(II), Pd(II)/Pd(IV), and Pd(II)/Pd(0). While few examples have been reported in the activation of alkane C-H bonds, many C(sp³)-H activation/C-C and C-heteroatom bond forming reactions have been developed by the use of directing group strategies to control regioselectivity and build structural patterns for synthetic chemistry. A number of mono- and bidentate ligands have also proven to be effective for accelerating C(sp³)-H activation directed by weakly coordinating auxiliaries, which provides great opportunities to control reactivity and selectivity (including enantioselectivity) in Pd-catalyzed C-H functionalization reactions.

Auxiliary-assisted palladium-catalyzed halogenation of unactivated C(sp3)–H bonds at room temperature

The direct transformation of unactivated C(sp³)–H bonds into C-halogen bonds was achieved by palladium catalysis at room temperature with good functional group tolerance.

Pd-catalyzed direct oxidative mono-aroyloxylation of O-aralkyl substituted acetoxime ethers

A highly site-selective palladium-catalyzed ortho-mono-aroyloxylation of O-aralkyl substituted acetoxime ethers via direct Csp²–H bond activation has been developed with simple exo-acetoxime as a directing group.

Palladium-catalyzed bisarylation of 3-alkylbenzofurans to 3-arylalkyl-2-arylbenzofurans on water: tandem C(sp3)–H and C(sp2)–H activation reactions of 3-alkylbenzofurans

A protocol involving facile sequential C(sp³)–H and C(sp²)–H activation reactions of 3-alkylbenzofurans catalyzed by Pd(OAc)₂ in the presence of pivalic acid, silver salt, and tricyclohexylphosphine ‘on water’ was developed.

Rhodium-catalyzed oxidative cyclization of arylphosphonic acid monoethyl esters with alkenes: efficient synthesis of benzoxaphosphole 1-oxides

Rhodium-catalyzed tandem oxidative alkenylation and an intramolecular oxy-Michael reaction were developed using arylphosphonic acid monoethyl esters and alkenes under aerobic conditions, which produced benzoxaphosphole 1-oxides in good to excellent yields.

If C-H bonds could talk: selective C-H bond oxidation

C-H oxidation has a long history and an ongoing presence in research at the forefront of chemistry and interrelated fields. As such, numerous highly useful articles and reviews have been written on this subject. Logically, these are generally written from the perspective of the scope and limitations of the reagents employed. This Minireview instead attempts to emphasize chemoselectivity imposed by the nature of the substrate. Consequently, many landmark discoveries in the field of C-H oxidation are not discussed, but hopefully the perspective taken herein will allow C-H oxidation reactions to be more readily incorporated into synthetic planning.

Palladium-catalyzed C–H bond functionalization of C6-arylpurines

A highly regioselective Pd-catalyzed C_Ar–H bond activation method was developed for the modification of purines (nucleosides) with different functional groups by using purine as a directing group. This approach provides a new access to a variety of functionalized purines (nucleosides).

Investigation of the mechanism of C(sp3)-H bond cleavage in Pd(0)-catalyzed intramolecular alkane arylation adjacent to amides and sulfonamides

The reactivity of C(sp(3))-H bonds adjacent to a nitrogen atom can be tuned to allow intramolecular alkane arylation under Pd(0) catalysis. Diminishing the Lewis basicity of the nitrogen lone pair is crucial for this catalytic activity. A range of N-methylamides and sulfonamides react exclusively at primary C(sp(3))-H bonds to afford the products of alkane arylation in good yields. The isolation of a Pd(II) reaction intermediate has enabled an evaluation of the reaction mechanism with a focus on the role of the bases in the C(sp(3))-H bond cleaving step. The results of these stoichiometric studies, together with kinetic isotope effect experiments, provide rare experimental support for a concerted metalation-deprotonation (CMD) transition state, which has previously been proposed in alkane C(sp(3))-H arylation. Moreover, DFT calculations have uncovered the additional role of the pivalate additive as a promoter of phosphine dissociation from the Pd(II) intermediate, enabling the CMD transition state. Finally, kinetic studies were performed, revealing the reaction rate expression and its relationship with the concentration of pivalate.

Amide-Directed Arylation of sp C-H Bonds using Pd(II) and Pd(0) Catalysts

Protocols to effect β-arylation of sp(3) C-H bonds via Pd(II)/(IV) and Pd(0)/(II) catalytic cycles have been achieved using a newly developed monodentate CONHC(6)F(5) directing group. These reactions provide an unprecedented means to functionalize sp(3) C-H bonds in aliphatic carboxylic acid-derived substrates.

Palladium-catalyzed oxidative arylhalogenation of alkenes: synthetic scope and mechanistic insights

This article describes the development of a Pd-catalyzed reaction for the arylhalogenation (halogen = Cl or Br) of diverse alpha-olefins by oxidatively intercepting Mizoroki-Heck intermediates. These transformations afford synthetically useful 1,2- and 1,1-arylhalogenated products in good yields with good to excellent selectivities that can be modulated by changing the nature of the halogenating reagent and/or the reaction conditions. The selectivity of these reactions can be rationally tuned by (i) controlling the relative rates of oxidative functionalization versus beta-hydride elimination from equilibrating Pd(II)-alkyl species and (ii) stabilization of organometallic Pd(II) intermediates through the formation of pi-benzyl adducts. These arylhalogenations exhibit modest to excellent levels of stereoselectivity, and the key carbon-halogen bond-forming step proceeds with predominant retention of stereochemistry at carbon.

O-acetyl oximes as transformable directing groups for Pd-catalyzed C-H bond functionalization

O-Acetyl oximes serve as effective directing groups for Pd-catalyzed sp(2) and sp(3) C-H functionalization reactions. The C-H functionalization products can be subsequently transformed into ortho- or beta-functionalized ketones, alcohols, amines, and heterocycles.

TBDPS and Br-TBDPS protecting groups as efficient aryl group donors in Pd-catalyzed arylation of phenols and anilines

It is shown that the TBDPS protecting group can serve as an efficient phenyl group donor for o-bromophenols via Pd-catalyzed C-H arylation followed by a routine TBAF deprotection of the resulting silacycles. Employment of the newly designed Br-TBDPS protecting group in the same sequence allows for a facile introduction of a phenyl group at the ortho position of phenols and anilines. Alternatively, switching desilylation to oxidation in the last step allows the conversion of forming silacycles into valuable o-biphenols.

Synthesis of Fluorenes via the Palladium-Catalyzed 5-Exo-dig Annulation of o-Alkynyl Biaryls

The direct Pd-catalyzed intramolecular rapidly with electron-deficient benzene ring, which, in hydroarylation of o-alkynyl biaryls proceeded in highly combination with a substantial isotope effect observed, stereoselective manner producing fluorenes 2, the products of 5-exo-dig cyclization, in excellent yields. The cascade intermolecular arylation, incorporated in this transformation, allowed for efficient synthesis of fully-substituted fluorenes 12. These cyclizations proceed more rapidly with electron-deficient benzene ring, which, in combination with a substantial isotope effect observed, strongly supports a C-H activation mechanism for the key annulation step.