Overcoming the Limitations of γ- and δ-C-H Arylation of Amines through Ligand Development

Pd-Catalyzed γ-C(sp3)-H Fluorination of Free Amines

The first example of free amine γ-C(sp³)-H fluorination is realized using 2-hydroxynicotinaldehyde as the transient directing group. A wide range of cyclohexyl and linear aliphatic amines could be fluorinated selectively at the γ-methyl and methylene positions. Electron withdrawing 3,5-disubstituted pyridone ligands were identified to facilitate this reaction. Computational studies suggest that the turnover determining step is likely the oxidative addition step for methylene fluorination, while it is likely the C-H activation step for methyl fluorination. The explicit participation of Ag results in a lower energetic span for methylene fluorination and a higher energetic span for methyl fluorination, which is consistent with the experimental observation that the addition of silver salt is desirable for methylene but not for methyl fluorination. Kinetic studies on methyl fluorination suggest that the substrate and PdL are involved in the rate-determining step, indicating that the C-H activation step may be partially rate-determining. Importantly, an energetically preferred pathway has identified an interesting pyridone-assisted bimetallic transition state for the oxidative addition step in methylene fluorination, thus uncovering a potential new role of the pyridone ligand.

Total Synthesis of (-)-Maximiscin

A short, enantioselective synthesis of (-)-maximiscin, a structurally intriguing metabolite of mixed biosynthetic origin, is reported. A retrosynthetic analysis predicated on maximizing ideality and efficiency led to several unusual disconnections and tactics. Formation of the central highly oxidized pyridone ring through a convergent coupling at the end of the synthesis simplified the route considerably. The requisite building blocks could be prepared from feedstock materials (derived from shikimate and mesitylene). Strategies rooted in hidden symmetry recognition, C-H functionalization, and radical retrosynthesis played key roles in developing this concise route.

Ligand-Enabled β-Methylene C(sp3 )-H Arylation of Masked Aliphatic Alcohols

Despite recent advances, reactivity and site-selectivity remain significant obstacles for the practical application of C(sp3 )-H bond functionalization methods. Here, we describe a system that combines a salicylic-aldehyde-derived L,X-type directing group with an electron-deficient 2-pyridone ligand to enable the β-methylene C(sp3 )-H arylation of aliphatic alcohols, which has not been possible previously. Notably, this protocol is compatible with heterocycles embedded in both alcohol substrates and aryl coupling partners. A site- and stereo-specific annulation of dihydrocholesterol and the synthesis of a key intermediate of englitazone illustrate the practicality of this method.

Palladium-Catalyzed Oxidation of β-C(sp3)-H Bonds of Primary Alkylamines through a Rare Four-Membered Palladacycle Intermediate

Site-selective functionalizations of C-H bonds are often achieved with a directing group that leads to five- or six-membered metallacyclic intermediates. Analogous reactions that occur through four-membered metallacycles are rare. We report a challenging palladium-catalyzed oxidation of primary C-H bonds β to nitrogen in an imine of an aliphatic amine, a process that occurs through a four-membered palladacyclc intermediate. The success of the reaction relies on the identification, by H/D exchange, of a simple directing group (salicylaldehyde) capable of inducing the formation of this small ring. To gain insight into the steps of the catalytic cycle of this unusual oxidation reaction, a series of mechanistic experiments and density functional theory (DFT) calculations were conducted. The experimental studies showed that cleavage of the C-H bond is rate-limiting and formation of the strained four-membered palladacycle is thermodynamically uphill. DFT calculations corroborated these conclusions and suggested that the presence of an intramolecular hydrogen bond between the oxygen of the directing group and hydroxyl group of the ligating acetic acid is crucial for stabilization of the palladacyclic intermediate.

Systematic Investigation of the Scope of Transannular C-H Heteroarylation of Cyclic Secondary Amines for Synthetic Application in Medicinal Chemistry

Transannular C-H heteroarylation of amines provides rapid access to complex scaffolds that are otherwise difficult to synthesize. Wide adaptation of this emerging reaction for medicinal chemistry requires a broad understanding of substrate scope and more robust experimental conditions. In this article, we report a new ligand to promote the transannular reaction of a range of fused- and bridged-bicyclic secondary amines with a broad set of heteroarenes. The method was also successfully applied to the arylation of one spiro-bicyclic amine, a class of substrates that has not been studied in the context of transannular C-H activation reactions. The broad application of this transannular C-H heteroarylation methodology is currently hampered by the difficulty of removing the directing group. The development of a new directing group that is easier to remove will expand the utility of this reaction.

Ligand-Enabled Pd(II)-Catalyzed C(sp3)-H Lactonization Using Molecular Oxygen as Oxidant

Pd(II)-catalyzed C-H lactonization of o-methyl benzoic acid substrates has been achieved using molecular oxygen as the oxidant. This finding provides a rare example of C-H oxygenation through Pd(II)/Pd(0) catalysis as well as a method to construct biologically important benzolactone scaffolds. The use of a gas mixture of 5% oxygen in nitrogen demonstrated the possibility for its application in pharmaceutical manufacturing.

Rational Development of Remote C-H Functionalization of Biphenyl: Experimental and Computational Studies

A simple and efficient nitrile-directed meta-C-H olefination, acetoxylation, and iodination of biaryl compounds is reported. Compared to the previous approach of installing a complex U-shaped template to achieve a molecular U-turn and assemble the large-sized cyclophane transition state for the remote C-H activation, a synthetically useful phenyl nitrile functional group could also direct remote meta-C-H activation. This reaction provides a useful method for the modification of biaryl compounds because the nitrile group can be readily converted to amines, acids, amides, or other heterocycles. Notably, the remote meta-selectivity of biphenylnitriles could not be expected from previous results with a macrocyclophane nitrile template. DFT computational studies show that a ligand-containing Pd-Ag heterodimeric transition state (TS) favors the desired remote meta-selectivity. Control experiments demonstrate the directing effect of the nitrile group and exclude the possibility of non-directed meta-C-H activation. Substituted 2-pyridone ligands were found to be key in assisting the cleavage of the meta-C-H bond in the concerted metalation-deprotonation (CMD) process.

Functionalization of Piperidine Derivatives for the Site-Selective and Stereoselective Synthesis of Positional Analogues of Methylphenidate

Rhodium‐catalyzed C−H insertions and cyclopropanations of donor/acceptor carbenes have been used for the synthesis of positional analogues of methylphenidate. The site selectivity is controlled by the catalyst and the amine protecting group. C−H functionalization of N‐Boc‐piperidine using Rh₂(R‐TCPTAD)₄, or N‐brosyl‐piperidine using Rh₂(R‐TPPTTL)₄ generated 2‐substitited analogues. In contrast, when N‐α‐oxoarylacetyl‐piperidines were used in combination with Rh₂(S‐2‐Cl‐5‐BrTPCP)₄, the C−H functionalization produced 4‐susbstiuted analogues. Finally, the 3‐substituted analogues were prepared indirectly by cyclopropanation of N‐Boc‐tetrahydropyridine followed by reductive regio‐ and stereoselective ring‐opening of the cyclopropanes.

Ligand-Controlled Direct γ-C-H Arylation of Aldehydes

The first example of Pd^II -catalyzed γ-C(sp³ )-H functionalization of aliphatic and benzoheteroaryl aldehydes has been developed using a transient ligand and an external ligand, concurrently. A wide array of γ-arylated aldehydes were readily accessed without preinstalling internal directing groups. The catalytic mechanism was studied by performing deuterium-labelling experiments, which indicated that the γ-C(sp³ )-H bond cleavage is the rate-limiting step during the reaction process. This reaction could be performed on a gram scale, and also demonstrated its potential application in the synthesis of new mechanofluorochromic materials with blue-shifted mechanochromic properties.

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds

During the past decades, synthetic organic chemistry discovered that directing group assisted C-H activation is a key tool for the expedient and siteselective construction of C-C bonds. Among the various directing group strategies, bidentate directing groups are now recognized as one of the most efficient devices for the selective functionalization of certain positions due to fact that its metal center permits fine, tunable, and reversible coordination. The family of bidentate directing groups permit various types of assistance to be achieved, such as N,N-dentate, N,O-dentate, and N,S-dentate auxiliaries, which are categorized based on the coordination site. In this review, we broadly discuss various C-H bond functionalization reactions for the formation of C-C bonds with the aid of bidentate directing groups.

Photoredox-Mediated Remote C(sp3)-H Heteroarylation of N-Alkyl Sulfonamides

A Minisci-type δ-selective C(sp³)-H heteroarylation of sulfonyl-protected primary aliphatic amines with N-heteroarenes under photoredox-catalyzed conditions was developed. The reaction typically uses a slight excess of amine reactant. The use of benziodoxole acetate (BI-OAc) oxidant and hexafluoroisopropanol solvent is critical to achieve high yield. Besides methylene C-H bonds, heteroarylation reactions of δ methyl C-H bonds also worked under more forced conditions. The reactions show a broad scope for both amine and N-heteroarene substrates, offering a straightforward method for synthesis of complex δ-heteroarylalkylmines from simple precursors.

1-Aminopyridinium Ylides as Monodentate Directing Groups for sp3 C-H Bond Functionalization

1-Aminopyridinium ylides are efficient directing groups for palladium-catalyzed β-arylation and alkylation of sp³ C-H bonds in carboxylic acid derivatives. The efficiency of these directing groups depends on the substitution at the pyridine moiety. The unsubstituted pyridine-derived ylides allow functionalization of primary C-H bonds, while methylene groups are unreactive in the absence of external ligands. 4-Pyrrolidinopyridine-containing ylides are capable of C-H functionalization in acyclic methylene groups in the absence of external ligands, thus rivaling the efficiency of the aminoquinoline directing group. Preliminary mechanistic studies have been performed. A cyclopalladated intermediate has been isolated and characterized by X-ray crystallography, and its reactivity was studied.

Pd-Catalyzed Heck-Type Reaction: Synthesizing Highly Diastereoselective and Multiple Aryl-Substituted P-Ligands

In this work, an efficient palladium-catalyzed Heck-type reaction was successfully used to synthesize a wide range of monoaryl- or diaryl-substituted P-ligands with excellent yields and diastereoselectivity (up to 98% yield, dr >20:1). The preliminary mechanistic studies demonstrated that it possibly underwent a cationic Heck reaction that was assisted by silver salt, and it revealed the significant role of P(O)Ar₂ for excellent yields and diastereoselectivity.

Palladium-Catalyzed Site-Selective C(sp3)-H Arylation of Phenylacetaldehydes

We describe a Pd-catalyzed selective C-H arylation reaction of phenylacetaldehydes using l-valine as the transient directing group. This process showed a broad substrate scope and excellent selectivity in which a ligand-controlled functionalization of the unactivated β-C(sp³)-H bond. In addition, enantioselective arylation of phenylacetaldehydes was preliminarily explored by utilizing a bulky chiral transient directing group.

Transient Ligand-Enabled Transition Metal-Catalyzed C-H Functionalization

Transition metal-catalyzed C-H bond functionalization is among the most efficient and powerful strategies in synthetic organic chemistry to derivatize otherwise inert sites of organic molecules for the construction of C-C and C-heteroatom bonds. However, additional steps are often required to install the directing groups to realize selective C-H bond functionalization of the substrates. These tedious steps run counter to the step-economical nature of the C-H activation. In contrast, direct functionalization of the substrate by using transient ligands avoids the unnecessary steps for the pre-functionalization of the substrates. This Minireview provides a short overview of the major progress made in this field for C-H functionalization at sp² and sp³ carbon centers with different transient working modes, including covalent, hydrogen, and ionic bonds.

Intramolecular palladium(ii)/(iv) catalysed C(sp3)-H arylation of tertiary aldehydes using a transient imine directing group

Palladium catalysed β-C(sp3)-H activation of tertiary aldehydes using a transient imine directing group enables intramolecular arylation to form substituted indane-aldehydes. A simple amine bearing a methyl ether (2-methoxyethan-1-amine) is the optimal TDG to promote C-H activation and reaction with an unactivated proximal C-Br bond. Substituent effects are studied in the preparation of various derivatives. Preliminary mechanistic studies identify a reversible C-H activation, product inhibition and suggest that oxidative addition is the turnover limiting step.

Pd-catalysed selective C(sp3)-H arylation and acetoxylation of alcohols

A palladium catalyzed selective C(sp3)-H arylation and acetoxylation of alcohols using a practical bidentate auxiliary were developed. Masked alcohols were selectively arylated at the β-position with diverse aryl iodides for the first time. Moreover, an efficient and site-selective acetoxylation of various primary methyl, methylene, and benzylic C(sp3)-H bonds was performed by using cheap K2S2O8 as the external oxidant.

The Pd-catalyzed C–H alkylation of ortho-methyl-substituted aromatic amides with maleimide occurs preferentially at the ortho-methyl C–H bond over the ortho-C–H bond

The reaction of ortho-methyl-substituted aromatic amides with maleimides in the presence of Pd(OAc)₂ and AgOAc results in C–H alkylation only at the ortho-methyl C–H bond.