Ligand-Enabled δ-C(sp3 )-H Borylation of Aliphatic Amines

Palladium-Catalyzed Regioselective Insertion of Carbenes into γ-C(sp3)-H Bonds of Aliphatic Amines

A migratory insertion of carbenes into distal γ-C(sp3)-H bonds of aliphatic amines has been successfully developed. The synergistic interplay among a palladium catalyst, picolinamide directing group, a carefully selected base additive, and an essential ligand proved crucial in achieving high yields. These findings hold significant value for advancing the exploration of regioselective carbene insertions into nonactivated C(sp3)-H bonds.

Transition-Metal-Catalyzed Silylation and Borylation of C-H Bonds for the Synthesis and Functionalization of Complex Molecules

The functionalization of C-H bonds in organic molecules containing functional groups has been one of the holy grails of catalysis. One synthetically important approach to the diverse functionalization of C-H bonds is the catalytic silylation or borylation of C-H bonds, which enables a broad array of downstream transformations to afford diverse structures. Advances in both undirected and directed methods for the transition-metal-catalyzed silylation and borylation of C-H bonds have led to their rapid adoption in early-, mid-, and late-stage of the synthesis of complex molecules. In this Review, we review the application of the transition-metal-catalyzed silylation and borylation of C-H bonds to the synthesis of bioactive molecules, organic materials, and ligands. Overall, we aim to provide a picture of the state of art of the silylation and borylation of C-H bonds as applied to the synthesis and modification of diverse architectures that will spur further application and development of these reactions.

Simplifying the Synthesis of Nonproteinogenic Amino Acids via Palladium-Catalyzed δ-Methyl C-H Olefination of Aliphatic Amines and Amino Acids

Transition metal-catalyzed directing group assisted C-H functionalizations provide a straightforward access to a wide variety of nonproteinogenic amino acids. While altering the side chain of an existing natural amino acids is one way, introducing a functional group to an aliphatic amine to synthesize versatile unnatural amino acids is another exciting avenue. In this work, we explore both the possibilities by the palladium-catalyzed δ-C(sp³)-H olefination of aliphatic amines and amino acids. A diverse substrate scope including sequential difunctionalizations followed by post synthetic transformations were achieved to understand the applicability of the current protocol. An in-depth mechanistic study was carried out to learn the mode of the reaction pathway.

Transition metal-free visible light photoredox-catalyzed remote C(sp3)-H borylation enabled by 1,5-hydrogen atom transfer

The borylation of unreactive carbon-hydrogen bonds is a valuable method for transforming feedstock chemicals into versatile building blocks. Here, we describe a transition metal-free method for the photoredox-catalyzed borylation of unactivated C(sp³)-H bond, initiated by 1,5-hydrogen atom transfer (HAT). The remote borylation was directed by 1,5-HAT of the amidyl radical, which was generated by photocatalytic reduction of hydroxamic acid derivatives. The method accommodates substrates with primary, secondary and tertiary C(sp³)-H bonds, yielding moderate to good product yields (up to 92%) with tolerance for various functional groups. Mechanistic studies, including radical clock experiments and DFT calculations, provided detailed insight into the 1,5-HAT borylation process.

Chiral Phosphoric Acid-Palladium(II) Complex Catalyzed Asymmetric Desymmetrization of Biaryl Compounds by C(sp3)-H Activation

Desymmetrization is an essential method for the synthesis of chiral compounds, particularly chiral biaryls. We have developed an enantioselective synthesis of axially chiral biaryls by desymmetrization using C(sp³)-H activation catalyzed by chiral palladium phosphate. Mechanistic studies show that C-H activation is the rate- and enantiomer-determining step. To the best of our knowledge, this is the first report of asymmetric desymmetrization of axially chiral compounds by C(sp³)-H activation.

Copper-Catalyzed Regioselective Remote C-H Bond Chalcogenation of Aromatic Amine Derivatives without Using Any Large Template

A mild and convenient strategy has been developed for the para-selective chalcogenation of anilide scaffolds via C-H bond functionalization. This methodology employs one of the most earth-abundant and inexpensive Cu(II) catalysts and a commercially available simple aryl chalcogen source without any complex directing template, exogenous ligand, acid/base, oxidant, or other additives. The key feature of this methodology is an impressive regioselectivity along with a wide range of functional group tolerance with good to excellent yields under aerobic conditions.

Silver Ions Promoted Palladium-Catalyzed Inactive β-C(sp3)-H Bond Arylation in Batch and Continuous-Flow Conditions

A palladium(II)-catalyzed protocol for inactive β-C(sp³)-H bond functionalization has been first accomplished. The reaction proceeds through five-membered carbocycles for the formation of C-C bonds via the Pd(II)/Pd(IV) cycle. This reaction was carried out with various aryl iodides and benzothiazoles/benzoxazoles/benzimidazoles, which were well-tolerated in this reaction and successfully generated β-C(sp³)-H arylated products. Further implementation of this batch protocol to continuous flow by utilizing a PTFE (polytetrafluoroethylene) capillary reactor enhanced the reaction efficiency and decreased the reaction time (18.4 min) as compared to batch conditions (8 h). Even on the gram scale, the process produced excellent yield with negligible diarylations. Functional group tolerance, a continuous-flow approach, and easy-to-handle reaction conditions make this inactive β-C(sp³)-H bond functionalization protocol very attractive.

Asymmetric C(sp3)-H borylation: an update

Chiral organoboronates have emerged as a key intermediate in the development of pharmaceuticals and materials science. Therefore, several attempts have been made to design various synthetic methods to easily furnish these compounds during the past few decades. Inter alia, asymmetric catalysis has been increasing rapidly as a viable, practical and beneficial strategy for their preparation. In this respect, recent years have witnessed significant progress in aliphatic C-H borylation as the generated carbon-boron bonds are largely utilized to produce other carbon-carbon, carbon-nitrogen and carbon-oxygen bonds. This review presents a detailed overview and analysis of transition metal-catalyzed asymmetric C(sp³)-H borylation strategies. Overall, it assembles all the recent developments in this particular synthetic avenue up to March 2022.

Pd-catalyzed bidentate auxiliary assisted remote C(sp3)-H functionalization

Pd-catalyzed C-H functionalisation affords effective synthetic tools to construct C-C and C-X bonds. Despite the challenges, the distal functionalization of C(sp³)-H bonds has witnessed significant developments and the use of bidentate auxiliaries has garnished this area by providing an opportunity to control reactivity as well as selectivity beyond proximal sites. This article covers the recent developments on the Pd-catalyzed bidentate auxiliary-assisted distal C(sp³)-H functionalization and is categorized based on the nature of functionalizations.

Saturated Boronic Acids, Boronates, and Trifluoroborates: An Update on Their Synthetic and Medicinal Chemistry

This review discusses recent advances in the chemistry of saturated boronic acids, boronates, and trifluoroborates. Applications of the title compounds in the design of boron-containing drugs are surveyed, with special emphasis on α-amino boronic derivatives. A general overview of saturated boronic compounds as modern tools to construct C(sp³ )-C and C(sp³ )-heteroatom bonds is given, including recent developments in the Suzuki-Miyaura and Chan-Lam cross-couplings, single-electron-transfer processes including metallo- and organocatalytic photoredox reactions, and transformations of boron "ate" complexes. Finally, an attempt to summarize the current state of the art in the synthesis of saturated boronic acids, boronates, and trifluoroborates is made, with a brief mention of the "classical" methods (transmetallation of organolithium/magnesium reagents with boron species, anti-Markovnikov hydroboration of alkenes, and the modification of alkenyl boron compounds) and a special focus on recent methodologies (boronation of alkyl (pseudo)halides, derivatives of carboxylic acids, alcohols, and primary amines, boronative C-H activation, novel approaches to alkene hydroboration, and 1,2-metallate-type rearrangements).

Pyrimidine-directed metal-free C-H borylation of 2-pyrimidylanilines: a useful process for tetra-coordinated triarylborane synthesis

Convenient, easily handled, laboratory friendly, robust approaches to afford synthetically important organoboron compounds are currently of great interest to researchers. Among the various available strategies, a metal-free approach would be overwhelmingly accepted, since the target boron compounds can be prepared in a metal-free state. We herein present a detailed study of the metal-free directed ortho-C–H borylation of 2-pyrimidylaniline derivatives. The approach allowed us to synthesize various boronates, which are synthetically important compounds and various four-coordinated triarylborane derivatives, which could be useful in materials science as well as Lewis-acid catalysts. This metal-free directed C–H borylation reaction proceeds smoothly without any interference by external impurities, such as inorganic salts, reactive functionalities, heterocycles and even transition metal precursors, which further enhance its importance.

We present the metal-free ortho-C–H borylation of 2-pyrimidylanilines to afford synthetically important boronic esters and tetra-coordinated triarylboranes, which could be useful in materials science as well as Lewis-acid catalysts.