Transition metal-catalyzed arylation of unactivated C(sp3)-H bonds

Palladium-catalyzed C(sp3)-H arylation of diarylmethanes at room temperature: synthesis of triarylmethanes via deprotonative-cross-coupling processes

Although metal-catalyzed direct arylation reactions of non- or weakly acidic C-H bonds have recently received much attention, chemists have relied heavily on substrates with appropriately placed directing groups to steer reactivity. To date, examples of intermolecular arylation of unactivated C(sp(3))-H bonds in the absence of a directing group remain scarce. We report herein the first general, high-yielding, and scalable method for palladium-catalyzed C(sp(3))-H arylation of simple diarylmethane derivatives with aryl bromides at room temperature. This method facilitates access to a variety of sterically and electronically diverse hetero- and nonheteroaryl-containing triarylmethanes, a class of compounds with various applications and interesting biological activity. Key to the success of this approach is an in situ metalation of the substrate via C-H deprotonation under catalytic cross-coupling conditions, which is referred to as a deprotonative-cross-coupling process (DCCP). Base and catalyst identification were performed by high-throughput experimentation (HTE) and led to a unique base/catalyst combination [KN(SiMe(3))(2)/Pd-NiXantphos] that proved to efficiently promote the room-temperature DCCP of diarylmethanes. Additionally, the DCCP exhibits remarkable chemoselectivity in the presence of substrates that are known to undergo O-, N-, enolate-, and C(sp(2))-H arylation.

Asymmetric Pd-NHC*-catalyzed coupling reactions

Very high asymmetric inductions result in the Pd-catalyzed intramolecular arylation of amides to give 3,3-disubstituted oxindoles when new in situ-generated chiral N-heterocyclic carbene (NHC*) ligands are employed. Structural studies show that conformational locking to minimize allylic strain is the key to understanding the function of these ligands. New applications of these ligands in the frontier area of asymmetric coupling reactions involving C(sp3)–H bonds are detailed. Highly enantioenriched fused indolines are accessible using either preformed- or in situ-generated Pd-NHC* catalysts. Remarkably, this occurs at high temperature (140–160 °C) via excellent asymmetric recognition of an enantiotopic C–H bond in an unactivated methylene unit.

Heterocycle Formation via Palladium-Catalyzed C-H Functionalization

Heterocyclic compounds are ubiquitous in natural products, pharmaceuticals, and agrochemicals. Therefore, the design of novel protocols to construct heterocycles more efficiently is a major area of focus in the organic chemistry. In the past several years, cyclization reactions based upon palladium-catalyzed C-H activation have received substantial attention due to their capacity for expediting heterocycle synthesis. This review discusses strategies for heterocycle synthesis via palladium-catalyzed C-H bond activation and highlights recent examples from the literature.

Ruthenium(0)-catalyzed sp3 C-H bond arylation of benzylic amines using arylboronates

A Ru-catalyzed direct arylation of benzylic sp(3) carbons of acyclic amines with arylboronates is reported. This highly regioselective and efficient transformation can be performed with various combinations of N-(2-pyridyl) substituted benzylamines and arylboronates. Substitution of the pyridine directing group in the 3-position proved to be crucial in order to achieve high arylation yields. Furthermore, the pyridine directing group can be removed in high yields via a two-step protocol.

C-C, C-O and C-N bond formation via rhodium(III)-catalyzed oxidative C-H activation

Rhodium(III)-catalyzed direct functionalization of C-H bonds under oxidative conditions leading to C-C, C-N, and C-O bond formation is reviewed. Various arene substrates bearing nitrogen and oxygen directing groups are covered in their coupling with unsaturated partners such as alkenes and alkynes. The facile construction of C-E (E = C, N, S, or O) bonds makes Rh(III) catalysis an attractive step-economic approach to value-added molecules from readily available starting materials. Comparisons and contrasts between rhodium(III) and palladium(II)-catalyzed oxidative coupling are made. The remarkable diversity of structures accessible is demonstrated with various recent examples, with a proposed mechanism for each transformation being briefly summarized (critical review, 138 references).