Cross-coupling of C(sp)-H Bonds with Organometallic Reagents via Pd(II)/Pd(0) Catalysis**

Catalytic functionalization of C(sp2)-H and C(sp3)-H bonds by using bidentate directing groups

C-H bonds are ubiquitous in organic compounds. It would, therefore, appear that direct functionalization of substrates by activation of C-H bonds would eliminate the multiple steps and limitations associated with the preparation of functionalized starting materials. Regioselectivity is an important issue because organic molecules can contain a wide variety of C-H bonds. The use of a directing group can largely overcome the issue of regiocontrol by allowing the catalyst to come into proximity with the targeted C-H bonds. A wide variety of functional groups have been evaluated for use as directing groups in the transformation of C-H bonds. In 2005, Daugulis reported the arylation of unactivated C(sp(3))-H bonds by using 8-aminoquinoline and picolinamide as bidentate directing groups, with Pd(OAc)2 as the catalyst. Encouraged by these promising results, a number of transformations of C-H bonds have since been developed by using systems based on bidentate directing groups. In this Review, recent advances in this area are discussed.

C(sp3)-H bond arylations catalyzed by well-defined [Ru(O2CMes)2(p-cymene)]

The well-characterized ruthenium(II) biscarboxylate complex [Ru(O2CMes)2(p-cymene)] enabled versatile direct (hetero)arylations of C(sp(3))-H bonds with low (co)catalyst loading and ample substrate scope. Detailed mechanistic studies provided strong support for a facile and reversible C(sp(3))-H bond metalation.

Palladium(0)-catalyzed alkynylation of C(sp3)-H bonds

The alkynylation of β-C(sp(3))-H bonds in aliphatic amides with alkynyl halides has been enabled using Pd(0)/N-heterocyclic carbene (NHC) and Pd(0)/phosphine (PR3) catalysts. This is the first example of utilizing [AlkynylPd(II)L(n)] complexes to activate C(sp(3))-H bonds.

Ligand-enabled methylene C(sp3)-H bond activation with a Pd(II) catalyst

Pd(II) insertion into β-methylene C(sp(3))-H bonds was enabled by a mutually repulsive and electron-rich quinoline ligand. Ligand tuning led to the development of a method that allows for installation of an aryl group on a range of acyclic and cyclic amides containing β-methylene C(sp(3))-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.

Pd(II)-catalyzed enantioselective C-H activation of cyclopropanes

Systematic ligand development has led to the identification of novel mono-N-protected amino acid ligands for Pd(II)-catalyzed enantioselective C-H activation of cyclopropanes. A diverse range of organoboron reagents can be used as coupling partners, and the reaction proceeds under mild conditions. These results provide a new retrosynthetic disconnection for the construction of enantioenriched cis-substituted cyclopropanecarboxylic acids.

Ligand-accelerated cross-coupling of C(sp2)-H bonds with arylboron reagents

A ligand-accelerated Pd(II)-catalyzed C(sp(2))-H/arylboron cross-coupling reaction of phenylacetic acid substrates is reported. Using Ac-Ile-OH as the ligand and Ag(2)CO(3) as the oxidant, a fast, high-yielding, operationally simple, and functional group-tolerant protocol has been developed for the cross-coupling of phenylacetic acid substrates with aryltrifluoroborates. This ligand scaffold has also been shown to improve catalysis using 1 atm O(2) as the sole reoxidant, which sheds light on the path forward in developing optimized ligands for aerobic C-H/arylboron cross-coupling.