Understanding the Reactivity of Pd0/PR3-Catalyzed Intermolecular C(sp3)–H Bond Arylation

Anilide formation from thioacids and perfluoroaryl azides

A metal-free method for fast and clean anilide formation from perfluoroaryl azide and thioacid is presented. The reaction proved highly efficient, displaying fast kinetics, high yield, and good chemoselectivity. The transformation was compatible with various solvents and tolerant to a wide variety of functional groups, and it showed high performance in polar protic/aprotic media, including aqueous buffer systems.

Transition metal-catalyzed C–H bond functionalizations by the use of diverse directing groups

In this review, a summary of transition metal-catalyzed C–H activation by utilizing the functionalities as directing groups is presented.

Synthesis of [POCOP]-pincer iron and cobalt complexes via Csp3–H activation and catalytic application of iron hydride in hydrosilylation reactions

C_sp3–H bond activation in pincer ligand (Ph₂PO(o-C₆H₂-(4,6-^tBu₂)))₂CH₂ (1) (POCH₂OP) was achieved by Fe(PMe₃)₄ and CoMe(PMe₃)₄ to afford (POCHOP)Fe(H) (PMe₃)₂ (2) and (POCHOP)Co(PMe₃)₂ (4).

Palladium-catalyzed methylene C(sp3)–H arylation of the adamantyl scaffold

A palladium-catalyzed C(sp³)–H arylation of adamantane at the methylene position with the assistance of an amide group was developed.

The catalytic aerobic synthesis of quaternary α-hydroxy phosphonates via direct hydroxylation of phosphonate compounds

A highly efficient Cu-catalyzed direct hydroxylation of phosphonate compounds has been developed. This transformation provides a powerful method for the synthesis of quaternary α-hydroxy phosphonates in good yields.

Key mechanistic features of Ni-catalyzed C-H/C-O biaryl coupling of azoles and naphthalen-2-yl pivalates

The mechanism of the Ni-dcype-catalyzed C-H/C-O coupling of benzoxazole and naphthalen-2-yl pivalate was studied. Special attention was devoted to the base effect in the C-O oxidative addition and C-H activation steps as well as the C-H substrate effect in the C-H activation step. No base effect in the C(aryl)-O oxidative addition to Ni-dcype was found, but the nature of the base and C-H substrate plays a crucial role in the following C-H activation. In the absence of base, the azole C-H activation initiated by the C-O oxidative addition product Ni(dcype)(Naph)(PivO), 1B, proceeds via ΔG = 34.7 kcal/mol barrier. Addition of Cs2CO3 base to the reaction mixture forms the Ni(dcype)(Naph)[PivOCs·CsCO3], 3_Cs_clus, cluster complex rather than undergoing PivO(-) → CsCO3(-) ligand exchange. Coordination of azole to the resulting 3_Cs_clus complex forms intermediate with a weak Cs-heteroatom(azole) bond, the existence of which increases acidity of the activated C-H bond and reduces C-H activation barrier. This conclusion from computation is consistent with experiments showing that the addition of Cs2CO3 to the reaction mixture of 1B and benzoxazole increases yield of C-H/C-O coupling from 32% to 67% and makes the reaction faster by 3-fold. This emerging mechanistic knowledge was validated by further exploring base and C-H substrate effects via replacing Cs2CO3 with K2CO3 and benzoxazole (1a) with 1H-benzo[d]imidazole (1b) or quinazoline (1c). We proposed the modified catalytic cycle for the Ni(cod)(dcype)-catalyzed C-H/C-O coupling of benzoxazole and naphthalen-2-yl pivalate.

Mechanistic study of chemoselectivity in Ni-catalyzed coupling reactions between azoles and aryl carboxylates

Itami et al. recently reported the C-O electrophile-controlled chemoselectivity of Ni-catalyzed coupling reactions between azoles and esters: the decarbonylative C-H coupling product was generated with the aryl ester substrates, while C-H/C-O coupling product was generated with the phenol derivative substrates (such as phenyl pivalate). With the aid of DFT calculations (M06L/6-311+G(2d,p)-SDD//B3LYP/6-31G(d)-LANL2DZ), the present study systematically investigated the mechanism of the aforementioned chemoselective reactions. The decarbonylative C-H coupling mechanism involves oxidative addition of C(acyl)-O bond, base-promoted C-H activation of azole, CO migration, and reductive elimination steps (C-H/Decar mechanism). This mechanism is partially different from Itami's previous proposal (Decar/C-H mechanism) because the C-H activation step is unlikely to occur after the CO migration step. Meanwhile, C-H/C-O coupling reaction proceeds through oxidative addition of C(phenyl)-O bond, base-promoted C-H activation, and reductive elimination steps. It was found that the C-O electrophile significantly influences the overall energy demand of the decarbonylative C-H coupling mechanism, because the rate-determining step (i.e., CO migration) is sensitive to the steric effect of the acyl substituent. In contrast, in the C-H/C-O coupling mechanism, the release of the carboxylates occurs before the rate-determining step (i.e., base-promoted C-H activation), and thus the overall energy demand is almost independent of the acyl substituent. Accordingly, the decarbonylative C-H coupling product is favored for less-bulky group substituted C-O electrophiles (such as aryl ester), while C-H/C-O coupling product is predominant for bulky group substituted C-O electrophiles (such as phenyl pivalate).

Versatile reactivity of Pd-catalysts: mechanistic features of the mono-N-protected amino acid ligand and cesium-halide base in Pd-catalyzed C–H bond functionalization

The C–H functionalization strategies, complexity in Pd-catalyzed chemical transformations, unprecedented Pd-clustering, base (Cs-halide) and weakly coordinated amino acid ligand effects.

Experimental and computational studies on the mechanism of the Pd-catalyzed C(sp3)–H γ-arylation of amino acid derivatives assisted by the 2-pyridylsulfonyl group

Experimental and computational studies on the mechanism of the Pd(OAc)₂-catalyzed C(sp³)–H γ-arylation of amino acid derivatives are reported.

Palladium-catalyzed meta-selective C-H bond activation with a nitrile-containing template: computational study on mechanism and origins of selectivity

Density functional theory investigations have elucidated the mechanism and origins of meta-regioselectivity of Pd(II)-catalyzed C-H olefinations of toluene derivatives that employ a nitrile-containing template. The reaction proceeds through four major steps: C-H activation, alkene insertion, β-hydride elimination, and reductive elimination. The C-H activation step, which proceeds via a concerted metalation-deprotonation (CMD) pathway, is found to be the rate- and regioselectivity-determining step. For the crucial C-H activation, four possible active catalytic species-monomeric Pd(OAc)2, dimeric Pd2(OAc)4, heterodimeric PdAg(OAc)3, and trimeric Pd3(OAc)6-have been investigated. The computations indicated that the C-H activation with the nitrile-containing template occurs via a Pd-Ag heterodimeric transition state. The nitrile directing group coordinates with Ag while the Pd is placed adjacent to the meta-C-H bond in the transition state, leading to the observed high meta-selectivity. The Pd2(OAc)4 dimeric mechanism also leads to the meta-C-H activation product but with higher activation energies than the Pd-Ag heterodimeric mechanism. The Pd monomeric and trimeric mechanisms require much higher activation free energies and are predicted to give ortho products. Structural and distortion energy analysis of the transition states revealed significant effects of distortions of the template on mechanism and regioselectivity, which provided hints for further developments of new templates.