The Ni-catalyzed oxidation of unactivated alkanes, including the oxidation of polyethylenes, by meta-chloroperbenzoic acid (mCPBA) occur with high turnover numbers under mild conditions, but the mechanism of such transformations has been a subject of debate. Putative, high-valent nickel-oxo or nickel-oxyl intermediates have been proposed to cleave the C–H bond, but several studies on such complexes have not provided strong evidence to support such reactivity toward unactivated C­(sp3)–H bonds. We report mechanistic investigations of Ni-catalyzed oxidations of unactivated C–H bonds by mCPBA. The lack of an effect of ligands, the formation of carbon-centered radicals with long lifetimes, and the decomposition of mCPBA in the presence of Ni complexes suggest that the reaction occurs through free alkyl radicals. Selectivity on model substrates and deuterium-labeling experiments imply that the m-chlorobenzoyloxy radical derived from mCPBA cleaves C–H bonds in the alkane to form an alkyl radical, which subsequently reacts with mCPBA to afford the alcohol product and regenerate the aroyloxy radical. This free-radical chain mechanism shows that Ni does not cleave the C­(sp3)–H bonds as previously proposed; rather, it catalyzes the decomposition of mCPBA to form the aroyloxy radical.

Site-selective functionalizations of C-H bonds are often achieved with a directing group that leads to five-or sixmembered 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 a study on H/D exchange, of a simple directing group (salicylaldehyde) capable of inducing the formation of this small ring. To gain insight into the mechanism of this unusual oxidation reaction, a series of mechanistic experiments and 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 that is crucial for stabilization of the palladacyclic intermediate.

The reactivity of (PNP)Rh(Ph) (PNP = 2,6bis(dialkylphosphinomethyl)pyridine) toward a variety of electrophiles (Ar−I, ArCH 2 Cl, O 2 , I 2 , B 2 pin 2 , and ArSO 3 H) was explored, and several new modes of oxidative reactivity were observed. Substituting t Bu 2 P for i Pr 2 P provided 100-fold rate enhancement toward C−H bond activation and addressed the previously reported challenge of N 2 inhibition. Studying the stoichiometric reactivity of (PNP)Rh complexes toward C−H cleavage and oxidative functionalization led to (PNP)Rhcatalyzed cross-coupling of aryl iodides with sp 2 and sp 3 C−H bonds.

We report the transmetalation of hydrocarbyl fragments (Me, Bn, Ph) from a variety of organometallic complexes relevant to C–H activation (Ir, Rh, W, Mo) to Pt­(II) electrophiles. The scope of suitable hydrocarbyl donors is remarkable in that three different classes of organometallics with widely varying reactivity all undergo the same general reaction with Pt­(II) electrophiles. A competitive substituent effect experiment reveals faster transmetalation of more electron-rich hydrocarbyl groups. This study suggests that transmetalation could provide a viable path for catalytic functionalization of stable complexes resulting from C–H bond activation and other processes.

A heterobimetallic rhodium-pincer complex bearing a phenylzinc ligand was synthesized and characterized by multinuclear NMR, COSY, NOESY, and X-ray crystallography. The crystal structure of this complex shows that it possesses a bridging Rh− Zn−C fragment with a geometry similar to the Rh−H−C fragment in a proposed transition state for metal to ligand proton transfer during redox-neutral C−H activation with dearomatized rhodium pincer complexes. Bonding analysis indicates that these fragments are isolobal, suggesting that the transition state analogue models not only the structure but also the bonding interactions that underlie metal−ligand cooperativity in the C−H activation transition state. The similarity of the transition state and its analogue prompted re-evaluation of the relevant rate equations to determine the relative contributions of viable proton transfer pathways. Parallel analysis of the transition state and its isolobal analogue thus serves as a bridge between theory and experiment that is rarely available in studies of bonding in transition states.