The mechanism for the palladium-catalyzed allylic C–H activation was investigated using a combination of experimental and theoretical methods. A Hammett study revealed a buildup of a partial negative charge in the rate-determining step, and determination of the kinetic isotope effect (KIE) indicated that the C–H bond is broken in the turnover-limiting transition state. These experimental findings were further substantiated by carrying out a detailed density functional theory (DFT)-based investigation of the entire catalytic cycle. The DFT modeling supports a mechanism in which a coordinated acetate acts as a base in an intramolecular fashion during the C–H activation step. The reoxidation of palladium was found to reach an energy level similar to that of the C–H activation. Calculations of turnover frequencies for the entire catalytic cycle for the C–H alkylation were used to acquire a better understanding of the experimental KIE value. The good correspondence between the experimental KIE and the computed KIE values allows discrimination between scenarios where the acetate is acting in an intramolecular fashion (C–H alkylation) and an intermolecular fashion (C–H acetoxylation and C–H amination).

A bimetallic Rh(II) catalyst promoted the C-H alkenylation of simple arenes at 1.0 equivalent without the use of a directing group. A phosphine ligand as well as cooperative reoxidation of Rh(II) with Cu(TFA)2 and V2O5 proved essential in providing monoalkenylated products in good yields and selectivities, especially with di- and trisubstituted arenes.

The atom-efficiency of one of the most widely used catalytic reactions for forging C-C bonds, the Tsuji-Trost reaction, is limited by the need of preoxidized reagents. This limitation can be overcome by utilization of the recently discovered palladium-catalyzed C-H activation, the allylic C-H alkylation reaction which is the topic of the current review. Particular emphasis is put on current mechanistic proposals for the three reaction types comprising the overall transformation: C-H activation, nucleophillic addition, and re-oxidation of the active catalyst. Recent advances in C-H bond activation are highlighted with emphasis on those leading to C-C bond formation, but where it was deemed necessary for the general understanding of the process closely related C-H oxidations and aminations are also included. It is found that C-H cleavage is most likely achieved by ligand participation which could involve an acetate ion coordinated to Pd. Several of the reported systems rely on benzoquinone for re-oxidation of the active catalyst. The scope for nucleophilic addition in allylic C-H alkylation is currently limited, due to demands on pKa of the nucleophile. This limitation could be due to the pH dependence of the benzoquinone/hydroquinone redox couple. Alternative methods for re-oxidation that does not rely on benzoquinone could be able to alleviate this limitation.

A bimetallic Rh II catalyst promoted the C À H alkenylation of simple arenes at 1.0 equivalent without the use of a directing group. A phosphine ligand as well as cooperative reoxidation of Rh II with Cu(TFA) 2 and V 2 O 5 proved essential in providing monoalkenylated products in good yields and selectivities, especially with di-and trisubstituted arenes.

Cooperative reoxidation of Rh(II) with cupric trifluoroacetate and vanadium pentoxide proves essential in providing monoalkenylated products, especially with di‐ and trisubstituted arenes.