Facile Functionalization of a Metal Carbon Bond by O-Atom Transfer

Ligand-induced decarbonylation in diphosphine-ligated palladium acetates [CH3CO2Pd((PR2)2CH2)]+ (R = Me and Ph)

Isotope labelling, IR spectroscopy and DFT calculations reveal a novel ligand-induced decarbonylation reaction.

Oxy-functionalization of Group 9 and 10 transition metal methyl ligands: use of pyridine-based hemi-labile ligands

Hemi-labile ligands (HLLs) are intriguing candidates for catalysts since they may facilitate bond activation and bond formation through facile ligand dissociation/association.

A homolytic oxy-functionalization mechanism: intermolecular hydrocarbyl migration from M–R to vanadate oxo

Carbon–oxygen bond formation proceeds via intermolecular phenyl migration from OVCl₂Ph to the oxo of OVCl₃.

Computational Hammett analysis of redox based oxy-insertion by Pt(II) complexes

A computational Hammett analysis of oxy-insertion into platinum-aryl bonds is performed. Modeled transformations involve the two-step conversion of [((X)bpy)Pt(R)(OY)](+) (R = p- or m-X-C(6)H(4); Y = 4- or 3-X-pyridine; (X)bpy = 4,4'- or 5,5'-X-bpy; X = NO(2), H, OMe, NMe(2)) proceeding through a Pt-oxo intermediate to form aryloxide [((X)bpy)Pt(OR)(Y)](+), which contrasts a one-step non-redox (Baeyer-Villiger) oxy-insertion. A structural connection is proposed between redox and non-redox transition states, linked to, among other parameters, oxidant identity. The electronic impact of the catalytic components is compared to previous Hammett studies on OMBV transformations. The Hammett sensitivity for aryl migration is diminished for the migrating group (R) and leaving group (Y), components as compared to OMBV transitions, while the bipyridine supporting ligand (L(n)) has an increased impact. The Hammett impact of R, Y and L(n) upon the aryl migration transition state is small in a global sense, ca. 5 kcal mol(-1); therefore, we conclude that the metal and oxidant are the most important factors in controlling oxy-insertion kinetics for these late metal systems. These results also point to a possible mechanistic advantage for redox over non-redox functionalization of hydrocarbons to alcohols.

Designing catalysts for functionalization of unactivated C-H bonds based on the CH activation reaction

In an effort to augment or displace petroleum as a source of liquid fuels and chemicals, researchers are seeking lower cost technologies that convert natural gas (largely methane) to products such as methanol. Current methane to methanol technologies based on highly optimized, indirect, high-temperature chemistry (>800 °C) are prohibitively expensive. A new generation of catalysts is needed to rapidly convert methane and O(2) (ideally as air) directly to methanol (or other liquid hydrocarbons) at lower temperatures (~250 °C) and with high selectivity. Our approach is based on the reaction between CH bonds of hydrocarbons (RH) and transition metal complexes, L(n)M-X, to generate activated L(n)M-R intermediates while avoiding the formation of free radicals or carbocations. We have focused on the incorporation of this reaction into catalytic cycles by integrating the activation of the CH bond with the functionalization of L(n)M-R to generate the desired product and regenerate the L(n)M-X complex. To avoid free-radical reactions possible with the direct use of O(2), our approach is based on the use of air-recyclable oxidants. In addition, the solvent serves several roles including protection of the product, generation of highly active catalysts, and in some cases, as the air-regenerable oxidant. We postulate that there could be three distinct classes of catalyst/oxidant/solvent systems. The established electrophilic class combines electron-poor catalysts in acidic solvents that conceptually react by net removal of electrons from the bonding orbitals of the CH bond. The solvent protects the CH(3)OH by conversion to more electron-poor [CH(3)OH(2)](+) or the ester and also increases the electrophilicity of the catalyst by ligand protonation. The nucleophilic class matches electron-rich catalysts with basic solvents and conceptually reacts by net donation of electrons to the antibonding orbitals of the CH bond. In this case, the solvent could protect the CH(3)OH by deprotonation to the more electron-rich [CH(3)O](-) and increases the nucleophilicity of the catalysts by ligand deprotonation. The third grouping involves ambiphilic catalysts that can conceptually react with both the HOMO and LUMO of the CH bond and would typically involve neutral reaction solvents. We call this continuum base- or acid-modulated (BAM) catalysis. In this Account, we describe our efforts to design catalysts following these general principles. We have had the most success with designing electrophilic systems, but unfortunately, the essential role of the acidic solvent also led to catalyst inhibition by CH(3)OH above ~1 M. The ambiphilic catalysts reduced this product inhibition but were too slow and inefficient. To date, we have designed new base-assisted CH activation and L(n)M-R fuctionalization reactions and are working to integrate these into a complete, working catalytic cycle. Although we have yet to design a system that could supplant commercial processes, continued exploration of the BAM catalysis continuum may lead to new systems that will succeed in addressing this valuable goal.