Synthetic and Mechanistic Studies of Ruthenium Catalyzed C-C, C-N and C-O Bond Activation Reactions

Transition metal catalyzed selective C-C, C-N and C-O bond activation reactions are fundamentally important in organometallic chemistry and organic synthesis. Catalytic C-C, C-N and C-O activation are highly valuable for reforming processes of crude oils. Significant research has been devoted to transition metal mediated C-C, C-N and C-O bond cleavage reactions to form new compounds as these processes are expected to provide novel ways to transformation of inexpensive hydrocarbons into more commercially valuable products such as pharmaceuticals, agrochemicals and polymers. A few examples of transition metal catalyzed cross coupling reactions involving C-N bond cleavage have been reported. A well-defined Ru catalytic system has been developed for oxidative alkylation of alcohol by deaminative coupling reactions of amines to form alkylated ketones. The catalytic method was successfully applied to the decarboxylative and deaminative coupling of amino acids with ketones. Reductive deoxygenation of aldehydes and ketones has attracted considerable attention due to its many applications in fine-chemical synthesis and biofuel production. Classical methods for the deoxygenation of carbonyl compounds are generally associated with harsh reaction conditions and the use of stoichiometric amounts of toxic reagents, and poor functional-group tolerance. A well-defined Ru-H catalyst was found to mediate the reductive deoxygenation of carbonyl compounds to produce aliphatic compounds. Two different mechanistic pathways have been investigated in detail to probe the electronic nature of the catalysts and ligands. Reductive etherification of ketones/aldehydes and alcohols have been studied intensively as cheaper and greener ways to synthesize ethers. A method for the reductive coupling of carbonyl compounds with alcohols has been developed, which involved a highly chemoselective formation of unsymmetrically substituted ether products. The catalytic etherification method employs cheaply available molecular hydrogen as the reducing agent, tolerates a number of common functional groups, and uses environmentally benign water as the solvent

Chemoselective Formation of Unsymmetrically Substituted Ethers from Catalytic Reductive Coupling of Aldehydes and Ketones with Alcohols in Aqueous Solution

A well-defined cationic Ru–H complex catalyzes reductive etherification of aldehydes and ketones with alcohols. The catalytic method employs environmentally benign water as the solvent and cheaply available molecular hydrogen as the reducing agent to afford unsymmetrical ethers in a highly chemoselective manner

Scope and Mechanistic Analysis for Chemoselective Hydrogenolysis of Carbonyl Compounds Catalyzed by a Cationic Ruthenium Hydride Complex with a Tunable Phenol Ligand

A cationic ruthenium hydride complex, [(C6H6)(PCy3)(CO)RuH]+BF4– (1), with a phenol ligand was found to exhibit high catalytic activity for the hydrogenolysis of carbonyl compounds to yield the corresponding aliphatic products. The catalytic method showed exceptionally high chemoselectivity toward the carbonyl reduction over alkene hydrogenation. Kinetic and spectroscopic studies revealed a strong electronic influence of the phenol ligand on the catalyst activity. The Hammett plot of the hydrogenolysis of 4-methoxyacetophenone displayed two opposite linear slopes for the catalytic system 1/p-X-C6H4OH (ρ = −3.3 for X = OMe, t-Bu, Et, and Me; ρ = +1.5 for X = F, Cl, and CF3). A normal deuterium isotope effect was observed for the hydrogenolysis reaction catalyzed by 1/p-X-C6H4OH with an electron-releasing group (kH/kD = 1.7–2.5; X = OMe, Et), whereas an inverse isotope effect was measured for 1/p-X-C6H4OH with an electron-withdrawing group (kH/kD = 0.6–0.7; X = Cl, CF3). The empirical rate law was determined from the hydrogenolysis of 4-methoxyacetophenone: rate = kobsd[Ru][ketone][H2]−1 for the reaction catalyzed by 1/p-OMe-C6H4OH, and rate = kobsd[Ru][ketone][H2]0 for the reaction catalyzed by 1/p-CF3-C6H4OH. Catalytically relevant dinuclear ruthenium hydride and hydroxo complexes were synthesized, and their structures were established by X-ray crystallography. Two distinct mechanistic pathways are presented for the hydrogenolysis reaction on the basis of these kinetic and spectroscopic data