Ruthenium complexes, reactions catalytic cycle

Since the vinylcarbenes la-c and the aryl substituted carbene (pre)catalyst Id, in the first turn of the catalytic cycle, both afford methylidene complex 3 as the propagating species in solution, their application profiles are essentially identical. Differences in the rate of initiation are relevant in polymerization reactions, but are of minor importance for RCM to which this chapter is confined. Moreover, the close relationship between 1 and the ruthenium allenylidene complexes 2 mentioned above suggests that the scope and limitations of these latter catalysts will also be quite similar. Although this aspect merits further investigations, the data compiled in Table 1 clearly support this view. 

Water has been shown to enhance the activity of ruthenium and rhodium catalysts in both the TEAF and potassium formate systems [34, 36, 52]. The aqueous systems enable much simpler control of pH this is important, as Xiao has found that a low pH markedly slows the reaction [52]. The pH at which this occurs corresponds with the pKa of formic acid (i.e., 3.7), implying that the formate anion is required for complexation with the catalyst. Xiao has proposed two possible catalytic cycles - one that provides poor ee-values at low pH as a result of ligand decomplexation, and another that gives high ee-values at high pH. 

In order to get a catalytic cycle it is necessary that the metal sulfide intermediate can react with hydrogen to form the reduced metal complex (or compound) and H2S. For highly electropositive metals (non-noble metals) this is not possible for thermodynamic reasons. The co-ordination chemistry and the oxidative addition reactions that were reported mainly involved metals such as ruthenium, iridium, platinum, and rhodium. 

An electrophilic palladation by a phenyl palladium intermediate at C(3) and a C(3) to C(2) migration of a palladium species, followed by reductive elimination, is indicated. 2-Phenylpyridine has been formed by the reaction of pyridine and iodobenzene at 150 °C in the presence of phosphido-bridged ruthenium dimer complexes.49 A catalytic cycle involving one of the complexes in the system was proposed. Optimum conditions for the efficient and regioselective palladium-catalysed C(2) arylation of ethyl 4-oxazolecarboxylate (47) with iodobenzene have been presented.50... 

Photocatalytic enantioselective oxidative arylic coupling reactions have been investigated by two different groups. Both studies involved the use of ruthenium-based photocatalysts [142, 143]. In 1993, Hamada and co-workers introduced a photostable chiral ruthenium tris(bipyridine)-type complex (A-[Ru(menbpy)3]2+) 210 possessing high redox ability [143]. The catalytic cycle also employed Co(acac)3 211 to assist in the generation of the active (A-[Ru(menbpy)3]3+) species 212. The authors suggested that the enantioselection observed upon binaphthol formation was the result of a faster formation of the (R)-enantiomer from the intermediate 213 (second oxidation and/or proton loss), albeit only to a rather low extent (ee 16 %) (Scheme 54). 

If bi-2-naphthol and Co(acac)3 exist in excess in the reaction solution, the photoexcited ruthenium(II) complex undergoes oxidative quenching by Co(acac)3 to afford the ruthenium(III) complex, as shown by the catalytic cycle of Scheme... 

Organic synthesis via transition metal complex-catalyzed electrochemical and photochemical reduction of CO2 has been developed [2,122b, 145-147]. Among transition metal complexes, ruthenium bipyridine complexes show high catalytic activity a typical reaction is shown in Eq. 11.79. [Ru(bpy)2(CO)2] and [Ru(bpy)2(CO)Cl] efficiently catalyze the electrochemical reduction of CO2 to CO and HC02. The nature of the products is dependent upon the pH of the solution. A catalytic cycle involving [Ru(bpy)2(CO)]°, ]Ru(bpy)2(C0)(C02 )] and [Ru(bpy)2(C0)C02H] was proposed (Eq. 11.79) [1461]. 

The same catalyst precursor, generated from [(EDTA)RuCI] which is also water soluble, was used for the hydroformylation of allylic alcohol under the same reaction conditions (//). At 50 bar and 130°C, in water as solvent, 4-hydroxybutanal was produced [Eq. (5)], together with about 2% of formaldehyde. However, the reaction proceeded further to give butane-1,4-diol by hydrogenation and y-butyrolactone as well as dihydrofuran by cyclization [Eq. (6)]. The same catalytic cycle as that proposed in Scheme 3 can be considered. A kinetic investigation revealed a first-order dependence on the ruthenium complex concentration and on the allyl alcohol... 

The proposed mechanism for the oxidation reaction is presented in Scheme 14.44. The formation of Ru-alcoholate (species 11) by the reaction of catalyst I with benzyl alcohol is considered to be the first step of the catalytic cycle, followed by P-hydride elimination to produce the corresponding carbonyl compound and probably a ruthenium hydride species. Subsequent reaction of complex 111 with may afford the complex Ru-hydroperoxide IV. The uptake of alcohol again completes the cycle with the formation of and H O. 

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