Metal-catalyzed water oxidation ruthenium catalysts

Directly supported clusters of type Os3H(CO)10(O—metal oxide) break down at quite low temperatures to give species which have a high selectivity to methane from CO and H2 (381,400). Similar behavior has been reported for Os3(CO)12 itself (401), but it is difficult to rule out metal as the catalyst. Os3(CO)12 also leads to methanol, methyl and ethyl formate, and acetone by reaction with CO and H 2 (190° C, 180 atm) in glyme solvents (402). The water-gas-shift reaction is catalyzed by Os3(CO)12, using KOH or even sodium sulfide in methanol as the base (403), although ruthenium catalysts are better (404). 

Alternatively, a ruthenium catalyst could be applied in phthalide preparation. In 2011, Ackermann s group developed a ruthenium-catalyzed cross-dehydrogenative C-H bond alkenylation reaction. The methodology used water as a solvent, benzoic acids and terminal alkenes as substrates good yields of the desired phthalides were isolated (Scheme 2.164). The reaction sequence consisted of cross-dehydrogenative alkenylation and a subsequent intramolecular oxa-Michael reaction. Mechanistic studies provided strong evidence that the oxidative alkenylation proceeds by an irreversible C-H bond metalation via acetate assistance. 

Some other catalytic events prompted by rhodium or ruthenium porphyrins are the following 1. Activation and catalytic aldol condensation of ketones with Rh(OEP)C104 under neutral and mild conditions [372], 2. Anti-Markovnikov hydration of olefins with NaBH4 and 02 in THF, a catalytic modification of hydroboration-oxidation of olefins, as exemplified by the one-pot conversion of 1-methylcyclohexene to ( )-2-methylcycIohexanol with 100% regioselectivity and up to 90% stereoselectivity [373]. 3. Photocatalytic liquid-phase dehydrogenation of cyclohexanol in the presence of RhCl(TPP) [374]. 4. Catalysis of the water gas shift reaction in water at 100 °C and 1 atm CO by [RuCO(TPPS4)H20]4 [375]. 5. Oxygen reduction catalyzed by carbon supported iridium chelates [376]. - Certainly these notes can only be hints of what can be expected from new noble metal porphyrin catalysts in the near future. 

A process for the coproduction of acetic anhydride and acetic acid, which has been operated by BP Chemicals since 1988, uses a quaternary ammonium iodide salt in a role similar to that of Lil [8]. Beneficial effects on rhodium-complex-catalyzed methanol carbonylation have also been found for other additives. For example, phosphine oxides such as Ph3PO enable high catalyst rates at low water concentrations without compromising catalyst stability [40—42]. Similarly, iodocarbonyl complexes of ruthenium and osmium (as used to promote iridium systems, Section 3) are found to enhance the activity of a rhodium catalyst at low water concentrations [43,44]. Other compounds reported to have beneficial effects include phosphate salts [45], transition metal halide salts [46], and oxoacids and heteropolyacids and their salts [47]. 

While, there are many similarities between the mechanism of the Ir-catalyzed process and that of the Rh-catalyzed stystem, there are important differences. Unlike the dependence of the rate of the Monsanto process on only [Rh] and [CH I], the dependence of BP s iridium system on CO pressure, water, methyl acetate, methyl iodide, ruthenium promoter, and iridium are more complex and nonlinear. In situ IR spectroscopy of the iridium catalyst shows that the predominant species is the anionic Ir(III) methyl complex/flc,ds-[Ir(CH3)(CO)2y (2100 and 2047 cm" ). Instead of occurring by turnover-limiting oxidative addition of Mel, the iridium-catalyzed process occurs by turnover-limiting insertion of CO into the metal-methyl complex. 

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