Olefin oxygenation, rhodium/iridium

Organometallic Chemistry of Rhodium and Iridium Directed towards Olefin Oxygenation... 

Rhodium complexes were generally found to be more effective than iridium, but on the whole they show moderate activity in alkene oxygenation reactions. Significantly, epoxides, a typical product of the oxidation of olefins catalyzed by the middle transition metals, have rarely been evoked as products [18-22]. Although allylic alcohols [23] or ethers [24] have sometimes been described as products, the above cited rhodium and iridiiun complexes are characterized by an excellent selectivity in the oxygenation of terminal alkenes to methyl ketones. 

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. 

The metal complexes [MCl(CO)(Ph3P)2] (M = Rh, Ir) catalyze epoxidation of tetramethylethylene with tert-hutyl hydroperoxide in good yield and the selectivity was better 90%) with rhodium than iridium, equation (285). In this case a reasonable mechanism for epoxide formation involves epoxidation of unreacted olefin with the intermediate aUylic hydroperoxide, XXXIX. The allylic hydroperoxide was found to reach levels in excess of 11% during the course of the metal catalyzed reactions of tetramethylethylene with oxygen [470], equation (284). 

Ruthenium(III), d, is ruthenium s most stable oxidation state and resembles rhodium(III) and iridium(III) more than osmium(III). The salts inelude the halides, hydroxides, and oxides RuCls SHaO is most important because it is a good starting material for other compounds and reacts readily with olefins and phosphines. Complexes of this oxidation state are known with water, eyanide, oxygenated organies, sueh as diketones and earboxylates, pyridines, earbonyls, ey-elopentadienyls, phosphine, and arsine ligands. A notable differenee between ruthenium(II) and ruthenium(III) is the absenee of ruthenium(III) nitrosyl complexes. 

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