Ruthenium salen

Chiral (nitrosyl)ruthenium(salen) complexes have been found to be efficient catalysts for aerobic oxidative desymmetrization of mc.vo-diols under photoirradiation to give optically active lactols. With the suitable catalysts, high enantioselectivities up to 93% has been achieved. The kinetics of the oxidation depend on the nature of the ligand. On the basis of kinetic parameters and the kinetic isotope effect, it has been suggested that when a ligand with an apical hydroxy group is used, the hydrogen atom... 

Ruthenium Salen, Ruthenium Porphyrin, and Related Catalysts. 87... 

In addition to epoxides, three-membered nitrogen heterocycles, aziridines, can be obtained by means of catalytic asymmetric aziridinations (Eq. 30). To this aim, chiral ruthenium(salen) complexes 67 [56] and 68 [57] were useful (Fig. 1). The former phosphine complexes 67 gave the aziridine from two cy-cloalkenes with 19-83% ee [56]. On the other hand, terminal alkenes selectively underwent aziridination in the presence of the latter carbonyl complex 68 with 87-95% ee [57]. In these examples, N-tosyliminophenyliodinane or N-tosyl azide were used as nitrene sources. Quite recently, catalytic intramolecular ami-dation of saturated C-H bonds was achieved by the use of a ruthenium(por-phyrin) complex (Eq. 31) [58]. In the presence of the ruthenium catalyst and 2 equiv iodosobenzene diacetate, sulfamate esters 69 were converted into cyclic sulfamidates 70 in moderate-to-good yields. 

Fig. 19 Ruthenium salen type complexes active as catalysts in alcohol oxidation under illumination...

Other reagents are also employed as nitrene precursors, primarily in an effort to avoid the practical problems associated with PhI=NTs <2001TL8089>, such as expense of the reagent and the generation of iodobenzene as a by-product. For example, tosyl azide has been used in combination with the chiral ruthenium(salen) catalyst 522 to effect the enantioselective aziridination of terminal alkynes with very good ee s <2003CL354>. Another... 

Ruthenium salen nitrosyl complexes react catalytically with thiiranes and convert them to olefins and 1,2,3-trithiolanes 163 (Scheme 34) or 1,2,3,4-tetrathianes 164 (Scheme 35) <2001JA4770>. 

Apart from the commonly used NaOCl, urea—H2O2 has been used/ With this reaction, simple alkenes can be epoxi-dized with high enantioselectivity. The mechanism of this reaction has been examined.Radical intermediates have been suggested for this reaction, polymer-bound Mn -salen complex, in conjunction with NaOCl, has been used for asymmetric epoxidation. Chromium-salen complexes and ruthenium-salen complexes have been used for epoxidation. Manganese porphyrin complexes have also been used. Cobalt complexes give similar results. A related epoxidation reaction used an iron complex with molecular oxygen and isopropanal. Nonracemic epoxides can be prepared from racemic epoxides with salen-cobalt(II) catalysts following a modified procedure for kinetic resolution. 

A number of other transition metal(salen) complexes have been investigated. A ruthenium(salen)-catalysed asymmetric epoxidation of wide scope has been developed by Katsuki that proceeds under irradiation with visible light. Trans-and terminal olefins are epoxidised with good ee but the selectivities obtained with ds-substrates are not as good as those achieved using Mn(salen) complexes. The use of palladium(salen) complexes as epoxidation catalysts has also been explored, but relatively poor selectivities have been obtained to date. ... 

Works CF, Jocher CJ et al (2002) Photochemical nitric oxide piecuisois synthesis, photochemistry, and ligand substitution kinetics of ruthenium salen nitrosyl and ruthenium salophen nitrosyl complexes. Inorg Chem 41 3728-3739... 

Enantioselective cyclopropanation is currently being explored. The ruthenium complex shown previously in Figure 7 also reacts with EDA and styrene to afford a transxis ratio of 4 1, with 46% ee of the trans isomer. The cis isomer is nearly racemic (<10% ee). The use of four-substituted stjrrene derivatives dramatically increases the diastereoisomeric excess of the trans isomer, with 4-fluorostyrene giving an 11 1 ratio, with 50% ee (74). Conversely, as shown in Figure 21, the porphyrin-like [RuCl(PNNP)]+ precatalyst reacts with EDA/ styrene to afford the cis isomer at a ratio of 10 1, with an enantiomeric excess of 87% (76). These types of ruthenium complexes have also been described as epoxi-dation catalysts above clearly there are mechanistic similarities between the oxo-and carbene- intermediates, which could help elucidate the reasons behind such variable enantioselectivity. Other ruthenium complexes that catalyze cyclopropanation include CpRu(II) catalysts, arene ruthenium complexes, and ruthenium-salen complexes. Cp Ru(cod)Cl is also known to catalyze the related reaction of diazo compounds with alkynes, affording the corresponding 1,3-diene (Figure 22) (77). 

Shimizu, H., Onitsuka, S., Egami, H., et al. (2005). Ruthenium(Salen)-Catalyzed Aerobic Oxidative Desymmetrization of Meso-Diols and its Kinetics, J. Am. Chem. Soc., 127, pp. 5396-5413. 

Jing, H. Chang, T Jin, L. Wu, M. Qiu, W. Ruthenium Salen/Phenyl Trimethyl Ammonium Tribromide Catalyzed Coupling Reaction of Carbon Dioxide and Epoxides. Catal. Comm. 2007, 8, 1630-1634. 

Ruthenium-salen complex 263 was examined. Achiral salen complex 263 in the presence of chiral sulfoxide 264 progressed the cyclopropanation of diazoacetate in a highly enantioselective manner (Scheme 1.127) [184]. Chiral sulfoxide served as an axial ligand that showed good asymmetric induction. C2-symmetric chiral ruthenium-salen complex 265 contained in metal-organic frameworks worked as a chiral catalyst for cyclopropanation (Scheme 1.128) [185]. 

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