Complex, Ruthenium-salene

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... 

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...

Works CF, Ford PC. Photoreactivity of the ruthenium nitrosyl complex, ru(salen)(cl)(no). solvent effects on the back reaction of NO with the Lewis acid Rum(salen)(Cl). J Am Chem Soc 2000 122 7592. 

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. ... 

Epoxidation of alkenes with iodosylbenzene can be effectively catalyzed by the analogous salen or chiral Schiff base complexes of manganese(in), ruthenium(II), or ruthenium(III). For example, the oxidation of indene with iodosylbenzene in the presence of (/ ,5)-Mn-salen complexes as catalysts affords the respective (15,2/ )-epoxyindane in good yield with 91-96% ee [704]. Additional examples include epoxidation of alkenes with iodosylbenzene catalyzed by various metalloporphyrins [705-709], corrole metal complexes, ruthenium-pyridinedicarboxylate complexes of terpyridine and chiral bis(oxazoUnyl)pyridine [710,711]. 

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... 

The electropolymerization method of functional monomers has proved successful in many cases, in particular to incorporate various ligands and their metallic complexes, like salen [66,245], porphyrin [246], diphosphine [247], pyridine [71,80,248,249], crown ether [250,251], metallofullerene [252], tetraazacyclotetradecane [253], or Prussian blue type [254] in a conjugated organic material like PPy, PTh, or PANE Thick films are likely to be obtained provided that the polymer is redox active at the deposition potential due to this, Zotti et al. demonstrated that 5,5 -bis(3,4-(ethylenedioxy)thien-2-yl)-2,2 -bipyridine could be electropolymerized when complexed by iron and ruthenium, but not in the case of complexation by nickel or copper [71]. 

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). 

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]. 

Other Metal Complexes Apart from metal complexes derived from BINOL, other metal complexes, such as the lithium-aluminum amiuo diol complex, " aluminum and nickel salen complex, ruthenium diamine complex, and ruthenium phosphinite diamine complex were also found applicable for the asymmetric Michael addition of 1,3-dicarbonyl compound to cyclic enone. All these metal complexes afforded about 90% of asymmetric induction in the Michael reaction of 2-cyclohexen-l-one and malonate. 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

An asymmetric C-H insertion using a chiral 3,3, 5,5 -tetrabromosubstituted (salen)manganese(m) complex 107 with TsN=IPh afforded insertion products with ee up to 89%.258 Che reported the first amidation of steroids such as cholesteryl acetate with (salen)ruthenium(n) complexes 108.259... 

Although salen complexes of chromium, nickel, iron, ruthenium, cobalt, and manganese ions are known to serve as catalysts for epoxidation of simple olefins, the cationic Mn-salen complex is the most efficient. 

The asymmetric epoxidation of /i-alkenes and terminal alkenes proved to be more difficult, though a recent finding, describing the use of a modified salen complex to epoxidize ( )-0-methylstyrene to form the corresponding epoxide in 83% ee, represents another important step forward. Alternatively, chiral (D2-symmetric) porphyrins have been used, in conjunction with ruthenium or iron, for efficient asymmetric oxidation of trans- and terminal alkenes[92]. 

The enantioselective oxidative coupling of 2-naphthol itself was achieved by the aerobic oxidative reaction catalyzed by the photoactivated chiral ruthenium(II)-salen complex 73. 2 it reported that the (/ ,/ )-chloronitrosyl(salen)ruthenium complex [(/ ,/ )-(NO)Ru(II)salen complex] effectively catalyzed the aerobic oxidation of racemic secondary alcohols in a kinetic resolution manner under visible-light irradiation. The reaction mechanism is not fully understood although the electron transfer process should be involved. The solution of 2-naphthol was stirred in air under irradiation by a halogen lamp at 25°C for 24 h to afford BINOL 66 as the sole product. The screening of various chiral diamines and binaphthyl chirality revealed that the binaphthyl unit influences the enantioselection in this coupling reaction. The combination of (/f,f )-cyclohexanediamine and the (R)-binaphthyl unit was found to construct the most matched hgand to obtain the optically active BINOL 66 in 65% ee. 

Kureshy et al. have reported that ruthenium-Schiff base complexes 27 serve as a catalyst for enantioselective epoxidation of styrene derivatives (Scheme 6B.26) [71], An electronic effect similar to that described in the Mn-salen-catalyzed epoxidation (vide supra) is observed in this epoxidation, that is, an electron-donating group on the catalyst and electron-withdrawing group on the substrate lead to higher enantioselectivity. For example, the epoxidation of styrene with 27c shows modest enantioselectivity (38% ee), whereas that of m-nitrostyrene with 27a exhibits much higher enantioselectivity (80% ee). 

It was recently found that (ON+)(salen)ruthenium(II) complex 28 was an efficient catalyst for the epoxidation of conjugated trans-, cis- and terminal olefins. These reactions were remarkably accelerated by irradiation. It is noteworthy that trans- and cis-olefins gave the corresponding trans- and civ-epoxides, respectively (Scheme 6B.27) [72]. 

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