Ruthenium complexes aerobic oxidation

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. 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

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 compounds are widely used as catalysts for hydrogen-transfer reactions. These systems can be readily adapted to the aerobic oxidation of alcohols by employing dioxygen, in combination with a hydrogen acceptor as a cocatalyst, in a multistep process. For example, Backvall and coworkers [85] used low-valent ruthenium complexes in combination with a benzoquinone and a cobalt Schiff s base complex. The proposed mechanism is shown in Fig. 14. A low-valent ruthenium complex reacts with the alcohol to afford the aldehyde or ketone product and a ruthenium dihydride. The latter undergoes hydrogen transfer to the benzoquinone to give hydroquinone with concomitant... 

Other ruthenium-based catalysts for the aerobic oxidation of alcohols have been described where it is not clear if they involve oxidative dehydrogenation by low-valent ruthenium, to give hydridoruthenium intermediates, or by high-valent oxoruthenium. Masutani et al. [107] described (nitrosyl)Ru(salen) complexes, which can be activated by illumination to release the NO ligand. These complexes demonstrated selectivity for oxidation of the alcoholic group versus epoxidation, which was regarded as evidence for the intermediacy of Ru-oxo moieties. Their excellent alcohol coordination properties led to a good enantiomer differentation in the aerobic oxidation of racemic secondary alcohols (Fig. 19) and to a selective oxidation of primary alcohols in the presence of secondary alcohols [108]. 

The present hydrogen transfer reaction is extended to the aerobic oxidation of alcohols. Thus, the oxidation of alcohols can be carried out with a catalytic amount of hydrogen acceptor under an O2 atmosphere by a multistep electron-transfer process. As shown in Scheme 3.4, the ruthenium dihydrides formed during the hydrogen transfer can be regenerated by a multistep electron-transfer process including hydroquinone, ruthenium complex, and molecular oxygen. 

Compared to the multistep electron-transfer process shown in Scheme 3.4, more simple aerobic oxidations of alcohols were reported with various homogeneous and heterogeneous ruthenium catalysts. The aerobic oxidation of alcohols with metal catalysts is an attractive method for economical and environmental reasons. Aerobic oxidation of alcohols can be carried out using RUCI3 catalyst with moderate conversion and selectivities (Eq. 3.13) [31]. Since this reaction was first reported, an arduous search for suitable catalysts has been continuing using various ruthenium complexes (Table 3.1). 

James et al. reported that aerobic oxidation of primary amines in the presence of a ruthenium porphyrin complex Ru(TMP)(0)2 (TMP = tetramesitylporphyrinato) gives nitriles (100%) (Eq. 3.31) [66]. 

Aqua(phosphine)ruthenium(II) complexes [121] are useful for activation of molecular oxygen, and catalytic oxidation of cyclohexene can be carried out with 1 atm of O2 [121a,bj. The ruthenium catalyst bearing perfluorinated 1,3-diketone ligands catalyzes the aerobic epoxidation of alkenes in a perfluorinated solvent in the presence of i-PrCHO [122]. Asymmetric epoxidations of styrene and stilbene proceed with 56-80% e.e. with ruthenium complexes 38-40 (Figure 3.2) and oxidants such as PhI(OAc)2, PhIO, 2,6-dichloropyridine N-oxide, and molecular oxygen [123-125]. 

Nonporphyrin-based oxo-metal species can be generated by the reaction of a low-valent ruthenium complex with molecular oxygen in the presence of an aldehyde [141]. Thus, the ruthenium-catalyzed oxidation of alkanes with molecular oxygen in the presence of acetaldehyde gives alcohols and ketones efficiently [155]. These aerobic oxidations can be rationalized by assuming the sequence shown in Scheme 3.10. 

Recently, we found that a copper catalyst - as well as ruthenium - is effective for the oxidation of alkanes with molecular oxygen in the presence of acetaldehyde [157]. The catalytic system CUCI2 and 18-crown-6 has proved to be efficient [157cj. Furthermore, we found that specific copper complexes derived from copper salts and acetonitrile are convenient and highly useful catalysts for the aerobic oxidation of unactivated hydrocarbons [158], For example, oxidation of cyclohexane with molecular oxygen (1 atm of O2 diluted with 8 atm of N2) in the presence of acetaldehyde and Cu(OAc)2 catalyst (0.0025 mol%) in CH3CN/CH2CI2 (3 2) at 70°C in an... 

Aerobic oxidation of alkanes.1 Various metal complexes arc known to catalyze air oxidation of unactivatcd C-—H bonds. Murahashi et al. have found that both ruthenium and iron complexes arc useful catalysts for aerobic oxidation in combination with an aldehyde and an acid. Iron powder is the most effective catalyst, but FeCl3 6H2(), RuCI3 H20, and RuCI2[P(C6H5)3]3 can be used. Useful aldehydes arc hcptanal, 2-mcthylpropanal, and even acetaldehyde. A weak acid is suitable thus acetic acid is preferred to chloroacetic acid. By using the most satisfactory conditions, cyclohexane... 

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