Ruthenium complex transfer reaction

The main methodologies developed until now for enantioselective oxidation of sulfides are effective only in the oxidation of alkyl aryl sulfoxides. Dialkyl sulfoxides on the other hand are generally oxidized with only poor selectivity. In an attempt to solve this problem, Schenk s group69 recently reported a stereoselective oxidation of metal-coordinated thioethers with DMD. The prochiral thioether is first coordinated to a chiral ruthenium complex by reaction with the chloride complexes [CpRu[(S,S)-chiraphos]Cl], 36. Diastereoselective oxygen transfer from DMD produces the corresponding sulfoxides in high yield and selectivity. The chiral sulfoxides 37 are liberated from the complexes by treatment with sodium iodide. Several o.p. aryl methyl sulfoxides have been obtained by this method in moderate to high ee (Scheme 12). 

Previously, examples of chiral Lewis acid coordination of either the radical or radical acceptor involved coordination to Lewis basic sites, typically oxygen or nitrogen. However, certain transition metals are also capable of coordination to al-kenes and, if complexed to a chiral ligand, can also afford chiral addition products. This has been illustrated in the enantioselective atom transfer additions of alkane and arene-sulfonyl chlorides and bromotrichloromethanes to olefins using chiral ruthenium complexes. These reactions are thought to follow a radical redox chain process detailed in Eq. (20). 

Let us now examine sample sets of data. We shall consider two reactions, the formation of a biradical1 [Eq. (7-10)] and an electron transfer reaction between two ruthenium complexes [Eq. (7-11)], in which LN represent nitrogen-donor ligands specified in the original reference.2 The chemical equations are... 

It has been demonstrated that group 6 Fischer-type metal carbene complexes can in principle undergo carbene transfer reactions in the presence of suitable transition metals [122]. It was therefore interesting to test the compatibility of ruthenium-based metathesis catalysts and electrophilic metal carbene functionalities. A series of examples of the formation of oxacyclic carbene complexes by metathesis (e.g., 128, 129, Scheme 26) was published by Dotz et al. [123]. These include substrates where double bonds conjugated to the pentacarbonyl metal moiety participate in the metathesis reaction. Evidence is... 

D KR of allylic alcohols can be also performed using ruthenium complexes for the racemization that occurs through hydrogen transfer reactions (vide infra) [16]. 

Ruthenium porphyrin complexes are also active in cyclopropanation reactions, with both stoichiometric and catalytic carbene transfer reactions observed for Ru(TPP)(=C(C02Et)2> with styrene. Ru(Por)(CO)orRu(TMP)(=0)2 catalyzed the cyclopropanation of styrene with ethyidiazoacetate, with aiiti.syn ratios of 13 1... 

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

In order to obtain further information on the magnitude of the overall reaction volume and the location of the transition state along the reaction coordinate, a series of intermolecular electron-transfer reactions of cytochrome c with pentaammineruthenium complexes were studied, where the sixth ligand on the ruthenium complex was selected in such a way that the overall driving force was low enough so that the reaction kinetics could be studied in both directions (153, 154). The selected substituents were isonicotinamide (isn), 4-ethylpyr-idine (etpy), pyridine (py), and 3,5-lutidine (lut). The overall reaction can be formulated as... 

Sabbatini N, Dellonte S, Bonazzi A et al (1986) Photoinduced electron-transfer reactions of poly(pyridine)ruthenium(II) complexes with europium(III/II) cryptates. Inorg Chem 25 1738-1742... 

The inverted region in electron transfer reactions is studied for the reaction of electronically-excited ruthenium(II) tris-bipyridyl ions with various metal(III) tris-bipyridyl complexes. Numerical calculations for the diffusion-reaction equation are summarized for the case where electron transfer occurs over a range of distances. Comparison is made with the experimental data and with a simple approximation. The analysis reveals some of the factors which can cause a flattening of the In k versus AG curve in the inverted... 

Aromatics occur as ligands in ruthenium complexes that are used for hydrogen transfer reaction, i.e. two hydrogen atoms are transferred from a donor molecule, e.g. an alcohol, to a ketone, producing another alcohol. Especially the enantiospecific variant has become important, see Chapter 4.4. The substitution pattern of the aromatic compound influences the enantioselectivity of the reaction. 

Hydrogen Transfer Reactions Catalyzed by Ruthenium Complexes Linked to P-Cyclodextrin 43... 

The control of enantioselectivity in the reduction of carbonyl compounds provides an opportunity for obtaining the product alcohols in an enantiomerically enriched form. For transfer hydrogenation, such reactions have been dominated by the use of enantiomerically pure ruthenium complexes [33, 34], although Pfaltz and coworkers had shown by 1991 that high levels of enantioselectivity could be obtained using iridium(I) bis-oxazoline complexes [35]. 

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. 

After extensive screening of various aldehydes to optimize the reaction conditions, it was found that aromatic aldehydes were able to serve as a carbon monoxide source, in which the electronic nature of the aldehydes is responsible for their ability to transfer CO efficiently [24]. Consequently, aldehydes bearing electron-withdrawing substituents are more effective than those bearing electron-donating substituents, with pentafluoro-benzaldehyde providing optimal reactivity. Interestingly, for all substrates tested the reaction is void of any complications from hydroacylation of either the alkene or alkyne of the enyne. Iridium and ruthenium complexes, which are known to decarboxylate aldehydes and catalyze the PK reaction, demonstrated inferior efficiency as compared to... 

Another focus of this chapter is the alkynol cycloisomerization mediated by Group 6 metal complexes. Experimental and theoretical studies showed that both exo- and endo- cycloisomerization are feasible. The cycloisomerization involves not only alkyne-to-vinylidene tautomerization but alo proton transfer steps. Therefore, the theoretical studies demonstrated that the solvent effect played a crucial role in determining the regioselectivity of cycloisomerization products. [2 + 2] cycloaddition of the metal vinylidene C=C bond in a ruthenium complex with the C=C bond of a vinyl group, together with the implication in metathesis reactions, was discussed. In addition, [2 + 2] cycloaddition of titanocene vinylidene with different unsaturated molecules was also briefly discussed. 

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