Ruthenium complexes nitrogen ligands

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

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... 

In the transition metal-catalyzed reactions described above, the addition of a small quantity of base dramatically increases the reaction rate [17-21]. A more elegant approach is to include a basic site into the catalysts, as is depicted in Scheme 20.13. Noyori and others proposed a mechanism for reactions catalyzed with these 16-electron ruthenium complexes (30) that involves a six-membered transition state (31) [48-50]. The basic nitrogen atom of the ligand abstracts the hydroxyl proton from the hydrogen donor (16) and, in a concerted manner, a hydride shift takes place from the a-position of the alcohol to ruthenium (a), re-... 

Optically active metal complexes have been recognized as excellent catalysts for the enantioselective cyclopropanation of carbenes with alkenes. Normally, diazo compounds react under metal catalysts in the dark to afford carbenoid complexes as key intermediates. Katsuki et al. have reported the ds-selective and enantioselective cyclopropanation of styrene with a-diazoacetate in the presence of optically active (R,R)-(NO + )(salen)ruthenium complex 80, supported under illumination (440 nm light or an incandescent bulb) [59]. The irradiation causes dissociation of the apical ligand ON + in 80, and thus avoids the splitting of nitrogen from the a-diazoacetate. 

The observation that the Ru(amidinate)C5Me5 complex could generate the first allyl ruthenium(IV) complex containing a nitrogen ligand led to the use of this complex as catalyst for simple allyl substitution of allylcarbonates [111]. Re-... 

Here, I focus on application of ruthenium complexes as catalysts for the cyclopropanation of olefins with diazoesters to describe their catalytic activity, stereoselectivity, and enantioselectivity together with structural analysis of intermediary carbene complexes, especially with nitrogen-based ligands including porphyrin derivatives [4,5]. 

Knifton has also shown (36 - 38,40) that nitrogen- or phosphorus-ligand modified ruthenium complexes, in a phosphonium salt matrix, can conveniently catalyze the hydroformylation of terminal alkenes with high selec-tivities in linear oxo products. Usually selectivities better than 80% were achieved. In the best case (160°C, 95 bar. CO/H2= 1/2) a linearity in nonanol of 94% was obtained starting from [Ru3(CO),2], 2,2 -bipyridine. and [PBu4]Br. The main products were alcohols and not aldehydes. However, it is often difficult to reduce the isomerization of oct-l-ene as well as its hydrogenation. The [Ru3(CO),2l/2,2 -bipyridine (bipy) system has been extensively explored. Two equilibria have been proposed to account for the infrared data and the effects of the bipy ligand [eqs. (8) and (9)]. 

Based on the finding that ruthenium complexes catalyzed the Michael addition of cyanoesters, Ito developed a system of RhH(CO)(PPh3)3 and chiral bi-dentated phosphine, (S,S)-(P,P)-TRAP. The catalyst promoted the asymmetric addition of 66 to 7 giving R)-67 [64, 65, 66]. In the case of a reactive acceptor, acrolein, even 0.1 mol % of the complex effectively catalyzed the reaction. An enantiomeric excess of up to 93% was attained with the diisopropylmethyl ester. Since BINAP, DIOP, CHIRAPHOS, etc., did not induce such high stereoselectivities, the frans-coordinated structure constructed by the TRAP was considered to be critical. The structure of the ruthenium complex obtained by X-ray analysis indicated the interaction of the metal with the nitrile nitrogen atom. The frans-coordinated ligand might be required to affect the remote reaction site. 

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