Ruthenium complex main group

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. 

These reports sparked off an extensive study of metalloporphyrin-catalyzed asymmetric epoxidation, and various optically active porphyrin ligands have been synthesized. Although porphyrin ligands can make complexes with many metal ions, mainly iron, manganese, and ruthenium complexes have been examined as the epoxidation catalysts. These chiral metallopor-phyrins are classified into four groups, on the basis of the shape and the location of the chiral auxiliary. Class 1 are C2-symmetric metalloporphyrins bearing the chiral auxiliary at the... 

As invented by Wender,196,197 a variant of the second transformation can take place if the alkene partner is substituted by a participating group such as a strained cyclopropyl or a cyclobutanone (in the case of a 1,6-diene).198 The whole process, which mainly relies on the use of rhodium or ruthenium complexes,1 9 results in the formal... 

Ruthenium complexes catalyse the two main oxidative reactions for alkenes those in which oxygen atoms or hydroxyl groups span the erstwhile double bond without C=C rupture (e.g. epoxidation, ctT-dihydroxylation, ketohydroxylation), and cleavage reactions in which the C=C bond is broken. Although RuO has recently been shown to be effective for c/x-dihydroxylation and ketohdroxylation, epoxidations are in general effected by Ru complexes of lower oxidation states, while RuO excels at cleavage reactions. 

A review has appeared on the synthesis of enantiomerically enriched aziridines by the addition of nitrenes to alkenes and of carbenes to imines.45 A study of the metal-catalysed aziridination of imines by ethyl diazoacetate found that main group complexes, early and late transition metal complexes, and rare-earth metal complexes can catalyse the reaction.46 The proposed mechanism did not involve carbene intermediates, the role of the metal being as a Lewis acid to complex the imine lone pair. Ruthenium porphyrins were found to be efficient catalysts for the cyclopropana-tion of styrenes 47 High diastereoselectivities in favour of the /ifr-product were seen but the use of chiral porphyrins gave only low ees. 

Following on from this initial publication of Backvall, many groups have reported on a variety of ruthenium-based systems for the DKR of secondary alcohols [9-17] mainly with the goal of eliminating the need for added base and ketone and reducing the reaction time by increasing the rate of racemization. Some examples of ruthenium complexes (1-8) which have been used as the racemization catalysts in these systems are depicted in Fig. 9.5. 

In comparison with classic Lewis acids derived from main group halides (e.g., B, Al, Sn), f-elements, and early transition metal halides, late transition metal Lewis acids often are more inert to ubiquitous impurities such as water, offer higher stability, tunable properties by ligand modification, and a well-defined structure and coordination chemistry, thus allowing detailed studies of reaction mechanisms, and a rational basis for catalyst optimization. Among this new class of late transition metal Lewis acids, ruthenium complexes - the subject of this chapter - display remarkable properties... 

Treatment of (80) and (81) with Ru3(CO)12 gave the >/4-silatrimethylenemethane-ruthenium complexes in 9% and 22% yield, respectively. The major product of the Z-alkylidenesilacyclopropane reaction was trinuclear ruthenium carbonyl cluster (82), whose structure was established by x-ray diffraction (Equation (37)). This appears to be the first example of a main group metal-bound carbonyl inserting into a silacyclopropane <9lJA279i, 94OM4606). 

Catalytic oxidation of electron rich alkenes such as styrene with ruthenium complexes gave mainly benzaldehyde rather than the expected epoxide. When styrene was oxidised with CHP as the oxidising agent benzaldehyde was the major detectable product. Polymerisation of styrene occurs in solvents like methanol. Table-4 shows the oxidation of styrene with CHP. Substitution of methyl groups in the pyridine ring increases the yield of benzaldehyde. When RuCl2(4-Mepy)4 was the catalyst, negligible amounts of styrene oxide was also detected. However with other catalysts benzaldehyde was the only detectable product. Other oxidation products like phenyl acetaldehyde and acetophenone were not detected [10,11]. 

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