Ruthenium complexes 97 halides

In contrast to the situation with copper-based catalysis, most studies on ruthenium-based catalysts have made use of preformed metal complexes. The first reports of ruthenium-mediated polymerization by Sawamoto and coworkers appeared in I995.26 In the early work, the square pyramidal ruthenium (II) halide 146 was used in combination with a cocatalyst (usually aluminum isopropoxide). 

The three steps 32-34 have been suggested77 to be equilibria, and the overall equilibrium must lie far to the left because no adduct 23 is found in the reaction mixture when the reaction of sulfonyl chloride with olefin is carried out in the absence of a tertiary amine. A second possible mechanism involving oxidative addition of the arenesulfonyl halide to form a ruthenium(IV) complex and subsequent reductive elimination of the ruthenium complex hydrochloride, [HRulvCl], was considered to be much less likely. 

The bond dissociation energy of fluoromethane is 115 kcal mol , which is much higher than the other halides (C-Cl, C-Br and C-1, respectively 84, 72 and 58 kcal mol ) [6], Due to its strength, the carbon-fluorine (C-F) bond is one of the most challenging bonds to activate [7], A variety of C-F bond activation reactions have been carried out with different organometallic complexes [8], Among them, nickel [9] and ruthenium complexes have proven to proceed selectively under mild conditions [10],... 

This type of hydrodehalogenation has been performed generally in the presence of organic or inorganic bases to neutralize the hydrogen halides formed. Among published results, the use of rhodium complexes as catalysts dominates, but palladium and ruthenium complexes have also been applied on a frequent basis. 

A special application of bimetallic ruthenium complexes was found in the olefin metathesis reaction vide infra) The two metal centers were closely attached to one another through /r-halide anions. The labile assembly was the key feature to the formation of highly active catalysts. 

Ruthenium complexes in nonreactive solvents such as sulfolane and NMP in the presence of halide promoters are found to possess high activity for the nium catalysts in many respects—rates, selectivities, catalytic species observed, and mechanism—it is addressed separately in this section. 

The organometallic complexes are quite various cobaltocene/bis(ethylaceto-acetato) copper (II) [366], CuCl or CuBr/bipyridyl [367, 368], cobaltoxime complex [369], reduced nickel/halide system [370], organoborane [371], ruthenium complex/trialkoxyaluminum system [372]. 

A recent study showed that 152 behaves mechanistically different from other catalysts in addition reactions of more activated halides 140, such as trichloroacetate to styrene [222]. After initial reduction to Ru(II), chlorine abstraction from substrates 140 is in contrast to all other ruthenium complexes not the rate limiting step (cf. Fig. 36). ESR spectroscopic investigations support this fact. The subsequent addition to styrene becomes rate limiting, while the final ligand transfer step is fast and concentration-independent. For less activated substrates 140, however, chlorine abstraction becomes rate-determining again. Moreover, the Ru(III) complex itself can enter an, albeit considerably slower Ru(III)-Ru(IV) Kharasch addition cycle, when the reaction was performed in the absence of magnesium. This cycle operates, however, for only the most easily reducible halides, such as trichloroacetate. 

Since the discovery of catalysts 2 and 3 containing one NHC ligand, the attractive family of NHC-ruthenium complexes has been rapidly expanded. In the following section, the different structural modifications of complexes 2 and 3 reported in the literature will be presented (phosphine and halide ligands, benzylidene ligand, NHC ligand). 

Ruthenium catalysts can participate in electron-transfer processes. Thus, a variety of radical reactions of organic halides have been catalyzed by ruthenium complexes, as in the following example [ 126] (Eq. 95). 

Reduction with sodium naphthalene of easily obtained diazadiene ruthenium ) complexes of type 65 (X = Cl) affords (rj6-arene)Ru(0) complexes 64. The latter are very reactive and undergo oxidative addition with iodine or alkyl halides to produce complexes 65 (X = I and X = CH2R, respectively) (51) [Eq. (26)]. Other ruthenium(O) complexes have been... 

To date, several jt-allylmthenium complexes have been prepared and reported. The representative methods for introducing an allyl group to a ruthenium complex are quite similar to those for other transition metals for example, (1) the reaction of ruthenium halides with allyl Grignard reagents (2) the insertion of conjugated dienes into a hydrido-ruthenium bond and (3) the oxidative addition of several allylic compounds to low-valence ruthenium complexes. 

A similar oxidative addition of allyl halides to zerovalent ruthenium complex, RuJj/ -cod)( 7 -cot), in the presence of PMe3 has been independently reported to give RuX(n-C3Hs)(PR3)3 (X = Br, Cl) (Eq. 5.11). 

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... 

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