Ruthenium-catalysed reactions

Reactions with aikynes may aiso result in the formation of cyclized products. Thus a ruthenium-catalysed reaction with aromatic amides has been used to give isoquinoline derivatives. On the basis of values of the kinetic isotope effect, A h/ d> proposed that the rate-determining step is a concerted acetate-assisted metalation-deprotonation process to yield the intermediate (30), followed by aikyne insertion and ring closure. The reaction of arylketones with alkynes may yield indenols when catalysed by rhodium cyclopentadienyl in the presence of silver and copper salts. The reaction proceeds regioselectively, so that reaction of acetophenone with 1-phenyl-1-propene... 

A study has been reported regarding the ruthenium-catalysed reaction of benza-mides with alkynes, which yields ort/io-alkenylated derivatives. Here, the mechanism is likely to involve rate-limiting metalation, followed by alkyne insertion to form intermediates such as (63) which on protonolysis yield the alkenylated products. An allylic carbon-carbon double bond has also been used as a coordination site in palladium-catalysed alkenylation reactions, as shown in Scheme 3. Here measurement of kinetic isotope effects suggests that coordination of the palladium with the allylic double bond occurs before palladation to give (64). Insertion of the alkene into the carbon-palladium bond gives (65) and -hydride elimination " leads to the product... 

Ruthenium-NHC complexes exhibit activity in a very wide field of applications. Due to their unique ability to break and reassemble olefin bonds under reaction conditions very favourable to design simple processes, applications in nearly any chemical discipline can be foreseen. This field may span from manufacturing of specialty polymers and rabbers to pharmaceuticals, pharmaceutical intermediates, agrochemicals, fragrances, dyes, specialty chemicals for electronic applications or fine chemicals from natural feedstock and many more. Below are described Ru-NHC catalysed reactions applied from pilot to full commercial scale. 

Representatives of the bridged sulfone system 70 have been subjected to ruthenium catalysed ring-closing metathesis reactions (Grubbs catalyst) and shown to afford, in low yields, a few selected cyclic dimers and trimers, of all the possibilities available. The diastereoselectivities observed were rationalised in terms of kinetic control involved with internal ruthenium/sulfonyl oxygen coordination . 

As an alternative to the use of hydrogen gas, asymmetric ruthenium-catalysed hydrogen transfer reactions have been explored with significant success [381. 

Enantioselective reduction of acetophenone was achieved in a ruthenium-catalysed hydrogen transfer reaction using isopropanol as the hydrogen source in the presence of mono-tosylated (R, R)-diphenylethylenediamine, ephedrine or norephedrine as chiral auxiliary ligands. Under optimised conditions, ( R)-l-phenylethanol was obtained in 90% yield and 82% enantiomeric excess (ee) within 9 min. f-Butylphenylketone was reduced under similar conditions in almost quantitative yield but in moderate ee... 

Until recently, intermolecular enyne metathesis received scant attention. Competing CM homodimerisation of the alkene, alkyne metathesis and polymerisation were issues of concern which hampered the development of the enyne CM reaction. The first report of a selective ruthenium-catalysed enyne CM reaction came from our laboratories [106]. Reaction of various terminal alkynes 61 with terminal olefins 62 gave 1,3-substituted diene products 63 in good-to-excellent yields (Scheme 18). It is interesting that in these and all enyne CM reactions subsequently reported, terminal alkynes are more reactive than internal analogues, and 1,2-substituted diene products are never formed thus, in terms of reactivity and selectivity enyne CM is the antithesis of enyne RCM. The mechanism of enyne CM is not well understood. It would appear that initial attack is at the alkyne however, one report has demonstrated initial attack at the alkene (substrate-dependent) is also possible, see Ref. [107]. 

The formation of butadiynes from bis(alkynyl)mercury compounds in the presence of a catalytic amount of RuHCl(CO)(PPh3)3 is closely related to the ruthenium-catalysed dimerization reaction in terms of the mechanism [60]. The proposed catalytic cycle involves the formation of a (alkynyl)(vinyli-... 

Another example is the ruthenium-catalysed alkenylation of pyridine which is performed in the presence of the same catalyst precursor RuCl(Cp)(PPh3)2 (20 mol %)/NaPF6 (20 mol %) at 150 °C [63]. The use of trimethylsilylalkynes, which are also known to produce vinylidene complexes rather than terminal alkynes, avoids the dimerization of the alkyne and favours the formation of the (E)-vinylpyridine (Scheme 17). The reaction has been applied to a variety of silylated alkynes and substituted pyridines (Fig. 8). 

A number of different ionic liquids have been screened in the ruthenium-catalysed oxidation of secondary alcohols (see Scheme 5.18). Three different ruthenium compounds, RuC13, RuCl2(PPh3)3 and [RuCFl/i-cymene) were compared and best results were obtained with RuCl2(PPh3)3.[76] While imidazolium-based ionic liquids gave only poor results (anion = Cl) or suppressed the reaction completely (anion = [BF4] or [PF6] ), tetraalkylammonium-based solvents such as Aliquat 336 (tricaprylmethylammonium chloride) or tetramethylammonium hydroxide afforded much better yields. 

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