Phenylacetylene, reaction with ruthenium

Ru-vinylidene complexes can be easily prepared by reaction of low-valent ruthenium complexes with terminal acetylenes. Treatment of the Ru(ii) complex 117 with phenylacetylene gave the Ru(iv)-vinylidene complex 118 in 88% yield (Scheme 41 ).60 The reaction of 118 with C02 in the presence of Et3N afforded selectively the Ru-carboxylate complex 120, probably via the terminal alkynide intermediate 119. 

Recently, a proposal has been put forth that a /raor-addition process may be possible through dinuclear ruthenium intermediates.34 As shown in Scheme 5, reaction of tetraruthenium aggregate A with phenylacetylene results in the fully characterized bridging dinuclear alkenyl complex B. The authors propose a direct /ra .r-dclivcry of hydride through a dinuclear intermediate may be active in the hydrosilylation catalyzed by A, though compound B itself is unreactive to Et3SiH. 

From the results presented here, one could get the impression that such allenes with hydroxyl groups in the substituents will always form heterocydes in the presence of transition metal catalysts, but in the presence of other substrates even allenylcarbinols can react to give different products. Examples are the rhodium-catalyzed reaction of allenylcarbinol 78 and phenylacetylene 79 to 80 [42], the palladium-catalyzed reaction of 81 and pyrrolidine 82 to 83 [43] and the ruthenium-catalyzed reaction of 78 and 79 to 84, an isomer of the rhodium-catalyzed reaction of the same substrates mentioned above [44] (Scheme 15.19). 

Recently, it was shown that the metathesis catalyst RuCl2(PCy3)2(=CHPh), where Cy is cyclohexyl, reacted in refluxing toluene with phenylacetylene to produce a ruthenium vinylidene species which promoted the regioselective dimerization of phenylacetylene into ( )-1,4-diphenylbutenyne [56]. The addition of 1 Eq acetic acid did not lead to enol esters but to a faster reaction and the stereoselective dimerization of phenylacetylene into the Z dimer. 

The interaction of an alkyne with (tj5-C5H5)(PR3)2RuX can result in the formation of a wide variety of ruthenium complexes. The nature of the products formed depends on the conditions used and the type of alkyne reacted. Reactions between I and terminal alkynes in the presence of ammonium hexafluorophosphate lead to the formation of cationic monosubstituted ruthenium vinylidene complexes in high yield, as shown for phenylacetylene in Eq. (61) (4,67,68). 

Similar reactions of 34 and 35 with phenylacetylene at room temperature are also stereospecific, and they are presumed to occur with retention of configuration at the metal center by analogy to the propyne reactions. When these reactions are performed in refluxing methanol, both chemo-selectivity and stereospecificity are lost, with almost equal amounts of the two benzylidene diastereomers (65 and 66) and a small amount (10-15%) of the methanol adducts (67) (vide infra) being formed from 34 [Eq. (63)]. The individual benzylidene epimers 65 and 66 do not epimerize at the ruthenium center in refluxing methanol, which indicates that the loss of stereochemical integrity occurs prior to addition of the acetylene. 

The monosubstituted vinylidene complexes are readily deprotonated with a variety of mild bases (e.g., MeO-, C032 ), and this reaction constitutes the most convenient route to ruthenium acetylide complexes. Experimentally the deprotonation is most easily achieved by passing the vinylidene complex through basic alumina. Addition of a noncomplexing acid (e.g., HPF6) to the acetylide results in the reformation of the vinylidene complex [Eq. (66)]. Reaction of 1 and terminal alkynes such as phenylacetylene in methanol followed by the addition of an excess of... 

In an extension of this work, either zinc(II), palladium(II), rhodium(I) or copper(I) salts were immobilised in an ionic liquid film (SILP, vide supra) onto diatomic earth and the catalysts tested for activity in the reaction between phenylacetylene and 4-isopropyl-phenylaminc.1 391401 The supported rhodium, ruthenium and zinc complexes afford higher rates and selectivities relative to their use under homogenous reaction conditions. Lower rates are, however, observed with the copper salt, which is rationalised by strong complexation of the ionic liquid to the Cu(I) centre. 

Water-soluble ruthenium vinylidene and allenylidene complexes were also synthetized in the reaction of [ RuC12(TPPMS)2 2] and phenylacetylene or diphenylpropargyl alcohol [29], The mononuclear Ru-vinylidene complex [RuCl2 C=C(H)Ph (TPPMS)2] and the dinuclear Ru-allylidene derivative [ RuCl( x-Cl)(C=C=CPh2)(TPPMS)2 2] both catalyzed the cross-olefin metathesis of cyclopentene with methyl acrylate to give polyunsaturated esters under mild conditions (Scheme 7.10). 

In order to further explore the reactivity of the homobimetallic ruthenium complexes, the reaction of 4 with terminal alkynes was investigated. Thus, when phenylacetylene or ferf-butylacetylene was added to a solution of complex 4 in CH2CI2 or benzene, the rapid and quantitative formation of the corresponding rathenium- vinylidene complexes, 8 and 9, respectively, was observed (13). The formation of 8 and 9 can be rationalised by the displacement of the ethylene ligand by the respective acetylene followed by an alkyne-to-vinylidene transformation. 

A real example following the H2-decoordination mechanism is provided by the selective hydrogenation of 1-alkynes to alkenes with the ruthenium(II) complex [(PP3)RuH(Ti2-H2)]BPh4 [PP3 = P(CH2CH2PPh2)3l [25]. A kinetic study of the hydrogenation of phenylacetylene at ambient or sub-ambient pressure showed that the reaction is first-order in catalyst concentration, second order in H2 pressure and independent of substrate concentration. At very low 1-alkyne concentration (<0.12 M), a first order dependence with respect to the substrate was observed. 

Table 4 shows clearly that if 1, activated with phenylacetylene, is used as catalyst, yields dramatically decrease when the monomer is diluted. This is also the case for completely inert solvents like decaline and therefore cannot be due to catalyst-solvent interactions. Solution polymerizations with 5 do not show this effect. The obvious explanation is that the initiation must be the rate determining step, which occurs by the formation of a ruthenium carbene from 1 and phenylacetylene (or other monosubstituted acetylenes) in situ. This process is strongly disfavored if the catalyst mixture is diluted. If ruthenium carbenes like 5 are used as ROMP initiators, such a "dilution effect" is not observed. Only the smaller reaction rate due to monomer dilution is seen, as usual. 

Ru (Tp)(Cl)(PPh3)(MeCN)] serves as a starting material for the synthesis of new ruthenium complexes. Water was also involved in the reactions of this Ru—Tp complex with phenylacetylene and l-ethynyl-4-fluorobenzene that yields the alkenyl ketone compound [Ru(Tp)(PPh3)(C(CH2Ph)= CHC(O)Ph)] and the acyl complex [Ru(Tp)(PPh3)(C(0)CH2(C6H5)F)]. ... 

An example of this rearrangement is the reaction of the ruthenium cyclopenta-dienyl complex (ti -C5H5)RuCl(PPh3)2 with phenylacetylene to give [(q -CsHs) Ru(=C=CHPh)(PPh3)2] ". 

Conjugated dienes can be selectively hydrated to ketones in the presence of cationic ruthenium complexes with bipyridyl ligands. The role of ruthenium is to catalyze the isomerization of allylic alcohols formed by the addition of water to diene. This method allows one to convert butadiene to methyl ethyl ketone in high yield [187]. Hydration of triple bonds is one of the oldest catalytic processes of organic chemistry. Though this reaction has no industrial value, it can serve as a tool of fine organic synthesis. The hydration can be catalyzed by rhodium salts under phase-transfer conditions [188]. The more exotic process of the hydrolysis of phenylacetylene to toluene and carbon monoxide catalyzed by ruthenium complex should also be mentioned [189] ... 

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