Ruthenium alkyne dimerization

Scheme 10.20 Examples of ruthenium catalysts active for terminal alkynes dimerization.

Cp Ru [14] and TpRu [20] complexes have also been studied in depth. As represented in Scheme 2c, the catalytic alkyne dimerization proceeds via coordinatively unsaturated ruthenium alkynyl species. Either a direct alkyne insertion and/or previous vinylidene formation are feasible pathways that determine the selectivity. The head-to-tail dimer cannot be formed by the vinylidene mechanism, whereas the E or Z stereochemistry is controlled by the nature of the alkynyl-vinylidene coupling. 

Typical Procedure for Ruthenium-Catalyzed Dimerization of Terminal Alkynes with Monocarboxylic Acids [23]... 

Terminal alkynes can undergo several types of interaction with ruthenium centres. In addition to the formation of ruthenium vinylidene species, a second type of activation provides alkynyl ruthenium complexes via oxidative addition. When these two types of coordination take place at the same metal centre, the migration of the alkynyl ligand onto the Ca atom of the vinylidene can occur to form enynyl intermediates, which upon protonation by the terminal alkyne lead to the formation of enynes corresponding to alkyne dimerization... 

Aryl acetylenes undergo dimerization to give 1-aryl naphthalenes at 180 °C in the presence of ruthenium and rhodium porphyrin complexes. The reaction proceeds via a metal vinylidene intermediate, which undergoes [4 + 2]-cycloaddition vdth the same terminal alkyne or another internal alkyne, and then H migration and aromatization furnish naphthalene products [28] (Scheme 6.29). 

Most of these catalytic systems are able to dimerize either aromatic alkynes, such as phenylacetylene derivatives, or aliphatic alkynes, such as trimethylsilylacetylene, tert-butylacetylene and benzylacetylene. The stereochemistry of the resulting enynes depends strongly on both the alkyne and the catalyst precursor. It is noteworthy that the vinylidene ruthenium complex RuCl(Cp )(PPh3)(=C=CHPh) catalyzes the dimerization of phenylacetylene and methylpropiolate with high stereoselectivity towards the ( )-enyne [65, 66], and that head-to-tail dimerization is scarcely favored with this catalyst. It was also shovm that the metathesis catalyst RuCl2(P-Cy3)2(=CHPh) reacted in refiuxing toluene with phenylacetylene to produce a... 

Finally, it can be noted that some cross-dimerization of terminal alkynes with internal alkynes, where ruthenium vinylidene intermediates are postulated, have also been reported [74, 75]. 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

Reports on ruthenium catalytic activity focus more on mechanistic consideration of the prototypical phenylacetylene dimerization than in establishing its synthetic applicability. It is not unusual that changing the alkyne substituents results in reversed selectivity (i.e. R = Ph or SiMe3 gave ( )- or (Z)- isomers, respectively) [27]. Competitive alkyne cyclotrimerization (R = COOMe) [27] or butatriene formation (R= CH2Ph, Bu) [10, 21] have occasionally been reported as possible drawbacks in enyne synthesis. The operating mechanism restricts the reaction to terminal alkynes. 

The first example involves the dimerization of terminal alkynes. It takes place via initial activation of the alkyne C-H bond, but several examples involve a vinylidene intermediate. In most cases, conjugated enynes are obtained by ruthenium-catalyzed tail-to-tail dimerization [84,85], as in the following example [85] (Eq. 63). 

The precatalyst Cp RuCl(COD) allowed the head-to-head oxidative dimerization of terminal alkynes and the concomitant 1,4-addition of carboxylic acid to stereoselectively afford 1-acyloxy-l,3-dienes in one step under mild conditions [89] (Eqs. 67,68). The first step of the reaction consists in the oxidative head-to-head alkyne coupling via the formation of a ruthenacycle intermediate that behaves as a mixed Fischer-Schrock-type biscarbene ruthenium complex, allowing protonation and nucleophilic addition of the carboxylate. 

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). 

Cyclopentadienyl dicarbonyl ruthenium dimer 132 reacts with silver tetrafluoroborate and diphenylacetylene to afford the cyclobutadiene ruthenium complex 133 (Scheme 12). Irradiation of 133 in dichloro-methane in the presence of several alkynes leads to the arene cyclopentadienyl ruthenium complexes 125 in high yield. This reaction appears to be a general route to sterically crowded ruthenium arene cations (55). 

Cyclic seven-membered vinyl silanes 161 were obtained by regio- and stereoselective hydrosilylation of internal alkynes catalyzed by the ruthenium complex [Cp Ru(MeCN)3]PF6, as shown in Equation (33) <2005JA10028>. Hydrosilylation of 2,2-divinyladamantane with bis(hydrosilane) species 162 in the presence of Zeise s dimer [Pt2Cl4(CH2CH2)2] gave the disilacyclic 163 in high yields (Equation 34) <19980M4267>. 

We had established in previous catalytic reactions involving complex 24 that this precatalyst was activated by the removal of the cod (1,5-cyclooctadiene) from the ruthenium by its reaction with the alkyne substrate via a [2 + 2 + 2] cydization as illustrated in Equation 1.64 [57]. Thus, not only does this reaction constitute an activation of the Ru complex 24 by reacting off the cod, it also serves as a novel atom economic reaction in its own right. Both internal and terminal alkynes participate. The overall atom economy of this process is outstanding since cod itself is simply available by the nickel-catalyzed dimerization of butadiene. Thus, the tricyclic product is available by the simple addition to two molecules of butadiene and an alkyne with anything else only needed catalytically. 

Dien-4-ynes 136 (R -R = alkyl) are produced from propargylic carbonates 135 and terminal alkynes in the presence of a palladium-phosphine complex and copper(I) iodide. The linear co-dimerization of terminal acetylenes and 1,3-dienes is catalyzed by ruthenium(cyclooctadiene)(cyclooctatriene)(trialkylphosphine) (alkyl = Et, Bu or octyl) thus 1-hexyne and methyl penta-2,4-dienoate give a mixture of the eneynes 137 and 138. Coupling of octa-l,7-diyne (139) with the acetylenic bromo acid 140 in aqueous THF-methanol containing butylamine, hydroxylamine hydrochloride and copper(I) chloride gave a mixture of the triynyl acids 141 and 142. ... 

The dimerization process in Scheme 7.22 involves migration of the alkynyl group to the alkenylidene ligand to give a-enynyl-ruthenium complex. Its further reaction with the alkyne, possibly via a-bond metathesis or protonation by the alkyne affords enyne with regeneration of an alkynyl-ruthenium complex that carries the catalysis. The a-enynyl-ruthenium complex, on the other hand, may be isomerized to butatrienyl-ruthenium complex, which on further reaction with the alkyne liberates butatriene with regeneration of the alkynylruthenium species. Sto-... 

Various situations are analyzed where the two metal centers play a role in one of the coordination modes A-E. There are many cases in which bimetallic catalysis can occur with the two metals acting cooperatively, for instance, in the dimerization of alkynes at two ruthenium metal centers, where a ruthenium-vinylidene species is formed, which is able to subsequently activate the second alkyne reactant through a C-H cleavage on the second ruthenium center. The coupling of these two moieties occurs on this dinuclear platform to provide the enyne product molecule. Examples are also presented where bimetallic catalysts cooperatively activate substituted alkynes in the catalyzed formation of heterocycles. 

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