Metathesis catalyst alkyne

The olefin metathesis reaction is also suitable for alkynes. In the presence of a metathesis catalyst alkynes can undergo one of the three following reactions (1) cyclo-trimerization (e. g., trimerization of propyne into 1,3,5- and 1,2,4-trimethylbenzene) (2) polymerization, e. g., of phenylalkynes (eq. (9)) and (3) metathesis (rupture and reformation of carbon-carbon triple bonds), e. g., eq. (10). 

Moulijn et al. (33) studied the reactions of some linear alkynes over a W08-Si02 catalyst in a fixed-bed flow reactor. Besides metathesis, cyclotrimerization to benzene derivatives occurred. Thus, propyne yielded, in addition to metathesis products, a mixture of trimethylbenzenes. From this an indication of the mechanism of the metathesis of alkynes can be obtained. 

The metathesis of ene-ynamides has been investigated by Mori et al. and Hsung et al. [80]. Second-generation ruthenium catalysts and elevated temperatures were required to obtain preparatively useful yields. Witulski et al. published a highly regioselective cyclotrimerization of 1,6-diynes such as 98 and terminal alkynes using the first-generation ruthenium metathesis catalyst 9... 

An obvious drawback in RCM-based synthesis of unsaturated macrocyclic natural compounds is the lack of control over the newly formed double bond. The products formed are usually obtained as mixture of ( /Z)-isomers with the (E)-isomer dominating in most cases. The best solution for this problem might be a sequence of RCAM followed by (E)- or (Z)-selective partial reduction. Until now, alkyne metathesis has remained in the shadow of alkene-based metathesis reactions. One of the reasons maybe the lack of commercially available catalysts for this type of reaction. When alkyne metathesis as a new synthetic tool was reviewed in early 1999 [184], there existed only a single report disclosed by Fiirstner s laboratory [185] on the RCAM-based conversion of functionalized diynes to triple-bonded 12- to 28-membered macrocycles with the concomitant expulsion of 2-butyne (cf Fig. 3a). These reactions were catalyzed by Schrock s tungsten-carbyne complex G. Since then, Furstner and coworkers have achieved a series of natural product syntheses, which seem to establish RCAM followed by partial reduction to (Z)- or (E)-cycloalkenes as a useful macrocyclization alternative to RCM. As work up to early 2000, including the development of alternative alkyne metathesis catalysts, is competently covered in Fiirstner s excellent review [2a], we will concentrate here only on the most recent natural product syntheses, which were all achieved by Fiirstner s team. 

In addition to transition metals, recent work has demonstrated that strong Lewis acids will catalyze the addition of silanes to alkynes in both an intra- and an intermolecular fashion.14,14a-14c The formation of vinylsilanes from alkynes is possible by other means as well, such as the synthetically important and useful silylcupration15,15a of alkynes followed by cuprate protonation to afford vinylsilanes. These reactions provide products which can be complementary in nature to direct hydrometallation. Alternatively, modern metathesis catalysts have made possible direct vinylsilane synthesis from terminal olefins.16,16a... 

Quite recently, ruthenium carbene complexes more typically known as olefin metathesis catalysts have been shown to act as alkyne hydrosilylation catalysts.78,79 7Vzz r-addition is the major product with trialkylsilanes, even in a single example with an internal alkyne.78 This result represents one of the very few examples of fra r-hydrosilylation of internal alkynes. 

Alkylidyne complexes can be used as catalysts for the metathesis of alkynes. For a classic review see Schrock [18],... 

Initially alkynes were polymerised by trial and error with the use of Ziegler type recipes and the mechanism for these reactions may well be an insertion type mechanism. Undefined metathesis catalysts of ETM complexes were known to give poly-acetylene in their reaction with alkynes (acetylene) [45] and metallacycles were proposed as intermediates. Since the introduction of well-defined catalysts far better results have been obtained. The mechanism for this reaction is shown in Figure 16.24 [46], The conductive polymers obtained are soluble materials that can be treated and deposited as solutions on a surface. 

In addition to reactions characteristic of carbonyl compounds, Fischer-type carbene complexes undergo a series of transformations which are unique to this class of compounds. These include olefin metathesis [206,265-267] (for the use as metathesis catalysts, see Section 3.2.5.3), alkyne insertion, benzannulation and other types of cyclization reaction. Generally, in most of these reactions electron-rich substrates (e.g. ynamines, enol ethers) react more readily than electron-poor compounds. Because many preparations with this type of complex take place under mild conditions, Fischer-type carbene complexes are being increasingly used for the synthesis [268-272] and modification [103,140,148,273] of sensitive natural products. 

Homogeneous, single-component catalysts such as, e.g., W(=CCMe3)(OCMe3)3 or W(=CMe)(OCMe2CF3)3, cannot only be used for exchange metathesis of alkynes but also for ROMP of cycloalkynes, ADMET of a,to-diynes, and RCM of a,co-diynes [751]. 

Zhang, W. Moore, J. S. Alkyne metathesis Catalysts and synthetic applications. Adv. Synth. Catal. 2007,349, 93-120. 

A Mo(VI) alkylidyne silsesquioxane complex has been synthesized and compared to a silica-supported alkyne metathesis catalyst [140],... 

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

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... 

New applications continue to demonstrate the enormous versatility of RCM for organic synthesis. Examples include triple ring closing (Eq. 48) and alkyne metathesis, an example being that of cross-metathesis that provides an efficient synthetic strategy for prostaglandin E2 (Eq. 49). Amines and alcohols deactivate metathesis catalysts, but their protection as ethers, esters, and amides allows them to be incorporated into the designated transformation. 

In each of the alkyne metatheses outlined here, the byproduct is the volatile 2-butyne. The alkyne metathesis can only be carried out on internal alkynes, since the metathesis catalysts cyclotrimerize terminal alkynes such as 11 to benzene derivatives. In this context, it may prove useful that readily-available terminal alkynes such as 14 are easily isomerized specifically to methyl alkynes such as 15... 

In 1975, it was discovered that WCk, which is a typical metathesis catalyst, is capable to catalyze the polymerization of phenylacetyl-ene. Subsequently, various substituted acetylenes have been polymerized by this type of catalyst. In 1983, poly(l-(trimethylsilyl)-l-propyne)) was synthesized in the presence of Tads and NbCls (35). The alkyne polymerization has many similarities with ROMP. 

Metathesis of alkynes is a much less general reaction, and only a few catalysts were found to be active. Terminal alkynes characteristically undergo cyclotrimeri-zation to yield benzene derivatives in the presence of most metathesis catalysts7,8... 

The compounds (Bu 0)3 W=CR are alkyne metathesis catalysts.280 281 Probable intermediates in these reactions are tungstacyclobutadienes and the latter are seen in the presence of Cl ligands.282 Addition of alkynes to the latter leads to the formation of substituted r/ -cyclopentadiene derivatives... 

Acetylenic monomers also appeared to undergo polymerisation with conventional olefin metathesis catalysts. This relates to monosubstituted highly branched alkylacetylenes and arylacetylenes as well as disubstituted acetylenes (internal alkynes) [16-18], It has been demonstrated that acetylene itself may also be polymerised using olefin metathesis catalysts [19,20]. The polymerisation of alkynes [scheme (2)] involves a metathesis reaction [scheme (5) of Chapter 2] analogously to that of cycloolefins [21] ... 

The mechanism of polymerisation of alkynes with metathesis catalysts requires that the original triple bond of the acetylenic monomer becomes a single bond in the polymer [scheme (5) of Chapter 2], in contrast to the insertion mechanism of acetylene polymerisation with Ziegler-Natta catalysts, where the triple bond becomes a double bond [scheme (1)]. Ideas about the mechanism of metathesis polymerisation of cycloolefins suggested that isolable metal carbenes might promote the polymerisation of cycloolefins suggested that isolable metal carbenes might promote the polymerisation of alkynes, as indeed turned out to be true, as several metal carbenes were found [22-24] to cause alkyne polymerisation. 

It is important to note that this reaction works only with W2(OCMe3)6 and not with any other M2(OR)6 molecule and that while it appears to be general for aliphatic R it does not proceed for R = QH5. It goes under very mild conditions and is virtually quantitative. Since the (Me3CO)3W=CR compounds are alkyne metathesis catalysts, still other tungsten alkylidyne compounds, not obtainable in the above reaction, can be obtained by the following reaction ... 

The authors of this work did not consider I-IV as cluster compounds nor did they view II and IV as cluster isomers. Let us see if we can relate the observed chemistry to cluster isomerization albeit one in which the forward (C apart to C adjacent) and reverse (C adjacent to C apart) have similar barriers. That is, an effective alkyne metathesis catalyst cannot have any species in the catalytic cycle of high stability (Bunz, 2005). 

Alternatively, with known metathesis catalysts such as (PCy3)2Cl2Ru=CHPh, a metal carbene mechanism of alkyne trimerization has been proposed.141... 

Zhang W, Moore JS. Alkyne metathesis catalysts and synthetic... 

The reaction of the rhenium alkylidyne complex 277 with diisopropyl-acetylene and with diethylacetylene [Eq. (196)] demonstrates the sensitivity of metathesis reactions toward steric factors (57). With diisopropylace-tylene an alkylidyne complex is obtained whereas the reaction with diethylacetylene gives a metallacyclobutadiene. In the metathesis reactions the alkyne with the bulkiest groups cleaves most easily from intermediate metallacyclobutadiene complexes. The rhenacyclobutadienes with the smallest substituents thus become sinks and slow down the effective rate of metathesis. The alkylidyne alkylidene rhenium complex 278 is an active olefin metathesis catalyst (52). Reaction with hexene transforms the neo-pentylidene group into a propylidene group as shown in Eq. (197). 

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