Alkyne metathesis reaction

The second example involves the synthesis of ortho-dipropynylated arenes (Scheme 4.12b), which serve as precursors to tribenzocyclyne by way of an alkyne metathesis reaction (see also Scheme 6.31). Here, a Sonogashira reaction was carried out in a pre-pressurized (propyne at ca. 2.5 bar) sealed microwave vessel in a standard single-mode microwave reactor. Double-Sonogashira coupling of the dibromodiiodo-benzene was completed within 20 min at 110 °C [30]. 

An interesting series of ring-closing alkyne metathesis reactions (RCAM) has recently been reported by Fiirstner and coworkers (Scheme 6.72) [152], Treatment of biaryl-derived diynes with 10 mol% of a catalyst prepared in situ from molybdenum hexacarbonyl and 4-(trifluoromethyl)phenol at 150 °C for 5 min led to a ca. 70% iso-... 

Scheme 6.72 Ring-closing alkyne metathesis reactions.

Alkyne metathesis, a mechanistic cousin of alkene metathesis, has thus far found only limited exploration. In 2004, Zhang and Moore reported that precipitation-driven alkyne metathesis reactions could efficiently produce arylene ethynylene macrocycles [50]. This was explored further in a 2005 report, verifying that the products obtained were indeed the result of thermodynamic self-selection [51]. 

Scheme 8.5 Alkyne metathesis reactions of polymer 35 prepared from the (a) Sonogashira condition and (b) hexacycle 36 and diphenylacetylene [35].

This mechanism was later confirmed experimentally in 1981 by Schrock and others, who reported the first example of alkyne metathesis by tungsten(vi)-alkylidyne complex. They have prepared tungsten alkylidyne complex 120 (Equation (21)) and found that it reacts with diphenylacetylende to give tungsten alkylidyne complex 121 and another alkyne 122 (lequiv.) (Equation (22)). Furthermore, complex 121 works as a catalyst for the alkyne metathesis reaction. 

The organic reactivity of the alkylidyne complexes has been extensively studied especially with regard to alkene and alkyne metathesis reactions.353,356... 

SCHEME 1. Types of alkene and alkyne metathesis reactions. DBC, double bond cleavage TBC, triple bond cleavage ADMET, acyclic diene metathesis RCM ring-closing metathesis ROMP ring-opening metathesis polymerization... 

In yet another successful application of the traditional Noyori three-component reaction, Ftirstner and co-workers took advantage of their remarkable alkyne metathesis reaction [21] to synthesize therapeutically promising cyclic analogues of prostaglandins [22] such as PGE2-l-15-lactone 34 (Figure 12.1) [23]. 

Schrock and co-workers, in their pioneering studies of alkyne metathesis reactions, have actually isolated and structurally characterized one such compound (I) (14,15) and shown that addition of a nitrogen base can convert the metallacyclobutadiene to II, which is formally a metallatetrahedrane or a... 

Grubbs, R. H., Alkene and Alkyne Metathesis Reactions , in Comprehensive Organometallic Chemistry, Pergamon Press, Oxford, 1982, Vol. 8, pp. 499-551. 

The original synthesis of this compound was described in Ref. [36b]. The ringclosing alkyne metathesis reaction was carried out under argon in flame-dried... 

Storm, C., and Madsen, R. 2003. Enyne metathesis catalyzed by ruthenium carbene complexes (review of alkyne metathesis reactions). Synthesis 1—19. 

Addition of pyridine and acetylene results in generation and trapping of the complex shown in equation (87), which has a reduced bond order. In this complex, the acetylene bridges the two metals. This type of chemistry was initially developed by Schrock and coworkers for alkyne metathesis (see Alkyne Metathesis) reactions but has been subsequently developed by a number of other researchers. 

Perhaps the most remarkable illustration of the ability of metals to activate alkynes comes from reactions in which complete scission of the carbon-carbon triple bond occurs. On the stoichiometric level these include examples in which carbyne complexes are produced from alkyne completes as in the melt-thermolysis of CpCo(PPh3)(RCsCR) [112] or from reactions of alkynes with unsaturated metal species (Scheme 4-34) [113]. The remarkable alkyne metathesis reaction (Scheme 4-35), which involves overall cleavage and regeneration of two o-and four rt-bonds, is conceptually related. A variety of functionalized alkynes can be tolerated as metathesis substrates [114] and especially effective catalysts for these reactions are Mo(VI)-and W(VI)-carbyne complexes. Metallacyclobutadienes 64, formed by the reaction of the alkyne with a metal-carbyne complex, appear to be central intermediates in these reactions and the equilibrium between metallacycle and alkyne/metal-carbyne is observable in some cases [115]. 

A number of authors have reported successful alkyne metathesis reactions catalysed by either tungsten " molybdenum" complexes. 

Fig. 4.23. A variation of the alkyne metathesis reaction ring-closing alkyne metathesis (RCAM).

Fig. 4.24. Another variation of the alkyne metathesis reaction alkyne ring-opening metathesis polymerization (alkyne ROMP).

Olefin metathesis reactions cleave carbon-carbon double bonds and reassemble tiiem to generate products containing new carbon-carbon double bonds. This process requires a catalyst and is largely controlled by thermodynamics (Equation 21.1). Alkyne metathesis reactions cleave carbon-carbon triple bonds and reassemble them to form products containing new carbon-carbon triple bonds (Equation 21.2). The observation of complete cleavage of strong carbon-carbon multiple bonds by a catalytic process was remarkable when first discovered, but many transition metal complexes are now known that catalyze these reactions with fast rates. One might expect that the equilibrium control of this reaction would limit its use, but olefin metathesis has become one of the most useful reactions catalyzed by transition metal complexes. 

Metallabenzenes have been invoked as possible intermediates in several other reaction types. Schrock, " for example, proposed tungstenabenzenes as possible intermediates in certain alkyne metathesis reactions that proceed by associative mechanisms. Shown in Scheme 32 is a proposed sequence for the metathesis of 3-heptyne to 3-hejQTie and 4-octyne using a tungstenacyclobutadiene complex as catalyst. The postulated metallabenzenes are formed by alkyne insertion into the metal carbon bonds of the metallacyclobutadienes. Of course, it is also possible to envisage a catalytic cycle based on Dewar metallabenzene intermediates. 

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