Metathesis with alkynes

Metathesis with alkynes is also quite useful in synthesis,particularly for internal alkynes although terminal alkynes are not good partners in this reaction.358 internal metathesis reactions with alkynes are known,including the conversion of 439 to 440 (in 73% yield) in FUrstner s synthesis of prostaglandin E2-1,15-lactone.360 Note the use of a molybdenum metathesis catalyst for this reaction. Diynes also react with alkynes in an intermolecular reaction to form aromatic rings. An example is the conversion of 441 to a 6 1 mixture of 442/443, in 82% yield.36la a similar, palladium-catalyzed cycloaromatization is also known.362 The metathesis disconnections are... 

A few publications have appeared concerning the metathesis of alkynes so far only heterogeneous systems with acyclic alkynes have been reported (31-33). From experiments with [l-14C]2-hexyne this reaction was found to be analogous to the metathesis of alkenes, because it turned out to be a transalkylidynation reaction (33) ... 

The currently known carbometallation chemistry of the group 6 metals is dominated by the reactions of metal-carbene and metal-carbyne complexes with alkenes and alkynes leading to the formation of four-membered metallacycles, shown in Scheme 1. Many different fates of such species have been reported, and the readers are referred to reviews discussing these reactions.253 An especially noteworthy reaction of this class is the Dotz reaction,254 which is stoichiometric in Cr in essentially all cases. Beyond the formation of the four-membered metallacycles via carbometallation, metathesis and other processes that may not involve carbometallation appear to dominate. It is, however, of interest to note that metallacyclobutadienes containing group 6 metals can undergo the second carbometallation with alkynes to produce metallabenzenes, as shown in Scheme 53.255 As the observed conversion of metallacyclobutadienes to metallabenzenes can also proceed via a Diels-Alder-like... 

In a noteworthy series of studies, Herndon has shown that cyclopropylcarbenes can be used as four-carbon components in molybdenum- and tungsten-mediated [4 + 2 + l]-reactions with alkynes and carbon monoxide (CO). These reactions give cycloheptadienones in moderate yields and with moderate selectivity (Equations (26)—(28)). The mechanism of this reaction is proposed to proceed through a series of steps involving metathesis, GO insertion, ketene formation, cyclopropane cleavage, and finally reductive elimination (Scheme 43).133... 

A unique bis-silylation system, in which a bis(silyl)palladium intermediate is generated via recombination of two Si-Si bonds, has been developed.8,97 A bis(disilanyl)dithiane reacts with alkynes in the presence of a palladium/ isocyanide catalyst, giving five-membered ring bis-silylation products in high yield with elimination of hexamethyl-disilane (Scheme 14). The recombination, that is, bond metathesis, is so efficient that no product derived from direct insertion of acetylene into the Si-Si bonds of the bis(silyl)dithiane is formed at all. 

The reactions catalyzed by cationic palladium complexes are believed to proceed via a different mechanism (Scheme 67).273 Initially, a cationic silylpalladium(n) species is generated by cr-bond metathesis of the Br-Pd+ with a silylstannane. Subsequently, the alkyne and alkene moieties of the 1,6-diyne successively insert into the Pd-Si bond to form a cationic alkylpalladium(n), which then undergoes bond metathesis with silylstannane to liberate the product and regenerate the active catalyst species, S/-Pd+. 

The potential synthetic utility of titanium-based olefin metathesis and related reactions is evident from the extensive documentation outlined above. Titanium carbene complexes react with organic molecules possessing a carbon—carbon or carbon—oxygen double bond to produce, as metathesis products, a variety of acyclic and cyclic unsaturated compounds. Furthermore, the four-membered titanacydes formed by the reactions of the carbene complexes with alkynes or nitriles serve as useful reagents for the preparation of functionalized compounds. Since various types of titanium carbene complexes and their equivalents are now readily available, these reactions constitute convenient tools available to synthetic chemists. 

G. Brizius, N.G. Pschirer, W. Steffen, K. Stitzer, H.-C. zur Loye, and U.H.F. Bunz, Alkyne metathesis with simple catalyst systems efficient synthesis of conjugated polymers containing vinyl groups in main side chain, J. Am. Chem. Soc., 122 12435-12440, 2000. 

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 1985, Dbtz et al. reported during a study on the reaction of Fischer-type carbene complexes with alkynes [10] that 2-oxacyclopentylidene chromium complex 24 was obtained as a side product. Thus, treatment ofmethyl(methoxy)carbene complex with 3-butynol at 70 °C in dibutyl ether gave the cyclic carbene complex 24 in 23% yield along with the desired metathesis product 23. The authors briefly commented that the cyclic carbene complex 24 might be obtained through the vinylidene complex 25, generated by the reaction of the alkyne with the liberated pentacarbonylchromium species (Scheme 5.7). 

Alkyne Metathesis with Tungsten Alkylidyne Complex 301... 

Alkyne Metathesis with Tungsten Aikyiidyne Compiex... 

Although much emphasis has been placed on the importance of alkene metathesis in organic synthesis, alkyne metathesis is also significant. Metathesis reactions of alkynes with alkencs and metathesis reactions of alkynes with alkynes have both been carried out efficiently. 

Regiospecific addition took place affording good yields of thietanes 143503 and, with alkynes, diene systems were formed504. Fischer has also described the reactions of thioketones with (CO)sW=NPh which undergo metathesis with these substrates to yield N-phenyl imines 144505 as shown in equation 154. 

Alkynes can also undergo total metathesis, with cleavage of all three C=C bonds, catalysed by metal carbyne complexes at room temperature and proceeding through met-allacyclobutadiene intermediates as indicated by the framework in equation 43-6. 

Acyclic carbenes, with palladium isocyanides, 8, 258 Acyclic diene metathesis polymerization alkynes, 1, 191 characteristics, 1, 138 in ionic liquids, 1, 869... 

Reaction of the carbene complex 148 with alkyne affords vinylcarbene 150 via metallacyclobutene 149. In the intramolecular reaction of enyne 152, catalysed by carbene complex 151, the triple bond is converted to vinylcarbene 153 which then reacts with the double bond to give the conjugated diene 154. Generation of 154 is expected by the formation and cleavage of cyclobutene 155 as a hypothetical intermediate. Based on this reaction, Ru-catalysed intramolecular metathesis of enyne 156 gave the N-containing cyclic diene 157, from which (—)-stemoamide (158) was synthesiszed. The reaction can be understood by assuming the formation of the hypothetical cyclobutene 159 from 156 [52],... 

Scheme 3. Simple examples of natural products synthesis by alkyne metathesis with the Mortreux catalyst.

Sequential Grubbs metathesis/Diels-Alder strategies can be used to produce tetrahydroquinolines. For example, intramolecular enyne metathesis followed by reaction with alkynes followed by oxidation serves as a useful route to multisubstituted systems (Scheme 78) <2000CC503>. 

In 1995, Mori revived alkyne metathesis with simple Mo(CO)6-based catalyst systems, and successfully prepared a series of diaryl- and arylalkylacetylenes (Scheme 5) [12]. The yields of the dimerization reactions were not particularly high. The simplicity of the method utilizing off-the-shelf catalyst precursors in combination with technical quality solvents made these systems potentially valuable for the synthesis of novel aromatic oligomers and polymers (Scheme 5). Mori offered an alternative mechanistic picture as to how alkyne metathesis might work in these simple catalyst systems, which invoked a transient molybdenacyclo-pentadiene 9a the classic interpretation of alkyne metathesis according to Schrock involves an intermediate 9b as shown in Scheme 5. 

By analogy with alkene metathesis, carbyne complexes might be expected to mediate the metathesis of alkynes, which indeed they do, but with some specific limitations. The basic mechanism parallels that for alkene metathesis, with the key intermediate being a metallacyclobutadiene which may break down in one of two possible directions (Figure 5.44). 

Formation of polycyclic ring systems via intermolecular reactions of Fischer carbenes with alkynes can be achieved. In a metathesis-type reaction, reactions of 1,6-enynes afford... 

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