Metathesis of diynes

The acyclic diyne metathesis polymerization of dodeca-2,10-diyne is catalyzed by W(=CEt)(OCMc3)3 eqn. (19). But-2-yne is eliminated and the product is an off-white insoluble powder with the same T as the polymer prepared from cyclooctyne (Section 10.2.3) (Krouse 1989). 

3 Metathesis reactions involving cleavage of two of the three C=C bonds 

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To identify the truly active species for the alkyne metathesis, various experiments are carried out for ring-closing alkyne metathesis of diynes (Table 5). Activation of complex 140 with CH2CI2 and evaporation of all the volatiles is shown to yield Mo[N(Ar)( Bu)]3Cl 141a and alkylidyne complex 141c as major components. The former complex 141a, that is also accessible by treatment of 140 with CI2 (Equation (24)), had an equal catalytic activity (entry 6), but... 

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... 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

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+. 

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]. 

Diyne-ene metathesis has also been reported. The reaction of diyne-ene 133a with Ic in the presence of terminal alkene gives triene 134a [Eq. (6.103)]. Intramolecular diyne-ene metathesis gives tricyclic compounds 134b... 

Using this catalyst system for ring-closing alkyne metathesis, Fiirstner successfully synthesized ambrettolide and yuzu lactone from alcohol 125. " " Treatment of diyne 126a with Mo(CO)6 (5 mol%) and /i-chlorophenol (1 equiv.) in chlorobenzene at 140 °C leads to cycloalkyne 127a in 69% yield. Subsequent Lindlar reduction proceeds smoothly in a stereoselective manner to afford ambrettolide. Similarly, 126b affords cycloalkyne 127b in 62% yield. From this compound, yuzu lactone has been synthesized (Scheme 45). " " ... 

Ring expansion of vinylcyclopro-penes and cyclobutenes 8-34 Ring expansion of vinylcycloal-kanes cyclization of diynes 8-39 Metathesis of dienes 8-40 Metal-ion-catalyzed o-bond rearrangements... 

Cycloalkynes may be formed by applying ring-closing metathesis to diynes.214 Partial hydrogenation of the product provides a stereoselective route to cis cyclo-alkenes that cannot be directly prepared in pure form by ring-closing metathesis ... 

The metathesis polymerization of diynes having four single bonds between the triple bonds (dipropargyl compounds) yields cyclopolymers. The structural units may contain a cyclohexene ring (equation 63) or a cyclopentene ring (equation 64) with the possibility... 

Intramolecular [2+2+2] cyclotrimerizations of diynes and triynes possessing heteroatom tethers furnish benzoheterocycles. The cyclization of triynes 88 using the Grubbs catalyst 76 proceeds via cascade metathesis as shown in Eq. (35) to yield a tricyclic product 89 [88]. This novel type of catalytic alkyne cyclotrimerization can be applied to the cycloaddition of 1,6-diynes with monoalkynes [89]. 

These titanium systems mediate the metathesis of G-G bonds and recombination processes with disubstituted butadyines.1461 Intramolecular cyclization of terminal disubstituted ct,a -diynes with successive unusual Gp cleavage and new intramolecular G-G coupling processes have been observed in the reaction with the terminal disubstituted diynes RC=C-(CH2) -C=CR (Scheme 567). The stability of the resulting products is determined by the spacer length (n = 2, 4, 5, 6). An increase in spacer length (n > 4) provides undefined secondary and decomposition products. The X-ray structures of the some of the resulting products have been reported.1469... 

The tris[Al-aryl-iV-(t-butyl)amino]molybdenum(0) complexes (3) are good catalyst precursors readily activated in situ for metathesis of alkynes and diynes. ... 

Alkyne metathesis is generally restricted to internal alkynes, and is driven to completion by the evolution of gaseous 2-butyne. Representative examples of alkyne cross-metathesis are depicted in Scheme 33, and include (i) dimerization of alkynylbenzoic acid derivative 287, (ii) alkyne cross-metathesis of 289 and bis(trimethylsilyl)acetylene, (iii) cross-metathesis of enyne 291 and various alkynes and metathesis dimerization of 291, and (iv) formation of high molecular weight alkyne-containing polymers through acyclic diyne metathesis (ADIMET) polymerization of acyclic diynes (e.g., 293) " using the molybdenum hexacarbonyl/2-chlorophe-nol system. 

Many ruthenium-carbene five-membered ring compounds have recently been reported to show good activity for metathesis [51-71]. Many types of reactions, such as ring-closing metathesis, ring-opening metathesis, ctoss metathesis, enyne metathesis, and diyne metathesis, proceed with the help of these catalysts, as shown in Eqs. (8.15), (8.16), (8.17), (8.18), and (8.19) [58]. 

ACYCLIC DIYNE METATHESIS CONDENSATION, ADIMET, AND RING CLOSING REACTIONS OF DIYNES... 

In 1995, we started to study acyclic diyne metathesis condensation of nonconjugated diynes with the catalyst ( BuO)3W=C Bu which catalyzes metathesis of disubstituted alkynes but not of 1-alkynes. Therefore we tested diynes with methyl or ethyl endgroups for Acyclic Diyne Metathesis (ADIMET). W2( BuO)6 was used as catalyst which is easier to synthesize. Figure 4. 

We found that methyl or ethyl substituted diynes give Acyclic Diyne Metathesis Condensation (ADIMET) which is similar to the Acyclic Diene Metathesis (ADMET) of dienes. The catalysts we used for ADIMET reactions are the Schrock type carbyne complex ( BuO)3W=C Bu or W2( BuO)6. For ADIMET reactions we used no or only small amonts of solvent. With higher amounts of solvents Ring Closure Metathesis (RCM) of diynes occured. 

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