Tungsten-carbyne metathesis catalyst

Scheme 17. Formation of poly(arylenethynylene)s via a tungsten-carbyne metathesis catalyst.

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. 

Reaction of a reduced Philipps catalyst with Fischer-type molybdenum or tungsten carbene or carbyne complexes led to very active bimetallic, heterogeneous olefin metathesis catalysts. Surface metal ions might be involved in bonding interactions with the organometallic complex, possibly leading to heterometallic species on inorganic oxides. ... 

The general equation and mechanism for alkyne metathesis is depicted in Scheme 31. Alkyne metathesis is considerably less well developed in comparison to alkene metathesis. Garbyne complexes or carbyne complex precursors are among the most effective alkyne metathesis catalysts representative catalysts are depicted in Scheme 32. Tungsten carbyne complex 276 is one of the earliest alkyne metathesis catalysts, and has frequently been employed to initiate... 

The Schrock type tungsten (VI) carbyne complexes Cl3(dme)W=C Bu (A), ( BuO)3W=C Bu (B), Np3W=C Bu (C) and the heterogeneous catalyst Si02/Np2W=CH Bu (D) are very active olefin metathesis catalysts. They catalyze ROMP reactions of mono and polycyclic olefins and ADMET or ringclosure (RCM) reactions of a,co-dienes. They also tolerate many heteroatom substituted monomers. 

Figure 2. Synthesis of heterogeneous highly active metathesis catalysts by reaction of Schrock type carbyne tungsten (VI) complexes with silicagel...

Heterogeneous, bimetallic metathesis catalysts are formed by reactions of Fischer type carbyne tungsten or molybdenum complexes with the reduced Phillips catalyst, a suface chromium(II) compound on silica (14).(scheme 5). The bimetallic surface compounds can result from 2+1 cycloaddition reactions. Similar reactions are well known by the work of Stone (15). 

In the preparative section 3.2 devoted to metal-carbene complexes, it is shown how the a-elimination reaction from high oxidation state early-transition-metal-alkyl complexes is one of the general methods of synthesis of Schrock s Ta and Nb alkylidene complexes. The other direction, formation of an alkylidene from an alkylidyne complex, can also be a valuable route to metal alkylidenes. For instance, Schrock s arylamino-tungsten-carbynes can be isomerized to imido-tungsten-carbene by using a catalytic amount of NEts as a base. These compounds are precursors of olefin metathesis catalysts by substitution of the two Cl ligands by bulky alkoxides (dimethoxyethane then decoordinates for steric reasons), and this route was extended to Mo complexes ... 

Synthesis of Schrock s W tungsten-carbyne complex, the prototype of active alkyne metathesis catalysts... 

By contrast, the Fischer carbyne tnms-bromotetracarbonyl(phcnylmcthyli-dene)tungsten, Br(CO)4W = CPh [140-142], will polymerise monosubstituted acetylenes by a metathesis mechanism [143] but will not give transalkylidynation products. The Fischer carbyne and related carbynes are effective catalysts for the metathesis polymerisation of not only monosubstituted acetylenes, whose polymerisation can be brought about by a number of catalysts [108, 109, 118, 120-125, 128-134], but also of acetylenes polymerised previously by only a few or no catalysts, i.e. even disubstituted acetylenes [112-115, 126, 127, 144-148] and unsubstituted acetylene [19, 20, 42, 119]. 

In the early 1980s, Schrock prepared a series of tungsten- and molybdenum-based carbyne complexes, and demonstrated that they are viable catalysts for performing stoichiometric and catalytic alkyne metathesis [7]. With the defined carbyne complexes, he laid the foundation for the mechanistic understanding of alkyne metathesis, and was the first to demonstrate that vinyl-substituted carbyne complexes are stable [8] and that alkyne metathesis could be performed in the presence of C=C double bonds. 

TABLE 4. Ring opening metathesis polymerization of dicyclopentadiene (DCPD) with the carbyne tungsten (VI) complexes Cl3(dme)WsC Bu (A), ( BuO)3WsC Bu (B), Np3W=C Bu (C) as catalysts reaction time 24 h, pa = polymerization activity [mmol DCPD mmol catalyst h ] (calculated from the turnover of monomer after 1 h), GPC Polystyrene standard solvent THE, temperature 25°C. 

In 1976 Fischer had tested the ROMP reactions of cycloalkanes with carbyne tungsten(O) complexes as catalysts (6). With addition of Lewis acids as cocatalysts the Fischer type carbyne complexes were active in cycloalkane metathesis polymerisation. Fischer type carbyne complexes are also active catalysts for alkyne polymerisations, as found by Katz in 1984 (7). The catalytic reactions of Schrock type carbyne tungsten(VI) or molybdenum(VI) complexes were focussed on alkyne metatheses reactions (8). 

Metathesis of Alkenes with the Carbyne-Tungsten-(VI)-Coinplex Cl3(dme)WCCMe3 (A) and the Heterogeneous Catalysts Si02/Np2WCHtBu (B) and Si02/Cr/Cl(C0)4WCPh (C)... 

Schrock and Fischer type carbyne tungsten or molybdenum complexes are very interesting catalysts for alkene metathesis or alkyne polymerisation reactions. Within the first reaction steps they form carbene complexes and on these carbene complexes further metathesis or polymerisation occur. 

Alkyne metathesis, using either preformed carbyne complexes, or carbyne complexes generated in situ, has started to find use in recent years, particularly as more reactive catalysts have been developed. The introduction of stable carbyne complexes, such as tungsten complex 8.483, as more-active catalysts started to make the reaction practicable for organic synthesis. ... 

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