Carbyne complexes Catalysts

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. 

The related dihydride-dichloro complex OsH2Cl2(P Pr3)2 is also an active catalyst for the hydrogenation of olefins, diolefms, and a-(3-unsaturated ketones,14 but attempts to hydrogenate phenylacetylene show a rapid deactivation of the catalyst due to formation of a hydride-carbyne complex.54... 

Table 3.16. Alkylidene and carbyne complexes as single-component catalysts for homogeneous-phase alkene metathesis.

The reaction is thought to proceed with the dissociation of CT followed by release of the extra charge of the mthenium complex by dissociating a proton from the alkyhdene hgand. Such an exchange in itself does not lead to the decomposition of the alkyhdene complex. Nevertheless, both the formation of the charged species, both the intermediate existence of the carbyne complex (Scheme 9.5) may open new ways to the deterioration of the ROMP catalysts. 

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. 

Following the Fischer procedure, alkynyl carbyne complexes trans-X(CO)4M=C—CPh 189 have been obtained in 30-60% yields by reaction of (l-alkynyl)carbene complexes la,b (M = Cr, W) with BX3 (X = Cl, Br, I). To date, (l-alkynyl)carbyne compounds have found application as catalysts as well as stochiometric reagents in organic synthesis.205c-206 Among the transformations reported thus far is the formation of a 4-amino-l-metalla-l-yne-3-ene (= enamino carbyne complex) 190 by addition of HNMe2 to compound 189 (Scheme 79).207... 

In addition to those listed in Tables 1 and 2, many more active catalyst systems have been reported, including W-carbyne complexes [68], and metallacycles such as Ti or Ta cyclobutanes [8, 69], which are initiators for ROMP. The ROMP of functionalized monomers such as 7-oxanorbomene derivatives can also be performed successfully using certain Ru initiators, affording polymers with a high molecular weight (eq. (18)) [70]. Moreover, in water alone, polymerization proceeds very rapidly in nearly quantitative yields in the presence of Ru catalysts under an atmosphere of air [71] (cf. Section 3.3.10.1). 

To this day, the most effective alkyne metathesis catalysts are Group 6 alkylidynes however, there are drawbacks to use of these carbyne complexes. 

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]. 

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. ... 

Since then catalysts have been found which allow the reaction to proceed at 25 C (Bencheick 1982). Especially significant is the fact that the reaction can be initiated by metal carbyne complexes (McCullough 1984). Details are given in Ch. 10. 

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... 

However, the new phenoxide-ligated molybdenum carbyne complexes have been used to form unsaturated macrocycles. In particular, the molybdenum catalyst containing phe-noxide ligands catalyzes the reaction in Equation 21.32. ° This reaction occurs in high yield because the macrocycUc product precipitates from solution. 

ROMP AND ADMET POLYMERISATION WITH CARBYNE COMPLEXES AS CATALYSTS... 

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. 

Cl3(dme)W=C Bu (A) as catalyst in the ROMP of cyclopentene and in 1-octene metathesis [3]. We found that this carbyne complex is very active in both metathesis reactions. 

The three Schrock type carbyne complexes A, B and C are also active catalysts for ROMP reactions of heteroatom substituted norbornene derivatives, Table 5. 

CARBYNE COMPLEXES AS CATALYSTS IN ACYCLIC DIENE METATHESIS CONDENSATION, ADMET. 

We tested the homogeneous carbyne complex Cl3(dme)W=CtBu (A) and heterogeneous catalyst Si02/Np2W=CH Bu (D) as ADMET catalysts for dienes [8]. The heterogeneous catalyst produced with 1,9-decadiene or with l,3-di(5-hexenyl)-1,1,3,3-tetramethyldisiloxane) polyalkenameres with higher molecular weights [7], Table 6. 

The carbyne complex Cl3(dme)W=C Bu produce only low molecular weight oligomeric polyalkynes with 1-alkynes. But the catalysts Np3W=C Bu and ( BuO)3 V=C Bu give high molecular weight polyalkynes in high yields [2, 3], Table 1. In contrast to monosubstituted alkynes the catalyst Cl3(dme)W=C Bu was active in the polymerization of disubstituted alkynes [3], Table 2. 

Figure 5. Ring Closure Metathesis (RCM) of 2,10-Dodecadiyne Amount of (cyclo-) dimeres and (cyclo-) trimeres built by the Ring Closure Metathesis (RCM) of 2,10-dodecadiyne with the carbyne complex ( BuO)3WsC Bu as catalyst solvent 25 ml octane pressure 12 hPa molar ratio monomer/catalyst 50.

Not only the carbyne complex ( BuO)3W=C Bu but also W2( BuO)6, which is easier to prepare, is a very active catalyst for ADIMET of conjugated and non conjugated diynes. 

The Schrock type carbyne complexes Np3W=C Bu, ( BuO)3W=C Bu and Cl3(dme)WsC Bu are very active catalysts for the polymerization of alkylsubstituted 1-alkynes and also for 1-alkynes with heteroatom containing substituents. The polymerization of disubstituted alkynes was only achieved with Cl3(dme)W=C Bu as catalyst. 

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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