Molybdenum alkylidene catalyst

Figure 10-10 Polymerization of 1,6-diynes using a molybdenum alkylidene catalyst [Rp is (CFjljCHjC] [67]. The 1,6-diyne monomer is drawn in two different exaggerated conformations to illustrate that head-tail polymerization leads to six-membered rings, and tail-tail polymerization leads to five-membered rings. See Fig. 10-8 for a more mechanistic diagram of acetylene metathesis.

The most recent application of olefin metathesis to the synthesis of polyenes has been described by Tao and Wagener [105,117], They use a molybdenum alkylidene catalyst to carry out acyclic diene metathesis (ADMET) (Fig. 10-20) on either 2,4-hexadiene or 2,4,6-octatriene. The Wagener group had earlier demonstrated that, for a number of nonconjugated dienes [118-120], these polymerizations can be driven to high polymer by removal of the volatile product (e. g., 2-butene). To date, insolubility limits the extent of polymerization of unsaturated monomers to polyenes containing 10 to 20 double bonds. However, this route has some potential for the synthesis of new substituted polyacetylenes. Since most of the monomer unit is preformed before polymerization, it is possible that substitution patterns which cannot be incorporated into an alkyne or a cyclic olefin can be built into an ADMET monomer. 

Figure 10-20 The early stages of acyclic diene metathesis (ADMET). The reaction is driven by pumping off volatile products. When X is a double bond and R is methyl (i. e., 2,4,6-octatetraene), oligomeric polyenes are formed using a molybdenum alkylidene catalyst [105, 117],...

Molybdenum alkylidene catalysts, three of the four polymers have indeed been successfully synthesized. 

Schrock and co-workers (yclopolymerized diethyl dipropar l-malonate (X = C(C02Et)2) with a well-defined molybdenum alkylidene catalyst [145]. Through NMR spectroscopy, equal... 

As catalysts, ruthenium- or molybdenum-alkylidene complexes are often employed, e.g. commercially available compounds of type 7. Various catalysts have been developed for special applications. " ... 

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

Initial reports of cross-metathesis reactions using well-defined catalysts were limited to simple isolated examples the metathesis of ethyl or methyl oleate with dec-5-ene catalysed by tungsten alkylidenes [13,14] and the cross-metathesis of unsaturated ethers catalysed by a chromium carbene complex [15]. With the discovery of the well-defined molybdenum and ruthenium alkylidene catalysts 3 and 4,by Schrock [16] and Grubbs [17],respectively, the development of alkene metathesis as a tool for organic synthesis began in earnest. 

In 1995 Crowe and co-workers underlined the potential of the molybdenum alkylidene 3 as a catalyst for cross-metathesis when they reported the first examples of productive acrylonitrile metathesis [27] (for example Eq. 10). 

Although the Grubbs ruthenium benzylidene 17 has a significant advantage over the Schrock catalyst 3 in terms of its ease of use, the molybdenum alkylidene is still far superior for the cross-metathesis of certain substrates. Acrylonitrile is one example [28] and allyl stannanes were recently reported to be another. In the presence of the ruthenium catalyst, allyl stannanes were found to be unreactive. They were successfully cross-metathesised with a variety of alkenes, however, using the molybdenum catalyst [39] (for example Eq. 20). 

A subsequent publication by Blechert and co-workers demonstrated that the molybdenum alkylidene 3 and the ruthenium benzylidene 17 were also active catalysts for ring-opening cross-metathesis reactions [50]. Norbornene and 7-oxanorbornene derivatives underwent selective ring-opening cross-metathesis with a variety of terminal acyclic alkenes including acrylonitrile, an allylsilane, an allyl stannane and allyl cyanide (for example Eq. 34). 

Since the late seventies efforts were directed toward the development of well-defined catalysts that would be active without addition of additives or further modification. A wide variety of tungsten and molybdenum alkylidene complexes have been prepared. Many of them show some activity, but only few are good catalysts. The synthesis is often not straightforward and a range of synthetic procedures varying solvents, alkylating reagents, anions, and alkylidene moieties have to be tried before a desired compound will be obtained. 

In order to understand the polymer structures that are obtained in the polymerization of 1,6-heptadiynes, one needs to consider all possible polymerization mechanisms. If 1,6-hep tadiynes are subject to cyclopolymerization using well-defined Schrock catalysts, polymerization can proceed via two mechanisms. One is based on monomer insertion, where the first alkyne group adds to the molybdenum alkylidene forming a disubstituted alkylidene, which then reacts with the second terminal alkyne group to form poly(ene)s consisting of five-membered rings. Analogous to 1-alkyne polymerization, one refers to this type of insertion as a-insertion (Scheme 4). 

Fig. 3. Three well-defined metathesis catalysts Schiock s molybdenum alkylidene (1) and Grubbs first generation (2) and second generation (3) benzylidene cataiysts.

Schrock, R. R. Olefin metathesis by molybdenum imido alkylidene catalysts. Tetrahedron 1999, 55, 8141-8153. 

In this context it is worth mentioning that (Tp(PPh3)( / -O2CCHPh2)Ru (=CHPh) is a (poor) catalyst for the ROMP of NBE without a co-catalyst, while molybdenum alkylidenes prepared from the Tp-ligand of the formula Mo(Tp)(CHCMe2Ph)(N-2,6-i-Pr2-C6H3)(OTf) require a co-catalyst (AlCl,) in order to be ROMP active [214]. 

The molybdenum catalyst 2 has been used extensively for ADMET polymerization. This complex is easier to handle than the tungsten analog and is more tolerant of functionality. This complex has allowed the synthesis of polymers containing esters, carbonates, ethers, sulfides, aromatic amines, boronates, dichlorosilanes, siloxanes, acetals, and conjugated carbon-carbon double bonds [38-45]. Aldehydes, ketones, and protic functionahty are not tolerated. The molybdenum alkylidene will react with aldehydes and ketones, but not esters, in a Wittig fashion [64]. 

Molybdenum imido alkylidene catalysts supported by sihca gel remain stable and highly active for alkene metathesis. ... 

---