Tungstacyclobutane

The unsubstituted metallacyclobutane formed from Ti(=CH2)(Cl)2 +CH2=CH2 is calculated to have a planar but easily puckered ring. Even a substituent in the 2-position (opposite to Ti which is numbered 4) is known to cause very little puckering124. However, in 1,3-disubstituted tungstacyclobutanes, extended Hiickel calculations show that the ring has a puckered ee configuration, as required by the interpretation of the cis/trans stereoselectivity in the metathesis reactions of alk-2-enes148 (see Section IV). 

Photochemical activation of tungstacyclobutanes lead to metathesis like products Ephritikine, M., and Green, M. L.H., J.C.S. Chem. Comm., 926 (1976). Thermal decomposition leads to homologation. 

There are a number of differences between the tungsten and molybdenum complexes. The main generality drawn from the data is that the tungsten complexes promote olefin metathesis quite a bit faster than the molybdenum complexes, but the tungsten complexes are less tolerant of functionality. The tungstacyclobutane is more stable than the molybdacyclobutane. Several tungstacyclobutanes have... 

Reacting an enol ether serves to isolate a new carbene derivative, thus confirming the tungstacyclobutane structure of the intermediate ... 

In another approach, cyclopentanone enol ether reacts with a pentacarbonyldiphen-ylcarbene complex to give an unsaturated carbene, X (the metathesis product) and a cyclopentanone derivative formed by rearangement of the cyclopropane intermediate, both coming from the nonisolated tungstacyclobutane intermediate ... 

Many stable tungstacyclobutane complexes are known, but few will initiate the metathesis of internal olefins or ROMP of cyclic olefins. Yet many will undergo exchange reactions with ethene or terminal olefins by a mechanism which must involve dissociation to a tungsten carbene complex. A great deal can therefore be learnt about the olefin metathesis mechanism from a study of such reactions. The following is a short summary. 

The tungstacyclobutane complex formed by the addition of W(=CHCMe3) (=NAr)(OCMe3)2 to 2,3-bis(trifluoromethyl)norbomadiene at 0°C has been characterized at low temperature and, as expected, has the transoid structure and square-pyramidal geometry. It rearranges to the tungsten carbene complex trans double bond), but is unstable and does not polymerize the monomer smoothly (Bazan 1990). 

Extremely high stereoselectivity in the metathesis of c/s-pent-2-ene catalyzed by the [W]=CHCMej complex (see Table 6.3) is maintained up to near-equilibrium conversion (Fig. 6.5). This can be explained either in terms of additional stabilization of the 1,3-ee structure of the tungstacyclobutane intermediate by the bulky ligands on tungsten, or by the rigidity generated by the cyclometallated ligand, or both (Couturier 1992). 

Compounds of type 7 proved to be remarkably active catalysts for the metathesis of internal olefins. [44,68,69] The activity of such species for the metathesis of ordinary internal olefins (e.g., c 5 -2-pentene) appeared to maximize for the OCMe(CF3)2 species. New alkylidene complexes such as W(NAr)(CHPh)[OCMe(CF3)2]2 could be isolated, and in some cases trigonal bipyramidal (TBP) tungstacyclobutane intermediates were stable enough to be observed and isolated. On the basis of this work it was proposed that the rate of reaction of alkylidene complexes with olefins correlated directly with the electron-withdrawing ability of the alkoxide, as found in acetylene metathesis systems described earlier. In many circumstances trigonal bipyramidal or square pyramidal tungstacyclobutane intermediates could be observed. [44] In any system in which ethylene could be formed, unsubstituted metallacycles could... 

Scheme 8.3 Tungstacyclobutane directly employed in the homocoupling of a-olefins.

Figure 8.3 (a) Tungstacyclobutane with a modified square-planar (SP ) geometry and (b) selected bond lengths and angles from the crystal structure of 13. 

FIGURE 20.19 Tungstacyclobutane derived from 2,3-bis(trifluoro)norbomadiene. 

It is to be noted, however, that the existence of monomer/catalyst combinations in which the syn rotamer can also contribute to the formation of trans C=C structures, and the anti rotamer can produce some cis-vinylene units, can currently not be excluded. For example, tungstacyclobutane compound 19 (Figure 20.19) was obtained from the reaction of monomer 2,3-(CF3)2NBD with... 

The reactivity of 3-methyl-1-hexene was explored to determine the steric effect of the allylic methyl group. It was found that the olefin undergoes no reaction with [W]2 but undergoes nearly complete dimerization with [Mo]2 [56]. NMR studies revealed that the tungsten catalyst s lack of activity was due to the exclusive formation of the a,a -disubstituted tungstacyclobutane [8bj. 

This method is specific for metallacyclobutane complexes. For stability reasons this method has been mostly applied to the preparation of high oxidation state tungstacyclobutane derivatives. Given the equilibrium shovm in Scheme 32, the use of excess aUcene may result in further exchange processes. The preparation of 82 in Scheme 35 is a two-step process involving the elimination of 3,3-dimethylbut-l-ene.l ... 

Benzobicyclo[4.2.0]octa-3,7-diene reacts in the presence of tungsten carbene and tungstacyclobutane catalysts [27] to give a living ring-opened polymer [Eq. (87)]. 

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