Detection of propagating metal-carbene complexes

The reactions of metal carbene complexes with olefins can be readily followed by H NMR spectroscopy. An early example was the reaction of W(=C//CMe3)(=0)(Cl)2(PEt3)2/AlCl3 (5 12.03) with but-l-ene to give 

Another clear example of an equilibrium metathesis reaction is the ROMP of cyclopentene initiated by the tungsten analogue of 2. The reaction may be totally reversed by placing the system under vacuum thereby stripping off the monomer units from the living metal carbene complexes (Schrock 1989). 

Stable propagating metal carbene complexes may also be observed when certain initiators of the type listed in Table 2.2 are used without a Lewis acid cocatalyst. The propagating species are living and addition of successive batches of different monomers can be used to make block copolymers see Ch. 14. The conversion of the living polymer derived from the first monomer, into the propagating species of the second monomer can be readily followed by H NMR. 

When the reaction is sufficiently slow, separate signals for the first addition product and subsequent addition products are often seen, either for the carbene proton resonance (Kress 1987c Feast 1994d) or elsewhere in the spectrum (Ivin 1988). The concentration of first-addition product [Pj] passes through a maximum 

As mentioned earlier, initiator 2 has rotational isomers in which the alkylidene substituent is turned either towards the second multiple bond (5y ) or away from it anti)-, eqn. (11). In this case the equilibrium position is very much in favour of the syn rotamer K= 1450 in toluene at 25°C), making it difficult to detect the anti rotamer in routine spectra. However, the equilibrium can be displaced by UV irradiation (366 nm) of the solution for several hours at — 80°C to yield a mixture containing about 33% of the anti rotamer as determined from the resonances syn, 6 12.11,7ch = 120.3 Hz anti, 5 13.30, Jch= 153.3 Hz (Oskam 1992, 1993a). On adding 0.33 equivalents of 2,3-bis(trifluoromethyl)norbomadiene to this solution and running the spectrum again at — 80°C it is found that the anti rotamer has been completely consumed, giving the syn first-addition product, eqn. (12), while the syn rotamer has scarcely reacted at all. It is estimated that the 

---