Metallacyclobutane complexes propagating

Intermediate molybdacyclobutane complexes have also been detected in the reactions of 7 with 21-24115. Only in the case of 21 is the ultimate product a long-chain polymer, but in all cases one may observe, at 0-60 °C, a clean first-order rearrangement of the initial metallacyclobutane complex to the first metal carbene adduct, consisting of an equilibrium mixture of syn and anti rotamers in the ratio 9 1 (see below). Except in the case of 21, the metal carbene complexes do not survive for very long. For 21, however, ROMP is propagated, and distinct H NMR signals are seen for the longer-chain metal carbene complexes in both syn and anti forms. 

The most important advance over the past 15 years has been the preparation of numerous well-defined metal carbene complexes which can act directly as initiators of all types of olefin metathesis reaction. These second-generation catalysts allow much closer control and better understanding of the mechanism of the olefin metathesis reaction. The initiating and propagating species can be closely monitored and in some cases the intermediate metallacyclobutane complexes can also be observed. Well-defined metallacyclobutane complexes also can sometimes be used as initiators. 

The metal carbene/metallacyclobutane mechanism of olefin metathesis, as outlined in Section 1.3, was first proposed by Herisson and Chauvin in 1971. By 1975 the evidence in its favour had become so compelling that the earlier pairwise mechanism had been totally discarded. From 1980 onwards well-defined carbene complexes of Ta, Mo, W, Re, and Ru were discovered which would act as initiators without the need for activation by heat, light, or cocatalyst. This in turn led to the spectroscopic detection of the propagating metal-carbene complexes in many systems, to the detection of the intermediate metallacyclobutane complexes in a few cases, and in one case to the detection of the metal-carbene-olefin complex that precedes the formation of the metallacyclobutane complex. In no individual case have all three intermediates been detected at most two have been observed, sometimes one, more often none. After 1980 metallacyclobutane complexes of Ti and Ta were found which would act as initiators at 60°C, but where the intermediate metal carbene complexes could not be detected. 

With experimental support for the metal-carbene-mediated mechanism of olehn metathesis, a number of groups initiated studies with isolated metal-carbene and metallacyclobutane complexes. Early work by Chauvin and Katz on the polymerization of strained olefins using Fischer-type carbenes demonstrated the success of such an approach [56], The introduction of high oxidation state alkylidene complexes led to well-defined catalyst in which the propagating species could be observed and studied, such as the tungsten-based systems developed by Osborn, Schrock, and Basset [59,60], The best-studied and useful of these have been the Schrock arylimido alkylidene complexes, and we will return to these later in this chapter. 

In 1964 Fischer prepared the first stable metal carbene complexes [11], such as W[=C(OMe)Me](CO)5. Six years later Herisson and Chauvin proposed that olefin metathesis reactions were initiated and propagated by complexes of this type in a chain reaction involving the intermediate formation of metallacyclobutane complexes, eq. (4) where [Mt] is a transition-metal atom surrounded by various ligands [12]. 

Besides the heterocyclic monomers, mostly discussed in this chapter, this volume is also devoted to the ring-opening metathesis polymerization (ROMP). This polymerization is initiated and propagated by metal carbene complexes and metallacyclobutane complexes as shown in Scheme 2. 

The tungsten analogue of 149a adds to 139 to form a transoid square-pyramidal metallacyclobutane eomplex, which can be isolated and characterized at low temperature. Its rearrangement to the propagating metal carbene complex can also... 

The corresponding metallacyclobutane can be isolated when the Tebbe complex reacts with one equivalent of norbornene (Fig. 4.19) [62]. This metallacycle continues to react with excess norbornene to generate poly (norbornene), and then carbene transfer to acetone can be used to remove the propagating metal species and end-cap the polymer chain. Subsequent studies estabhshed that these systems are living polymerizations. The experimental proof for living character includes the observations that (i) the propagating species can be observed for extended periods, (ii) the propagating species remains active for extended periods, (iii)... 

Detection of a rare alkylidene-alkene complex is possible by NMR spectioscopy using 22. The metallacyclobutane remains undetected (the ROMP propagation rate is higher than that of initiation). These ideas are supported by the kinetics studies of norbomene derivatives using 22.290... 

---