Metallacyclobutane mechanism

The stoichiometry of this conversion is in accordance with a carbene starting structure. An alternating alkyUdene/metallacyclobutane mechanism [102, 131-133], which has precedent in the ethylene polymerization catalyzed by a Ta(III) neopentilydene complex [134], has been proposed where the chro-miiun alkyUdenes undergo [2-1-2] cycloaddition to give chromacyclobutane intermediates (mechanism III in Scheme 7). 

The metallacyclobutane mechanism of olefin metathesis has been discussed in Sections 1.3 and 3.1.7. For metathesis of acetylenes carbyne complexes are generally required (Figure 3.44), and both heterogeneous and homogeneous catalytic systems have been developed for this purpose. 

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. 

In this chapter we present the historical development of the metal carbene/ metallacyclobutane mechanism of olefin metathesis. 

The Metal Carbene/Metallacyclobutane Mechanism 3.2 Evidence from cross-metathesis reactions... 

Schrock has demonstrated that the Ta-based ethylene polymerization system shown in Figure 19 is likely to proceed via the Green-Rooney alkylidene mechanism. They based this on their observation of the metallacyclobutane intermediate at low temperatures in the NMR. However, the alkylidene-metallacyclobutane mechanism does not appear to operate for most metal-based polymerization catalysts. 

In accordance with this alkylidene—metallacyclobutane mechanism, metathesis of alkynes has been performed using tungsten metallacarbyne complexes [36]. The analogy between the reactions of olefins and acetylenic hydrocarbons has led to the assumption that metallacyclobuta-dienes could be intermediates for this reaction. 

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