Metal carbene 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. 

When the propagating metal carbene complex does not have a predetermined vacant ligand position, but is instead trigonal-bipyramidal or tetrahedral, it may still behave like the octahedral model provided that the ligands other than the carbene offer an asymmetric environment which controls the direction of approach of the monomer. If this is not the case there will not be a favoured direction of approach unless the chain-end effect comes into play. 

These two pieces of evidence show that cyclohexene can add to metal carbene complexes to some extent, but that at low temperature backbiting to give oligomers is preferred to propagation, while at room temperature the product of addition can be trapped, at least for a time, by reaction with norbornene358. 

The understanding of the reaction mechanism is directly related to the role of the catalyst, i.e., the transition metal. It is universally accepted that olefin metathesis proceeds via the so-called metal carbene chain mechanism, first proposed by Herisson and Chauvin in 1971 [25]. The propagation reaction involves a transition metal carbene as the active species with a vacant coordination site at the transition metal. The olefin coordinates at this vacant site and subsequently a metalla-cyclobutane intermediate is formed. The metallacycle is unstable and cleaves in the opposite fashion to afford a new metal carbene complex and a new olefin. If this process is repeated often enough, eventually an equilibrium mixture of alkenes will be obtained. 

In view of the fact that the propagation center in metathesis closely resembles metal carbene complexes, it is not surprising that preformed carbene complexes show catalytic activity for metathesis. Table 2 gives examples of carbene complexes that are effective as metathesis catalyst. 

It is now well established that ring-opening polymerization of cycloalkanes and bicycloalkenes, initiated with olefin metathesis catalysts, is propagated by metal carbene complexes (1). 

Propagation is assumed to occur by a [2+23 reaction between the metal carbene complex and the monomer. The approach required for the formation of a cis double bond is illustrated in Scheme 1. The cis double bond is formed at following the rupture of the transition state at bonds and c. For the formation of a trans double bond the norbomene molecule must approach Mt C with at the back instead of at the front. 

Conversely with and 9, using (mes)W(CO) 3/EtAlCl2/EPO as catalyst, the cis content is markedly lower than for the other polymers. This may be attributed to the increased hindrance to cis double bond formation caused by the syn-methyl group in and the six-membered ring in 9. This catalyst is in fact the only one that will polymerize and 9 and in the latter case the additional presence of Mei Sn is necessary to suppress side reactions. The propagation of the polymerization of the syn compound will clearly be much more hindered than that of the anti compound if it is required to present its exo face to the metal carbene complex (Scheme 1). Hence it is not surprising that with most catalyst... 

NMR spectra of the ring-opened polymers of norbomene and its derivatives give information about their cis content, cis/trans distribution, HT or XN bias, and tacticity with respect to both cis and trans double bonds. The results indicate chat the propagating metal carbene complex sometimes contains a chiral reaction site and in other cases may be epimerized or achiral with respect to the reaction site. Relaxation processes other than epimerization must also sometimes be occurring, for example a change of geometry with respect to the metal centre. 

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. 

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. 

For the reactions of RC=CH with Me3CC=W(OCMc3)3, metathesis products can be detected in the early stages, but the metathesis reaction is rapidly overtaken by polymerization (Bray 1993 Mortreux 1995). This results from elimination of a hydrogen atom from the intermediate metallacyclobutadiene, leading to the formation of a metal carbene complex which then propagates the polymerization reaction Scheme 10.3 (also see McCullough 1983). 

It was first observed by Woon (1974) and Farona (1974) that acetylenes could be polymerized by catalysts of the type Mo(CO)3(toluene). This was followed by the discovery that conventional metathesis catalysts such as M0CI5 (Masuda 1974) and WCls (Navarro 1976 Masuda 1976), with or without a cocatalyst, could also bring about polymerization of acetylenes. At first there was some doubt as to whether these polymerizations were being propagated by the metathesis mechanism (Scheme 10.2) or whether a Ziegler-Natta mechanism was operating. However, the observation that metal carbene complexes could react with acetylenic molecules to form simple adducts as in reaction (20) (Fischer, H. 1980), and the fact that such complexes could initiate the polymerization of acetylenes, albeit somewhat slowly, but cleanly and in fair yield, soon allayed these doubts. 

Metal carbene complexes used as initiators (I) are not always fully consumed before the monomer (M) has been completely polymerized. This can happen when the propagation rate constant kp is somewhat greater than the initiation rate constant... 

If the propagating metal carbene complex does not have a predetermined vacant position, but is instead trigonal-bipyramidal or tetrahedral, it may still behave as in... 

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