Metallacyclobutanes Subject

Metathesis, which is reversible and can be catalyzed by a variety of organometallic complexes, has been the subject of considerable investigation, and many reviews on this topic have been published.In 1970, Herisson and Chauvin proposed that these reactions are catalyzed by carbene (alkylidene) complexes that react with alkenes via the formation of metallacyclobutane intermediates, as shown in Figure 14-20. This mechanism, now known as the Chauvin mechanism, has received considerable support and is believed to be the pathway of the majority of transition metal-catalyzed olefin metathesis reactions. 

As already mentioned for rhodium carbene complexes, proof of the existence of electrophilic metal carbenoids relies on indirect evidence, and insight into the nature of intermediates is obtained mostly through reactivity-selectivity relationships and/or comparison with stable Fischer-type metal carbene complexes. A particularly puzzling point is the relevance of metallacyclobutanes as intermediates in cyclopropane formation. The subject is still a matter of debate in the literature. Even if some metallacyclobutanes have been shown to yield cyclopropanes by reductive elimination [15], the intermediacy of metallacyclobutanes in carbene transfer reactions is in most cases borne out neither by direct observation nor by clear-cut mechanistic studies and such a reaction pathway is probably not a general one. Formation of a metallacyclobu-tane requires coordination both of the olefin and of the carbene to the metal center. In many cases, all available evidence points to direct reaction of the metal carbenes with alkenes without prior olefin coordination. Further, it has been proposed that, at least in the context of rhodium carbenoid insertions into C-H bonds, partial release of free carbenes from metal carbene complexes occurs [16]. Of course this does not exclude the possibility that metallacyclobutanes play a pivotal role in some catalyst systems, especially in copper-and palladium-catalyzed reactions. 

The stereochemistry of the olefin metathesis has been the subject of numerous publications which tried to rationalize the behaviour of various catalysts with acyclic olefins [1-5]. It had been noticed during these studies that a cis olefin gave preferentially a cis olefin and that a trans olefin led essentially to a trans olefin. This was explained in terms of stability of the metallacyclobutanes involved in the catalytic cycle (Scheme 1) the most favoured metallacyclobutanes are those where the substituents in positions 1 and 3 are equatorial. This simple rule allowed the explanation of most experimental results and the configuration retention. 

Pseudotetrahedral metallacycles 17 are formed in the reaction of [Mg(CH2)2SiMe2ln with MCI2CP2 (M = Ti, Zr, Nb, or Mo). All four molecules are subjects of -ray crystal analysis. Their thermal stability is greater than the corresponding metallacyclobutanes. The electronic configuration of the metal exerts an influence upon the conformational structure of the MC2Si ring in the series 17 M = Zr, Nb, and Mo). An increase in the number of metal valence... 

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