Metallacycles metallacyclobutadiene

The currently known carbometallation chemistry of the group 6 metals is dominated by the reactions of metal-carbene and metal-carbyne complexes with alkenes and alkynes leading to the formation of four-membered metallacycles, shown in Scheme 1. Many different fates of such species have been reported, and the readers are referred to reviews discussing these reactions.253 An especially noteworthy reaction of this class is the Dotz reaction,254 which is stoichiometric in Cr in essentially all cases. Beyond the formation of the four-membered metallacycles via carbometallation, metathesis and other processes that may not involve carbometallation appear to dominate. It is, however, of interest to note that metallacyclobutadienes containing group 6 metals can undergo the second carbometallation with alkynes to produce metallabenzenes, as shown in Scheme 53.255 As the observed conversion of metallacyclobutadienes to metallabenzenes can also proceed via a Diels-Alder-like... 

It is perhaps premature to attempt to delineate the factors that determine whether symmetrical or distorted metallacyclobutadiene or t 3-cycopropenyl coordination is observed, given the comparitive sparsity of directly comparable examples, and the observation that tautomerism appears to operate between coordination modes in some cases. A similar situation arises for cyclobutadiene coordination vs. metallacyclopentadi-ene formation (Figures 6.38, 6.39, 7.19). In both cases the metallacycles are important intermediates in catalytic manifolds (alkyne metathesis and oligomerization, respectively) and in both cases the polyhapto variant represents a tangent to the productive catalytic cycle, formation of which may be reversible or in some cases may lead to termination. 

Metal carbenes are also initiators for the metathetical polymerization of alkynes by, e. g., catalysts such as M0CI5 and WClg. Reaction of the metal carbene with an alkyne gives a metallacyclobutene intermediate, followed by opening of this intermediate metallacycle into a new metal carbene that in its turn can interact with another alkyne molecule, and so on eq. (13). Metathesis of acetylenes proceeds through reactions between an alkylidyne complex and an alkyne via a metallacyclobutadiene intermediate (eq. (14)). 

Perhaps the most remarkable illustration of the ability of metals to activate alkynes comes from reactions in which complete scission of the carbon-carbon triple bond occurs. On the stoichiometric level these include examples in which carbyne complexes are produced from alkyne completes as in the melt-thermolysis of CpCo(PPh3)(RCsCR) [112] or from reactions of alkynes with unsaturated metal species (Scheme 4-34) [113]. The remarkable alkyne metathesis reaction (Scheme 4-35), which involves overall cleavage and regeneration of two o-and four rt-bonds, is conceptually related. A variety of functionalized alkynes can be tolerated as metathesis substrates [114] and especially effective catalysts for these reactions are Mo(VI)-and W(VI)-carbyne complexes. Metallacyclobutadienes 64, formed by the reaction of the alkyne with a metal-carbyne complex, appear to be central intermediates in these reactions and the equilibrium between metallacycle and alkyne/metal-carbyne is observable in some cases [115]. 

The preparation and study of metallacycles has been a subject of active investigation for organometallic chemists. We have just seen one example where metallacycle formation is a key step in a catalytic process and there are several others most notably, olefin metathesis. The metal acts as a geometrical and electronic template in these reactions. For unsaturated metallacycles there are interesting questions concerning delocalization [29]. Certain metal carbynes can react with acetylene to give metallacyclobutadienes as intermediates [30]. One such example of an insoluble molecule is the tungstenacyclobutadiene complex, 18.36 [31]. The compound is quite stable and not very reactive (in contrast to cyclobutadienes... 

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