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... 

A particularly striking structural feature of the vanadium complex is the exceptional long distance (R) of 171 pm between the ring carbons closest to the metal compared to that in the Pt and Pd complexes (158 pm)315. This suggests that the vanadium complex approaches a metallacyclobutadiene structure, found in the analogous trigonal-bipyrami-dal rhenium complexes315 (Section V. A.2). 

A potential side reaction is the formation of a metallatetrahedrane complex by tau-tomerization of the metallacyclobutadiene intermediate or by its direct formation from the reactants756. 

In reaction 71 the products are (i) EtC=CCMe3, resulting from the metathesis reaction, and (ii) a metallacyclobutadiene complex, produced by addition of a second molecule of... 

The crystal structure of a Fischer-type metallacyclobutadiene complex of rhenium 6 has also been reported <1993JA9986, 1990JOMC1>. The bond lengths between Re and the -carbons (2.18 and 2.13 A) are very similar, but the two C-C bond lengths are considerably different (C(l)-C(2) 1.36 A, C(2)-C(3) 1.45 A). In addition, the C(3)-0(4) bond distance (1.30 A) is much longer than C(5)-0(6) (1.19 A) but shorter than C(5)-0(7) (1.34 A). Therefore, the structure of rhenacyclobutadiene 6 is expected to be best represented by significant contributions from the resonance structures 6a and 6b and a smaller contribution from structure 6c. 

The reversible [2+2] cycloaddition of metal alkylidyne or Fischer-type metal carbyne complexes remains the only general methodology for the synthesis of metallacyclobutadiene complexes. Recent literature revolves principally around the heavier group 6 metals and the investigation of intermediates in catalytic alkyne metathesis (Scheme 25 Equation 45) <1996CHEC-II(lb)887> (W <2005OM4684>, Mo <2003JOM56>). 

Schrock and co-workers, in their pioneering studies of alkyne metathesis reactions, have actually isolated and structurally characterized one such compound (I) (14,15) and shown that addition of a nitrogen base can convert the metallacyclobutadiene to II, which is formally a metallatetrahedrane or a... 

The essential features seen in both structural determinations are as follows (1) There is an essentially planar X4W2 unit (X = C or O) and perpendicular to this is the M2(/t-C) plane associated with the /r-CSiMe3 ligand. (2) One tungsten atom lies in a plane of the /t-C3 ring, that is, forms part of a metallacyclobutadiene, while the other tungsten atom is rc-bonded to this ring. 

By analogy with alkene metathesis, carbyne complexes might be expected to mediate the metathesis of alkynes, which indeed they do, but with some specific limitations. The basic mechanism parallels that for alkene metathesis, with the key intermediate being a metallacyclobutadiene which may break down in one of two possible directions (Figure 5.44). 

Metallacyclobutadienes and dehydrometallacyclobutadienes relevance to alkyne metathesis (see also Figure 7.18)... 

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. 

The M2(OR)6 compounds are also templates for cluster synthesis and the reversible coupling of two bonds to give a metallacyclobutadiene, where M = W,... 

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