Cyclobutadiene complexes Subject

The CpCo complexes, on the other hand, should be more stable due to the presence of the robust and bulky Cp-shield. Unfortunately, however (tetraiodo-cyclobutadiene)CpCo is not available, and there is no obvious synthetic path to make it. But maybe another way to produce CpCo-stabiHzed tetraethynylated cyclobutadiene complexe exists It is known, that 22 a undergoes a rearrangement to 22 d when subjected to the conditions of flash vacuum pyrolysis at elevated temperatures [24]. The driving force behind this rearrangement is twofold first, the steric strain between the two adjacent TMS groups is removed in 22d and second, the TMS groups in 22d are not bound to an -hybridized center but to an sp-hybridized one, which is a more favorable situation from a thermodynamic point of view. 

Acetylenes are well known to undergo facile trimerizations to derivatives of benzene in the presence of various transition metal catalysts 23). A number of mechanisms for this process have been considered including the intervention of metal-cyclobutadiene complexes 24). This chemistry, however, was subjected to close examination by Whitesides and Ehmann, who found no evidence for species with cyclobutadiene symmetry 25). Cyclotrimeri-zation of 2-butyne-l,l,l-d3 was studied using chromium(III), cobalt(II), cobalt(O), nickel(O), and titanium complexes. The absence of 1,2,3-trimethyl-4,5,6-tri(methyl-d3) benzene in the benzene products ruled out the intermediacy of cyclobutadiene-metal complexes in the formation of the benzene derivatives. The unusual stability of cyclobutadiene-metal complexes, however, makes them dubious candidates for intermediates in this chemistry. Once formed, it is doubtful that they would undergo sufficiently facile cycloaddition with acetylenes to constitute intermediates along a catalytic route to trimers. 

Gibson and co-workers have introduced a well-designed latent cyclobutadiene moiety. Compound 49 reacts with a phenylethyne-cobalt complex to give PKR product 50 as a single diastereomer that resulted from the reaction at the less sterically hindered site, and 50 was subjected to the retro-Diels-Alder reaction at 205 °G under a vacuum of 6 torr to give 51 (Equation (23)). ... 

Compounds with a narrow HOMO-LUMO gap (Figure 5.5d) are kinetically reactive and subject to dimerization (e.g., cyclopentadiene) or reaction with Lewis acids or bases. Polyenes are the dominant organic examples of this group. The difficulty in isolation of cyclobutadiene lies not with any intrinsic instability of the molecule but with the self-reactivity which arises from an extremely narrow HOMO-LUMO gap. A second class of compounds also falls in this category, coordinatively unsaturated transition metal complexes. In transition metals, the atomic n d orbital set may be partially occupied and/or nearly degenerate with the partially occupied n + 1 spn set. Such a configuration permits exceptional reactivity, even toward C—H and C—C bonds. These systems are treated separately in Chapter 13. 

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... 

Cyclobutadiene is a key compound in the study of antiaromaticity since it is the smallest neutral example and it is planar. Its chemistry has been the subject of several reviews. It was first observed in an argon matrix, being formed by the photolysis of a-pyrone. Subsequently, it was prepared from a variety of other precursors. It is highly reactive, and it readily dimerizes when the matrix softens and molecular diffusion becomes important. The dimerization process has been studied theoretically. Although cyclobutadiene cannot be isolated in the pure form, it can be stabilized by the formation of metal complexes. 

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