Cyclobutadienes from alkenes

A rare example of a ferracycloheptane 108 was obtained as the product of the photochemical reaction of a Petitt s cyclobutadiene iron complex with dimethyl-maleate [Eq. (43)].118 The ferracycloheptane arises from the insertion of a maleate into each of two Fe-C bonds and might therefore be considered a special case of alkene trimerisation (vide infra). The cyclobutene fragment in the final metallacycle remains coordinated to iron, as established crystallographically (Fig. 34). 

There is much evidence that cyclic conjugated systems of An electrons show no special stability. Cyclobutadiene dimerises at extraordinarily low temperatures (>35K).28 Cyclooctatetraene is not planar, and behaves like an alkene and not at all like benzene.29 When it is forced to be planar, as in pentalene, it becomes unstable to dimerisation even at 0 °C.30 [12]Annulene and [16]annulene are unstable with respect to electrocyclic reactions, which take place below 0 °C.31 In fact, all these systems appear on the whole to be significantly higher in energy and more reactive than might be expected, and there has been much speculation that they are not only lacking in extra stabilisation, but are actually destabilised. They have been called antiaromatic 32 as distinct from nonaromatic. The problem with this concept is what to make the comparisons with. We can see from the arguments above that we can account for the destabilisation... 

Early evidence for the singlet nature of cyclobutadiene derived from trapping experiments using dimethyl maleate or dimethyl fumarate, both of which added stereospecifically to the diene [42,43]. Furthermore, if 1,1-diphenylethylene was present in addition to either of the esters, cyclobutadiene reacted exclusively with the esters and not with the alkene, suggesting that it is reacting as a diene and not as a biradical [43]. 

Intermediates with one coordinated alkene are often seen (e.g., Eq. 6.49), but the bis-alkcne species is probably the immediate precursor of the coupled product." The products from alkynes are often stable and are known as metalloles (Eq. 6.51) but they can also reductively eliminate to give cyclobutadiene complexes (Eq. 6.47). 

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