Electrocyclic cyclobutadiene

Figure 7.2 The frontier orbitals of s-cis- 1,3-butadiene are the four n orbitals (jt2 is the specific example shown). If these orbitals are followed in a diabatic sense along the electrocyclization reaction coordinate, they correlate with the indicated orbitals of cyclobutadiene...

The practical exploitation of the proposed criterion can be very simply demonstrated by the example of the electrocyclic transformation of cyclobutadiene to cyclobutene, for which the structure of the intermediate can be quite reliably estimated from the available results of quantum chemical calculations [74] (Scheme 2). 

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... 

To explain the increase in the rate of an electrocyclic ring opening like 6.443 > 6.444, we need to remember that the conrotatory pathway will have a Mobius-like aromatic transition structure, not the antiaromatic Hiickel cyclobutadiene that we saw in Fig. 1.46. We have not seen the energies for this system expressed in 8 terms, nor can we do it easily here, but the numbers are in Fig. 6.55, where we can see that a donor, a withdrawing group, and a C-substituent on C-3 can each accelerate the reaction—the numbers on the right, —4.29 and —4.06, are more negative than for the unsubstituted system, 3.66. 

Matrix photolysis of 2i/-pyran-2-one at 8 K leads to electrocyclic ring opening of the bond O/C-2 and formation of the aldoketene 8 which, by addition of CH3OH, can be intercepted as ester 10. Photolysis at higher temperatures or in ether leads to electrocyclization of the 1,3-diene system and formation of the p-lactone 9 which, by addition of CH3OH, yields the enol ether 12. Further photolysis of the P-lactone 9 causes decarboxylation resulting in cyclobutadiene 11 and its products, e.g. the dimer 14 (CHAPMAN) [12] ... 

The [ 2s + 2sJ electrocyclic photochemical cyclization is important from the preparative viewpoint. The interesting but labile compounds Dewar benzene (19) and cyclobutadiene have been prepared by such means. Equations (6.19) and (6.20). The anti-aromatic cyclobutadiene is too reactive to be isolated, but is conveniently trapped as the tricarbonyliron(0) complex from which it can be later regenerated under oxidative conditions, for example by using cerium (IV) salts. 

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