Rearrangement of s-cis-Butadiene to Bicyclobutane

Let us return to Fig. 5.4 and compare two closely related synunetry coordinates illustrated in it (b) is the familiar conrotation that has been confirmed to be essential for allowing the cyclization of s-czs-butadiene to cyclobutene, whereas (c) leads directly to bicyclobutane. The irrep of both coordinates in is 02] i.e. both are conrotatory. They differ by virtue of the concerted out-of-plane motion of Ci and C4, which brings them within bonding distance of C3 and C2 respectively, rather than of each other. 

Woodward and Hoffmann consider this reaction to be a [2 + 2]-cycloaddition [1, p. 76], which - if it occurs in one step - the Rules require to be [J2g + 72a], and cite confirmatory stereochemical evidence [21]. They note, however, that the experimentally observed activation energy (41 kcal/mol) is rather high, and 

The question arises whether it is indeed justified to treat the x bond between Cl and C2 as if it were completely isolated from that between C3 and C4. [1, p. 34] Might not two correlation diagrams between butadiene and bicyclobutane, or an OCAMS correspondence diagram similar to Fig. 5.5, afford additional insight into the reaction The latter, displayed in Fig. 5.8, is constructed in the common global symmetry of the two isomers, C2V  

In bicyclobutane, the CH bonds to Ci and C4 all lie in the xz plane and are not interconvertible. Accordingly, two of the four CH-bonding combinations are totally symmetric (ai) and the remaining two are labelled 61. In contrast, the CC bonds that form the four-membered ring are inter con verted by the sym-ops of C2VJ and their combinations span its four irreps. For completeness, we add the fifth CC-bonding orbital. In zeroth order, it is localized in the central bond between C2 and C3, but it interacts - favorably and unfavorably - with the totally symmetric acc combination to produce two ai orbitals, Ci and (T5. Since the analysis of the reaction is being formulated as an 18-electron problem, butadiene too has to be represented by nine doubly-occupied orbitals To the six in Fig. 5.5 we add three (Tcc orbitals to house the three electron pairs in the 

As in Fig. 5.5, the two totally symmetric CH-bonding orbitals correlate across the diagram, but the other two, both of them bi on one side and 62 on the other, can only be induced to correspond by displacement along an U2 coordinate, viz, a conrotation. The five CC-bonding orbitals correlate across the diagram without requiring any non-totally symmetric displacement at all. Evidently, the reaction has to be characterized formally as allowed in the subgroup C2, the kernel of fl2- This conclusion not only differs from that of the Rules but stands in apparent contradiction to experiment Bicyclobutene is thermodynamically much less stable than butadiene if its conversion to the latter can take place with conservation of orbital symmetry, why is it so difficult  

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