Bonding orbital 1, 3-butadiene-cyclobutene

The 7T bonding orbital of cyclobutene arises from reorganization of the butadiene tt system. This process is considered in Woodward, R. B. Hoffmann, R. The Conservation of Orbital Symmetry, Verlag Chemie Weinheim, 1970 p. 38 ff. and references cited therein. 

Analysis of the conrotatory process is carried out in exactly the same way. In this case the element of symmetry that is maintained throughout the reaction process is the twofold rotation axis. The resulting correlation diagram is shown in Figure 10.24. The conrotatory reaction is symmetry allowed, since the bonding orbitals of butadiene correlate with the bonding orbitals of cyclobutene and vice versa. Figure 10.25 is a pictorial representation of the orbital in the reactant, transition structure, and product. 

The resulting correlation diagram is shown in Fig. 10.5. This reaction is symmetry allowed, since the bonding orbitals of butadiene correlate with the bonding orbitals of cyclobutene and vice versa. 

For the butadiene-cyclobutene interconversion, the transition states for conrotatory and disrotatory interconversion are shown below. The array of orbitals represents the basis set orbitals, i.e., the total set of 2p orbitals involved in the reaction process, not the individual MOs. Each of the orbitals is tc in character, and the phase difference is represented by shading. The tilt at C-1 and C-4 as the butadiene system rotates toward the transition state is different for the disrotatory and conrotatory modes. The dashed line represents the a bond that is being broken (or formed). 

In the reverse transformation, the ring opening of cyclobutene to gi butadiene, the conversion involves net transformation of a c-bonding orbital to --bonding orbital. The 4 orbitals of cyclobutene are a, a and tc, tS (Figure 8.6). The 4 orbitals of the product are the familiar MO s of... 

The correlation diagrams for the conrotatory process show that there is a good correlation between the bonding orbitals v 1 and v 2 of butadiene and ct- and TT-orbitals of cyclobutene (Fig. 8.40). Thus, the ring opening of cyclobutene to butadiene or the reverse reaction is thermally allowed and occurs by the conrotatory process. The reaction proceeds with conservation of orbital symmetry. The photochemical conrotatory process in this case will be symmetry forbidden. 

If we examine the classification of orbitals of starting material and product with respect to each of the two symmetry elements in turn, we see that the bonding orbitals, and for starting material and product share symmetry only with respect to the C2 axis. (With respect to the a plane, both bonding orbitals of the cyclobutene system are symmetric, but in the butadiene system, only is symmetric with respect to the a plane.)... 

In conrotatory ring opening, the reoriented a orbitals derived from cyclobutene look like part of the butadiene molecular orbitals 1 2 and %. The orbitals derived from the double bond of cyclobutene look like part of the butadiene molecular orbitals and 3. Because the signs of the cyclobutene orbitals can be correlated with bonding orbitals of butadiene by... 

The forbidden nature of the disrotatory process rests in the crossing of the 7t( 3) and 7t (02) molecular orbitals. The n electrons in cyclobutene occupy an orbital correlated with an antibonding orbital ( 3) of butadiene. To construct butadiene from cyclobutene, electron pairs must flow into the two bonding orbitals, tfii and 2. Similar to cyclobutanation, then, this process could conceivably be rendered allowed if an electron pair were removed from the 1/13) orbital and an electron pair added to the 7r ifiz) orbital as the disrotatory process proceeded. A transition metal could effect these operations through properly ordered dyz and dzx orbitals. This process is illustrated in Fig. 9. 

Let us consider again the cyclobutene-butadiene reaction. The molecular orbitals of cyclobutene that undergo a radical change in the course of reaction are a and a, the bonding and antibonding orbitals of the bond that is broken, and n and n the bonding and antibonding orbitals of the double bond. 

Electrocyclic reactions are not really different from cycloadditions. Figure 20.27 compares the equilibration of 1,3-butadiene and cyclobutene with the 2 + 2 dimerization of a pair of ethylenes. The only difference is the extra o bond in butadiene, and this bond is surely not one of the important ones in the reaction—it seems to be just going along for the ride. Why should its presence or absence change the level of detail av able to us through an orbital symmetry analysis It shouldn t, and in fact, it doesn t. 

Longuet-Higgins and Abrahamson [6] suggested that in any concerted process, the orbitals of the starting material and product have the same symmetry. This is also supported by Woodward and Hoffmann [5]. The cyclobutene-butadiene intercon-version may be considered as an example to verify the fact by constraclion of a correlation diagram. For cyclobutene, the bonding orbitals are o and ti, while the... 

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

The stereochemistry of the cyclobutene isomerizations and the reverse processes of this type, involving the formation of a bond between the ends of a linear system containing a number of 7i--electrons, has been discussed by Woodward and Hoffmann (1965). They term such processes electrocyclic and consider that their steric course is determined by the symmetry of the highest occupied molecular orbital of the open-chain isomer. In an open-chain system containing 4 7T-electrons (such as butadiene), the symmetry of the highest occupied ground-state orbital is such that bonding interaction between the ends of the chain must involve overlap between orbital envelopes on opposite faces of the system, and this can only occur in a conrotatory process ... 

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