Butadiene, orbitals

In this case, the ti-MOs resemble those of butadiene. Relative to the butadiene orbitals, however, the acrolein orbitals lie somewhat lower in energy because of the effect of the more electronegative oxygen atom. This factor also increases the electron density at oxygen relative to carbon. 

Fig. 11.4. Correlation diagram for cyclobutene and butadiene orbitals (symmetry-forbidden disrotatory reaction).

The FMOs of acrolein to the left in Fig. 8.2 are basically slightly perturbed butadiene orbitals, while the FMOs of protonated acrolein resemble those of an allyl cation mixed in with a lone-pair orbital on the oxygen atom (Fig. 8.2, right). Based on the FMOs of protonated acrolein, Houk et al. [2] argued that the predominant interaction in a normal electron-demand carbo-Diels-Alder reaction is between the dienophile LUMO and diene HOMO (Fig. 8.1, left). This interaction is greatly... 

It is a straightforward task to carry the procedure one step further. In order to find the butadiene orbitals, it is easiest to allow two ethylene units to come together end to end. Figure 10.20 illustrates the results. The important interactions are between orbitals of the same energy we simply treat the ethylene orbitals as the basis, and combine the bonding pair in the two possible ways and the antibonding pair in the two possible ways. 

The butadiene orbitals are denoted F and the orbital representing the donor substituent D. The MO diagram (c) below shows that the HOMO of the 1-substituted... 

This means that, during the course of a reaction, maximum bonding overlap can be maintained only if all of the intermediates along the reaction pathway also have C2 symmetry in other words, the whole process must be symmetrical with respect to C2. If we look at symmetry correlations with respect to the a plane, we see that to maintain symmetry with respect to the <7 plane, the butadiene orbital 2 correlate with the tt orbital. This... 

A slightly different monomer has also been studied bicyclopropyle, composed of two cyclopropanes linked by a o- C-C bond (M10). The conjugation between the two cyclopropanes of the monomer is similar to that appearing between the butadiene orbitals. 

With suitable initiators, the two conformers may yield polymers of different structures. The existence of a pseudoconjugation between the two cyclopropanes, which may be compared with those of the butadiene orbitals, theoretically favors a polymerization by simultaneous opening of the two conjugated orbitals. 

Orbital symmetries of 1,3-butadiene orbitals. In the ground state, HOMO is in the excited state, HOMO is i/f3. 

Not only must we consider the symmetry properties of 1,3-butadiene orbitals, but we must also consider the s)mtunetry properties of both cyclobutene and the transition structure expected for the conversion of the reactant to product. The two pathways for the closure of 13-butadiene to cyclobutene are illustrated in Figure 11.16. The Cl—C2, C2—C3, and C3—C4 a bonds are shown as solid lines. The p orbitals of 1,3-butadiene and cyclobutene, as well as the sp orbitals of the C3—C4 cr bond of cyclobutene, are represented by the shapes of the atomic p or sp orbitals. This is therefore only a basis set representation, not an illustration of a particular molecular orbital. Although there are many symmetry elements present in the representations of both 1,3-butadiene and cyclobutene, in the conrotatory reaction the only symmetry element that is present continuously from reactant through transition structure to product is the C2 rotation. Similarly, only the a reflection is present from reactant through transition structure to product for the disrotatory pathway. 

One aspect of Figure 11.18 that may not be intuitively obvious is the process by which the lowest energy orbital of cyclobutene (a), which has electron density only on Cl and C4, is transformed into a 1,3-butadiene orbital... 

Fig. 10.3. Symmetry properties of cyclobutene and butadiene orbitals with respect to the disrotatory...

The symmetry properties of the cyclobutene and butadiene orbitals with respect to this twofold axis are shown in Fig. 10.5 and the derived correlation diagram, in Fig. 10.6. This reaction is symmetry-allowed, since the bonding orbitals of cyclobutene correlate with the bonding orbitals of butadiene, and vice versa. 

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