where the sigma bond was lost and a pi bond was formed on an open ring structure. The reactant had sigma bonds and a pi double bond, while the product gained a pi double bond after the reaction took place. Refer to Figure 8 to see the out of phase orientation of the two p orbitals on the HOMO’s ends.
Refer to Figure 10 to see the in phase orientation of the p orbitals at the end of the HOMO. The rightmost figure shows the result of a photochemical reaction with electrocyclization. However, since the highest occupied molecular orbital became the excited state HOMO, the odd number of pi double bonds led to conrotation. Conrotatory reactions with 2,4,6-octatriene led to a specific stereochemistry as well. This product is known as trans-5,6-dimethyl- 1,3-cyclohexadiene because the methyl groups are on opposite sides of the molecular plane. Refer to Figure 10 to see the out of phase orientation of the two p orbitals at the end of the excited state HOMO. From this information, it can be concluded that in a thermal electrocyclic process with (2E,4Z,6E)-2,4,6octatriene, the stereospecific nature of the reaction will only yield cis products, and in a photochemical electrocyclic process, (2E,4Z,6E)-2,4,6-octatriene will only yield trans products.
Figure 10. (2E,4Z,6E)-2,4,6-octatriene molecular orbitals.
Figure 11. (2E,4Z,6E)-2,4,6-octatriene reactant and products (The NC HS Server, n.d.). The three molecules shown in Figure 11 are those involved in the thermal and photochemical electrocyclic reactions using 2,4,6-octatriene as the reactant. The leftmost molecule is the reactant of the electrocyclic reaction. It is important to observe this molecule because it shows the two pi orbitals that are on the verge of forming a sigma bond. The sigma bond completion is shown in two separate reactions, the products of which are shown as the middle and rightmost molecules of Figure 11. In the middle diagram, the molecule was formed from a reaction that underwent a thermal electrocyclic process. Due to the odd number of pi double bonds in the reactant, the reaction must proceed using disrotation. As disrotatory changes occur, a sigma bond forms in the opening and is shown in the middle molecule by the two red orbitals that extend across the bond. To further characterize this product, it can be named a cis product because its methyl groups are on the same side of the molecular plane. This product is known as cis-5,6-dimethyl- 1,3-cyclohexadiene. 22 | 2016-2017 | Broad Street Scientific
Figure 12. Provitamin D3 molecular orbitals. Figure 12 shows the pi orbitals for provitamin D3. Because this reaction is a photochemical reaction, the light that interacts with the molecule causes one of the electrons from the HOMO to move to its excited state, causing what would have been the LUMO to be an excited state HOMO. This became the orbital analyzed to determine the behavior of the reaction. Because the two orbitals at the ends of the excited state HOMO are out of phase, it was determined that conrotatory behavior was required to complete the electrocyclic reaction converting 7-dehydrocholesterol to provitamin D3.
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