Page 911 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

The analysis for the 1,3,5-triene system according to FMO theory proceeds in 895
the same way as for a diene, but leads to the conclusion that a bonding interaction
between C(1) and C(6) of the triene will require a disrotatory motion. This is because SECTION 10.5
the HOMO, , has positive lobes on the same face of the system and these must Electrocyclic Reactions
3
overlap to permit bond formation. The symmetry properties of other six -electron conju-
gated triene systems are the same, so disrotatory ring closure (or opening) is general for
conjugated trienes. The orbitals for the hexatriene system are shown in Figure 10.22.
When we recall the symmetry patterns for linear polyenes that were discussed in
Chapter 1 (see p. 29), we can further generalize the predictions based on the symmetry
of the polyene HOMO. The HOMOs of the 4n systems are like those of 1,3-dienes in
having opposite phases at the terminal atoms. The HOMOs of other 4n + 2 systems
are like trienes and have the same phase at the terminal atoms. Systems with 4n
electrons will undergo electrocyclic reactions by conrotatory motion, whereas systems
with 4n+2 electrons will react by the disrotatory mode.
The analysis of electrocyclic reactions can also be done using orbital corre-
lation diagrams. 171 This approach focuses attention on the orbital symmetries of both
reactants and products and considers the symmetry properties of all the orbitals. In any
concerted process, the orbitals of the starting material must be smoothly transformed
into orbitals of product having the same symmetry. If this process of orbital conversion
leads to the ground state electronic configuration of the product, the process will have
a relatively low activation energy and be an allowed process. If, on the other hand, the
orbitals of the reactant are transformed into a set of orbitals that does not correspond to
the ground state of the product, a high-energy TS occurs and the reaction is forbidden,
since it would lead to an excited state of the product.
The cyclobutene-butadiene interconversion can serve as an example of the
construction of an orbital correlation diagram. For this reaction to occur, the four
orbitals of butadiene must be converted smoothly into the two and two orbitals of
the ground state of cyclobutene. The orbitals of butadiene are , , , and . For
3
4
1
2
cyclobutene, the four orbitals are , , *, and *, with each of them classified with
respect to the symmetry elements that are maintained in the course of the transform-
ation. The relevant symmetry features depend on the structure of the reacting system.
The most common elements of symmetry to be considered are planes of symmetry and
rotation axes. An orbital is classified as symmetric, S, if it is unchanged by reflection
in a plane of symmetry or by rotation about an axis of symmetry. If the orbital changes
sign (phase) at each lobe as a result of the symmetry operation, it is called antisym-
metric, A. Proper molecular orbitals must be either symmetric or antisymmetric. If an
orbital is neither S nor A, it must be adapted by combination with other orbitals to
meet this requirement.

ψ 1 (no nodes) ψ (one node) ψ 3 (two nodes)
2 disrotatory
closure
Fig. 10.22. Symmetry properties of the occupied orbitals of a conjugated triene.
171
H. C. Lon` guet-Higgins and E. W. Abrahamson, J. Am. Chem. Soc., 87, 2045 (1965).