Page 914 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

898 Correlation diagrams can be constructed in an analogous manner for the disro-
tatory and conrotatory modes for interconversion of 1,3,5-hexatriene and cyclohexa-
CHAPTER 10
diene. They lead to the prediction that the disrotatory mode is an allowed process,
Concerted Pericyclic whereas the conrotatory reaction is forbidden, which is in agreement with the experi-
Reactions
mental results on this reaction. Other electrocyclizations can be analyzed by the same
method. Substituted derivatives of polyenes obey the orbital symmetry rules, even in
cases where the substitution pattern does not correspond in symmetry to that of the
orbital system. It is the symmetry of the participating orbitals, not of the molecule as
a whole, that is crucial to the analysis.
Electrocyclic reactions can also be analyzed on the basis of the idea that transition
states can be classified as aromatic or antiaromatic, just as is the case for ground
state molecules. 172 A stabilized aromatic TS results in a low activation energy, i.e., an
allowed reaction. An antiaromatic TS has a high energy barrier and corresponds to
a forbidden process. The analysis of electrocyclizations by this process consists of
examining the array of basis set orbitals that is present in the transition structure
and classifying the system as aromatic or antiaromatic. For the butadiene-cyclobutene
interconversion, the TSs for conrotatory and disrotatory interconversion are shown
below. The array of orbitals represents the basis set orbitals, that is, the complete set
of 2p orbitals involved in the reaction process, not the individual molecular orbitals.
The tilt at C(1) and C(4) as the butadiene system rotates toward the TS is different for
the disrotatory and conrotatory modes. The dashed line represents the bond that is
being broken (or formed).

basis set orbitals basis set orbitals
for conrotation; 4 for disrotation; 4
electrons, one node electron, zero nodes
aromatic antiaromatic

For the cyclobutene-butadiene TS, the conrotatory closure results in a Mobius system,
whereas a disrotatory TS gives a Hückel system. The same rules of aromaticity apply as
for ground state molecules. A Hückel system is aromatic when it has 4n+2 electrons.
A Mobius system is aromatic when it has 4n electrons. In the case of the cyclobutene-
butadiene interconversion, which involves four electrons, it is the conrotatory Mobius
TS that is the favored aromatic transition state.
Basis set orbital analysis of the hexatriene-cyclohexadiene system leads to the
conclusion that the disrotatory process will be favored. The basis set orbitals for the
conrotatory and disrotatory transition states are shown below. Here, with six electrons
involved, it is the disrotatory mode (Hückel system) that gives a stabilized TS.

172
H. E. Zimmerman, Acc. Chem. Res., 4, 272 (1971); M. J. S. Dewar, Angew. Chem. Int. Ed. Engl., 10,
761 (1971).