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CEM 311
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The simplest way to think of a disrotatory ring opening/closing is to think of the HOMO of the ring-opened product. The ends of the HOMO rotate, and the p-orbitals need to be in phase when they form the new sigma bond. Thus, disrotatory ring-closing of a triene is allowed.
Disrotatory ring-closing of a diene attempts to form the sigma bond from p orbitals which are out of phase, and so the reaction is forbidden.
For the correlation diagram, remember that the symmetry element preserved during a disrotatory ring closing is the mirror plane. Orbitals of the starting material and the product are classified as symmetrical and unsymmetrical with respect to this plane, and orbital symmetry must be conserved. This means that occupied molecular orbitals in the starting material must transform into occupied molecular orbitals in the product. For a correlation diagram showing that disrotatory motion is allowed for ring-closing of a triene to a cyclohexadiene, click here. For a correlation diagram showing that disrotatory motion is forbidden for ring-opening of a cyclobutene to a butadiene, click here. |
Geometry of disrotatory ring-opening
Transition state for disrotatory ring-opening
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Copyright © 2001 by Daniel J. Berger. This work may be copied without limit if its use is to be for non-profit educational purposes. Such copies may be by any method, present or future. The author requests only that this statement accompany all such copies. All rights to publication for profit are retained by the author. |
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