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CEM 311
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The simplest way to think of a conrotatory 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, conrotatory ring-closing of a diene is allowed.
Conrotatory ring-closing of a triene 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 conrotatory ring closing is the two-fold rotational axis. Orbitals of the starting material and the product are classified as symmetrical and unsymmetrical with respect to this axis, 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 conrotatory motion is allowed for ring-opening of a cyclobutene to a butadiene, click here. For a correlation diagram showing that conrotatory motion is forbidden for ring-closing of a triene to a cyclohexadiene, click here. |
Geometry of conrotatory ring-opening
Transition state for conrotatory 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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