Stereochemical consequences of dynamic gearing in substituted bis(9-triptycyl) methanes and related molecules.
Empirical force-field calculations show that bis(9-triptycyl)methane has a C(s) ground state and that the securely meshed triptycyl groups undergo nearly unhindered cogwheel rotation through a C(2) transition state, with an activation energy of approximately 1.0 kcal mol(-1). These features lead to note-worthy stereochemical attributes in appropriately substituted derivatives and analogs. Bis(9-triptycyl)carbinol has been prepared as an example of a compound that is chemically achiral, even though all conformations are chiral under the constraint of gear meshing. It is shown that, under the same constraint, ring substitution of either compound (or analogous systems) may lead to residual stereoisomerism. A permutational analysis of chemical isomers and isomerizations for every possible type of substitution pattern gives an enumeration of residual stereoisomers under the operation of various gearing and nongearing modes and provides information essential for a choice among mechanistic alternatives for a given isomer count.
Hounshell
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Molecular Design, Ltd., 1122 B Street, Hayward, California 94541.
Johnson
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Guenzi
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Cozzi
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Mislow
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