Relative Stability for the Structural Isomers of Producing Fused-Ring Systems from Acyclic Substrates : Ab Initio Study

Abstract

cyclic compounds calculated with the MP2/6-311+G ** level. Three schematic potential energy diagrams and optimized structures for the structural isomers of the 6-, 7-, and 8membered cyclic compounds are drawn in Figure 1. All optimized geometrical structures of the cyclic compounds are local minima without an imaginary frequency. The geometrical isomers of the 6-, 7-, and 8-membered cyclic compounds with higher potential energy are not represented. The structures of the cyclic compounds are classified to be chair and boat types except for 5-methoxy-1-methylcyclohexene with fused-ring form. In these compounds, the optimized structures of the chair types are more stable than those of the corresponding boat types, respectively. The cyclic structures of the methylcycloalkenes with a methyl group are more symmetric than those of the corresponding methylenecycloalkanes with a methylene group. A methoxy group is substituted as a horizontal or vertical direction on the cyclic skeleton. In the most stable structures of 6-, 7-, and 8-membered cyclic compounds, the geometrical conformation between three hydrogen atoms of a methoxy group and a carbon atom on the ring bonded to a oxygen atom is optimized as an eclipsed form. The structures of the 6- and 8-membered cyclic compounds are more symmetric than those of the 7-membered cyclic compounds. As shown in Figure 1, the lowest potential energies of the 6-, 7-, and 8-membered cyclic compounds are set equal to zero, respectively. The chair types of these cyclic compounds are more stable than the boat types. In the 6- and 7-membered cyclic compounds, the methylenecyclohexanes with a methylene group are more stable than the methylcyclohexenes with a methyl group. That is, the structures with an exocyclic double bond are more stable than those with a double bond on the ring. While, in the 8-membered cyclic compound, the methylcyclohexenes with a methyl group are more stable than methylenecyclohexanes with a methylene group. In the 6- and 8-membered cyclic compounds, the structures substituted by a methoxy group as a horizontal direction are more stable than those substituted by a methoxy group as a vertical direction. While, in the 7-membered cyclic compounds, the structures substituted by a methoxy group as a vertical direction are more stable than those substituted by a methoxy group as a horizontal direction.