Steric Consequences on the Conformation of Medium-Sized Rings:  Solution NMR, Solid-State Crystallographic, ab Initio Molecular Orbital Calculations, and Molecular Mechanics Studies on Substituted Eight-Membered Organosilicon Ring Systems1

The conformation of the eight-membered membered 12H-dibenzo[d,g][1,3,2]dioxasilocin ring system was investigated both in the solid-state by X-ray crystallography and in solution by NOE experiments. Ab initio Hartree−Fock calculations were performed to locate all stationary points for the unsubstituted 12H-dibenzo[d,g][1,3,2]dioxasilocin ring system. The MM2* force field was parametrized to reproduce our ab initio results, and these data were compared to the experimental data. The transition states for conformational interchange were identified. The pseudoequatorial preference for a methyl substituent on either the C(12) carbon atom or silicon is greater than the difference in energy between a boat-chair and twist-boat conformation in 2,4,8,10-tetra-tert-butyl-substituted 12H-dibenzo[d,g][1,3,2]dioxasilocins. The conformations observed in the solid-state X-ray crystal structures of 12H-dibenzo[d,g][1,3,2]dioxasilocins are sensitive to crystal-packing forces and may be different from that in solution.