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 8-MEMBERED ORGANOSILICON RING-SYSTEMS

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 Ha rtree-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 res ults, and these data were compared to the experimental data. The trans ition states for conformational interchange were identified. The pseud oequatorial preference for a methyl substituent on either the C(12) ca rbon 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 ob served 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 b e different from that in solution.