Synthesis and Conformational Analysis of Torsionally Constrained 1,3,2-Dioxaphosphepanes

Article abstract of DOI:10.1021/jo00036a011

A series of benzo-fused and norbornene-fused 1,3,2-dioxaphosphepanes have been synthesized and conformationally studied by proton and phosphorus-31 NMR, infrared spectroscopy, and X-ray crystallography.The NMR data are best interpreted in terms of chair-chair equilibria in which substituents on phosphorus (phenoxy, dimethylamino, and phenyl) show a remarkable preference for axial positions.Twist conformations cannot be completely ruled out, however.X-ray crystal structures have been determined for two of the norbornene-fused compounds,trans-5,6-(endo,endo-2',3'-bicyclo<2.2.1>hept-5'-eno)-2-oxo-2-phenoxy-1,3,2-dioxaphosphepane (trans-2a) and trans-5,6-(endo,endo-2',3'-bicyclo<2.2.1>hept-5'-eno)-2-oxo-2-phenyl-1,3,2-dioxaphosphepane (trans-2b), and one of the benzo-fused compounds, 5,6-benzo-2-oxo-2-phenyl-1,3,2-dioxaphosphepane (1b).The X-ray structures of the two norbornene-fused dioxaphosphepanes confirm the stereochemical assignments for these materials and show the seven-membered ring to adopt a chair conformation.A twist conformation is seen in the structure of the benzo-fused dioxaphosphepane, on the other hand.