IN-CYCLOPHANES CONTAINING 2ND-ROW ELEMENTS POISED ABOVE AROMATIC RINGS

The silicon- and phosphorus-containing cyclophanes 2 and 3 were prepared by base-promoted condensation of tris(2-mercaptophenyl)silane and tris(2-mercaptophenyl)phosphine, respectively, with 1,3,5-tris(bromomethyl)benzene. The structures of both cyclophanes were determined by X-ray crystallography. In compound 2, the distance from the apical silicon to the center of the basal aromatic ring was 3.34 angstrom, and in 3 the corresponding phosphorus to ring distance was 2.90 angstrom. The H-1 NMR spectrum of 2 was characterized by a high-field Si-H resonance (delta-1.04, 5 ppm upfield from the corresponding resonance in an acyclic model), and the IR spectrum showed a high-frequency Si-H stretching band (2457 cm-1, 280 cm-1 above the acyclic model); both features result from the proximity of the in-hydrosilane to the aromatic ring. For cyclophane 3, the C-13 NMR spectrum exhibited ''through-space'' spin-spin coupling of the phosphorus atom with the carbons of the basal aromatic ring (J(PC)(methine) = 7.5 Hz and J(PC)(quaternary) = 3.5 Hz) and in the P-31 NMR spectrum the phosphorus resonance was 32 ppm downfield from that of an acyclic model, both features suggesting an unusual electronic interaction of the phosphine with the base. Due to a strong preference for an in geometry (pyramidalization of the phosphine toward the basal ring) and the consequent steric shielding, the phosphine of 3 was quite unreactive: It was not protonated by anhydrous HBr, and even when 3 was heated in refluxing hydrogen peroxide and acetic acid, the corresponding trisulfone was obtained without formation of the phosphine oxide.