The steric structure of 1,3,6,2-dioxazaphosphocanes and 1,3,6,2-dioxazasilocanes in solution was established by high-resolution 1 H, 13 C, and 31 P NMR spectroscopy and PM3 calculations. It is shown that the preferred conformation of the eight-membered heterocycle is chair!chair (crown).The structure of eight-membered heterocycles containing both electron-donor and electron-acceptor centers attracts researcher's attention because, together with solving conformational problems (ring conformation, orientation of exocyclic substituents), one can expect to reveal transannular intramolecular interactions in these compounds. Prerequisite for such interactions is that the heteroring conformation allows proximity of the heteroatoms to each other. Fairly scarce data are available in the literature, concerning the structure of 1,3,2-dioxaphosphocanes [133] or diphosphocanes with P3C bonds [4,5]. Hence, an almost ideal crown was found in 1,3,5,7-tetraphenyl-1,5,3,7-diazadiphosphocane with two trivalent phosphorus atoms on the opposite ends of the eightmembered ring [4]. This molecule preserves the same conformation in benzene solutions [5], unlike the corresponding derivative of four-coordinate phosphorus [5] that has the distorted boat!boat conformation in crystal (according to X-ray data) and prefers the crown conformation in solution. The preferred conformation of the eight-membered heteroring in 1,3,6,2-dioxaphosphasilocanes, as found by high-resolution NMR spectroscopy, dipole moment and Kerr effect measurements, and semiempirical quantumchemical calculations, is crown independently of the coordination of the phosphorus atom (3 or 4) [6]. The X-ray analysis of one of these molecules with an exocyclic P=Se bond, too, established that the eightmembered P,Si-containing heteroring has the crown conformation of [7]. However, MeN(CH 2 CH 3 O) 2 . ÄÄÄÄÄÄÄÄÄÄÄÄ K Deceased.
