Conformational analysis of 1,3,6,2-dioxazaphosphocanes and 1,3,6,2-dioxazasilocanes

Abstract

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.

Theoretical Conformational Analysis of Cyclic Organophosphorus and Organosilicon Compounds

“…Thus, transannular interactions N→P, N→Si, or P→Si are absent in 1,3,2,6-dioxasilaazacyclooctanes despite the favorable crown conformation. 3,15 This is evident from the absence of exaltation of the dipole moments as well as the data of NMR spectroscopy and theoretical calculations. Kemme and Bleidelis 16 have established the absence of transannular bonding for azadioxasilocin Ph 2 Si(OCH 2 CH 2 ) 2 NPh, though the geometrical parameters of its cycle (the crown conformation, N .…”

“…[1][2][3] However, interest in these compounds has not decreased. In the last 10 years, a search for new polyfunctional heterocyclic systems of phosphorus and silicon that present theoretical interest and can have useful properties has been conducted.…”

Synthesis and conformational study in solution and solid state of 1,3-dioxa-6-aza-2(O-trimethylsilyl ester)- and 1,3-dioxa-6-aza-2(hydroxy)-σ4λ4phosphacyclooctanes

“…This particular distance was sometimes used to deduce a transannular interaction between N and P when, as in dioxazaphosphocane 2, it was found shorter than the sum of the corresponding Van der Walls radii (3.4 Å). Even if this explanation has generated some controversy 4 , it is worth noting here that in the case of 4b and 4c the distance between N and P is too long and excludes any hypothetical transannular interaction. Therefore, as it was already observed in solution through NMR analysis, the atom doubly bonded to P (S, Se) and the replacement of the small H in 2 by the bulkier −OSiMe 3 group in 4b and 4c have no influence at all on the heterocyclic conformation in the solid state.…”

“…However, those eight-membered heterorings, named dioxoazaphosphocanes, are also interesting for conformational aspects and many researchers have worked on determining their ring conformation, the orientation of the exocyclic substituents and the presence of a hypothetical intramolecular transannular interaction between the N and P atoms 4 . Up to this point, the existing NMR and X-ray data analyses show a slight preference for the chair-chair (crown) conformation in saturated eight-membered phosphorus heterocycles 5 .…”

Experimental and theoretical conformation analysis of eight-membered silocines with planar fragments

“…New polyfunctional eight-membered heterocyclic systems are sought for, taking into account that some of them possess useful properties [5][6][7]. Such structures are characterized by versatile conformations and electronic interactions [8][9][10]; primarily, intramolecular dative interactions S→P, N→Si, S→Si, etc., are observed [11,12]. Interest in eight-membered heterocycles is also determined by their similarity to structural units of silicon derivatives of calix [4]arenes and thiacalix [4]arenes that are capable of selectively recognizing various species (see, e.g., [13][14][15]).…”