Conformations of Saturated Six-Membered Ring Phosphorus Heterocycles. 2-Aryl-1,3,2λ5-oxazaphosphorinanes

Article abstract of DOI:10.1021/ja00229a027

A series of 2-aryl-2-oxo- and 2-thioxo-5-tert-butyl-1,3,2λ⁵-oxazaphosphorinanes has been prepared (6-12).Assignments of cis or trans geometries to individual diastereomers were made by 31P and 1H NMR criteria combined wit X-ray crystallographic structures for cis-8 and trans-12. 1H NMR analysis at 300 MHz of chair ->/<- twist equilibria leads to the conclusion, based mostly on the 2-oxo derivatives, that cis chair conformations (13) with 5-tert-butyl equatorial and Ar substituents on phosphorus axial are strongly destabilized by N3Ph/PAr steric interactions.Thus, one finds an increasingly more favorable estimate of ΔG⁰ (chair -> twist) for the 2-oxo series cis-6, cis-8, cis-11, and cis-12 (N3H/PPh, N3H/PMes, N3Ph/PPh, N3Ph/PMes).Substitution of H on N3 by Ph (cis-11) changes the ΔG⁰ (chair -> twist) value for the PPh compound (cis-6) by 0.9 kcal/mol in favor of the twist form.The same substitution for Mes case (cis-8) results in an even greater change (>/= 2.2 kcal/mol) in ΔG⁰ (chair -> twist) with cis-12 in fact essentially completely in twist conformation 15.Methyl substitution at N3, cis-9, likewise increases the population of twist conformation, clearly an effect of destabilizing N3Me/PPh interactions in the chair, which are relieved in the twist conformation.These results point to the major role of N3R/PZ steric repulsive effects in the equilibria of 1,3,2λ⁵-oxazaphosphorinanes.Excluded is the possibility that Ph substitution at N3 merely reduces the importance of n/?* anomeric effects involving the N3 lone pair and axial substituent Z (Ar, Me2N) on phosphorus.The trans diastereomers also readily depopulate the chair conformation to give twist form 16, however, instead of 15 (trans-6,8, and 11).The pseudoaxial-seeking tendencies of substituents on phsophorus for trans diastereomers are shown to be CH3O > Ph > Me2N.A notable finding for the trans-2-aryl-2-oxo-5-tert-butyl-1,3,2λ⁵-oxazaphosphorinanes is the destabilizing effect of equatorial PMes compared to equatorial PPh, which results in greater depopulation of the diequatorially substituted chair, 13.