Apical versus equatorial disposition of substituents in tetraoxyphosphoranes bearing a 1,3,2-dioxaphosphocin ring: implications on apicophilicity in trigonal bipyramidal phosphorus

Abstract

New spirocyclic (amino/amido)tetraoxyphosphoranes CH₂(6-t-Bu-4-Me-C₆H₂O)₂P(NRR')(O₂C₆Cl₄[R = Me, R' = Ph (1), R = R' = i-Pr (2); R = R' = H (3); R = H, R' = Ph (4)] and the isothiocyanatotetraoxyphosphorane CH₂(6-t-Bu-4-Me-C₆H₂O)₂P(NCS)(O₂C₆Cl₄(5) have been synthesized. X-ray crystallography for these compounds reveals that -N(Me)Ph, -N(i-Pr)₂, and -NCS groups occupy an apical position whereas -NH₂ and -NHPh groups occupy an equatorial position in a trigonal bipyramidal geometry around phosphorus. These results are in contrast with the common assumption that a sterically bulky and less electronegative substituent [e.g. -N(i-Pr)₂] should be less apicophilic than a sterically small and more electronegative substituent (e.g. -NH₂). The possible rationalization for these results is discussed. Variable-temperature (¹H, ³¹P) NMR spectra of these compounds show some unusual features not reported before for pentacoordinate phosphorus. Probable intramolecular processes involving (i) apical-equatorial ↔ equatorial-apical exchange, (ii) apical-equatorial ↔ equatorial-equatorial exchange, and (iii) boat-chair ↔ tub (for the eight-membered ring) interconversion as well as cessation of the P-N bond rotation have been invoked to explain the spectral features.