Apical versus Equatorial Disposition of Substituents in Tetraoxyphosphoranes Bearing a 1,3,2-Dioxaphosphocin Ring: Implications on Apicophilicity in Trigonal Bipyramidal Phosphorus

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.