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 CH(2)(6-t-Bu-4-Me-C(6)H(2)O)(2)P(NRR')(O(2)C(6)Cl(4)) [R = Me, R' = Ph (1), R = R' = i-Pr (2); R = R' = H (3); R = H, R' = Ph (4)] and the isothiocyanatotetraoxyphosphorane CH(2)(6-t-Bu-4-Me-C(6)H(2)O)(2)P(NCS)(O(2)C(6)Cl(4)) (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 -NH(2) 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. -NH(2)). The possible rationalization for these results is discussed. Variable-temperature ((1)H, (31)P) 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.