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-C6 H2O)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.