Stereomutation and Experimental Determination of the Relative Stability of Diastereomeric <i>O</i>‐Equatorial Anti‐Apicophilic Spirophosphoranes

Abstract

Abstract A novel bidentate ligand based on 1,1,1,3,3,4,4,4‐octafluoro‐2‐phenyl‐2‐butanol ( 16 ) has been used to synthesize a diastereomeric pair of hydrophosphoranes ( 18‐ exo and 18‐ endo ). A comparison of the structures of a series of hydrophosphoranes ( 18‐ exo , 18‐ endo , 19 , and 20‐ exo ) obtained by X‐ray crystallography indicated that the apical P–O bond lengths were affected by the electronic properties of the oxygen atom, which can be interpreted by the “single bond/no bond resonance” concept. From the hydrophosphoranes, O ‐apical ( 13‐ exo and 13‐ endo ) and anti‐apicophilic O ‐equatorial ( 12‐ exo and 12‐ endo ) phosphoranes were synthesized. The O ‐equatorial phosphoranes were irreversibly converted into diastereomeric pairs of O ‐apical isomers. Kinetic measurements implied that the electronic properties of the pentafluoroethyl group are comparable to those of the trifluoromethyl group. The activation enthalpies calculated for the stereomutations enabled us for the first time to experimentally show the stability of an O ‐apical isomer to be greater than that of an O ‐equatorial isomer by 13.7 kcal mol –1 .(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)