Synthesis and Application of a Bidentate Ligand Based on Decafluoro-3-phenyl-3-pentanol: Steric Effect of Pentafluoroethyl Groups on the Stereomutation of O-Equatorial C-Apical Spirophosphoranes

1,1,1,2,2,4,4,5,5,5-Decafluoro-3-phenyl-3-pentanol (13) was prepared via the Cannizzaro-type disproportionation reaction, and dimetallated 13 (i.e., 15) was used as a bidentate ligand, which was bulkier than the Martin ligand (1,1,1,3,3,3-hexafluoro-2-phenyl-2-propanol). P-H spirophosphorane (16) was synthesized utilizing the new bidentate ligand, and the structure of 16 was essentially the same as that of the P-H phosphorane with the Martin ligands (1b). Phosphoranes which exhibit reversed apicophilicity (9: O-equatorial) were also synthesized and could be converted to the corresponding stable stereoisomers (10: O-apical). The crystal structures of O-equatorial phosphoranes (9) and those of O-apical isomers (10) were slightly affected by the steric repulsion of pentafluoroethyl groups. Kinetic measurements revealed that the stereomutation of O-equatorial methylphosphorane (9a) to the O-apical isomer (10a) was slowed. The activation enthalpy for the stereomutation of 9a→10a (24.4 kcal mol–1) was higher than that of the phosphorane bearing the Martin ligands (3a→4a: 19.3 kcal mol–1) by 5.1 kcal mol–1.