Characterization of an optically active pentacoordinate phosphorane with asymmetry only at phosphorus

Catalyst control over pentavalent stereocentres

A monumental diversity of catalytic methods imparts the ability to select one of two configurations of tetravalent stereocentres. Conversely, catalyst control over pentavalent stereocentres, where a fifth moiety bound to the central atom encodes an expanded stereochemical space, remained a challenge to be accomplished. Herein, we report the feasibility of the catalytic tractability of pentavalent stereocentres. A bifunctional iminophosphorane thiourea catalyst enables enantio- and diastereocontrol over pentavalent phosphoranes to differentiate configurationally stable enantiomers and ensembles of diastereomers which emerge together from a single stereocentre. The desired dioxophosphorane stereoisomers are obtained with excellent yield and selectivity (up to 99% yield, 96:4 e.r. and 99:1 d.r.), while stereodivergent catalysis reroutes the reaction for selective access to each of the viable stereoisomeric states of pentavalent phosphoranes. Considering the diversity of high-valent main group species, it is expected that catalyst control over pentavalent stereocentres significantly increases the synthetically addressable stereochemical space.

Chiral Hypervalent, Pentacoordinated Phosphoranes

This review presents synthetic procedures applied to the preparation of chiral (mainly optically active) pentacoordinated, hypervalent mono and bicyclic phosphoranes. The mechanisms of their stereoisomerization and their selected interconversions are also presented.

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 was prepared by a Cannizzaro-type disproportionation reaction, and the dimetallated compound was used as a bidentate ligand, which is bulkier than the Martin ligand (1,1,1,3,3,3-hexafluoro-2-phenyl-2-propanol). A P-H spirophosphorane was synthesized by utilizing the new bidentate ligand, and the structure of the product was essentially the same as that of the P-H phosphorane with Martin ligands. Phosphoranes that exhibit reversed apicophilicity (O-equatorial) were also synthesized and could be converted into the corresponding stable stereoisomers (O-apical). The crystal structures of O-equatorial phosphoranes and the O-apical isomers were slightly affected by the steric repulsion of pentafluoroethyl groups. Kinetic measurements revealed that the stereomutation of O-equatorial methylphosphorane to the O-apical isomer was slowed. The activation enthalpy for the stereomutation of the former to the latter was higher than that of the phosphorane with Martin ligands by 5.1 kcal mol(-1).

Reactivity of 1-hydro-5-carbaphosphatrane based on tautomerization between pentavalent phosphorane and trivalent cyclic phosphonite

The reaction behavior of 1-hydro-5-carbaphosphatrane (1 a) was examined. Treatment of 1 a with oxidants such as 3-chloroperoxybenzoic acid (mCPBA) and tBuOCl gave cyclic phosphonate 2 and 1-chloro-5-carbaphosphatrane (4), respectively, via cyclic phosphonite 3, a tautomer of 1 a. Compound 4 was readily hydrolyzed to afford 2. Compound 1 a was also sulfurized via 3 by elemental sulfur to afford cyclic thioxophosphonate 5, which was also obtained by reactions in the presence of bases. Treatment of 1 a with bases also proceeded through 3 to give an equilibrium mixture of the corresponding phenoxide anion 10 and the phosphoranide anion 9, which was quenched with MeI to afford a mixture of 11 and 1-methyl-5-carbaphosphatrane (1 b). Such reactivities are typical for neutral phosphoranes. Theoretical investigations of these reactivities were also performed.

First optically active selenurane oxide: Resolution of C2-symmetric 3,3,3?,3?-tetramethyl-1,1?-spirobi [3h,2,1]-benzoxaselenole oxide

The enantiomers of the first optically active selenurane oxide ever reported, C(2)-symmetric 3,3,3',3'-tetramethyl-1,1'-spirobi[3h,2,1]-benzoxaselenole oxide, were isolated via enantioselective liquid chromatography of the racemate or by spontaneous resolution that occurs during the slow evaporation of its acetonitrile solution or the slow crystallization from the same solvent.

Characteristic reactions and properties of C-apical O-equatorial (O-cis) spirophosphoranes: effect of the sigma(P)(-)(O) orbital in the equatorial plane and isolation of a hexacoordinate oxaphosphetane as an intermediate of the Wittig type reaction of 10-P-5 phosphoranes

Novel spirophosphoranes (O-cis) that exhibit reversed apicophilicity having an apical carbon-equatorial oxygen array in a five-membered ring showed enhanced reactivity toward nucleophiles such as n-Bu(4)N(+)F(-) or MeLi in comparison with the corresponding stable isomeric spirophosphoranes (O-trans) having an apical oxygen-equatorial carbon configuration. The enhanced reactivity of the O-cis isomer could be explained by the presence of a lower-lying sigma(P)(-)(O(equatorial)) orbital as the reacting orbital in the equatorial plane, whereas the corresponding orbital is a higher-lying sigma(P)(-)(C(equatorial)) in the O-trans isomer. Density functional theory (DFT) calculation on the actual compounds provided theoretical support for this assumption. In addition, we found that the benzylic anion alpha to the phosphorus atom in O-cis benzyl phosphorane is much more stable than that generated from the corresponding O-trans compounds. The experimental results were considered to be due to the n(C) --> sigma(P)(-)(O) interaction in the O-cis anion, and this was confirmed by DFT calculations. Furthermore, the hexacoordinate anionic species derived from the reaction of the benzylic anion from O-cis benzylphosphorane with an aldehyde was also found to be stabilized as compared with analogous species from the corresponding O-trans isomer. The first X-ray structural characterization of a hexacoordinate phosphate intermediate in the Wittig type reaction using pentacoordinate phosphoranes is reported.