Characteristic Reactions and Properties of C-Apical O-Equatorial (O-cis) Spirophosphoranes: Effect of the σ*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

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Abstract
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-Bu4N+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 σ*P-O(equatorial) orbital as the reacting orbital in the equatorial plane, whereas the corresponding orbital is a higher-lying σ*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 α 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 nC → σ*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.

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