The stereochemical mechanism of the nucleophilic substitution reaction at pentacoordinate phosphorus (P–V) atom is rarely studied. Here, we report the Atherton-Todd-type reaction of pentacoordinate hydrospiro­phosphorane with phenolic compounds in detail. The stereochemical mechanism of nucleophilic substitution at P–V atom was proposed by 31P NMR tracing experiment, X-ray diffraction analysis, and density functional theory calculations. The first step of the Atherton-Todd-type reaction is the formation of halogenated spirophosphorane intermediate with retention of configuration at phosphorus definitely. The second step is a nucleophilic substitution reaction at P–V atom of halogenated spirophosphorane. When using CCl4 as a halogenating agent, the reaction of chlorinated spirophosphorane proceeds via SN2­(P–V) mechanism, and the backside attack of P–Cl bond is the main pathway. For chlorinated spirophosphorane with ΔP configuration, the completely P-inverted product is normally obtained. As for chlorinated spirophosphorane with ΛP configuration, which has larger steric hindrance behind P–Cl bond, the proportion of P-retained products apparently increases and a pair of diastereoisomers is acquired. Furthermore, if CBr4 is used as a halogenating agent, the nucleophilic substitution reaction of brominated spirophosphorane may go through a SN1­(P–V) mechanism to afford a pair of diastereoisomers.

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PR3 efficiently catalyzes the addition of H‐spirophosphorane to electron‐deficient alkenes to give the spirophosphoranes with a pentacoordinate P−C bond in moderate to excellent yields. This practical procedure was conducted by using PR3 as an organocatalyst, which avoided the use of an additional base, traditional heating, and metal reagents. The investigation of the reaction mechanism showed that the zwitterion intermediate served as the base to further catalyze hydrospirophosphorane. Then the spirophosphoranide formed as a nucleophile attacked electron‐deficient alkene to give the addition products. This reaction is a first and simple hydrophosphorylation reaction of alkenes to construct pentacoordinate P−C bond under organocatalysis