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.
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
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