Unexpected Insertion of CO2 into the Pentacoordinate P–N Bond: Atherton–Todd-Type Reaction of Hydrospirophosphorane with Amines

The Atherton–Todd-type reaction of pentacoordinate hydrospirophosphoranes with amines was investigated, and a novel CO2 insertion reaction into the pentacoordinate P–N bond under mild conditions was developed. The mechanism and stereochemistry of the CO2 insertion reaction between hydrospirophosphoranes and secondary amines were proposed via a carbon-13 labeling experiment, a 31P NMR tracing experiment, and X-ray diffraction analysis. The chlorinated spirophosphorane intermediate was first generated with stereoretention of the configuration at phosphorus and subsequently was attacked by a carbamate anion formed from CO2 and a secondary amine. It was found that rear attack of nucleophilic substitution with stereoinversion at pentacoordinate phosphorus was the preferred route, although front attack happened for sterically hindered reactants. The configuration of the CO2 insertion product depended mainly upon the original phosphorus configuration of the hydrospirophosphoranes.