Molecular-Strain Driven Phosphinidene Reactivity of a Phosphanorcaradiene

Herein we report straightforward synthesis, characterization and reactivity studies of a phosphanorcaradiene, in which one of the benzene rings in the flanking fluorenyl substituents is intramolecularly dearomatized through attachment to the phosphorus atom. It was facilely obtained by the reduction of phosphorus(III) dichloride MsFluidtBu-PCl2 supported by a hydrindacene substituent with potassium graphite. Despite being thermally robust, it serves as an elegant precursor for transient phosphinidene. It reacted with trimethylphosphine and isonitrile to yield phosphanylidene-phosphorane and 1-phospha-3-azaallene, respectively. Moreover, it was capable of activating ethylene and 4-tertbutylphenylacetylene to afford [1+2] cycloaddition products, as well as oxidative cleavage of Si–H and N–H bonds to yield secondary phosphines. All the reactions proceeded smoothly at room temperature without the presence of transition metals. The driving force for these reactions is most likely the high ring-constraint of the three-membered PC2 ring and recovery of the aromaticity of the benzene ring.