Benzo-Fused Phosphepines: Synthesis by Gold(I)-Catalyzed Intramolecular Hydroarylation and Ring Inversion

Gold(I)-catalyzed intramolecular hydroarylation of dialkynyl(biaryl)phosphine oxides provided versatile benzo-fused phosphepine oxides. O-exo adducts were obtained as the major product, and O-endo adducts were the minor product. O-exo and O-endo indicate the position of an oxygen atom with respect to the central phosphepine framework. The isomers of dibenzo[b,d]phosphepine oxides with methyl and phenyl groups in an adjacent position (shown as 2 a and 3 a in the main text) were determined by X-ray crystallographic analysis. O-exo and O-endo adducts are diastereomers owing to the central chirality of the phosphorus atom and the planar chirality of phosphepine, and they are interconverted through a ring inversion of phosphepine via a transition state with a planar structure. The inversion barrier, fully estimated through theoretical calculations, drastically varied depending on the substituents in the phosphepine framework and/or the fusion of benzene and thiophene rings. Furthermore, the kinetics of the thermal isomerization of less stable O-endo product 3 a into O-exo product 2 a was examined.