Reversible Boron-Insertion into Aromatic C-C Bonds

Formation of borabicyclo[3.2.0]heptadiene derivatives was achieved via boron-insertion into aromatic C-C bonds in the photo-promoted skeletal rearrangement reaction of triarylboranes bearing an ortho-phosphino substituent (ambiphilic phosphine-boranes). The borabicyclo[3.2.0]heptadiene derivatives were fully characterized by NMR and X-ray analyses. The dearomatized products were demonstrated to undergo the reverse reaction in the dark at room temperature, realizing photochemical and thermal interconversion between triarylboranes and boron-doped bicyclic systems. Experimental and theoretical studies revealed that sequential two electrocyclic reactions involving E/Z-isomerization of an alkene moiety proceed via a highly strained trans-borepin intermediate. Unprecedented formation of borabicyclo[3.2.0]heptadiene derivatives was achieved via reversible boron-insertion into aromatic C-C bonds in the photo-promoted skeletal rearrangement reaction of ambiphilic phosphine-boranes. The reaction mechanism was clarified to consist of sequential two electrocyclic reactions with E/Z-isomerization of an alkene moiety through a highly strained trans-borepin intermediate.image