Strong C─B Hyperconjugation in a Structurally Distorted Benzoanellated Tricyclic Boratirene

The benzoanellated tricyclic boratirene 2⋅Li(Et2O) was synthesized by reductive dehalogenation of C─borylborirene 1 with Lithium powder, and its distorted structure was determined by X‐ray analysis. The formation of compound 2⋅Li(Et2O) can be described as a result of the insertion of the carbene‐like boron atom into the C─H bond of a methyl group at a duryl ring (duryl ring = 2,3,5,6‐tetramethylphenyl) in the ortho position. This new tricyclic compound contains a highly shielded, formally three‐coordinated boron atom with δ11B = 27 ppm. However, the 11B chemical shift in the range of 65 to 85 ppm is expected for carbon‐bonded three‐coordinated boron atoms. The reason for the high shielding of this boron atom in 2⋅Li(Et2O) is a very strong C─B hyperconjugation. The strained, and negatively charged three‐membered ring in 2⋅Li(Et2O) is an optimal component for a strong hyperconjugative interaction, leading to the formation of a three‐center two‐electron (3c‐2e) σ bond with the empty p orbital of the boron atom, resulting in a strong distortion of the six‐membered ring. The distorted structure of the benzoanellated tricyclic boratirene was determined by X‐ray analysis. The formation of this compound can be described as a result of the insertion of a carbene‐like boron atom into a C─H bond. The 11B chemical shift of the formally three‐coordinated boron atom (B2) shows a signal at 27 ppm (typically, the 11B chemical shift for carbon‐bonded three‐coordinated boron atom is in the range of 65 to 85 ppm). The reason for the high shielding of the B2 atom is a strong hyperconjugative interaction, leading to the formation of a 3c‐2e σ bond with the empty p orbital of the B2 atom, resulting in a strong distortion of the six‐membered ring.