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

Abstract The benzoanellated tricyclic boratirene 2 ⋅ Li(Et 2 O) 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(Et 2 O) 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 δ 11 B = 27 ppm. However, the 11 B 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(Et 2 O) is a very strong C─B hyperconjugation. The strained, and negatively charged three‐membered ring in 2 ⋅ Li(Et 2 O) 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.