Homobenzene: Homoaromaticity and Homoantiaromaticity in Cycloheptatrienes

Cycloheptatriene (C s ) is firmly established to be a neutral homoaromatic molecule based on detailed analyses of geometric, energetic, and magnetic criteria. Substituents at the 7 (methylene) position, ranging from the electropositive BH2 to the electronegative F, favor the equatorial conformation but influence the aromaticity only to a small extent. By the same criteria, the planar transition state (C 2v ) for cycloheptatriene ring inversion is clearly antiaromatic. This is attributed to the involvement of the pseudo-2π-electrons of the CH2 group with the 6π-electrons of the ring to give an 8π-electron system. Similarly, the participation of the CH2 groups into C2v cyclopentadiene and cyclononatetraene lead to significant 4n + 2 π electron aromaticity. The cyclization of cycloheptatriene to norcaradiene proceeds via a highly aromatic transition structure, but norcaradiene itself is less aromatic than cycloheptatriene. An annelated cyclopropane ring does not function as effectively as a double bond in promoting cyclic electron delocalization.