Regioselective Migratory Ring Expansion of Heterole-Fused Norcaradienes for Modular Access to Azulene-Incorporated...

In this study, we report a sterically steered, thermally driven [1,5]-C shift ("carbon-walking") of congested heterole-fused norcaradienes (NCDs), generated in situ by tetramethylguanidine (TMG) catalyzed intramolecular hydroheteroarylation of heterole-substituted alkynylcyclopropanes, coupled by Cope ring expansion to give highly functionalized cycloheptatrienes (CHTs). Further cyclopentannulation of the CHT ring with the aroyl substituents via either formal reductive or oxidative Nazarov cyclization enables modular access to diverse azulene-incorporated thienoacenes as promising candidates for organic materials. DFT and experimental mechanistic investigations reveal that this organocatalytic intramolecular hydroheteroarylation is initiated by an unprecedented aromatic yne Cope rearrangement facilitating by a subsequent rate-determining TMG-mediated proton transfer. The resulting dearomatized heterole-fused CHT species then isomerize via Cope ring contraction to isolable heterole-fused NCDs. Upon heating, the differential steric environments around the two shared cyclopropyl carbon centers steer a stereoinvertive carbon-walking process of such heterole-fused NCDs, coupled with Cope ring expansion, to furnish the final skeleton-rearranged CHTs. Beyond its synthetic versatility, the synergy between the challenging aromatic yne Cope rearrangement and base-mediated proton transfer unlocks an otherwise inaccessible base-catalyzed intramolecular hydroheteroarylation of propargylic-proton-free alkynes.