Palladium-Catalyzed [3 + 2] Annulation of Alkynes with Concomitant Aromatic Ring Expansion: A Concise Approach to (Pseudo)azulenes

The
construction of (pseudo)­azulenes represents an appealing yet
challenging task in organic synthetic chemistry. Herein, we disclose
a palladium-catalyzed [3 + 2] annulation technique of alkynes with
concomitant aromatic ring expansion driven by a diboron reagent and
iodide, affording a concise approach to azulenes (7-fused-5 bicycle)
and pseudoazulenes (6-fused-5 bicycle). Compared with the documented
synthetic strategies, the route to (pseudo)­azulenes developed herein
is applicable for both homo- and cross-annulation processes and exhibits
a broad substrate scope. It is worth noting that this transformation
is not only suitable for the ring expansion of the phenyl moiety to
afford azulenes but also applicable to the ring expansion of the electron-rich
five-membered heterocycles to deliver pseudoazulenes. Experimental
and computational investigations on the mechanism support the formal
trans-palladium-boration across the alkyne, cis-addition of the alkyne,
dearomative spiroannulation, and aromatic ring expansion process.