N‑Heterocyclic Carbene Catalyzed Switchable Reactions of Enals with Azoalkenes: Formal [4 + 3] and [4 + 1] Annulations for the Synthesis of 1,2-Diazepines and Pyrazoles

A regio- and enantioselective formal
[4 + 3] annulation reaction
between enals and in situ formed azoalkenes has been achieved. A diverse
set of 1,2-diazepine derivatives were synthesized in good yields with
excellent enantioselectivities (often 99% ee). Alternatively,
modifying the standard NHC catalyst switched the reactivity toward
a formal [4 + 1] annulation to afford functionalized pyrazoles. The
electronic and steric properties of the N-heterocyclic carbene organocatalyst
play a vital role in controlling the reaction pathway (homoenolate
vs acyl-anion reactivity of enal), allowing selective access to diverse
1,2-diazepine and pyrazole derivatives from identical substrates.