Taming Keteniminium Reactivity by Steering Reaction Pathways: Computational Predictions and Experimental Validations

Keteniminium ions,
the nitrogen analogues of ketenes, exhibit high
reactivity toward olefins and π-systems. Previous results from
the Maulide group demonstrated an unexpected propensity for an alternative
intramolecular Belluš–Claisen-type rearrangement rather
than an expected intramolecular (2 + 2) cycloaddition. We have conducted
a cooperative density functional theory/experimental investigation
of this process, seeking insights into the competition between the
observed Claisen-type reaction and the historically expected (2 +
2) cyclization. Our calculations revealed a surprisingly small difference
in the free energy barrier between these two intramolecular reactions.
Further theoretical and experimental investigations probe the electronics
of the substrate, rationalize a competing deallylation side reaction,
and demonstrate the proof-of-concept for an enantioselective (2 +
2) variant.