On the Mechanism of the α‐Alkynone Cyclization: A Theoretical Study

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Abstract
The acetylene → carbene rearrangement of three model a‐alkynones, namely propynal (1a), 2‐butynal (1b) and butynone (1c) is studied by ab initio double‐zeta, double‐zeta plus polarization and double‐zeta plus self‐consistent electron pairs calculations. Transition states are located by force‐gradient geometry optimization. Calculated minimum energy reaction paths reveal substituent effects on the activation parameters, which indicate that a H‐atom or an alkyl group competes favorably with an acyl group in the [1,2]‐shift from a‐alkynones 1 to the corresponding acylvinylidenes 5.