Mechanistic insights into the rhodium catalysed dehydrogenative cycloaddition of cyano-yne-allene substrates†

The continued progress in creating novel acceptorless dehydrogenations for environmentally friendly and efficient chemical synthesis is anticipated to be significantly shaped by the design of efficient catalysts that can do the job. In this study, we present a comprehensive mechanistic investigation utilizing DFT calculations to elucidate the mechanism of the [RhCl(PPh₃)₃]-catalysed dehydrogenative cycloaddition of cyano-yne-allene substrates. Overall, the reaction can be described as a cycloaddition involving the three unsaturations, followed by a hydrogen shift, 6π electrocyclization and acceptorless H₂ elimination. The crucial factors contributing to the observed reactivity are as follows: (i) the favoured cycloaddition pathway involves the external double bond of the allene in the formation of the initial metallacycle; (ii) the stabilization of the alkyne insertion intermediate that occurs through η³-coordination to the rhodium; and (iii) the presence of a weak base that assists in deprotonation.