Intramolecular competition between alkenes in ketene-alkene [2+2] cycloaddition. A theoretical study

In this paper, we describe the results of the intramolecular ketene-alkene [2+2] cycloaddition in 2-pent-4-enyl-octa-1,7-dien-1-one, at HF/6-31+G∗ and DFT-B3LYP/6-31+G∗ computational levels of theory. For this reaction, we have explored the four possible pathways at a supra-antara approach in which four possible products: 5-4-, and 6-4-membered fused [n.2.0] bicycle rings; 6-4-, and 7-4-membered fused [n.1.1] bicycle rings. In all cases, an asynchronous process was found with an appreciable interaction of the ketene central carbon with both carbons from the alkenes. We demonstrated that the reaction is driven towards the formation of 5-4 member rings instead of 6-4 member rings. In addition, the internal molecular orbital HOMO-2 was identified as the molecular orbital with higher nucleophilic reactivity.