Regioselective Cyclocarboxylation of Nonconjugated Dienes to Cyclic Keto Esters

The carboxylation of 1,5-hexadiene in the presence of (PPh₃)₂PdCl₂ in 1-butanol at 100 °C gives butyl 2-(3-methyl-2-oxocyclopentyl)acetate (1) in good yield. Small amounts of linear diesters are also produced. A number of other olefins were also carboxylated to give mixtures of cyclic and linear products. Although carboxylation of monoolefins usually gives a mixture of regioisomers, cyclopentanone 1 is produced in >25:1 selectivity over the cyclohexanone regioisomer. Deuterium labeling and model studies were carried out in an attempt to explain the unusual regioselectivity observed. These studies suggest that although the reversible initial insertion of 1,5-hexadiene into Pd−H occurs with poor regioselectivity, the conversion of the regioisomer resulting from a 2,1-insertion to product is much faster than conversion of the other regioisomer to product.