REGIOSELECTIVE CYCLOCARBOXYLATION OF NONCONJUGATED DIENES TO CYCLIC KETO ESTERS

The carboxylation of 1,5-hexadiene in the presence of (PPh(3))(2)PdCl2 in 1-butanol at 100 degrees C gives butyl 2-(3-methyl-2-oxocyclopenty l)acetate (1) in good yield. Small amounts of linear diesters are also produced. A number of other olefins were also carboxylated to give mi xtures of cyclic and linear products. Although carboxylation of monool efins usually gives a mixture of regioisomers, cyclopentanone 1 is pro duced in >25:1 selectivity over the cyclohexanone regioisomer. Deuteri um labeling and model studies were carried out in an attempt to explai n the unusual regioselectivity observed. These studies suggest that al though the reversible initial insertion of 1,5-hexadiene into Pd-H occ urs with poor regioselectivity, the conversion of the regioisomer resu lting from a 2,1-insertion to product is much faster than conversion o f the other regioisomer to product.