In conclusion, we report that 4-chlorobenzoic acid is a stable, general catalyst for the aliphatic Claisen rearrangement to give γ,δunsaturated ketones. Monitoring for catalyst decay was essential in identifying the most viable catalyst. A wide range of allylic alcohols are tolerated as substrates under either open flask conditions or under microwave heating. Table . The calculated free energies at 120 °C for the transition states (TS1_4), (TS4_5) and the final Claisen rearrangement step (TS5_6), and stationary points for selected substrates.