Treatment of alkenyl and aryl iodides and bromides containing an appropriateα, β-unsaturatedcarbonyl group with a catalytic amount (3–5 mol %) of either a Pd(O) or a Pd(II) complex, e.g., Pd(PPh3)4, and a base, e.g., NEt3, can induce highly regioselective cyclization via intramolecular carbopalladation; formation of exocyclic alkenes can also be highly stereoselective.