Heck Reactions with Very Low Ligandless Catalyst Loads Accelerated by Microwaves or Simultaneous Microwaves/Ultrasound Irradiation

Heck couplings were carried out ligandless in air with very low catalyst loads under microwave or simultaneous microwave/ultrasound irradiation. Using ligand‐free palladium(II) acetate [Pd(OAc) 2 ] in the range of 0.01–0.1 mol % or palladium‐on‐carbon (Pd/C) 10 % in the range of 1.0–2.0 mol %, most aryl iodides and bromides gave high yields under conventional heating (120 °C) in 18 h. Microwave irradiation alone or, better still, combined with high‐intensity ultrasound, strongly promotes the reaction, generally decreasing reaction times to 1 h. Electron‐poor aryl chlorides such as 4‐chloroacetophenone and 1‐chloro‐4‐nitrobenzene reacted with styrene to afford high product yields in the presence of 0.25 mol % Pd(OAc) 2 or 2.0–3.0 mol % Pd/C. In several cases the addition of a co‐catalyst, either rhodium tris(triphenylphosphine) chloride, 0.005 mol %, or a copper(I) salt (iodide or bromide), 2.0–4.0 mol %, proved very advantageous. 4‐Bromo‐ and 4‐chloroacetophenone afforded up to 15 % of oxidation products, namely the corresponding 4‐halobenzoic acid and 4‐styrylbenzoic acid, a drawback that was avoided by working under a nitrogen atmosphere.