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 [PdACHTUNGTRENUNG(OAc)2] in the range of 0.01–0.1 mol% or palladium-oncarbon (Pd/C) 10% in the range of 1.0–2.0 mol%, most aryl iodides and bromides gave high yields under conventional heating (120 8C) 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% PdACHTUNGTRENUNG(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.