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Keywords:

  • alcohols;
  • Heck reaction;
  • hydrogen bonds;
  • palladium catalysis;
  • regioselectivity

Abstract

A general method for the regioselective Heck reaction of electron-rich olefins is presented. Fast, highly regioselective Pd-catalysed α-arylation of electron-rich olefins, vinyl ethers (1 a–d), hydroxyl vinyl ethers (1 e, f), enamides (1 g, h) and a substituted vinyl ether (1 i) has been accomplished with a diverse range of aryl bromides (2 a–r), for the first time, in cheap, green and easily available alcohols with no need for any halide scavengers or salt additives. The reaction proceeds with high efficiency, leading exclusively to the α-products, in 2-propanol and particularly in ethylene glycol. In the latter, the arylation can be catalysed at a palladium loading of 0.1 mol % and finish in as short a time as 0.5 h. The remarkable performance of the alcohol solvents in promoting α regiocontrol is attributed to their hydrogen-bond-donating capability, which is believed to facilitate the dissociation of halide anions from PdII, and hence, the generation of a key ionic PdII–olefin intermediate responsible for the α product. This belief is further strengthened by the study of a benchmark arylation reaction in 21 different solvents. The study revealed that exclusive α-regioselective arylation takes place in almost all of the protic solvents, and there is a rough correlation between the α-arylation rates and the solvent parameter ETN. The method is simpler, cleaner and more general than those established thus far.