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RESEARCH PRODUCT

Influence of the Ligand of Palladium(0) Complexes on the Rate of the Oxidative Addition of Aryl and Activated Alkyl Bromides: Csp2 −Br versus Csp3 −Br Reactivity and Selectivity

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Kinetic data by means of electrochemical techniques are used to characterize the reactivity of aryl bromides and activated alkyl bromides in oxidative addition to palladium(0) complexes generated from three precursors: Pd0(PPh3)4, {Pd0(dba)2 + 2 PPh3} and {Pd0(dba)2 + 2 P(o Tol)3} in DMF at 25 °C. It is established that, for the investigated substrates 1-6 and 7-11, the oxidative addition at the Csp3 Br bond is much faster than that at the Csp2 Br bond when the palladium(0) is ligated by two PPh3. This explains why the regioselectivity in Suzuki-Miyaura reactions performed from substrates bearing both Csp2 Br and Csp3 Br groups is in favour of the substitution at the Csp3 Br bond. It is in agreement with DFT calculations which propose a SN2-type mechanism from the bis-ligated Pd0(PPh3)2. The oxidative addition at the Csp2 Br bond is much faster when the palladium(0) is ligated by P(o-Tol)3 than by PPh3 in agreement with the DFT calculations which support a concerted mechanism involving the mono-ligated Pd0[P(o-Tol)3] as the reactive species. This explains why the regioselectivity in Suzuki-Miyaura reactions performed from substrates bearing both Csp2 Br and Csp3 Br groups might be in favour of the substitution at Csp2 Br bond when the ligand is P(o-Tol)3.