Mechanism of the copper-free palladium-catalyzed Sonagashira reactions: multiple role of amines.
Amines used as bases in copper-free, palladium-catalyzed Sonogashira reactions play a multiple role. The oxidative addition of iodobenzene with [Pd(0)(PPh(3))(4)] is faster when performed in the presence of amines (piperidine>morpholine). Amines also substitute one ligand L in trans-[PdI(Ph)(L)(2)] (L=PPh(3), AsPh(3)) formed in the oxidative addition. This reversible reaction, which gives [PdI(Ph)L(R(2)NH)], is favored in the order AsPh(3)>PPh(3) and piperidine>morpholine. Two mechanisms are proposed for Sonogashira reactions, depending on the ligand and the amine. When L=PPh(3), its substitution by the amine in trans-[PdI(Ph)(PPh(3))(2)] is less favored than that of the alkyne. A mechanism involving prior coordination of the alkyne is suggested, followed by deprotonation of the ligated alkyne by the amine. When L=AsPh(3), its substitution in trans-[PdI(Ph)(AsPh(3))(2)] by the piperidine is easier than that by the alkyne, leading to a different mechanism: substitution of AsPh(3) by the amine is followed by substitution of the second AsPh(3) by the alkyne to generate [PdI(Ph)(amine)(alkyne)]. Deprotonation of the ligated alkyne by an external amine leads to the coupling product. This explains why the catalytic reactions are less efficient with AsPh(3) than with PPh(3) as ligand.
Tougerti
Asma
A
Ecole Normale Supérieure, Département de Chimie, UMR CNRS-ENS-UPMC 8640, 24 Rue Lhomond, 75231 Paris Cedex 5, France.
Negri
Serge
S
Jutand
Anny
A