Mechanism of the copper-free palladium-catalyzed Sonagashira reactions: multiple role of amines |
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Authors: | Tougerti Asma Negri Serge Jutand Anny |
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Institution: | Ecole Normale Supérieure, Département de Chimie, UMR CNRS-ENS-UPMC 8640, 24 Rue Lhomond, 75231 Paris Cedex 5, France. |
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Abstract: | 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. |
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Keywords: | alkynes amines palladium reaction mechanisms Sonogashira reaction |
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