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Mechanism of the Copper‐Free Palladium‐Catalyzed Sonagashira Reactions: Multiple Role of Amines
Author(s) -
Tougerti Asma,
Negri Serge,
Jutand Anny
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200600574
Subject(s) - alkyne , chemistry , morpholine , piperidine , deprotonation , sonogashira coupling , amine gas treating , medicinal chemistry , ligand (biochemistry) , oxidative addition , substitution reaction , palladium , catalysis , organic chemistry , ion , biochemistry , receptor
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.