Res. Chem. Intermed.
, Vol. 31, No. 9, pp. 759–767 (2005)
Also available online - www.vsppub.com
Ligand effects on migratory insertion by the Heck reaction
QIAN CHEN, BO-LIN LIN, YAO FU, LEI LIU
and QING-XIANG GUO
Department of Chemistry, University of Science and Technology of China, Hefei 230026, P.R. China
Received 31 May 2004; accepted 2 December 2004
Abstract—Ligand effects on ethene insertion into a series of cationic phenylpalladium complexes
with diverse bidentate phosphine ligands were studied by using the density functional methods. For
the complexes with n-membered ring ligands (n = 4–6), a correlation was found between the ring
size and the insertion barrier. This behavior was explained by considering the P
Pd P bond angle.
In the case of complexes with ligands of different rigidity, almost no difference was found for the
insertion barriers. Furthermore, the bidentate phosphine ligand was systematically substituted by Me,
t-Bu, F and Ph groups. It was found that the electron-donating substituents increased the insertion
barrier, whereas the electron-withdrawing groups decreased it. The substantial increase of insertion
barrier by the t-Bu group indicated that steric effect also had great effect on the migratory insertion.
Keywords: Heck reaction; migratory insertion; palladium; catalysis; ligand effects.
The arylation of oleﬁns catalyzed by transition metals is called the Heck reaction
[1, 2] (Scheme 1). It is one of the most powerful tools for organic chemists to
C bonds [3– 5]. Generally, the Heck reaction is catalyzed by a Pd(0)
species stabilized by phosphine ligands. The classical mechanism of the Heck
reaction is composed of four steps: (1) oxidative addition of Pd(0) to the C
bond; (2) migratory insertion of alkene into the Pd
aryl bond; (3) elimination of
β-hydrogen and release of the cross-coupling product; (4) regeneration of the
catalyst in the presence of base.
Although chelating bidentate phosphines were believed to be inappropriate lig-
ands for the Heck reactions in the early days, a recent discovery demonstrated that
bidentate phosphines could perform equally well as monodentate phosphines [6–8].
Compared to monodentate phosphine, bidentate phosphine-assisted Pd(0) catalysts
could switch the neutral mechanism to the cationic mechanism and achieve different
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