COMMUNICATION
DOI: 10.1002/asia.201000721
Sulfonamidoquinoline/Palladium(II)-Dimer Complex As a Catalyst
Precursor for Palladium-Catalyzed g-Selective and Stereospecific Allyl–Aryl
Coupling Reaction between Allylic Acetates and Arylboronic Acids
Yusuke Makida, Hirohisa Ohmiya,* and Masaya Sawamura*[a]
Dedicated to Professor Eiichi Nakamura on the occasion of his 60th birthday
À
Transition-metal-catalyzed allylic substitution reactions
dium(II) intermediate across the C C double bond of the
allylic acetate followed by syn-b-acyloxy elimination, with
the assistance of intramolecular coordination of the acyloxy
group to the cationic palladium center.[2b] However, the
À
with carbon nucleophiles are powerful carbon carbon bond-
formation methods because of their broad substrate scope
under mild reaction conditions. For example, Tsuji–Trost al-
lylic substitution involving a (p-allyl)metal intermediate has
made impressive progress in this regard.[1] However, the al-
lylic substitution of unsymmetrically substituted allylic sub-
strates occurs competitively at the a- and g-positions owing
to formation of a (p-allyl)metal intermediate: the regiose-
lectivity is highly dependent on the substitution pattern of
the allylic substrates. Therefore, most previous studies in
this area have focused on cases in which the allylic system is
either located at the terminus of a molecule or is highly un-
symmetric because of electronic and/or steric substituent ef-
fects.
allyl–aryl coupling reaction with the PdACTHNUTRGENUG(N OAc)2/phenanthro-
line/AgSbF6 catalytic system required relatively high catalyst
loading to obtain reasonable product yields, presumably
owing to low catalytic activity and short catalyst lifetime, as
catalyst decomposition competed with the formation of the
coupling products. This observation prompted us to investi-
gate the nature of the reaction in more detail and to look
for more-robust and effective catalyst systems.
Herein, we report that a new palladium(II)-dimer catalyst
system involving anionic sulfonamidoquinoline ligands is ef-
fective for the g-selective and stereospecific allyl–aryl cou-
pling reaction between acyclic (E)-allylic acetates and aryl-
boronic acids. Although the catalytic performance of the
new system was only comparable to the previous one, this
system takes advantage of the anionic nature of the support-
ing nitrogen ligand in avoiding the use of a silver salt as a
co-catalyst. A mechanistic model is also proposed based on
a monomeric, uncharged palladium center; this catalytic
cycle is consistent with the previously reported one which
contained a cationic palladium center. Our studies revealed
that the cationic nature of the palladium center of a (s-al-
kyl)palladium(II) complex is not a prerequisite for the pre-
dominance of syn-b-acetoxy elimination over b-hydride
elimination.[8–11]
To address the issue of regiochemical control in the allylic
substitution reaction, we recently developed a new palladi-
um-catalyzed allylic substitution methodology: coupling re-
actions between acyclic (E)-allylic acetates and arylboronic
acids in the presence of a palladium catalyst prepared from
PdACHTUNGTRENNUNG(OAc)2, 1,10-phenanthroline, and AgSbF6 (1:1.2:1) pro-
ceeded with excellent g-selectivity to afford allyl–aryl cou-
pling products with an E configuration.[2–8] From mechanistic
studies, we knew that a (s-aryl)palladium(II) intermediate
with a vacant site was initially generated by a transmetala-
tion reaction between an arylboronic acid and the cationic
mono
ACHTUNGTRENNUNG
the phenanthroline-ligated Pd
AHCTUNGTRENNUNG
subsequent steps involved the addition of the (s-aryl)palla-
In designing a robust catalyst system, we reasoned that re-
placing the neutral nitrogen donors and one acetoxo ligand
in the PdACTHNUTRGENNG(U OAc)2/phenanthroline system with a monoanionic
N,N-bidentate ligand would afford a vacant coordination
site on the palladium center without the need for a silver
[a] Y. Makida, Dr. H. Ohmiya, Prof. Dr. M. Sawamura
Department of Chemistry, Faculty of Science
Hokkaido University, Sapporo, 060-0810 (Japan)
Fax : (+81)11-706-3749
salt: in the previous PdACHTNUTRGNE(UNG OAc)2/phenanthroline/AgSbF6
system,[2] a vacant site was provided by abstracting one of
the acetoxo ligands from the palladium center through the
metathesis with AgSbF6.
Supporting information for this article is available on the WWW
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Chem. Asian J. 2011, 6, 410 – 414