PAPER
111
Cross-Coupling of Aryl Halides and Triflates with Group 13-Metal Alkylating
Reagents by Palladium–Molybdenum and Palladium–Tungsten Mixed-Metal
Catalysts
a
b
a
C
M
ross-Coupling by Pal
a
ladium–Mo
r
lybdenu
g
m
and Pallad
a
ium–Tungs
r
ten Cata
i
lysts ta Shenglof, Gary A. Molander, Jochanan Blum*
a
Department of Organic Chemistry, Hebrew University, Jerusalem 91904, Israel
Fax +972(2)6513832; E-mail: jblum@chem.ch.huji.ac.il
Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA
Fax +1(215)5737165; E-mail: gmolandr@sas.upenn.edu
b
Received 27 June 2005
mmol), PPh (0.20 mmol) and 1 (0.05 mmol) for two
hours followed by quenching with 5% hydrochloric acid,
3
Abstract: The palladium-containing clusters {MoPdcp[m-
(
CO) ][m -(CO)](PPh )}
(1)
and
{PdWcp[m-(CO) ][m -
2
3
3
2
2
3
9
8% of 1-methylnaphthalene is isolated. Further exam-
(
CO)](PPh )} (2) catalyze the cross-methylation and ethylation of
3
2
aryl halides and triflates with the respective intramolecularly stabi- ples of cross-coupling reactions catalyzed by 1 as illustrat-
lized dialkylaluminum, gallium and indium reagents 3–7 at temper- ed in Scheme 1 are summarized in Table 1. Similar
atures between 78 °C and 120 °C. Unlike the conventional
mononuclear palladium catalysts, complexes 1 and 2 promote the
coupling of aryl chlorides and show no signs of homocoupling or
hydrogenolysis even when the alkylating moieties contain b-hydro-
gen atoms. The catalytic activity and selectivity of the mixed-metal
clusters is attributed to synergism between the different metal nuclei
of the catalysts.
experiments were conducted in the presence of cluster 2.
Representative results from this study are listed in
Table 2.
(
1) 1 or 2, PR'3, C6H6, 78 °C
ArX + (R2ML)2
Ar + MCl3 + L'
(2) aq HCl
Key words: cross-coupling, mixed-metal catalysts, palladium,
molybdenum, tungsten
Scheme 1 M = Al, Ga, In; L = OCH CH NMe ; L¢ = ligand residue
2
2
2
after hydrolysis; X = halogen or triflate group; R = alkyl group of
reagents 3–7; R¢ = Ph or t-Bu
In the course of our studies on cross-coupling of aryl and
vinyl halides with intramolecularly stabilized group 13-
metal alkylating reagents, we have shown that the mixed-
Tables 1 and 2 indicate a general resemblance between
the activities of 1 and 2 although a few differences, such
as the level of activation of 1-chloronaphthalene (entries
metal complexes [(CO) Fe(m-PPh )Pd(m-Cl)]
and
4
2
2
1
in both Tables) are remarkable. There are also some
[
(CO) Co](m-CO)Pd[m-Ph PCH )] are not only efficient
3
2
2
2
similarities to the mixed Pd–Fe and Pd–Co heteronuclear
catalysts described in our previous paper, but in sharp
and selective catalysts for the cross-alkylation of bromo-
and iodoarenes but, in contrast to the common conven-
tional mononuclear palladium complexes, they affect also
1
contrast, neither 1 nor 2 ever led to the formation of any
hydrogenolysis products of the aryl halides. While bromo-
and iodoarenes, as well as aryl triflate, smoothly couple
with 3 at 78 °C during a few hours, electron-neutral and
electron-rich chloroarenes hardly react under these condi-
tions but require a temperature of 120 °C (see Table 1, en-
tries 1,6 and Table 2, entries 1 and 3). Chloroarenes with
electron-attracting substituents react at 78 °C but require
usually ≥ 20 hours for the completion of the process. The
cross-coupling of aryl triflates depends on the structure of
the aromatic skeleton; while naphthyl triflate reacts rather
fast (e.g., Table 1, entry 5) derivatives of phenol react
slowly even in the presence of electron-attracting substit-
uents (see entries 15 in both Table 1 and Table 2).
1
chloroarenes. We have now found that some other palla-
dium-containing heteronuclear clusters are equally effi-
cient. We investigated two such complexes: (4Mo–
5
Pd)(Pd–Pd)tetra-m-carbonyldi-m -carbonylbis(h -2,4-cy-
3
clopentadien-1-yl)bis[(triphenylphosphine)palladium]di-
molybdenum (1) and (Pd–Pd)(4Pd–W)tetra-m-carbonyldi-
5
m -carbonylbis(h -2,4-cyclopentadien-1-yl)bis[(triphenyl-
3
2
phosphine)palladium]ditungsten (2). These clusters
proved even more stable, easier to handle and more selec-
tive than the aforementioned Pd–Fe and Pd–Co complex-
es. They, and some of their analogs, have already been
shown to act as catalysts in various hydrogen-transfer re-
actions (alkene and alkyne hydrogenation, hydrosilation,
3
The analogous gallium and indium compounds 5–7 can
successfully replace the aluminum-containing alkylating
reagent 3. With the exception of one case in which 4-
IC H CF is used as substrate (Table 2, entry 13) the
hydroformylation and oligomerization), as well as in car-
4
bonylation of nitroarenes. Typically, when a solution of
1
-bromonaphthalene (0.96 mmol) in anhydrous benzene
6
4
3
is heated at reflux with [Me AlOCH CH NMe ] (3; 1.94
2
2
2
2 2
cross-coupling with the gallium reagent takes place only
slightly slower than with 3. Alkylation with the indium re-
agents 6 and 7 proceeds equally well as, or even better
than, the corresponding aluminum compounds, 3 and 4
SYNTHESIS 2006, No. 1, pp 0111–0114
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Advanced online publication: 16.12.2005
DOI: 10.1055/s-2005-921749; Art ID: Z12205SS
©
Georg Thieme Verlag Stuttgart · New York