Homogeneous and heterogenized Au(III) Schiff base-complexes as selective and general catalysts for self-coupling of aryl boronic acids

Article abstract of DOI:10.1039/b418897g

A series of homogeneous and heterogenized gold metal complexes show high activity and selectivity for the homo-coupling of a large variety of aryl boronic acids, being of general utility for the synthesis of C ₂-symmetric biaryls. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b418897g

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Homogeneous and heterogenized Au(III) Schiff base-complexes as
selective and general catalysts for self-coupling of aryl boronic acids{
C. Gonza´lez-Arellano,ac A. Corma,*b M. Iglesiasc and F. Sa´ncheza
Received (in Cambridge, UK) 16th December 2004, Accepted 3rd February 2005
First published as an Advance Article on the web 18th February 2005
DOI: 10.1039/b418897g
The preparation of immobilized ligands and complexes involves
the anchoring of precursors bearing triethoxysilyl groups onto
pure mesoporous silica8 and on a delaminated zeolitic material
(ITQ-2), as shown in Scheme 1.
A series of homogeneous and heterogenized gold metal
complexes show high activity and selectivity for the homo-
coupling of a large variety of aryl boronic acids, being of
general utility for the synthesis of C2-symmetric biaryls.
The organically modified8 silica based supports, i.e., MCM-419
and delaminated zeolitic materials,10 were prepared in the
following way: A solution of 2 mmol of 5-azidomethyl-3-tert-
butyl-2-hydroxybenzaldehyde in toluene (5 ml) was added to a
suspension of the siliceous support (2 g) in toluene (25 ml), and
stirred at room temperature for 30 min. The slurry was heated at
100 uC for 16 h and the yellow solid was filtered off and washed
successively with toluene, petroleum ether, ethanol and ether.
Then, 1 mmol of the anchored hydroxyaldehyde 2–(Sil, MCM-41,
ITQ-2) was suspended in ethanol (10 ml) under Ar at room
temperature. After 10 min, 125 mg (1.1 mmol) of the amine were
added and stirred for 16 h. The deep yellow solid was filtered and
washed thoroughly with ethanol and then with ether. The solid
was dried under vacuum to afford the corresponding anchored
ligands [ligand–(support)]. The heterogenized complexes were
synthesized as follows. An ethanolic solution of HAuCl4
(0.5 mmol, 15 ml) was added to a toluene (20 ml) suspension of
heterogenized ligand at room temperature. The resulting mixture
was stirred under reflux for 12 h, cooled to room temperature and
filtered. The solid was washed several times with ethanol, dried
and filtered to afford the respective heterogenized complexes,
complex–(support), in almost quantitative yields. Supported
complexes are characterized by their spectroscopic and analytical
Coupling of aryl boronic acids and esters with organic halides is an
excellent method for preparing symmetrical and unsymmetrical
biaryls, which are important building blocks in natural products
and in materials sciences.1 Biaryls exhibit a wide variety of physical
and chemical properties2 with versatile applications in pharma-
ceuticals, polymers, nonlinear optics,3 liquid crystals,4 and optically
active ligands.5 The diverse applications of these molecules have
recently led to a number of transition metal catalyzed approaches
to yield the biaryl moiety. Suzuki coupling has become increasingly
popular due to its compatibility with a variety of functional
groups, the stability of the organoboron precursors, and the ease
of working up the reaction mixture. As such, a wide variety of aryl
boronic acids are available commercially. Self-coupling of aryl
groups of the aryl boronic acids is very slow.6 We have observed
that selective formation of self-coupling products occur under
Suzuki-catalyzed reaction conditions when using gold(III), and
therefore our attention has been directed towards the application
of the self-coupling reaction as a synthetic method for allowing a
wide variety of C2-symmetric biaryls.7 Selective homocoupling
with halogenated aryl boronic acids represents a new way for the
synthesis of halide biaryls difficult to prepare by typical Pd-Suzuki
catalyzed reaction.
Herein we present a very easy and widely applicable approach
using Au(III)-complexes and supported Au(III)-complexes on silica
based mesoporous and laminar inorganic solids for the homo-
coupling of aryl boronic acids.
Metal complexes were prepared under dinitrogen by conven-
tional Schlenk-tube techniques, and solvents were carefully
degassed before use. Then, in the case of gold complexes, an
ethanolic solution of anhydrous HAuCl4 (97 mg, 0.27 mmol) was
added to a solution of the ligand in EtOH (30 ml) at room
temperature. The resultant mixture was stirred under reflux for
10 h, cooled to room temperature and then concentrated under
vacuum. The residue was washed several times with diethyl ether,
dried and filtered to afford the respective complexes in almost
quantitative yields. Complexes are characterized by their analytical
and spectroscopic data, their stoichiometry is confirmed by ESI-
mass spectrometry that shows ion molecular peaks.{
{ Electronic supplementary information (ESI) available: experimental
data, analytic and spectroscopic methods and results. See http://
www.rsc.org/suppdata/cc/b4/b418897g/
*acorma@itq.upv.es
Scheme 1 Synthesis of soluble and heterogenized Au-complexes.
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data, ICP-analysis showed a metal content of 0.30–0.60 mmol g21
;
with TON 53.3–66.0 mol converted per mol gold. The results for
complex 7Au and its corresponding heterogenized 7Au–(MCM-41)
are given, as an example, in Table 1.
these values were used in catalytic experiments.{ In the IR spectro-
scopic characterization, the peaks due to the support dominate
the spectra of the heterogenized catalyst. Nevertheless, some of the
bands characteristic of the complexes could be distinguished. The
1600 cm21 band can be assigned to CLC and azomethine CLN
vibrations, shifted to lower wavenumbers (relative to the free
ligands) due to N2 coordination of the imine. Bands in the 500–
600 cm21 region are present and ascribed to d(M–O).
The effect of electron donating and electron withdrawing
substituents on conversion and yields was studied, and it can be
concluded that, in general, electron withdrawing or electron
donating substituents did not significantly change the yield of the
substituted biphenyl. Furthermore, no significant steric effect was
exhibited for the aryl boronic acids tested.
The DFTR spectra for all complexes were obtained in the 200–
800 nm range. The complexes show several band maxima in the
UV region which agree with the assignment of the bands as
intraligand p A p*, n A p* transitions in the aromatic ring,
azomethine group and charge-transfer transitions. The bands in
the 400–500 nm region correspond to d–d transitions expected
for planar complexes11 and MLCT bands. The diffuse reflectance
spectra of M–(ligand) complexes are almost identical before and
after heterogenization, indicating that the complexes maintain their
geometry and electronic characteristics after heterogeneization
without significant distortion.
In an additional experiment phenyl boronic acid was reacted
with 4-bromophenyl boronic acid under the same conditions
than above, and the results showed a ratio (1 : 1 : 0.5) for
phenyl–phenyl, 4-bromophenyl–4-bromophenyl and phenyl–4-
bromophenyl products respectively indicating that the homo-
coupling products were obtained in the same ratio (1 : 1) and
the aryl boronic cross-coupling product is obtained as a minor
product.
We have also examined the prepared 7Au complexes (homo-
geneous and heterogenized) catalyzed reaction for the typical
Suzuki conditions between BrPh and p-BrPhB(OH)2, and the
expected cross-coupling product was not formed while the aryl
boronic homocoupling compound was obtained as the exclusive
product (Table 2). The same results, i.e., 100% selectivity towards
the homocoupling product were obtained when the reaction was
carried out in air. We have investigated the general utility of
the catalyst and reaction conditions with a variety of aryl boronic
acid substrates (aryl 5 phenyl, 4-BrPh, 4-OMePh, 4-MePh)
and aryl halides (BrPh, IPh, 1-bromonaphthalene, 1-bromo-4-
methoxybenzene) and we have obtained 100% homocoupling
selectivity for all tested reactions.
Diamagnetic gold complexes have also been characterized by
13C NMR spectroscopy. In all cases, the spectra show the
simultaneous occurrence of two sets of signals which can be related
on one hand to the substituted benzaldimine entity, and on the
other hand to the aliphatic part of the ligand. The 13C NMR
spectra showed the signals assigned to CLN carbon highfield
shifted and C1 at d y162 ppm downfield shifted confirming that
metallation has occurred.
The FTIR, DFTR and 13C NMR spectra for complex 7Au–
MCM-41 are given as an example in supplementary material.
The soluble gold complexes and complexes bound on MCM-41
and delaminated zeolites have been tested in the homocoupling
reaction between aryl boronic acids, using o-xylene as solvent. All
prepared catalysts give high yields of the homocoupling reaction
It has to be remarked that when the corresponding homo-
geneous palladium catalysts bound on MCM-41 and delaminated
zeolites have been tested for the same Suzuki reactions, under
the same reaction conditions, the results (Table 2) show that
Table 1 7Au and 7Au–(MCM-41)-catalyzed homocoupling of selected reaction partnersa
Yield (%)b
Product
7Au
7Au–(MCM-41)
6Au
6Au–(MCM-41)
2Au
2Au–(MCM-41)
93
97
97
99
95
96
85
81
80
96
94
94
97
96
95
97
97
95
92
92
90
92
92
91
a
b
Conditions: aryl boronic acid (10 mmol.), gold catalyst (0.3 equiv.), and K2CO3 (20 mmol) in xylene at 130 uC. Yields were determined
from isolated samples of the reaction after 24 h.
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Table 2 Summary for all prepared Au(III)- and Pd(II)-complex
catalyzed cross-coupling at Suzuki’s reaction conditionsa
Aub
Pdb
Ar
X Ar9
A(%) B(%) C(%) A(%) B(%) C(%)
Ph
Ph
Br Ph
Ph
90–92 0
.90
95–99 0
3–5
0
0
0
85–90 traces
65–70 0
I
0
4-OMePh Br Ph
Naphth
traces 5–10 40–50 0
Br Ph
91–98 5–10 0 2–5 45–50 5–8
Ph
Ph
Ph
Ph
a
Br 4-MePh 91–95 0
4-MePh .90
Br 4-OMePh 90–95 0
Br 4-BrPh 90–93 0
traces 0
50–60 traces
40–50 traces
50–60 0
I
0
0
0
traces 0
traces 20
60–65 0
Conditions: aryl boronic acid (10 mmol.), aryl halide (15 mmol),
Au or Pd catalyst (0.3 equiv.), and K2CO3 (20 mmol) in xylene
at 130 uC. Yields were determined, for various catalysts, by GC
Scheme 2 Tentative mechanism for self-coupling of aryl boronic acids
with Au complex.
b
analysis of the crude reaction mixtures at 24 h. Range of yields for
C. Gonza´lez-Arellano,ac A. Corma,*b M. Iglesiasc and F. Sa´ncheza
aInstituto de Qu´ımica Orga´nica General, CSIC, C/ Juan de la Cierva 3,
28006 Madrid, Spain. E-mail: felix-iqo@iqog.csic.es;
Fax: +34915644872
all catalysts in each reaction.
bInstituto de Tecnolog´ıa Qu´ımica, UPV-CSIC, Universidad Polite´cnica
de Valencia, Avda. de los Naranjos s/n, 46022 Valencia, Spain.
E-mail: acorma@itq.upv.es; Fax: +34963877809
Pd-catalysts are highly selective towards cross-coupling reactions,
and only when the reaction was very slow, some homocoupling
reaction between boronic acids also occurred. It is worth
mentioning that under the same reaction conditions and with the
same amount of gold but in the form of AuHCl4 only 12% yield
for homocoupling was observed after 24 h. At present we can only
speculate on the mechanistic differences between Au and Pd
catalysts. Nevertheless, it is well accepted12,13 that Suzuki cross-
coupling involves an oxidative addition of phenyl halide to the Pd
as the first step. Since the Au complex is unable to perform the
cross-coupling, but produces the homocoupling instead, we may
assume in a first approximation that the oxidative addition does
not occur on gold and the homocoupling may occur through
an ‘‘aromatic transmetallation’’ from the boron to the gold; in a
second transmetallation the gold complex will bind a second
aromatic ring which finally will give biphenyl by reductive
elimination while regenerating the original Au complex through
ligand rearrangement In Scheme 2 we have included this tentative
mechanism.
cInstituto de Ciencia de Materiales de Madrid, CSIC, C/ Sor Juana Ine´s
de la Cruz, s/n Cantoblanco, 28049 Madrid, Spain.
E-mail: marta.iglesias@icmm.csic.es; Fax: +34913720623
Notes and references
1 (a) J. Hassan, M. Se´vignon, Ch. Gozzi, E. Schulz and M. Lemaire,
Chem. Rev., 2002, 102, 1359; (b) A. Suzuki, Metal-Catalysed Cross-
Coupling Reactions, in Cross-Coupling Reaction of Organoboron
Compounds with Organic Halides, ed. F. Diederich and P.T. Stang,
Wiley-VCH, Weinheim, 1998, pp. 49–97 and references therein.
2 (a) D. J. Koza and E. Carita, Synthesis, 2002, 2183; (b) G. Bringmann,
R. Walter and R. Weirich, Angew. Chem., Int. Ed. Eng., 1990, 29, 977.
3 Non Linear Optical Properties of Organic Molecules and Crystals, ed.
D. S. ChemLa and J. Zyss, Academic Press, Orlando, 1987.
4 G. W. Gray, in Molecular Structures and the Properties of Liquid
Crystals, Academic Press, London, New York, 1962.
5 C. Rosini, L. Franzini, A. Rafael and P. Dalvadori, Synthesis, 1992, 503.
6 (a) E. M. Campi, W. R. Jackson and S. M. Marcuccio, J. Chem. Soc.,
Chem. Commun., 1994, 2395; (b) T. Gillmann and T. Weeber, Synlett,
1994, 649; (c) Z. Z. Song and H. N. C. Wong, J. Org. Chem., 1994, 59,
33.
7 D. J. Koza and E. Carita, Synthesis, 2002, 2183.
8 C. Gonza´lez-Arellano, A. Corma, M. Iglesias and F. Sa´nchez, Adv.
Synth. Catal., 2004, 346, 1316.
9 J. S. Beck, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt,
C. T.-W. Chu, K. H. Olson, E. Sheppard, S. B. McCullen, J. B. Higgins
and J. L. Schlenk, J. Am. Chem. Soc., 1992, 114, 10834.
10 (a) A. Corma, V. Forne´s and S. B. Pergher, Nature, 1998, 396, 353; (b)
A. Corma, U. Diaz, M. E. Domine and V. Forne´s, Angew. Chem., Int.
Ed., 2000, 39, 1499.
In summary we have obtained selectively a gold-catalyzed aryl
boronic acid homocoupling reaction using a protocol similar to the
well documented cross-coupling reaction. The general conditions
used here are applicable to a variety of substituted boronic acids.
This method has a number of advantages including the mild
reaction conditions, easily isolable products, and characteristic
yields associated with the homocoupling versus cross-coupling
reaction.
11 A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam,
1984.
12 A. de Meijere and F. Diederich, Metal-Catalyzed Cross-Coupling
Reactions, Wiley-VCH, Weinheim, 2004, vol. 1–2, .
13 G. C. Fu and A. F. Littke, Angew. Chem., Int. Ed., 2002, 41, 4176–4211.
Financial support by the Direccio´n General de Investigacio´n
Cient´ıfica y Te´cnica of Spain (Project MAT2003-07945-C02-02)
and Auricat EU-Network (HPRN-CT-2002-00174) is gratefully
acknowledged.
1992 | Chem. Commun., 2005, 1990–1992
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