Efficient and practical cross-coupling of arenediazonium tetrafluoroborate salts with boronic acids catalyzed by palladium(0)/barium carbonate

Article abstract of DOI:10.1002/adsc.200800025

The cross-coupling reaction of arenediazonium tetrafluoroborate salts with boronic acids catalyzed by the unusual palladium(0)/barium carbonate catalyst is described as an extremely practical and highly efficient alternative to classical homogeneous conditions. Reactions are conducted under mild conditions at room temperature without any base and ligand. The opportunity of preparing unsymmetrical terphenyls in a one-pot process is also demonstrated. Finally, the power of this methodology is highlighted by the synthesis of Bifenazate.

Full text of DOI:10.1002/adsc.200800025

FULL PAPERS
DOI: 10.1002/adsc.200800025
Efficient and Practical Cross-Coupling of Arenediazonium
Tetrafluoroborate Salts with Boronic Acids Catalyzed by
Palladium(0)/Barium Carbonate
FranÅois-Xavier Felpina,* and Eric Fouqueta
a
UniversitØ Bordeaux 1, Institut des Sciences MolØculaires, CNRS, 351 Cours de la LibØration, 33405 Talence, France
Fax : (+33)-5-4000-6286; e-mail: fx.felpin@ism.u-bordeaux1.fr
Received: January 14, 2008; Published online: March 20, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The cross-coupling reaction of arenediazo- terphenyls in a one-pot process is also demonstrated.
nium tetrafluoroborate salts with boronic acids cata- Finally, the power of this methodology is highlighted
lyzed by the unusual palladium(0)/barium carbonate by the synthesis of Bifenazate.
catalyst is described as an extremely practical and
highly efficient alternative to classical homogeneous
conditions. Reactions are conducted under mild con- Keywords: arenediazonium tetrafluoroborate salts;
ditions at room temperature without any base and bifenazate; boronic acids; palladium(0)/barium car-
ligand. The opportunity of preparing unsymmetrical bonate; practical cross-coupling
Introduction
procedures for the cross-coupling of boronic acids
and arenediazonium tetrafluoroborate salts with
have Pd(0)/C as catalyst.[6,7] The procedure developed
Organometallic-mediated
transformations
gained a pivotal role in synthetic chemistry from ma- allows for extremely fast reactions (few minutes)
terials science to medicine. Among them, palladium- under mild, base-free and ligandless conditions with
catalyzed cross coupling reactions have become ex- an easily removable heterogeneous catalyst. The pro-
tremely popular in laboratories all over the world.[1] tocol features inexpensive reagents and solvents with
In the last years, a high level of sophistication has low toxicity, rendering the method environmentally
been reached with the design of new ligands and pal- benign, very practicable, and scalable. However, a
ladium catalysts, allowing the cross-coupling of an in- fairly high catalyst loading (5 mol%) was required for
credible vast variety of substrates. However, the lack optimum conversions and acceptable yields. More-
of practicality remained one of the major drawbacks over, we also noticed that some diazonium salts, espe-
as exemplified by the use of costly, sensitive, and cially when ortho-substituted or substituted with nitro
sometimes not commercially available ligands or pal- groups, were prone to decomposition and thereby
ladium complexes requiring glove-boxes, inert atmos- gave poor yields of cross-coupled products. With the
phere, and anhydrous solvents.
aim of resolving these issues we report, in this paper,
In this context, the use of easy to handle heteroge- that the Pd(0)/BaCO3 system allows the cross-cou-
neous catalysts emerged as a useful alternative for the pling of arenediazonium salts with boronic acids at
development of metal-catalyzed reactions in an as room temperature in technical grade methanol under
easy as possible way.[2] Obviously, these simplified base- and ligand-free conditions. Moreover, we dem-
protocols usually show lower catalytic activity but still onstrate that our procedure tolerates the lowest cata-
have salient advantages in many cases in terms of cost lyst loading (0.1–2 mol%) reported to date for such a
and waste products.
cross-coupling (i.e., arenediazonium salts with boronic
Over the last few years, we have reported our own acid under ligandless conditions).
contributions to this concept in the context of the
Tsuji–Trost[3] and Suzuki–Miyaura[4] reactions taking
advantage of the useful reactivity of Pd/C[5] as catalyst
under ligandless, non-anhydrous and non-degassed
conditions. Recently, we reported one of the simplest
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FranÅois-Xavier Felpin and Eric Fouquet
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Results and Discussion
noting that Pd(0)/BaCO3 is not a commonly used cat-
ꢀ
alyst for C C bond formation and Pd(0)/CaCO3 has
Optimization of Reaction Conditions
been rarely used for Suzuki–Miyaura,[8] Stille[9] and
Heck[10] reactions. Finally, in terms of safety issues,
Initial studies focused on the cross-coupling of the, Pd(0)/BaCO3 should be much more appropriate for
usually reluctant, 2-nitrobenzenediazonium tetrafluor- large-scale coupling than dry Pd/C, for which the self-
oborate 1 with 4-methoxybenzeneboronic acid 2. The ignition in the presence of methanol and oxygen
challenge presented by this cross-coupling led us to could be a major concern.
select it as our model reaction for the screening of
various sources of inexpensive and commercially
available heterogeneous palladium in technical grade Scope of the Reaction
2-propanol or methanol at room temperature under
air (Table 1).
To further explore the scope of this new process, we
As already observed in our previous studies, Pd(0)/ examined the cross-coupling of a variety of arenedia-
C or Pd(OH)2/C proved to be unproductive for such a zonium salts and boronic acids (Table 2). All arene-
coupling, leading to the formation of large amounts diazonium tetrafluoroborate salts were easily accessed
of undesired nitrobenzene by reduction of the diazo- by reaction of the corresponding anilines with sodium
nium function (entries 1 and 2). Conversely, other nitrite and tetrafluoroboric acid.[11] Importantly, the
supports for palladium (i.e., Al2O3, BaSO4, CaCO3 prepared arenediazonium tetrafluoroborate salts
and BaCO3) were substantially more efficient than showed excellent stability over years when stored at
charcoal at a loading of 5 mol% Pd. Studies to mini- ꢀ208C. Diazonium electrophiles compete favorably
mize the necessary quantity of catalyst further dem- with less reactive and more expensive halogenated
onstrated the high activity of Pd(0)/CaCO3 and Pd(0)/ electrophiles as exemplified by reported industrial ap-
BaCO3 at 1.5 mol% (entries 9 and 12); Pd(0)/BaCO3 plications.[12] As already observed in the optimization
being a slightly more efficient source of Pd in terms studies, all reactions can be conducted at room tem-
of yield. We were even able to work at only 1 mol% perature in technical grade methanol under air irre-
Pd, simply by using MeOH instead of i-PrOH spective of the nature of the coupling partners. For
(entry 14), while a lower loading (0.5 mol% Pd) was each reaction, the catalyst loading has been optimized
deleterious for this cross-coupling although still re- in order to minimize the cost of the process. Although
taining a respectable yield (80%, entry 15). It is worth the catalyst loading is substrate-dependent, it can be
as low as 0.1 mol% Pd for unchallenging partners
(entry 10). For more problematic substrates higher
loadings are required, without exceeding, in any case,
Table 1. Optimization of the catalyst.
2 mol% Pd. This point is particularly relevant since a
detailed survey of the literature results showed that
the cross-coupling of arenediazonium salts with bor-
onic acid under ligandless conditions requires higher
palladium loadings (5 to 10 mol%).[13]
ortho- and even ortho ortho’-disubstitued biphenyls
can be prepared in good yield using the developed
protocol (entries 7 and 9). Significantly, these results
contrast with literature precedents that noticed the
unsuitability of diazonium salts for the preparation of
ortho ortho’-disubstitued biphenyls with good yield-
s.[13e,f]
We have also conducted a close inspection of the
crude reaction mixtures and observed only trace
amounts of side products in the studied cases. This
result contrasts with our method making use of Pd(0)/
C where variable amounts of reduced diazonium salts
were often observed.[6]
Importantly, an excellent level of selectivity is ob-
served for the diazonium group in the cross-coupling
of halogenated arenes (entries 3, 9–12). Not only the
chlorine and bromine atoms are unaffected, but also
the usually very reactive iodine atom. Such a high se-
lectivity towards the diazonium group when an iodine
[a]
Yields are an average of two runs.
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Efficient and Practical Cross-Coupling of Arenediazonium Tetrafluoroborate Salts
FULL PAPERS
Table 2. Cross-coupling studies.
[a]
Yields are an average of two runs.
1.5 equivalents of diazonium salt were used.
[b]
atom is also present has not been reported at a similar lyst, solvent, purification materials and time. It should
level in the literature[13e,14] and, thereby, seems specific be noted that the second cross-coupling reaction pro-
to our newly developed conditions. Thus, it was possi- ceeds smoothly in refluxing methanol in the presence
ble to take advantage of the high chemoselectivity for of aqueous Na2CO3. It should be mentioned that ter-
the diazonium group with the bifunctional 4-iodoben- phenyls are significant targets due to a wide range of
zenediazonium tetrafluoroborate salt 16 to make the significant biological activities including potent immu-
unsymmetrical terphenyl 18 in a single reaction vessel nosuppressant, neuroprotective, antithrombotic, anti-
(Scheme 1). In order to achieve full conversion, 5 coagulant, specific 5-lipoxygenase inhibitory, and cy-
mol% Pd are required. However multiple transforma- totoxic activities.[15] In addition, terphenyl and poly-
tions performed in a one-pot process are economical- phenyl systems are also important structural elements
ly advantageous considering the outlays for the cata- in liquid crystals[16] and fluorescent compounds.[17]
In the light of these results, the practical benefits of
the Pd(0)/BaCO3-catalyzed reaction of arenediazoni-
um salts with boronic acid must be briefly noted. Re-
action are unsensitive to oxygen and moisture and
there is no need to use inert atmosphere or distilled
solvents. Another notable feature of the process is the
robustness of the palladium catalyst used and the ab-
sence of any ligand and base. To date, the only limita-
tion lies in the fact that Pd(0)/BaCO3 cannot be recy-
cled due to the partial decomposition of the support
in the reaction media. Further studies concerning this
Scheme 1. One pot synthesis of terphenyl.
point are underway in order to assess whether Pd(0)/
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FranÅois-Xavier Felpin and Eric Fouquet
FULL PAPERS
tions. These advantages, associated with the low cost
and high stability of Pd(0)/BaCO3, render the method
very practicable and competitive compared to existing
processes.
Experimental Section
Characterization of all compounds is available in the Sup-
porting Information.
General Procedure for the Preparation of Biphenyls
To a solution of diazonium salt (1.2 mmol) in MeOH
(4.8 mL) were added ArB(OH)2 (1 mmol) and 5% Pd(0)/
BaCO3 (0.1–2 mol%, see Table 2). The resulting mixture
was stirred for 12 h at 258C and then concentrated under re-
duced pressure. The crude was purified by flash chromatog-
raphy to give the corresponding cross-coupled product.
4-Methoxy-2’-nitrobiphenyl (3): Purification by flash chro-
matography (5% EtOAc-petroleum ether) gave a yellow
solid.
4-Methoxy-4’-nitrobiphenyl (4): Purification by flash chro-
matography (5% EtOAc-petroleum ether) gave a yellow
solid.
4-Chloro-2’-methoxy-3’-nitrobiphenyl (5): Purification by
flash chromatography (10% EtOAc-petroleum ether) gave a
white solid.
2-Methoxy-4-Nitrobiphenyl (6): Purification by flash chro-
matography (10% EtOAc-petroleum ether) followed by re-
crystallization (Et2O-petroleum ether) gave a pale yellow
solid.
3,4-Dimethoxy-3’-trifluoromethylbiphenyl (7): Purification
by flash chromatography (10% EtOAc-petroleum ether)
gave a pale yellow oil.
2,3-Dimethoxybiphenyl (8): Purification by flash chroma-
tography (10% EtOAc-petroleum ether) gave a colorless
oil.
2,3-Dimethoxy-2’-nitrobiphenyl (9): Purification by flash
chromatography (20% EtOAc-petroleum ether) gave a
yellow solid.
Scheme 2. Synthesis of bifenazate.
BaCO3 could act as a reservoir for soluble and stable
Pd(0) species.[10c]
This methodology has been extended to the synthe-
sis of bifenazate, an effective acaricide against pest
species such as European red mites and two-spotted
spider mites, but with low toxicity to predaceous
mites and other beneficial arthropods.[18] Different
synthetic approaches have been described for the
preparation of bifenazate.[19] Our strategy relies on a
cross-coupling reaction of an appropriate diazonium
salt with phenylboronic acid (Scheme 2). Starting
from inexpensive 4-fluoro-3-nitroaniline 19, aromatic
nucleophilic substitution of the fluorine atom with
sodium methoxide followed by diazotation of aniline
furnished 20 in high yield. Diazonium tetrafluorobo-
rate salt 20 was reacted with phenylboronic acid 15 to
give the cross-coupled product 6 which was reduced
in situ to give aniline 21.
It is important to note that, in this two-step one-pot
transformation (20!21), the dual reactivity of Pd(0)/
ꢀ
BaCO3 (i.e., C C bond formation and reduction) has
been exploited to shorten the overall process. The last
steps were conducted following procedures reported
in the patent literature and, consequently, were not
fully optimized. Transformation of aniline 21 into the
3,4-Methylenedioxy-4’-methoxybiphenyl (10): Purification
by flash chromatography (5% EtOAc-petroleum ether) gave
a white solid.
2-Bromo-2’-methylbiphenyl (11): Purification by flash
chromatography (100% petroleum ether) gave a colorless
oil.
4-Bromobiphenyl (12): Purification by flash chromatogra-
phy (100% petroleum ether) gave a white solid.
4-Bromo-3’-nitrobiphenyl (13): Purification by flash chro-
matography (5% EtOAc-petroleum ether) gave a white
solid.
required hydrazine via the reduction of the corre-
sponding diazonium chloride salt with SnCl2
lowed by carbamatation with isopropyl chlorofor-
[18a]
fol-
ACHTREUNG
steps.
4-Iodobiphenyl (14): Purification by flash chromatography
(100% petroleum ether) gave a white solid.
Conclusions
In summary, we have found that Pd(0)/BaCO3 can ef-
ficiently catalyze the cross-coupling of arenediazoni-
um tetrafluoroborate salts with boronic acids. To our
knowledge it is the first time that Pd(0)/BaCO3 has
been used for such a cross-coupling. The process can
be conducted at room temperature with technical
3,4-Dimethoxy-p-terphenyl (18)
To a solution of 4-iodobenzenediazonium tetrafluoroborate
salt 16 (1.2 mmol) in MeOH (4.8 mL) were added
PhB(OH)2 15 (122 mg, 1 mmol) and Pd/BaCO3 (106 mg, 5
mol%). The reaction mixture was stirred for 8 h at 258C.
grade methanol under base-free and ligandless condi- Then 2M aqueous Na2CO3 solution (1.5 mL) and 3,4-dime-
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Adv. Synth. Catal. 2008, 350, 863 – 868

Efficient and Practical Cross-Coupling of Arenediazonium Tetrafluoroborate Salts
thoxyboronic acid (1.5 mmol) were added dropwise. The re-
FULL PAPERS
Acknowledgements
sulting mixture was stirred for 12 h at 708C. The catalyst
was filtered, washed with CH2Cl2 (2”5 mL) and water
(5 mL). The aqueous layer was extracted with CH2Cl2 (3”),
the collected organic extracts were dried (MgSO4) and con-
centrated under reduced pressure. Purification by flash chro-
matography (50% CH2Cl2-petroleum ether) gave 18 as a
white solid; yield: 239 mg (92%).
The authors greatly appreciated the efficient technical assis-
tance of Mrs. Marie-HØl›ne Lescure (UniversitØ Bordeaux 1)
for GC-MS Analyses. F.-X.F. thanks Mrs. Marion Zanese
(UniversitØ Bordeaux 2) for fruitful discussions. The Univer-
sitØ Bordeaux 1 and the Centre National de la Recherche Sci-
entifique (CNRS) are gratefully acknowledged for financial
support.
4-Methoxy-3-nitroaniline
This compound was prepared according a reported proce-
dure.[20] To a solution of 3-nitro-4-fluoroaniline (1.67 g,
10.7 mmol) in distilled MeOH (11 mL) at room temperature
was added MeONa (2.31 g, 42.8 mmol) and the resulting
mixture was stirred for 20 h at 708C. After cooling to 08C,
the reaction was quenched successively with concentrated
HCl (1.34 mL) and water (25 mL). The aqueous phase was
extracted with Et2O (3”). The collected organic extracts
were dried (MgSO4) and concentrated under reduced pres-
sure to give the title compound as a dark orange solid which
was used without further purification; yield: 1.65 g (92%).
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