A novel transmetallation of arylzinc species into arylboronates from aryl halides in a barbier procedure

Article abstract of DOI:10.1039/b707056j

A variety of functionalized arylboronates are obtained in moderate to excellent yield by a one-step chemical procedure from the corresponding halides and a haloboronic ester via an intermediate arylzinc species. The Royal Society of Chemistry.

Full text of DOI:10.1039/b707056j

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A novel transmetallation of arylzinc species into arylboronates from
aryl halides in a barbier procedure{
Ste´phanie Claudel,a Corinne Gosmini,{*a Jean Marc Parisb and Jacques Pe´richona
Received (in Cambridge, UK) 10th May 2007, Accepted 14th June 2007
First published as an Advance Article on the web 26th June 2007
DOI: 10.1039/b707056j
with high temperature. Furthermore, transition metal-catalyzed
direct borylation of aromatic rings has been achieved using
expensive Re, Rh, Ir or other catalysis.6 In addition to these
methods utilizing aromatic precursors, benzannulation or cycload-
dition involving unsaturated organoboron reagents has allowed
the assembly of highly substituted arylboronic acid frameworks.7
During the course of our work, based on the cobalt-catalyzed
formation of organometallic compounds, we have found a new
method for the preparation of functionalized arylboronic esters
from the corresponding arylzinc species.8 Contrary to arylboronate
esters, arylzinc species can neither be stored for a long time nor
isolated because of their instability. Recently, we have established
that arylzinc compounds9 are convenient for the facile preparation
of arylstannanes from the corresponding arylbromides.10 In this
case, transmetallation between the arylzinc species, formed in the
medium, and tributylstannyl chloride occurs, as described in
the literature. However, the toxicity of organotin reagents limits
the use of such compounds. Unlike these tin-containing com-
pounds, boronic derivatives present a lower toxicity, but to our
knowledge, there are no reports in the literature concerning
transmetallation between an organozinc species and a boron
compound. Conversely, organozinc compounds are prepared via a
boron–zinc exchange reaction.11 We supposed that the use of a
halogenocatecholborane might transmetallate arylzinc com-
pounds. Therefore, a coupling reaction of arylzinc species, formed
via cobalt catalysis, with a halogenocatecholborane was explored.
Herein, we wish to disclose our results. Initially, we verified that
the two-step coupling reaction of arylzinc bromides and chloro-
catecholborane led to the corresponding arylboronic ester at room
temperature without the further addition of a catalyst (eqn. 1).
A variety of functionalized arylboronates are obtained in
moderate to excellent yield by a one-step chemical procedure
from the corresponding halides and a haloboronic ester via an
intermediate arylzinc species.
Arylboronic acids and their derivatives have become very
attractive reagents in modern organic synthesis,1 and they are
used in a wide variety of significant organic transformations. In
fact, the availability of these reagents and their mild reaction
conditions both contribute to the versatility of their reactions. For
example, the Suzuki–Miyaura palladium-catalyzed cross-coupling
reaction between organoboron compounds and organic halides or
triflates has become a cornerstone of organic synthesis, providing a
carbon–carbon bond-forming reaction.2 The key advantages of
boronic derivatives over other organometallic derivatives are the
commercial availability of diverse boronic acids, their wide ranging
functional group tolerance, and the low toxicity of the reagents
and their by-products, especially compared to tin-containing
compounds. Arylboronates are playing an increasing prominent
role in Suzuki–Miyaura cross-coupling reactions as important
alternatives to arylboronic acids, particularly when the correspond-
ing boronic acids are not readily available, as they are as easy to
handle as boronic acids. They also continue to attract attention as
versatile functional group-tolerant cross-coupling substrates in
organic synthesis. However, the difficulty of accessing such
compounds constitutes their main drawback. Different approaches
to their preparation have been reported in the literature. Generally,
they are conventionally prepared from a reaction of arylmagne-
sium or -lithium reagents with boron halides or trialkylborates.3
This approach is limited in scope because of its incompatibility
with sensitive functionalities. To avoid this drawback, alternative
synthetic methodologies, allowing the direct transition metal-
catalyzed coupling of aryl halides, triflates or aryldiazonium
tetrafluoroborate salts with tetraalkoxydiboranes4 by the cleavage
of the B–B bond or the more readily available dialkoxyboranes,5
have been successfully performed to afford arylboronic esters.
Generally, these methodologies require the use of a base associated
ð1Þ
The behaviour of our arylzinc compounds, synthesized via a
cobalt catalysis in acetonitrile, towards a halogenocatecholborane
is similar to that of Bu3SnCl. For example, p-MeOC6H4B-catechol
(70% GC yield) and p-EtOCO-C6H4B-catechol (80% GC yield)
have been readily synthesized in a two-step sequence from the
corresponding aryl bromides and chlorocatecholborane using the
reaction conditions detailed elsewhere for the preparation of
ArZnBr: 0.1 equiv. of CoBr2, 1.5 equiv. of zinc dust activated by
50 ml of trifluoroacetic acid and a 1.5 M aryl bromide solution.
Prior to the addition of aryl bromide in acetonitrile, a preliminary
step of 5 min stirring in the presence of 0.3 equiv. of allyl chloride
(3 equiv. vs. cobalt) is required in order to enhance the yield of the
organozinc species and to decrease the formation of by-products,
especially ArH. Once the organozinc compound has formed,
aLaboratoire d’Electrochimie, Catalyse et Synthe`se Organique, UMR
7582, Universite´ Paris 12-CNRS, 2 Rue Henri Dunant, 94320 Thiais,
France
bRhodia, Research Center of Lyon, 85 rue des Fre`res perret, BP 62,
69192 saint fons Cedex, France
{ Electronic supplementary information (ESI) available: Synthetic
methods and characterisation data. See DOI: 10.1039/b707056j
{ Corresponding author’s present address: De´partement de Chimie,
Laboratoire ‘‘He´te´roe´le´ments et Coordination’’, UMR CNRS 7653,
Ecole Polytechnique, 91128 Palaiseau Cedex, France. E-mail: corinne.
gosmini@polytechnique.edu; Fax: +33 1 69 33 39 90; Tel: +33 1 69 33 45
76
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1 equiv. of chlorocatecholborane is added to the solution at room
temperature and the mixture stirred for 30 min. Hydrolysis with
NH4Cl, followed by diethyl ether extraction, gives the crude
arylboronic ester in good yields. As reported in the case of
arylstannanes, it seemed interesting to develop a convenient
method for the one-step synthesis of arylboronic esters from aryl
bromides or iodides. This procedure follows the protocol already
described, but after the usual preliminary stirring in the presence of
0.3 equiv. of allyl chloride, the aryl halide (1 equiv.) and
halogenocatecholborane are simultaneously added in acetonitrile
at room temperature. The nature of the halogenocatecholborane
depends on the substituent on aromatic ring. The mixture is stirred
until total consumption of the aryl bromide has taken place (in
about 30 min). The corresponding arylboronic ester is rapidly
detected by GC using an internal standard (alkane). In several
instances, the arylboronate products were accompanied by the
formation of a small amount of the reduction product in the
reaction mixture. An inert atmosphere is not required.
Consequently, we applied this method to various aromatic
bromides or iodides bearing electron-donating or -withdrawing
groups in the para/meta/ortho series. With an electron-withdrawing
group on the aromatic ring, the aryl halide is more reactive with
low valency cobalt, and the corresponding arylzinc species is less
nucleophilic than an electron-donating group substituent.
Therefore, a more reactive halogenocatecholborane, such as
bromocatecholborane, has to be used in the one-step coupling.
Moreover, the resulting catechol esters are sensitive to moisture
and chromatography, whereas the corresponding inert pinacol
esters are more stable.5a This is the reason why the quantitative
transesterification with pinacol was performed at room tempera-
ture in 30 min (eqn. 2). On the basis of this result, subsequent
experiments with a number of representative aryl halides bearing
an electron-withdrawing group were carried out to give the
corresponding products. The results are summarized in Table 1.
small amounts. Satisfactory-to-excellent yields are observed when
the electron-withdrawing substituent is in para or meta position.
However, it can be pointed out that substituents in the ortho
position have an important influence on the yields, as revealed in
the case of ethyl bromobenzoate (Table 1, entries 1, 2 and 3) and
bromobenzonitrile (Table 1, entries 7 and 8). This reflects steric
and chelation effects.
In the case where an aromatic ring is substituted by an electron-
donating group, the chlorocatecholborane is sufficiently reactive
for the transmetallation to occur with the corresponding
intermediate arylzinc species (eqn. 3). The results are reported in
Table 2.
ð3Þ
Again, satisfactory yields were observed, and the ortho position
was less compatible with this process (Table 2, entry 2).
In order to confirm that cobalt salts were not involved in
transmetallation between the organozinc species and the halogen-
ocatecholborane, benzylzinc bromide was prepared in acetonitrile
in the absence of a cobalt catalyst. The organozinc species thus
obtained was allowed to react with chlorocatecholborane at room
temperature, giving rise almost quantitatively to the benzylboro-
nate ester without supplementary cobalt catalysis.
The standard conditions developed for aromatic halides bearing
either electron-donating or -withdrawing groups have also been
applied to the conversion of heteroaromatic bromides into the
corresponding boronate esters. The resulting products have been
isolated and show that the preceding method affords
3-thienylboronate esters from their corresponding 3-bromothio-
phene in good yield (75% isolated). With a more reactive
bromothiophene, such as 2-bromothiophene, the corresponding
thienylboronate is detected in small amounts (18%). 2-Thienyl
bromide gives a large amount of the reduction product.
Unfortunately, results were disappointing with bromopyridines.
As already described for organozinc species formed by cobalt
catalysis in acetonitrile, no organoboronate ester could be detected
from 2- or 3-bromopyridine. These results were not surprising.
Indeed, the synthesis of 2-pyridyl organometallics remains a
significant problem, probably due to their frequent instability and
difficult synthesis.12
ð2Þ
These results show that this original method gives the expected
arylboronates bearing an electron-withdrawing group. In all cases,
the only by-product is that from reduction, contrary to the two-
step procedure where the homocoupling product was detected in
In conclusion, we have developed a new method to prepare
arylboronate esters from the corresponding aryl iodide or bromide
via an intermediate arylzinc species. To the best of our knowledge,
we have shown for the first time that transmetallation between an
arylzinc species and a halogenoborane occurs without a transition
Table 1 One-step synthesis at room temperature of arylboronates
bearing an electron-withdrawing group from ArBr
Entry
Aryl halide
Isolated yield of ArBP (%)a GC yield (%)
1
2
3
4
5
6
7
8
9
2-EtOCOPhBr
3-EtOCOPhBr
4-EtOCOPhBr
4-EtOCOPhI
4-F3CPhBr
4-MeOCOPhBr
2-NCPhBr
4-NCPhBr
47
95
90
65
48
69
5
52
51
50
71
95
90
65
61
80
33
79
88
75
Table 2 One-step synthesis of arylboronates from ArBr bearing an
electron-donating group at room temperature
Entry Aryl bromide Isolated yield of ArBP (%)a GC yield (%)
1
2
3
4
4-MeOPhBr
2-MeOPhBr
4-MeSPhBr
4-(Me)2NPhB
71
45
56
43
80
60
60
76
4-ClPhBr
4-MeCOPhBr
10
a
Based on initial ArX. All products gave satisfactory 1H, 13C NMR
and mass spectra.
Based on initial ArX. All products gave satisfactory 1H, 13C NMR
and mass spectra.
a
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