Decarboxylative Suzuki-Miyaura coupling of (hetero)aromatic carboxylic acids using iodine as the terminal oxidant

Article abstract of DOI:10.1039/c9cc01817d

A novel methodology for the decarboxylative Suzuki-Miyaura-type coupling has been established. This process uses iodine or a bromine source as both the decarboxylation mediator and the terminal oxidant, thus avoiding the need for stoichiometric amounts of transition metal salts previously required. Our new protocol allows for the construction of valuable biaryl architectures through the coupling of (hetero)aromatic carboxylic acids with arylboronic acids. The scope of this decarboxylative Suzuki reaction has been greatly diversified, allowing for previously inaccessible non-ortho-substituted aromatic acids to undergo this transformation. The procedure also benefits from low catalyst loadings and the absence of stoichiometric transition metal additives.

Full text of DOI:10.1039/c9cc01817d

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Decarboxylative Suzuki–Miyaura coupling of
(hetero)aromatic carboxylic acids using iodine
Cite this:DOI: 10.1039/c9cc01817d
as the terminal oxidant†
Received 18th March 2019,
Accepted 7th May 2019
Jacob M. Quibell, ‡a Guojian Duan,‡ab Gregory J. P. Perry
and
a
a
Igor Larrosa
*
DOI: 10.1039/c9cc01817d
rsc.li/chemcomm
A novel methodology for the decarboxylative Suzuki–Miyaura-type of biaryls. However, despite the merits of decarboxylative couplings,
coupling has been established. This process uses iodine or a bromine most procedures come with some limitations. For example, ortho-
source as both the decarboxylation mediator and the terminal oxidant, substituents and high temperatures are often necessary to ensure
thus avoiding the need for stoichiometric amounts of transition metal efficient reactivity and stoichiometric transition metal additives,
salts previously required. Our new protocol allows for the construction such as Cu- or Ag-salts, are required in many transformations.11,13
of valuable biaryl architectures through the coupling of (hetero)- Furthermore, despite the popularity of organoboronic acids as
aromatic carboxylic acids with arylboronic acids. The scope of this coupling partners in cross-coupling reactions, only a handful of
decarboxylative Suzuki reaction has been greatly diversified, allowing for examples of decarboxylative Suzuki–Miyaura couplings have been
previously inaccessible non-ortho-substituted aromatic acids to undergo reported to date (Scheme 1a).15,16 Furthermore, these methods
this transformation. The procedure also benefits from low catalyst suffer from extremely poor substrate scope (only ortho-substituted
loadings and the absence of stoichiometric transition metal additives.
benzoic acids and often only 2,6-dimethoxy benzoic acid reacts
efficiently), the need for high catalyst loadings (10–20 mol% of Pd),
Over the last few decades, transition metal catalysed cross-coupling and super-stoichiometric silver salt oxidants (2–3 equiv.). Given the
chemistry has redefined chemists’ approaches to target synthesis.1 wide availability of aryl boronic acids, a general methodology for
Among all the methods developed, the Suzuki–Miyaura reaction, decarboxylative Suzuki–Miyaura couplings using low Pd-catalyst
coupling an organohalide with an organoboronic acid, has loadings and avoiding stoichiometric transition metal oxidants
undoubtedly proved the most applicable in organic synthesis, would be highly desirable.
as demonstrated by its wide use in the production of pharma-
We have recently reported a novel approach to decarboxylative
ceuticals, natural products and materials.2 The synthetic utility activation that uses iodine and bromine sources to mediate a decarb-
of aryl boronic acids has led to the development of a wide variety oxylative iodination and bromination, respectively (Scheme 1b).17,18
of methodologies for their synthesis,3 including borylation of
organo(pseudo)halides through transition metal catalysed4 or
lithiation5 procedures, hydroboration,6 direct C–H borylation7
and more recently decarboxylative borylation.8
Over the last decade, kick-started by the pioneering work of
Gooßen,9 decarboxylative activation has paved the way for the
use of readily available (hetero)aromatic carboxylic acids as coupling
partners in cross-coupling reactions.10 Thus, a plethora of new
methods have been developed for the decarboxylative coupling of
these acids with aryl halides,11 pseudohalides,12 unactivated arenes
through C–H activation,13 and even double decarboxylative
couplings;14 rapidly expanding the tools available for the synthesis
a School of Chemistry, University of Manchester, Oxford Road, Manchester,
M13 9PL, UK. E-mail: igor.larrosa@manchester.ac.uk
b College of Pharmacy, Gansu University of Chinese Medicine,
Lanzhou 730000, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cc01817d
Scheme 1 Coupling methods.
‡ Jacob Quibell and Guojian Duan contributed equally to this work.
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These transformations proceed efficiently on electron-rich aryl and
heteroaryl carboxylic acids, even in the absence of ortho-substituents.
Based on this novel decarboxylative activation mode, we now report
the development of a decarboxylative Suzuki–Miyaura coupling
allowing for the first time the use of non-ortho-substituted benzoic
acids with arylboronic acids as coupling partners, using only iodine
or a bromine source as terminal oxidants (Scheme 1c).
Our strategy for a decarboxylative Suzuki–Miyaura coupling
required the combination of three distinct steps in a one-pot
process: (1) decarboxylative halogenation, (2) quench of any
excess halogen and (3) coupling of the resulting aryl halide
with a boronic acid. We initiated our studies by attempting the
coupling of 4-methoxybenzoic acid (1a) with phenylboronic
acid (2a, Table 1). Accordingly, 1a was treated with 3 equiv. of
I2 in the presence of K3PO4 until complete formation of the
corresponding aryl iodide. Addition of Et3N efficiently removed
the excess of I2. This step was followed by addition of the
arylboronic acid 2a along with a palladium catalyst, a base and
any additional solvent required. Optimization of the coupling
Scheme 2 Scope of the boronic acid coupling partner.a a Reaction con-
ditions: (1) 1a (0.5 mmol), I2 (3 equiv.), K3PO4 (1 equiv.), MeCN (0.66 M),
100 1C, 16 h. (2) Et3N (4.5 equiv.), 120 1C, 5 h. (3) Pd-A = Pd(N,N-dimethyl-
step revealed that common commercially available catalysts such b-alaninate)2 (2 mol%),
2 (1.5 equiv.), K3PO4 (1.5 equiv.), EtOH/H2O
(1 : 1, 1.5 mL). Yields given are isolated yields. b Reaction run on 10 mmol
scale. c Boronic acid (2.0 equiv.), 15 h reaction time in 3rd step.
as Pd(OAc)2, PdCl2, PdCl2dppf and Pd(PPh3)4 gave only modest
yields (Table 1, entries 1–4). Remarkably, the use of Pd(N,N-
dimethyl-b-alaninate)2,19 Pd-A, gave an excellent yield using only
2 mol% loading of this catalyst (Table 1, entry 5). Lower catalyst
loadings and reaction temperature led to lower conversions
(Table 1, entries 6 and 7).
This decarboxylative Suzuki–Miyaura reaction tolerates a wide
range of aryl boronic acids; electron withdrawing para-cyano (3b),
nitro (3d), chloro (3e) and acetyl (3f) substituents all gave
excellent yields, although when the cyano group was placed in
the ortho-position (3c) a slight drop in the yield was observed.
Electron rich p-tolyl (3g) and o-anisole boronic acids (3h) again
showed excellent reactivity and even heteroaromatic boronic acids
(3i and 3j) gave moderate to excellent yields. Using (E)-styrylboronic
acid as the coupling partner led to the corresponding (E)-stilbene
(3k) with complete retention of stereochemistry. Notably, all of these
couplings were performed using 4-methoxybenzoic acid as the
coupling partner. Previous decarboxylative Suzuki couplings have
been limited to ortho-substituted benzoic acids. Satisfyingly, a
simple scaling up of the standard conditions allowed an excellent
yield of 3a to be obtained on a 10 mmol scale, presenting this
methodology as an easily scalable attractive process for industry
(3a, Scheme 2, footnote b).
Next our attention turned to the scope of the benzoic acid
(Scheme 3). As we have reported previously, the decarboxylative
iodination is limited to electron rich (hetero)aromatic carboxylic
acids.17 Nevertheless, the range of carboxylic acid substrates
Table 1 Optimization of an iodine-mediated decarboxylative Suzuki–
Miyaura reactiona
Entry
[Pd] source
T (1C)
Yieldb 3a (%)
1
2
3
4
5
6
7
Pd(OAc)2
PdCl2
PdCl2dppf
Pd(PPh3)4
Pd-A
100
100
100
100
100
80
60
49
52
46
92
87
79
Pd-A
Pd-Ac
100
Scheme 3 Scope of the (Hetero)aromatic acid coupling partner.a a Reaction
conditions: (1) 1a (0.5 mmol), I2 (3 equiv.), K3PO4 (1 equiv.), MeCN (0.66 M),
100 1C, 16 h. (2) Et3N (4.5 equiv.), 120 1C, 5 h. (3) Pd-A = Pd(N,N-dimethyl-b-
alaninate)2 (2 mol%), 2 (1.5 equiv.), K3PO4 (1.5 equiv.), EtOH/H2O (1 : 1, 1.5 mL).
Yields given are isolated yields. b 2 (2.0 equiv.). c 15 h reaction time in 3rd step.
d B20 1C reaction temperature in 1st step.
a
Reaction conditions: 1. 1a (0.5 mmol), I2 (3 equiv.), K3PO4 (1 equiv.),
MeCN (0.66 M), 100 1C, 16 h. 2. Et3N (4.5 equiv.), 120 1C, 5 h. 3. [Pd]
(2 mol%), 2a (1.5 equiv.), K3PO4 (1.5 equiv.), EtOH/H2O (1 : 1, 1.5 mL).
Pd-A = Pd(N,N-dimethyl-b-alaninate)2. Yields calculated by 1H NMR
b
c
with CH2Br2 as an internal standard. 1 mol% of Pd-A.
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compatible with this system proved to be diverse. The ortho-
Using iodine or a bromine source as the decarboxylation
methoxy substituted acid 1l reacted well to give 3l. Similarly, the mediator and terminal oxidants has allowed the development
highly electron rich substrates 1m and 1n gave good yields of of a novel decarboxylative Suzuki–Miyaura cross-coupling. The
3m and 3n, respectively, and due to the highly reactive nature of scope of this protocol vastly improves upon previous examples
these acids to decarboxylative iodination the first step can be of the methodology: the common requirement of an ortho-
carried out at room temperature. The 3-methyl-4-methoxy sub- substituent is now avoided and heteroaromatic substrates can
stituted benzoic acid showed good conversion to 3o despite its now be chosen as either coupling partner. This strategy has also
lack of ortho-substituents and the polymethyl-derivative 3p was allowed for low catalyst loadings and the avoidance of stoichio-
isolated in good yield despite its propensity to react sluggishly metric transition metals. In addition the reaction can be scaled
under traditional decarboxylative conditions.20 Electron with- up with minimal loss of reactivity.
drawing halogen substituents (3q, 3r) were tolerated, provided a
We gratefully acknowledge the University of Manchester
methoxy group was present. Polyfluorinated acids such as 1s School of Chemistry for funding (to G. J. P. P.). We also thank
also undergo this transformation smoothly, to yield 3s; it is the National Natural Science Foundation of China (No. 21762001)
likely that in this case the decarboxylative iodination step occurs and the China Scholarship Council for a visiting scholarship
via a different mechanism to the electron rich aromatic acids.17a (to G. D.).
Heteroaromatic benzoic acids, such as nicotinic acid (to 3t–v),
benzofuran- (to 3w–y) and benzo[b]thiophene-carboxylic acids,
(to 3z) gave excellent yields under these conditions. Chromone
and pyrazole substrates (to 3aa and 3ab, respectively) gave also
good yields although 2 equiv. of phenyl boronic acid and longer
Conflicts of interest
There are no conflicts to declare.
reaction times were necessary in these cases. Finally, the furan
carboxylic acid substrate 1ac afforded moderate yields of 3ac. The
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Scheme 4 Scope of the decarboxylative Suzuki–Miyaura coupling
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