A general rhodium-catalyzed cyanation of aryl and alkenyl boronic acids

Article abstract of DOI:10.1002/anie.201006044

Give me a cyanide: N-cyano-N-phenyl-p-methylbenzenesulfonamide as a cyanation reagent allows the synthesis of aryl(alkenyl) nitriles in good yield under mild conditions (see scheme; cod=cycloocta-1,5-diene). Combination of the procedure with the direct borylation of arenes and hydroboration of alkynes leads to the straightforward synthesis of various nitriles.

Full text of DOI:10.1002/anie.201006044

DOI: 10.1002/anie.201006044
Homogeneous Catalysis
AGeneral Rhodium-Catalyzed Cyanation of Aryl and Alkenyl Boronic
Acids**
Pazhamalai Anbarasan, Helfried Neumann, and Matthias Beller*
Dedicated to Professor Henri Brunner on the occasion of his 75th birthday
Transition-metal-catalyzed functionalization of organoboron
derivatives has emerged as a powerful tool for the selective
formation of carbon–carbon and carbon–heteroatom bonds in
achieved in the transition-metal-catalyzed synthesis of ben-
[9]
zonitriles. Notable examples include palladium-, copper-,
[10]
and nickel-catalyzed cyanations of aryl halides,
direct
[
1]
modern organic synthesis. In addition to palladium-cata-
lyzed coupling reactions, such as the famous Suzuki–Miyaura
reaction, rhodium-catalyzed additions of aryl or alkenyl
cyanation of heteroarenes, as well as chelation-assisted
[
11]
reactions. Furthermore, electrophilic cyanation of organo-
lithium, organomagnesium, and organozinc reagents are
[
2]
[3]
[4]
[12]
boronic acids to alkynes, alkenes, carbonyl compounds,
imines, and nitriles
known. Most recently, we disclosed a general cyanation of
[5]
[6,7]
are in particular frequently explored
Grignard reagents through the use of N-cyanobenzimidazole
[
13]
nowadays for the straightforward construction of pharma-
ceutically important scaffolds. Inspired by previous studies
(4).
Despite all these studies, the cyanation of more stable and
(
Scheme 1, cycle A), we envisioned a novel catalytic synthesis
readily available organometallic reagents such as organo-
boron derivatives has only been rarely studied. So far, only
two reports have described this type of reaction. While Zhang
and Liebeskind reported the palladium-catalyzed and copper-
mediated cyanation of aryl boronic acids with benzylthiocya-
of (hetero)aryl and alkenyl nitriles from readily available aryl
and alkenyl boronic acids in the presence of suitable
cyanation reagents (Scheme 1, cycle B).
[14]
nate, Hartwig and co-workers demonstrated the combina-
tion of iridium-catalyzed borylation and copper-mediated
[
15]
cyanation of arenes. However, stoichiometric amounts of
the metal source were used in these reactions, and thus there
is great interest in developing the corresponding catalytic
variants. Based on our long-standing interest in the synthesis
[16]
of benzonitriles,
we herein disclose the first rhodium-
catalyzed cyanation of aryl and alkenyl boronic acids.
The cyanation of phenylboronic acid (1) to yield benzoni-
trile (2) was chosen at the start of our investigations as a
model reaction. To find an appropriate cyanation reagent,
reactions were performed in the presence of 1 mol% of
Scheme 1. Rhodium-catalyzed 1,2-addition of aryl boronic acids to
nitriles: A novel strategy for the synthesis of benzonitriles.
[
{Rh(OH)(cod)} ], 1 equivalent of the cyanation reagent, and
2
K CO in 1,4-dioxane at 808C (Scheme 2).
2
3
The resulting aromatic and vinyl nitriles constitute an
integral part of many dyes, herbicides, agrochemicals, phar-
maceuticals, and natural products. Furthermore, the nitrile
group serves as a valuable intermediate for other diverse
functional-group transformations that lead to the generation
of benzoic acid derivatives, amines, aldehydes, heterocycles,
Cyanation with p-toluenesulfonyl cyanide (3), a commer-
cially available cyanation reagent, gave only trace amounts
(< 1%) of the desired product 2. Similar results were
observed with N-cyanobenzimidazole (4) and N-cyanophtha-
limide (6). No product was observed in the reaction with N-
cyano-4-(N,N-dimethylamino)pyridinium tetrafluoroborate
(5). However, to our delight, an excellent yield of benzonitrile
[
8]
etc. In the last decade, significant progress has been
(2) was obtained by using N-cyano-N-phenyl-p-methylbenze-
[
*] Dr. P. Anbarasan, Dr. H. Neumann, Prof. M. Beller
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
Fax: (+49)381-1281-51113
E-mail: matthias.beller@catalysis.de
Homepage: http://www.catalysis.de
nesulfonamide (7). Notably, 7 is readily synthesized in an
environmentally friendly manner from inexpensive phenyl-
urea by dehydrative tosylation with p-toluenesulfonyl chlo-
[17]
ride.
Next, we examined various rhodium precursors to under-
stand the effect of the metal precursor on the reaction
[
**] This work was funded by the Alexander-von-Humboldt-Stiftung, the
(Table 1). No reaction was found in the absence of rhodium,
State of Mecklenburg–Western Pomerania, the BMBF, and the DFG
thus proving that the model reaction is catalyzed by rhodium
(Table 1, entry 2). Changing the hydroxide ligand to either
methoxide or chloride resulted in a comparable yield
(
Leibniz Prize). We thank S. Leiminger, Dr. W. Baumann, and Dr. C.
Fischer (LIKAT) for analytical support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006044.
(Table 1, entries 3 and 4), while [{Rh(acac)(cod)} ] afforded
2
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519

Communications
Table 2: Rhodium-catalyzed cyanation of phenylboronic acid: Variation
[
a]
of additive and temperature.
[
b]
Entry
[{Rh(OH)(cod)} ] [mol%]
Additive
T [8C]
Yield [%]
89
27 (44)
94
79
64
2
1
2
3
4
5
6
7
8
9
1
1
2.5
1
1
1
1
1
1
K2CO3
–
K2CO3
KF
80
80
80
80
80
80
50
50
RT
RT
[
c]
K3PO4
H BO
92
3
3
[
[
[
[
d]
e]
f]
f]
K2CO3
K2CO3
K2CO3
K2CO3
71
64
57
78
10
2.5
Scheme 2. Rhodium-catalyzed cyanation of phenylboronic acid using
different NÀCN reagents. Reaction conditions: CN source (0.5 mmol),
phenylboronic acid (0.6 mmol), [{Rh(OH)(cod)} ] (1 mol%), K CO
[
(
a] 7 (0.5 mmol), phenylboronic acid (0.6 mmol), [{Rh(OH)(cod)}2]
x mol%), additive (0.5 mmol), 1,4-dioxane (1 mL), 808C, 4 h.
2
2
3
[b] Yields were determined by GC analysis with hexadecane as the
internal standard. [c] 24 h. [d] 8 h. [e] THF, 12 h. [f] 16 h.
(
0.5 mmol), 1,4-dioxane (1 mL), 808C, 4 h. Yields given were deter-
mined by GC analysis with hexadecane as the internal standard.
cod=cycloocta-1,5-diene.
Based on these results we chose the following conditions
for further exploration of substrates: 1 mol% of [{Rh(OH)-
(cod)} ], one equivalent of K CO , 1,4-dioxane, 808C, 4 h.
2 2 3
Table 1: Rhodium-catalyzed cyanation of phenylboronic acid: Variation
[
18]
[
a]
of rhodium precursors.
As shown in Table 3, the rhodium-catalyzed cyanation of
simple aryl boronic acids such as o-tolyl- and p-methoxyphe-
nylboronic acids gave the corresponding benzonitriles in
excellent yield (Table 3, entries 1 and 2). Sterically demand-
ing (Table 3, entries 3 and 6) as well as nonhindered aryl
boronic acids (Table 3, entries 2, 4, and 5) were also cyanated
effectively under these conditions. Halogenated benzonitriles,
which are useful substrates in organic synthesis and have
appropriate functional groups, were readily synthesized from
the corresponding boronic acids in good to excellent yields
(Table 3, entries 7–9). Pharmaceutically interesting fluori-
nated and trifluoromethylated benzonitriles were synthesized
in reasonable yields (70% and 58%; Table 3, entries 9 and
[
b]
Entry
[Rh]
Yield [%]
1
2
3
4
5
6
7
8
9
[{Rh(OH)(cod)}2]
89
0
78
76
47
0
0
0
0
–
[{Rh(OMe)(cod)}2]
[{RhCl(cod)}2]
[{Rh(acac)(cod)}2]
[RhCl(PPh ) ]
RhCl ·3H O
5% Rh/C
3
3
3
2
5% Rh nanoparticle/alumina
1
0). Furthermore, electronically different aryl boronic acids
[
a] 7 (0.5 mmol), phenylboronic acid (0.6 mmol), [Rh] source (2 mol%
reacted smoothly and afforded the corresponding nitriles in
good yield (Table 3, entries 2–4 and 9–11). Moreover, more
challenging functionalized aryl boronic acids are also com-
patible with the present cyanation conditions (Table 3,
entries 11–14). For example, 5-cyano-N-methylindole was
formed in 76% yield from the commercially available
indolylboronic acid. It is interesting to note that an alkenyl
boronic acid was also converted effectively into cinnamylni-
trile in 69% yield (Table 3, entry 15).
Having found suitable conditions for the cyanation of a
variety of aryl(alkenyl) boronic acids, we next investigated
readily accessible boronates. While the respective pinacolate
did not undergo cyanation, the neopentylglycol ester was
transformed effectively into benzonitrile in 69% yield
(Table 3, entries 16 and 17).
with respect to [Rh]), K CO (0.5 mmol), 1,4-dioxane (1 mL), 808C, 4 h.
[
internal standard. acac=acetylacetonate.
2
3
b] Yields were determined by GC analysis with hexadecane as the
benzonitrile (2) in only 47% yield (Table 1, entry 5). No
product was observed on using [RhCl(PPh ) ], RhCl , Rh/C,
or Rh nanoparticles (Table 1, entries 6–9).
After identifying a suitable cyanation reagent and rho-
dium precursor, we optimized other critical parameters such
as additive, solvent, and temperature. As shown in Table 2,
catalytic cyanation in the absence of potassium carbonate was
sluggish and afforded 2 in only 44% yield after 24 h (Table 2,
entries 1 and 2). The use of other basic additives such as KF
and K PO afforded 2 in 79% and 64% yield, respectively
3
3
3
3
4
(
Table 2, entries 4 and 5).
Next, we thought that our rhodium-catalyzed cyanation
might easily be combined with the straightforward prepara-
tion of organoboron substrates to provide new efficient
reaction sequences towards aromatic and vinyl nitriles. Thus,
an iridium-catalyzed borylation of 1,3-dimethoxybenzene
with bis(pinacolato)diboron (B pin ) followed by hydrolysis
Surprisingly, the use of boric acid (H BO ) as an additive
3
3
[
6c]
afforded 2 in an excellent yield (Table 2, entry 6). Notably,
this novel cyanation also proceeded at 508C and even at room
temperature to give 71% (8 h) and 57% (16 h) of benzoni-
trile, respectively (Table 2, entries 7 and 9).
2
2
5
20
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Angew. Chem. Int. Ed. 2011, 50, 519 –522

Table 3: Rhodium-catalyzed cyanation of aryl boronic acids: Scope and
[
a]
limitations.
[
b]
Entry
Aryl boronic acid
Aryl nitriles
Yield [%]
1
2
3
4
R=2-Me
84
90
64
71
R=4-OMe
R=2-OMe
R=3-OEt
[
c]
Scheme 3. Functionalization of arene(alkyne) to aryl(alkenyl) nitirles.
dtbpy=4,4’-di-tert-butyl-2,2’-bipyridine.
5
83
alkyne through hydroboration with catecholborane, hydroly-
sis, and rhodium-catalyzed cyanation.
6
7
86
74
[
20]
On the basis of previous reports on rhodium-catalyzed
[2–7]
additions of aryl boronic acids to an unsaturated system,
we postulate the following mechanism for our reaction
Scheme 4). Transmetalation of the aryl boronic acid with
(
[
c]
8
9
76
70
[
[
[
c]
c]
c]
10
11
12
58
65
72
[
c]
1
3
81
1
4
5
76
69
1
Scheme 4. Proposed mechanism for the rhodium-catalyzed cyanation
of aryl boronic acids. Ts=4-toluene sulfonyl.
[
d]
1
6
7
0
an active rhodium(I) species would generate the aryl–
rhodium species I, which on coordination with the NÀCN
[
d]
1
69
reagent would form intermediate II. The formation of
rhodium species III can be envisaged by transfer of the aryl
motif to the nitrile carbon atom. Rearrangement of III
produces the benzonitrile along with the reactive rhodium
species IV. Finally, transmetalation affords the aryl–rhodium I
to complete the catalytic cycle.
[
(
a] 7 (1 mmol), aryl boronic acid (1.1 mmol), [{Rh(OH)(cod)}2]
1 mol%), K CO (1 mmol), 1,4-dioxane (2 mL), 808C, 4 h. [b] Yield of
2
3
isolated product. [c] 2.5 mol% of [{Rh(OH)(cod)} ]. [d] Yields were
determined by GC analysis with hexadecane as the internal standard.
2
of the boronate was carried out according to the recent report
In conclusion, we have demonstrated the first rhodium-
catalyzed cyanation of aryl and alkenyl boronic acids. The use
[
19]
by Hartwig and co-workers.
Subsequent reaction of the
resultant 3,5-dimethoxyphenylboronic acid under rhodium-
catalyzed cyanation conditions produced the 3,5-dimethoxy-
benzonitrile in 78% yield (Scheme 3). Similarly, 2-(4-meth-
ylphenyl)vinylnitrile was obtained from the corresponding
of N-cyano-N-phenyl-p-methylbenzenesulfonamide as
a
cyanation reagent enables a variety of aromatic and vinyl
nitriles to be obtained in good yields under mild conditions.
The developed procedure can easily be combined with direct
Angew. Chem. Int. Ed. 2011, 50, 519 –522
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521

Communications
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Received: September 27, 2010
Published online: December 5, 2010
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Keywords: benzonitriles · boronic acids · cyanation ·
.
homogeneous catalysis · rhodium
6
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