Synthesis of α-aryl esters and nitriles: Deaminative coupling of α-aminoesters and α-aminoacetonitriles with arylboronic acids

Article abstract of DOI:10.1002/anie.201406765

Transition-metal-free synthesis of α-aryl esters and nitriles using arylboronic acids with α-aminoesters and α-aminoacetonitriles, respectively, as the starting materials has been developed. The reaction represents a rare case of converting C(sp³)-N bonds into C(sp³)-C(sp²) bonds. The reaction conditions are mild, demonstrate good functional-group tolerance, and can be scaled up. Touch base: A transition-metal-free protocol for the synthesis of α-aryl esters and nitriles by deaminative coupling is presented. Strong bases and transition-metal catalysts are not needed. The new synthetic method uses readily available starting materials and demonstrates wide substrate scope.

Full text of DOI:10.1002/anie.201406765

Angewandte
Chemie
DOI: 10.1002/anie.201406765
Synthetic Methods
Synthesis of a-Aryl Esters and Nitriles: Deaminative Coupling of a-
Aminoesters and a-Aminoacetonitriles with Arylboronic Acids**
Guojiao Wu, Yifan Deng, Chaoqiang Wu, Yan Zhang, and Jianbo Wang*
Abstract: Transition-metal-free synthesis of a-aryl esters and
nitriles using arylboronic acids with a-aminoesters and a-
aminoacetonitriles, respectively, as the starting materials has
been developed. The reaction represents a rare case of
Alternatively, the synthesis of a-aryl esters and nitriles can
be achieved through a cross-coupling reaction of arylboron
reagents with a-halocarbonyl compounds[14] and a-haloni-
triles,[15] respectively (Scheme 1b). Recently, Liu and co-
workers developed a palladium-catalyzed decarboxylative
coupling to access a-aryl esters and nitriles,[16] and similar
reactions have also been reported by the groups of Kwong[17]
and Xu[18] (Scheme 1c). Although significant progress has
been made in transition-metal-catalyzed synthesis of a-aryl
esters and nitriles, further development of alternative
approaches toward these important compounds is still
highly desirable.
Herein, we report a transtion-metal-free process to
synthesize a-aryl esters and nitriles by deaminative coupling
of a-aminoesters and a-aminoacetonitriles, respectively, with
boronic acids (Scheme 2). a-Aminoesters and a-aminoaceto-
nitriles attracted our attention because of their ready avail-
ability.[19] We previously reported the transtion-metal-free
reaction of boronic acids with diazo compounds.[20,21] This
type of transformation follows a simple process involving the
coordination of electron-rich diazo carbon atoms to the
electron-deficient boron center and subsequent 1,2-shift to
form a carbon–carbon bond.[22] As shown in Scheme 2, with
electron-withdrawing groups adjacent to the amino group, the
a-aminoesters and a-aminonitriles can be converted into the
3
3
2
À
À
converting C(sp ) N bonds into C(sp ) C(sp ) bonds. The
reaction conditions are mild, demonstrate good functional-
group tolerance, and can be scaled up.
N
itriles and a-aryl esters are versatile intermediates
because of their easy conversion into amides, carboxylic
acids, aldehydes, and primary amines, as well as their
applications in the synthesis of heterocyles.[1] Additionally,
a-aryl carboxylic acid derivatives are found as important
moieties in various medicinal and natural products.[2] Because
of their importance, the synthesis of a-aryl esters and nitriles
have attracted attention over the past decades.[3] Traditional
strategies for the synthesis of a-aryl nitriles include Friedel–
Crafts reactions,[4] cyanation of benzylic alcohols or halides,[5]
and dehydration of a-aryl amides and oximes.[6] These
traditional methods generally suffer from drawbacks which
include the need for toxic reagents, harsh reaction conditions,
and multistep operations.[7]
In 1997 the groups of Miura, Buchwald, and Hartwig
reported the palladium-catalyzed arylation of ketones.[8]
Subsequently, the synthesis of a-aryl esters and nitriles
through palladium-catalyzed coupling reactions was devel-
oped (Scheme 1a).[3,9] Such coupling reactions require strong
base, such as LiHMDS or NaHMDS, to deprotonate the
substrate to generate enolates. The use of strong base imposes
some limits to the reaction, including the functional-group
tolerance, side reactions from Claisen condensations, and
diarylations. To obviate the need for a strong base, zinc
enolates and silyl enol ethers,[10–12] and a-silyl nitriles and zinc
cyanoalkyls[13] have been used as the enolate precursors to
achieve arylation under mild reaction conditions.
Scheme 1. Synthesis of a-aryl esters and nitriles through transition-
metal-catalyzed coupling reactions. Tf=trifluoromethanesulfonyl.
[*] G. Wu, Y. Deng,[+] C. Wu,[+] Dr. Y. Zhang, Prof. Dr. J. Wang
Beijing National Laboratory of Molecular Sciences (BNLMS) and
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education, College of Chemistry, Peking University
Beijing 100871 (China)
E-mail: wangjb@pku.edu.cn
Prof. Dr. J. Wang
The State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences, Shanghai 200032 (China)
[+] These authors contributed equally to this work.
[**] The project is supported by the 973 Program (No. 2012CB821600)
and NSFC (Grant Nos. 21272010 and 21332002).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201406765.
Scheme 2. Transition-metal-free process for the synthesis of a-aryl
esters and nitriles.
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Table 1: Optimization of the reaction conditions.[a]
Eentry
Solvent
Additive
1a/2a
Yield [%][b]
1
2
3
4
5
6
7
8
toluene
toluene
toluene
toluene
PS[c]
–
1:2
1:2
1:2
1:2
1:2
1:2
1:1.5
1:1.25
1.25:1
1:1.5
<15
<20
80
74
0
66
88
79
73
NH4F
NH4Cl
HCO2NH4
NH4Cl
NH4Cl
NH4Cl
NH4Cl
NH4Cl
NH4Cl
DCE
toluene
toluene
toluene
toluene
9
10[d]
94
[a] Reaction was carried out with 1a and 2a in solvent (0.5 mmol, 0.2m)
at 1008C for 24 h. [b] Yield of isolated product. [c] PS=polar solvents,
including 1,4-dioxane, THF, DMF, and MeCN. [d] The reaction was
carried out in toluene and water (0.4m). DCE=1,2-dichloroethane,
DMF=N,N-dimethylformamide, THF=tetrahydrofuran.
Scheme 3. Reaction of arylboronic acids with glycine ethyl ester
hydrochloride. If not otherwise noted, the reaction was carried out with
1 (0.5 mmol) and 2a (0.75 mmol) in PhMe/H2O (20:1; 1.25 mL) at
1008C. All the yields refer to those of the isolated products. [a] The
reaction was carried out at 1208C. [b] Reaction was scaled up to
10 mmol of 1n and 15 mmol of 2a.
corresponding diazo compounds through diazotization and
subsquent deprotonation.[23] The in situ generated diazo
compounds react with arylboronic acids to achieve trans-
tion-metal-free synthesis of a-aryl esters and nitriles.
The original study was carried out with 4-chlorophenyl
boronic acid (1a) and glycine ethyl ester hydrochloride (2a;
Table 1). With toluene as the solvent, the desired product 3a
could be obtained, albeit in low yield (entry 1). Then a series
of additives were investigated in the reaction (entries 2–4).[20b]
The reaction was not significantly improved with NH4F as an
additive. To our delight, the yield was dramatically improved
with NH4Cl and HCO2NH4 as the additives, and the former
gave a better result with an 80% yield upon isolation
(entry 3). To further optimize the reaction, a series of solvents
were screened. Polar solvents, such as 1,4-dioxane, MeCN,
THF, and DMF, gave no desired product, whereas DCE
resulted in a diminished yield as compared that obtained with
toluene (entries 5 and 6). Finally, by tuning the ratio of 1a to
2a (entries 7–9) and increasing the reaction concentration,
the optimal result was obtained with 94% yield upon isolation
of 3a (entry 10).
With the optimized reaction conditions in hand, we next
carried out a study on the reaction scope with various
arylboronic acids. As shown in Scheme 3, the arylboronic
acids bearing both electron-rich and electron-poor substitu-
ents, such as methoxy, methylthio, olefin, ketone, ester, and
cyano, all can be successfully converted into the desired
products, thus indicating that the reaction is only marginally
affected by electronic effects. The arylboronic acids bearing
a halide group gave the desired a-aryl esters with moderate to
excellent yields (3a–c and 3s–v). Remarkably, the iodo
substituent could tolerate the reaction (3c), and thus provides
the possibility for additional transformations through tran-
sition-metal-catalyzed coupling reactions. It is also notewor-
thy that a base-sensitive ketone group can tolerate the
reaction (3q). For arylboronic acids bearing electron-with-
drawing substituents, high temperature is needed to promote
the reaction (3m, 3q, and 3r). A gram-scale experiment was
carried out with 1n, thus affording 3n in similar yield as
a small scale experiment.
Furthermore, the scope with respect to the a-aminoesters
was investigated by using phenylboronic acid (1n). However,
with the same protocol, the deaminative coupling with
alanine methyl ester hydrochloride afforded the desired
product with only low yield (< 10%). We observed the a-
chloro ester formation in the reaction.[23] Therefore, the
additive was changed from NH4Cl to HCO2K to avoid the a-
chloro ester formation. Under such reaction conditions,
satisfactory results were obtained. With the modified proto-
col, a range of a-aminoesters were examined. The deami-
native coupling proved to be effective and broadly applicable,
as shown in Scheme 4. Substituted glycine ethyl ester hydro-
chlorides all showed good reactivity (4b–g and 4m). The
reaction was not affected by substituents such as iodo, nitro,
and cyano on the aryl ring of phenylalanine methyl ester
hydrochloride (4h–j). The tryptophan methyl ester hydro-
chloride bearing a heterocycle with a free NH also afforded
the desired product 4k, though with a lower yield. The
diminished yield of 4l may be attributed to steric hindrance of
the a-aminoester. To our delight, the olefin and alkynyl
moieties are compatible with the reaction (4n and 4o).
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Scheme 4. Reaction of phenylboronic acid (1n) with a-aminoester
hydrochlorides. If not otherwise noted, the reaction was carried out
with 1n (0.5 mmol) and 2 (1.0 mmol) in PhMe/H2O (20:1; 1.25 mL) at
808C. All the yields refer to those of the isolated products. [a] The
reaction was carried out in the absence of water.
Scheme 5. Reaction of arylboronic acids with a-aminoacetonitrile hy-
drochloride (5a). If not otherwise noted, the reaction was carried out
with 1 (0.5 mmol) and 5a (1.0 mmol) in PhMe/H2O (20:1; 1.25 mL) at
508C. All the yields refer to those of the isolated products. [a] Reaction
was scaled up to 10 mmol of 1a and 20 mmol of 5a. [b] The reaction
was carried out at 1008C with DCE as solvent instead of PhMe. [c] The
reaction was carried out at 1008C.
Having the synthetic protocol for a-aryl esters estab-
lished, we next turned our attention to the synthesis of a-aryl
nitriles with the same strategy of deaminative coupling.
Gratefully, under the same reaction conditions employed for
the synthesis of a-aryl esters, the reaction of 1a with
aminoacetonitrile hydrochloride (5a) afforded the expected
product in 79% yield at 758C. A 93% yield of the isolated
product could be obtained at a reaction temperature of 508C,
while the yield was decreased to 66% at 308C. Interestingly,
99% yield could be obtained when carrying out the reaction
without NH4Cl.
With the modified reaction conditions, a series of
substitued arylboronic acids were submitted to investigate
the scope of this deaminative coupling for the synthesis of a-
aryl nitriles. As demonstrated in Scheme 5, a series of
arylboronic acids reacted smoothly to give moderate to
excellent yields of the corresponding a-aryl nitriles. It was
illustrated that halide substituents on the aryl ring of the
boronic acid was compatible with this protocol (6a–c, 6m, and
6s–v). This transformation is not significantly affected by both
electron-withdrawing and electron-donating substituents. The
terminal alkenyl (6e), ester (6o), and ketone (6q,r) functional
groups are also tolerated under the reaction conditions.
Moreover, arylboronic acids with steric hindrance undergo
successful transformation, although in some cases with
diminished yields (6k–m). Notably, the reaction with vinyl-
boronic acid afforded the desired product with a moderate
yield (6w). Similarly, for the electron-poor arylboronic acids
both a higher reaction temperature and DCE as the solvent
enhance the yields (6o–r). A gram-scale experiment has also
been carried out with 1a, thus affording 6a in similar yield.
Finally, the scope of a-aminoacetonitrile derivatives was
explored. As summarized in Scheme 6, a-aminoacetonitrile
derivatives with monoalkyl, dialkyl, cycloalkyl, and benzyl
substituents all showed good reactivities under the same
reaction conditions (7a–d and 7m–n). The functional groups
such as iodo, cyano, methoxyl, and ester attached to the
benzyl group does not significantly affect the reaction (7e–h).
The heterocyclic substituent is also compatible with the
reaction conditions (7i). An alkynyl substituent, especially
a terminal alkyne, and terminal alkene remain intact, thus
giving moderate yields of the desired products (7k,l).
Experiments were carried out to gain some insight into
the reaction mechanism. Since the diazo compound has been
proposed as the intermediate in the reaction, ethyl diazoace-
tate (EDA), instead of glycine ethyl ester hydrochloride, was
used as the substrate to react with phenylboronic acid under
the standard reaction conditions in the absence of NaNO2.
The reaction gave 3n in 93% yield [Eq. (1)]. This result is
consistent with the mechanism shown in Scheme 2.
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expect that this method will find wide applications in organic
synthesis.
Received: July 1, 2014
Published online: && &&, &&&&
Keywords: arenes · boron · diazo compounds · nitriles ·
.
synthetic methods
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4
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Communications
Synthetic Methods
G. Wu, Y. Deng, C. Wu, Y. Zhang,
J. Wang*
&&&& — &&&&
Touch base: A transition-metal-free pro-
tocol for the synthesis of a-aryl esters and
nitriles by deaminative coupling is pre-
sented. Strong bases and transition-
metal catalysts are not needed. The new
synthetic method uses readily available
starting materials and demonstrates wide
substrate scope.
Synthesis of a-Aryl Esters and Nitriles:
Deaminative Coupling of a-Aminoesters
and a-Aminoacetonitriles with
Arylboronic Acids
6
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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