Copper-Mediated ortho-Arylation of Benzamides with Arylboronic Acid

Article abstract of DOI:10.1002/adsc.201500884

The copper-mediated direct ortho C-H bond arylation of benzamide derivatives with arylboronic acids was achieved by employing an 8-aminoquinoline moiety as the bidentate directing group. Various biaryls were synthesized in good yields with excellent regioselectivity. The reaction shows good functional group compatibility and proceeds in a highly selective manner at the ortho-position of the benzamides. Deuterium-labelling experiments indicated that the ortho C-H bond cleavage of benzamide was involved in the rate-determining step of the arylation.

Full text of DOI:10.1002/adsc.201500884

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DOI: 10.1002/adsc.201500884
Copper-Mediated ortho-Arylation of Benzamides with
Arylboronic Acid
Qingwen Gui,a Xiang Chen,a Liang Hu,a Dadian Wang,a Jidan Liu,a and Ze Tana,*
a
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan
University, Changsha 410082, Peopleꢀs Republic of China
Fax: (+86)-731-8882-2400; e-mail: ztanze@gmail.com
Received: September 21, 2015; Revised: December 22, 2015; Published online: February 9, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500884.
À
Abstract: The copper-mediated direct ortho C H
bond arylation of benzamide derivatives with aryl-
boronic acids was achieved by employing an 8-ami-
noquinoline moiety as the bidentate directing
group. Various biaryls were synthesized in good
yields with excellent regioselectivity. The reaction
shows good functional group compatibility and pro-
ceeds in a highly selective manner at the ortho-posi-
tion of the benzamides. Deuterium-labelling experi-
Among the myriad of metal promoters and catalysts
developed, copper has attracted significant attention
because of its relatively low-cost and abundancy. The
combination of copper salts with directing groups
such as 8-aminoquinoline has emerged as an innova-
À
À
tive strategy for the construction of C C or C hetero-
[8–14]
À
atom bonds through C H bond cleavage.
Recent-
ly, copper-catalyzed or mediated, chelation-assisted
À
À
À
C H bond activations to construct C C and C X
bonds (X=S, O, N, F) with the aid of an 8-aminoqui-
noline directing group have been developed by the
groups of Daugulis,[6] Hirano and Miura,[8] Ge,[9] Kuni-
nobu and Kanai,[10] Stahl[11] and others.[12] We and
other groups also independently developed the selec-
tive ortho-sulfonylation and nitration of benzamides
bearing the 8-aminoquinoline group.[13] It should be
À
ments indicated that the ortho C H bond cleavage
of benzamide was involved in the rate-determining
step of the arylation.
À
Keywords: arylation; biaryl; C H activation;
copper; selectivity
mentioned that, besides copper catalysis, construction
[6,14]
Ni,[15] Co,[6,16] and
À
À
of C C and C X bonds via Pd,
Fe[17] catalyzed C H activation has also been devel-
The development of efficient methods for making the oped with the aid of an 8-aminoquinoline group.
À
privileged biaryl motif has always been a center of at-
Organoboronic acids are a group of unique organo-
tention in organic synthesis.[1] Among the available metallic reagents because of their stability and ease of
À
methods, the formation of C C bonds between the handling as well as their low toxicity. Their synthetic
two aryls by transition metal-catalyzed functionaliza- utility has been well manifested in the widely used
2
tion of C(sp ) H bonds is rather straightforward due Suzuki–Miyaura coupling.[18] Consequently, chemists
À
to the need for fewer steps and its overall high atom have aimed to develop transition metal-catalyzed or
economy.[2] The huge challenge in this field is to ach- promoted C H activation-based C C and C X bond
ieve the catalytic and highly regioselective functional- formation processes using organoboronic acids as the
À
À
À
À
ization of one specific inert C H bond in a complexly coupling partner. In 2014, Yu and Dai reported an el-
substituted benzene derivative that contains many C
À
egant Cu(OAc)2-catalyzed ortho-arylation of benz-
H bonds and possibly an array of functional groups. amide derivatives with ArBpin reagents using a 2-
To achieve this goal, chemists have utilized directing aminophenyloxazoline as the directing group. In
groups to direct the metal-catalyzed C H activation order to effect the transformation using only a catalyt-
À
process, mostly through chelation. Nitrogen-contain- ic amount of copper salt, a stoichiometric amount of
ing directing groups such as amides,[3] imines,[4] and ni- Ag2O was utilized as the oxidant [Scheme 1, Eq.
trogen heterocycles[5] are the most commonly used di- (1)].[19] In continuation of our research efforts on the
recting groups. Since Daugulisꢀ pioneering work,[6] 8- development of new C C and C X bond formation
À
À
[13a,b]
À
aminoquinoline has been shown to be one of the most processes via C H cleavage,
we wondered wheth-
powerful and versatile directing groups for transition er similar transformations could be achieved using 8-
[7]
À
metal-catalyzed functionalization of C H bonds.
aminoquinoline as the directing group. Furthermore,
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Table 1. Screening of reaction conditions.[a]
Scheme 1. Arylation of benzamides with organoboron re-
agents.
since more arylboronic acids are commercially avail-
able than arylBpins and they also tend to be cheaper,
À
replacing arylBpins with arylboronic acids in these C
H activation processes would be highly desirable.
Herein we disclose that not only ortho-selective aryla-
tion of benzamides with phenylboronic acids can
indeed be successfully achieved with the assistance of
an 8-aminoquinoline directing group, but also that the
reaction does not need any Ag additive at all. Instead,
copper salts alone are sufficient for the reaction to
proceed efficiently [Scheme 1, Eq. (2)].
[a]
Reaction conditions: amide 1a (0.3 mmol), 2a (0.6 mmol),
Cu salt (0.3 mmol), base (0.6 mmol), solvent (1 mL)
under air for 4 h.
Isolated yield.
Reaction run under N2.
PhBpin was used as arylboron reagent.
PhBF3K was used as arylboron reagent.
(PhBO)3 was used as arylboron reagent.
10% Cu(OAc)2 was used, reaction run under O2 (1 atm).
10% Cu(OAc)2 was used, 1 equiv. of AgOAc was used as
the oxidant.
We began our studies by reacting benzamide deriv-
ative (1a) with phenylboronic acid (2a) in the pres-
ence of one equivalent of Cu(OAc)2 in DMF at
1208C (Table 1, entry 1). We were pleased to observe
that the desired product 3a was formed in 11% yield.
Subsequent studies indicated that the addition of
a base had a drastic effect on the product yield
(Table 1, entries 2–5). Evaluation of bases used re-
vealed that Na2CO3 was superior to other alkali car-
bonates and the yield can be elevated to 81%
(Table 1, entries 6–8). To our delight, the desired
product 3a could be isolated in 90% yield when the
solvent was switched from DMF to DMSO (Table 1,
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
10% Cu(OAc)2 was used, 1 equiv. of Ag2O was used as
the oxidant.
entry 9). Other solvents such as DME, toluene, of Cu(OAc)2 (10 mol%) using O2 or Ag salts as oxi-
CH3CN, diglyme only gave 3a in dismal yields dant all led to disappointing results (Table 1, en-
(Table 1, entries 10–13). A yield of 71% was obtained tries 21–23).
when the reaction was performed under N2, which
With the optimized reaction conditions in hand, we
suggests that O2 was not essential for the reaction to examined the scope of the reaction. The results of
proceed (Table 1, entry 14). While the use of CuCl2 or coupling amide 1a with various arylboronic acids
Cu(OAc)2 hydrate as the metal promoter gave 3a in under the standard reaction conditions are shown in
slightly lower yields, the use of CuI only furnished 3a Table 2. From this, we can see that a variety of aryl-
in 21% yield (Table 1, entries 15–17). It is pleasing to boronic acids were compatible with this transforma-
see that PhBpin was also able to participate in the de- tion. Both electron-donating and electron-withdraw-
sired coupling even though the yield was lower by 8% ing groups were well tolerated in this reaction. For ex-
(Table 1, entry 18). On the other hand, the use of ample, arylboronic acids containing electron-with-
PhBF3K or (PhBO)3 in place of PhB(OH)2 gave much drawing groups such as chloro, fluoro, trifluorometh-
lower yields (Table 1, entries 19 and 20). Attempts to yl, carbonyl, cyano groups were able to provide the
run the reaction in the presence of a catalytic amount coupling products in 68–81% yields (3b, 3c and 3i–
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Copper-Mediated ortho-Arylation of Benzamides
Table 2. Copper-mediated arylation of 1a with various phe-
ucts in even better yields (82–95%, 3d–3h and 3n). In
addition, the arylboronic acid bearing a methyl group
at the ortho-position could also be used as a viable
substrate to deliver the desired product 3p, showing
the reaction is not sensitive to steric hindrance.
Heteroaromatic boronic acids such as pyridine-4-bor-
onic acid also participated in this arylation reaction to
give 3r in 52% yield. Coupling with 1- or 2-naphthyl-
boronic acid also proceeded uneventfully, albeit in
moderate yields (3s and 3t).
nylboronic acids.[a]
We next set out to explore the substrate scope of
benzamides and the results are summarized in
Table 3. As shown in Table 3, halogen substituents
such as chloro, fluoro and bromo groups (4b, 4c, 4e,
4f, 4h, 4l and 4n) were well tolerated whereas sub-
strates bearing an iodo substituent gave complex mix-
tures. This is to be expected since iodoarenes have
been shown to have excellent reactivities in copper-
À
catalyzed C N bond formation reactions. Other sub-
stituents such as phenyl, tert-butyl, trifluoromethyl are
also compatible with the coupling as well. It should
be noted that in cases where diarylation can easily
take place (meta- and para-substituted),[9,10,18] our re-
action provided the monosubstituted products only.
When a nitro group was placed on the para-position
of the phenylcarboxamide, we did not obtain any de-
[a]
Reaction conditions: amide (0.3 mmol), phenylboronic
acid (0.6 mmmol), Cu(OAc)2 (0.3 mmol), Na2CO3
(0.6 mmol) in DMSO (1 mL) at 1208C for 4 h.
6 h.
À
sired C H activation–coupling product 4o, instead,
only N-arylation took place. This result showed that
the electron density of the phenyl ring is very impor-
tant for the reaction to take place.
[b]
We also carried out the desired coupling with some
3m). On the other hand, couplings of 1a with elec- heteroaromatic amides. Much to our disappointment,
tron-rich arylboronic acids afforded the arylated prod- a pyridine-derived amide gave no reaction at all
Table 3. Copper-mediated arylation of various aromatic amides.[a]
[a]
Reaction conditions: amide (0.3 mmmol), arylboronic acid (0.6 mmol), Cu(OAc)2 (0.3 mmol), Na2CO3(0.6 mmol) in
DMSO (2 mL) at 1208C for 4 h.
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Qingwen Gui et al.
promoted oxidation of B.[20] C will next react with
phenylboronic acid to afford copper complex D by
transmetallation. Subsequent reductive elimination of
D provides the desired arylated product 3a.
In order to demonstrate the synthetic utility of our
procedure, we successfully removed the 8-aminoqui-
noline directing group by N-methylation of the secon-
whereas two thiophene- and furan-derived amides all dary amide followed by base hydrolysis. In the end, 2-
led to decomposition (see below).
phenylbenzoic acid was obtained in a good overall
To gain insights into the mechanism of this new yield of 58% (Scheme 4).It should be mentioned that
transformation, a series of control experiments were hydrolysis failed when 3a was directly subjected to
conducted. When radical inhibitor 2,2,6,6-tetramethyl- the same hydrolysis conditions.
piperidine N-oxyl (TEMPO) was employed in the
In conclusion, we have developed a novel copper-
À
standard reaction, the yield was only slightly de- mediated, 8-aminoquinoline-assisted ortho C H bond
creased, suggesting that the reaction does not involve arylation of benzoic acid derivatives with a variety of
radical steps. Kinetic studies showed that the intermo- arylboronic acids. Notable features of this newly de-
lecular kH/kD of 1a to 1a-d5 was 2.2 when the competi- veloped protocol include high monoarylation selectiv-
tion reactions were run in the same flask or 3.8 when ity, excellent regioselectivity and broad functional
the reactions were run in parallel (Scheme 2). The
large KIE observed clearly indicates that the ortho
À
C H bond cleavage of benzamide was involved in the
rate-determining step of the arylation.
Although the exact mechanism of this reaction is
still unclear at this moment, based on the above in-
vestigations and literature precedents,[6–13] a tentative
reaction mechanism is proposed in Scheme 3. Firstly,
coordination of 1a to Cu(OAc)2 followed by ligand
exchange provides intermediate A, which undergoes
À
C H activation to form the arylCu(II) species B.
Next copper(III) complex C is formed via Cu(OAc)2- Scheme 4. Removal of the 8-aminoquinoline auxiliary.
Scheme 2. Deuterium-labelling experiments.
Scheme 3. Plausible mechanism.
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Copper-Mediated ortho-Arylation of Benzamides
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which requires the use of a stoichiometric amount of
Ag additive, our method utilized copper salts only. In-
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sis of complex biaryl motifs are currently underway in
our lab and the results will be reported in due course.
Experimental Section
General Information
1H NMR and 13C NMR were recorded in CDCl3 at room
temperature on
a
Varian INOVA-400 spectrometer
(400 MHz, 1H). The chemical shift scale is based on internal
TMS. The peak patterns are indicated as follows: s, singlet;
d, doublet; t, triplet; q, quartet; m, multiplet; qui, quintet;
sxt, sextet. The coupling constants, J are reported in Hertz
(Hz). Mass spectroscopy data were collected on an HR-MS-
EI instrument. Unless otherwise noted, all reagents were ob-
tained from commercial suppliers and used without further
purification. Anhydrous Cu(OAc)2 was purchased from Alfa
Aesar. All solvents were purified and dried according to
standard methods prior to use. Products were purified by
flash column chromatography on 200–300 mesh silica gel,
SiO2.
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General Procedure for Copper-Mediated ortho-
Arylation of Benzamides
Benzamide 1a (0.3 mmol), anhydrous Cu(OAc)2 (55 mg,
0.3 mmol), Na2CO3 (64 mg, 0.6 mmol), arylboronic acid 2a
(0.6 mmol) and anhydrous DMSO (1 mL) were added to
a 25-mL Schlenk flask equipped with a high-vacuum PTFE
valve-to-glass seal. Then the flask was sealed under air and
the contents stirred at 1208C for 4 h. After the completion
of the reaction, the solvent was evaporated under reduced
pressure. The mixture was extracted with ethyl acetate, and
the combined organic layer was dried over sodium sulfate.
Concentration under vacuum followed by silica gel column
purification with petroleum ether/ethyl acetate elutent (4/
1
1 to 6/1) gave the desired product 3a; yield: 90%; H NMR
(400 MHz, CDCl3): d=9.63 (s, 1H), 8.75 (d, J=8 Hz, 1H),
8.54 (d, J=4 Hz, 1H), 7.96 (d, J=8 Hz, 1H), 7.51 (d, J=
12 Hz, 2H), 7.43 (t, J=8 Hz, 1H), 7.36 (t, J=8 Hz, 2H),
7.26–7.22 (m, 3H), 7.17 (t, J=8 Hz, 2H), 7.04 (t, J=8 Hz,
1H), 2.50 (s, 3H); 13C NMR (100 MHz, CDCl3): d=168.3,
147.9, 140.3, 139.6, 138.2, 136.7, 135.9, 135.7, 134.2, 129.4,
129.1, 128.6, 128.1, 127.7, 127.5, 127.2, 127.0, 121.7, 121.4,
116.4, 19.7.
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