Mild Copper-Catalyzed Addition of Arylboronic Esters to Di- tert -butyl Dicarbonate: An Easy Access to Methyl Arylcarboxylates

Article abstract of DOI:10.1055/a-1377-7369

An efficient copper-catalyzed addition of arylboronic esters to (Boc) ₂O was developed. The reaction can be conducted under exceedingly mild conditions and is compatible with a variety of synthetically relevant functional groups. It therefore represents a useful alternative route for the synthesis of methyl arylcarboxylates. A preliminary mechanistic study indicated the involvement of an addition-elimination mechanism.

Full text of DOI:10.1055/a-1377-7369

SYNLETT
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart
2021, 32, 833–837
letter
833
en
J.-D. Xu et al.
Letter
Synlett
Mild Copper-Catalyzed Addition of Arylboronic Esters to Di-tert-
butyl Dicarbonate: An Easy Access to Methyl Arylcarboxylates
Jin-Di Xu
O
O
B
O
O
MeI
cat. Cu
Xiao-Bo Su
Cai Wang
O
R
O
+
O
O
O
R
Li-Wei Yao
Jing-Hui Liu*
Guo-Qin Hu*
aryl or heteroaryl
up to 96% yield
28 examples
School of Chemical Engineering, Zhengzhou University,
Zhengzhou, Henan 450001, P. R. of China
jhliu@zzu.edu.cn
huguoqin@zzu.edu.cn
Corresponding Authors
Received: 29.12.2020
Accepted after revision: 29.01.2021
Published online: 29.01.2021
been reported, mainly involving transformations of arylbo-
ronic acid derivatives (Scheme 1), including (a) Suzuki-type
coupling reactions with organic halides or pseudohalides,6–
8 (b) 1,4-additions to ,-unsaturated compounds,9 (c) ring-
opening reactions with epoxides to afford -substituted al-
cohols,10 (d) inter- or intramolecular additions to aldehydes
or ketones,11 and (e) carboxylation reactions with CO2 to af-
ford aromatic carboxylic acids.12 These elegant works not
only provide ready access to specific synthetically import-
ant structural motifs, but also show intriguing promise in
the exploration of novel organoboron-based copper-cata-
lyzed procedures.
DOI: 10.1055/a-1377-7369; Art ID: st-2020-u0651-l
Abstract An efficient copper-catalyzed addition of arylboronic esters
to (Boc)2O was developed. The reaction can be conducted under ex-
ceedingly mild conditions and is compatible with a variety of syntheti-
cally relevant functional groups. It therefore represents a useful alterna-
tive route for the synthesis of methyl arylcarboxylates. A preliminary
mechanistic study indicated the involvement of an addition–elimina-
tion mechanism.
Key words copper catalysis, arylboronic esters, addition, carboxyl-
ation, methyl arylcarboxylates
OR2
R3
O
cat. Cu
B
OR2
a)
b)
Organocopper reagents are commonly used to deliver
carbon-containing nucleophiles in modern C–C bond-form-
ing reactions.1 Conventional transformations involve the
use of an excess of an organocopper reagent (R2CuLi) to
achieve high reaction efficiency,2 which entails unneces-
sary waste of valuable organometallic species. It is not sur-
prising that alternative catalytic procedures have been de-
veloped to generate active organocopper reagents in situ
from a combination of a copper catalyst and a stoichiomet-
ric strongly nucleophilic organometallic reagent, such as a
Grignard reagent.3 However, owing to the robust reactivity
and limited commercial availability of these reagents, such
procedures usually suffer from harsh reaction conditions
and poor functional-group tolerance. In contrast, organobo-
ron derivatives exhibits a high stability and controllable re-
activity, are readily available from commercial sources, and
are well known to serve as excellent reaction partners for
introducing desirable functionalities into organic scaf-
folds.4,5 Consequently, recent researches on copper catalysis
have paid increasing attention to the use of organoboron
reagent as substitutes for organometallic compounds in C–
C bond-forming reactions. Several successful examples have
+
R3-X/OTs
R1
R1
O
B
O
cat. Cu
R2
O
R1
+
R2
R1
O
B
R2
OH
cat. Cu
O
O
+
R1
c)
R1
R2
OR2
B
OH
R4
R3
cat. Cu
O
OR2
+
R1
R1
d)
e)
R3 R4
OR2
B
H+
COOH
cat. Cu
OR2
R1
R1
+
CO2
This work:
O
B
COOMe
cat. Cu
MeI
O
(Boc)2O
+
R
R
Scheme 1 Copper-catalyzed arylboron-based C–C bond-forming reac-
tions
© 2021. Thieme. All rights reserved. Synlett 2021, 32, 833–837
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

834
J.-D. Xu et al.
Letter
Synlett
In this context, we report our finding that copper can
promote the addition of arylboronic esters to di-tert-butyl
carbonate [(Boc)2O] under mild conditions to give moderate
to high yields of methyl arylcarboxylates after simple post-
processing. Preliminary mechanistic studies revealed that
an unusual addition–elimination mechanism might be in-
volved in this transformation. Our method therefore pro-
vides a powerful additional route for the synthesis of aryl-
carboxylic acid derivatives and expands the scope of orga-
noboron-based copper catalysis.
Table 1 Copper-Catalyzed Cross-Coupling under Various Conditionsa
O
O
B
cat. Cu MeI
O
O
+
(Boc)2O
1a
2
3a
Et
N
R
R
R = H, L4
R = OMe, L5
R = tBu, L6
R = Me, L7
Et
N
Ph2P
PPh2
N
Et
N
Et
N
N
L1
L2
L3
Our initial study focused on examining the feasibility of
copper-catalyzed addition of arylboronic ester 1a to various
carboxylic anhydrides, in the hope that this might provide a
useful complement to conventional synthetic procedures
involving Grignard reagents. After numerous unsuccessful
attempts, we serendipitously obtained a 23% yield of meth-
yl benzoate (3a) from a reaction system with (Boc)2O (2) as
a coupling partner, CuCl as a catalyst, LiOMe as a base, and
1,2-bis(diphenylphosphino)ethane (L1) as a ligand in N,N-
dimethylacetamide (DMA) at 50 ℃. It appeared that LiOMe
can further attack the intermediate addition product of 1a
to 3a to afford a methylated product. Subsequent analysis of
the reaction residue also showed the presence of a 10%
yield of benzoic acid together with a tiny amount of tert-
butyl benzoate, which might have resulted from the de-
composition of the addition intermediate under the alka-
line conditions. On quenching the reaction with MeI, the to-
tal yield of 3a reached 33% (Table 1, entry 1). Excitingly,
lowering the reaction temperature to 30 ℃ had no signifi-
cant effect on the reaction (entry 2). The choice of ligand
played a crucial role in this reaction. Without a ligand, the
yield of 3a was only 8% (entry 3). Replacing L1 with
N,N,N′,N′-tetraethylethane-1,2-diamine (L2) improved the
yield to 54% (entry 4), whereas the use of 1,10-phenanthro-
line (L3) dramatically inhibited the reaction (entry 5).
When 2,2′-bipyridine (L4) was used as the ligand, an excel-
lent catalytic performance was observed, with a yield of
71% (entry 6). This encouraged us to test a series of bipyri-
dine ligands (entries 6–9). As a result, the electron-rich bi-
pyridine ligand bearing two methyl groups L7 gave the op-
timal result (80% HPLC yield; 78% isolated yield). Replacing
LiOMe base with KOMe or NaOMe led to lower yields (en-
tries 10 and 11). Replacement of CuCl with other copper
catalyst also failed to improve the yield (entries 12–15). A
screening of DMF, NMP, or DMSO as solvent also negatively
affected the reaction (entries 16–18). After further optimi-
zation of the reaction time, a period of six hours was select-
ed as optimal (entries 19 and 20). Exposing the reaction
system to air led to a sharp decrease in the reaction efficien-
cy (entry 21). Finally, in a control experiment without CuCl,
the cross-coupling failed to occur, indicating that the copper
catalyst was necessary for the transformation (entry 22).
Entry Catalyst
Ligand Base
Solvent Temp (℃)Time (h) Yieldb(%)
1
2
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuI
L1
L1
–
LiOMe
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMF
NMP
DMSO
DMA
DMA
DMA
DMA
50
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
4
8
6
6
33
34
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
KOMe
NaOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
LiOMe
3
8
4
L2
L3
L4
L5
L6
L7
L7
L7
L7
L7
54
5
11
6
71
7
74
8
75
9
80 (78)c
68
17
36
10
11
12
13
14
15
16
17
18
19
20
CuBr
69
Cu(OTf)2 L7
Cu(OAc)2 L7
25
50
CuCl
CuCl
CuCl
CuCl
CuCl
L7
L7
L7
L7
L7
L7
L7
45
61
50
51
79
21d CuCl
22
5
–
trace
a Reaction conditions: 1a (0.375 mmol), 2 (0.25 mmol), catalyst (10 mol%),
ligand (13 mol%), LiOMe (2.5 equiv), solvent (0.5 mL) under Ar, then MeI (5
equiv), stirring, 2 h.
b Yields (average of two runs) were determined by HPLC with benzophenone
as the internal standard.
c Isolated yield.
d The reaction was conducted under air.
With the optimized conditions in hand, we then exam-
ined the scope of the reaction with respect to the arylbo-
ronic ester. As shown in Scheme 2, a wide range of arylbo-
ronic esters with electron-donating or electron-withdraw-
ing substituents smoothly participated in the reaction to
afford the desired products 3 in moderate to good yields.
Besides phenylboronic esters, naphthylboronic esters were
also suitable coupling partners (3b and 3c). Substituents in
the meta- (3d) or para-positions (3e) of the phenyl rings
had no significant effect on the reaction efficiency. A more
sterically hindered ortho-substituted arylboronic ester
© 2021. Thieme. All rights reserved. Synlett 2021, 32, 833–837

835
J.-D. Xu et al.
Letter
Synlett
could also be successfully converted into the corresponding
methyl arylcarboxylate 3g. A variety of synthetically im-
portant functional groups such as alkyl (3d–g), phenyl (3h),
methoxy (3i), fluoro (3j), chloro (3k), bromo (3l), iodo (3m),
trifluoromethyl (3n), trifluoromethoxy (3o), cyano (3p),
terminal olefin (3q), methylsulfanyl (3r), sulfone (3s), ester
(3t), and piperonyl (3u) were tolerated. A nitro-substituted
arylboronic ester gave ester 3v in a moderate yield of 45%.
This protocol showed good compatibility with a trimethyl-
silyl group (3w), thereby offering additional opportunities
for downstream structural modification. Notably, an aryl-
boronic ester with an aldehyde substituent (3x) was also
compatible, and no carbonyl-addition product was ob-
served under our optimized reaction conditions. Finally,
our catalytic procedure could be applied to the coupling of
active hydrogen-containing arylboronic esters. A substrate
with an unprotected hydroxy group reacted with (Boc)2O to
provide the desired product 3y in a relatively low yield of
45%.
of this transformation (Scheme 3). Both oxygen- and sulfur-
containing hetarylboronic esters including furyl (5a), thie-
nyl (5b and 5c), and benzothienyl (5d) esters were effec-
tively converted into the corresponding products in moder-
ate yields. Unfortunately, with a pyridine-based boronic es-
ter as a substrate, none of the desired product 5e was
observed, and most of substrate remained unreacted.
CuCl (10 mol%)
L7 (13 mol%)
O
O
MeI
HetAr
O
B
(Boc)2O
+
HetAr
O
LiOMe, DMA
30 °C, Ar, 6 h
O
5
4
2
O
O
O
O
O
S
O
O
S
O
O
O
S
N
5a, 43%
5b, 58%
5c, 57%
5d, 57%
5e, trace
Scheme 3 Copper-catalyzed cross-coupling with hetarylboronic es-
ters. Reagents and conditions: 4 (0.375 mmol), 2 (0.25 mmol), CuCl (10
mol%), L7 (13 mol%), LiOMe (2.5 equiv), DMA (0.5 mL), 30 °C, 6 h, un-
der Ar, then MeI (5 equiv), stirring, 2 h. Isolated yields are reported.
Next, several hetarylboronic esters were synthesized
and subjected to the reaction system to test the generality
Our catalytic protocol could also be applied to the cou-
pling of alkenylboronic esters. The -styrylboronic ester 6
reacted smoothly with (Boc)2O to give ,-unsaturated
methyl cinnamate (7) in 60% yield (Scheme 4).
O
O
CuCl (10 mol%)
L7 (13 mol%)
B
MeI
O
O
(Boc)2O
+
R
R
LiOMe, DMA
30 °C, Ar, 6 h
1
2
3
O
O
O
O
O
CuCl (10 mol%)
L7 (13 mol%)
O
B
MeI
O
O
+
(Boc)2O
O
O
O
O
O
O
O
LiOMe, DMA
30 °C, Ar, 6 h
3b, 92%
3c, 91%
3d, 87%
3e, 88%
6
2
7, 60%
O
O
O
O
Scheme 4 Copper-catalyzed cross-coupling of an alkenylboronic ester
O
O
O
O
tBu
Ph
3h, 94%
3f, 89%
3g, 85%
3i, 91%
To further explore the synthetic practicability of our
new reaction, we conducted a gram-scale (5 mmol) experi-
ment with the 2-naphthylboronic ester 1b as a reaction
partner (Scheme 5). This copper-catalyzed coupling also
worked well, and afforded product 3b with a negligible
change in yield compared with that of the smaller-scale ex-
periment.
O
O
O
O
Cl
O
O
O
O
F
Br
I
3j, 52%
3n, 58%
3k, 88%
3l, 89%
3m, 92%
O
O
O
NC
F3CO
O
O
O
F3C
O
3p, 54%
3q, 74%
3o, 41%
CuCl (10 mol%)
L7 (13 mol%)
O
B
O
O
MeI
O
+
(Boc)2O
O
O
O
LiOMe, DMA
30 °C, Ar, 6 h
O
O
S
O
O
O
O
S
O
3b, 89%
1b
2
O
COOMe
3t, 91%
5 mmol, 1.09 g
3r, 96%
3s, 91%
3u, 89%
Scheme 5 A copper-catalyzed gram-scale cross-coupling
O
O
O
O
O2N
O
O
O
O
Si
HO
OHC
Next, several control experiments were performed to
probe the mechanism of this reaction (Scheme 6). When
(Boc)2O was replaced with CO2, no carboxylation product
was observed under the standard reaction conditions. This
suggested that a mechanism involving the reaction of 1a
with CO2 generated in situ by decomposition of (Boc)2O can
3v, 45%
3w, 92%
3x, 64%
3y, 45%
Scheme 2 Copper-catalyzed cross-coupling reactions of arylboronic
esters. Reagents and conditions: 1 (0.375 mmol), 2 (0.25 mmol), CuCl
(10 mol%), L7 (13 mol%), LiOMe (2.5 equiv), DMA (0.5 mL), 30 °C, 6 h,
under Ar, then MeI (5 equiv), stirring, 2 h. Isolated yields are reported.
© 2021. Thieme. All rights reserved. Synlett 2021, 32, 833–837

836
J.-D. Xu et al.
Letter
Synlett
be excluded. An addition–elimination mechanism, unlike
those of previously reported carboxylation reactions of ar-
ylboronic acid derivatives,12 is more likely to be involved in
this transformation. To further identify the actual active in-
termediate we synthesized two possible elimination prod-
ucts 7 and 813 and we subjected these to the reaction condi-
tions. Substrate 7 remained unchanged under the current
reaction conditions, indicating that it is not the key product
of the addition–elimination step. However, carbonate 8 de-
composed quickly under basic conditions and afforded a
44% yield of 3a, a 38% yield of benzoic acid (9) and a small
amount of tert-butyl benzoate (7) after quenching with
HCl; this result was consistent with the analysis of reaction
residue from Table 1, entry 1.
CuCl + L
O
B
MeI
base
O
3a +
9 7
+
LCu-OMe
1a
I
base
O
O
O
B
base
Ph
O
8
OtBu
MeO
O
CuL
O
O
tBuO
Ph
LCu-Ph
II
O
O
2
Scheme 7 Plausible reaction mechanism
O
CuCl (10 mol%)
L7 (13 mol%)
O
B
MeI
+
CO2
O
(a)
O
O
LiOMe, DMA
30 °C, 6 h
Funding Information
1 atm
1a
3a, trace
Financial support from Key Scientific and Technological Project of
Henan Province (182102210184), and the Postdoctoral Science Foun-
dation of Henan Scientific Committee is acknowledged.()
CuCl (10 mol%)
L7 (13 mol%)
O
O
O
MeI
(b)
LiOMe, DMA
30 °C, Ar, 6 h
7
3a, trace
Supporting Information
O
O
Supporting information for this article is available online at
+
CuCl (10 mol%)
L7 (13 mol%)
O
O
Ph
O
Ph
OH
https://doi.org/10.1055/a-1377-7369.
S
u
p
p
orti
n
gI
n
f
orm
a
ti
o
n
S
u
p
p
orti
n
gI
n
f
orm
a
ti
o
n
H+
3a, 44%
9, 38%
O
O
(c)
O
LiOMe, DMA
30 °C, Ar, 6 h
tBu
+
Ph
O
8
References and Notes
7, 18%
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Scheme 6 Control experiments for mechanistic studies
On the basis of these results and previous reports,11,12,14
we proposed a plausible reaction mechanism for this cop-
per-catalyzed protocol (Scheme 7). First, CuCl reacts with
ligand L to afford the LCu–OMe complex I, which undergoes
transmetalation with the arylboronic ester 1a to give inter-
mediate II. Addition of intermediate II to the carbonyl
group of (Boc)2O (2) is followed by elimination to afford in-
termediate 8, along with the regeneration of the copper cat-
alyst. Finally, base-promoted decomposition of 8 gives rise
to a mixture of arylcarboxylic acid derivatives. A high yield
of 3a is obtained after quenching the reaction system with
MeI.
In conclusion, we found that addition of arylboronic es-
ters to (Boc)2O could be achieved under mild conditions
with a copper catalyst.15 Further explorations showed that
this reaction might proceed through an addition–elimina-
tion pathway and that it was compatible with a range of
aryl, hetaryl, and alkenylboronic esters. It therefore pres-
ents a conceptually new approach to the synthesis of aryl-
carboxylic acid derivatives. We hope the findings reported
above will inspire future research aimed at exploring the
synthetic versatility of organoboron-based copper catalysis.
© 2021. Thieme. All rights reserved. Synlett 2021, 32, 833–837

837
J.-D. Xu et al.
Letter
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(15) Methyl (Het)arylcarboxylates 3a–y, 5a–e; General Procedure
A 15 mL Schlenk tube equipped with a stirrer bar was charged
with CuCl (10 mol%), L7 (13 mol%), LiOMe (2.5 equiv), and the
appropriate boronic ester 1 or 4 (0.375 mmol). The vessel was
then evacuated and filled with Ar (three cycles). DMA (0.5 mL)
and (Boc)2O (0.25 mmol) were added sequentially under Ar, and
the mixture was stirred at 30 ℃ for 6 h. MeI (5 equiv) was then
added in air, and the mixture was stirred at 30 ℃ for additional
2 h. The mixture was finally diluted with EtOAc and washed
with sat. aq NaCl (20 mL). The aqueous phase was further
extracted with EtOAc (3 × 20 mL), and the combined organic
phases were dried (Na2SO4) and concentrated. The residue was
purified by column chromatography [silica gel EtOAc–hexane
(1:100 to 1:50)].
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Methyl 2-Naphthoate (3b)
(10) (a) Lu, X.-Y.; Yang, C.-T.; Liu, J.-H.; Zhang, Z.-Q.; Lu, X.; Lou, X.;
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Prepared by following the general procedure as a white solid;
yield: 91% (by HPLC). 1H NMR (400 MHz, CDCl3):  = 8.62 (s, 1
H), 8.08–8.05 (m, 1 H), 7.96 (d, J = 8.0 Hz, 1 H), 7.88 (d, J = 8.7 Hz,
2 H), 7.63–7.49 (m, 2 H), 3.98 (s, 3 H). 13C NMR (101 MHz,
CDCl3):  = 167.29, 135.53, 132.51, 131.09, 129.37, 128.25,
128.17, 127.78, 127.41, 126.66, 125.24, 52.26. The NMR spectral
data agreed with the reported values.16
(16) Tobisu, M.; Yamakawa, K.; Shimasaki, T.; Chatani, N. Chem.
Commun. 2011, 47, 2946.
© 2021. Thieme. All rights reserved. Synlett 2021, 32, 833–837