Copper-Catalyzed Highly Efficient Esterification of Aldehydes with N -Hydroxyphthalimide via Cross-Dehydrogenative Coupling in Water at Room Temperature

Article abstract of DOI:10.1055/s-0036-1588760

A copper-catalyzed cross-dehydrogenative coupling reaction between N -hydroxyphthalimide and aldehydes using PhI(OAc) ₂ as an oxidant is described. It is reported for the first time to synthesize NHPI esters in water, providing the corresponding NHPI esters in moderate to good yields. This facile and efficient method is eco-friendly and possesses the advantages of mild conditions, short reaction time, and broad substrate scope.

Full text of DOI:10.1055/s-0036-1588760

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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–F
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A
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Z. Guo et al.
Letter
Synlett
Copper-Catalyzed Highly Efficient Esterification of Aldehydes
with N-Hydroxyphthalimide via Cross-Dehydrogenative Coupling
in Water at Room Temperature
Zhicheng Guoa
Xinpeng Jiangb
Can Jin*b
Jiadi Zhoua
Bin Suna
O
O
O
H
Cu (10 mol%)
N
Selectfluor (10 mol%)
O
+
R
O
PhI(OAc)2 (1.2 equiv)
rt, 20 min
O
R
H
Weike Su*a
24 examples
up to 91% yield
water as solvent
N
O
a Collaborative Innovation Center of Yangtze River Delta Region
Green Pharmaceuticals, Zhejiang University of Technology,
Hangzhou 310014, P. R. of China
O
pharmlab@zjut.edu.cn
b College of Pharmaceutical Sciences, Zhejiang University of
Technology, Hangzhou 310014, P. R. of China
jincan@zjut.edu.cn
Received: 18.01.2017
Accepted after revision: 27.02.2017
Published online: 27.03.2017
uncovered. Tan and co-workers developed an n-Bu4NBr-cat-
alyzed cross-coupling reaction of aldehydes with N-hy-
droxyimides,9 Dinda and co-workers reported the same re-
action catalyzed by ruthenium under visible light.10 Select-
fluor-mediated intermolecular C–O cross-coupling reaction
for the synthesis of N-hydroxyimide esters has been report-
ed by Lv.11 However, using of the above-mentioned meth-
ods still suffers from several drawbacks, such as high reac-
tion temperature or long reaction time in toxic organic sol-
vents, which may limit their practical application. Hence,
practical, efficient, and environmentally friendly approach-
es to afford N-hydroxyimide esters from readily available
starting materials are urgently needed.
DOI: 10.1055/s-0036-1588760; Art ID: st-2017-w0053-l
Abstract A copper-catalyzed cross-dehydrogenative coupling reaction
between N-hydroxyphthalimide and aldehydes using PhI(OAc)2 as an
oxidant is described. It is reported for the first time to synthesize NHPI
esters in water, providing the corresponding NHPI esters in moderate to
good yields. This facile and efficient method is eco-friendly and pos-
sesses the advantages of mild conditions, short reaction time, and
broad substrate scope.
Key words copper-catalyzed, esterification, cross-dehydrogenative
coupling reaction, water, NHPI esters
Selectfluor is a mild, stable, inexpensive, and commer-
cially available electrophilic fluorination reagent.12 It has
been widely employed for introducing fluorine atom into
organic compounds.13 Besides its fluorination ability, this
electrophilic reagent could oxidize copper powder into cop-
per(II) salts.14 Moreover, our previous work has shown that
the copper(II) salts can react with N-hydroxyphthalimide
(NHPI) to produce PINO radical,15 a fairly stable but highly
reactive free radical, which plays an important role in the
CDC reaction between NHPI and aldehydes. In addition, in
the past decades, chemists have begun to re-investigate the
possibility of using water as solvent for organic reactions. It
is not only because water is safety and environmental
friendly but also provides opportunities for clean process-
ing and pollution prevention.16 To the best of our knowl-
edge, there is no precedence of such a methodology with
readily available aldehydes and NHPI as precursors for the
synthesis of N-hydroxyimide esters via a CDC approach in
water. Herein, we disclosed an example of copper-catalyzed
intermolecular C–O cross-coupling reaction of simple alde-
hydes with NHPI in water under mild conditions.
Esterification is a fundamental and important reaction
widely employed in organic synthesis, especially in the syn-
thesis of a variety of pharmaceuticals and natural prod-
ucts.1 The development of synthetic protocols to esters has
therefore emerged as a research topic of great interest.2 In
recent years, much attention has been paid to N-hydroxy-
imide esters, which are widely used in organic chemistry.
N-Hydroxyimide esters could be readily converted into am-
ides or O-substituted hydroxylamine species,3 which are
essential for the synthesis of biologically and pharmacolog-
ically active compounds.4–7 In view of the importance of N-
hydroxyimide esters, many simple and efficient methods
have been developed. Traditionally, N-hydroxyimide esters
were prepared via coupling of N-hydroxyphthalimide
(NHPI) with carboxylic acids under the action of N,N′-di-
cyclohexylcarbodiimide.8 However, this method suffers
from several drawbacks, such as the formation of urea as a
byproduct and a poor atom economy. In this context, metal
or metal-free catalyzed oxidative cross-dehydrogenative
couplings (CDC) between aldehydes and NHPI have been
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

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Z. Guo et al.
Letter
Synlett
Table 1 Optimization of the Reaction Conditionsa
sources screened such as Cu(OAc)2, CuCl2, CuBr2, Cu(OTf)2,
CuCl, CuBr, and CuI, giving NHPI esters 3b in 48%, 53%, 50%,
43%, 38%, 35%, 30% yield, respectively (Table 1, entries 4–
10). Interestingly, Cu/PhI(OAc)2 system is sensitive to the
Selectfluor according to the control experiments (Table 1,
entry 11 vs. 22), giving 3b in 75% yield (Table 1, entry 11).
However, increasing the reaction to 45 °C or using
Cu(OAc)2·H2O and CuBr instead of Cu failed to improve the
yield of 3b (Table 1, entries 12–14). The reaction also was
performed in organic solvent, such as MeCN, giving 3b in
68% yield (Table 1, entry 15). Other widely used common
oxidants such as [dibmin]+[BF4]–, 2-iodoxybenzoic acid
(IBX), TBHP, K2S2O8, and 2,3-dichloro-5,6-dicyano-1,4-ben-
zoquinone (DDQ) were also tested under the similar reac-
tion conditions and did not show better results (Table 1, en-
tries 16–20), thus highlighting the unique oxidizing ability
of PhI(OAc)2 for the present reaction. Control experiment
showed that no desired product 3b was detected in the ab-
sence of PhI(OAc)2 (Table 1, entry 21).
To examine the scope of this transformation, various al-
dehydes were investigated as substrates to react with NHPI
under the optimized reaction conditions (Table 1, entry 11).
As shown in Table 2, aryl aldehydes with various functional
groups were effective in this transformation. Furthermore,
electronic properties and the positions of the substituents
on the aromatic ring had an effect on reaction reactivity
when using copper as a catalyst and yields ranged from 51–
91%. In addition, halo-substituted aryl aldehydes were tol-
erated in this CDC reaction, providing the corresponding
NHPI esters in good yields (3b–d,i,k–m,r). Aryl aldehyde
substrates bearing electron-withdrawing substituents, such
as nitro and carboxylate, were smoothly converted into the
corresponding products 3o and 3p, respectively. Most of the
substrates with electron-donating substituents on the aro-
matic ring could afford the desired products (3e,f,j,n,q) in
good yields. Polysubstituted arylaldehydes were also
achieved for this reaction, giving the desired products 3s
and 3t in 63% and 60% yield, respectively. Starting reactants
from 1-naphthaldehyde and phenylacetaldehyde, NHPI es-
ters 3u and 3x could be obtained in 51% and 60% yield, re-
spectively. Reactants with heteroaromatic groups also af-
forded the desired product 3v in excellent yield. Further-
more, conjugated aldehydes were also suitable reaction
partners and gave rise to the corresponding NHPI esters 3w
in 78% yield. Remarkably, alkyl aldehyde was also effective
in providing esters 3y in 62% yields. The reaction of N-hy-
droxysuccinimide (NHSI) with 2-fluorobenzaldehyde was
also carried out in present transformation system, unfortu-
nately, no desired product was isolated. Overall, this reac-
tion system exhibited a good scope with respect to various
aldehydes in synthetically useful yields of NHPI esters.
O
O
F
O
O
N
catalyst
H
O
N
OH
oxidant, H2O
O
F
O
1
2
3b
Entry Catalyst
Oxidant
Additive
Temp
Yield (%)b
1
2
–
Selectfluor
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
–
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
N.D.
25
–
–
3
Cu(OAc)2·H2O
–
62
4
Cu(OAc)2
CuCl2
CuBr2
Cu(OTf)2
CuCl
CuBr
CuI
–
48
5
–
53
6
–
50
7
–
43
8
–
38
9
–
35
10
11c
12
13
14
15d
16e
17
18
19
20
21
22
–
30
Cu
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
75
Cu
45 °C
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
72
Cu(OAc)2·H2O
CuBr
Cu
65
40
68
Cu
[dibmin]+[BF4]– Selectfluor
<10
38
Cu
IBX
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
–
Cu
TBHP
K2S2O8
DDQ
–
trace
N.D.
trace
N.D.
27
Cu
Cu
Cu
Cu
PhI(OAc)2
a Reactions were carried out with oxidant (1.2 mmol), 2-fluorobenzaldehyde
(1, 1.2 mmol), NHPI (2, 1 mmol), catalyst (10 mol%), additive (10 mol%) in 8
mL water for 1 h at r.t., unless otherwise noted.
b Yield of the isolated product 3b based on NHPI.
c Reaction performed for 20 min.
d MeCN as reaction solvent.
e [dibmin]+[BF4]– is ion-supported PhI(OAc)2.
The reaction of 2-fluorobenzaldehyde (1) with NHPI (2)
was selected as a model reaction. For an initial study, a mix-
ture of 2-fluorobenzaldehyde and NHPI was treated with
Selectfluor alone in water at room temperature for one
hour, unfortunately, no desired NHPI ester 3b was detected
(Table 1, entry 1). Gratifyingly, the reaction occurred to pro-
duce NHPI ester 3b in 25% yield when the substrates were
stirred with 1.2 equivalents of PhI(OAc)2 (Table 1, entry 2).
It was found that Cu(OAc)2·H2O can significantly improve
the reaction yield (Table 1, entry 3), which prompted us to
optimize the other copper salts. In a set of other copper
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

C
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Letter
Synlett
Table 2 Scope of the Reaction of Aldehydes with NHPIa
O
O
O
O
Cu(0) (10 mol%)
N
Selectfluor (10 mol%)
H
+
O
N
OH
PhI(OAc)2 (1.2 equiv)
H2O, rt, 20 min
O
R
O
R
Entry
1
Substrate
Product
Yield (%)b
O
O
O
N
3a 81
H
H
O
O
O
3b R1 = F, 75
O
2
3
4
5
6
O
O
3c R1 = Cl, 70
3d R1 = Br, 72
3e R1 = Me, 78
3f R1 = OMe, 80
N
O
O
R1
R1
O
O
7
8
R2
3h R2 = OMe, 55
3i R2 = Br, 79
R2
N
H
H
O
O
9
10
11
12
13
14
15
3j R3 = Me, 80
3k R3 = F, 90
O
O
O
3l R2 = Cl, 88
3m R3 = Br, 91
3n R3 = OMe, 85
3o R3 = NO2, 50
3p R3 = COOMe, 75
N
O
R3
O
R3
O
O
O
O
O
N
16
3q 81
H
O
O
O
O
O
Cl
O
Cl
O
N
17
18
3r 80
3s 63
H
O
O
Cl
Cl
O
O
O
N
H
O
O
O
O
O
O
O
O
N
H
O
19
20
3t 60
3u 51
O
O
O
O
O
O
H
O
N
O
O
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

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Table 2 (continued)
Entry
Substrate
Product
Yield (%)b
O
H
O
N
21
3v 80
S
S
O
O
O
O
O
O
22
23
3w 78
3x 60
N
H
O
O
O
H
O
N
O
O
O
O
O
O
H
24
3y 62
N
O
O
a Standard reaction conditions: Reactions were carried out with PhI(OAc)2 (1.2 mmol), aldehydes (1.2 mmol), NHPI (1 mmol), Cu (10 mol%), Selectfluor (10 mol%)
in water (8 mL) for 20 min at r.t.
b Yield of the isolated products based on NHPI.
Several control experiments were carried out to probe
the reaction mechanism (Scheme 1). When the radical-
trapping reagent 2,2,6,6-tetramethylpiperidine N-oxide
(TEMPO) was added to the model reaction under the opti-
mal conditions, no desired product 3b was detected
(Scheme 1, a). The model reaction was also completely in-
hibited by another radical scavenger, 2,6-di-tert-butyl-4-
methylphenol (BHT) (Scheme 1, b). These results indicated
that the present reaction may proceed via a radical path-
way. Moreover, 2-fluorobenzoic acid was obtained from the
model reaction. To further gain insights into the reaction
mechanism, the reaction of 2-fluorobenzoic acid and NHPI
was applied to determine whether 2-fluorobenzoic acid
was an active intermediate of this transformation (Scheme
1, c). However, no desired product 3b was obtained under
the optimal conditions. In addition, our precious work has
shown that the copper(II) salts can react with NHPI to pro-
duce PINO radical, which plays an import role in the CDC
reaction between aldehydes and NHPI.
A plausible reaction mechanism based on the literature
precedent and our current control experimental results
have been proposed (Scheme 2). Initially, copper powder
reacts with Selectfluor to afford XCuIIOH (X = F or BF4), an
active copper species.14 Next, the reaction of XCuIIOH with
NHPI to produce PINO radical, CuIX and H2O (X = F or BF4).
Then the copper(I) salts were oxidized to copper(II) species
O
O
O
O
Cu(0) (10 mol%)
N
O
Selectfluor (10 mol%)
PhI(OAc)2 (1.2 equiv)
TEMPO (2 equiv)
a)
H
H
N
OH
OH
O
O
O
F
trace
F
O
O
O
O
O
Cu(0) (10 mol%)
Selectfluor (10 mol%)
N
O
b)
c)
N
PhI(OAc)2 (1.2 equiv)
BHT (2 equiv)
F
F
trace
O
O
O
O
O
Cu(0) (10 mol%)
Selectfluor (10 mol%)
N
O
OH
N
OH
PhI(OAc)2 (1.2 equiv)
F
F
N.D.
O
Scheme 1 Control experiments
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

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Synlett
Selectfluor
XCuII(OH), X=F or BF4
O
Cu
H2O
O
O
F
N
O
XCuII(OH)
NHPI
PINO
O
path a
F
3b
1a
1
PhI(OAc)2
O
H
F
PINO
CuIX + H2O
PhI(OAc)2
path b
PINO
O
O
Cl
OH
O
F
N
N
N
N
–
2BF4
Selectfluor
O
O
O
O
F
3
F
Scheme 2 Postulated mechanism
by PhI(OAc)2 to complete the catalytic cycle. The PINO radi-
cal induces the homolysis of the C–H bond from the alde-
hyde group to afford the acyl radical 1a,17 the capture of the
acyl radical 1a by PINO radical gives the desired NHPI esters
3b (Scheme 2, path a). On the other hand, we could not ex-
clude another pathway: The PINO radical was then inter-
cepted by aldehyde to hemiaminal radical 3,10 which could
be further oxidized to the desired NHPI ester 3b by
PhI(OAc)2 (Scheme 2, path b).
In summary, we have developed a convenient and eco-
friendly protocol for the synthesis of NHPI esters by the oxi-
dative cross-coupling reaction of aldehydes with NHPI, of-
fering a new possibility of the NHPI esters forming method-
ology in synthetic chemistry. A wide range of aldehydes
were good coupling partners, providing NHPI esters in
moderate to good yields. This reaction could be effective in
water at room temperature, the short reaction time and the
use of the inexpensive copper as the catalyst would make
this process attractive.
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(18) General Experimental Procedure to Synthesize 1,3-Dioxoiso-
indolin-2-yl 2-fluorobenzoate (3b)
A flame-dried flask was charged with Cu (10 mol%) and Select-
fluor (10 mol%) in H2O (8 mL) was added and stirred at r.t. for 5
min, then 2-fluorobenzaldehyde (1, 1.2 mmol), NHPI (2, 1
mmol), PhI(OAc)2 (1.2 mmol) were added, the resulting mixture
was whisked for a further 15 min in water at r.t. After the reac-
tion was finished (monitored by TLC), CH2Cl2 (10 mL) was
added, then the organic layer was separated and dried over
Na2SO4. The solvent was removed under vacuum. The crude
product was purified by column chromatography on silica gel
(n-hexane–EtOAc, 10:1) to afford the corresponding product 3b
in 75% yield (214 mg) as white solid; mp 170–172 °C. 1H NMR
(500 MHz, CDCl3): δ = 8.16–8.13 (m, 1 H), 7.95–7.91 (m, 2 H),
7.84–7.81 (m, 2 H), 7.12–7.67 (m, 1 H), 7.33–7.31 (m, 1 H),
7.28–7.24 (m, 1 H). 13C NMR (126 MHz, CDCl3): δ = 162.5 (1JC–F
=
264.6 Hz), 161.9, 160.2 (4JC–F = 5.0 Hz), 136.7 (3JC–F = 10.1 Hz),
134.9, 132.7, 128.9, 124.4 (4JC–F = 3.8 Hz), 124.0, 117.4 (2JC–F
21.4 Hz), 113.9 (3JC–F = 10.0 Hz).
=
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F