Copper-Catalyzed O-Methylation of Carboxylic Acids Using DMSO as a Methyl Source

Article abstract of DOI:10.1055/s-0035-1560967

A copper-catalyzed O-methylation of carboxylic acids using dimethyl sulfoxide (DMSO) as the methyl source is disclosed. This transformation exhibits a broad substrate scope and excellent functional group tolerance. Mechanistic studies indicate that a methyl radical is generated from dimethyl sulfoxide in the reaction process.

Full text of DOI:10.1055/s-0035-1560967

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2016, 48, 421–428
paper
421
J. Jia et al.
Paper
Synthesis
Copper-Catalyzed O-Methylation of Carboxylic Acids Using DMSO
as a Methyl Source
Jing Jia
O
O
S
O
CuCl2•2H2O, CaCl2, K2CO3
Qing Jiang
An Zhao
CH3
H
+
R
O
H3C
CH3
H2O2, O2, 80 °C, 15 h
R
O
24 examples
up to 82% yield
Bin Xu
R = aromatic, aliphatic, alkenyl
Qiang Liu
Wei-Ping Luo
Can-Cheng Guo*
cheap copper salt
DMSO as methyl source and solvent
H2O2 and O2 as oxidant
College of Chemistry and Chemical Engineering,
Hunan University, Changsha 410082, P. R. of China
ccguo@hnu.edu.cn
Received: 18.09.2015
Accepted after revision: 27.10.2015
Published online: 19.11.2015
and co-workers developed a method for the palladium-
catalyzed cyanation of indole with dimethyl sulfoxide and
ammonium hydrogencarbonate.8 Many groups, such as the
Suzuki,9 Cheng,10 Cao,11 and Zhang groups,12 have success-
fully introduced the formyl group into various substrates
using dimethyl sulfoxide as a formyl source. In recent years,
dimethyl sulfoxide has attracted increasing attention as a
methylthiolation13 and methyl sulfone source.14 Although
the use of dimethyl sulfoxide as a carbon source has been
well studied, to the best of our knowledge the use of di-
methyl sulfoxide as a methyl source has been less fruitful.
There are only a few reports on the use of dimethyl sulfox-
ide as a methyl source, for example Bansal et al.15 developed
an efficient methylation reaction of 1,4-quinones, Li and co-
workers16 reported a palladium-catalyzed alkylation of iso-
quinoline N-oxides, and Xiao and co-workers17 disclosed a
method for the N-methylation of amines and nitro com-
pounds with dimethyl sulfoxide. Herein, we report a cop-
per-catalyzed O-methylation of carboxylic acids using di-
methyl sulfoxide as the methyl source.
We envisioned that dimethyl sulfoxide could act as
methyl donor with the help of a copper salt and hydrogen
peroxide, and the O-methylation reaction of carboxylic
acids could be realized under basic conditions. Initially,
benzoic acid and dimethyl sulfoxide were chosen as the
model substrates to optimize the reaction conditions. After
extensively screening the reaction parameters (Table 1),
benzoic acid (1a) gave the O-methylation product methyl
benzoate (2a) in good yield using: benzoic acid (0.5 mmol,
1.0 equiv), CuCl2·2 H2O (10%), CaCl2 (1.0 equiv), 30% H2O2
(3.0 equiv), K2CO3 (1.0 equiv), DMSO (2.0 mL), 80 °C, 15 h
under an O2 atmosphere in a sealed tube (entry 1); these
conditions are referred to as the ‘standard conditions’. Oth-
er reaction factors were investigated and the results are list-
ed in entries 2–23. Using the standard conditions but re-
placing the oxidant hydrogen peroxide with di-tert-butyl
DOI: 10.1055/s-0035-1560967; Art ID: ss-2015-h0547_op
Abstract A copper-catalyzed O-methylation of carboxylic acids using
dimethyl sulfoxide (DMSO) as the methyl source is disclosed. This trans-
formation exhibits a broad substrate scope and excellent functional
group tolerance. Mechanistic studies indicate that a methyl radical is
generated from dimethyl sulfoxide in the reaction process.
Key words carboxylic acids, O-methylation, dimethyl sulfoxide, hy-
drogen peroxide, methyl radical, esterification, esters
O-Methylation of carboxylic acids is a very common
step in natural products synthesis.1 Classical methods often
involve electrophilic reagents such as diazomethane,
(trimethylsilyl)diazomethane, dimethyl sulfate, and methyl
iodide,2 which are generally hazardous and/or unstable. Re-
cently, the groups of Mao3 and Chen4 independently report-
ed copper-catalyzed O-methylation reactions with organic
peroxides as the methyl source. The groups of Selva, Aricò,
and Gorin have reported that methyl could transfer from di-
methyl carbonate to carboxylic acids.5 Other O-methylation
reactions with (diacetoxyiodo)benzene or methylboronic
acid have been reported.6 Despite the great progress being
made in this field, safe and effective methyl sources are still
highly desirable.
Dimethyl sulfoxide is widely used as a solvent in organic
synthesis due to its low toxicity, low cost, great dissolving
capacity, and relative stability. Moreover, it is also a versa-
tile reactant and it has shown its importance in contempo-
rary organic chemistry. For example, dimethyl sulfoxide is
used as oxidant in the well-known Swern oxidation and
Kornblum reaction. Recently, the Jiao group reported an ef-
ficient α-hydroxylation of ketones and the transformation
of alkyl bromides or alkenes to bromohydrins using di-
methyl sulfoxide as an oxidant and oxygen source.7 Cheng
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peroxide (DTBP) or 3-chloroperoxybenzoic acid gave a trace
amount of product 2a only (entries 2 and 3). Performing the
reaction under standard conditions but without the use of
hydrogen peroxide did not give 2a (entry 4), suggesting that
hydrogen peroxide is vital in this transformation. Using the
standard conditions but replacing calcium chloride by 10%
1,10-phenanthroline or 2,2′-bipyridine gave 2a in very low
yield (entries 5 and 6), and a similar result was obtained
without the use of calcium chloride (entry 7). Using the
standard conditions but varying the catalyst showed that
iron(II) chloride tetrahydrate was not as effective as cop-
per(II) chloride dihydrate in this system (entry 8) and using
manganese(II) chloride tetrahydrate, nickel(II) chloride tet-
rahydrate, or cobalt(II) chloride hexahydrate as the catalyst
gave almost no product 2a (by GC, entries 9–11). In addi-
tion, no product was obtained in the absence of copper(II)
chloride dihydrate (entry 12). Raising or lowering the tem-
perature under the standard conditions gave lower yields of
2a (entries 13 and 14). Using the standard conditions but
varying the base suggested that it plays a very important
role in this transformation as a very low yield or only trace
amounts of 2a were obtained using different bases or no
base (entries 15–18). An oxygen atmosphere was not vital
to this reaction, and changing from the use of an oxygen at-
mosphere to the use of an oxygen balloon or leaving the
flask open to air had little influence on the reaction effi-
ciency, but using a low oxygen concentration under nitro-
gen resulted in a low yield of 2a (entry 1 vs. entries 19 and
20). The solvent in the standard conditions could not be
changed and the use of acetonitrile–dimethyl sulfoxide
gave only trace amounts of 2a (entry 21). Under the stan-
dard conditions, replacing potassium carbonate/calcium
chloride with potassium chloride/calcium carbonate gave
2a in only 32% yield (entry 22). Finally, under the standard
conditions, increasing the time from 15 hours to 24 hours
did not improve the yield (entry 23).
With the optimized conditions in hand, we explored the
scope of this copper-catalyzed O-methylation reaction of
carboxylic acids with dimethyl sulfoxide. The results are
summarized in Scheme 1. A variety of carboxylic acids were
tolerated well in this system and this transformation afford-
ed the corresponding products 2a–x with moderate to good
yields (14–82%). Benzoic acid substrates with electron-
donating groups were well tolerated under the standard
conditions and gave products 2b–e in 76–82% yields, while
those bearing electron-withdrawing groups gave 2f,g in
lower 64–67% yields. Unfortunately, when terephthalic acid
was used as the substrate, dimethyl terephthalate (2h) was
isolated in only 14% yield. Steric hindrance did not affect
the reaction efficiency, 2,4,6-trimethylbenzoic acid (1i) re-
acted smoothly to afford 2i in 65% yield. It is noteworthy
that halo-substituted benzoic acids gave halo-substituted
products 2j–l in 61–72% yields, which could be used for fur-
ther transformations. Interestingly, when 4-hydroxybenzo-
Table 1 Screening the Reaction Conditionsa
CuCl2•2H2O (10%)
CaCl2 (1.0 equiv)
O
O
O
O
S
30% H2O2 (3.0 equiv)
+
OH
K2CO3 (1.0 equiv)
O2, 15 h, 80 °C
0.5 mmol
2.0 mL
1a
2a
standard conditions
Entry Variation from the standard conditionsa
Yieldb
(%)
1
2
none (or with an O2 balloon)
replace 30% H2O2 with DTBP
replace 30% H2O2 with MCPBA
without 30% H2O2
83 (80)
trace
trace
0
3
4
5
replace CaCl2 with 10% 1,10-phenanthroline
replace CaCl2 with 10% 2,2′-bipyridine
without CaCl2
8
6
14
7
6
8
replace CuCl2·2 H2O with FeCl2·4 H2O
replace CuCl2·2 H2O with MnCl2·4 H2O
replace CuCl2·2 H2O with NiCl2·4 H2O
replace CuCl2·2 H2O with CoCl2·6 H2O
without CuCl2·2 H2O
35
9
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
0
trace
0
change 80 °C to 70 °C
71
change 80 °C to 90 °C
79
replace K2CO2 with Na2CO3
replace K2CO2 with KHCO3
27
12
replace K2CO2 with KOH
trace
trace
71 (70)
39
without base
replace O2 with air (or open to air)
replace O2 with N2
replace DMSO with MeCN (2.0 mL)–DMSO (4.0 equiv)
trace
replace CaCl2–K2CO3 with KCl (2.0 equiv), CaCO3 (1.0 equiv) 32
24 h instead of 15 h 82
a All reactions took place under the standard conditions with the variation of
catalyst, reagent, solvent, temperature, and time noted.
b Determined by GC using 1,4-dichlorobenzene as internal standard.
ic acid (1m) was used as the substrate, only trace amounts
of 2b were observed by GC-MS, and methyl 4-hydroxyben-
zoate (2m) was obtained in 63% yield together with recov-
ered 1m (32%). Benzoic acids bearing CN and CHO groups
were also tolerated in this transformation, generating 2n
and 2o in 70% and 69% yields, respectively. The reaction of
2-aminobenzoic acid (1p) under the standard conditions
gave methyl 2-aminobenzoate (2p) (43%) together with
methyl 2-(methylamino)benzoate (2p′) (14%) and trace
amounts of methyl 2-(dimethylamino)benzoate (by GC-
MS). Increasing the reaction time had no influence on the
ratio of 2p′/2p, while increasing or decreasing the amount
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of hydrogen peroxide affected the yields and the ratio of
2p′/2p. In addition, when 2-hydroxy-2-phenylacetic acid
(1q) was reacted under the standard conditions, the sole
methylation product was methyl 2-hydroxy-2-phenylace-
tate (2q) together with some other byproducts, such as 2a
(GC yield: 8%), methyl 2-oxo-2-phenylacetate (GC yield:
<5%), and benzaldehyde (GC yield: 13%).
Furthermore, vinyl carboxylic acids also gave the de-
sired products 2r,s in low 32–37% yields. Other aromatic
carboxylic acids also readily underwent the reaction to gen-
erate the expected products 2t–u. Although quinoline-2-
carboxylic acid (1v) did not work well in this transforma-
tion, no other methylation products other than 2v were ob-
served. Aliphatic acids such as 2-phenylacetic acid (1w) and
hexanoic acid (1x) were also ideal substrates giving 2w,x in
70% and 73% yield, respectively. Changing dimethyl sulfox-
ide to tetrahydrothiophene 1-oxide, dibutyl sulfoxide, or di-
phenyl sulfoxide disappointingly gave almost no desired
product under the standard conditions, and only trace
amounts of 2a were detected by GC-MS when methyl phe-
nyl sulfoxide and 1a were utilized.
O
O
O
S
standard conditions
OH
O
O
+
1
2
O
O
O
O
O
O
O
3
2a: 76% (31%a)
2b: 82%
2c: 81%
O
O
O
O
O
O2N
2d: 78%
2e: 76%
2f: 64%
To get a good understanding of the reaction mechanism,
control experiments were conducted and the results are
displayed in Scheme 2. When the radical inhibitor 2,2,6,6-
tetramethylpiperidin-1-oxy (TEMPO) was added under the
standard conditions, the reaction was completely inhibited
[Scheme 2 (1)]. Using GC-MS to analyze the reaction mix-
ture, showed that the major product was 1-methoxy-
2,2,6,6-tetramethylpiperidine (3b), and it was isolated in
O
O
O
O
O
O
O
NO2
O
2g: 67%
2h: 14%
2i: 65%
O
O
O
O
O
O
F
Cl
Br
OH
2j: 61%
2k: 72%
2l: 70%
O
O
O
O
O
O
standard
conditions
O
S
+
+
O
O
O
O
N
TEMPO
(2.0 equiv)
(1)
O
HO
NC
OHC
2m: 63% (32%b)
2n: 70%
2o: 69%
O
O
O
2b
3b
OH
1b
trace isolated yield: 25%
based on TEMPO
O
O
O
O
O
standard
conditions
O
NH2
O
S
2q: 51% (23%b)
2r: 32%
+
+
OH
OH
OH
O
(2)
(3)
(4)
2p: 43%; 2p': 14%; 2p'/2p = 0.33
BQ
(2.0 equiv)
2p: 49%; 2p': 16% (27%b); 2p'/2p = 0.33c
2p: 30%; 2p': 6%; 2p'/2p = 0.20d
2p: 25%; 2p': 7%; 2p'/2p = 0.28e
O
O
O
O
2b
trace
O
1b
1b
O
O
O
O
O
standard
conditions
O
S
O
O
O
O
O
BHT
(2.0 equiv)
O
2s: 37%
2t: 75%
2u: 81%
2b
GC yield: 27%
O
O
O
O
standard
conditions
O
S
N
CD3
O
O
O
+
D3C
O
2v: 29% ( 65%b)
CD3
2w: 73%
2x: 70%
O
Scheme 1 Scope of the carboxylic acid substrate in O-methylation
2b'
1b
with dimethyl sulfoxide; the yields are isolated yields. a Using benzalde-
hyde instead of benzoic acid, GC yield. b Unreacted substrate. c Reaction
time 24 h. d Using H2O2 (2.0 equiv). e Using H2O2 (4.0 equiv).
isolated yield: 84%
Scheme 2 Control experiments
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25% yield based on TEMPO. Other radical inhibitors, such as
benzoquinone (BQ) and 2,6-di-tert-butyl-4-methylphenol
(BHT), had a similar effect on the reaction [Scheme 2 (2)
and (3)]. In addition, an isotope-labeling experiment was
conducted and the result confirmed that the methyl was
from dimethyl sulfoxide [Scheme 2 (4)]. These results indi-
cate that the reaction may take place by a radical pathway
with dimethyl sulfoxide as the methyl donor.
wise addition of 30% aq H2O2 (1.5 mmol, 3.0 equiv). The tube was
sealed with a Teflon-lined cap and the mixture was stirred at 80 °C
under O2 for 15 h. The mixture cooled and water (10 mL) was added;
the mixture was extracted with EtOAc (3 × 10 mL). After evaporation
of the solvent, the residue was purified by column chromatography
(silica gel) to obtain the product.
Methyl Benzoate (2a)21
[CAS Reg. No. 93-58-3]
Although the precise mechanism of the methyl transfer
remains unknown. On the basis of these results and litera-
ture reports, a plausible mechanism for the copper-cata-
lyzed O-methylation of carboxylic acids is proposed in
Scheme 3. Firstly, the benzoic acid forms intermediate 3 by
reacting with base and the copper salt, and the dimethyl
sulfoxide is decomposed to form a methyl radical by the hy-
droxyl radical,18 which is generated from hydrogen perox-
ide in the presence of a copper ion.19 Then the methyl radi-
cal reacts with the intermediate 3 to obtain the desired
product 2a.3,4,20
Purified by flash chromatography (PE–EtOAc, 10:1); colorless oil;
yield: 51.7 mg (76%).
1H NMR (400 MHz, CDCl3): δ = 8.04 (d, J = 7.9 Hz, 2 H), 7.53 (t, J = 7.4
Hz, 1 H), 7.42 (t, J = 7.6 Hz, 2 H), 3.90 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 167.1, 132.9, 130.1, 129.6, 128.3, 52.1.
LRMS (EI, 70 eV): m/z (%) = 137 (M+ + 1, 3), 136 (M+, 33), 105 (100), 77
(71).
Methyl 4-Methoxybenzoate (2b)21
[CAS Reg. No. 121-98-2]
Purified by flash chromatography (PE–EtOAc, 10:1); colorless solid;
yield: 68.1 mg (82%); mp 48–50 °C.
1H NMR (400 MHz, CDCl3): δ = 7.99 (d, J = 8.9 Hz, 2 H), 6.91 (d, J = 8.8
Hz, 2 H), 3.88 (s, 3 H), 3.84 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 166.8, 163.3, 131.6, 122.6, 113.6, 55.4,
base
PhCO2–[CuII]
PhCO2H
[CuII]
1a
3
+ [CuI]
(1)
PhCO2CH3
51.8.
[Cu]
2a
DMSO
H2O2
•OH
CH
3
•
LRMS (EI, 70 eV): m/z (%) = 167 (M+ + 1, 2), 166 (M+, 25), 135 (100),
107 (15).
[O]
[CuI]
[CuII]
(2)
Methyl-d3 4-Methoxybenzoate (2b′)22
Scheme 3 Proposed mechanism
[CAS Reg. No. 79825-71-1]
Purified by flash chromatography (PE–EtOAc, 10:1); colorless solid;
yield: 71 mg (84%); mp 49–52 °C.
1H NMR (400 MHz, CDCl3): δ = 7.91 (d, J = 8.8 Hz, 2 H), 6.83 (d, J = 8.8
Hz, 2 H), 3.77 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 165.8, 162.3, 130.5, 121.5, 112.5, 54.4,
In conclusion, we have demonstrated a novel, simple,
and environmentally friendly copper-catalyzed O-methyla-
tion of carboxylic acids with dimethyl sulfoxide. This prac-
tical method offers a strategy for replacing toxic, electro-
philic alkylation reagents. In this protocol, dimethyl sulfox-
ide not only serves as the solvent, but it is also a convenient
methyl donor, and hydrogen peroxide is used as the oxidant
which produces no toxic byproduct only water. In addition,
a wide range of substrates, including some substrates bear-
ing oxidant-sensitive groups, are well tolerated. Further-
more, this transformation is not sensitive to moisture and
oxygen.
50.8 (m, CD3).
LRMS (EI, 70 eV): m/z (%) = 170 (M+ + 1, 3), 169 (M+, 29), 135 (100),
107 (14).
Methyl 4-Butylbenzoate (2c)21
[CAS Reg. No. 20651-69-8]
Purified by flash chromatography (PE–EtOAc, 10:1); colorless oil;
yield: 77.8 mg (81%).
1H NMR (400 MHz, CDCl3): δ = 7.94 (d, J = 8.1 Hz, 2 H), 7.23 (d, J = 8.0
Hz, 2 H), 3.89 (s, 3 H), 2.65 (t, J = 7.7 Hz, 2 H), 1.65–1.56 (m, 2 H),
1.40–1.30 (m, 2 H), 0.92 (t, J = 7.3 Hz, 3 H).
13C NMR (100 MHz, CDCl3): δ = 167.2, 148.5, 129.6, 128.4, 127.7, 51.9,
35.7, 33.3, 22.3, 13.9.
All reagents were commercially available and used without further
purification. 1H NMR spectra were recorded at 400 MHz and 13C NMR
at 100 MHz in CDCl3 using TMS as the internal standard. The yield of
methyl benzoate was measured by GC using 1,4-dichlorobenzene as
internal standard when optimizing the reaction conditions. MS were
recorded using EI. Column chromatography was performed on silica
gel (200–300 mesh). Petroleum ether = PE.
LRMS (EI, 70 eV): m/z (%) = 193 (M+ + 1, 6), 192 (M+, 45), 161 (58), 91
(100).
Methyl 4-Methylbenzoate (2d)21
[CAS Reg. No. 99-75-2]
Methyl Esters; General Procedure
Carboxylic acid (0.5 mmol, 1.0 equiv), K2CO3 (0.5 mmol, 1.0 equiv),
and CaCl2 powder (0.5 mmol, 1.0 equiv) were added to a 25-mL tube.
10% CuCl2·2 H2O in DMSO (2.0 mL) was added, followed by the drop-
Purified by flash chromatography (PE–EtOAc, 10:1); colorless oil;
yield: 58.5 mg (78%).
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1H NMR (400 MHz, CDCl3): δ = 7.93 (d, J = 8.1 Hz, 2 H), 7.23 (d, J = 8.0
13C NMR (100 MHz, CDCl3): δ = 170.6, 139.3, 135.2, 130.9, 128.4, 51.8,
Hz, 2 H), 3.89 (s, 3 H), 2.40 (s, 3 H).
21.1, 19.8.
13C NMR (100 MHz, CDCl3): δ = 167.2, 143.5, 129.6, 129.1, 127.4, 51.9,
LRMS (EI, 70 eV): m/z (%) = 179 (M+ + 1, 7), 178 (M+, 53), 147 (100),
21.6.
119 (64).
LRMS (EI, 70 eV): m/z (%) = 151 (M+ + 1, 3), 150 (M+, 28), 119 (100), 91
(53).
Methyl 4-Fluorobenzoate (2j)21
[CAS Reg. No. 403-33-8]
Methyl 3-Methylbenzoate (2e)21
Purified by flash chromatography (PE–EtOAc, 10:1); colorless oil;
yield: 47.0 mg (61%).
[CAS Reg. No. 99-36-5]
Purified by flash chromatography (PE–EtOAc, 10:1); colorless oil;
yield: 57.0 mg (76%).
1H NMR (400 MHz, CDCl3): δ = 8.11–8.00 (m, 2 H), 7.11 (t, J = 8.6 Hz, 2
H), 3.91 (s, 3 H).
1H NMR (400 MHz, CDCl3): δ = 7.92–7.78 (m, 2 H), 7.37–7.29 (m, 2 H),
3.90 (s, 3 H), 2.39 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 167.0 (d, JC-F = 252.1 Hz), 166.1, 132.1
(d, JC-F = 9.2 Hz), 126.4 (d, JC-F = 2.9 Hz), 115.6 (d, JC-F = 21.8 Hz), 52.17.
13C NMR (100 MHz, CDCl3): δ = 167.3, 138.1, 133.7, 130.1, 130.1,
19F NMR (376 MHz, CDCl3): δ = –105.8.
128.2, 126.7, 52.0, 21.2.
LRMS (EI, 70 eV): m/z (%) = 151 (M+ + 1, 3), 150 (M+, 28), 119 (100), 91
LRMS (EI, 70 eV): m/z (%) = 155 (M+ + 1, 3), 154 (M+, 25), 123 (100), 95
(44).
(69).
Methyl 4-Chlorobenzoate (2k)21
Methyl 4-Nitrobenzoate (2f)3
[CAS Reg. No. 1126-46-1]
[CAS Reg. No. 619-50-1]
Purified by flash chromatography (PE–EtOAc, 10:1); colorless oil;
yield: 61.2 mg (72%).
1H NMR (400 MHz, CDCl3): δ = 7.98 (d, J = 8.3 Hz, 2 H), 7.42 (d, J = 8.3
Purified by flash chromatography (PE–EtOAc, 10:1); colorless solid;
57.9 mg (64%); mp 94–96 °C.
1H NMR (400 MHz, CDCl3): δ = 8.30 (d, J = 8.9 Hz, 2 H), 8.22 (d, J = 8.9
Hz, 2 H), 3.92 (s, 3 H).
Hz, 2 H), 3.99 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 165.2, 150.5, 135.5, 130.7, 123.6, 52.9.
LRMS (EI, 70 eV): m/z (%) = 182 (M+ + 1, 2), 181 (M+, 23), 164 (23), 150
13C NMR (100 MHz, CDCl3): δ = 166.2, 139.4, 131.0, 128.7, 128.6, 52.3.
LRMS (EI, 70 eV): m/z (%) = 172 (M+ + 2, 7), 170 (M+, 23), 139 (100),
111 (44).
(100).
Methyl 4-Bromobenzoate (2l)21
Methyl 2-Nitrobenzoate (2g)3
[CAS Reg. No. 619-42-1]
[CAS Reg. No. 606-27-9]
Purified by flash chromatography (PE–EtOAc, 10:1); colorless solid;
yield: 74.9 mg (70%); mp 78–80 °C.
1H NMR (400 MHz, CDCl3): δ = 7.89 (d, J = 8.5 Hz, 2 H), 7.57 (d, J = 8.5
Purified by flash chromatography (PE–EtOAc, 4:1); colorless oil; yield:
60.6 mg (67%).
1H NMR (400 MHz, CDCl3): δ = 7.91 (d, J = 7.9 Hz, 1 H), 7.74 (d, J = 7.4
Hz, 2 H), 3.91 (s, 3 H).
Hz, 1 H), 7.72–7.60 (m, 2 H), 3.92 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 165.8, 148.2, 132.9, 131.8, 129.8,
127.5, 123.9, 53.2.
LRMS (EI, 70 eV) m/z (%):182 (M+ + 1, 2), 181 (M+, 12), 150 (100), 92
(18), 77 (27).
13C NMR (100 MHz, CDCl3): δ = 166.3, 131.7, 131.1, 129.0, 128.0, 52.3.
LRMS (EI, 70 eV): m/z (%) = 216 (M+ + 2, 41), 214 (M+, 42), 185 (96),
183 (100).
Methyl 4-Hydroxybenzoate (2m)24
[CAS Reg. No. 99-76-3]
Dimethyl Terephthalate (2h)21
Purified by flash chromatography (PE–EtOAc, 4:1); colorless solid;
yield: 47.9 mg (63%); mp 127–130 °C.
[CAS Reg. No. 120-61-6]
Purified by flash chromatography (PE–EtOAc, 10:1); colorless solid;
yield: 13.6 mg (14%); mp 138–140 °C.
1H NMR (400 MHz, CDCl3): δ = 8.10 (s, 4 H), 3.95 (s, 6 H).
13C NMR (100 MHz, CDCl3): δ = 166.3, 133.9, 129.5, 52.4.
1H NMR (400 MHz, CDCl3): δ = 7.95 (d, J = 8.7 Hz, 2 H), 6.90 (d, J = 8.7
Hz, 2 H), 6.74 (s, 1 H), 3.90 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 167.6, 160.4, 132.0, 122.1, 115.3, 52.2.
LRMS (EI, 70 eV): m/z (%) = 153 (M+ + 1, 3), 152 (M+, 30), 121 (100), 93
LRMS (EI, 70 eV): m/z (%) = 195 (M+ + 1, 3), 194 (M+, 23), 163 (100),
(25).
135 (31).
Methyl 4-Cyanobenzoate (2n)6b
Methyl 2,4,6-Trimethylbenzoate (2i)23
[CAS Reg. No. 1129-35-7]
[CAS Reg. No. 2282-84-0]
Purified by flash chromatography (PE–EtOAc, 10:1); colorless solid;
yield: 56.4 mg (70%); mp 63–65 °C.
1H NMR (400 MHz, CDCl3): δ = 8.15 (d, J = 8.3 Hz, 2 H), 7.76 (d, J = 8.3
Purified by flash chromatography (PE–EtOAc, 10:1); colorless oil;
yield: 57.9 mg (65%).
1H NMR (400 MHz, CDCl3): δ = 6.85 (s, 2 H), 3.89 (s, 3 H), 2.28–2.27 (d,
Hz, 2 H), 3.97 (s, 3 H).
9 H).
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13C NMR (100 MHz, CDCl3): δ = 165.4, 133.9, 132.2, 130.1, 118.0,
13C NMR (100 MHz, CDCl3): δ = 167.7, 145.2, 139.4, 129.8, 118.5, 51.4,
116.4, 52.8.
18.6.
LRMS (EI, 70 eV): m/z (%) = 162 (M+ + 1, 2), 161 (M+, 19), 130 (100),
LRMS (EI, 70 eV): m/z (%) = 127 (M+ + 1, 5), 126 (M+, 57), 111 (92), 95
102 (54).
(62), 67 (100).
Methyl 4-Formylbenzoate (2o)5c
Methyl Cinnamate (2s)6b
[CAS Reg. No. 1571-08-0]
[CAS Reg. No. 103-26-4]
Purified by flash chromatography (PE–EtOAc, 10:1); colorless solid;
yield: 56.6 mg (69%); mp 60–62 °C.
Purified by flash chromatography (PE–EtOAc, 10:1); colorless solid;
yield: 30.0 mg (37%); mp 35–37 °C.
1H NMR (400 MHz, CDCl3): δ = 10.11 (s, 1 H), 8.20 (d, J = 8.0 Hz, 2 H),
7.96 (d, J = 8.1 Hz, 2 H), 3.97 (s, 3 H).
1H NMR (400 MHz, CDCl3): δ = 7.70 (d, J = 16.0 Hz, 1 H), 7.52 (dd, J =
6.7, 3.0 Hz, 2 H), 7.42–7.34 (m, 3 H), 6.44 (d, J = 16.0 Hz, 1 H), 3.81 (s, 3
H).
13C NMR (100 MHz, CDCl3): δ = 191.7, 166.1, 139.1, 135.1, 130.2,
129.5, 52.6.
13C NMR (100 MHz, CDCl3): δ = 167.4, 144.9, 134.4, 130.3, 128.9,
128.1, 117.8, 51.7.
LRMS (EI, 70 eV): m/z (%) = 165 (M+ + 1, 4), 164 (M+, 41), 133 (100),
105 (38).
LRMS (EI, 70 eV): m/z (%) = 163 (M+ + 1, 4), 162 (M+, 39), 131 (100),
103 (71).
Methyl 2-Aminobenzoate (2p)25
Methyl 2-Naphthoate (2t)3
[CAS Reg. No. 134-20-3]
[CAS Reg. No. 2459-25-8]
Purified by flash chromatography (PE–EtOAc, 4:1); colorless oil; yield:
32.5 mg (43%).
1H NMR (400 MHz, CDCl3): δ = 7.85 (d, J = 8.0 Hz, 1 H), 7.34–7.22 (m, 1
H), 6.72–6.59 (m, 2 H), 5.69 (s, 2 H), 3.87 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 168.6, 150.4, 134.1, 131.2, 116.7,
116.3, 110.8, 51.52.
LRMS (EI, 70 eV): m/z (%) = 152 (M+ + 1, 5), 151 (M+, 50), 119 (100), 92
(61).
Purified by flash chromatography (PE–EtOAc, 10:1); colorless solid;
yield: 69.8 mg (75%); mp 76–78 °C.
1H NMR (400 MHz, CDCl3): δ = 8.60 (s, 1 H), 8.05 (d, J = 8.6 Hz, 1 H),
7.93 (d, J = 8.0 Hz, 1 H), 7.86 (d, J = 8.8 Hz, 2 H), 7.59–7.50 (m, 2 H),
3.97 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 167.3, 135.5, 132.5, 131.1, 129.4,
128.3, 128.2, 127.8, 127.4, 126.7, 125.2, 52.3.
LRMS (EI, 70 eV): m/z (%) = 187 (M+ + 1, 7), 186 (M+, 52), 155 (87), 127
Methyl 2-(Methylamino)benzoate (2p′)26
(100).
[CAS Reg. No. 85-91-6]
Methyl Furan-2-carboxylate (2u)3
Purified by flash chromatography (PE–EtOAc, 4:1); colorless oil; yield:
10.7 mg (14%).
[CAS Reg. No. 611-13-2]
Purified by flash chromatography (PE–EtOAc, 10:1); colorless oil;
yield: 51.0 mg (81%).
1H NMR (400 MHz, CDCl3): δ = 7.64–7.50 (m, 1 H), 7.19 (d, J = 3.5 Hz, 1
H), 6.52 (dd, J = 3.6, 1.7 Hz, 1 H), 3.90 (s, 3 H).
1H NMR (400 MHz, CDCl3): δ = 7.89 (dd, J = 8.0, 1.6 Hz, 1 H), 7.63 (s, 1
H), 7.38 (m, 1 H), 6.66 (d, J = 8.5 Hz, 1 H), 6.58 (t, J = 7.5 Hz, 1 H), 3.85
(s, 3 H), 2.91 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 169.1, 152.0, 134.6, 131.6, 114.3,
110.7, 109.9, 51.4, 29.6.
13C NMR (100 MHz, CDCl3): δ = 159.1, 146.3, 144.6, 117.9, 111.9, 51.9.
LRMS (EI, 70 eV): m/z (%) = 166 (M+ + 1, 7), 165 (M+, 67), 132 (52), 105
LRMS (EI, 70 eV): m/z (%) = 127 (M+ + 1, 2), 126 (M+, 29), 95 (100), 39
(100), 77 (65).
(18).
Methyl 2-Hydroxy-2-phenylacetate (2q)27
Methyl Quinoline-2-carboxylate (2v)29
[CAS Reg. No. 4358-87-6]
[CAS Reg. No. 19575-07-6]
Purified by flash chromatography (PE–EtOAc, 4:1); colorless oil; yield:
42.3 mg (51%).
Purified by flash chromatography (PE–EtOAc, 3:1); colorless solid;
yield: 27.1 mg (29%); mp 174–177 °C.
1H NMR (400 MHz, CDCl3): δ = 7.44–7.30 (m, 5 H), 5.17 (s, 1 H), 3.75
(s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 174.1, 138.3, 128.6, 128.5, 126.6, 72.9,
53.0.
1H NMR (400 MHz, CDCl3): δ = 8.31 (d, J = 8.6 Hz, 2 H), 8.20 (d, J = 8.5
Hz, 1 H), 7.88 (d, J = 8.2 Hz, 1 H), 7.80 (t, J = 7.6 Hz, 1 H), 7.66 (t, J = 7.5
Hz, 1 H), 4.09 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 166.0, 147.9, 147.5, 137.3, 130.7,
130.3, 129.4, 128.7, 127.6, 121.0, 53.2.
LRMS (EI, 70 eV): m/z (%) = 187 (M+, 5), 157 (14), 129 (100), 101 (18).
LRMS (EI, 70 eV): m/z (%) = 166 (M+, 6), 107 (100), 79 (91), 51 (20).
Methyl (2E,4E)-Hexa-2,4-dienoate (2r)28
Methyl 2-Phenylacetate (2w)5c
[CAS Reg. No. 689-89-4]
[CAS Reg. No. 101-41-7]
Purified by flash chromatography (PE–EtOAc, 10:1); colorless oil;
yield: 20.2 mg (32%).
1H NMR (400 MHz, CDCl3): δ = 7.30–7.22 (m, 1 H), 6.24–6.10 (m, 2 H),
5.78 (d, J = 15.4 Hz, 1 H), 3.74 (s, 3 H), 1.85 (d, J = 5.7 Hz, 3 H).
Purified by flash chromatography (PE–EtOAc, 10:1); colorless oil;
yield: 54.8 mg (73%).
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1H NMR (400 MHz, CDCl3): δ = 7.36–7.24 (m, 5 H), 3.69 (s, 3 H), 3.63
(s, 2 H).
13C NMR (100 MHz, CDCl3): δ = 172.1, 134.0, 129.3, 128.6, 127.1, 52.1,
41.2.
Angew. Chem. Int. Ed. 2014, 53, 13253. (n) Mita, T.; Higuchi, Y.;
Sato, Y. Org. Lett. 2014, 16, 14. (o) Smith, J. M.; Moreno, J.; Boal,
B. W.; Garg, N. K. J. Org. Chem. 2015, 80, 8954.
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(3) Zhu, Y.; Yan, H.; Lu, L.; Liu, D.; Rong, G.; Mao, J. J. Org. Chem.
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3326.
LRMS (EI, 70 eV): m/z (%) = 150 (M+, 17), 91 (100), 65 (15), 43 (20).
Methyl Hexanoate (2x)30
[CAS Reg. No. 106-70-7]
(5) (a) Selva, M.; Tundo, P. J. Org. Chem. 2006, 71, 1464. (b) Aricò, F.;
Tundo, P. Russ. Chem. Rev. 2010, 79, 479. (c) Ji, Y.; Sweeney, J.;
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Purified by flash chromatography (CH2Cl2); colorless oil; yield: 45.5
mg (70%).
1H NMR (400 MHz, CDCl3): δ = 3.67 (s, 3 H), 2.31 (t, J = 7.6 Hz, 2 H),
1.67–1.59 (m, 2 H), 1.34–1.28 (m, 4 H), 0.90 (t, J = 6.8 Hz, 3 H).
13C NMR (100 MHz, CDCl3): δ = 174.4, 51.5, 34.1, 31.3, 24.7, 22.3, 13.9.
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6725.
LRMS (EI, 70 eV): m/z (%) = 101 (9), 99 (20), 87 (31), 74 (100).
1-Methoxy-2,2,6,6-tetramethylpiperidine (3b)31
[CAS Reg. No. 34672-84-9]
Purified by flash chromatography (PE–EtOAc, 10:1); colorless oil;
yield: 43 mg (25%).
1H NMR (400 MHz, CDCl3): δ = 3.61 (s, 3 H), 1.45 (m, 6 H), 1.17 (s, 6 H),
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1.08 (s, 6 H).
13C NMR (100 MHz, CDCl3): δ = 65.3, 59.7, 39.6, 33.0, 20.0, 17.0.
LRMS (EI, 70 eV): m/z (%) = 171 (M+, 5), 156 (100), 125 (7), 109 (23).
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Acknowledgment
This work was supported by the National Natural Science Foundation
of China (Grant No. 21372068 and 21572049).
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1560967.
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