Catalyst-Free Photodriven Reduction of α-Haloketones with Hantzsch Ester

Article abstract of DOI:10.1055/s-0037-1610629

Catalyst-free dehalogenation of α-haloketones under visible light irradiation is studied. The reactions were carried out in common organic solvent. The outcomes of dechlorination are excellent in yields up to 92%, and it is also applicable to bromides, which give even higher yields. The reaction is tolerable to a broad spectrum of substrates, especially to aromatic ketones, including various aryl and hetaryl groups. There are two examples of aliphatic ketones presented in the paper, although their reactivities are not as high as that of the aromatic ketones.

Full text of DOI:10.1055/s-0037-1610629

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2018, 50, A–H
paper
A
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Z. Lu, Y.-Q. Yang
Paper
Synthesis
Catalyst-Free Photodriven Reduction of α-Haloketones with
Hantzsch Ester
.
Zheng Lu
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Yong-Qing Yang*
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EtO
OEt
School of Pharmacy, Jiangsu University, Zhenjiang,
Jiangsu 212013, People’s Republic of China
yqy@ujs.edu.cn
O
O
N
H
R2
R2
R1
R1
AcOH
X
Dedicated to Professor Xiyan Lu on the occasion of
his 90th birthday.
blue LED, 3 W
R
R
1 = aryl, hetaryl, alkyl
2 = H, Ph, alkyl
X = Cl, Br
26 examples, yield up to 98%
Received: 20.05.2018
Accepted after revision: 04.08.2018
Published online: 29.08.2018
The report on the reduction of α-haloaromatic ketone
could be traced back to 1962,4 since then, extensive studies
have been done on these reactions with numerous papers
published.5 Reiser’s group has developed a photoreduction
method catalyzed by Ru(bpy)3Cl2 with ascorbic acid as the
reducing agent and 1,5-dimethoxynaphthalene (DMN) as
an electron transporter to achieve dehalogenation of α-
haloketones.6 Since the bond dissociation energy of car-
bon–bromine bond is often lower than that of the carbon–
chlorine bond,7 bromides are more favored substrates that
have been studied, whereas only a handful of papers on re-
duction of iodides8 and fluorides9 have been presented.
Hantzsch ester is widely used as a reductive quencher in
photoinduced reactions. Interestingly, it has been reported
in the 1970s that some Hantzsch dihydropyridines are light
sensitive.1c Recently, research work on reduction with
Hantzsch ester under light without added photocatalyst is
showing up.5d,10 Cho’s group has reported that bromometh-
yl aromatic ketones could be effectively reduced with
Hantzsch ester under catalyst-free photoreduction condi-
tions although only limited to bromoacetylaromatic com-
pounds.5d However, there are still some issues to be ad-
dressed in the arena of photoreduction of α-halo aromatic
ketones, such as broader halogen spectrum, toleration of α-
substitution and more complexed aromatic and hetero aro-
matic system. Herein we report our findings towards solv-
ing these problems (Scheme 1).
DOI: 10.1055/s-0037-1610629; Art ID: ss-2018-h0351-op
Abstract Catalyst-free dehalogenation of α-haloketones under visible
light irradiation is studied. The reactions were carried out in common
organic solvent. The outcomes of dechlorination are excellent in yields
up to 92%, and it is also applicable to bromides, which give even higher
yields. The reaction is tolerable to a broad spectrum of substrates, es-
pecially to aromatic ketones, including various aryl and hetaryl groups.
There are two examples of aliphatic ketones presented in the paper, al-
though their reactivities are not as high as that of the aromatic ketones.
Key words Hantzsch ester, catalyst-free, visible light, dehalogena-
tion, environment-benign
There is no doubt that reduction is significant in organic
chemistry, and strategies to achieve it could be classified
mainly as the following three types: catalyzed hydrogena-
tion or transfer hydrogenation, reduction with inorganic
hydrides, and reduction with organic hydride donors. As an
important organic hydride donor, Hantzsch dihydropyri-
dines1 have been widely explored as reductants in a variety
of reductions either catalyzed or uncatalyzed (Figure 1).2,3
O
.
O
G
G
N
H
The bond dissociation energy difference between C–Cl
and C–Br bonds is around 60 kJ·mol–1,7 which means, to the
best of our knowledge, that extra energy or catalysts are re-
quired for dechlorination than debromination. Initially, 2-
chloroacetophenone (2a) was selected as the standard sub-
strate for our exploration in the photodriven reduction on
chlorides. First, blue LED was used as the light source,
1
1a, G = OEt Hantzsch 18811a
1b, G = OMe Kurtz 19611b
1c, G = Oi-Pr Uldrikis 19751c
1d, G = Ot-Bu Uldrikis 19751c
1e, G = NHR Gelbard 19921d
Figure 1 Hantzsch dipyridines reported
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H

B
Z. Lu, Y.-Q. Yang
Paper
Synthesis
Table 1 Optimization of Reaction Conditions with 2-Chloroacetophe-
O
O
.
nonea
EtO
OEt
EtO2C
CO2Et
O
O
N
H
Br
O
N
H
1a
O
CF3CH2OH
CFL lamp, 14 W
Cho's group5d
G
G
Cl
blue LED 3 W
2a
3a
Time (h)b Yield (%)c
O
O
Entry
Solvent
Additive
Ru(bpy)3(PF6)2 (0.05 mol%) 2.5
EtO
OEt
1
2
3
4
5
6
7
MeCN
MeCN
DMF
76
38
73
32
58
80
35
O
–
6.5
3
O
N
H
R2
R2
R1
–
R1
AcOH
X
toluene
CF3CH2OH
AcOH
–
7.5
1
blue LED, 3 W
this work
R1 = aryl, hetaryl, alkyl
2 = H, Ph, alkyl
–
R
–
2.1
1.5
X = Cl, Br
AcOH
open to air
Scheme 1 Catalyst-free photoreduction of α-haloketones
a The reactions were carried out under N2, 1 mmol of compound 2a (0.4 M)
and 1.2 equiv of Hantzsch ester were used.
b The irradiation was terminated when the mixture turned off-white to col-
orless solution. TLC showed that compound 2a or 1a was consumed.
c Isolated yield.
Ru(byp)3 was added5b,11 as the photocatalyst (Table 1, en-
try 1), and acetonitrile was selected as the optimal solvent
as it was found that the ruthenium reagent [Ru(bpy)3(PF6)2]
dissolves well in it.12 To our delight, the dehalogenation
worked smoothly in a good yield within a reasonable peri-
od even with a low catalysis loading (0.05 mol%). Driven by
our curiosity, a control experiment was carried out to see
whether the photocatalyst is essential (entry 2). Surprising-
ly, there was a remarkable level of background reaction, al-
beit not as good as that with photocatalyst. Subsequently,
solvents were screened to find out the optimal one for this
catalyst-free photodriven reduction on chlorides. It turns
out that in trifluoroethanol reduction occurred fastest (en-
try 5), although the yield was just moderate. The outcome
of the uncatalyzed reaction in DMF is comparable to that
under ruthenium complex catalyst (entry 3 vs. entry 1) in
terms of yield and reaction frame. Considering that the re-
duction might benefit from enhancing the activity of the
ketone via hydrogen bonding, we have tried acetic acid,
which is a green organic solvent13 (entry 6). To our gratifi-
cation, the yield was better than that with ruthenium pho-
tocatalyst.
2+
action of tetralone chloride was rather sluggish (entry 6),
which could mean that planar intermediate is not of desired
energy profile.
Table 2 Reduction of 2-Chloroaromatic Ketonesa
EtO2C
CO2Et
O
O
N
H
1a
Cl
Ar
Ar
AcOH
R
R
blue LED 3 W
3
2
Entry
Ar
Ph
R
Time (h)b
Yield (%)c
1
2
3
4
5
6
H (2a)
2.1
3
80 (3a)
79 (3b)
92 (3c)
88 (3d)
75 (3e)
48 (3f)
4-ClC6H4
4-BrC6H4
4-MeOC6H4
Ph
H (2b)
H (2c)
3
H (2d)
1.5
4.5
5
Me (2e)
(CH2)2 (2f)
Under the optimal condition, more α-chloro aromatic
ketones were subjected to the reductive dechlorination.
Most of the chloroacetyl ketones gave good results while
substituents on the phenyl ring made some difference on
the reaction duration. Introduction of a methoxy group
could accelerate the reaction and improve the yield (Table 2,
entry 4). This indicates that an electron-deficient interme-
diate might be generated in the reaction. When the chlorine
atom is on a secondary carbon (entry 5), the reaction was
retarded although the yield was still good. However, the re-
o-C6H4
a The reactions were carried out under N2, 1 mmol of compound 2 (0.4 M)
and 1.2 equiv of Hantzsch ester were used.
b The irradiation was terminated when the mixture turned off-white to col-
orless solution. TLC showed that compound 2 or 1a was consumed.
c Isolated yield.
The reaction with chlorides went quite well, and we
were keen to find out whether this condition could be ap-
plied on bromides. A spectrum of bromoacetophenones
was screened and the results were better than their coun-
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H

C
Z. Lu, Y.-Q. Yang
Paper
Synthesis
terparts bearing chloride as we would expect. Whereas
2-bromo-1-(4-trifluoromethyl)phenylethanone (2l) gave
rather poor results, which shows that the reaction interme-
diate could be destabilized by electron-withdrawing group
on the phenyl ring (Table 3, entry 7). Interestingly, when
the reaction was carried out open in air, the outcome re-
mains as good as that of the reaction done under N2 (entries
7 and 8). Comparing these results with that from compound
2a (Table 1, entry 7), it could be inferred that the bromide
has a higher oxidative potential than that of the chloride.
Table 4 Reduction of Various 2-Bromoketonesa
EtO2C
CO2Et
O
O
N
H
R2
1a
R2
R1
R1
AcOH
blue LED 3 W
Br
2
3
Entry R1
R2
Time (h)b Yield (%)c
1
2
Ph
Ph
Ph
Me (2m)
1.5
1.3
6.5
1.2
5.5
1
83 (3e)
98 (3i)
29 (3j)
73 (3k)
64 (3l)
89 (3m)
69 (3n)
47 (3f)
94 (3o)
53 (3p)
90 (3q)
31 (3r)
50 (3s)
39 (3t)
Et (2n)
Me2 (2o)
Et (2p)
Ph (2q)
H (2r)
Table 3 Reduction of 2-Bromoacetophenonesa
3
EtO2C
CO2Et
4
4-BrC6H4
Ph
5
O
O
N
H
1a
6
2-naphthyl
Br
AcOH
7
2-benzofuranyl
o-C6H4
H (2s)
1.5
5
G
G
blue LED 3 W
8
(CH2)2 (2t)
H (2u)
H (2v)
3
2
9
9-anthracenyl
2,4,6-Me3C6H2
1-tosylindol-3-yl
pyridin-3-yld
1-admantyl
0.7
5.5
2
10
11
12
13
14
Entry
G
Time (h)b
Yield (%)c
H (2w)
H (2x)
1
2
3
4
5
6
7
8
H (2g)
1
94 (3a)
85 (3g)
87 (3b)
88 (3c)
98 (3d)
96 (3d)
63 (3h)
67 (3h)
6.5
3
Me (2h)
Cl (2i)
1
H (2y)
1.1
1.1
0.5
0.3
3
2-bromo-4-phenylcyclohexanone (2z)
5
Br (2j)
a The reactions were carried out under N2. 1 mmol of compound 2 (0.4 M)
and 1.2 equiv of Hantzsch ester were used.
MeO (2k)
MeO (2k)d
CF3 (2l)
CF3 (2l)d
b The irradiation was terminated when the mixture turned off-white to color-
less solution. TLC showed that compound 2 or 1a was consumed.
c Isolated yield.
d HBr salt was used.
2
a The reactions were carried out under N2, 1 mmol of compound 2 (0.4 M)
and 1.2 equiv of Hantzsch ester were used.
group, electron-rich rings like benzofuranyl and tosylindo-
lyl gave reasonable results (entries 7 and 11), whereas elec-
tron-deficient ring pyridine disfavored the reaction with
low yield and retarded reaction time (entry 12). To further
explore the generality of this reduction, the nonaromatic
bromides have also been tested, it was exciting to find that
α-bromoaliphatic ketones (entries 13 and 14) could also
participate in the reaction, although only moderated yields
were achieved while the reaction time were relatively long.
Besides all these successful results, two substrates are
inert to this catalyst-free photodriven reduction, which are
the bromoacetylferrocene and 3-bromocamphor. For bro-
moacetylferrocene, it might be because the substrate itself
absorbs too much light to prevent Hantzsch dihydropyri-
dine from being excited. 4-Me-Hantzsch ester 1f (Figure 2)
is reported to have an increased light absorption than
Hantzsch ester,5d however, when it was applied to this reac-
tion, still no dehalogenation product was observed.
b The irradiation was terminated when the mixture turned off-white to col-
orless solution. TLC showed that compound 2 or 1a was consumed.
c Isolated yield.
d The reaction was open to air.
As it was shown that the substituent on the aromatic
ring could affect the reaction, we were curious about the
influence of the substituent on the α-carbon in alkyl side.
The study was carried out on diverse bromoketones as it is
more facile to prepare bromides. Introduction of an ethyl
group slowed down the reaction slightly, while the yield re-
mained excellent (Table 4, entry 2). Surprisingly, introduc-
tion of a methyl group decreased the yield of the reaction
(entry 1) and introduction of two methyl groups further re-
duced the yield of the reaction and retarded the reaction
significantly (entry 5). A phenyl group also disfavored the
reaction with decreased yield and prolonged reaction time
(entry 5).
When the phenyl ring is replaced with a naphthyl or an-
thracenyl group, the results remain almost the same (Table
4, entries 6 and 9). If it is replaced with a mesityl group, the
reaction speed is reduced with inferior yield (entry 10).
When the phenyl ring is replaced with heteroaromatic
From the results in Tables 2–4, we can see that the reac-
tion rate of bromide is about 2 to 3 times that of chloride.
However, for the halotetralones, the reaction rates were of
little difference (Table 2, entry 6 vs. Table 4, entry 8). These
results indicate that the reaction of the open chain com-
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H

D
Z. Lu, Y.-Q. Yang
Paper
Synthesis
O
O
.
Common solvents and reagents were used as received from commer-
cial sources. Melting points were recorded on a WRS-1B digital melt-
ing point recorder from Shanghai Precision Scientific Instrument Cor-
poration or on a hot-stage micro-melting point apparatus equipped
with a microscope. Both were corrected by benzoic acid (analytical
grade from Sinopharm Chemical, mp 119.8–120.7 °C) and p-bromoa-
cetophenone (>98% from Energy Chemical, mp 48.5–48.9 °C). IR spec-
tra were recorded on a Nicolet Avatar 370 DTGS Instrument. NMR
spectra were recorded on a Bruker AV400 or Bruker AV500 spectrom-
eter, the offset of 1H NMR was positioned by TMS as 0 ppm, while the
peak of CDCl3 at 77.16 ppm was used for 13C NMR (as the peak for
TMS is often too small). ESI-MS data were recorded on a Shimadzu
LCMS-2010EV mass spectrometer. EI-MS data were recorded on an
Angilent 7890A/5973N GC/MS spectrometer. EI-HR was recorded on a
Premier CAB088 instrument. Column chromatography was per-
formed on Jiangyou silica gel (200–300 mesh).
EtO
OEt
N
H
1f
Figure 2 4-Me-Hantzsch ester
pounds and that of the cyclic compounds follow different
pathways. The open chain compounds are more favored in
terms of energy. The compound with α,α-dimethyl pre-
sented the lowest yield after the longest reaction period
(Table 4, entry 3).
Cho’s group has proposed a mechanism via the radical
of acetophenone (3*) (Figure 3),5d which perfectly ex-
plained their findings from debromonation of bromometh-
yl aromatic ketones. However, some of our results are rath-
er incompatible with this mechanism.
Reduction of α-Haloketones 2; General Procedure
Into a flask charged with the respective α-haloketone 2 (1 mmol) and
ethidine (Hantzsch’s ester, 1a; 1.2 mmol), was added the solvent as
per Tables 2–4 (2.5 mL). The flask was then attached to a balloon
filled with N2 and irradiated with 3 W blue LED at a distance of 5 cm.
The workup was followed when TLC showed that ethidine or the
haloketone was consumed. When AcOH was used as the solvent, the
reaction was worked up as follows: the reaction mixture was parti-
tioned between EtOAc (40 mL) and H2O (10 mL), then the organic
phase was washed with sat. aq NaHCO3 (3 × 15 mL) and brine (15
mL), and dried (anhyd Na2SO4). After concentration under reduced
pressure, the residue was subjected to flash chromatography for puri-
fication eluting with petroleum ether (PE) and CH2Cl2. When aprotic
polar solvents were used, the reaction mixture was diluted with EtO-
Ac (40 mL) and the organic layer was washed with H2O (3 × 15 mL)
and brine (15 mL), and dried (anhyd Na2SO4). When volatile solvents
were used, the mixture was concentrated under reduced pressure
and the residue was subjected to flash chromatography for purifica-
tion.
O
3*
Figure 3 Acetophenone radical
First, for compound 2u (Table 4, entry 9), albeit, the an-
thracenyl moiety and the bromoacetyl moiety are almost
perpendicular to each other,14 its reaction rate is even faster
than that of compound 2g (Table 3, entry 1). Furthermore,
9-bromoanthracene (4) was obtained, although the yield is
very low (2%). These two phenomena are rather weird
based on Cho’s mechanism.5d
Acetophenone (3a)15
Table 2-1: Colorless liquid; yield: 100 mg (80%); Rf = 0.25 (CH2Cl2/PE
1:1).
Second, the outcome of compound 2o is pretty poor, in
terms of yield and reaction rate (Table 4, entry 3). This re-
sult is just opposite to the deduction from the reported
mechanism.5d Because if the reaction proceeds via 3* type
radical, this reaction would be much favored compared
with compound 2m regarding the tendency to release the
steric hindrance via debromonation and the stability of the
generated radical intermediates (3° radical vs. 2° radical).
In summary, photodriven reduction of α-chloro aromat-
ic ketones with Hantzsch ester as reducing agent in an envi-
ronmentally benign solvent is realized, and this condition
could be extended to α-bromo aromatic ketones as well.
Different aromatic and hetero aromatic rings are tolerable.
Substituent on the α-carbon is tolerable, further substitu-
ent leads to low yields and prolonged reaction time. A lim-
ited scope of α-bromo aliphatic ketones could work under
this condition with moderate yields. Some of our results are
hard to explain via simple radical 3* approach, therefore
the reaction mechanism needs to be further explored.
1H NMR (400 MHz, CDCl3): δ = 7.98–7.95 (m, 2 H), 7.59–7.55 (m, 1 H),
7.49–7.45 (m, 2 H), 2.62 (s, 3 H).
Table 3-1: Colorless liquid; yield: 122 mg (94%).
IR (KBr, film): 1684 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 7.97–7.95 (m, 2 H), 7.57 (t, J = 7.6 Hz, 1
H), 7.46 (t, J = 7.6 Hz, 2 H), 2.61 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 198.26, 137.26, 133.21, 128.68,
128.42, 26.71.
ESI-MS: m/z = 121.0 ([M + H]+).
1-[(4-Chloro)phenyl]ethanone (3b)15
Table 2-2: Colorless crystals; yield: 154 mg (79%); Rf = 0.32 (CH2Cl2/PE
2:5); mp 17.0–18.5 °C (Lit.16 mp 18 °C).
IR (KBr, film): 1687 cm–1
.
1H NMR (500 MHz, CDCl3): δ = 7.90 (dt, J = 9.0, 2.0 Hz, 2 H), 7.44 (dt,
J = 9.0, 2.0 Hz, 2 H), 2.59 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 196.92, 139.71, 135.59, 129.86,
129.03, 26.69.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H

E
Z. Lu, Y.-Q. Yang
Paper
Synthesis
ESI-MS: m/z = 155.0 ([M + H]+), 157.8 ([M + 2 + H]+).
Table 3-3: Colorless liquid; yield: 140 mg (87%).
Table 4-8: Light yellow liquid; yield: 76 mg (47%).
IR (KBr, film): 1683 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 7.90 (ddd, J = 9.2, 2.4, 2.0 Hz, 2 H), 7.44
(ddd, J = 9.2, 2.4, 2.0 Hz, 2 H), 2.60 (s, 3 H).
1H NMR (500 MHz, CDCl3): δ = 8.03 (dd, J = 7.5, 1.0 Hz, 1 H), 7.46 (td,
J = 7.5, 1.5 Hz, 1 H), 7.30 (td, J = 7.5, 0.5 Hz, 1 H), 7.25 (d, J = 8.0 Hz, 1
H), 2.97 (t, J = 6.0 Hz, 2 H), 2.66 (dd, J = 7.0, 6.0 Hz, 2 H), 2.14 (quint,
J = 6.5 Hz, 2 H).
1-(4-Bromophenyl)ethanone (3c)17
13C NMR (126 MHz, CDCl3): δ = 198.47, 144.60, 133.50, 132.76,
Table 2-3: White solid; yield: 154 mg (92%); Rf = 0.21 (CH2Cl2/PE 1:1).
128.89, 127.29, 126.75, 39.31, 29.84, 23.42.
ESI-MS: m/z = 146.9 ([M + H]+).
1H NMR (400 MHz, CDCl3): δ = 7.82 (dt, J = 8.4, 2.0 Hz, 2 H), 7.61 (dt,
J = 8.8, 2.0 Hz, 2 H), 2.59 (s, 3 H).
Table 3-4: White solid; yield: 147 mg (88%); mp 47.1–48.2 °C (Lit.16
mp 48–52 °C).
1-[(4-Methyl)phenyl]ethanone (3g)15
Table 3-2: Light yellow liquid; yield: 141 mg (85%); Rf = 0.33 (CH2-
Cl2/PE 1:1).
IR (KBr, film): 1684 cm–1
1H NMR (400 MHz, CDCl3): δ = 7.82 (dt, J = 8.8, 2.0 Hz, 2 H), 7.61 (dt,
J = 8.8, 2.0 Hz, 2 H), 2.59 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 197.12, 135.97, 132.03, 129.97,
128.43, 26.66.
.
IR (KBr, film): 1683 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 7.86 (d, J = 8.0 Hz, 2 H), 7.25 (d, J = 8.0
Hz, 2 H), 2.57 (s, 3 H), 2.41 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 197.96, 143.99, 134.87, 129.36,
128.56, 26.63, 21.74.
ESI-MS: m/z = 198.9 ([M + H]+), 200.9 ([M + 2 + H]+).
ESI-MS: m/z = 135.0 ([M + H]+).
1-(4-Methoxyphenyl)ethanone (3d)17
Table 2-4: Off-white solid; yield: 134 mg (88%); Rf = 0.21 (CH2Cl2/PE
1:1).
1-[(4-Trifluoromethyl)phenyl]ethanone (3h)20
Table 3-7: White solid; yield: 132 mg (63%); mp 24.9–26.5 °C; Rf =
0.25 (CH2Cl2/PE 5:2)1H NMR (400 MHz, CDCl3): δ = 8.07 (d, J = 8.0 Hz,
2 H), 7.74 (d, J = 8.0 Hz, 2 H), 2.65 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 197.10, 139.84, 134.60 (d, J = 33.3 Hz),
128.77, 125.83 (q, J = 4.0 Hz), 123.75 (d, J = 273.7 Hz), 26.91.
MS (EI): m/z (%) = 188 (M+, 15), 173 (M – 15, 100), 169, 145, 125, 95,
75.
1H NMR (400 MHz, CDCl3): δ = 7.94 (d, J = 8.4 Hz, 2 H), 6.93 (d, J = 8.4
Hz, 2 H), 3.87 (br s, 3 H), 2.56 (br s, 3 H).
Table 3-5: Off-white solid; yield: 155 mg (98%); mp 33.5–35.2 °C
(Lit.16 mp 36–39 °C).
IR (KBr, film): 1676 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 7.94 (dt, J = 8.8, 2.4 Hz, 2 H), 6.94 (dt,
J = 8.8, 2.4 Hz, 2 H), 3.87 (s, 3 H), 2.56 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 196.89, 163.63, 130.72, 130.52,
113.82, 55.60, 26.46.
ESI-MS: m/z = 151.1 ([M + H]+).
Butyrophenone (3i)18
Table 4-2: Colorless liquid; yield: 151 mg (98%); Rf = 0.41 (CH2Cl2/PE
1:1).
IR (KBr, film): 1684 cm–1
.
Propiophenone (3e)18
1H NMR (400 MHz, CDCl3): δ = 7.98–7.95 (m, 2 H), 7.57–7.53 (m, 1 H),
7.48–7.44 (m, 2 H), 2.95 (t, J = 7.2 Hz, 2 H), 1.77 (sext, J = 7.2 Hz, 2 H),
1.01 (t, J = 7.2 Hz, 3 H).
13C NMR (101 MHz, CDCl3): δ = 200.52, 137.26, 132.97, 128.66,
128.16, 40.64, 17.90, 14.01.
Table 2-5: Light yellow liquid; yield: 115 mg (75%); Rf = 0.33
(CH2Cl2/PE 1:3).
1H NMR (400 MHz, CDCl3): δ = 7.98–7.96 (m, 2 H), 7.55 (t, J = 7.2 Hz, 1
H), 7.46 (t, J = 7.2 Hz, 2 H), 3.01 (q, J = 7.2 Hz, 2 H), 1.23 (t, J = 7.2 Hz, 3
H).
ESI-MS: m/z = 148.9 ([M + H]+).
Table 4-1: Light yellow liquid; yield: 101 mg (83%); mp 16.6–18.7 °C
(Lit.19 mp 18 °C).
2-Methyl-1-phenylpropan-1-one (3j)21
IR (KBr, film): 1688 cm–1
1H NMR (400 MHz, CDCl3): δ = 7.98–7.96 (m, 2 H), 7.55 (t, J = 7.2 Hz, 1
H), 7.46 (t, J = 7.2 Hz, 2 H), 3.01 (q, J = 7.2 Hz, 2 H), 1.23 (t, J = 7.2 Hz, 3
H).
13C NMR (101 MHz, CDCl3): δ = 200.93, 137.09, 132.98, 128.67,
128.10, 31.90, 8.37.
ESI-MS: m/z = 135.0 ([M + H]+).
.
Table 4-3: Colorless oil; yield: 51 mg (29%); Rf = 0.53 (CH2Cl2/PE 1:1).
IR (KBr, film): 1684 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 7.97–7.95 (m, 2 H), 7.58–7.53 (m, 1 H),
7.49–7.45 (m, 2 H), 3.56 (sept, J = 6.8 Hz, 1 H), 1.22 (d, J = 6.8 Hz, 6 H).
13C NMR (101 MHz, CDCl3): δ = 204.64, 136.41, 132.91, 128.74,
128.46, 35.51, 19.29.
ESI-MS: m/z = 149.0 ([M + H]+).
3,4-Dihydro-1(2H)-naphthalenone (3f)17
1-(4-Bromophenyl)-1-butanone (3k)15
Table 2-6: Light yellow liquid; yield: 90 mg (48%); Rf = 0.20 (CH2Cl2/PE
1:1).
1H NMR (400 MHz, CDCl3): δ = 8.04 (dd, J = 8.0, 0.8 Hz, 1 H), 7.47 (td,
J = 7.2, 1.6 Hz, 1 H), 7.31 (td, J = 7.6, 0.8 Hz, 1 H), 7.25 (d, J = 7.6 Hz, 1
H), 2.97 (t, J = 6.0 Hz, 2 H), 2.66 (dd, J = 7.2, 6.0 Hz, 2 H), 2.14 (quint,
J = 6.4 Hz, 2 H).
Table 4-4: White solid; yield: 163 mg (73%); mp 34.0–35.5 °C (Lit.22
mp 38–39 °C); Rf = 0.39 (CH2Cl2/PE 1:1).IR (KBr, film): 1683 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 7.82 (dt, J = 8.4, 2.4 Hz, 2 H), 7.60 (dt,
J = 8.8, 2.4 Hz, 2 H), 2.91 (t, J = 7.2 Hz, 2 H), 1.76 (sext, J = 7.2 Hz, 2 H),
1.00 (t, J = 7.2 Hz, 3 H).
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H

F
Z. Lu, Y.-Q. Yang
Paper
Synthesis
13C NMR (101 MHz, CDCl3): δ = 199.42, 135.98, 132.00, 129.73,
MS (EI): m/z (%) = 258 (M + 2, 90), 256 (M, 96), 178 (11), 177 (60), 176
128.13, 40.61, 17.83, 13.97.
(100), 151, 88.
ESI-MS: m/z = 226.90 ([M + H]+), 228.75 ([M + 2 + H]+).
1-(2,4,6-Trimethylphenyl)ethanone (3p)16
1,2-Diphenylethanone (3l)23
Table 4-10: Light yellow liquid; yield: 87 mg (53%); Rf = 0.24
(CH2Cl2/PE 1:3).
IR (KBr, film): 1699 cm–1
Table 4-5: Colorless oil; yield: 126 mg (64%); mp 52.5–54.1 °C (Lit.23
mp 54 °C); Rf = 0.57 (CH2Cl2/PE 1:1).
.
IR (KBr, film): 1683 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 6.84 (s, 2 H), 2.46 (s, 3 H), 2.28 (s, 3 H),
2.22 (s, 6 H).
1H NMR (400 MHz, CDCl3): δ = 8.03–8.00 (m, 2 H), 7.57–7.52 (m, 1 H),
7.47–7.43 (m, 2 H), 7.34–7.30 (m, 2 H), 7.27–7.23 (m, 3 H), 4.28 (s, 2
H).
13C NMR (101 MHz, CDCl3): δ = 208.7, 140.0, 138.4, 132.4, 128.6, 32.4,
21.1, 19.2.
13C NMR (101 MHz, CDCl3): δ = 197.73, 136.74, 134.67, 133.28,
MS (EI): m/z = 162 (M+), 147 (M – 15, 100%), 119, 91, 77.
129.59, 128.79, 128.76, 128.74, 127.01, 45.62.
1-{1-[(4-Methylphenyl)sulfonyl]-1H-indol-3-yl}ethanone (3q)23
Table 4-11: White solid; yield: 172 mg (90%); mp 148–149.5 °C (Lit.23
ESI-MS: m/z = 197.0 ([M + H]+).
1-(Naphthalen-2-yl)ethanone (3m)15
mp 145–146 °C); Rf = 0.32 (CH2Cl2/PE 1:1).
Table 4-6: Off-white solid; yield: 151 mg (89%); mp 49.0–50.2 °C
(Lit.24 mp 49–51 °C); Rf = 0.30 (CH2Cl2/PE 1:1).
IR (KBr, film): 3126, 1668, 1539, 1383 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 8.34–8.32 (m, 1 H), 8.21 (s, 1 H), 7.94–
7.91 (m, 1 H), 7.84 (d, J = 8.4, 2 H), 7.39–7.31 (m, 2 H), 7.28 (d, J = 8.4
Hz, 2 H), 2.57 (s, 3 H), 2.36 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 193.6, 146.1, 135.0, 134.7, 132.3,
130.4, 127.6, 127.3, 125.9, 124.9, 123.2, 121.7, 113.2, 27.9, 21.8.
IR (KBr, film): 1679 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 8.47 (s, 1 H), 8.04 (dd, J = 8.8, 1.6 Hz, 1
H), 7.98 (d, J = 8.0 Hz, 1 H), 7.91–7.87 (m, 2 H), 7.63–7.54 (m, 2 H),
2.74 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 198.23, 135.76, 134.69, 132.69,
MS (EI): m/z = 314 (M + 1), 313 (M+), 298 (M – 15), 155, 91 (100%).
130.33, 129.70, 128.62, 128.58, 127.94, 126.93, 124.07, 26.83.
1-(3-Pyridyl)ethanone (3r)29
ESI-MS: m/z = 171.0 ([M + H]+).
Table 4-12: Yellow oil; yield: 53 mg (31%); Rf = 0.38 (EtOAc/PE 1:1).
1-(Benzofuran-2-yl)ethanone (3n)25
Table 4-7: White solid; yield: 110 mg (69%); mp 66.5–67.5 °C (Lit.26
IR (KBr, film): 3360, 2921, 1698, 1452, 1350, 1223 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 9.18 (d, J = 1.6 Hz, 1 H), 8.79 (dd,
J = 4.4, 1.6 Hz, 1 H), 8.24 (dt, J = 8.0, 2.0 Hz, 1 H), 7.44 (d, J = 8.0, 4.8 Hz,
1 H), 2.66 (s, 3 H).
mp 69–71 °C); Rf = 0.20 (CH2Cl2/PE 1:1).
IR (KBr, film): 1685 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 7.71 (d, J = 8.0 Hz, 1 H), 7.58 (d, J = 8.4
Hz, 1 H), 7.50 (s, 1 H), 7.48 (td, J = 8.4, 1.2 Hz, 1 H), 7.32 (t, J = 8.0 Hz, 1
H), 2.61 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 196.8, 153.7, 150.1, 135.6, 132.4,
123.7, 26.8.
MS (EI): m/z = 121 (M+), 106 (100%), 78.
13C NMR (101 MHz, CDCl3): δ = 188.75, 155.80, 152.78, 128.39,
(1-Admantyl) Methyl Ketone (3s)30
127.18, 124.03, 123.42, 113.14, 112.59, 26.58.
ESI-MS: m/z = 161.0 ([M + H]+).
Table 4-13: Colorless oil; yield: 77 mg (50%); isolated together with
recovered 1-bromoacetyladmantane (104 mg, 47%); Rf
(CH2Cl2/PE 1:1).
= 0.40
1-(Anthracen-9-yl)ethanone (3o)16
Table 4-9: Light yellow solid; yield: 93 mg (94%); mp 68.4–68.7 °C
(Lit.16 mp 75–76 °C); Rf = 0.41 (CH2Cl2/PE 1:2).
IR (KBr, film): 2906, 1698, 1452, 1350, 1223 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 2.10 (s, 3 H), 2.05 (br s, 3 H), 1.81 (d,
J = 2.4 Hz, 6 H), 1.77 (d, J = 12.4 Hz, 3 H), 1.69 (d, J = 12.4 Hz, 3 H).
IR (KBr, film): 1697 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 8.46 (s, 1 H), 8.02–8.00 (m, 2 H), 7.85–
7.82 (m, 2 H), 7.52–7.45 (m, 4 H), 2.80 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 208.20, 136.86, 131.20, 128.95,
13C NMR (101 MHz, CDCl3): δ = 214.2, 46.6, 38.4, 36.7, 28.1, 24.4.
MS (EI): m/z = 178 (M+), 135 (100%), 93, 79.
4-Phenylcyclohexanone (3t)17
128.33, 126.89, 126.73, 125.61, 124.45, 33.96.
ESI-MS: m/z = 221.0 ([M + H]+).
Table 4-14: Off-white oil; yield: 28 mg (39%); isolated together with
recovered 2-bromo-4-phenylcyclohexanone (46 mg, 45%); Rf = 0.57
(EtOAc/PE 1:6).
9-Bromoanthracene (4)27,28
Off-white solid obtained as a side product alongside compound 3o;
yield: 2 mg (2%); Rf = 0.85 (CH2Cl2/PE 1:2).
1H NMR (400 MHz, CDCl3): δ = 8.53 (dd, J = 8.9, 0.7 Hz, 2 H), 8.46 (s, 1
H), 8.01 (d, J = 8.4 Hz, 2 H), 7.62 (dd, J = 6.6, 1.2 Hz, 1 H), 7.60 (dd,
J = 6.6, 1.3 Hz, 1 H), 7.52 (dd, J = 6.6, 1.0 Hz, 1 H), 7.50 (dd, J = 6.6, 0.9
Hz, 1 H).
IR (KBr, film): 2934, 1716, 1496 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 7.35–7.31 (m, 2 H), 7.26–7.22 (m, 3 H),
3.03 (tt, J = 12.0, 3.2 Hz, 1 H), 2.54–2.50 (m, 4 H), 2.26–2.20 (m, 2 H),
2.01–1.90 (m, 2 H).
13C NMR (101 MHz, CDCl3): δ = 211.2, 144.9, 128.7, 126.8, 126.7, 42.9,
41.5, 34.1.
MS (EI): m/z = 174 (M+), 145, 119, 117, 104 (100%), 91.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H

G
Z. Lu, Y.-Q. Yang
Paper
Synthesis
2-Bromo-1-(4-bromophenyl)butan-1-one (2p)
gram Development of Jiangsu Higher Education Institutions (PAPD),
and Jiangsu University (No. 10JDG042 and No. 14JDG018) for their
This compound was prepared following the procedure reported in the
literature.31
generous support.
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A mixture of 1-(4-bromophenyl)butan-1-one (2.304 g, 10.14 mmol),
N-bromosuccinimide (2.253 g, 12.66 mmol), and p-TsOH·H2O (190
mg, 1.00 mmol) in MeCN (20 mL) was heated at 40 °C for 12 h. Sat. aq
NaHSO4 (15 mL) was added to the cooled reaction mixture and stirred
for 0.5 h. The aqueous layer was separated and extracted with CH2Cl2
(2 × 15 mL). The combined organic phases were washed with sat. aq
NaHCO3 (10 mL) and brine (10 mL), and dried (anhyd Na2SO4). The
solution was concentrated to give a light yellow liquid. The residue
was subjected to flash chromatography eluting with PE/CH2Cl2 (10:1
to 7:1) to afford compound 2q as a colorless oil (1.268 g, 41%), which
solidified over standing to a white solid; yield: 1.268 g (41%); mp
26.0–26.6 °C; Rf = 0.39 (CH2Cl2/PE 2:5).
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1610629.
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References
(1) (a) Hantzsch, A. Ber. Dtsch. Chem. Ges. 1881, 14, 1637. (b) Kurz, J.
L.; Hutton, R.; Westheimer, F. H. J. Am. Chem. Soc. 1961, 83, 584.
(c) Uldrikis, Y. R.; Dubur, G. Y.; Dipan, I. V.; Chekavichus, B. S.
Chem. Heterocycl. Compd. 1975, 11, 1070. (d) Gelbard, G.; Lin, J.;
Roques, N. J. Org. Chem. 1992, 57, 1789.
IR (KBr, film): 2972, 2934, 2875, 1685, 1585, 1568, 1484, 1459, 1398,
1264, 1071, 1011, 986, 899, 844, 739 cm–1
.
(2) (a) Faisca Phillips, A. M.; Pombeiro, A. J. Org. Biomol. Chem.
2017, 15, 2307. (b) Herrera, R. P. Top. Curr. Chem. 2016, 374, 40.
(c) McSkimming, A.; Colbran, S. B. Chem. Soc. Rev. 2013, 42,
5439. (d) Zheng, C.; You, S.-L. Chem. Soc. Rev. 2012, 41, 2498.
(3) (a) Huang, W.; Chen, J.; Hong, D.; Chen, W.; Cheng, X.; Tian, Y.;
Li, G. J. Org. Chem. 2018, 83, 578. (b) He, R.; Cui, P.; Pi, D.; Sun, Y.;
Zhou, H. Tetrahedron Lett. 2017, 58, 3571. (c) Lee, K. N.; Lei, Z.;
Ngai, M. Y. J. Am. Chem. Soc. 2017, 139, 5003. (d) Pi, D.; Zhou, H.;
Cui, P.; He, R.; Sui, Y. ChemistrySelect 2017, 2, 3976. (e) Weng,
G.; Ma, X.; Fang, D.; Tan, P.; Wang, L.; Yang, L.; Zhang, Y.; Qian,
S.; Wang, Z. RSC Adv. 2017, 7, 22909. (f) Tang, W.; Zhao, X.; Xiao,
J. Synthesis 2017, 49, 3157. (g) Carlone, A.; Bernardi, L. Org.
Chem. Front. 2017, 4, 1651.
1H NMR (400 MHz, CDCl3): δ = 7.88 (ddd, J = 8.6, 2.3, 1.8 Hz, 2 H), 7.63
(ddd, J = 8.6, 2.3, 1.8 Hz, 2 H), 4.99 (dd, J = 7.7, 6.4 Hz, 1 H), 2.24 (dqd,
J = 21.7, 7.3, 6.4 Hz, 1 H), 2.13 (ddq, J = 22.1, 7.7, 7.3 Hz, 1 H), 1.09 (t,
J = 7.3 Hz, 3 H).
13C NMR (101 MHz, CDCl3): δ = 192.37, 133.41, 132.25, 130.49,
129.06, 26.91, 12.29.
MS (EI): m/z (%) = 308 (M + 4, 0.34), 306 (M + 2, 0.65), 304 (M+, 0.33),
227 (M + 2 – Br, 7), 225 (M – Br, 7), 185 (99), 183 (100), 157 (19), 155
(19), 76.
HRMS (EI): m/z ([M+]), calcd for C10H10BrO2: 303.9098; found:
303.9096.
(4) Kuivila, H. G.; Menapace, L. W.; Warner, C. R. J. Am. Chem. Soc.
1962, 84, 3584.
2-Bromo-1-(anthracen-9-yl)ethanone (2u)32
The compound was prepared following the procedure reported in the
literature,32 except that CH2Cl2 was used to replace benzene.
(5) (a) Martinez-Haya, R.; Miranda, M. A.; Marin, M. L. Eur. J. Org.
Chem. 2017, 2164. (b) Li, X.; Hao, Z.; Zhang, F.; Li, H. ACS Appl.
Mater. Interfaces 2016, 8, 12141. (c) Dong, W.-L.; Cai, W.-X.; Wu,
R.; Li, Z.-M.; Zhao, W.-G.; Liu, X.-H. Phosphorus, Sulfur Silicon
Relat. Elem. 2016, 191, 980. (d) Jung, J.; Kim, J.; Park, G.; You, Y.;
Cho, E. J. Adv. Synth. Catal. 2016, 358, 74. (e) Luo, J.; Zhang, X.;
Zhang, J. ACS Catal. 2015, 5, 2250.
(6) Maji, T.; Karmakar, A.; Reiser, O. J. Org. Chem. 2011, 76, 736.
(7) Lin, C. Y.; Peh, J.; Coote, M. L. J. Org. Chem. 2011, 76, 1715.
(8) (a) Peng, P.; Wu, J.; Liang, J.; Zhang, T.; Huang, J.; Wu, F. RSC Adv.
2017, 7, 56034. (b) Nobuta, T.; Hirashima, S.; Tada, N.; Miura, T.;
Itoh, A. Org. Lett. 2011, 13, 2576.
(9) (a) Tanner, D. D.; Diaz, G. E.; Potter, A. J. Org. Chem. 1985, 50,
2149. (b) Tanner, D. D.; Singh, H. K. J. Org. Chem. 1986, 51, 5182.
(10) Chen, W.; Tao, H.; Huang, W.; Cheng, X.; Li, G.; Wang, G.; Li, S.;
Li, G. Chem. Eur. J. 2016, 22, 9546.
(11) Narayanam, J. M. R.; Tucker, J. W.; Stephenson, C. R. J. J. Am.
Chem. Soc. 2009, 131, 8756.
To a stirred suspension of AlCl3 (1.625 g, 12.19 mmol) in CH2Cl2 (50
mL) at –5 °C was added anthracene (2.288 g, 12.84 mmol) followed by
CH2Cl2 (50 mL). The golden mixture was allowed to stir for 20 min be-
fore bromoacetyl bromide (2.397 g, 11.88 mmol in 10 mL of CH2Cl2)
was added dropwise. The mixture turned red upon the addition of
bromoacetyl bromide. The mixture was allowed to react at that tem-
perature for 3 h. The reaction was quenched with aq 10% HCl (30 mL).
The organic phase was separated and washed sequentially with aq
10% HCl (30 mL), H2O (2 × 30 mL), sat. aq NaHCO3 (30 mL) and brine.
The CH2Cl2 solution was then dried (anhyd Na2SO4) before concentra-
tion. The residue was subjected to flash chromatography to give com-
pound 2u as a yellow solid; yield: 839 mg (23%); mp 92.0–92.6 °C
(Lit.32 mp 104.5–107 °C).
IR (KBr, film): 3053, 2929, 2852, 1931, 1717, 1446, 1390, 1160, 1086
cm–1
.
1H NMR (400 MHz, CDCl3): δ = 8.52 (s, 1 H), 8.03 (d, J = 7.6 Hz, 2 H),
7.78–7.61 (m, 2 H), 7.54 (ddd, J = 15.2, 6.8, 1.6 Hz, 2 H), 7.51 (ddd,
J = 14.8, 6.4, 1.5 Hz, 2 H), 4.61 (s, 2 H).
13C NMR (101 MHz, CDCl3): δ = 199.72, 132.62, 131.05, 129.73,
129.08, 127.84, 127.60, 125.86, 124.16, 38.03.
(12) Lu, Z.; Yang, Y.-Q.; Li, H.-X. Synthesis 2016, 48, 4221.
(13) Denis Prat, D.; Hayler, J.; AndyWells, A. Green Chem. 2014, 16,
4546.
(14) Kubo, K.; Watanabe, K.; Sakurai, T. Acta Crystallogr., Sect. E 2007,
63, o1300.
(15) Ruan, J.; Li, X.; Saidi, O.; Xiao, J. J. Am. Chem. Soc. 2008, 130,
2424.
ESI-MS: m/z = 321.1 ([M + Na]+), 323.0 ([M + 2 + Na]+).
(16) Sharghi, H.; Jokar, M.; Doroodmand, M. M.; Khalifeh, R. Adv.
Synth. Catal. 2010, 352, 3031.
(17) Maji, T.; Karmakar, A.; Reiser, O. J. Org. Chem. 2011, 76, 736.
(18) Zhao, B.; Lu, X. Org. Lett. 2006, 8, 5987.
Funding Information
The authors are grateful to the National Natural Science Foundation of
China (No. 21572118 & No. 21572080), the Priority Academic Pro-
(19) Pandey, P. N.; Purkayastha, M. L. Synthesis 1982, 876.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H

H
Z. Lu, Y.-Q. Yang
Paper
Synthesis
(20) Liu, T.; Shen, Q. Org. Lett. 2011, 13, 2342.
(26) Farrar, M. W.; Levine, R. J. Am. Chem. Soc. 1950, 72, 4433.
(21) Cutulic, S. P. V.; Findlay, N. J.; Zhou, S.-Z.; Chrystal, E. J. T.;
Murphy, J. A. J. Org. Chem. 2009, 74, 8713.
(27) House, H. O.; Ghali, N. I.; Haack, J.; VanDerveer, D. J. Org. Chem.
1980, 45, 1807.
(22) Oelshläger, H.; Schreiber, O. Justus Liebigs Ann. Chem. 1961, 641,
81.
(28) Seetula, J. A. J. Mol. Catal. A: Chem. 2005, 231, 153.
(29) Piller, F. M.; Pradad, A.; Gavryushin, A.; Helm, M.; Knochel, P.
Angew. Chem. Int. Ed. 2008, 36, 6802.
(30) Wright, J. A.; Gaunt, M. J.; Spencer, J. B. Chem. Eur. J. 2006, 12,
949.
(23) Kurihara, T.; Toshiro, F.; Shinya, H.; Ryuji, Y. Synthesis 1987, 396.
(24) Zoeller, R. R.; Sumner, C. E. J. J. Org. Chem. 1990, 55, 319.
(25) Paizs, C.; Toşa, M.; Majdik, C.; Moldovan, P.; Novák, L.; Kolonits,
P.; Marcovici, A.; Irimie, F.-D.; Poppe, L. Tetrahedron: Asymmetry
2003, 14, 1495.
(31) Vyas, V. K.; Bhanage, B. M. Org. Lett. 2016, 18, 6436.
(32) May, E. L.; Mosettig, E. J. Am. Chem. Soc. 1948, 70, 686.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H