Catalyst-free formation of 1,4-diketones by addition of silyl enolates to oxyallyl zwitterions in situ generated from α-haloketones

Article abstract of DOI:10.1039/c5ra12244a

Reported here is the exclusive formation of 1,4-diketones by the uncatalyzed reaction of silyl enolates and α-haloketones. Enolates I are inherently more likely to react with α-haloketones II at the carbonyl carbon to produce halohydrin derivatives III or

Full text of DOI:10.1039/c5ra12244a

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Catalyst-free formation of 1,4-diketones by
addition of silyl enolates to oxyallyl zwitterions in
situ generated from a-haloketones†
Cite this: RSC Adv., 2015, 5, 67901
a
a
ab
Juan Luo,ab Qihua Jiang, Hao Chen and Qiang Tang
*
Reported here is the exclusive formation of 1,4-diketones by the uncatalyzed reaction of silyl enolates and
a-haloketones. Enolates I are inherently more likely to react with a-haloketones II at the carbonyl carbon to
produce halohydrin derivatives III or 2-(2-oxoethyl)-oxiranes IV. Thus, a variety of metal-catalyzed coupling
reactions have been developed to avoid the undesired reaction when attempting the preparation of 1,4-
diketones. We found that the oxyallyl zwitterions in situ generated from a-haloketones enabled the
addition of silyl enolates to the a-carbonyl position to exclusively form 1,4-diketones in weakly basic
conditions. Various types of silyl enolates and a-haloketones were applied to the catalyst-free coupling.
Received 25th June 2015
Accepted 3rd August 2015
DOI: 10.1039/c5ra12244a
www.rsc.org/advances
indoles and a-halo ketones via in situ generated oxyallyl zwit-
terions provides a-indolylketones.13 Upon further exploration,
Introduction
we have also found an efficient catalytic-free method for the
coupling of naphthols with oxyallyl zwitterions to produce a-
naphtholylketones.14 Additionally, MacMillan's report has also
demonstrated that oxyallyl zwitterions allow the addition of p-
nucleophiles or even neutral heteroatoms to the a-carbonyl
position under mild, weakly basic conditions.15
1,4-Diketones are common substructures of natural products
and pharmaceuticals,1 like maoecrystal V1a and herquline,1d as
well as highly useful synthetic building blocks of various
carbocyclic and heterocyclic compounds, such as cyclo-
pentenones,2 furans,3 thiophenes,4 pyrroles5 and pyridazine
derivatives.6 Therefore, signicant efforts have been directed
toward the synthesis of those highly valuable synthons.7 The
most straightforward method for their preparation would be the
C–C bond formation between carbonylmethyl anion and cation
equivalents.8 Although versatile enolates have been developed
as carbonylmethyl anions, selecting appropriate candidates as
carbonylmethyl cation units is still a challenging problem. a-
Haloketones might be used as carbonylmethyl cation equiva-
lents, but there is a regioselectivity problem resulting from the
two reactive sites respectively located at the carbonyl carbon
and the a-carbonyl position.9 Without metal catalysts, enolates I
inherently are more likely to react with a-haloketones II at the
carbonyl carbon to produce halohydrin derivatives III or 2-(2-
oxoethyl)-oxiranes IV (Scheme 1, Path a).10 Thus, a variety of
metal-catalyzed coupling reactions have been developed to
avoid the undesired reaction for the preparation of 1,4-dike-
tones V (Scheme 1, Path b).11
It is notable that the Harmata group has reported an efficient
ene-like reaction between alkyl enol ethers VII and a special
Scheme 1 Reaction approaches of enolates with a-haloketones.
Our interest in this chemistry stems from our work on the
interrupted cycloadditions of oxyallyl zwitterions.12 We have
recently reported that the direct coupling of unprotected
aDepartment of Medical Chemistry, College of Pharmacy, Chongqing Medical
University, Chongqing 400016, P. R. China. E-mail: tangqiang@cqmu.edu.cn
bThe Faculty of Laboratory Medicine, Chongqing Medical University, Chongqing
401331, P. R. China
† Electronic supplementary information (ESI) available: 1H NMR, 13C NMR, and
IR spectral data for compounds. See DOI: 10.1039/c5ra12244a
Scheme 2 Reaction approaches of enolates with oxyallyl zwitterions.
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oxyallyl zwitterion VI yielded from a specic a-chloroketone With the merits of the mildness of the desilylation process,
(Scheme 2).12b It demonstrates that the leaving group, the the ease of preparation and the cleanliness of reactions, silyl
Me2PhSi moiety substituted on the oxyallyl zwitterion VI, takes enolates have long been known as weak nucleophiles in the
part in a crucial role in the hydride shiing process and nally Mukaiyama aldol addition, Michael addition, and alkylation
benets the reaction efficiency.
reaction.16 However, to our knowledge, this is the rst report on
Based on the above ndings, we hypothesized that the their application in the synthesis of 1,4-diketones without any
course of the reaction might be changed if the leaving group is catalyst.17
located on enolates XI, and that simple oxyallyl zwitterion X
generated in weakly basic conditions might allow the exclusive
addition of silyl enolates XI to the a-carbonyl position of X and
Background
nally form 1,4-diketones XIII aer desilylation (Scheme 2).
Oxyallyl zwitterions and cyclopentanone intermediates, which
can be transformed to each other, were rst proposed as tran-
sient electrophilic intermediates in the Favorskii rearrange-
ment in 1894.18 More specically, under strongly basic
conditions, a variety of nucleophiles add to the carbonyl carbon
of the incipient cyclopentanone intermediates induced from a-
haloketones and subsequently produce carboxylic acids and
their derivatives aer bond migration.19 Furthermore, the oxy-
allyl zwitterions have also been known as dienophiles in [4 + 3]
cycloadditions to construct seven-membered carbocycles across
a wide range of unique applications.20
To shed light on the reaction mechanism of nucleophiles
with oxyallyls (cyclopentanone intermediates and oxyallyl
zwitterions), different types of reactions are summarized in
Scheme 3 and 4. Under typical Favorskii rearrangement
conditions (Scheme 3),21 strong bases induce the formation of
Scheme 3 The reaction of oxyallyls with p-nucleophiles.
Scheme 4 The reaction of oxyallyl zwitterions with p-nucleophiles using TFE or HFIP as solvent.
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cyclopentanone intermediates XV from a-haloketones and then entry 4), as a higher amount of silyl enolate could lead to the
the ring-contracted product XVII,22 XVIII,23 or XIX 24 is produced generation of quite a few byproducts containing the silyl group.
depending on the p-nucleophile we used. Remarkably, treat- Remarkably, we discovered that the basicity of bases shows a
ment of 2-chlorocyclopentanone with the weak base (sodium signicant effect on the reaction cleanliness and yield. In fact, an
carbonate) instead of the strong base (sodium hydroxide) organic base Et3N, could also effectively initiate the reaction
affords an excellent yield of the substituted product XX. (Table 1, entries 8). However, when employing a relatively weak
However, in the presence of a weaker base, such as sodium base, i.e. sodium bicarbonate, only a trace of the desired product
bicarbonate, or without any bases, most of the starting material was detected (Table 1, entry 6). On the contrary, when a strong
remains unreacted, and only a trace of the substituted product base, i.e. NaOH, was used, the reaction became messy (Table 1,
XX is detected.14 Based on those observations, we proposed that entry 7).
the generation of oxyallyl zwitterions XVI (the valence tautomers
of the cyclopropane intermediates XV) under the weakly basic
conditions is the key step, which permits the subsequent
Scope and generality of the substrates
addition of the p-nucleophiles to render the substituted prod-
ucts (XX–XXIII).25
With the optimized conditions in hand, we next examined other
silyl enolates in this new synthetic protocol. Gratifyingly, a
variety of silyl enolates functioned well in this nucleophilic
reaction. In general, since alkyl enol ethers (Table 2, entries 7–
12) are more stable than aryl enol ethers (Table 2, entries 1–6) in
TFE, a little excess amount of alkyl enol ethers led to better
yields in the reaction system. No steric effect was observed for
the terminal enol ethers (Table 2, entries 1–4 and 7–9) and the
disubstituted enol ethers (Table 2, entries 5–6 and 10–12)
showed nearly the same reaction efficiency as the mono-
substituted enol ethers.
In addition, we found that the electron density of silyl eno-
lates plays an important role in the reaction efficiency. As a
matter of fact, the reaction with silyl enolate (2c) bearing an
electron-donating substitutent proceeded efficiently by this
uncatalyzed system (Table 2, entry 3). On the contrary, no
desired product was isolated for this reaction of 2-chlor-
ocyclopentanone with silyl enolates bearing a strong electron-
withdrawing substitutent, such as trimethyl((1-(4-nitrophenyl)
vinyl)oxy)silane and 4-(1-((trimethylsilyl)oxy)vinyl)benzonitrile.
Silyl enolates generated from aldehydes, such as trimethylsi-
loxyethylene, 2-phenyl-1-trimethylsiloxyethylene and 2-benzyl-1-
It should be noted that both oxyallyl zwitterions and cyclo-
pentanone intermediates are formed in the reaction of a-hal-
oketones with furan using sodium 2,2,2-triuoroethoxide as a
base in 2,2,2-triuoroethanol (NaTFE/TFE), and that the two
corresponding types of products, the ring-contracted products
(XVIII, Scheme 3) and the [4 + 3] cycloadducts (Scheme 4) are
eventually generated.26
It is well known that oxyallyl zwitterions27 tend to react with
enes either in a stepwise or a concerted fashion so as to generate
the [4 + 3]28 or [3 + 2]29 cycloadducts (Scheme 4). However,
several valuable interrupted cycloadditions of oxyallyl zwitter-
ions have been reported in recent years. The mechanism of the
interrupted cycloadditions depends on (i) the nucleophilicity of
p-nucleophiles, (ii) the positions of the leaving groups on the
intermediates XXIV–XXX and (iii) the difficulty of removing the
leaving groups from the intermediates XXIV–XXX. Compared
with indole or silyl enolate, the low nucleophilicity of styrene
and the difficult deprotonation of the intermediate XXIV
(denoted in red color) result in the complexity of the reaction of
styrene with 2-chlorocyclopentanone in TFE.30 Whereas, an
efficient reaction takes place when the leaving groups become
easy to release from the intermediates XXVII–XXX. Moreover,
the course of the reaction is changed just by changing the Table 1 Model reaction optimizationa
positions of the leaving groups (the silyl group) on the inter-
mediates, and thus the product of the intermediate XXX is 1,4-
diketone, entirely different from the product of the interme-
diate XXIX.
Results and discussion
Optimization of reaction conditions
Base
[1.2 eq.]
1a
[equiv.]
2a
[equiv.]
Entry
Time [h]
Yieldb [%]
42
—
61
70
65
Trace
Complex
60
The proposed transformation was rst examined using 1-phenyl-
1-trimethylsiloxyethylene 2a and 2-chlorocyclopentanone 1a in
the presence of sodium carbonate as a base, and triuoroethanol
(TFE) as a solvent (Table 1). To our delight, the desired 1,4-
diketone 3a was obtained efficiently in one step without hal-
ohydrin III or cycloaddition compound being detected, thereby
demonstrating the feasibility of our proposal (the structure of 3a
was characterized by 2D NMR spectroscopy). However, an excess
amount of silyl enolate was needed because of its instability in
TFE (Table 1, entry 2). Moreover, three equivalents of silyl enolate
were eventually found to be optimal in terms of yield (Table 1,
1
2
3
4
5
6
7
8
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
NaHCO3
NaOH
1
0
1
1
1
1
1
1
1
1
2
3
4
3
3
3
6
12
12
12
12
24
3
c
Et3N
12
a
Reaction conditions: 1a (1.0 mmol), 2a (1.0–4.0 mmol), base (1.2
b
c
mmol) in TFE (2 mL) at 25 ꢀC. Isolated yield. Silyl enolate was not
detected by TLC aer twelve hours.
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trimethylsiloxyethyl-ene, yielded complicated reaction mixtures
that were not studied further.
Table 2 Coupling reaction: scope of silyl enolatesa
We next examined the scope of a-haloketones in the catalyst-
free formation of 1,4-diketones. This transformation is not
limited to ve-membered rings since both six-membered rings
and seven-membered rings are competent substrates (Table 3,
entries 1–5). Additionally, both a-chloro- and a-bromocyclo-
hexanones gave the same products with similar reaction rate
and efficiency (Table 3, entries 1 and 2). For the reaction of a-
iodocyclohexanone, the reaction rate was faster but the yield
was lower in comparison with the other two halocyclohex-
anones. Use of acyclic a-haloketones afforded the correspond-
ing 1,4-diketones in comparable yields (Table 3, entries 6–7).
Furthermore, the reaction of dibromoketones (1h–1i) with an
equimolar or excess molar amount of silyl enolates gave the
monosubstituted products (Table 3, entries 8–9). The coupling
reaction shows a high regioselectivity, as only one regioisomer
(3q) was obtained, whose structure was conrmed by 2D NMR.
Entry
1
Enolate
Product
Yieldb,c [%]
70
2
3
4
65
78
69
Table 3 Coupling reaction: scope of a-haloketonesa
Entry
1
Haloketone
Enolate
Product
Yieldb,c [%]
62
5
6
7
73(1 : 1 dr)
67(5 : 2 dr)
72
2
3
4
5
6
60
56
71(1 : 1 dr)
63(1 : 1 dr)
39 (5 : 1 dr)
8
9
73
76
10
11
81(1 : 1 dr)
80(1 : 1 dr)
7
8
51(10 : 1 dr)
32(10 : 1 dr)
12
71(1 : 1 dr)
9
31
a
Reaction conditions: 1a (1.0 mmol), silicon enolates (3.0 mmol in
entries 1–3, 2.0 mmol in entries 4–11), Na2CO3 (1.2 mmol) in TFE
b
c
(2 mL) at 25 ꢀC. Isolated yield. The diastereomer ratio was
determined by 1H NMR spectroscopic analysis of the crude material.
a
Reaction conditions: a-haloketones (1.0 mmol), silicon enolates (3.0
mmol in entries 1,2,7,8; 2.0 mmol in entries 3–6), Na2CO3 (1.2 mmol)
b
c
in TFE (2 mL) at 25 ꢀC. Isolated yield. The diastereomer ratio was
determined by 1H NMR spectroscopic analysis of the crude material.
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126.5, 123.0, 45.0, 38.7, 37.4, 29.6, 20.8; HRMS (ESI) calcd for
C
Experimental section
General information
13H14BrO2 (M + 1)+: 281.0172, found: 281.0179.
2-(2-(4-Methoxyphenyl)-2-oxoethyl)cyclopentan-1-one (3c).
Nuclear magnetic resonance spectra (1H and 13C) were recorded
on 300, 400, and 500 MHz spectrometers with tetramethylsilane
(TMS) as an internal standard. The splitting patterns are
designated as singlet (s), doublet (d), triplet (t), quartet (q), dd
(doublet of doublets); m (multiplets), and etc. All rst-order
splitting patterns were assigned on the basis of the appear-
ance of the multiplet. Splitting patterns that could not be easily
interpreted are designated as multiplet (m) or broad (br). High
resolution mass spectral analysis (HRMS) was performed on
ESI-QTOP mass spectrometer. Visualization was performed
using a UV lamp or chemical stains like KMnO4 and 2,4-dini-
trophenyl hydrazine solutions.
Commercially available materials were used as received, except
a-haloketones that were further puried via distillation or column
chromatography over silica gel prior to use. Some of the a-chlor-
oketones (2-bromocyclohexanone, 2-chlorocycloheptanone and
1,3-dibromo-3-methylbutan-2-one) were prepared using literature
method.31
The title compound was prepared as colorless oil in 78% yield
according to the general procedure as described above; 1H NMR
(400 MHz, CDCl3) d 8.03–7.83 (m, 2H), 7.01–6.82 (m, 2H), 3.86
(s, 3H), 3.47 (dd, J ¼ 20.0, 4.0 Hz, 1H), 2.99 (dd, J ¼ 17.8, 8.0 Hz,
1H), 2.73–2.50 (m, 1H), 2.48–2.18 (m, 3H), 2.12–2.02 (m, 1H),
1.88–1.72 (m, 1H), 1.66–1.53 (m, 1H); 13C NMR (100 MHz,
CDCl3) d 220.5, 196.5, 163.5, 130.3, 129.7, 113.7, 55.4, 45.2, 38.3,
37.6, 29.7, 20.8; IR (KBr, cmꢂ1): 2971, 2897, 1738, 1670, 1603,
1260, 1177, 811, 562, 497; HRMS (ESI) calcd for C14H17O3 (M +
1)+: 233.1172, found: 233.1175.
2-(2-(Naphthalen-1-yl)-2-oxoethyl)cyclopentanone (3d). The
title compound was prepared as white solid in 69% yield
according to the general procedure as described above; 1H NMR
(500 MHz, CDCl3) d 8.59 (d, J ¼ 8.5 Hz, 1H), 7.99 (d, J ¼ 8.2 Hz,
1H), 7.93–7.85 (m, 2H), 7.64–7.46 (m, 3H), 3.59 (dd, J ¼ 17.7, 3.8
Hz, 1H), 3.12 (dd, J ¼ 17.7, 7.8 Hz, 1H), 2.73 (d, J ¼ 8.1 Hz, 1H),
2.42 (ddd, J ¼ 18.8, 10.3, 4.8 Hz, 2H), 2.36–2.24 (m, 1H), 2.15–
2.08 (m, 1H), 1.92–1.85 (m, 1H), 1.75–1.68 (m, 1H); 13C NMR
(125 MHz, CDCl3) d 220.2, 202.3, 135.5, 133.9, 132.7, 130.1,
128.4, 127.9, 127.5, 126.5, 125.7, 124.3, 45.6, 42.0, 37.5, 29.6,
20.8; HRMS (ESI) calcd for C17H17O2 (M + 1)+: 253.1223, found:
253.1225.
Typical procedure for catalyst-free coupling of silyl enolates
with a-haloketones
A 4 mL vial equipped with a magnetic stir bar was charged with
2-(2-Oxocyclopentyl)-3,4-dihydronaphthalen-1(2H)-one (3e).
freshly distilled a-haloketone 1 (0.5 mmol), anhydrous Na2CO3 Two diastereomers were prepared as white solid in 73% total
(0.6 mmol) and TFE (1.0 mL). Silyl enolate 2 (1.0–1.5 mmol) was yield according to the general procedure as described above.
added in portionwise to the reaction mixture at 25 ꢀC. Aer The upper isomer: 1H NMR (400 MHz, CDCl3) d 8.02 (d, J ¼ 7.8
completion of the reaction (about 12–24 h, monitored by TLC or Hz, 1H), 7.46 (dd, J ¼ 10.8, 4.1 Hz, 1H), 7.41–7.15 (m, 2H), 3.27–
crude 1H NMR analysis), the reaction was quenched with water 3.06 (m, 2H), 3.06–2.86 (m, 2H), 2.40 (dd, J ¼ 17.6, 7.8 Hz, 1H),
(1.0 mL) and stirred for 30 min. Extraction with CH2Cl2 (3 ꢁ 10 2.29–1.98 (m, 4H), 1.87 (dddd, J ¼ 18.5, 11.6, 8.1, 5.2 Hz, 2H),
mL), drying of the combined organic layers with Na2SO4, 1.70 (ddd, J ¼ 18.2, 11.8, 5.7 Hz, 1H); 13C NMR (100 MHz, CDCl3)
ltration, and evaporation of the solvent in vacuum gave a d 220.6, 198.5, 144.1, 133.4, 132.4, 128.7, 127, 126.6, 49.6, 47.7,
residue which was puried by column chromatography on silica 38.5, 29.6, 26.1, 25.5, 20.8; IR (KBr, cmꢂ1): 3447, 2961, 1737,
gel with petroleum ether and ethyl acetate (v/v ¼ 10 : 1 to 1 : 1) 1714, 1162, 1020, 802, 472, 418; HRMS (ESI) calcd for C15H17O2
as the eluent to afford the desired product.
(M + 1)+: 229.1223, found: 229.1233; the lower isomer: 1H NMR
2-(2-Oxo-2-phenylethyl)cyclopentanone (3a).32 The title (400 MHz, CDCl3) d 7.97 (d, J ¼ 7.8 Hz, 1H), 7.46 (t, J ¼ 7.4 Hz,
compound was prepared as colorless oil in 70% yield according 1H), 7.34–7.20 (m, 2H), 3.36–3.26 (m, 1H), 3.13 (ddd, J ¼ 17.0,
to the general procedure as described above. 1H NMR (400 MHz, 12.0, 5.3 Hz, 1H), 3.00 (dt, J ¼ 16.6, 3.5 Hz, 1H), 2.72–2.57 (m,
CDCl3) d 7.95 (dd, J ¼ 5.2, 3.4 Hz, 2H), 7.61–7.52 (m, 1H), 7.45 1H), 2.35 (dd, J ¼ 17.5, 6.7 Hz, 1H), 2.24 (dd, J ¼ 14.0, 5.8 Hz,
(dd, J ¼ 10.5, 4.7 Hz, 2H), 3.53 (dd, J ¼ 18.1, 3.3 Hz, 1H), 3.04 1H), 2.21–2.01 (m, 4H), 1.89–1.57 (m, 2H); 13C NMR (100 MHz,
(dd, J ¼ 18.1, 8.0 Hz, 1H), 2.69–2.60 (m, 1H), 2.47–2.20 (m, 3H), CDCl3) d 219.5, 197.4, 144.0, 133.4, 132.4, 128.7, 127.4, 126.6,
2.09 (ddd, J ¼ 6.4, 4.0, 2.0 Hz, 1H), 1.91–1.82 (m, 1H), 1.71–1.50 50.4, 49.3, 37.8, 29.6, 28.3, 25.1, 21.1; IR (KBr, cmꢂ1): 3447, 2924,
(m, 1H); 13C NMR (100 MHz, CDCl3) d 220.4, 198.0, 136.6, 133.2, 1733, 1716, 1162, 761, 418; HRMS (ESI) calcd for C15H17O2 (M +
128.6, 128.0, 45.1, 38.6, 37.5, 29.7, 20.8; IR (KBr, cmꢂ1): 2962, 1)+: 229.1223, found: 229.1231.
2879, 1738, 1684, 1596, 1448, 1263, 1001, 753, 690; HRMS (ESI)
calcd for C13H15O2 (M + 1)+: 203.1067, found: 203.1070.
2-(1-Oxo-1-phenylpentan-2-yl)cyclopentanone (3f). The title
compound was prepared as a brown mixture of two diastereo-
2-(2-(3-Bromophenyl)-2-oxoethyl)cyclopentanone (3b). The mers in 67% yield according to the general procedure as
title compound was prepared as light brown solid in 65% yield described above; 1H NMR (400 MHz, CDCl3) d 7.98 (d, J ¼ 7.4 Hz,
according to the general procedure as described above; 1H NMR 2H), 7.64–7.52 (m, 1H), 7.46 (dd, J ¼ 15.2, 7.7 Hz, 2H), 3.87 (dt, J
(500 MHz, CDCl3) d 8.09 (t, J ¼ 1.7 Hz, 1H), 7.91–7.85 (m, 1H), ¼ 9.3, 4.7 Hz, 1H), 2.69–2.50 (m, 1H), 2.39–2.21 (m, 1H), 2.09
7.69 (ddd, J ¼ 7.9, 1.9, 0.9 Hz, 1H), 7.35 (t, J ¼ 7.9 Hz, 1H), 3.49 (tdd, J ¼ 11.5, 10.0, 5.9 Hz, 3H), 1.91–1.59 (m, 4H), 1.47–1.28 (m,
(dd, J ¼ 18.2, 3.4 Hz, 1H), 3.01 (dd, J ¼ 18.2, 7.9 Hz, 1H), 2.68– 1H), 1.28–1.08 (m, 1H), 0.86 (t, J ¼ 7.2 Hz, 3H); 13C NMR (100
2.61 (m, 1H), 2.46–2.32 (m, 2H), 2.32–2.21 (m, 1H), 2.16–2.05 MHz, CDCl3) d 218.8, 202.7, 136.9, 132.9, 128.7, 128.3, 51.7,
(m, 1H), 1.92–1.82 (m, 1H), 1.61 (td, J ¼ 12.0, 6.8 Hz, 1H); 13C 45.4, 37.9, 31.0, 25.9, 21.0, 20.8, 14.2; IR (KBr, cmꢂ1): 3746, 3396,
NMR (125 MHz, CDCl3) d 220.0, 196.6, 138.3, 136.0, 131.1, 130.2,
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2962, 2877, 1816, 1733, 1164, 1051, 1010, 418; HRMS (ESI) calcd (m, 2H), 1.86–1.66 (m, 2H), 1.59 (qd, J ¼ 12.0, 6.7 Hz, 2H); 13C
for C16H21O2 (M + 1)+: 245.1536, found: 245.1540.
NMR (125 MHz, CDCl3) d 219.0, 49.2, 38.2, 26.7, 20.9; IR (KBr,
2-(2-Oxo-4-phenylbutyl)cyclopentanone (3g). The title cmꢂ1): 3446, 2962, 2873, 1733, 1449, 1133, 1000, 599, 418;
compound was prepared as colorless oil in 72% yield according HRMS (ESI) calcd for C10H15O2 (M + 1)+: 167.1067, found:
to the general procedure as described above; 1H NMR (500 MHz, 167.1070.
CDCl3) d 7.28 (dd, J ¼ 10.3, 4.6 Hz, 2H), 7.26–7.13 (m, 3H), 2.90
2-(2-Oxocyclopentyl)cycloheptanone (3l). The title compound
(t, J ¼ 7.7 Hz, 3H), 2.75 (dd, J ¼ 16.0, 8.1 Hz, 2H), 2.55–2.40 (m, was prepared as a white mixture of two diastereomers in 71%
2H), 2.40–2.30 (m, 1H), 2.27–2.13 (m, 2H), 2.10–2.00 (m, 1H), yield according to the general procedure as described above; 1H
1.89–1.74 (m, 1H), 1.49 (dd, J ¼ 11.9, 6.7 Hz, 1H); 13C NMR (125 NMR (400 MHz, CDCl3) d 3.09 (dt, J ¼ 7.6, 4.3 Hz, 1H), 2.92 (ddd, J
MHz, CDCl3) d 220.1, 207.9, 140.8, 128.5, 128.3, 126.1, 44.8, ¼ 12.9, 9.5, 6.5 Hz, 1H), 2.69–2.58 (m, 2H), 2.52 (ddd, J ¼ 16.2,
44.3, 42.5, 37.4, 29.7, 29.4, 20.7; HRMS (ESI) calcd for C15H19O2 6.5, 2.8 Hz, 1H), 2.47–2.30 (m, 3H), 2.30–2.23 (m, 1H), 2.23–2.09
(M + 1)+: 31.1380, found: 231.1381.
(m, 2H), 2.09–1.95 (m, 4H), 1.81 (qdd, J ¼ 25.2, 11.4, 6.2 Hz, 10H),
2-(4-Methyl-2-oxopentyl)cyclopentanone (3h).33 The title 1.71–1.47 (m, 6H), 1.47–1.17 (m, 3H); 13C NMR (100 MHz, CDCl3)
compound was prepared as colorless oil in 73% yield according d 220.0, 220.0, 214.5, 213.7, 52.4, 52.2, 51.7, 51.2, 43.9, 43.7, 37.9,
to the general procedure as described above; 1H NMR (500 MHz, 37.9, 30.1, 29.8, 29.7, 29.2, 29.1, 28.8, 26.3, 25.7, 23.9, 23.6, 21.0,
CDCl3) d 2.86 (d, J ¼ 16.6 Hz, 1H), 2.43 (ddd, J ¼ 61.0, 21.5, 8.9 20.7; IR (KBr, cmꢂ1): 3447, 2930, 2874, 1733, 1696, 1453, 1148,
Hz, 2H), 2.38–2.10 (m, 6H), 2.10–2.00 (m, 1H),1.81 (d, J ¼ 11.2 502, 418; HRMS (ESI) calcd for C12H19O2 (M + 1)+: 195.1380,
Hz, 1H), 1.53 (dd, J ¼ 11.5, 6.9 Hz, 1H), 0.92 (d, J ¼ 6.6 Hz, 6H); found: 195.1388.
13C NMR (125 MHz, CDCl3) d 220.2, 208.7, 51.8, 44.8, 42.9, 37.4,
2-(2-Oxo-2-phenylethyl)cyclohexanone (3m).36 The title
29.4, 24.7, 22.5, 22.5, 20.7; HRMS (ESI) calcd for C11H19O2 (M + compound was prepared as white solid in 62% yield according
1)+: 183.1385, found: 183.1386. to the general procedure as described above; 1H NMR (400 MHz,
2-(2-Oxotridecyl)cyclopentanone (3i). The title compound CDCl3) d 7.99 (dd, J ¼ 5.2, 3.3 Hz, 2H), 7.61–7.52 (m, 1H), 7.52–
was prepared as brown oil in 76% yield according to the general 7.41 (m, 2H), 3.61 (dd, J ¼ 17.7, 6.6 Hz, 1H), 3.17 (dd, J ¼ 12.7,
procedure as described above; 1H NMR (500 MHz, CDCl3) d 2.87 6.3 Hz, 1H), 2.69 (dd, J ¼ 17.7, 5.7 Hz, 1H), 2.49–2.39 (m, 2H),
(d, J ¼ 16.7 Hz, 1H), 2.57–2.13 (m, 7H), 2.12–1.98 (m, 1H), 1.98– 2.33–2.07 (m, 2H), 1.89 (dd, J ¼ 9.9, 6.4 Hz, 1H), 1.79 (dt, J ¼
1.70 (m, 1H), 1.61–1.39 (m, 3H), 1.25 (s, 16H), 0.88 (t, J ¼ 6.7 Hz, 12.7, 3.4 Hz, 1H), 1.73–1.64 (m, 1H), 1.46 (qd, J ¼ 12.8, 3.9 Hz,
3H); 13C NMR (125 MHz, CDCl3) d 220.3, 209.1, 44.8, 42.9, 42.3, 1H); 13C NMR (100 MHz, CDCl3) d 211.5, 198.6, 137.0, 133.0,
37.4, 31.9, 29.6, 29.5, 29.4, 29.3, 29.3, 29.1, 23.8, 22.6, 20.7, 14.1; 128.5, 128.0, 46.4, 41.9, 38.3, 34.3, 27.9, 25.3; HRMS (ESI) calcd
HRMS (ESI) calcd for C18H33O2 (M + 1)+: 281.2475, found: for C14H17O2 (M + 1)+: 217.1223, found: 217.1226.
281.2484.
2-(3-Oxopentan-2-yl)cyclohexanone
(3n).37
The
title
2-(2-Oxocyclopentyl)cyclohexanone (3j).34 Two diastereomers compound was prepared as brown oil in 39% yield according to
1
were prepared as white solid in 81% total yield according to the the general procedure as described above; H NMR (400 MHz,
general procedure as described above. The upper isomer: 1H CDCl3) d 2.90 (p, J ¼ 7.0 Hz, 1H), 2.65 (dd, J ¼ 13.0, 6.4 Hz, 1H),
NMR (400 MHz, CDCl3) d 2.85 (dd, J ¼ 7.7, 3.8 Hz, 1H), 2.60–2.47 2.61–2.54 (m, 1H), 2.48 (dt, J ¼ 10.8, 7.2 Hz, 1H), 2.42–2.25 (m,
(m, 1H), 2.42 (dd, J ¼ 14.0, 1.7 Hz, 1H), 2.39–2.25 (m, 2H), 2.25– 2H), 2.09–1.98 (m, 2H), 1.91–1.84 (m, 1H), 1.77–1.54 (m, 3H),
2.10 (m, 2H), 2.10–1.97 (m, 2H), 1.96–1.74 (m, 3H), 1.73–1.60 1.13 (d, J ¼ 7.1 Hz, 3H), 1.10–0.96 (m, 3H); 13C NMR (100 MHz,
(m, 4H); 13C NMR (100 MHz, CDCl3) d 220.7, 211.0, 50.5, 48.7, CDCl3) d 214.4, 211.7, 53.8, 44.8, 42.2, 35.4, 30.8, 27.7, 25.1, 14.8,
42.0, 38.4, 30.3, 27.4, 26.2, 25.3, 20.9; IR (KBr, cmꢂ1): 2938, 2863, 7.5; IR (KBr, cmꢂ1): 2938, 2864, 1759, 1708, 1450, 1281, 1167,
1734, 1706, 1465, 1316, 1141, 502, 463; HRMS (ESI) calcd for 1019, 974, 890; HRMS (ESI) calcd for C11H19O2 (M + 1)+:
C
11H17O2 (M + 1)+: 181.1223, found: 181.1226; the lower isomer: 183.1380, found: 183.1387.
1H NMR (400 MHz, CDCl3) d 3.09–2.94 (m, 1H), 2.54 (ddd, J ¼
2-(2-Oxo-1,3-diphenylpropyl)cyclohexanone (3o). The title
18.2, 11.9, 9.1 Hz, 1H), 2.45–2.21 (m, 3H), 2.16–1.95 (m, 5H), compound was prepared as brown oil in 51% yield according to
1
1.89 (dddd, J ¼ 18.0, 14.9, 7.4, 4.0 Hz, 2H), 1.79–1.52 (m, 4H); the general procedure as described above; H NMR (400 MHz,
13C NMR (100 MHz, CDCl3) d 220.3, 210.0, 51.2, 49.9, 41.9, 38.0, CDCl3) d 7.41–7.15 (m, 8H), 7.11–7.04 (m, 2H), 4.34 (d, J ¼ 7.3
31.8, 27.1, 25.4, 25.2, 21.2; IR (KBr, cmꢂ1): 3735, 2951, 2874, Hz, 1H), 3.71 (q, J ¼ 15.9 Hz, 2H), 2.96–2.71 (m, 1H), 2.38–2.19
1734, 1706, 1506, 1148, 418, 408; HRMS (ESI) calcd for C11H17O2 (m, 2H), 2.03–1.81 (m, 2H), 1.81–1.62 (m, 3H), 1.60–1.47 (m,
(M + 1)+: 181.1223, found: 181.1232.
1H); 13C NMR (100 MHz, CDCl3) d 211.1, 206.9, 136.6, 134.0,
[1,10-Bi(cyclopentane)]-2,20-dione (3k).35 Two diastereomers 129.7, 129.3, 128.7, 128.5, 127.3, 126.9, 56.3, 54.4, 49.6, 42.1,
were prepared as white solid in 80% total yield according to the 30.4, 27.5, 25.0; IR (KBr, cmꢂ1): 3059, 3028, 2924, 2852, 1768,
general procedure as described above. The upper isomer: 1H 1717, 1496, 1426, 1154, 1072, 736, 698, 521; HRMS (ESI) calcd
NMR (500 MHz, CDCl3) d 2.73–2.59 (m, 2H), 2.41–2.29 (m, 2H), for C21H23O2 (M + 1)+: 307.1693, found: 307.1695.
2.13–1.98 (m, 6H), 1.86–1.74 (m, 2H), 1.72–1.59 (m, 2H); 13C
5-Bromo-3-methyl-1-phenylhexane-1,4-dione (3p). The title
NMR (125 MHz, CDCl3) d 220.0, 48.5, 38.0, 25.3, 20.7; IR (KBr, compound was prepared as white solid in 32% yield according
cmꢂ1): 3447, 2963, 2878, 1734, 1405, 1145, 821, 488; HRMS (ESI) to the general procedure as described above; 1H NMR (400 MHz,
calcd for C10H15O2 (M + 1)+: 167.1067, found: 167.1071; the CDCl3) d 8.01–7.93 (m, 2H), 7.57 (dd, J ¼ 10.4, 4.3 Hz, 1H), 7.46
1
lower isomer: H NMR (500 MHz, CDCl3) d 2.60–2.48 (m, 2H), (t, J ¼ 7.6 Hz, 2H), 4.73 (q, J ¼ 6.8 Hz, 1H), 3.92–3.60 (m, 2H),
2.32 (ddd, J ¼ 18.6, 8.4, 1.1 Hz, 2H), 2.25–2.10 (m, 4H), 2.09–1.96 3.52 (dd, J ¼ 17.8, 7.0 Hz, 1H), 3.12 (dd, J ¼ 17.8, 5.8 Hz, 1H),
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1.83 (d, J ¼ 6.9 Hz, 2H), 1.29 (d, J ¼ 7.2 Hz, 3H); 13C NMR (100
MHz, CDCl3) d 206.8, 197.9, 136.5, 133.2, 128.6, 128.0, 45.8,
42.0, 38.1, 20.5, 17.6; HRMS (ESI) calcd for C13H16BrO2 (M + 1)+:
283.0328, found: 283.0337.
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5-Bromo-3,3-dimethyl-1-phenyl-2-propylpentane-1,4-dione (3q).
The title compound was prepared as brown solid in 31% yield
1
according to the general procedure as described above; H NMR
(500 MHz, CDCl3) d 8.02–7.94 (m, 2H), 7.58 (t, J ¼ 7.4 Hz, 1H), 7.48
(t, J ¼ 7.7 Hz, 2H), 4.19 (q, J ¼ 13.6 Hz, 2H), 3.98 (dd, J ¼ 10.6,
3.2 Hz, 1H), 1.76 (ddd, J ¼ 18.2, 10.1, 5.4 Hz, 1H), 1.43 (d, J ¼ 5.6
Hz, 1H), 1.38 (d, J ¼ 18.1 Hz, 3H), 1.25 (d, J ¼ 7.0 Hz, 3H), 1.22–1.08
(m, 2H), 0.84 (t, J ¼ 7.3 Hz, 3H); 13C NMR (125 MHz, CDCl3) d
205.5, 204.6, 139.0, 133.2, 128.7, 128.3, 51.5, 50.3, 32.9, 31.9, 24.3,
22.3, 21.9, 14.2; IR (KBr, cmꢂ1): 2960, 2873, 1719, 1466, 1447, 1164,
1042, 713, 690; HRMS (ESI) calcd for C16H22BrO2 (M + 1)+:
325.0798, found: 325.0801.
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Conclusions
In conclusion, we have detailed a catalyst-free coupling reaction
of silyl enolates with a-haloketones via in situ generated oxyallyl
zwitterions in basic TFE. The reaction took place regiose-
lectively at the a-carbonyl position and nally produced the
useful 1,4-diketones. Further studies to dene the enolate scope
and the optimal catalyst are currently being pursued in our
laboratory.
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Acknowledgements
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(b) N. De Kimpe and R. Verhe, in a-Haloketones, a-
Haloaldehydes and a-Haloimines (1988), John Wiley & Sons,
Inc., 2010.
We appreciate the nancial support from the Fundamental and
Advanced Research Projects of Chongqing City (No.
cstc2014jcyjA10022), Starting Foundation for Talents Returning
from Overseas of Ministry of Education, and Starting Founda-
tion of College of Pharmacy, Chongqing Medical University.
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