Organocatalytic Redox Isomerization of Electron-Deficient Allylic Alcohols: Synthesis of 1,4-Ketoaldehydes

Article abstract of DOI:10.1021/acs.joc.6b00243

An organocatalytic redox isomerization strategy has been developed for the synthesis of 1,4-ketoaldehydes. DABCO was found to be the best catalyst for the isomerization of -hydroxy enones. With 20 mol % of DABCO as catalyst and DMSO as the solvent high yi

Full text of DOI:10.1021/acs.joc.6b00243

Note
pubs.acs.org/joc
Organocatalytic Redox Isomerization of Electron-Deficient Allylic
Alcohols: Synthesis of 1,4-Ketoaldehydes
Keshab Mondal, Buddhadeb Mondal, and Subhas Chandra Pan*
Department of Chemistry, Indian Institute of Technology Guwahati, North Guwahati, Assam 781039, India
S
* Supporting Information
ABSTRACT: An organocatalytic redox isomerization strategy
has been developed for the synthesis of 1,4-ketoaldehydes.
DABCO was found to be the best catalyst for the isomerization
of γ-hydroxy enones. With 20 mol % of DABCO as catalyst and
DMSO as the solvent high yields have been achieved for
different 1,4-ketoaldehydes.
ne of the current challenges in organic chemistry is to
Odevelop reactions that provide products with high levels 1).
of efficiency, selectivity, and atom economy.1 Isomerization
reactions are considered one of the perfect atom economic
methods that do not generate byproducts. Redox isomerization
is an intrinsically efficient process as it avoids external oxidants
and reductants, and formation of unwanted byproducts is
avoided.2 The redox isomerization of readily available allylic
alcohols to saturated carbonyl compounds is an attractive and
atom-economic process that circumvents use of highly reactive
and expensive reagents and also reduces the number of
protection−deprotection steps often required for such trans-
formations (Scheme 1).3
ization strategy for the synthesis of 1,4-ketoaldehydes (Scheme
We started our investigation by stirring 3-benzoylprop-2-en-
1-ol (1a)10 in DMSO at 60 °C with various organic base
catalysts (Table 1). As can be seen, 1,8-diazabicyclo[5.4.0]-
Table 1. Catalyst Screening and Optimization of Reaction
Conditions
a
b
entry
catalyst
yield (%)
1
2
3
4
5
6
7
8
DBU
0
45
52
0
Scheme 1. Redox Isomerization of Allylic Alcohols
Et3N
DIPEA
TMG
pyridine
K2CO3
DMAP
DABCO
DABCO
0
0
71
93
95
c
9
a
Reaction conditions: 0.2 mmol of 1a in 1.0 mL of solvent using 20
b
A variety of transition metal complexes have been utilized for
the transformations of allylic alcohols to ketones, aldehydes and
1,4-diketones.3 Interestingly, literature survey did not reveal the
synthesis of 1,4-ketoaldehydes using this strategy. In addition,
organocatalytic redox isomerization4,5 of allylic alcohols, an
attractive alternative strategy, has been less studied. Previously,
few acid- and base-catalyzed redox isomerization of γ-hydroxy
enones to 1,4-diketones have been reported.6 Taylor et al. also
observed formation of 1,4-diketones and 1,4-ketoaldehydes
during the reaction of 1,2-dioxines and phosphorus ylide
though lower yields (∼30%) were observed for 1,4-
ketoaldehydes.7 Miles and co-workers disclosed DABCO
catalyzed isomerization of 6-hydroxy-2H-pyranones to 1,2,5-
triketones.8 Realizing the potential of 1,4-ketoaldehydes as
important building blocks,9 we embarked in a redox isomer-
mol % of catalyst. Isolated yield after silica gel column
chromatography. Reaction temperature 90 °C and reaction time 5 h.
c
undec-7-ene (DBU) could not promote the isomerization
reaction after 20 h (entry 1). Pleasingly, a moderate yield
(45%) of the isomerized product 3-benzoylpropanal (2a) was
obtained with Et3N (entry 2). A small improvement in the yield
was observed with DIPEA (entry 3). However, 1,1,3,3-
tetramethylguanidine (TMG), pyridine, and inorganic base
such as potassium carbonate (K2CO3) failed to react under
identical conditions (entries 4−6). Gratifyingly, DMAP
Received: February 9, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.6b00243
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
provided the product 2a in a better yield of 71% (entry 7).
Finally, the best catalyst turned out to be 1,4-
diazabicyclo[2.2.2]octane (DABCO), which afforded 93%
yield of the product 2a (entry 8). The yield was further
enhanced to 95% by running the reaction at higher temperature
and the reaction time was shortened (entry 9). A lower yield
(89%) was observed with 10 mol % of the catalyst. Other
solvents were also screened, but DMSO was found to be the
best solvent (see the Supporting Information for details).
Having identified the optimized conditions, we ventured
toward the scope and generality of the reaction. Initially,
different γ-hydroxy aryl enones were prepared and treated
under the reaction conditions (Table 2). It turned out that the
11). Interestingly, ortho-substitution on the aryl group also did
not change the outcome of the reaction, and a good yield of
82% was attained with 2-methyl-substituted aryl enone 1l
(entry 12). 1-Naphthyl-substituted enone 1m was also engaged
in the reaction, and good yield of 75% was achieved for the
desired product 2m (entry 13). 4-Biphenyl-substituted enone
1n provided product 2n in a much higher yield of 95% (entry
14). Interestingly, 2,4-dimethylphenyl-substituted keto alde-
hyde 2o was obtained in 97% yield after the treatment of the
corresponding enone (entry 15). Then, a heteroaromatic group
was incorporated in the enone moiety, and product 2p was
isolated in 89% yield (entry 16). Finally, aliphatic enones were
examined under the reaction conditions, and gratifyingly, the
products are obtained in good yields although higher reaction
times were required (entries 17 and 18). Enone 1q having
hydrocinnamyl moiety provided product 2q in 65% yield (entry
17), whereas a higher yield of 77% was achieved for the
cyclohexyl-substituted product 2r (entry 18). Finally, enone 1s
having an isovaleryl moiety was used in our reaction, and a
good yield of 80% was obtained (entry 19).
Table 2. Substrate Scope: α-Unbranched Enone
The next phase of experiments concerned the preparation
and screening of different α-substituted enones. After some
experiments were performed, it was found that the α-methyl
group could be tolerated in this reaction. Thus, a variety of α-
methyl enones (1t−x) was prepared and engaged in the
reaction (Table 3, entries 1−5). In general, longer reaction
a
b
entry
R
time (h)
2
yield (%)
1
Ph
5
3.5
5
2a
2b
2c
2d
2e
2f
95
90
88
66
99
75
79
92
98
74
68
82
75
95
97
89
65
77
80
2
4-MeC6H4
4-iPrC6H4
4-tBuC6H4
4-OMeC6H4
4-FC6H4
3
4
4.5
6
5
6
3
Table 3. Substrate Scope: α- and β-Branched Enones
7
4-ClC6H4
3
2g
2h
2i
8
4-BrC6H4
3
9
3-OMeC6H4
3-ClC6H4
4.5
3
10
11
12
13
14
15
16
17
18
19
2j
3-MeC6H4
2-MeC6H4
1-naphthyl
4-biphenyl
2,4-(Me)2C6H3
2-thiophene-yl
hydroxycinnamyl
cyclohexyl
4
2k
2l
3.5
2.5
5
a
b
entry
Ar
R1
R2
time (h)
2
yield (%)
2m
2n
2o
2p
2q
2r
2s
1
2
3
4
5
6
Ph
Me
Me
Me
Me
Me
H
H
48
30
30
48
72
30
2t
51
67
62
59
47
48
3-ClC6H4
4-ClC6H4
4-BrC6H4
4-MeC6H4
Ph
H
2u
2v
2w
2x
2y
4
H
2
H
10
20
24
H
Me
isovaleryl
a
a
Reaction conditions: 0.2 mmol of 1 in 1 mL of solvent using 20 mol
b
Reaction conditions: 0.2 mmol of 1 in 1 mL of solvent using 20 mol
% of DABCO. Isolated yield after silica gel column chromatography.
b
% of DABCO. Isolated yield after silica gel column chromatography.
reaction is quite general for a variety of electron-rich and
electron-poor aryl enones. Substitutions at the ortho, para, and
meta positions of the phenyl group are also well tolerated. 4-
Methyl-substituted aryl enone provided 90% yield of the
product 2b (entry 2). A similar yield was observed with 4-
isopropyl-substituted aryl enone (entry 3), but a lower yield
was obtained with 4-tert-butyl-substituted aryl enone (entry 4).
Consequently, 4-methoxy-substituted aryl enone 1e was found
to be the best substrate, providing the product 2e in 99% yield
(entry 5). Then 4-halo-substituted aryl enones were examined
under the reaction conditions, and the desired products were
obtained in acceptable yields (entries 6−8). While 4-chloro-
and 4-fluoro-substituted enones provided similar yields, 4-
bromo-substituted enone 1h afforded an enhanced yield of 92%
(entry 8). Aryl enones having meta-substitutions were then
prepared and subjected to the reaction conditions. Excellent
yield (98%) was obtained with 3-methoxyaryl enone 1i (entry
9). 3-Chloro-substituted and 3-methyl-substituted aryl enones
afforded products 2j and 2k in moderate yields (entries 10 and
times were required for significant conversion to the products.
Product 2t was isolated in 51% yield after 48 h (entry 1).
Substitutions at the meta and para positions of the aryl group
did not alter the fate of the reaction. Moderate yields were
achieved for 3- and 4-chloro-substituted aryl ketoaldehydes 2u
and 2v (entries 2 and 3). Product 2w having a 4-bromo-
substituted aryl group was isolated in a similar yield of 59%
(entry 4). 4-Methyl-substituted aryl ketone 1x was found to
undergo isomerization slowly, and product 2x was attained in
47% yield after 72 h (entry 5). This indicates that electron
deficiency of the double bond is important for the rate of the
reaction for α-substituted substrates. Pleasingly, our method is
also applicable for β-methyl-substituted enones such as 1y,
providing a moderate yield of 48% for the product 2y (entry 6).
To understand the mechanism of our reaction, kinetic
experiments were performed using 1a in DMSO-d6 at 90 °C,
1
and the conversions were determined by H NMR spectros-
copy (Figure1a,b). The rate of the reaction was found to be
second order overall (Figure 1a) and first order with respect to
B
DOI: 10.1021/acs.joc.6b00243
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
Scheme 2. Plausible Mechanism
Figure 1. (a) Percent conversion vs time. (b) Initial reaction rate vs ln
[DABCO].
DABCO (Figure 1b). Thus, the rate-determining step involves
one molecule of 1 and one molecule of DABCO in the
transition state.
over anhydrous Na2SO4 and concentrated in a rotary evaporater under
reduced pressure. For column chromatography, silica gel (60−120
mesh size) were used. For TLC analyses precoated silica gel plates
were used.
To gain further insight into the mechanism of our redox
isomerization reaction, we performed a deuterium incorpo-
ration experiment with 1a. After treatment of 1a with 20 mol %
of DABCO in DMSO-d6/D2O (16:1),5b the exclusive
formation of 2,3-d2-2a was observed (Scheme 2, eq 1).11 This
result eliminated the possibility of 1,2-hydride shift from C4 to
C3 induced by the 4-OH deprotonation by DABCO as in this
case product 3-d-2a will be formed (eq 2). Similarly, any
mechanism involving the abstraction of 4-H of 1a by DABCO
was also ruled out since in this case the resulting enolate would
be quenched by more acidic protonated DABCO (pKa = 9 in
DMSO) than H2O (or D2O; pKa = 32 for H2O in DMSO)5b,12
as was found to operate in the isomerization of secondary
propargylic alcohol5b (eq 3). Thus, the remaining possibility
was the conjugate addition by DABCO to the enone to
generate intermediate 3 (eq 4). This intermediate would create
4 after quenching with D2O.
1H, 13C NMR spectroscopy: 400 MHz (at 298 K) and 600 MHz.
Chemical shifts, δ (in ppm), are reported in ppm relative to TMS δ
(1H) 0.0 ppm, δ (13C) 0.0 ppm, which was used as the inner reference
with multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet).
Otherwise, the solvents residual proton resonance and carbon
resonance (CHCl3, δ (1H) 7.26 ppm, δ (13C) 77.23 ppm were used
for calibration. IR: spectra were recorded on an FT-IR Instrument at
normal temperature making KBr pellet grinding the sample with KBr
(IR grade). Mass spectra were recorded on a Q-TOF mass
spectrometer. Melting points were obtained with a Mel-Tem capillary
melting point apparatus and are uncorrected.
General Procedure for the Synthesis of α-Unbranched trans-
γ-Hydroxy Enones 1a−s. α-Unbranched trans-γ-hydroxy enones
were prepared according to the reported procedure.10b,13
To a stirred solution of SnCl2·2H2O (1.5 equiv) and KI (3 equiv) in
H2O were added allyl bromide and aldehydes, satd aqueous NH4Cl
solution was added, and the resulting solution was stirred for 5 h at
room temperature to obtain the homoallylic alchol. β,γ-Unsaturated
ketone was obtained by PCC (1.5 equiv) oxidation. Then β,γ-epoxy
ketone was prepared from β,γ-unsaturated ketone using m-CPBA (1.1
equiv). Finally, ring opening occurred to afford the trans-γ-
hydroxyenones.
An E2 elimination of DABCO from 4 assisted by the general
base catalysis of another molecule of DABCO leaded to the
formation of enol 5, which then tautomerized after abstracting a
proton from residual H2O or a deuterium from D2O to afford
2,3-d2-2a.
General Procedure for the Synthesis of α-Branched trans-γ-
Hydroxy Enones 1t−y. α-Branched trans-γ-hydroxy enones were
prepared according to the reported procedure.10a
To a stirred solution of the appropriate stabilized keto ylide (1.2
mmol) in THF (5 mL) was added glycoaldehyde dimer (1 mmol), and
the resulting solution was heated under reflux for 3 h. The solution
was cooled and the solvent evaporated in vacuo. The product was
purified by silica gel column chromatography.
In summary, we have developed a facile synthesis of 1,4-
ketoaldehydes by redox isomerization strategy. DABCO was
found to be the best catalyst for this purpose. The 1,4-
ketoaldehyde motifs are difficult to obtain, and our method will
be very useful in natural product synthesis and drug discovery.
EXPERIMENTAL SECTION
■
General Information. All reactions were carried under air in oven-
Ylides were prepared by reacting the appropriate haloalkanone with
dried glassware with magnetic stirring. Organic solvents were dried
triphenylphosphine and deprotonating with 2 N NaOH solution.14,15
C
DOI: 10.1021/acs.joc.6b00243
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The Journal of Organic Chemistry
Note
General Procedure for the Synthesis of Products 2a−y. In a
10 mL round-bottom-flask were combined compound 1 (0.2 mmol)
and DABCO (20 mol %), and 1 mL of DMSO solvent was added. The
round bottom flask was sealed with a glass stopper and placed in a
heating block at 90 °C. After completion of the reaction, as
determined by TLC, the reaction mixture was allowed to cool at
room temperature and diluted with EtOAc. The organic layer was
washed with water and brine and dried (Na2SO4). The solvents were
concentrated in vacuo and purified by silica gel column chromatog-
raphy (15% EtOAc/hexane) to afford the title compound 2.
HRMS (+ESI): calcd for C10H10FO2 [M + H]+ 181.0659, found
181.0660.
4-(4-Chlorophenyl)-4-oxobutanal (2g). This compound was
prepared according to the general procedure. Reaction was completed
after 3 h. Analytical TLC on silica gel, 12% ethyl acetate/hexane (Rf =
1
0.52). Brown solid (31 mg, yield: 79%). Mp: 51−53 °C. H NMR
(600 MHz, CDCl3): δ 9.90 (s, 1H), 7.92 (d, J = 8.4 Hz, 2H), 7.45 (d, J
= 8.4 Hz, 2H), 3.28 (t, J = 6.3 Hz, 2H), 2.94 (t, J = 6.3 Hz, 2H);. 13C
NMR (150 MHz, CDCl3): δ 200.5, 196.8, 140.0, 134.9, 129.7, 129.2,
37.8, 31.1. FT-IR (thin film): 1710, 1682, 1589, 1400, 1244, 1207,
1091, 983, 833 cm−1. HRMS (+ESI): calcd for C10H10ClO2 [M + H]+
197.0364, found 197.0363.
4-Oxo-4-phenylbutanal (2a). This compound was prepared
according to the general procedure. Reaction was completed after 5
h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf = 0.55).
4-(4-Bromophenyl)-4-oxobutanal (2h). This compound was
prepared according to the general procedure. Reaction was completed
after 3 h. Analytical TLC on silica gel, 12% ethyl acetate/hexane (Rf =
0.53). White solid (44 mg, yield: 92%). Mp: 59−61 °C. 1H NMR (600
MHz, CDCl3): δ 9.90 (s, 1H), 7.85 (d, J = 8.4 Hz, 2H), 7.62 (d, J = 8.4
Hz, 2H), 3.29 (t, J = 6.0 Hz, 2H), 2.95 (t, J = 6.0 Hz, 2H). 13C NMR
(150 MHz, CDCl3): δ 200.6, 197.0, 135.3, 132.2, 129.8, 128.7, 37.7,
31.1. FT-IR (thin film): 1713, 1681, 1582, 1407, 1296, 1202, 1067,
998, 822, 715, 666 cm−1. HRMS (+ESI): calcd for C10H10BrO2 [M +
H]+ 240.9859, found 240.9858.
1
Brown liquid (31 mg, yield: 95%). H NMR (600 MHz, CDCl3): δ
9.91 (s, 1H), 7.99 (d, J = 7.2 Hz, 2H), 7.58 (t, J = 7.2 Hz, 1H), 7.47 (t,
J = 4.8 Hz, 2H), 3.33 (t, J = 6.3 Hz, 2H), 2.94 (t, J = 6.3 Hz, 2H). 13C
NMR (150 MHz, CDCl3): δ 200.8, 198.0, 136.7, 133.6, 128.9, 128.3,
37.9, 31.3. FT-IR (thin film): 1716, 1685, 1597, 1449, 1364, 1240, 983,
759, 691 cm−1. HRMS (+APCI): calcd for C10H11O2 [M + H]+
163.0754, found 163.0748.
4-Oxo-4-p-tolylbutanal (2b). This compound was prepared
according to the general procedure. Reaction was completed after
3.5 h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf =
4-(3-Methoxyphenyl)-4-oxobutanal (2i). This compound was
prepared according to the general procedure. Reaction was completed
after 4.5 h. Analytical TLC on silica gel, 18% ethyl acetate/hexane (Rf
1
0.57). Brown solid (32 mg, yield: 90%). Mp: 45−47 °C. H NMR
(400 MHz, CDCl3): δ 9.91 (s, 1H), 7.89 (d, J = 8.0 Hz, 2H), 7.27 (d, J
= 8.0 Hz, 2H), 3.31 (t, J = 6.4 Hz, 2H), 2.93 (t, J = 6.4 Hz, 2H), 2.42
(s, 3H). 13C NMR (100 MHz, CDCl3): δ 200.9, 197.6, 144.3, 134.2,
129.5, 128.4, 37.8, 31.1, 21.9. FT-IR (thin film): 1709, 1681, 1607,
1382, 1239, 1182, 977, 875, 814, 548 cm−1. HRMS (+ESI): calcd for
C11H13O2 [M + H]+ 177.0910, found 177.0908.
1
= 0.51). Pale yellow oil (38 mg, yield: 98%). H NMR (400 MHz,
CDCl3): δ 9.90 (s, 1H), 7.57 (d, J = 7.6 Hz, 1H), 7.50 (s, 1H), 7.38 (t,
J = 8.0 Hz, 1H), 7.12 (dd, J = 2.4, 8.4 Hz, 1H), 3.85 (s, 3H), 3.31 (t, J
= 6.4 Hz, 2H), 2.93 (t, J = 6.4 Hz, 2H). 13C NMR (100 MHz, CDCl3):
δ 200.8, 197.8, 160.0, 137.9, 129.8, 120,.9, 119.9, 112.5, 55.6, 37.8,
31.3. FT-IR (thin film): 1720, 1685, 1596, 1576, 1487, 1431, 1293,
1263, 1169, 1045, 788 cm−1. HRMS (+APCI): calcd for C11H13O3 [M
+ H]+ 193.0859, found 193.0858.
4-(3-Chlorophenyl)-4-oxobutanal (2j). This compound was
prepared according to the general procedure. Reaction was completed
after 3 h. Analytical TLC on silica gel, 12% ethyl acetate/hexane (Rf =
0.54). Pale yellow oil (29 mg, yield: 74%). 1H NMR (600 MHz,
CDCl3): δ 9.89 (s, 1H), 7.95 (s, 1H), 7.86 (d, J = 7.8 Hz, 1H), 7.54 (d,
J = 9.0 Hz, 1H), 7.42 (t, J = 7.8 Hz, 1H), 3.29 (t, J = 6.0 Hz, 2H), 2.94
(t, J = 6.0 Hz, 2H). 13C NMR (150 MHz, CDCl3): δ 200.4, 196.8,
138.2, 135.2, 133.4, 130.2, 128.4, 126.4, 37.7, 31.3. FT-IR (thin film):
1717, 1689, 1572, 1420, 1237, 1206, 1018, 997, 869, 789, 680 cm−1.
HRMS (+ESI): calcd for C10H10ClO2 [M + H]+ 197.0364, found
197.0363.
4-(4-Isopropylphenyl)-4-oxobutanal (2c). This compound was
prepared according to the general procedure. Reaction was completed
after 5 h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf =
0.60). Yellow oil (36 mg, yield: 88%). 1H NMR (400 MHz, CDCl3): δ
9.91 (s, 1H), 7.92 (d, J = 8.0 Hz, 2H), 7.32 (d, J = 8.0 Hz, 2H), 3.31 (t,
J = 6.4 Hz, 2H), 2.95−3.00 (m, 1H), 2.93 (t, J = 6.4 Hz, 2H), 1.27 (d,
J = 6.8 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ 200.9, 197.7, 155.1,
134.5, 128.5, 126.9, 37.8, 34.4, 31.1, 23.8. FT-IR (thin film): 1722,
1685, 1607, 1460, 1419, 1362, 1243, 1182, 1055, 981, 830 cm−1.
HRMS (+APCI): calcd for C13H17O2 [M + H]+ 205.1223, found
205.1213.
4-(4-Tert-butylphenyl)-4-oxobutanal (2d). This compound was
prepared according to the general procedure. Reaction was completed
after 4.5 h. Analytical TLC on silica gel, 10% ethyl acetate/hexane (Rf
1
= 0.50). Brown liquid (29 mg, yield: 66%). H NMR (400 MHz,
4-Oxo-4-m-tolylbutanal (2k). This compound was prepared
according to the general procedure. Reaction was completed after 4
h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf = 0.60).
CDCl3): δ 9.90 (s, 1H), 7.93 (d, J = 8.0 Hz, 2H), 7.48 (d, J = 8.0 Hz,
2H), 3.31 (t, J = 6.2 Hz, 2H), 2.92 (t, J = 6.2 Hz, 2H), 1.34 (s, 9H).
13C NMR (100 MHz, CDCl3): δ 201.1, 197.7, 157.3, 134.0, 128.2,
125.8, 37.8, 35.3, 31.3, 31.1. FT-IR (thin film): 1720, 1682, 1605,
1407, 1364, 1247, 1192, 1108, 1018, 986, 828 cm−1. HRMS (+APCI):
calcd for C14H19O2 [M + H]+ 219.1380, found 219.1372.
1
Brown liquid (24 mg, yield: 68%). H NMR (600 MHz, CDCl3): δ
9.91 (s, 1H), 7.78 (d, J = 9.0 Hz, 2H), 7.34−7.40 (m, 2H), 3.32 (t, J =
6.6 Hz, 2H), 2.93 (t, J = 6.6 Hz, 2H), 2.41 (s, 3H). 13C NMR (150
MHz, CDCl3): δ 200.9, 198.2, 138.7, 136.7, 134.3, 128.8, 128.7, 125.5,
37.9, 31.3, 21.6. FT-IR (thin film): 1722, 1685, 1603, 1587, 1362,
1255, 1161, 875, 785 cm−1. HRMS (+ESI): calcd for C11H13O2 [M +
H]+ 177.0910, found 177.0900.
4-Oxo-4-o-tolylbutanal (2l). This compound was prepared
according to the general procedure. Reaction was completed after
3.5 h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf =
0.59). Pale yellow oil (29 mg, yield: 82%). 1H NMR (600 MHz,
CDCl3): δ 9.90 (s, 1H), 7.73 (d, J = 7.8 Hz, 1H), 7.38 (t, J = 7.5 Hz,
1H), 7.25−7.29 (m, 2H), 3.23 (t, J = 6.3 Hz, 2H), 2.91 (t, J = 6.3 Hz,
2H), 2.49 (s, 3H). 13C NMR (150 MHz, CDCl3): δ 201.9, 200.8,
138.5, 137.5, 132.2, 131.7, 128.8, 125.9, 38.1, 33.9, 21.5. FT-IR (thin
film): 1718, 1686, 1456, 1384, 1237, 1207, 1018, 977, 760 cm−1.
HRMS (+ESI): calcd for C11H13O2 [M + H]+ 177.0910, found
177.0915.
4-(4-Methoxyphenyl)-4-oxobutanal (2e). This compound was
prepared according to the general procedure. Reaction was completed
after 6 h. Analytical TLC on silica gel, 18% ethyl acetate/hexane (Rf =
1
0.52). Pale yellow oil (38.5 mg, yield: 99%). H NMR (600 MHz,
CDCl3): δ 9.91 (s, 1H), 7.97 (d, J = 9.0 Hz, 2H), 6.94 (d, J = 9.0 Hz,
2H), 3.87 (s, 3H), 3.28 (t, J = 6.3 Hz, 2H), 2.91 (t, J = 6.3 Hz, 2H).
13C NMR (150 MHz, CDCl3): δ 201.1, 196.5, 163.9, 130.6, 129.7,
114.0, 55.7, 37.9, 30.9. FT-IR (thin film): 1720, 1675, 1601, 1575,
1511, 1419, 1363, 1246, 1212, 1171, 1028, 834 cm−1. HRMS (+ESI):
calcd for C11H13O3 [M + H]+ 193.0859, found 193.0857.
4-(4-Fluorophenyl)-4-oxobutanal (2f). This compound was
prepared according to the general procedure. Reaction was completed
after 3 h. Analytical TLC on silica gel, 12% ethyl acetate/hexane (Rf =
0.53). Colorless oil (27 mg, yield: 75%). 1H NMR (600 MHz,
CDCl3): δ 9.89 (s, 1H), 8.00 (dd, J = 5.1, 8.7 Hz, 2H), 7.13 (t, J = 8.7
Hz, 2H), 3.28 (t, J = 6.3 Hz, 2H), 2.92 (t, J = 6.3 Hz, 2H). 13C NMR
(150 MHz, CDCl3): δ 200.7, 196.4, 166.0 (d, J = 103.4 Hz), 133.1 (d,
J = 3 Hz), 130.9 (d, J = 9.3 Hz), 115.9, 37.8, 31.1. FT-IR (thin film):
1721, 1685, 1597, 1506, 1410, 1363, 1233, 1209, 1158, 986, 838 cm−1.
4-(Naphthalen-5-yl)-4-oxobutanal (2m). This compound was
prepared according to the general procedure. Reaction was completed
after 2.5 h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf
= 0.50). Brown oil (32 mg, yield: 75%). 1H NMR (600 MHz, CDCl3):
δ 9.95 (s, 1H), 8.58 (d, J = 8.4 Hz, 1H), 8.01 (d, J = 8.4 Hz, 1H), 7.97
D
DOI: 10.1021/acs.joc.6b00243
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The Journal of Organic Chemistry
Note
(d, J = 6.6 Hz, 1H), 7.88 (d, J = 7.8 Hz, 1H), 7.59 (t, J = 7.8 Hz, 1H),
7.50−7.55 (m, 2H), 3.39 (t, J = 6.0 Hz, 2H), 3.02 (t, J = 6.3 Hz, 2H).
13C NMR (150 MHz, CDCl3): δ 202.2, 200.9, 135.6, 134.1, 133.1,
130.3, 128.6, 128.2, 127.9, 126.7, 125.9, 124.6, 38.3, 34.5. FT-IR (thin
film): 1717, 1677, 1595, 1509, 1394, 1235, 1178, 1104, 940, 805, 773
cm−1. HRMS (+ESI): calcd for C14H13O2 [M + H]+ 213.0910, found
213.0912.
after 48 h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf =
0.51). Colorless oil (18 mg, yield: 51%). 1H NMR (600 MHz,
CDCl3): δ 9.81 (s, 1H), 7.98 (d, J = 7.8 Hz, 1H), 7.58 (t, J = 7.2 Hz,
1H), 7.48 (t, J = 7.2 Hz, 2H), 4.03−3.97 (m, 1H), 3.17 (dd, J = 18.3,
7.8 Hz, 1H), 2.62 (dd, J = 18.6, 4.8 Hz, 1H), 1.24 (d, J = 7.2 Hz, 3H).
13C NMR (150 MHz, CDCl3): δ 202.7, 200.7, 135.8, 133.4, 128.9,
128.7, 47.3, 35.3, 18.2. FT-IR (thin film): 1711, 1681, 1596, 1449,
1380, 1240, 1179, 979, 794, 704 cm−1. HRMS (+APCI): calcd for
C11H13O2 [M + H]+ 177.0910, found 177.0909.
4-(Biphenyl)-4-oxobutanal (2n). This compound was prepared
according to the general procedure. Reaction was completed after 5 h.
Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf = 0.51).
Pale yellow solid (45 mg, yield: 95%). Mp: 93−95 °C. 1H NMR (600
MHz, CDCl3): δ 9.93 (s, 1H), 8.06 (d, J = 8.4 Hz, 2H), 7.70 (d, J = 8.4
Hz, 2H), 7.63 (d, J = 7.2 Hz, 2H), 7.48 (t, J = 7.8 Hz, 2H), 7.41 (t, J =
7.2 Hz, 1H), 3.37 (t, J = 6.3 Hz, 2H), 2.97 (t, J = 6.3 Hz, 2H). 13C
NMR (150 MHz, CDCl3): δ 200.9, 199.6, 146.2, 140.0, 135.3, 129.1,
128.9, 128.8, 127.6, 127.4, 37.8, 31.2. FT-IR (thin film): 1711, 1680,
1448, 1405, 1384, 1245, 1197, 981, 766, 724, 690 cm−1. HRMS
(+APCI): calcd for C16H15O2 [M + H]+ 239.1067, found 239.1068.
4-(2,4-Dimethylphenyl)-4-oxobutanal (2o). This compound was
prepared according to the general procedure. Reaction was completed
after 4 h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf =
0.59). Brown liquid (37 mg, yield: 97%). 1H NMR (600 MHz,
CDCl3): δ 9.90 (s, 1H), 7.68 (d, J = 7.8 Hz, 1H), 7.07 (d, J = 11.4 Hz,
2H), 3.24 (t, J = 6.3 Hz, 2H), 2.89 (t, J = 6.0 Hz, 2H), 2.48 (s, 3H),
2.35 (s, 3H). 13C NMR (150 MHz, CDCl3): δ 201.1, 200.9, 142.5,
139.2, 134.3, 133.2, 129.4, 126.6, 38.2, 33.6, 21.9, 21.6. FT-IR (thin
film): 1720, 1680, 1611, 1566, 1448, 1234, 1206, 1140, 981, 814 cm−1.
HRMS (+APCI): calcd for C12H15O2 [M + H]+ 191.1067, found
191.1068.
4-Oxo-4-(thiophene-2-yl)butanal (2p). This compound was
prepared according to the general procedure. Reaction was completed
after 2 h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf =
0.45). Brown oil (30 mg, yield: 89%). 1H NMR (600 MHz, CDCl3): δ
9.88 (s, 1H), 7.77 (d, J = 4.2 Hz, 1H), 7.65 (d, J = 4.8 Hz, 1H), 7.14 (t,
J = 4.2 Hz, 1H), 3.26 (t, J = 6.6 Hz, 2H), 2.93 (t, J = 6.6 Hz, 2H). 13C
NMR (150 MHz, CDCl3): δ 200.5, 190.9, 143.7, 134.0, 132.3, 128.4,
37.8, 31.7. FT-IR (thin film): 1723, 1662, 1518, 1416, 1246, 1054, 928,
854, 728 cm−1. HRMS (+ESI): calcd for C8H9O2S [M + H]+
169.0318, found 169.0319.
4-(3-Chlorophenyl)-3-methyl-4-oxobutanal (2u). This compound
was prepared according to the general procedure. Reaction was
completed after 30 h. Analytical TLC on silica gel, 15% ethyl acetate/
1
hexane (Rf = 0.53). Yellow oil (28 mg, yield: 67%). H NMR (600
MHz, CDCl3): δ 9.78 (s, 1H), 7.94 (s, 1H), 7.85 (d, J = 7.8 Hz, 1H),
7.55 (d, J = 7.8 Hz, 1H), 7.42 (d, J = 7.8 Hz, 1H), 3.95−3.89 (m, 1H),
3.19 (dd, J = 18.6, 8.4 Hz, 1H), 2.64 (dd, J = 18.6, 4.8 Hz, 1H), 1.22
(d, J = 7.2 Hz, 3H). 13C NMR (150 MHz, CDCl3): δ 201.5, 200.4,
137.5, 135.3, 133.3, 130.3, 128.8, 126.7, 47.3, 35.4, 18.0. FT-IR (thin
film): 1726, 1685, 1570, 1239, 1198, 1075, 801, 736, 670 cm−1. HRMS
(+ESI): calcd for C11H12ClO2 [M + H]+ 211.0520, found 211.0522.
4-(4-Chlorophenyl)-3-methyl-4-oxobutanal (2v). This compound
was prepared according to the general procedure. Reaction was
completed after 30 h. Analytical TLC on silica gel, 15% ethyl acetate/
1
hexane (Rf = 0.52). Yellow oil (26 mg, yield: 62%). H NMR (400
MHz, CDCl3): δ 9.79 (s, 1H), 7.92 (d, J = 8.8 Hz, 2H), 7.45 (d, J = 8.8
Hz, 2H), 3.98−3.89 (m, 1H), 3.18 (dd, J = 18.8, 8.4 Hz, 1H), 2.62
(dd, J = 18.8, 4.8 Hz, 1H), 1.22 (d, J = 6.8 Hz, 3H). 13C NMR (100
MHz, CDCl3): δ 201.5, 200.5, 139.8, 134.2, 130.1, 129.3, 47.3, 35.2,
18.1. FT-IR (thin film): 1724, 1681, 1590, 1488, 1459, 1401, 1238,
1092, 1013, 980, 842, 748, 525 cm−1. HRMS (+ESI): calcd for
C11H12ClO2 [M + H]+ 211.0520, found 211.0522.
4-(4-Bromophenyl)-3-methyl-4-oxobutanal (2w). This compound
was prepared according to the general procedure. Reaction was
completed after 48 h. Analytical TLC on silica gel, 15% ethyl acetate/
1
hexane (Rf = 0.51). Yellow oil (30 mg, yield: 59%). H NMR (600
MHz, CDCl3): δ 9.78 (s, 1H), 7.84 (d, J = 8.4 Hz, 2H), 7.62 (d, J = 8.4
Hz, 2H), 3.95−3.89 (m, 1H), 3.17 (dd, J = 18.6, 8.4 Hz, 1H), 2.63
(dd, J = 18.6, 4.8 Hz, 1H), 1.22 (d, J = 7.2 Hz, 3H). 13C NMR (150
MHz, CDCl3): δ 201.7, 200.5, 134.7, 132.3, 130.2, 128.6, 47.3, 35.2,
18.1. FT-IR (thin film): 1713, 1681, 1585, 1397, 1071, 1010, 978, 838
cm−1. HRMS (+ESI): calcd for C11H12BrO2 [M + H]+ 255.0015,
found 255.0017.
3-Methyl-4-oxo-4-p-tolylbutanal (2x). This compound was
prepared according to the general procedure. Reaction was completed
after 72 h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf =
0.54). Yellow oil (18 mg, yield: 47%). 1H NMR (600 MHz, CDCl3): δ
9.80 (s, 1H), 7.88 (d, J = 8.4 Hz, 2H), 7.28 (d, J = 8.4 Hz, 2H), 3.99−
3.96 (m, 1H), 3.14 (dd, J = 18.3, 8.1 Hz, 1H), 2.60 (dd, J = 18.6, 5.4
Hz, 1H), 2.42 (s, 3H), 1.23 (d, J = 8.4 Hz, 3H). 13C NMR (150 MHz,
CDCl3): δ 202.3, 200.8, 144.2, 133.3, 129.6, 128.8, 47.3, 35.3, 21.9,
18.3. FT-IR (thin film): 1724, 1680, 1608, 1456, 1381, 1261, 1183,
1019, 801, 747 cm−1. HRMS (+ESI): calcd for C12H15O2 [M + H]+
191.1067, found 191.1065.
4-Oxo-6-phenylhexanal (2q). This compound was prepared
according to the general procedure. Reaction was completed after 10
h. Analytical TLC on silica gel, 12% ethyl acetate/hexane (Rf = 0.52).
1
Brown liquid (25 mg, yield: 65%). H NMR (400 MHz, CDCl3): δ
9.79 (s, 1H), 7.28 (t, J = 7.6 Hz, 2H), 7.19 (t, J = 8.0 Hz, 3H), 2.91 (t,
J = 7.2 Hz, 2H), 2.80 (t, J = 7.2 Hz, 2H), 2.77−2.74 (m, 2H), 2.71−
2.68 (m, 2H). 13C NMR (100 MHz, CDCl3): δ 207.9, 200.6, 141.0,
128.7, 128.5, 126.3, 44.4, 37.6, 35.0, 29.9. FT-IR (thin film): 1712,
1599, 1435, 1409, 1263, 1100, 877, 751, 700 cm−1. HRMS (+ESI):
calcd for C12H15O2 [M + H]+ 191.1067, found 191.1066.
4-Cyclohexyl-4-oxobutanal (2r). This compound was prepared
according to the general procedure. Reaction was completed after 20
h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf = 0.55).
1
Pale yellow oil (26 mg, yield: 77%). H NMR (600 MHz, CDCl3): δ
9.80 (s, 1H), 2.73−2.77 (m, 4H), 2.39 (t, J = 11.1 Hz, 1H), 1.88 (d, J
= 11.4 Hz, 2H), 1.78 (d, J = 13.2 Hz, 2H), 1.23−1.37 (m, 5H), 1.20 (t,
J = 12.6 Hz, 1H). 13C NMR (150 MHz, CDCl3): δ 212.1, 200.9, 50.9,
37.6, 32.8, 28.7, 26.0, 25.8. FT-IR (thin film): 1705, 1632, 1448, 1386,
1146, 1018 cm−1. HRMS (+ESI): calcd for C10H17O2 [M + H]+
169.1223, found 169.1223.
2-Methyl-4-oxo-4-phenylbutanal (2y). This compound was
prepared according to the general procedure. Reaction was completed
after 30 h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf =
0.51). Brown oil (17 mg, yield: 48%). 1H NMR (600 MHz, CDCl3): δ
9.80 (s, 1H), 7.98 (d, J = 7.8 Hz, 2H), 7.58 (t, J = 7.2 Hz, 1H), 7.48 (t,
J = 8.4 Hz, 2H), 3.50 (dd, J = 18.0, 7.2 Hz, 1H), 3.16−3.10 (m, 1H),
3.26 (dd, J = 17.7, 6.0 Hz, 1H), 1.25 (d, J = 7.2 Hz, 3H). 13C NMR
(150 MHz, CDCl3): δ 203.7, 199.0, 136.7, 133.6, 128.9, 128.3, 48.9,
39.6, 14.0. FT-IR (thin film): 1725, 1683, 1597, 1448, 1359, 1263,
1217, 1003, 913, 751, 690 cm−1. HRMS (+ESI): calcd for C11H13O2
[M + H]+ 177.0910, found 177.0912.
6-Methyl-4-oxoheptanal (2s). This compound was prepared
according to the general procedure. Reaction was completed after 24
h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf = 0.49).
1
Yellow oil (23 mg, yield: 81%). H NMR (600 MHz, CDCl3): δ 9.80
(s, 1H), 2.72 (d, J = 16.8 Hz, 4H), 2.33 (d, J = 6.0 Hz, 2H), 2.18−2.10
(m, 1H), 0.91 (d, J = 3.6 Hz, 6H). 13C NMR (150 MHz, CDCl3): δ
208.7, 200.7, 51.9, 37.6, 35.4, 24.9, 22.7. FT-IR (thin film): 1712, 1468,
1386, 1368, 1170, 1140, 1080, 1033, 872 cm−1. HRMS (+APCI): calcd
for C8H15O2 [M + H]+ 143.1067, found 143.1068.
Spectral data for compound 2,3-d2-2a. This compound was
prepared according to the general procedure. Reaction was completed
after 5 h. Analytical TLC on silica gel, 15% ethyl acetate/hexane (Rf =
1
0.55). H NMR (600 MHz, CDCl3): δ 9.91 (s, 1H), 7.99 (d, J = 7.8
3-Methyl-4-oxo-4-phenylbutanal (2t). This compound was
prepared according to the general procedure. Reaction was completed
Hz, 2H), 7.58 (t, J = 7.2 Hz, 1H), 7.47 (t, J = 7.8 Hz, 2H), 3.32(t, J =
6.6 Hz, 1H), 2.92 (t, J = 6.6 Hz, 1H). 13C NMR (150 MHz, CDCl3): δ
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DOI: 10.1021/acs.joc.6b00243
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The Journal of Organic Chemistry
Note
200.9, 198.0, 136.6, 133.5, 128.9, 128.3, 37.4 (t, J = 7.8 Hz), 30.9 (t, J =
7.8 Hz). FT-IR (thin film): 1724, 1682, 1597, 1448, 1280, 1220, 753,
690 cm−1. HRMS (+ESI): calcd for C10H19 D2O2 [M + H]+ 166.0957,
found 166.0953
(8) Miles, W. H.; Gildner, P. G.; Ahmed, Z.; Cohen, E. M. Synthesis
2010, 2010, 3977.
(9) For selected recent uses of 1,4-ketoaldehydes such as 2a, see:
(a) Uyeda, C.; Jacobsen, E. N. J. Am. Chem. Soc. 2008, 130, 9228.
(b) Zheng, P.; Gondo, C. A.; Bode, J. W. Chem. - Asian J. 2011, 6, 614.
(c) Lu, J.; Song, Z.; Zhang, Y.; Gan, Z.; Li, H. Angew. Chem., Int. Ed.
2012, 51, 5367. (d) Hasegawa, E.; Osawa, C.; Tateyama, M.; Miura,
K.; Tayama, E.; Iwamoto, H. Heterocycles 2012, 86, 1211. (e) Kumar,
I.; Mir, N. A.; Ramaraju, P.; Singh, D.; Gupta, V.; Kant, R. RSC Adv.
2014, 4, 34548.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.6b00243.
(10) For the synthesis of 1a, see: (a) Greatrex, B. W.; Kimber, M. C.;
Taylor, D. K.; Tiekink, E. R. T. J. Org. Chem. 2003, 68, 4239. (b) Sada,
M.; Ueno, S.; Asano, K.; Nomura, K.; Matsubara, S. Synlett 2009, 2009,
724.
(11) Under the reaction conditions of DMSO/D2O (16:1) at 90 °C,
2,3-(1.5d2)-2a was obtained.
(12) See Evans’ pKa table: http://evans.rc.fas.harvard.edu/pdf/
evans_pKa_table.pdf.
(13) Silva, F. P. L.; Cirqueira, M. L.; Martins, F. T.; Vasconcellos, M.
Optimization table and spectra of all the products (PDF)
AUTHOR INFORMATION
Corresponding Author
* Email: span@iitg.ernet.in.
■
Notes
The authors declare no competing financial interest.
L. A. A. J. Mol. Struct. 2013, 1052, 189.
(14) Borzęcka, W.; Lavandera, I.; Gotor, V. J. Org. Chem. 2013, 78,
7312.
(15) Facchin, G.; Bertani, R.; Berton, A. Inorg. Chim. Acta 1988, 147,
165.
ACKNOWLEDGMENTS
■
This work was supported by DST-SERB (file no EMR/2015/
001034). We thank the Central Instruments Facility, Indian
Institute of Technology Guwahati, for the instrumental help.
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F
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