Selective hydrogenation of conjugated unsaturated ketones containing a hydroxyaryl substituent in the β-position

Article abstract of DOI:10.1134/S1070428017010055

A high selectivity was achieved in the Ni2B-catalyzed hydrogenation of α,β-unsaturated ketones containing a hydroxyaryl (phenolic) substituent in the β-position. The developed hydrogenation procedure was used to synthesize natural compounds of the phenylpropane series and their structural analogs.

Full text of DOI:10.1134/S1070428017010055

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2017, Vol. 53, No. 1, pp. 24–28. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © V.N. Kovalenko, A.S. Pratsko, 2017, published in Zhurnal Organicheskoi Khimii, 2017, Vol. 53, No. 1, pp. 31–35.
Selective Hydrogenation of Conjugated Unsaturated Ketones
Containing a Hydroxyaryl Substituent in the β-Position
V. N. Kovalenkoa, b* and A. S. Pratskob
a Belarusian State University, pr. Nezavisimosti 4, Minsk, 220030 Belarus
*e-mail: kovalenkovn@rambler.ru
b Belarusian State Pedagogical University, ul. Sovetskaya 18, Minsk, 220030 Belarus
Received May 31, 2016
Abstract—A high selectivity was achieved in the Ni2B-catalyzed hydrogenation of α,β-unsaturated ketones
containing a hydroxyaryl (phenolic) substituent in the β-position. The developed hydrogenation procedure was
used to synthesize natural compounds of the phenylpropane series and their structural analogs.
DOI: 10.1134/S1070428017010055
Reduction of α,β-unsaturated carbonyl compounds
is widely used in organic synthesis when selective
transformation of functional groups is necessary.
However, despite a large number of known methods
and reagents [1], desired selectivity for some substrates
could not always be achieved [2, 3]. We previously
reported [3] the synthesis of raspberry ketone
(rheosmin) and structurally related zingerone by hy-
drogenation of α,β-unsaturated precursors in the
presence of a cheap catalyst (nickel boride Ni2B). The
selectivity for 1,4-reduction exceeded 98%. The pro-
posed procedure can be used to obtain other com-
pounds of the phenylpropane series. In this article
we describe hydrogenation of accessible aldolization/
crotonization products with the formation of a number
of rheosmin and zingerone analogs. The effect of the
substituent nature on the selectivity and rate of the
reduction of α,β-unsaturated ketones was also studied.
ing to our data, hydrogenation over a longer time in-
volves the carbonyl group up to its complete reduction
(Scheme 1). As a result, 4-hydroxybenzylideneacetone
(1a) under the same conditions can be converted to
1,4-reduction product 2a (rheosmin) or the corre-
sponding alcohol 3a (rhododendrol).
The hydrogenation of structurally related com-
pounds 1b–1g (Scheme 2) was carried out under
similar conditions, and the reaction completion was
determined by discoloration of the reaction mixture.
The corresponding saturated ketones 2b–2g were
isolated in no less than 65% yield. The selectivity for
1,4-reduction was evaluated by the molar ratio of
saturated ketone 2 and saturated alcohol 3 in the
reaction mixture according to the GLC data. The
selectivity in the hydrogenation of zingerone (2b) was
comparable with that in the reduction of rheosmin (2a,
98%). The hydrogenation of salicylideneacetone 1c
was characterized by a lower selectivity (93%). The
substituent on the double bond in the α-position (enone
1d), as well as tert-butyl substituent on the carbonyl
carbon atom (enones 1e, 1f), appreciably reduced the
hydrogenation rate, the selectivity being 96–97%. The
hydrogenation of camphor derivative 1g was the most
difficult to accomplish. In this case, increased load of
An interesting peculiarity of the hydrogenation of
enones containing a hydroxyaryl substituent in the
β-position is discoloration of the reaction mixture
when the initial unsaturated ketone is consumed com-
pletely (disappearance of conjugated bond system), so
that it is easy to detect the end of the reaction and
attain selective reduction of the double bond. Accord-
Scheme 1.
O
O
OH
H2 (1 equiv), Ni2B
MeOH, 1–1.5 h
H2, Ni2B, MeOH
18 h
Me
Me
Me
HO
HO
HO
1a
2a
3a
24

SELECTIVE HYDROGENATION OF CONJUGATED UNSATURATED KETONES
25
Scheme 2.
O
O
OH
H2, Ni2B (NaBH4 + NiCl2)
MeOH
Ar
R2
Ar
R2
+
Ar
R2
R1
1b–1g
R1
R1
2b–2g 100:0 to 93:7 3b–3g
Ar = 3-MeO-4-HOC6H3, R1 = H, R2 = Me (b); Ar = 2-HOC6H4, R1 = H, R2 = Me (c); Ar = 4-HOC6H4, R1 = R2 = Me (d);
Me
Ar = 4-HOC6H4, R1 = H, R2 = t-Bu (e); Ar = 3-MeO-4-HOC6H3, R1 = H, R2 = t-Bu (f); Ar = 4-HOC6H4, R1R2 =
(g).
Me
Me
the catalyst, longer reaction time, and higher tempera-
ture were necessary, but no side reduction of the
carbonyl group occurred.
noted previously [4]. By replacing sodium methoxide
by potassium methoxide we obtained the correspond-
ing potassium phenoxide which is readily soluble in
methanol; however, this in no way favored the reac-
tion. The condensation was successful only with
vanillin protected by a tetrahydropyranyl group. The
observed strong effect of the meta substituent on the
condensation may be rationalized by coordination of
the methoxy group to metal ion, which additionally
stabilizes phenoxide ion and hence reduces the reac-
tivity of the aldehyde carbonyl group.
In summary, we have proposed a simple procedure
for selective 1,4-reduction of unsaturated ketones
containing a hydroxyaryl substituent in the β-position.
A number of structurally related phenylpropane deriva-
tives, including natural raspberry ketone and zinge-
rone, have been synthesized via aldolization/crotoniza-
tion of aromatic aldehydes with ketones and subse-
quent hydrogenation in the presence of nickel boride.
Saturated ketone 2g was isolated as a mixture of
diastereoisomers at a ratio of 2 :1. Base-catalyzed
epimerization of 2g changed the diastereoisomer ratio
in the opposite direction (Scheme 3), presumably in
favor of the trans isomer (9:91). The epimer ratio was
determined from the intensities of signals from one
1
benzylic proton in the H NMR spectra, which were
observed as doublets of doublets at δ 3.16 and
3.09 ppm. The stereoisomer ratio no longer changed
after double recrystallization of the isomer mixture.
Scheme 3.
O
Me
*
HO
Me
Me
(1) MeONa, MeOH, reflux, 6 h
(2) 5% HCl
EXPERIMENTAL
2g
2g
dr 67:33
dr 9:91
The H and 13C NMR spectra were recorded from
1
solutions in CDCl3 on a Bruker Avance 500 spectrom-
eter at 500 and 125 MHz, respectively. The IR spectra
were measured on a Bruker Vertex 70 spectrometer.
The melting points were determined in capillaries. Gas
chromatographic–mass spectrometric analyses were
obtained on a Shimadzu GCMS-QP2010 instrument
using an Equity-5 capillary column. Silica gel (70–
230 mesh) was used for column chromatography; TLC
was performed on Silufol plates using hexane–ethyl
acetate as eluent; spots were detected by treatment
with a 10% alcoholic solution of phosphomolybdic
acid. The elemental analyses were obtained with
a Thermo Scientific Flash 2000 CHNS/O analyzer. The
solvents were dried by standard methods and distilled
prior to use.
Initial unsaturated ketones 1a–1g are readily avail-
able by condensation of aromatic aldehydes with the
corresponding ketones (see Experimental). Unexpected
difference in the reactivities of 4-hydroxybenzalde-
hyde and vanillin toward pinacolone was observed in
the synthesis of enones 1e and 1f, respectively. In the
first case, the reaction smoothly afforded the target
product on heating in methanol in the presence of
sodium methoxide with almost complete conversion of
the reactants, and there was no need of introducing
protecting groups. Vanillin failed to react with pina-
colone under analogous conditions, and only the initial
compounds were detected in the reaction mixture. The
low reactivity of vanillin may be related to the
formation of poorly soluble sodium phenoxide, as was
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 53 No. 1 2017

KOVALENKO, PRATSKO
26
(3E)-4-(4-Hydroxyphenyl)but-3-en-2-one (1a) was
dried over Na2SO4, and the solvent was distilled off
under reduced pressure. The residue was dissolved in
15 mL of methanol containing 20 mmol of sodium
methoxide, 2.5 g (25 mmol) of pinacolone was added,
and the mixture was stirred for 20 h at room tempera-
ture and concentrated under reduced pressure. The
residue was treated with 20 mL of water and 20 mL of
ethyl acetate, the organic layer was separated, and the
aqueous layer was extracted with ethyl acetate
(2×10 mL). The extracts were combined with the or-
ganic phase, washed with water (10 mL) and saturated
solutions of NaHCO3 and NaCl (10 mL each), and
dried over Na2SO4. The solvent was distilled off under
reduced pressure, the crude condensation product was
dissolved in 20 mL of methanol, 0.06 g (0.24 mmol) of
pyridinium p-toluenesulfonate was added, and the mix-
ture was kept for 3 h at room temperature. Triethyl-
amine, 0.1 mL, was added, the solvent was distilled off
under reduced pressure, the residue was treated with
10 mL of water and 10 mL of ethyl acetate, the organic
phase was separated, and the aqueous phase was
extracted with ethyl acetate (2×5 mL). The combined
extracts were vigorously shaken for 10 min with
a solution prepared from 4 g of Na2SO3, 20 mL of
water, and 0.8 g of 96% H2SO4, washed with water
(5 mL) and saturated solutions of NaHCO3 and NaCl
(5 mL each), and dried over Na2SO4. The solvent was
distilled off under reduced pressure to obtain 3.5 g of
the crude product with a purity of >90% (according to
synthesized as described in [5] and was purified by
recrystallization from toluene–ethyl acetate (2:1).
(3E)-4-(4-Hydroxy-3-methoxyphenyl)but-3-en-2-
one (1b) was synthesized according to [5] and was
purified by recrystallization from 75% isopropyl
alcohol.
(3E)-4-(2-Hydroxyphenyl)but-3-en-2-one (1c) was
prepared by condensation of salicylaldehyde with
acetone according to the same procedure as that used
for the synthesis of 1a and 1b [5], but the reaction
time was shortened to 6 h. The product was purified
by recrystallization from benzene with addition of
charcoal [6].
(3E)-4-(4-Hydroxyphenyl)-3-methylbut-3-en-2-one
(1d) was synthesized according to the procedure
reported in [7].
(1E)-1-(4-Hydroxyphenyl)-4,4-dimethylpent-
1-en-3-one (1e). 4-Hydroxybenzaldehyde, 5.0 g
(40.9 mmol), and pinacolone, 9.0 g (90 mmol), were
added to a solution of 100 mmol of sodium methoxide
in 45 mL of methanol. The mixture was refluxed for
9 h and concentrated under reduced pressure. The
residue was treated with 100 mL of 10% aqueous HCl
and extracted with ethyl acetate (4×25 mL). The com-
bined extracts were vigorously shaken for 30 min with
a solution prepared from 20 g of sodium sulfite,
100 mL of water, and 3.9 g of 96% sulfuric acid,
washed with water (10 mL) and saturated solutions of
NaHCO3 and NaCl (10 mL each), and dried over
Na2SO4. The solvent was distilled off under reduced
pressure, and the residue was recrystallized from
toluene. Yield 6.35 g (76%), light yellow crystals,
mp 125–128°C. IR spectrum (KBr), ν, cm–1: 3150
(OH), 1665 (C=O), 1600 (C=C). 1H NMR spectrum, δ,
ppm: 1.22 s (9H, t-Bu), 6.92–6.93 m (2H, Harom), 7.03 d
(1H, CH=, J = 15.5 Hz), 7.46–7.47 m (2H, Harom),
7.67 d (1H, CH=, J = 15.5 Hz), 7.83 br.s (1H, OH).
13C NMR spectrum, δC, ppm: 26.4 (3C, CH3), 43.2,
116.0 (2C, CH), 117.7 (CH), 126.9, 130.4 (2C, CH),
144.0 (CH), 158.7, 206.3 (C=O). Found, %: C 76.60;
H 7.84. C13H16O2. Calculated, %: C 76.44; H 7.90.
1
the H NMR data), which was purified by silica gel
column chromatography (hexane–ethyl acetate, 5:1).
Yield 3.35 g (87%), oily material. IR spectrum (KBr),
ν, cm–1: 3200 (OH), 1645 (C=O), 1590 (C=C).
1H NMR spectrum, δ, ppm: 1.22 s (9H, t-Bu), 3.92 s
(3H, CH3O), 6.11 br.s (1H, OH), 6.90–6.91 m (1H,
H
arom), 6.97 d (1H, CH=, J = 15.5 Hz), 7.02–7.03 m
(1H, Harom), 7.13–7.14 m (1H, Harom), 7.62 d (1H,
13
CH=, J = 15.5 Hz). C NMR spectrum, δC, ppm: 26.4
(3C, CH3), 43.1, 55.9 (CH3), 110.2 (CH), 114.8 (CH),
118.2 (CH), 122.8 (CH), 127.4, 143.2 (CH), 146.7,
147.9, 204.3 (C=O). Found, %: C 71.50; H 7.70.
C14H18O3. Calculated, %: C 71.77; H 7.74.
(3E)-3-(4-Hydroxybenzylidene)-1,7,7-trimethyl-
bicyclo[2.2.1]heptan-2-one (1g) was synthesized
according to modified procedure [8]. A solution of
2.0 g (16.4 mmol) of 4-hydroxybenzaldehyde, 2.1 g
(25.0 mmol) of dihydropyran, and 0.1 g (0.4 mmol) of
pyridinium p-toluenesulfonate in 10 mL of anhydrous
methylene chloride was stirred for 20 h at room tem-
perature. When the reaction was complete, the mixture
was washed with a saturated solution of sodium hydro-
(1E)-1-(4-Hydroxy-3-methoxyphenyl)-4,4-di-
methylpent-1-en-3-one (1f). A solution of 2.5 g
(16.4 mmol) of vanillin, 2.1 g (25.0 mmol) of dihydro-
pyran, and 0.1 g (0.4 mmol) of pyridinium p-toluene-
sulfonate in 10 mL of anhydrous methylene chloride
was stirred for 40 h at room temperature. When the
reaction was complete, the mixture was washed with
a saturated solution of sodium hydrogen carbonate and
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SELECTIVE HYDROGENATION OF CONJUGATED UNSATURATED KETONES
27
gen carbonate and dried over Na2SO4, and the solvent
was distilled off under reduced pressure. The residue
was dissolved in 10 mL of DMSO, 2.0 g (35.7 mmol)
of powdered potassium hydroxide, 2.20 g (14.5 mmol)
of (±)-camphor were added, and the mixture was
stirred for 20 h at room temperature, diluted with
100 mL of water, and extracted with ethyl acetate
(3×20 mL). The combined extracts were washed with
water (20 mL) and saturated solutions of NaHCO3 and
NaCl (20 mL each) and dried over Na2SO4, and the
solvent was distilled off under reduced pressure. The
tetrahydropyranyl protecting group was removed as
described above in the synthesis of 1f. The crude
product was purified by crystallization from methanol.
Yield 2.38 g (64%), colorless crystals, mp 207–212°C.
IR spectrum (KBr), ν, cm–1: 3230 (OH), 1660 (C=O),
Yield 1.14 g (75%), colorless crystals, mp 82–83°C.
The spectral parameters of 2a were consistent with
those given in [5].
1-(4-Hydroxyphenyl)propan-2-ol (3a) was ob-
tained in a similar way from 1.5 g (9.25 mmol) of
enone 1a; after disappearance of green color, the
hydrogenation was continued for 18 h. The product
was purified by crystallization from methanol–water
(1:2). Yield 1.0 g (65%), colorless crystals, mp 70–
73°C. The spectral parameters of 3a were consistent
with those given in [9].
4-(4-Hydroxy-3-methoxyphenyl)butan-2-on (2b)
was obtained from 1.78 g (9.26 mmol) of enone 1b at
25°C (reaction time 1.5 h). The product was purified
by silica gel column chromatography using petroleum
ether–ethyl acetate (1:5 to 1:3) as eluent. Yield 1.69 g
(94%), viscous liquid which crystallized on storage in
a refrigerator, mp 39–40°C. Alternatively, the crude
product was recrystallized from methanol–water (1:3)
using a few crystals of pure compound 2b to initiate
crystallization. Yield 1.22 g (68%), colorless crystals,
mp 41–42°C. The spectral parameters of the product
were consistent with those given in [5].
4-(2-Hydroxyphenyl)butan-2-one (2c) was ob-
tained from 1.5 g (9.25 mmol) of enone 1c at 25°C
(reaction time 1 h). The product was purified by silica
gel column chromatography using petroleum ether–
ethyl acetate (1:10 to 1:5) as eluent. Yield 1.33 g
(88%), viscous liquid which crystallized on storage in
a refrigerator, mp 38–40°C. The spectral parameters of
the product were consistent with those given in [10].
4-(4-Hydroxyphenyl)-3-methylbutan-2-one (2d)
was obtained from 1.63 g (9.25 mmol) of enone 1d at
25°C (reaction time 3 h). The product was purified by
silica gel column chromatography using petroleum
ether–ethyl acetate (1:10 to 1:3) as eluent. Yield 1.5 g
(91%), viscous liquid. The spectral parameters of the
product were consistent with those given in [11].
1
1592 (C=C). H NMR spectrum, δ, ppm: 0.81 s (3H,
CH3), 1.00 s (3H, CH3), 1.03 s (3H, CH3), 1.48–1.60 m
(2H), 1.75–1.81 m (1H), 2.13–2.20 m (1H), 3.08 br.d
(1H, HCC=CH, J = 4.1 Hz), 5.65 br.s (1H, OH), 6.86–
6.89 m (2H, Harom), 7.19 br.s (1H, C=CH) 7.38–7.41 m
(2H, Harom).
Hydrogenation of unsaturated ketones 1a–1g
(general procedure). Sodium tetrahydridoborate,
0.46 g (12.1 mmol), was added in portions with
stirring to a solution of 1.10 g (4.63 mmol) of NiCl2·
6 H2O in 20 mL of methanol, cooled to 0°C. The
resulting suspension was stirred for 15 min at 0°C and
refluxed for 20 min under argon. The mixture was then
cooled to room temperature, 9.25 mmol of ketone
1a–1g was added, and the miixture was stirred in
a hydrogen atmosphere. When the green color of the
reaction mixture disappeared, hydrogen supply was
turned off, and the mixture was filtered. The precip-
itate of nickel boride was additionally washed with
a small amount of methanol, and the filtrate was
evaporated under reduced pressure. The residue was
dissolved in ethyl acetate (20 mL), the solution was
washed with a saturated aqueous solution of ammo-
nium chloride (20 mL), the organic phase was separat-
ed, and the aqueous phase was extracted with ethyl
acetate (10 mL). The combined organic phases were
washed with saturated solutions of NaHCO3 and NaCl
(10 mL each), dried over Na2SO4, and evaporated
under reduced pressure. The residue was purified by
crystallization from appropriate solvent mixture or by
column chromatography.
1-(4-Hydroxyphenyl)-4,4-dimethylpentan-3-one
(2e) was obtained from 1.89 g (9.25 mmol) of enone
1e at 35–40°C (reaction time 3 h). The product was
purified by crystallization from methanol–water (4:3).
Yield 1.36 g (71%), colorless crystals, mp 84–86.5°C.
IR spectrum (KBr), ν, cm–1: 3050 (OH), 1700 (C=O).
1H NMR spectrum, δ, ppm: 1.11 s (9H, t-Bu), 2.76–
2.83 m (4H, CH2CH2), 6.36 br.s (1H, OH), 6.77–
6.79 m (2H, Harom), 7.02–7.04 m (2H, Harom). 13C NMR
spectrum, δC, ppm: 26.2 (3C, CH3), 29.2 (CH2), 38.8
(CH2), 44.1, 115.3 (2C, CH), 129.4 (2C, CH), 133.1,
4-(4-Hydroxyphenyl)butan-2-one (2a) was ob-
tained by hydrogenation of 1.5 g (9.25 mmol) of enone
1a at 25°C (reaction time 1 h). The product was
purified by crystallization from methanol–water (1:3).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 53 No. 1 2017

KOVALENKO, PRATSKO
28
154.1, 216.6 (C=O). Found, %: C 75.50; H 8.75.
C13H18O2. Calculated, %: C 75.69; H 8.80.
0.85 s (3H, CH3), 0.93 s (3H, CH3), 0.97 s (3H, CH3),
1.33–1.38 m (1H), 1.69–1.80 m (3H), 1.93–1.95 m
(1H), 2.44 d.d (1H, C6H4CH2, J = 14.4, 11.1 Hz),
2.65–2.69 m [1H, C(O)CH], 3.09 d.d (1H, C6H4CH2,
J = 14.4, 4.5 Hz), 5.82 br.s (1H, OH), 6.77–6.80 m
1-(4-Hydroxy-3-methoxyphenyl)-4,4-dimethyl-
pentan-3-one (2f) was obtained from 2.17 g
(9.26 mmol) of enone 1f at 35–40°C (reaction time
4 h). The product was purified by silica gel column
chromatography using petroleum ether–ethyl acetate
(1:15 to 1:10) as eluent. Yield 1.99 g (91%), viscous
liquid which crystallized on storage in a refrigerator,
mp 60–65°C. An analytical sample was obtained by
recrystallization from methanol–water (4:3), mp 66–
68.5°C. IR spectrum (KBr), ν, cm–1: 3062 (OH), 1701
13
(2H, Harom), 7.03–7.05 m (2H, Harom). C NMR spec-
trum (signals of the major isomer are given), δC, ppm:
9.51 (CH3), 19.21 (CH3), 19.47 (CH3), 20.19 (CH2),
30.93 (CH2), 31.76 (CH2), 45.68 (CH), 45.73, 52.06
(CH), 58.96, 115.35 (2C, CH), 129.46 (2C, CH),
131.84, 154.16, 221.91 (C=O). Found, %: C 79.40;
H 8.70. C17H22O2. Calculated, %: C 79.03; H 8.58.
This study was performed under financial support
by the Belarusian Republican Foundation for Basic
Research (project no. Kh15M-020) and by the Ministry
of Education of Belarus Republic (State Research
Program for 2016–2018, project no. 2.31).
1
(C=O). H NMR spectrum, δ, ppm: 1.10 s (9H, t-Bu),
2.74–2.82 m (4H, CH2CH2), 3.86 s (3H, CH3O),
5.56 br.s (1H, OH), 6.64–6.69 m (2H, Harom), 6.81–
13
6.82 m (1H, Harom). C NMR spectrum, δC, ppm: 26.2
(3C, CH3), 29.7 (CH2), 38.7 (CH2), 44.0, 55.8 (OCH3),
111.1 (CH), 114.2 (CH), 120.7 (CH), 133.4, 143.8,
146.3, 215.1 (C=O). Found, %: C 71.04; H 8.50.
C14H20O3. Calculated, %: C 71.16; H 8.53.
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 53 No. 1 2017