The enantioselective hydrogenation of 5,6-dihydro-2H-pyran-3-carboxylic acid over a cinchona alkaloid-modified palladium catalyst: Asymmetric synthesis of a cockroach attractant

Article abstract of DOI:10.1039/b808694j

A novel application of a cinchona-modified supported Pd catalyst is presented. The key step in the asymmetric synthesis of the cockroach attractant methyl (+)-tetrahydro-2H-pyran-3-carboxylate was the enantioselective hydrogenation of 5,6-dihydro-2H-pyran

Full text of DOI:10.1039/b808694j

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LETTER
www.rsc.org/njc | New Journal of Chemistry
The enantioselective hydrogenation of 5,6-dihydro-2H-pyran-3-carboxylic
acid over a cinchona alkaloid-modified palladium catalyst: asymmetric
synthesis of a cockroach attractant
a
a
ab
+
+ +
Kornel Szori, Gyorgy Szollosi* and Mihaly Bartok
´
¨
´
´
Received (in Montpellier, France) 22nd May 2008, Accepted 20th June 2008
First published as an Advance Article on the web 4th July 2008
DOI: 10.1039/b808694j
A novel application of a cinchona-modified supported Pd cata-
lyst is presented. The key step in the asymmetric synthesis of the
cockroach attractant methyl (+)-tetrahydro-2H-pyran-3-car-
boxylate was the enantioselective hydrogenation of 5,6-dihy-
dro-2H-pyran-3-carboxylic acid over 5% Pd/Al2O3 modified by
cinchonidine, which afforded the saturated product in up to 89%
optical purity.
only moderate optical purities were obtained in the asymmetric
hydrogenation of heteroaromatic furan carboxylic acids.13
Based on the high enantioselectivities obtained in the hydro-
genation of oxygen-containing 2-pyrone derivatives and the
promising results described in the hydrogenation of hetero-
aromatic furan carboxylic acids, we attempted to extend the
scope of this catalytic system to the asymmetric preparation of
a biologically-active cockroach attractant, methyl (+)-tetra-
hydro-2H-pyran-3-carboxylate. It is known that the enantio-
mers of tetrahydropyran-3-carboxylic acid methyl ester have
different bioactivities, the (+)-enantiomer being a much more
potent attractant than its optical antipode or a racemic
mixture.14 The enantioselective hydrogenation of either of
the dihydro derivatives, 5,6-dihydro-4H-pyran-3-carboxylic
acid (1) or 5,6-dihydro-2H-pyran-3-carboxylic acid (2), may
be the key step to an easy preparation procedure of the target
compound. Carboxylic acid 1 was prepared from 3,4-dihydro-
2H-pyran according to a modified literature procedure in
B60% total yield,15 while 2 was prepared by a two-step
process, starting from acrolein in 55% total yield,16 as
presented in Scheme 1.
Chiral carboxylic acids are frequently-used intermediates in
the production of optically-pure pharmaceuticals, agrochem-
icals, flavors and fragrances.1 The enantioselective hydrogena-
tion of prochiral a,b-unsaturated carboxylic acids is
a
convenient synthetic method for the preparation of optically
pure acids that gained increased practical importance due to
development of a large variety of efficient chiral metal complex
catalysts.2 Attractive alternatives to soluble metal complexes
may be heterogeneous chiral catalysts that take advantage of
the well known benefits of these catalytic systems. The simplest
way of preparing chiral heterogeneous catalysts for the hydro-
genation of prochiral unsaturated compounds is the modifica-
tion of conventional metal catalysts by optically-pure
substances, the so called modifiers.3 The simplicity of these
procedures and the availability of a large variety of modifiers,
usually natural chiral compounds, was the driving force for the
extensive studies, which led to the development of highly
effective heterogeneous catalytic systems, such as tartaric acid-
modified Ni and cinchona alkaloid-modified Pt catalysts.3–5
The most efficient modified catalysts for the enantioselective
hydrogenation of prochiral a,b-unsaturated carboxylic acids
are supported Pd catalysts in presence of cinchona alka-
loids.3,6–8 However, the reactions were found to be highly
substrate sensitive. Aliphatic unsaturated acids in presence of
a catalytic amount of cinchona alkaloids were hydrogenated in
up to B60% enantioselectivity,9,10 while the hydrogenation of
substrates bearing aromatic rings in a and b positions resulted
in higher, up to 92%, enantiomeric excesses (ee).11 The sup-
ported Pd-cinchona alkaloid catalytic system was also found to
be efficient in the enantioselective partial hydrogenation of
several 2-pyrone derivatives lacking acidic groups.12 However,
It was reasonable to expect that the above compounds would
be saturated faster than the aromatic and semi-aromatic oxygen-
containing heterocycles hydrogenated thus far using the Pd-
cinchona alkaloid catalytic system. Thus, the transformation of
the modifier during the reaction wouldn’t alter the optical purity
of the saturated product, as observed in the reaction of 2-pyrone
and furan derivatives.12,13 Surprisingly, under atmospheric H2
pressure and using methanol as the solvent, 1 was not hydro-
genated, even in the absence of any chiral modifier, and only 2
displayed the anticipated behaviour. This latter compound was
completely transformed in 5 min, yielding exclusively saturated
product 3, as presented in Scheme 2.
The hydrogenation of 2, even in presence of chiral modifier
cinchonidine (CD), took only 25 min, as shown in Table 1,
a Research Group of Stereochemistry of the Hungarian Academy of
´
´
Sciences, Dom ter 8, H-6720 Szeged, Hungary.
E-mail: szollosi@chem.u-szeged.hu; Fax: +36 62 544 200;
Tel: +36 62 544 514
´ ´
Department of Organic Chemistry, University of Szeged, Dom ter 8,
H-6720 Szeged, Hungary
b
Scheme 1 The preparation of 1 and 2.
ꢀc
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in lower ee values. In acetonitrile (AcCN), the enantioselec-
tivity approached the value obtained in MeOH in presence of
the BA additive (Table 1, entry 12). However, it is known that
AcCN itself is hydrogenated to give amines over supported Pd
catalysts and, opposite to the hydrogenation of 4-hydroxy-6-
methyl-2-pyrone,12 the presence of these products increased
the enantioselectivity of the hydrogenation of 2.
The ee was enhanced with increasing amounts of CD, as
presented in Table 2, entries 1–3. However, even using the
highest CD amount (10 mol%), the decrease in the amount of
BA to 0.5 equiv. with respect to 2 diminished the ee and
increased to more than double the Ri (Table 2, entry 4). Thus,
the addition of one equiv. of BA is essential so as to obtain
good enantiodifferentiation. Similar behavior concerning both
the effect of the BA amount on the ee and the extent of the ee
increase was observed in the hydrogenation of itaconic acid.10
Accordingly, similar to itaconic acid, 2 is transformed in the
liquid phase to a benzylammonium salt.10b The rate enhance-
ment of the stereoselective reaction in presence of BA may be
explained by the higher exchange rate of the adsorbed cinch-
onidinium salt of 3 and the dissolved benzylammonium salt of
2, compared to the exchange of the free unsaturated acid or
the cinchonidinium salt of 2 from solution. The drop in rate of
the racemic hydrogenation might be due to the hindered
adsorption or desorption of the benzylammonium salt of 2
or 3, compared with the acids.
Scheme 2 Hydrogenation of dihydropyran-3-carboxylic acids. Reac-
tion conditions: 50 mg Pd/Al2O3, 3 ml methanol, 0.78 mmol acid,
295 K.
entry 2. However, under these reaction conditions, the ee
obtained was rather low; (+)-3 being formed in excess, as
determined by measuring the optical rotation of the isolated,
crude, saturated acid after removing the modifier.
The addition of an achiral base additive to the reaction
slurry may lead to a significant increase in the enantioselec-
tivity during the hydrogenation of prochiral a,b-unsaturated
carboxylic acids over cinchona-modified Pd catalysts.7–11 The
origin of the ee enhancement seems to be different for acids of
different structures, expressively illustrated by the opposite
effect of the amine on the initial rate of the hydrogenation of
acids bearing either aromatic7,11 or aliphatic8–10 substituents.
The addition of benzylamine (BA) to the reaction mixture
resulted in a surprisingly high increase in the ee of the
hydrogenation of 2, and a decrease in the initial H2 uptake
rate (Ri) compared with the reaction in the absence of BA
(Table 1, entries 2 and 3). It must be noted that the rate of
production of both enantiomers decreased by the adding of
BA. However, based on the ee and Ri values, one can
approximate a more than threefold decrease in the racemic
reaction, while the asymmetric hydrogenation was accelerated
by a factor of 1.3. Thus, in the hydrogenation of 2, the ee
increase in the presence of BA was mainly due to a significant
deceleration of the racemic hydrogenation.
Similar to the hydrogenation of aliphatic a,b-unsaturated
carboxylic acids,6e,10 an increase in the H2 pressure resulted in
further ee enhancement (Table 2, entries 3, 5 and 6). The
pressure dependence of the ee resembled the behavior ob-
served in the hydrogenation of itaconic acid.10 The ee reached
a maxima at 5 MPa and then declined following further
pressure increase (10 MPa). The high (89%) ee obtained under
5 MPa H2 pressure, to the best of our knowledge, is the largest
value reported to date in the enantioselective hydrogenation of
an a,b-unsaturated carboxylic acid lacking aromatic substitu-
ents over a cinchona-modified Pd catalyst. The results ob-
tained using three other natural cinchona alkaloids are
presented in Table 2, entries 8–11. When using cinchonine
(CN), which has the opposite configuration at the C8 and C9
The optical purity was further enhanced, up to 74%, by
decreasing the reaction temperature (Table 1, entry 4). Chan-
ging the solvent from MeOH, even to another alcohol, resulted
Table 1 The enantioselective hydrogenation of 2 over a supported Pd catalyst modified by CDa
Entry
Solvent
Additive
Time/min
X (%)b
Ri/mmol hꢁ1 gꢁ1c
ee (%)
1d
2
MeOH
MeOH
MeOH
MeOH
MeOH
iPrOH
Toluene
THF
—
—
5
100
100
99
100
100
100
40
100
100
95
100
100
100
668.6
74.2
42.0
10.7
7.3
—
26
59
74
68
33
43
45
43
39
30
53
34
25
40
90
90
30
120
60
30
80
70
50
40
3
BA
BA
BA
BA
BA
BA
BA
BA
BA
—
4e
5e,f
6
48.0
3.1
7
8
9
10
11
12
13
28.8
58.3
18.8
20.3
29.6
39.7
1,4-DOg
DMFg
H2O
AcCN
AcOH
—
a
Reaction conditions: 50 mg 5% Pd/Al2O3, 3 ml solvent, 0.039 mmol CD, 0.78 mmol 2, 0.78 mmol benzylamine (BA) (when used), pH
b
=
0.1 MPa, 1000 rpm stirring, 295 K. X = conversion reached in the given time. Ri = initial rate calculated from the initial H2 uptake. Racemic
2
c
d
e
f
g
reaction in absence of chiral modifier. Reaction temperature = 273 K. Hydrogenation over 50 mg 5% Pd/TiO2. 1,4-Dioxane (1,4-DO) and
DMF containing 2.5 vol% water.
ꢀc
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a
Table 2 The effect of the reaction conditions and modifier structure on the enantioselective hydrogenation of 2 over cinchona-modified Pd/Al2O3
Entry
Modifier (amount (mol%))b
pH /MPa
Time/min
X (%)c
Ri/mmol hꢁ1 gꢁ1d
ee (%)e
2
1
2
CD (1)
CD (5)
0.1
0.1
0.1
0.1
1
70
90
100
70
60
60
60
120
60
100
100
96
94
83
100
100
95
100
100
97
12.7
10.7
8.4
19.0
19.2
n.d.
n.d.
7.8
65
74
79
42
82
89
70
32g
56g
19
6g
3
CD (10)
CD (10)
CD (10)
CD (10)
CD (10)
CN (10)
CN (10)
QN (10)
QD (10)
4f
5
6
7
8
9
10
11
5
10
0.1
5
0.1
0.1
n.d.
10.9
12.9
85
89
a
b
Reaction conditions: 50 mg catalyst, 3 ml MeOH, 0.78 mmol 2, 0.78 mmol BA, stirring 1000 rpm, 273 K. Modifier amount in relation to 2. CD
c
= cinchonidine, CN = cinchonine, QN = quinine and QD = quinidine. X = conversion reached in the given time. Initial rate calculated from
d
e
f
g
the initial H2 uptake. Unless otherwise noted, (+)-3 was formed in excess. 0.39 mmol of BA was used. (ꢁ)-3 was formed in excess.
chiral centers compared to CD, (ꢁ)-3 was formed in a lower ee
under both 0.1 and 5 MPa H2 pressure. The C60-methoxy
substituent of both quinine (QN) and quinidine (QD) de-
creased even more the enantioselectivity of the hydrogenation,
the latter giving the (ꢁ)-3 enantiomer only in a very low ee.
The results obtained using these cinchona alkaloids are in line
with those obtained in the hydrogenation of aliphatic a,b-
unsaturated carboxylic acids, such as trans-2-methyl-2-pente-
noic acid and itaconic acid.9,10
used in previously published bioactivity studies were prepared
in low, only up to 15%, yields by resolution of the racemic
mixture using an equivalent amount of QN.14 The cyclic
structure, the free carboxylic acid group and the position of
the ring oxygen atom relative to the prochiral CQC group of
the acid all have a crucial importance in determining the effect
of the achiral amine additive, and consequently on the attained
ee value. This is well illustrated by the lower ee obtained in the
hydrogenation of furan and indene carboxylic acids,13,17 and
in the effect of amine additives on the hydrogenation of
4-hydroxy-2-pyrone derivatives.12b Furthermore, in the hydro-
genation of aliphatic a-methyl-a,b-unsaturated carboxylic
acids, the ee was increased much less as result of the addition
of the same achiral amine, while ee enhancement to similar
extent was obtained in the hydrogenation of itaconic acid.10
Our present study shows that this effect is of kinetic origin.
Detailed kinetic and spectroscopic studies to gain further
insight into the origin of the enantiodifferentiation in this
catalytic system will be the subject of a future investigation.
This novel application of a cinchona-modified Pd catalyst
open up new perspectives, broadening the scope of this chiral
heterogeneous catalytic system.
In conclusion, we have reported the first enantioselective
hydrogenation of a dihydropyran carboxylic acid over a
cinchona alkaloid-modified, supported Pd catalyst. The use
of CD as a chiral modifier and BA as an achiral additive
afforded unexpectedly high enantioselectivities, up to 89%,
unprecedented in the hydrogenation of aliphatic or cycloali-
phatic unsaturated carboxylic acids in this heterogeneous
catalytic system. The optical purity of the product approached
the high values usually obtained in enantioselective hydroge-
nations of a,b-unsaturated carboxylic acids using chiral metal
complexes,2 though, to our knowledge, the asymmetric hydro-
genation of 2 using a homogeneous catalyst has not yet been
attempted. The reaction is applicable to the easy preparation
of an optically enriched cockroach attractant, as shown in
Scheme 3. We note that the optically pure enantiomers of 3
Financial support by the Hungarian National Science
Foundation (OTKA Grants T 048764 and K 72065) is highly
Scheme 3 The preparation of optically-enriched methyl (+)-tetrahydropyran-carboxylic acid by an enantioselective heterogeneous catalytic
hydrogenation.
ꢀc
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appreciated. Gy. Sz. is grateful for the financial support of
HAS through a Bolyai Janos Research grant.
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Cinchona alkaloids CD ( Z 98%), CN ( Z 98%), QN
( Z 98%), QD ( Z 98%) and BA ( Z 99.5%) are commercial
products (Fluka), and were used as received. H2 gas (99.999%)
was purchased from Linde AG. The catalyst used in most of
the studies was commercial 5% Pd/Al2O3 (Engelhard 40692),
having a 0.19–0.21 Pd dispersion, 185–200 m2 gꢁ1 BET surface
area and a 5.8 nm average Pd particle size.6e,18 The 5% Pd/
TiO2 (TiO2, Degussa P-25, 55 m2 gꢁ1 surface area) was
prepared by deposition–precipitation according to a pre-
viously described procedure.8 Commercial, high purity sol-
vents and reagents (Aldrich and Fluka products) were used
without purification.
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All products were identified by GC-MS (Agilent Technolo-
1
gies 6890N GC-5973 MSD), and H- and 13C-NMR (Bruker
AVANCE DRX-500 spectrometer) analysis. Their purity was
checked by gas chromatography (HP-5890 II GC-FID).
Hydrogenations were carried out using a glass hydrogena-
tion apparatus or a stainless steel autoclave equipped with a
glass liner and an automatic pressure recorder. In a typical
run, the catalyst was pre-treated by stirring (1000 rpm) in the
given solvent for 0.5 h under H2 at 297 K, followed by the
addition of specified amounts of the modifier, BA (when used)
and substrate. The reactor was flushed and filled with H2 to
the specified pressure, the temperature set to the chosen value
and the reaction started by stirring the slurry. After H2 uptake
ceased, the catalyst was filtered, and the solution analyzed
before and after work-up. The solvent was evaporated, the
product was then washed with 10% aqueous HCl to remove
the modifier and BA additive, and extracted with tert-butyl
methyl ether. Repeat experiments gave conversions and ee
values within ꢂ2%.
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´
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´
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+
+
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+
´
and M. Barto
+
´
k, Adv. Synth. Catal.,
2007, 349, 405; (b) Gy. Szollosi, T. Varga, K. Felfoldi, Sz. Cserenyi
´
The initial rates were calculated from the H2 uptake up to
20 ꢂ 2% conversion. The conversion and ee were determined
by GC analysis using a Cyclosil-B (30 m ꢃ 0.2 mm, J&W
Scientific Inc.) chiral capillary column. The ee was calculated
using the formula:
¨
+ +
´
k, Catal. Commun., 2008, 9, 421; (c) Gy. Szollosi,
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´
k, Appl. Catal., A,
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´
k, J. Catal., 2005, 231, 480; (b) Gy. Szollosi,
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´
´
k, Appl. Catal., A, 2007, 319, 193;
+ +
´
n, Gy. Szollosi, F. Fulop and M. Bartok, Appl.
¨
¨
´
where [(+)-3] and [(ꢁ)-3] are the concentrations of the dex-
trorotatory and levorotatory enantiomers of the product,
respectively. The sense of rotation of the excess enantiomer
was determined by measuring the optical rotation (Polamat A
polarimeter) of the crude product obtained after removal of
the modifier and BA.
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008
1358 | New J. Chem., 2008, 32, 1354–1358