Palladium nanocarbon catalysts (Pd/CNT) as potential enantioselective heterogeneous systems. the behavior in the hydrogenation and hydroarylation of unsaturated substrates

Article abstract of DOI:10.1007/s11172-015-0845-2

Hydroxy-CNT were modified with optically active α-amino acids. The resulting chiral derivatives were directly palladated with Pd₂dba₃ and used to catalyze the hydrogenation of α-acetamidostyrene and α-phenylcinnamic acid as well as t

Full text of DOI:10.1007/s11172-015-0845-2

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Russian Chemical Bulletin, International Edition, Vol. 64, No. 1, pp. 216—218, January, 2015
Brief Communications
Palladium nanocarbon catalysts (Pd/CNT)
as potential enantioselective heterogeneous systems.
The behavior in the hydrogenation and hydroarylation of unsaturated substrates
N. V. Abramova,a O. V. Turova, Yu. A. Davidovich, and V. I. Sokolov
b
a
a
aA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
8 ul. Vavilova, 119991 Moscow, Russian Federation.
2
Fax: +7 (499) 135 5085. Eꢀmail: stemos@ineos.ac.ru
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
b
4
7 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5328. Eꢀmail: turova@ioc.ac.ru
HydroxyꢀCNT were modified with optically active ꢀamino acids. The resulting chiral
derivatives were directly palladated with Pd dba3 and used to catalyze the hydrogenation of
2
ꢀacetamidostyrene and ꢀphenylcinnamic acid as well as the hydroarylation of norbornene
with iodoarenes. All the catalytic reactions afforded products containing a new chiral carbon
center in high yields, though as racemates. Apparently, asymmetric induction is precluded
because the palladium sites on the CNT surface are spatially apart from the chiral carbon
centers in amino acid groups.
Key words: carbon nanotubes, palladium nanoclusters, hydroarylation, hydrogenation, catalysis.
Enantioselective synthesis and catalysis are among the
synthesized. Many of these ligands show directional efꢀ
fects in the sense that they provide the desired stereochemꢀ
ical outcome only for a particular group of compounds. As
for heterogeneous catalysts, Oosthuizen and Nyamori in
their recent review concerned with carbonꢀsupported platꢀ
inum metals made no reference to chiral palladium nanoꢀ
carbon catalysts (Pd/CNT).
Novel nanocarbon materials (fullerenes, nanotubes,
etc.) make it possible to prepare catalysts with an active
metal as nanoclusters immobilized on a carbon material.
This material acts as both a catalyst carrier and a ligand.
most important challenges in organic chemistry, first of all
because chiral molecules are essential constituents of most
drugs and different enantiomers or diastereomers produce
substantially different effects on the human body. For this
reason, the synthesis of pure enantiomers is of primary
importance for medicinal chemistry and pharmaceutics.
The most intriguing results in this area have been obꢀ
tained in the last few decades when dealing with homoꢀ
geneous catalysts based on transition metal complexes.
Novel ligands for these catalysts have been proposed and
1
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 0216—0218, January, 2015.
066ꢀ5285/15/6401ꢀ0216 © 2015 Springer Science+Business Media, Inc.
1

Potential enantioselective Pd/CNT as catalysts
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 1, January, 2015
217
2
Earlier, we have proposed a new straightforward method
Here we presented the results of the use of catalysts
2a—d in those reactions that allow asymmetric induction
through the formation of a new chiral center on the carꢀ
bon atom. Such reactions include hydrogenation of proꢀ
chiral substituted olefins (Scheme 2) and hydroarylation
of norbornene (Scheme 3). The Suzuki and Heck reacꢀ
tions were not considered because of their poor stereoꢀ
chemical scope.
for the synthesis of a catalyst in a neutral medium from the
zeroꢀvalent palladium complex Pd (dba) . Originally, this
method has been used to deposit palladium on carbon
nanotubes (CNT). The resulting catalyst is highly effecꢀ
tive in crossꢀcoupling (Suzuki, Heck, and Sonogashira)
2
3
3—7
reactions. Later,
ities of palladium catalysts based on fullerene C , nanoꢀ
we have examined the catalytic activꢀ
6
0
diamond, graphene, and a purely superparamagnetic inꢀ
organic complex made up of Fe O and a double salt layer
3
4
Scheme 2
and studied the hydrogenation of unsaturated hydroꢀ
carbons. However, all those catalysts were a priori achiral.
To obtain potentially enantioselective catalysts, we perꢀ
formed chiral modification of singleꢀwalled hydroxyꢀCNT.
8
Earlier, we have described in detail the starting material
1
2
1
2
and its acylation with optically pure natural ꢀamino acꢀ
3: R = NHAc, R = R = H; 4: R = H, R = Ph, R = COOH
ids. The reaction sequence is shown in Scheme 1.
Reagents and conditions: i. catalyst, p(H ) = 19 atm, 60 C, 5 h.
2
Scheme 1
Scheme 3
CNT
Reagents and conditions: i. catalyst, HCOOH, Et N, DMSO.
3
Derivatives of Lꢀvaline, Lꢀmethionine, and Lꢀproline
were used as catalysts for the hydrogenation of such
prochiral substituted olefins as ꢀacetamidostyrene (3) and
CNT
ꢀphenylcinnamic acid (4). Conversions of these substrates
varied appreciably, probably because of the inhomogeneꢀ
ity of the catalyst. The best results obtained are summarized
in Table 1. One can see that the conversion of substrate 4
in the hydrogenation is lower than that of substrate 3.
LꢀProline and Lꢀphenylalanine derivatives were emꢀ
ployed for the hydroarylation of norbornene (Scheme 3).
The results obtained are given in Table 2
CNT
R = CH(Me)2 (a), CH CH SMe (b), (CH ) (proline) (c), CH Ph (d)
2
2
2 3
2
Table 1. Hydrogenation of ꢀacetamidostyrene and ꢀphenylcinnamic acid on the palladium catalysts prepared from
hydroxyꢀCNT acylated with enantiomeric ꢀamino acidsa
Entry
Catalyst
Substrate (S)
S/Catb
mol/gꢀatom)
MeOH
/mL
Conversion
(%)
(
1
2
3
4
5
6
3% Pd—CNT—Lꢀvalinec
3% Pd—CNT—Lꢀvaline
3% Pd—CNT—Lꢀvalinec
6% Pd—CNT—Lꢀmethionine
6.7% Pd—CNT—Lꢀproline
6.7% Pd—CNT—Lꢀproline
PhC(NHAc)=CH2
PhC(NHAc)=CH2
PhCH=C(Ph)COOH
PhC(NHAc)=CH2
PhC(NHAc)=CH2
PhCH=C(Ph)COOH
1000
1200
1200
1200
1200
1200
3
2
2
2
2
2
192
100
171
165
100
168
c
d
d
d
a
The hydrogenation conditions: p(H ) = 19 atm, 60 C, 5 h.
The substrate/catalyst ratio.
The amount of the catalyst is 4 mg.
The amount of the catalyst is 2 mg.
2
b
c
d

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Russ.Chem.Bull., Int.Ed., Vol. 64, No. 1, January, 2015
Abramova et al.
Table 2. Hydroarylation of norbornene on the palladium cataꢀ
lysts prepared from hydroxyꢀCNT acylated with enantiomeric
(10 mg) was added. The reaction mixture was heated with stirꢀ
ring for 7—10 h and diluted with water. Organic products were
extracted with diethyl ether. The extracts were dried with
Na SO , concentrated, and chromatographed on SiO with hexꢀ

ꢀamino acids
2
4
2
ane—AcOEt (1 : 1) as an eluent. The compounds obtained were
identified by comparing their H NMR spectra with the literaꢀ
Amino acid
residue
ArI
t/h
T/C Conversion (%)
1
ture data. The conversion was determined from the signal intenꢀ
sity ratios for the protons of the starting iodoarene and those of
LꢀProline
LꢀProline
LꢀProline
LꢀPhenylalanine C H MeI
LꢀPhenylalanine
C H MeI
17
17
17
17
10
65*
75*
169
197
110
165
100
6
4
1
C H MeI
the hydroarylation product in the H NMR spectrum.
6
4
C H MeI
120*
65*
6
4
We are grateful to M. G. Vinogradov for his consultaꢀ
tions and careful consideration of this study.
6
4
C H I
75*
6
5
*
The catalyst decomposes at 120 C.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 14ꢀ03ꢀ00467)
and the Division of Chemistry and Materials Sciences of
the Russian Academy of Sciences (Program OKhꢀ1).
It is worth noting that all reaction products are raceꢀ
mates. Therefore, the catalysts under discussion show no
enantioselectivity. Apparently, this is because palladium
clusters are bound to strained double bonds outside the
2
References
zone of catalytic reactions. Their arrangement along CNT
is random, regardless of the acylating amino acid as
a chiral inductor. Clearly, the steric relations between the
Pd atoms and the amino acid during the formation of the
catalyst cannot be controlled. Now we are studying an
alternative approach to the synthesis of a catalyst containꢀ
ing an enantiomeric acid directly bound to a palladium
nanocluster.
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acids. For the acylation of hydroxyꢀCNT as well as for the synꢀ
thesis and characteristics of compounds 1a—c, see Ref. 8. Comꢀ
pound 1d was obtained as described earlier.8 Found (%): C,
8
3.35; H, 1.29; N, 0.84.
2
012, 22, 237.
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V. I. Sokolov, B. ZümreogluꢀKaran, Inorg. Chem. Commun.,
8
characterized earlier. For the details of the synthesis of comꢀ
pound 2d, see Ref. 8. Found (%): C, 80.09; H, 1.07; N, 0.72;
Pd, 5.7.
2
013, 27, 64.
. V. I. Sokolov, Yu. A. Davidovich, N. V. Abramova, Russ.
Chem. Bull. (Int. Ed.), 2013, 62, 2088 [Izv. Akad. Nauk, Ser.
Khim., 2013, 2088].
Hydrogenation of unsaturated substrates was carried out as
described earlier.5
Reactions of norbornene with iodoarenes (general procedure).
Norbornene (3 mmol), iodoarene (1 mmol), formic acid (3 mmol),
and triethylamine (3 mmol) were added with stirring to DMSO
Received March 5, 2014;
(
5 mL). The mixture was stirred for 5 min, and then the catalyst
in revised form September 10, 2014