Communication
DOI: 10.1002/bkcs.10105
BULLETIN OF THE
B. S. Cho et al.
KOREAN CHEMICAL SOCIETY
Palladium-catalyzed Aerobic Oxidation Reaction of 1-Heptanol in Water
using Water-soluble N-Heterocyclic Carbene Ligands
‡,
Beom Shin Cho,† Il Gu Jung,† Young Keun Chung,†, and Jin Wook Han
*
*
†Intelligent Textile System Research Center, Department of Chemistry, College of Natural Sciences,
Seoul National University, Seoul 151-747, Korea. *E-mail: ykchung@snu.ac.kr
‡Department of Chemistry, College of Natural Sciences, Institute of Nanoscience and Technology, Hanyang
University, Seoul 133-791, Korea. *E-mail: jwhan@hanyang.ac.kr
Received November 5, 2014, Accepted November 19, 2014, Published online January 19, 2015
Keywords: Palladium, N-Heterocyclic carbene, Water-soluble, Alcohol oxidation, 1-Heptanol
Recent interest in the concept of “green chemistry” has led
many researchers to study and develop catalytic reactions
under aqueous conditions.1 The use of a water-soluble catalyst
is required to carry out catalytic reactions in water effectively.2
We have reported a methodology to form tetrasubstituted
ammonium salts from tertiary amines by a Ni nanoparticle-
catalyzed C Cl bond activation3 and found recently that
N-heterocyclic carbenes (NHCs) bearing a quaternary ammo-
nium moiety, which acts as a solubilizing agent in water, could
be easily prepared by the method. Herein, we wish to commu-
nicate our preliminary results on the syntheses of 1,3-
dialkylimidazolium compounds containing a quaternary
ammonium moiety and a catalytic oxidation reaction of alco-
hols to acids by their water-soluble NHC–palladium catalysts
in water.
generate the N-heterocyclic carbene (b), and the NHC-
palladium complex (c). The chemical shift value of the C-2
position of the imidazolium salt, the carbene carbon atom of
the NHC, and the carbene carbon atom with Pd complex were
observed at 136.3, 225.4, and 167.4 ppm, respectively. The
upfield shift of the carbene carbon indicated an NHC–
palladium complex was generated in situ.5
Palladium-catalyzed aerobic oxidation reaction of 1-
heptanol to 1-heptanoic acid using water-soluble NHC ligands
from 4 in water was investigated as shown in Table 1. The mix-
ture of 4, a base, and Pd(OAc)2 in water became homogeneous
within 30 min. The catalytic reaction was highly dependent on
the reaction conditions, including reaction temperature, base,
and NHC ligands. A reaction with ligand 4co and K2CO3 at
100 ꢀCgavethebestresult(entry6).Withoutabase,noreaction
was observedbecause palladium didnotforma compoundwith
4ao (entry 1). The optimized reaction temperature was found to
be 100 ꢀC; when the reaction temperature was elevated to 120 ꢀ
C, the color of the reaction mixture was rapidly darkened and
the yield was dropped dramatically, presumably due to decom-
position of the catalysts (entry 2).
Water-soluble 1,3-dialkylimidazolium compounds4 (4)
containing a quaternary ammonium moiety were prepared
from α,ω-dichloroalkanes (1) in excellent yields (Scheme 1).
The Ni nanoparticle-catalyzed coupling reaction of 1 and
triethylamine gave the corresponding quarternary ammonium
salts 2 in high yields. Substitution reactions between the chlor-
oalkylated quarternary ammonium salts (2) and 1-alkyl-1H-
imidazoles (3) afforded the desired imidazolium compounds
(4) in excellent yields. All compounds were oily except 4ao.
Structure of 4ao was confirmed by a single crystal X-ray spec-
troscopy (see the Supporting Information).
At 80 ꢀC, the reaction did not proceed at all only to
recover the reactant (entry 3). Ligands derived from 4 bp,
4bq, 4cp, and 4cq gave inferior results to ligands from 4bo
and 4co under the similar reaction conditions (entries 5–10).
At present stage, the reason is not clear but we suppose that
the long alkyl chain, especially for 4 having an octyl (m = 7)
or a dodecyl (m = 11) group, might obstruct the interaction
between the catalyst and the substrate. Actually, it was found
in reactions with 4cp and 4cq that the obscure borderline
between an aqueous and an organic layer led to a difficulty
in separation, eventually giving only poor isolated yields. In
conclusion, we have demonstrated that a design of water-
soluble imidazolium salts (4) allowed the formation of
water-solublepalladium complexes that could be usedas acat-
alyst in the oxidation of 1-heptanol to 1-heptanoic acid in high
yields in water. We are currently designing new imidazolium
salts and acquiring a deeper understanding of the roles of imi-
dazolium salts, including vesicle formation under the reaction
conditions.6
Formation of NHC–palladium complexes from the imida-
zolium 4 and palladium (II) acetate in water was proved by
a
13C-NMR study (Figure 1). Figure 1 shows the spectra of
a dialkylimidazolium (4ao) (a), after a base treatment to
[Ni]
Cl
Cl
N
N
m
N
N
NEt3 Cl
n
+ NEt3
+
Cl
n
NEt3
2a: n = 2 (90%)
2b: n = 3 (85%)
2c: n = 5 (82%)
3o: m = 0
3p: m = 7
3q: m = 11
4ao: m = 0, n = 2 (92%)
4bo: m = 0, n = 3 (92%)
4co: m = 0, n = 5 (95%)
4bp: m = 7, n = 3 (95%)
4bq: m = 7, n = 5 (95%)
4cp: m = 11, n = 3 (95%)
4cq: m = 11, n = 5 (95%)
1a: n = 2
1b: n = 3
1c: n = 4
Scheme 1. Preparation of 1,3-dialkylimidazolium compounds (4)
containing a quaternary ammonium moiety.
Bull. Korean Chem. Soc. 2015, Vol. 36, 451–452
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
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