An Air/water-stable tridentate N-heterocyclic carbene-palladium(II) Complex: Catalytic C-H activation of hydrocarbons via hydrogen/deuterium exchange process in deuterium oxide

Article abstract of DOI:10.1002/adsc.200800698

While developing novel catalysts for carbon-carbon or carbon-heteroatom coupling (nitrogen, oxygen, or fluorine), we were able to introduce tridentate N-heterocyclic carbene (NHC)-amidate-alkoxide palladium(II) complexes. In aqueous solution, these NHC-Pd(II) complexes showed high ability for C-H activation of various hydrocarbons (cyclohexane, cyclopentane, dimethyl ether, tetrahydrofuran, acetone, and toluene) under mild conditions.

Full text of DOI:10.1002/adsc.200800698

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DOI: 10.1002/adsc.200800698
An Air/Water-Stable Tridentate N-Heterocyclic Carbene-
À
Palladium(II) Complex: Catalytic C H Activation of
Hydrocarbons via Hydrogen/Deuterium Exchange Process in
Deuterium Oxide
Joo Ho Lee,a Kyung Soo Yoo,a Chan Pil Park,a Janet M. Olsen,a
Satoshi Sakaguchi,b G. K. Surya Prakash,a Thomas Mathew,a
and Kyung Woon Junga,*
a
Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles,
California 90089, USA
Fax : (+1)-213-821-4096; e-mail: kwjung@usc.edu
Department of Applied Chemistry, Kansai University, Osaka 564-8680, Japan
b
Received: November 12, 2008; Revised: February 5, 2009; Published online: March 4, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800698.
Abstract: While developing novel catalysts for
carbon-carbon or carbon-heteroatom coupling (ni-
trogen, oxygen, or fluorine), we were able to intro-
duce tridentate N-heterocyclic carbene (NHC)-ami-
date-alkoxide palladium(II) complexes. In aqueous
solution, these NHC-Pd(II) complexes showed high
under mild conditions have been discovered.[7] In
spite of these advances, practical catalysts for the C
À
H bond functionalization remain elusive, due to the
À
requirement of high energy to break C H bond and
subsequent vulnerability of the metal-carbon bond.
À
Additionally, the C H bond activation is often inhib-
À
ability for C H activation of various hydrocarbons
ited by water or by the product eliminated from the
(cyclohexane, cyclopentane, dimethyl ether, tetrahy-
drofuran, acetone, and toluene) under mild condi-
tions.
metal complex during the reaction.[8]
N-Heterocyclic carbene (NHC) ligands, which are
known to enhance s-donor coordination, have shown
À
improved behaviour toward C H bond activation
Keywords: C H activation; H/D exchange; inter-
during the past several years.[9] A number of organo-
metallic species at various oxidation states (Pd, Pt,
Ru, and Ir) are stabilized by the use of NHC ligands,
À
molecular reactions; N-heterocyclic carbene (NHC)
ligands; palladium complexes
increasing the capability to obtain efficient catalysts
[10]
À
for C H activation. However, most examples refer
to intramolecular processes, while intermolecular or
catalytic examples remain rare. With these challenges
À
Carbon-carbon or carbon-heteroatom cross-coupling in mind, we report herein the efficient C H bond ac-
reactions catalyzed by transition metal catalysts have tivation of hydrocarbons in aqueous solution contain-
been widely investigated and established because of ing catalytic amounts of tridentate NHC ligand-Pd
À
their significance in organic synthesis. In particular, complex via H/D exchange as a probe for direct C H
homogeneous Pd catalysis has been well studied in functionalization. Such reactions have been studied in
the most powerful and versatile synthetic processes heterogeneous Pd catalytic systems but are rarely ob-
such as Heck,[1] Suzuki,[2] Stille,[3] Sonogashira,[4] Ne- served in homogeneous Pd systems.[11]
gishi,[5] and Buchwald–Hartwig reactions.[6] However,
Recently, we have succeeded in preparing the air-
these reactions have required alkenyl/aryl halide sub- stable tridentate NHC-amidate-alkoxide ligand/palla-
strates. Alternatively, the more desirable direct func- dium complex 1 from an amino alcohol.[12] We found
À
tionalization of hydrocarbons via C H bond activa- that H/D exchange on benzene using D2O as both the
tion has still remained a challenge in cross-coupling solvent and deuterium source, occurred via C H
À
reactions.
bond activation in the presence of Pd complex 1 and
In recent years, a substantial number of metal com- silver tetrafluoroborate. As shown in Eq. (1), the H/D
À
plexes that are able to selectively activate C H bonds exchange reaction was demonstrated with benzene
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563

Joo Ho Lee et al.
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depending on the temperature of the aqueous solu-
tion.[14] The higher efficiency of the H/D exchange
process at 1008C compared to 558C is attributable to
the conversion of the catalyst into active mode 3
from complex 2 at the elevated temperature.
À
(20 mL), palladium complex 1 (5 mol%) and AgBF4 in
To further investigate the C H activation of this
deuterium oxide (700 mL) for 22 h at 558C and 1008C. class of Pd complexes, we extended the structural
The efficiency of the H/D exchange giving deuterated motifs of palladium to various tridentate ligands. To
isotopomers showed significant enhancement when circumvent the dimerization of monomeric species in
the temperature was increased from 558C to 1008C. aqueous solution at low temperatures, we employed a
While evaluating H/D exchange with palladium com- methyl ether (7a) or benzyl ether (7b) group instead
plex 1 and AgBF4, we found that a dimeric structure of the previously employed hydroxy moiety as
1
2 was present in the solution as indicated by H NMR oxygen-site of the ligands (Scheme 1). Compounds 4a
spectroscopy.[13] At an elevated temperature, however, and 4b were prepared from ethanolamine and valinol,
the concentration of monomeric species 3 was ob- respectively.[15] Bromoacetylation of the amino ethers
served to be higher than complex 2. Consequently, we was followed by amide formation and N-alkylation
À
confirmed that the H/D displacement via C H bond with benzimidazole to yield 5a and 5b. The benzimi-
activation worked well only in the presence of the dazole salts 6a and 6b were obtained by allowing 5a
monomeric species 3. As shown in Eq. (2), the NHC and 5b to react with CH3I in THF. For the coordina-
tion of the NHCs to palladium, metal exchange was
carried out through a silver NHC complex.[16] N-
Methyl iodide salts 6a and 6b were reacted with
Ag2O to give silver NHC complexes, and subsequent
treatment of the silver compounds with PdCl2
AHCTNUGTERN(GNNU CH3CN)2 in CH3CN provided the desired complexes
7a and 7b in 79% and 72% yields, respectively. The
structures of Pd/ligand complexes 7a and 7b were fur-
ther confirmed by the molecular ion peaks (ESI-MS)
at m/z=388.0029 [M+] (calcd.: 388.0044) and m/z=
506.0813 [M+] (calcd.: 506.0827).
À À
In addition, we designed an O C N tridentate
ligand system, an example of which is the alkoxy-
NHC-amidate
ligand/palladium
complex
11
(Scheme 2). Amide compound 9 was easily derived
from aniline and bromoacetyl bromide. By means of
ligand/palladium complex 1 was converted to the pal- N-alkylation with benzimidazole and compound 9, the
ladium complex 2 by treating with aqueous AgBF4 benzimidazole-amide adduct was obtained in DMF
À
for the C H bond activation, and subsequently the solution, which after second N-alkylation with iodo-
monomer/dimer equilibrium process (2Q3) took place ethanol gave the iodine salt 10 in 62% yield for two
Scheme 1.
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An Air/Water-Stable Tridentate N-Heterocyclic Carbene-Palladium(II) Complex
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Scheme 2.
Table 1. H/D exchange of benzene in D2O by 8a, 8b, and
12.[a]
steps. Subsequently, construction of alkoxy-NHC-ami-
date ligand/palladium complex 11 was successfully ac-
complished by treatment with half an equivalent of
Ag2O and one equivalent of PdCl2ACTHUNGRTNEUNG(CH3CN)2 sequen-
tially in CH3CN solution at room temperature.
First, to optimize the H/D exchange reaction condi-
tions, various reaction temperatures and catalyst load-
ings were screened with Pd complexes 8a, 8b and 12
for benzene in D2O. The H/D exchange can be con-
veniently monitored by 1H NMR spectroscopy
through observation of the decreasing proton signals
of benzene in the aqueous phase relative to the poly-
Entry Catalyst
(mol%)
H/D conversions [%][b]
558C
(6 h)
1008C
558C
1008C
(6 h)
(22 h)
(22 h)
1
2
3
4
5
6
8a (0.4)
8a (0.9)
8a (1.7)
8a (6.9)
8b (1.7)
12 (1.7)
48
81
90
89
69
23
–
–
–
–
–
–
89
89
8
95
–
–
96
–
–
ACHTUNGTRENNUNG(dimethylsiloxane) as an external reference standard.
Before H/D exchange, AgBF4 was added into a
MeCN solution of Pd complexes 7a, 7b and 11 to
remove Cl anion, so that Pd complexs 8a, 8b, and 12
86
–
–
[a]
All reactions were carried out with 20 mL of benzene in
0.7 mL of D2O.
À
could be ready for the C H activation. In addition,
unreacted AgBF4 and AgCl were removed by a celite
[b]
The H/D conversions were determined by1H NMR.
1
column. In D2O solution, the MeCN H-signal shifted
from 2.06 ppm (free MeCN in D2O) to 2.16 ppm with
a broadening of the signal indicating that there was tramolecular activation to generate 8c [Eq. (3)].[17]
weak interaction between MeCN and Pd metal.
Moreover, alkoxy-NHC-amidate ligand/palladium
As shown in Table 1, we found that the reaction
temperature could be lowered to 558C without caus-
ing a significant decrease in the extent of deuterium
incorporation relative to that observed in reactions
run at 1008C (entry 3). Interestingly, we discovered
that at 558C increasing the catalyst loading beyond
1.7 mol% (entry 4) did not lead to a detectable in-
crease in deuterium incorporation. While use of less
catalyst (entries 1 and 2) led to a decrease in the
extent of H/D exchange, 1.7 mol% is apparently
enough to saturate the system and achieve optimum
conversion. In addition, longer reaction times may complex 12 gave the lowest level of deuterium incor-
not affect the catalytic cycle dramatically (entry 3). poration into benzene at 558C because of the steric
When the ligand 8b possessing a benzyl ether group hindrance of the phenyl group as well as the existence
was utilized, deuterium incorporation slowed down of a dimer/monomer equilibrium in aqueous solution
somewhat at 558C (entry 5), however deuterium in- [Eq. (4)].[14] This result wherein the efficiency of the
corporation was barely seen at an elevated tempera- H/D exchange showed significant enhancement at a
ture (1008C) due to deactivation of 8b by coordina- higher temperature is in accordance with behaviour
À
tion between palladium and benzyl aromatic C H in- of the active catalyst derived from 1 [Eq. (1)].
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Joo Ho Lee et al.
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the H/D exchange reaction with palladium NHC-
ligand complex 8a. In general, effective multiple deu-
À
terium incorporation into the alkyl C H bond was
observed with D2O as both the solvent and deuterium
source. Table 2 lists the organic substrates examined
and their corresponding extents of deuterium incorpo-
Table 2. H/D exchange of various organic substrates in D2O
by Pd catalyst 8a.[a]
Entry 8a Substrate[b] T [8C] D conversion [%][c]/TON[d]
6 h
22 h
1
2
3.86
3.86
55
100
81/464
96/552
93/536
93/536
Additionally, in an experiment designed to study the
time dependence of the deuterium displacement, a so-
lution of benzene with catalyst 8a in D2O was heated
3
4
5
6
3.86
3.86
3.86
3.86
55
100
55
72/399
96/533
2/10
76/422
97/541
2/10
1
to 558C and monitored by H NMR spectroscopy. As
shown in Figure 1(A), over a period of 22 h, the reso-
nances for the benzene (d=6.71 at 558C) disappeared
without any significant change in the signal intensities
of the other resonances. Furthermore, to unequivocally
100
44/222
52/267
7
8
9
10
11
12
13
14
3.86
3.86
15.4
3.86
3.86
15.4
3.86
3.86
55
0.5/2
5/25
1/4
9/43
100
100
55
100
100
55
27/27
12/52
26/110
55/58
83/323
95/369
46/46
15/65
52/218
63/66
87/340
97/379
À
establish C H activation with Pd complex, we also
probed deuterium-proton displacement between C6D6
and H2O at 558C and H/D incorporation was moni-
tored by 2H NMR spectroscopy. As shown in Fig-
ure 1(B), the signal for benzene-d6 decreased in inten-
sity over a period of 16 h. Overall conversion of C6D6
to C6H6 by 3.6 mmol of 8a was 75%, which is lower
than the percent of conversion of C6H6 to C6D6 (89.5%
in 6 h, entry 2 in Table 1) as expected.
100
[a]
All reactions were performed with Pd complex 8a (mmol)
in sealed glass tubes containing D2O and substrate at 55
and 1008C.
20 mL.
For % D, H NMR spectroscopy was used for analysis.
[D conversionꢁmole of substrate]/[mole of Pdꢁ100].
[b]
[c]
[d]
1
On the basis of these results, various organic sub-
strates were investigated to explore the feasibility of
1
Figure 1. (A) H NMR spectra of a solution of 8a and C6H6 in D2O at 558C recorded over a period of 22 h. REF1 (internal
2
standard): poly(dimethylsiloxane). (B) H NMR spectra of solution of 8a and C6D6 in H2O at 558C recorded after 16 h REF2
(internal standard): CD3CN.
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An Air/Water-Stable Tridentate N-Heterocyclic Carbene-Palladium(II) Complex
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À
ration. Catalyst 8a showed deuteration ability for a product of benzene via C H activation to afford
wide range of molecules including saturated hydrocar- phenol as a product.
bons, ethers, and ketones. Remarkably, the catalyst
In summary, we have demonstrated efficient and
À
À
was also capable of C H activation of saturated hy- strong C H activation with water soluble NHC-Pd(II)
drocarbons such as cyclohexane and cyclopentane, complexes that showed efficient H/D exchange of var-
and excellent total deuterium incorporation (96–97%) ious organic substrates in D2O under mild conditions
was observed (entries 1–4). Recently, there have been (558C or 1008C). Importantly, the catalytic reaction
a few examples for the H/D exchange of cyclohexane was not inhibited by coordination of water to Pd.
and cyclopentane, in which efficient thermal catalysis Strongly electron-donating groups such as NHC,
was carried out by Ir or Pt species using deuterated amide N, and O would increase the electron density
organic solvents or D2 as the deuterium source.[18] of Pd, allowing a weak interaction between electro-
À
These C H activations were developed with modest philic Pd and water. In addition, we were able to
conversions, and they had additional drawbacks such obtain a preliminary result for the direct functionali-
as air/water sensitivity of the metal catalysts and high zation in aqueous media. This independent character
reaction temperature requirement. On the contrary, of the developed catalysts from water and polar sol-
our palladium complex 8a appeared to be superior for vents can open up a new avenue in Pd(II) catalysis.
À
C H activation of saturated hydrocarbons with signif- Accordingly, utilization of this system for the incorpo-
icant air/water stability.
ration of deuterium and other useful functional
À
Ether compounds such as THF and diethyl ether groups into the C H bonds of numerous classes of or-
showed lower levels of D-incorporation compared to ganic compounds will be extensively pursued and re-
aromatic and saturated hydrocarbons, however we ob- ported in due course.
served that it was also a facile process with catalyst 8a
under conditions of higher temperatures and longer
reaction times (entries 6 and 8) with increased catalyst Experimental Section
loading (entry 9). To the best of our knowledge, there
are only few examples for the H/D exchange of THF, Catalytic H/D Exchange Reaction
which were effected by Ir catalysis at lower levels of
Pd cationic compounds were prepared when required. Cata-
lyst (7a, 7b, or 11) and 1.5 equiv. of AgBF4 were stirred in
2 mL of MeCN solvent for 30 min. After passing through a
D-incorporation at longer reaction times and higher
temperatures.[19] Although the selective C H activa-
À
tion for the a- and b-positions of THF and diethyl
ether was not examined under these conditions, we
demonstrated H/D exchange for these ether com-
pounds with palladium complexes. These findings
were quite similar to the case of H/D exchange with
ketone as the substrate. While deuterium was incor-
porated into acetone at higher initial conversion
values than the ethers, an increase in the three afore-
mentioned reaction variables led to a significant im-
provement of percent D-incorporation for this sub-
strate (entries 10–12). Also, we examined the deuteri-
um incorporation of toluene in D2O to elucidate the
celite column, the filtrate was dried in a rotary evaporator.
The dried compound was dissolved in 0.7 mL of D2O and
placed in a J-Young NMR tube with an external standard
capillary consisting of C6F6 solution with the poly(dimethyl-
siloxane). Then the hydrocarbon substrate (20 mL) was
added in D2O solution. The resulting mixture was heated at
558C/1008C. Deuteration levels were monitored by
1H NMR using an external capillary standard consisting of a
solution of the poly(dimethylsiloxane).
Oxidation of Benzene [Eq. (5)]
The Pd cationic compound 2 was prepared when required.
The catalyst 1 (2 mg, 0.5 mol%) and AgBF4 (1.5 equiv.)
were stirred in 2 mL of MeCN solvent for 30 min. Then the
filtrate was dried in a rotary evaporator after passage
through celite column. The dried compound was dissolved
in 0.2 mL of H2O. Then 0.2 mL of H2O2 and 0.1 mL of C6H6
were added to the aqua solution. The resulting mixture was
heated at 608C for 20 h. After the catalytic reaction, 2 mL of
pyridine for the internal reference and 0.2 mL of D2O were
added. The production of 16 equivalents of phenol was de-
tected by the WET1D NMR technique.
À
possibility of selective C H activation for aryl and
alkyl groups. Similarly to other examined substrates,
toluene showed high H/D conversions (95–97%) at
1008C (entry 14), however we were not able to ob-
À
serve selective C H activation for CH3 (95% after
6 h) or C6H5 C H bonds (95% after 6 h).
À
Based on these studies, which tested the possibility
of direct oxidation of an sp C H bond, C6H6, was
added to a reaction mixture of 2 (10 mol%) in 30%
2
À
H2O2 solution [Eq. (5)]. We detected the oxidation
Acknowledgements
We acknowledge generous financial supports from the Na-
tional Institute of General Medical Sciences of the National
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Joo Ho Lee et al.
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Institute of Health (RO1 GM 71495) as well as the Hydrocar-
bon Research Foundation.
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