An efficient heterogenized palladium catalyst for N-alkylation of amines and α-alkylation of ketones using alcohols

Article abstract of DOI:10.1039/c5ra07225e

A silica supported palladium-NiXantphos complex is reported as an efficient and a high turnover heterogeneous catalyst for the N-alkylation of amines and the α-alkylation of ketones using readily available alcohols under neat conditions at 120-140 °C following hydrogen borrowing strategy. The catalyst is easily separable and offers negligible amount of palladium leaching (<0.01 ppm). A high turnover number of about 46000 for the N-alkylation of amines and 4400 for the α-alkylation of ketones were achieved in the respective single batch reactions. The catalyst is recyclable up to four times without appreciable change in catalytic performance.

Full text of DOI:10.1039/c5ra07225e

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An efficient heterogenized palladium catalyst for
N-alkylation of amines and a-alkylation of ketones
using alcohols†
Cite this: RSC Adv., 2015, 5, 42399
Tuan Thanh Dang, Siah Pei Shan, Balamurugan Ramalingam
*
and Abdul Majeed Seayad
A silica supported palladium–NiXantphos complex is reported as an efficient and a high turnover
heterogeneous catalyst for the N-alkylation of amines and the a-alkylation of ketones using readily
available alcohols under neat conditions at 120–140 ꢀC following hydrogen borrowing strategy. The
catalyst is easily separable and offers negligible amount of palladium leaching (<0.01 ppm). A high
turnover number of about 46 000 for the N-alkylation of amines and 4400 for the a-alkylation of
ketones were achieved in the respective single batch reactions. The catalyst is recyclable up to four
times without appreciable change in catalytic performance.
Received 21st April 2015
Accepted 5th May 2015
DOI: 10.1039/c5ra07225e
www.rsc.org/advances
these heterogeneous catalyst systems, the main drawback is the
low turnover numbers, which limit their potential applications.
Introduction
Supported metal nanoparticles, oxides and hydroxides of
Ru,13 Ir,14 Au,15 Ag,16 Pd,17 Ni,18 Cu,19 Fe,20 and Mn,21 as well as
unmodied magnetite22 were reported to be promising hetero-
geneous catalysts for N-alkylation using various alcohols at
temperatures of 100–180 ꢀC to achieve 50–98% yields. An
improved TON of about 11 800 was reported using Au/TiO2
catalyst at 180 ꢀC for 96 h.15e Under similar condition, a TON of
570 was reported using Pd/MgO17e as a solid catalyst for the
mono-alkylation of aniline in triuorotoluene. Bimetallic
heterogeneous catalysts23 were also reported for N-alkylation of
amines using alcohols as well as diols. The metal leaching and
the recyclability of many of these heterogeneous catalysts are
not well reported.
C–N and C–C bond forming reactions are two indispensable
transformations in organic synthesis as well as pharmaceutical
and ne chemical industries.1 Conventional alkylation methods
that utilize reactive and toxic alkyl halides have been in practice
for the past several decades as the most convenient method for
the synthesis of alkylated amines and a-alkylated ketones.2 This
method oen produces over-alkylated products causing selec-
tivity issues that not only reduces the overall yield of the desired
product but also poses separation issues apart from generating
toxic halogenated waste, which needs further downstream
processing. Under this context, utilization of readily available
and less-toxic alcohols as alkylating agents following the
recently developed catalytic hydrogen borrowing strategy3 offers
a green and sustainable alternative method for the direct
alkylation of amines and ketones.
Ru4 and Ir5 remain to be the preferred choice of metals for
developing promising catalysts for the direct alkylation of
amines using alcohols under homogeneous reaction condi-
tions. Other metals such as Rh,6 Au,7 Cu,8 Fe,9 Pd10 and Os11 were
also reported as alternative catalysts. Although, these develop-
ments in homogeneous catalysts are remarkable achieving turn-
over-numbers (TON) up to 1000,10a several heterogeneous cata-
lysts were also reported12 to address the catalyst-product sepa-
ration and recycle issues. Apart from the requirement of
stringent conditions, and limited substrate scope with most of
Heterogenization of active homogeneous catalysts is another
approach24 to circumvent the separation and recyclability
problems associated with efficient homogeneous catalysts.
Accordingly, heterogenization of an active N-heterocyclic car-
bene (NHC) iridium complex on mesoporous silica (SBA-15)14b
was reported for the N-alkylation of amines and b-alkylation of
secondary alcohols in the presence of 50 mol% a base. Though
the catalyst gave high yields at 110 ꢀC for 24–48 h and was
recyclable, the catalyst loading was high (1.5 mol%) that limits
the TON (<70). Recently, we have developed a reusable
ruthenium catalyst13a for the N-alkylation of amines at 120 C
ꢀ
under batch (5 h) and continuous ow conditions (30 min
residence-time). In our continued effort to develop efficient
heterogeneous catalyst systems, herein we report a silica sup-
ported-palladium NiXantphos complex as an efficient and high
turnover catalyst for the alcohol activation towards N- and C-
alkylation reactions.
Organic Chemistry, Institute of Chemical and Engineering Sciences (ICES),
8
Biomedical Grove, #07-01 Neuros, Singapore 138665. E-mail: abdul_seayad@ices.
a-star.edu.sg
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra07225e
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Results and discussion
In our previous work,10a we have reported an in situ formed
homogeneous palladium catalyst from PdCl2 and a bidentate
phosphine ligand such as dppe, Xantphos(t-Bu), etc., as an
efficient catalyst for the N-alkylation of amines under relatively
mild operating conditions. In order to select a suitable biden-
tate ligand for the facile anchoring on readily available and
inexpensive solid supports, such as silica, to develop the cor-
responding heterogeneous analogue, we have further screened
other promising bidendate ligands for the N-alkylation of
aniline using benzyl alcohol and the representative results are
given in Table 1. We have observed that Xantphos(t-Bu) is still
one of the best bidendate ligands screened giving near quan-
titative yield of the N-alkylated product at 100 ꢀC for 24 h (entry
3). The corresponding commercially available NiXantphos,
which is easy to anchor through the free N–H group, gave a
promising yield of 86% (entry 6). To our delight, quantitative
yield was achieved (entry 7) when the free N–H group of NiX-
antphos was protected using alkyl groups such as n-Bu to mimic
the tethering group for anchoring to the solid support. Under
homogeneous conditions, PdCl2 gave higher performance
compared to that of Pd(OAc)2 (entry 7 vs. 8). Encouraged by
these results, NiXantphos was selected as the potential ligand to
anchor on to the readily available solid support such as silica to
prepare the heterogenized palladium catalyst.
Scheme 1 One-pot synthesis of Pd–Si–Pr–NiXantphos/SiO2 catalyst.
intermediate alkylated Si–Pr–NiXantphos, which was subse-
quently anchored on to silica to obtain Si–Pr–NiXantphos/SiO2.
Addition of palladium(II) precursor such as PdCl2 or Pd(OAc)2 in
toluene at 80 ꢀC resulted in the formation of the solid Pd–Si–Pr–
NiXantphos/SiO2 catalysts A and B, which were ltered under
argon and washed with dry toluene to remove impurities.
The silica supported palladium catalysts were characterized
by solid state NMR, XPS and ICP methods. For example, catalyst
B gave 13C NMR signals in the region 10–50 ppm and 110–150
ppm in the solid state NMR spectrum (Fig. 1) indicating the
presence of aliphatic and aromatic carbons corresponding to
that of the Si–Pr–NiXantphos ligand. The presence of 3d5/2 peak
at 337.21 eV in XPS spectra corresponds to that of Pd(II)–phos-
phine complex. The amount of palladium present in the cata-
lysts A and B were found to be 2.31 wt% and 2.23 wt%
respectively by ICP analysis.
Immobilization of NiXantphos on solid supports was
reported earlier for the rhodium catalyzed hydrofomylation25
and for the palladium catalyzed allylic alkylation26 of diethyl 2-
methylmalonate. Adopting
a similar procedure, we have
prepared silica supported Pd–NiXantphos catalysts in a modi-
ed one-pot protocol as depicted in Scheme 1. The alkylation of
NiXantphos with 3-chloropropyl(trimethoxy)silane afforded the
To our surprise, we observed that only very small amount of
palladium is required to achieve good yield of the N-alkylated
product when using the solid catalysts A or B. Accordingly, up to
65% yield was observed with 0.043 mol% of palladium at 100 ꢀC
for 24 h that provided a high TON of 1511 as shown in Table 2
(entry 1) for the N-alkylation of aniline using benzyl alcohol with
catalyst A. Under these conditions, catalyst B gave better results
with a TON of 1928 (entry 2) and was selected for further
Table 1 Ligand screening for Pd-catalyzed N-alkylation of aniline
using benzyl alcohola
Entry
PdX2
Ligand
Product, 1b (%)
1
2
3
4
5
6
7
8
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
Pd(OAc)2
dppe
Xantphos
96
34
>99
95
89
86
>99
83
Xantphos(t-Bu)
dpePhos(Cy)
dppf(t-Bu)
NiXantphos
BuNiXantphos
BuNiXantphos
a
Reaction condition: aniline (2 mmol), benzyl alcohol (3 equiv.),
b
catalyst (1 mol%, Pd/L ¼ 1 : 1), LiOH (20 mol%) 100 ꢀC, 24 h. NMR
Fig. 1 Solid-state 13C NMR spectra of catalyst B.
yield using mesitylene as the internal standard.
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Table 2 Optimization of conditions for N-alkylation of aniline with benzyl alcohola
Entry
Cat. (mg, Pd mol%)
LiOH (equiv.)
Temp. (ꢀC)
Product, 1b (%)
TON
1
2
3
4
5
6
A (4, 0.043)
B (4, 0.042)
B (4, 0.042)
B (4, 0.042)
B (2, 0.021)
B (1, 0.0105)
0.2
0.2
0.2
0.1
0.2
0.2
100
100
120
120
120
120
65
81
>99
95
98
78
1511
1928
2381
2262
4666
7429
a
b
Reaction condition: aniline (2 mmol), benzyl alcohol (3 equiv.), LiOH (20 mol%) 120 ꢀC, 24 h. NMR yield using mesiltylene as an internal
standard. No alkylation reaction was observed in the absence of catalyst A or B.
optimization. Upon increasing the temperature to 120 ꢀC, obtained in 53% isolated yield when cyclohexanol was used as
quantitative yield with an improved TON of 2381 was observed. the alkylating agent. The alkylation of piperidine with 2-octa-
Decreasing LiOH from 20 mol% to 10 mol% had only a nol resulted in 65% isolated yield of the desired product 25. In
marginal decrease in the catalytic performance. Decreasing the most cases, higher TON was observed compared to the litera-
catalyst loading to 0.021 mol% and 0.0105 mol% of palladium ture reports.
gave further improved TON of 4666 and 7429 respectively
(entries 5 and 6).
Aer achieving the high TON, we have evaluated the
substrate scope and limitations using the catalyst B for the N-
alkylation of various amines using primary and secondary
alcohols. In most cases, the desired products were obtained in
good to excellent isolated yields as given in Table 3. Aniline and
4-methylaniline were benzylated to the corresponding mono
substituted anilines 1 and 2 in 94% and 98% isolated yields
respectively at 120 ꢀC. Less reactive and challenging substrates
required slightly higher temperature or increased catalyst
loading. Electron decient anilines having substituents such as
Cl, F and CF3 were benzylated at 130 ꢀC for 24 h to give the
corresponding products 3, 4 and 5 in excellent yields up to 99%.
Electron rich linear aliphatic amines such as n-hexyl amine and
n-octyl amine were also benzylated under similar conditions to
afford the secondary amines 6 (94%) and 7 (86%) in excellent
yields. a-Branched amines were also found to be good
substrates and the corresponding alkylated products 8 and 9
were obtained in 82% and 88% yields respectively. Less reactive
aliphatic alcohols such as n-octanol and 2-ethyl-1-butanol could
also be used as electrophilic partner for obtaining the secondary
amines 10–12 in 75–83% yields.
Table 3 Substrates scope for N-alkylation of amines using hetero-
geneous Pd catalysta
Involvement of unfavorable iminium intermediate makes
the cyclic amines generally less reactive under hydrogen
borrowing conditions. However, the current heterogeneous
Pd catalyst promotes the alkylation of cyclic amines such
as piperidine, pyrrolidine, azepane, morpholine, tetralin
and N-methyl piperazine to the corresponding teritary amines
13–20 in 75–92% isolated yields. The alkylation of amines
with challenging secondary alcohols such as 1-phenyl ethanol,
2-octanol and cyclohexanol also gave the corresponding alky-
lated products 21, 22 and 23 in very good isolated yields (81–
87%) in the case of cyclohexylamine as the substrate. With
aromatic amines such as aniline, the alkylated product 24 was
a
Reaction condition: amine (2 mmol), alcohol (3 equiv.), LiOH (20
b
c
mol%), 24 h. B (2 mg, 0.021 mol% of Pd; TON, 4476–4666). B (8
mg, 0.084 mol% of Pd; TON, 976–1179). B (20 mg, 0.21 mol% of Pd;
TON, 252–400).
d
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Transition metals catalyzed a-alkylation of ketones using Under these pre-optimized conditions acetophenone gave 1,3-
alcohols following hydrogen borrowing strategy also attracted a diphenylpropan-1-one (26) in 83% yield, while 98% and 96%
greater attention in recent years as it provides an environmen- yields were achieved with 4-methyl (27) and penta-methyl (28)
tally attractive protocol compared to the classical alkylation substituted acetophenones respectively, when benzyl alcohol
methods.27 Most of the catalysts reported were based on Ru and was used as the alkylating agent. Unprotected hydroxy group
Ir complexes and required stoichiometric amount of an inor- was tolerated and the corresponding alkylated product 29 was
ganic base to achieve very good yields. Palladium was also obtained in 81% yield. Other aromatic methyl ketones such as
explored as an alternative catalyst for a-alkylation using alco- naphthyl, ferrocenyl and furanyl ketones were also excellent
hols. For example, a homogeneous Pd–NHC catalyst28 was substrates and the corresponding benzylated products 30–32
reported for the a-alkylation of acetophenone using benzyl were obtained in 74–98% yield. Excellent yields were achieved
ꢀ
alcohol at 125 C giving a mixture of alcohol and ketone prod- in the case of cyclic (33) and acyclic (34) ketones. Less reactive
ucts in 20% and 56%, respectively. A heterogeneous Pd catalyst aliphatic alcohol such as 1-octanol was also found to be a very
(Pd/AlOOH)29 was reported that gave a TON of up to 490 for the good alkylating agent and the alkylated products 35 and 36
a-alkylation of ketones using a variety of alcohols at 80 ꢀC. were obtained in 67% and 73%, respectively. In addition, the
Palladium nanoparticles supported on viologen30 was reported alkylation of acetophenone with substituted benzyl alcohol
under solvent free condition, however the TON was found to be such as 3,5-dimethylbenzyl alcohol afforded the ketone
very low (<20). Recently, Pd/C was reported as a catalyst in water product 37 in 75% isolated yield.
as the solvent at temperatures of 140–160 ꢀC in the presence of
1–3 equivalents of base giving up to 93% yield with a TON of supported palladium–Ni-Xantphos catalyst B by taking N-alkyl-
Further, we have investigated the recyclability of the silica
46.31
ation of aniline using benzyl alcohol at 120 C for 24 h as the
ꢀ
We have explored the potential use of the silica supported representative reaction. 4 mg (0.042 mol%) of catalyst B was
palladium Ni-Xantphos catalyst B for the a-alkylation of used for easy handling and the catalyst was recovered by simple
ketones in the presence of sub-stoichiometric amounts of base ltration under argon and reused for subsequent reactions.
under solventless conditions. As given in Table 4, a-alkylation Unfortunately, the yield of the mono-alkylated amine decreased
ꢀ
of methyl ketones were achieved at 140 C in the presence of to 59% for the rst recycle and 22% in the second recycle
only 20 mol% of LiOH as a base providing excellent yields compared to 99% in the rst reaction (Fig. 2). This may be due
of the a-alkylated ketones 26–37 with a TON of up to 467. to the decomposition of the Pd(II) catalyst either in presence of
water produced in the reaction or in contact with small amount
of air during the recycling procedure.
Improved recyclability was observed when the reaction was
Table 4 Substrates scope for a-alkylation of ketones using primary
carried out in the presence of molecular sieves (50 mg) to
partially absorb the water produced during the reaction,
providing 99%, 98% and 45% yields respectively, in rst, second
and third runs.32 Though the addition of molecular sieves
improved the recyclability, higher amount of solids in the
reaction mixture may not be very much practicable. Hence, we
decided to improve the stability of the catalyst by increasing the
ligand to palladium ratio on the silica support and two new
catalysts C (0.95 wt% Pd) and D (0.52 wt% Pd) with ligand/Pd
alcoholsa
a
Reaction condition: ketone (2 mmol, 1 equiv.), alcohol (6 mmol, 3
Fig. 2 Recycle studies of silica supported Pd–NiXantphos catalysts B,
C and D.
equiv.), LiOH (20 mol%), catalyst B (20 mg, 0.21 mol% of Pd, TON,
319–467).
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ratios of 3 : 1 and 6 : 1 respectively were synthesized. Both the was cooled to room temperature and the calculated amount of
catalysts C and D were found to be active and not only gave near Pd precursor (Pd(OAc)2) was introduced under argon and the
quantitative yields under the standard reaction conditions, but stirring continued at 80 ꢀC for another 4 h. The reaction mixture
also provided higher recyclability compared to that of the was allowed to cool to RT and the brown solid catalyst was
catalyst B (Fig. 2). The catalyst D with a ligand/Pd ratio of 6 : 1 collected by ltration and washed with toluene (3 times, 20 mL)
was found to be more stable and could be recycled up to four under argon atmosphere. The solid catalyst was dried in a
times without signicant loss in activity as shown in Fig. 2. A vacuum oven at 50 ꢀC overnight, collected and stored under
total TON of about 16 000 was achieved aer 6 runs using the argon atmosphere.
catalyst D. The palladium leaching in each run was found to be
less than 0.01 ppm.32 In order to further investigate the stability
General procedure for the N-alkylation of amine with alcohol
of the catalyst D, the representative N-alkylation of aniline with
Catalyst B (2–20 mg), LiOH (20.0 mol%), alcohol (6.0 mmol),
benzyl alcohol was carried out with very low catalyst loading (2
and amine (2.0 mmol) were charged under argon-atmosphere in
mg, 0.00196 mol%) for 5 mmol of substrate and a surprisingly
a Radleys carrousel reaction tube. The reaction tube was closed
high TON of about 46 000 was achieved in 72 h of reaction with
with the Teon stopper and was heated to the desired temper-
91% yield.32 A high TON of about 4400 was also achieved in the
ature under stirring. Aer the desired reaction time, the reac-
case of a-alkylation of 4-methyl acetophenone with benzyl
tion mixture was allowed to cool to room temperature and
alcohol using 0.0146 mol% catalyst D for 5 mmol of substrate,
diluted with toluene (10 mL). SiO2 (400 mg) was added into the
providing 65% yield in 96 h.32
crude mixture. The organic solvent was removed under vacuum
The catalytic cycle may be initiated by the formation of an in
and the product was puried by column chromatography.
situ formed palladium alkoxide species on the silica support
and proceeds through hydrogen borrowing mechanism similar
to that reported for the homogeneous palladium catalyst.10a
General procedure for the a-alkylation of ketones with
alcohols
Catalyst B (20 mg), LiOH (20.0 mol%), alcohol (6.0 mmol), and
ketone (2.0 mmol) were charged under argon-atmosphere in a
Conclusions
Radleys carrousel reaction tube. The reaction tube was closed
with the Teon stopper and was heated to the desired temper-
ature under stirring. Aer the desired reaction time, the reac-
tion mixture was allowed to cool to room temperature and
diluted with toluene (10 mL). SiO2 (400 mg) was added into the
crude mixture. The organic solvent was removed under vacuum
and the product was puried by column chromatography.
In conclusion, we have reported silica supported palladium–
NiXantphos as an efficient and high turnover heterogeneous
catalyst for the N-alkylation of amines and a-alkylation of
ketones using a variety of alcohols including challenging
aliphatic and secondary alcohols as alkylating agents under
solvent free conditions. The catalyst can be readily recovered
from the reaction mixture for subsequent runs and offers a very
low palladium leaching of less than 0.01 ppm. The catalyst
provided very high turnover numbers (TON) compared to most
of the reported catalysts and a remarkably high TON of up to
46 000 was achieved in a single batch reaction for the N-alkyl-
ation of amine using alcohols. The catalyst can be recycled up to
four times without appreciable change in catalytic performance.
A high TON of about 4400 was also achieved for the a-alkylation
of ketone in the presence of only 20 mol% of base under
solventless conditions. Though the supported palladium cata-
lysts B, C and D provided similar catalytic performance in a
single batch reaction, the catalyst D having higher ligand to Pd
ratio is superior and is preferred in terms of stability and
recyclability.
Acknowledgements
This work was funded by “GSK—Singapore Partnership for
Green and Sustainable manufacturing and the Institute of
Chemical and Engineering Sciences (ICES), Agency for Science,
Technology and Research (A*STAR), Singapore.
Notes and references
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General procedure for the one-pot preparation of solid
catalysts
10 mL of dry toluene, 55.2 mg of NiXantphos (0.1 mmol), NaH
(0.3 mmol) were charged under argon-atmosphere in a Schlenk
tube followed by additions of (3-chloropropyl)triethoxysilane
(0.3 mmol). The reaction tube was closed with the Teon
stopper and was heated to 80 ꢀC under stirring for 12 h. 250 mg
of dry SiO2 was introduced in to the reaction mixture and stir-
ꢀ
ring continued at 80 C for another 12 h. The reaction mixture
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