Palladium nanoparticles stabilized by alkylated polyethyleneimine as aqueous biphasic catalysts for the chemoselective stereocontrolled hydrogenation of alkenes

Article abstract of DOI:10.1021/ol062052k

Palladium nanoparticles were prepared, stabilized, and dispersed in water by alkylated branched polyethyleneimine. The palladium nanoparticles were effective aqueous biphasic catalysts for the chemoselective hydrogenation of alkenes with preferential reduction of less hindered double bonds, such as reduction of 3-methylcyclohexene in the presence of 1-methylcyclohexene and 1-octene in the presence of 2-methyl-2-heptene.

Full text of DOI:10.1021/ol062052k

ORGANIC
LETTERS
2
006
Vol. 8, No. 24
445-5448
Palladium Nanoparticles Stabilized by
Alkylated Polyethyleneimine as Aqueous
Biphasic Catalysts for the
5
Chemoselective Stereocontrolled
Hydrogenation of Alkenes
Maxym V. Vasylyev, Galia Maayan, Yonathan Hovav, Adina Haimov, and
Ronny Neumann*
Department of Organic Chemistry, Weizmann Institute of Science,
RehoVot, Israel 76100
ronny.neumann@weizmann.ac.il
Received August 21, 2006
ABSTRACT
Palladium nanoparticles were prepared, stabilized, and dispersed in water by alkylated branched polyethyleneimine. The palladium nanoparticles
were effective aqueous biphasic catalysts for the chemoselective hydrogenation of alkenes with preferential reduction of less hindered double
bonds, such as reduction of 3-methylcyclohexene in the presence of 1-methylcyclohexene and 1-octene in the presence of 2-methyl-2-heptene.
Much recent research in catalysis has been related to
development of alternative reaction media in order to achieve
synthetic procedures with a low environmental load. Thus,
inexpensive solvent (water) and easy catalyst recovery and
recycle (phase separation). Unfortunately, low reaction rates
because of slow mass transfer of organic substrates into the
aqueous catalytic phase and the requisite synthesis of water-
soluble ligands for the metallorganic catalysts have been an
obstacle to more widespread application of aqueous biphasic
catalysis. Mass transfer of the substrate to the aqueous phase
may be facilitated by addition of amphiphiles to reduce
surface tension. In this context, besides the more often used
micellar and (micro)emulsion catalyst systems, the use of
alkylated quaternized polyethylenimines as amphiphiles has
led to their application in general acid-base and oxidation
1
instead of using classical organic solvents, efforts are being
2
3
4
made to apply aqueous, fluorous, ionic liquid, and super-
5
critical fluid as reaction solvents. In our opinion, the use of
water as solvent and aqueous biphasic catalysis is the most
potentially attractive alternative that combines a benign,
(
1) Adams, D. J.; Dyson, P. J.; Taverner, S. J. Chemistry in AlternatiVe
Reaction Media; Wiley: Weinheim, 2003.
(
2) (a) Li, C.-J.; Chen, L. Chem. Soc. ReV. 2006, 35, 68-82. (b) Cornils,
B. J. Mol. Catal. A. Chem. 1999, 143, 1-10. (c) Cornils, B. Org. Proc.
Res. DeV. 1998, 2, 121-127.
6
(3) Gladysz, J. A.; Curran, D. P.; Horvath, I. T. Handbook of Fluorous
reactions.
Chemistry; Wiley-VCH: Weinheim, 2004.
(
4) (a) Welton, T. Chem. ReV. 1999, 99, 2071-2083. (b) Sheldon, R. A.
Chem. Commun. 2001, 2399-2407. (c) Wasserscheid, P.; Welton, T. Ionic
Liquids in Synthesis; Wiley-VCH: Weinheim, 2002.
(5) Theyssen, N.; Hou, Z. Leitner, W. Chem. Eur. J. 2006, 12, 3401-
3409 and references therein.
1
0.1021/ol062052k CCC: $33.50
© 2006 American Chemical Society
Published on Web 11/01/2006

Recently, there has been an exponential growth in the use
7
of the colloids or nanoparticles as catalysts. Typically, in
Scheme 1. Cartoon Representation of Palladium Nanoparticles
Stabilized by Alkylated Polyethylenimine and Dispersed in
Water as Catalyst for Chemoselective Hydrogenation of
Hydrophobic Alkenes
such applications the nanoparticles have been either dispersed
in the reaction medium leading to a “semi-heterogeneous”
catalyst or supported on a matrix leading to a more classic
heterogeneous catalyst. Thus, aqueous biphasic catalysis that
classically has dealt with intrinsically homogeneous catalysts
2
with known molecular structure can now evolve, as de-
scribed herein, to aqueous biphasic catalysis with “semi-
heterogeneous” nanoparticle catalysts. An important, flexible
parameter that can be manipulated in the study of catalysis
with nanoparticles, especially in the “semi-heterogeneous”
reaction mode, is the identity of the stabilizing agent that
7
c
can vary from the more established use of polymers and
surfactants7 to the more recent use of dendrimers and
c
8
polyoxometalates.9
In this paper, we present our research on the use of a
simply prepared alkylated polyethylenimine as a stabilizer
for palladium nanoparticles, which upon dispersion into water
allows the aqueous biphasic hydrogenation of hydrophobic
alkenes. Beyond the practical and ecological advantage
indigenous to this hydrogenation method, it was very
surprising to observe that the hydrogenation was highly
chemoselective with reaction of less sterically hindered
alkenes being significantly preferred. Such considerable
chemoselectivity, typically a domain of homogeneous
catalysis is now demonstrated for catalysis by nano-
particles. The entire concept is pictorially summarized in
Scheme 1.
Palladium nanoparticles stabilized by alkylated poly-
10
ethylenimine (PEI) were prepared as follows. First, branched
PEI (M
w
) 60 000 with ∼25% primary and tertiary amines
and ∼50% secondary amines) was alkylated with 1-iodo-
dodecane, and the resulting alkylated PEI was treated with
a basic resin, Amberlite IR-900, to remove HI from the
15
polymer solution. The N chemical shifts (23-34 ppm) and
15
1
the correlations in the N- H HMBC NMR spectrum,
Figure 1, reveal that the alkylated PEI contains only
(6) (a) Klotz, I. M.; Royer, G. P.; Scarpa, I. S. Proc. Natl. Acad. Sci.
U.S.A. 1971, 68, 263-266. (b) Shu, J.; Scarpa, I. S.; Klotz, I. M. J. Am.
Chem. Soc. 1976, 98, 7060-7064. (c) Hollfelder, F.; Kirby, A. J.;
Tawfik, D. S. J. Am. Chem. Soc. 1997, 119, 9578-9579. (d) Liu, L.;
Breslow, R. J. Am. Chem. Soc. 2002, 124, 4978-4979. (e) Liu, L.;
Rozenman, M.; Breslow, R. J. Am. Chem. Soc. 2002, 124, 12660-12661.
(
3
(
f) Zhou, W. J.; Liu, L.; Breslow, R. HelV. Chim. Acta 2003, 86, 3560-
567. (g) Liu, L.; Breslow, R. Bioorg. Med. Chem. 2004, 12, 3277-3287.
h) Suh, J.; Hong, S. H. J. Am. Chem. Soc. 1998, 120, 12545-12552. (i)
Haimov, A.; Cohen, H.; Neumann, R. J. Am. Chem. Soc. 2004, 126, 11762-
1
1763.
(
7) For recent reviews, see: (a) Johnson, B. F. G. Top. Catal. 2003, 24,
1
(
47-159. (b) Kung, H. H.; Kung, M. C. Catal. Today 2004, 97, 219-224.
c) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44,
7
852-7872. (d) Schmid, G. Chem. ReV. 1992, 92, 1709-1727. (e) Lewis,
L. N. Chem. ReV. 1993, 93, 2693-2730. (f) Widegren, J. A.; Finke, R. G.
Mol. Catal. A: Chem. 2003, 198, 317-341.
(8) (a) Astruc, D.; Chardac, F. Chem. ReV. 2001, 101, 2991-3023. (b)
Twyman, L. J.; King, A. S. H.; Martin, I. K. Chem. Soc. ReV. 2002, 31,
6
9-82. (c) Jiang, Y.; Gao, Q. J. Am. Chem. Soc. 2006, 128, 716-717. (d)
Oh, A.-K.; Niu, Y.; Crooks, R. M. Langmuir 2005, 21, 10209-10213. (e)
Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2004, 108, 8572-8580.
(
f) Ye, H.; Scott, R. W. J.; Crooks, R. M. Langmuir 2004, 20, 2915-2920.
(g) Tabuani, D.; Monticelli, O.; Chincarini, A.; Bianchini, C.; Vizza, F.;
Moneti, S.; Russo, S. Macromolecules 2003, 36, 4294-4301. (h) Esumi,
K.; Isono, R.; Yoshimura, T. Langmuir 2004, 20, 237-243. (i) Gopidas,
K. R.; Whitesell, J.; Fox, M. A. Nano Lett. 2003, 3, 1757-1760. (j) Scott,
R. W. J.; Datye, A. K.; Crooks, R. M. J. Am. Chem. Soc. 2003, 125, 3708-
15
1
Figure 1. N- H HMBC NMR spectrum of alkylated PEI.
3
709. (k) Rahim, E. H.; Kamounah, F. S.; Frederiksen, J.; Christensen, J.
B. Nano Lett. 2001, 1, 499-501. (l) Reek, J. N. H.; de Groot, D.; Oosterom,
G. E.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. J. Biotech. 2002, 90,
secondary and tertiary amine moieties;11 all of the primary
amines in the original PEI were alkylated; no quaternary
ammonium moieties were observed using this alkylation
1
59-181.
(
9) (a) Finke, R. G.; Oezkar, S. Coord. Chem. ReV. 2004, 248, 135-
1
5
46. (b) Oezkar, S.; Finke, R. G. J. Am. Chem. Soc. 2002, 124, 5796-
810. (c) Finke, R. G. Metal Nanopart. 2002, 17-54. (d) Troupis, A.;
Hiskia, A.; Papaconstantinou, E. Angew. Chem., Int. Ed. 2002, 41, 1911-
procedure. Second, K
added to an aqueous solution of alkylated PEI, and the
resulting yellow solution was treated with NaBH leading
2 4
PdCl (10 wt % of alkylated PEI) was
1
914. (e) Kogan, V.; Aizenshtat, Z.; Popovitz-Biro, R.; Neumann. Org.
Lett. 2002, 4, 3529-3532. (f) Maayan, G.; Neumann, R. Chem. Commun.
005, 4595-4597.
2
4
5446
Org. Lett., Vol. 8, No. 24, 2006

to a brown-black suspension of palladium nanoparticles in
water that remained visibly unchanged for significant periods
of time (7-10 days). A transmission electron micrograph
of the dispersion prepared by freeze fracture of the aqueous
solution confirmed the formation of palladium nanoparticles
with particle sizes ranging from 2 to 4 nm (see Figure S1,
Supporting Information).
Aqueous biphasic catalytic hydrogenation of sterically
unhindered, hydrophobic, water-insoluble alkenes at 80 °C
using the palladium nanoparticles stabilized by alkylated PEI
as catalyst showed efficient hydrogenation to the correspond-
ing alkanes, Scheme 2.12 Thus, primary alkenes such as
catalytic reaction showed no significant change in nano-
particle size and no significant aggregation of the particles.
Interestingly, however, the hydrogenation of 2-methyl-1-
heptene under conditions described in Scheme 1, yielded only
57.4 mol % 2-methylheptane although the conversion was
>98%; the remaining product was the 2-methyl-2-heptene
isomer. Furthermore, 2-methyl-2-heptene did not react at all
as was observed also in a separate experiment. These results
indicated that sterically hindered alkenes did not undergo
hydrogenation; this was verified by competition reactions
between 1-octene/2-methyl-2-heptene and 3-methylcyclo-
hexene/1-methylcyclohexene as shown in Figure 2. Both
Scheme 2. Hydrogenation of Hydrophobic Alkenes in an
n
Aqueous Biphasic Reaction Medium with a Pd -Alkylated PEI
Catalyst
1-hexene, 1-heptene, 1-octene, 1-decene, and styrene all gave
high yields of the corresponding n-alkanes with up to 2%
yield of a mixture of alkene isomers. Similarly, cyclic alkenes
such as cyclohexene, cyclooctene, and cyclododecene also
gave very high yields of the corresponding cycloalkanes. It
should be noted that the nonamphiphilic polyethylenimine
itself was also very effective for the stabilization of palladium
nanoparticles, but the hydrogenation of 1-octene, under
identical reaction conditions as presented in Scheme 2,
proceeded poorly with only 21% 1-octene conversion at 58%
selectivity to n-octane (the remaining products were isomers
of linear octene). Importantly, using this aqueous biphasic
reaction system, it was simple to recover and recycle the
catalyst containing aqueous phase by decantation of the
product from the aqueous phase. Thus, over five reaction
cycles, using 1-octene as substrate, yields of >99% n-octane
were obtained. A transmission electron micrograph after a
Figure 2. Reaction profiles for competitive hydrogenation of
alkenes with different steric demand.
hydrogenation reactions showed entirely chemoselective
hydrogenation of the less hindered alkenes, 1-octene and
3-methylcyclohexene. Furthermore, the kinetic profiles show
in absolute terms that the more hindered, though reactive,
3-methylcyclohexene is reduced more slowly than 1-octene.
Additional manifestation of the chemoselectivity observed
can be noted in the competitive hydrogenation of 1-methyl-
cyclohexene and 1-phenylcyclohexene. Since the planar
phenyl moiety is sterically less demanding than the spherical
methyl moiety, 1-phenylcylohexene is selectively reduced.
There are three additional points worth mentioning. First,
there is no apparent selectivity in the hydrogenation of cis-
and trans-alkenes. Thus, hydrogenation of a 1:1 mixture of
cis- and trans-stilbene resulted in the formation of bibenzyl
with reduction of both substrates at the same rate and
completion of the hydrogenation within 2 h. Second, the
aqueous biphasic hydrogenation was faster than a mono-
phasic reaction attained by using a 1:1 water/methanol
solvent, though the chemoselectiVe nature of the hydrogena-
tion reactions was fully retained. Third, the classic Pd/C
catalyst (substrates, 0.5 mmol each; 5% Pd/C, 0.02 g; MeOH,
(10) Synthesis of the Alkylated PEI. Polyethelenimine 60,000 (4 g)
was dissolved in 200 mL of dry ethanol. 1-Iodododecane (3.33 mL, 13.5
mmol) was added, and the mixture was refluxed for 15 h. The reaction
mixture was cooled, 10 g of the Amberlite IR-900 was added, and the
suspension obtained was stirred for 3 h and then filtered. The filtrate was
concentrated under reduced pressure and dried under high vacuum for 24
h yielding a yellow glassy compound that was used without any further
1
purification: H NMR (D2O) δ 0.84 (br s), 1.25 (br s), 2.76 (br m) ppm;
1
5
N NMR (D2O) δ 25.35, 27.17, 31.03, 32.66 ppm; IR (film cast from THF)
ν 3408, 2954, 2923, 2852, 1552, 1466, 1376, 1361, 1304, 1101, 802, 721
-1
cm . Synthesis of Palladium Nanoparticles Stabilized by Alkylated PEI.
K2PdCl4 (0.031 g, 0.09 mmol) and alkylated PEI (0.32 g) were dissolved
in 10 mL of double-distilled water. To this vigorously stirred mixture was
added NaBH4 (0.1 mmol in 1 mL of water), and the mixture immediately
turned brown-black.
1
2
mL; H , 2 bar; 80 °C, 6 h) showed no evidence of
(11) Witanowski, M.; Stefaniak, L.; Webb, G. A. In Annual Reports of
NMR Spectroscopy; Webb, G. A., Ed.; Academic Press: New York, 1986;
Vol. 18.
chemoselectivity in the reduction of the pairs of alkenes
shown in Figure 2. These experiments would appear to
indicate that the source of the chemoselectivity in these
palladium nanoparticle-alkylated PEI catalytic systems is
a different accessibility of hindered versus nonhindered
alkenes to the palladium surface due to the structure of
(12) Typical Catalytic Hydrogenation. A 15 mL Ace Glass pressure
tube was loaded with 1 mL of the Pdn-alkylated PEI dispersion (0.0082
mmol of Pd) and 1 mmol of alkene and then pressurized to 2 bar with
hydrogen. The pressure tube was placed in an oil bath thermostated at 80
°
C to initiate the reaction. Aliquots of the organic phase were analyzed by
gas chromatography using a 5% phenylmethylsilicone capillary column.
Org. Lett., Vol. 8, No. 24, 2006
5447

alkylated PEI rather than mass transfer effects. Future
research will be devoted to better understanding the source
of the chemoselectivity.
Acknowledgment. The research was supported by the
Minerva Foundation, the Israeli Ministry of Science, and the
Helen and Martin Kimmel Center for Molecular Design. R.N.
is the Rebecca and Israel Sieff Professor of Organic
Chemistry.
Commonly, chemoselectivity is the purview of homoge-
neous catalysis and is realized by control of the ligand
environment around the active metal center as is the case
for alkene hydrogenation using the Wilkinson catalyst
Supporting Information Available: Transmission
electron micrograph of the Pd nanoparticles. This
material is available free of charge via the Internet
at http://pubs.acs.org.
1
3
RhCl(PPh
3
)
3
.
Here, we have observed that a “semi-
heterogeneous” metal cluster without a designed or defined
ligand environment also displayed very significant chemo-
selectivity in a rather elementary hydrogenation of simple
hydrophobic alkenes.14 The aqueous biphasic reaction me-
dium also allows for the ecologically preferred use of water
as solvent coupled with effective catalyst recovery and
recycle.
OL062052K
(14) Some interesting chemoselectivities in hydrogenation reactions
catalyzed by nanoparticles have also been recently observed in ionic liquid
solvents; cf. (a) Umpierre, A. P.; Machado, G.; Fecher, G. H., Morias, J.;
Dupont, J. AdV. Synth. Catal. 2005, 347, 1404-1412. (b) Fonseca, G. S.;
Domingos, J. B.; Nome, F.; Dupont, J. J. Mol. Catal. A Chem. 2006, 248,
10-16.
(
13) James, B. R. In ComprehensiVe Organometallic Chemistry; Wilkin-
son, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982.
5448
Org. Lett., Vol. 8, No. 24, 2006