Silica-supported dendrimer-palladium complex-catalyzed selective hydrogenation of dienes to monoolefins

Article abstract of DOI:10.1002/adsc.200505236

The selective hydrogenation of cyclic and acyclic dienes to monoolefins occurs under very mild conditions, in the presence of silica-supported PAMAM-Pd complexes. The activity and selectivity of this reaction is sensitive to the dendrimer structure. These dendritic complexes display excellent recycle properties, retaining activity for up to eight recycles.

Full text of DOI:10.1002/adsc.200505236

FULL PAPERS
DOI: 10.1002/adsc.200505236
Silica-Supported Dendrimer-Palladium Complex-Catalyzed
Selective Hydrogenation of Dienes to Monoolefins
Pumza P. Zweni, Howard Alper*
Centre for Catalysis Research and Innovation, Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa,
Ontario, Canada, K1N 6N5
Fax: (þ1)-613-562-5871, e-mail: halper@uottawa.ca
Received: June 6, 2005; Accepted: February 2, 2006
Abstract: The selective hydrogenation of cyclic and These dendritic complexes display excellent recycle
acyclic dienes to monoolefins occurs under very properties, retaining activity for up to eight recycles.
mild conditions, in the presence of silica-supported
PAMAM-Pd complexes. The activity and selectivity Keywords: alkenes; dienes; hydrogen; palladium; sili-
of this reaction is sensitive to the dendrimer structure. ca-supported PAMAM dendrimers.
Introduction
presence of alkynes in the reaction medium can result
in irreversible deactivation, while dienes simply slow
Recent publications have demonstrated that silica-sup- down the reaction, and enones cause temporary inactiv-
ported polyamidoamine (PAMAM) dendrimer-metal ity. The removal of these impurities from alkene feed-
complexes can catalyze a variety of reactions. For exam- stocks is crucial for extending the catalyst lifetime.
ple, dendrimer-rhodium complexes on silica or a resin, Moreover, as the alkene is often hydrogenated as well,
display excellent activity and recyclability for the hydro- the challenge is to find a catalytic system that can re-
[
1–3]
formylation of alkenes.
Dendrimer-palladium com- move these minute impurities without affecting the al-
plexes have been used to catalyze the Heck coupling re- kene feedstocks.
[
4]
action of bromoarenes, the carbonylation of haloar-
There are few examples of the homogeneously cata-
[5]
[6]
enes, as well as the hydroesterification, and oxida- lyzed selective hydrogenation of dienes. Work by Du-
tion of alkenes. It was anticipated that, under appro- pont and co-workers
[7]
[11]
using Pd(acac) dissolved in
2
priate conditions, the dendritic Pd complexes could the ionic liquid 1-n-butyl-3-methylimidazolium tetra-
effect selective reduction reactions. A particularly fasci- fluoroborate (BMIM.BF ) resulted in the hydrogena-
4
nating transformation would be to attain the semi-hy- tion of simple and functionalized dienes to their corre-
drogenation of dienes to monoolefins in fine selectivi- sponding monoolefins at 508C and 5–20 atm in selectiv-
ties.
ities ranging from 93 to 100%. Kaneda and co-workers
The catalytic selective hydrogenation of multiple un- demonstrated that the dendrimer-bound palladium
saturated hydrocarbons can be used for synthetic and complex of phosphonated poly(propyleneimine) can se-
non-synthetic purposes. For instance, the production of lectively hydrogenate conjugated cyclic dienes to mono-
[
12]
flavour components including hexen-1-ol and methyl olefins. The dendrimer complex gave better results
trans-2-hexenoate, from their parent diene compounds than the monomeric catalyst [PhN(CH PPh ) PdCl ].
2
2
2
2
[
8]
was effected by hydrogenation. For non-synthetic pur- The proposed reaction mechanism involves a PdꢀH spe-
poses, selective hydrogenation is used to remove dienes cies. The equation of the reaction rate indicated that the
and other polyunsaturated hydrocarbons from alkene presence of bases should speed up the reaction. Superior
feedstocks. van Leeuwen showed that the presence of results obtained with the dendrimer complex were at-
impurities such as alkynes, dienes and enones in alkene tributed to the fact that the large number of basic amino
feedstocks can bring about deactivation in certain cata- groups present in the dendrimer accelerated the reac-
[
9,10]
lytic reactions.
general, the insertion of dienes to metal complexes is
faster than the insertion of alkenes. Dienes tend to deac- as templates to prepare nanoparticles with well-control-
The challenge in this area is that, in tion.
In a different strategy, dendrimers have been utilized
[
13]
tivate the catalyst by forming p-allyl species that are of- led size, stability and solubility. Crooks and co-work-
ten inactive as catalysts for hydroformylation. For the ers have used generations G4, G6 and G8 of hydroxy-
same reaction, catalyzed by rhodium complexes, the terminated PAMAM to prepare Pd nanoparticles which
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Pumza P. Zweni, Howard Alper
were studied as catalysts for the hydrogenation of allyl Results and Discussion
alcohol and N-isopropylacrylamide. For the latter sub-
strate, there was a strong dependency between the hy- The dendrimer-Pd complexes 1–6 were synthesized fol-
drogenation activity and the dendrimer generation. G6 lowing the procedures reported in an earlier publica-
[
7]
and G8 gave TOFs that were 10 and 5% that of G4. Allyl tion. Our goal was to carry out the hydrogenation reac-
alcohols with substituents of different sizes on the a-po- tions under mild conditions. 1,3-Cyclohexadiene and
sition were catalyzed with the G4-Pd nanoparticles. The complex 1 were used to determine the optimum condi-
activity diminished with the increase in the size of the a- tions for the reaction. Since the solvent usually has an ef-
[
14]
substituent. Kaneda et al. functionalized third to fifth fect on the selectivity of hydrogenation reactions, we be-
generation PPI dendrimers with triethoxybenzamide, gan our investigation by testing different solvents (Ta-
prior to using them as templates for the synthesis of Pd ble 1). Three alcohols were screened, methanol, ethanol
nanoparticles. The catalytic performance of these nano- and isopropyl alcohol. The highest selectivity was ob-
catalysts was investigated towards the hydrogenation of served using methanol, with isopropyl alcohol being
various olefins. The hydrogenation rates were lower the least selective. Activity was highest for isopropyl al-
than those obtained with Pd/C and they decreased in cohol and lowest for methanol. Reasonable selectivity
the order G3>G4>G5 for all substrates. The Pd nano- and good activity was obtained in dichloromethane,
particles encapsulated with G5 dendrimer showed inter- while in acetonitrile and ethyl acetate, lower activities
esting molecular recognition properties. For example, and selectivities were found. Since selectivity took prior-
this system allowed the selective hydrogenation of cer- ity over activity, methanol was used for subsequent reac-
tain substances. Polar substrates reacted faster than tions. Parameters such as temperature, pressure and cat-
non-polar substrates when they were simultaneously alyst loading were also examined.
present in the reaction mixture.
After establishing the optimum conditions, we pro-
The need for more selective hydrogenation catalysts ceeded to examine the catalytic performance of the var-
coupled with the successful application of supported ious dendrimer complexes 2–6. Various spacer lengths
Pd catalysts in hydrogenation reactions prompted us were used within the dendrimer structure with the
to investigate the catalytic performance of our silica- hope that the resulting catalysts would effect different
supported,
palladium-based
dendrimers
1–6 activities. Cyclohexene was the major product for the
(
Scheme 1) for the selective hydrogenation of dienes hydrogenation of the 1,3-cyclohexadiene (Table 2). Cy-
to monoolefins. We wanted to determine if these cata- clohexane and benzene were minor products using some
lysts would display a positive or negative dendritic effect of these complexes. 1,3-Cyclohexadiene is known to dis-
in activity and selectivity. It was our hope to transform proportionate to cyclohexene and benzene in the pres-
the dienes to monoolefins without any deleterious effect ence of Pd particles. Therefore, the presence of benzene
on the monoolefins formed during the reaction. The po- in the product mixture may be an indication of traces of
tential to recycle the dendrimer–palladium catalysts for Pd particles within the catalyst matrix.
the reaction is an important additional feature, should
this be possible.
The G0 complex, 1 showed the highest activity with
76% selectivity to cyclohexene (Table 2, entry 1). The
second generation C12 catalyst, 6 gave complete conver-
sion after 5 hours (Table 2, entry 11), while the other cat-
Scheme 1. Dendritic catalyst.
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Selective Hydrogenation of Dienes to Monoolefins
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Table 1. Effect of solvent on the hydrogenation of 1,3-cyclohexadiene.
Reaction conditions: 5.25 mmol 1,3-cyclohexadine, 10 mmol Pd catalyst 1, 5 mL
MeOH, pressurized glass autoclave to 14 psi H , 258C. GC and NMR were used
2
to monitor the reaction.
Table 2. Hydrogenation of cyclohexadiene.
Reaction conditions: 5.25 mmol 1,3-cyclohexadiene, 10 mmol Pd catalyst, 5 mL
MeOH, pressurized glass autoclave to 14 psi H , 258C. GC and NMR were used
2
to monitor the reaction.
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Pumza P. Zweni, Howard Alper
Table 3. Hydrogenation of ethyl sorbate.
Reaction conditions: 5.4 mol ethyl sorbate, 10 mmol Pd catalyst , 5 mL MeOH, pressurized glass autoclave to
1
4 psi H , 258C. GC and NMR were used to monitor the reaction.
2
Table 4. Recycling of complex 1 (G0).
Reaction conditions: 5.4 mol ethyl sorbate, 10 mmol Pd catalyst 1, 5 mL MeOH, pressur-
ized glass autoclave to 14 psi H , 258 GC and NMR were used to monitor the reaction.
2
alysts required 20 hours (Table 2, entries 3, 5, 7 and 9). a reaction perspective, one can achieve excellent
Cyclohexene was the only product for all of the catalysts results.
at conversions below 30%. At higher conversions, the
The hydrogenation of ethyl sorbate catalyzed by
alkene formed began to compete with the diene for {[(t-Bu) PH]Pd[P(t-Bu) ]} pre-activated with O gave
2
2
2
2
[15]
the active site. Complexes 3 and 5 of the C6 series 70% methyl 2-hexenoate and 30% methyl hexanoate.
gave similar activity, but the former afforded better se- The dendrimer complex 4 was ineffective in catalyzing
lectivity. For the C12 series, the first generation catalyst the hydrogenation of ethyl sorbate. All the other den-
is less active than the second generation. The question as drimer-Pd complex catalysts displayed comparable se-
to what is responsible for this difference in activity and lectivity, i.e., there was no significant sensitivity to cata-
selectivity is not easy to answer. Nevertheless, from lyst structure for this substrate (Table 3).
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Table 5. Recycling of complex 6 (G2-C12).
Reaction conditions: 5.4 mol ethyl sorbate, 10 mmol Pd catalyst , 5 mL MeOH, pres-
surized glass autoclave to 14 psi H , 258C. GC and NMR were used to monitor the
2
reaction.
Table 6. Hydrogenation of acyclic dienes.
Standard reaction conditions: 5.5 mmol substrate, 10 mmol Pd, 5 mL methanol.
Pressurized glass autoclave to 14 psi H , 258C. GC and NMR were used to moni-
2
tor the reaction. Number in parenthesis refers to trans to cis ratio.
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Pumza P. Zweni, Howard Alper
Table 7. Hydrogenation of cyclic dienes.
Standard reaction conditions: 5.25 mmol substrate, 10 mmol Pd complex 1, 5 mL me-
thanol. Pressurized glass autoclave to 14 psi H , 258C. GC and NMR were used to
2
monitor the reaction.
Since complex 6 showed better activity than the other to the more substituted double bond. Up to 81% selec-
complexes for the hydrogenation of cyclohexadiene, it tivity was obtained with complex 3.
was chosen for an experiment where the recycle proper-
In Table 7 are shown the results for the hydrogenation
ties of the catalyst were tested for the hydrogenation of of cyclic dienes using the complex 1. 1-Methyl-1,4-cyclo-
ethyl sorbate. When compared to the G0 catalyst 1, the hexadiene and 1,2,3,4,5-pentamethyl-1,3-pentadiene
G2 complex 6 retained activity and selectivity for up to were transformed to their corresponding monoolefins
eight runs while 1 began to show a decrease after three in selectivities of 83% and 78%, respectively. Non-con-
runs (Tables 4 and 5). Complex 6 was therefore shown jugated 1,4-cyclohexadiene was converted to cyclohex-
to be an excellent catalyst for the hydrogenation of ethyl ene in 68% selectivity.
sorbate, with good selectivity for the g,d-double bond.
1,3-Cycloheptadiene has been selectively reduced to
For the partialreduction of acyclicdienes, the catalytic cycloheptene using SmI /H O/amine mixtures in THF
2
2
[
16]
activity decreased with the augmentation of the genera- in >99% yield.
At least 2.5 equivalents of SmI2
tion number (Table 6). The least substituted double were required with the reaction operating through a rad-
bond was selectively reduced during the hydrogenation, ical mechanism. When complex 1 was used, 92% of cy-
e.g., 2-methyl-1-phenyl-1,3-butadiene afforded 2-meth- cloheptene along with 8% of cycloheptane was ob-
yl-1-phenyl-1-butene in 52–67% selectivity using com- tained. 1,3-Cyclooctadiene was reduced with 98% selec-
plexes 1, 3, and 5. 2,4-Dimethyl-1,3-pentadiene gave al- tivity towards the monoolefin, which is comparable to
most equal amounts of the two monoene isomers, 2,4-di- the result obtained using {[(t-Bu) PH]Pd[P(t-Bu) ]}
2
2
2
[
15]
methyl-1-pentene and 2,4-dimethyl-2-pentene. For the pre-activated with O2. The cyclic conjugated dienes
partial hydrogenation of 3-methyl-1,3-hexadiene, the showed an increase in selectivity in the order C <C <
6
7
least substituted double bond was reduced in preference C with the complex 1.
8
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Selective Hydrogenation of Dienes to Monoolefins
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ration will preferentially bind to the metal. After mono-
ene expulsion, the PdꢀH species can be regenerated.
Conclusions
The results described herein show that silica-supported
PAMAM-palladium complexes can catalyze the hydro-
genation of conjugated dienes to olefins in good to ex-
cellent selectivity. The higher generation catalysts also
display good recycle properties; they could be reused
up to eight times without loss in selectivity. This method
is superior than the one reported by Dupont as it em-
ploys milder reaction conditions.
Experimental Section
Scheme 2. Proposed mechanism 1.
General Procedure for the Selective Hydrogenation of
Dienes
The diene substrate (5.5 mmol) in methanol (5 ml) was added to
1
0 mmol (referred to Pd) of the dendrimer-Pd complex in a glass
autoclave. The reaction mixture was purged three times with H2
before pressurizing to 14 psig with H . The autoclave was stirred
2
at 258C for the amount of time specified in the Results and Dis-
cussion section. The liquid phase was passed through a pad of
silica and then analyzed. Catalyst recycle was done after the cat-
alyst was washed with methanol and ether.
References
[
[
[
[
[
[
1] S. C. Bourque, F. Maltais, W.-J. Xiao, O. Tardif, H. Alper,
P. Arya, L. E. Manzer, J. Am. Chem. Soc. 1999, 121, 3035.
2] P. Arya, N. V. Rao, J. Singkhonrat, H. Alper, S. C. Bour-
que, L. E. Manzer, J. Org. Chem. 2000, 65, 1881.
3] S. C. Bourque, H. Alper, L. E. Manzer, P. Arya, J. Am.
Chem. Soc. 2000, 122, 956.
4] H. Alper, P. Arya, S. C. Bourque, G. R. Jefferson, L. E.
Manzer, Can. J. Chem. 2000, 78, 920.
5] S. Antebi, P. Arya, L. E. Manzer, H. Alper, J. Org. Chem.
2002, 67, 6623.
Scheme 3. Proposed mechanism 2.
Proposed Mechanism
6] J. P. K. Reynhardt, H. Alper, J. Org. Chem. 2003, 68,
8
353.
Two mechanisms are proposed for this reaction. Both in-
[
[
[
7] P. Zweni, H. Alper, Adv. Synth. Catal. 2994, 346, 849.
8] P. Kukula, L. Cerven y´ , App. Catal. A 1999, 177, 79.
9] P. van Leeuwen, App. Catal. A 2001, 212, 61.
[10] P. van Leeuwen, Angew. Chem. Int. Ed. 2003, 42, 4665.
11] J. Dupont, P. A. Suarez, A. P. Umpierre, R. F. J. de Sou-
za, J. Braz. Chem. Soc. 2000, 11.
12] T. Mizugaki, M. Ooe, K. Ebitani, K. Kaneda, J. Mol. Cat.
A: Chem. 1999, 145, 329.
13] R. M. Crooks, M. Zhao, L. Sun, V. Chechik, L. K. Yeung,
Acc. Chem. Res. 2001, 34, 181.
[14] M. Ooe, M. Murata, T. Mizugaki, K. Ebitani, K. Kaneda,
volve the initial generation of a PdꢀH species from the
reaction of the dendrimer complex with H through ox-
2
idative addition. In the first mechanism (Scheme 2), the
4
diene coordinates to the metal complex in a h -fashion.
[
[
[
After the transfer of hydrogen, a Pd-allyl species is
formed. It is at this stage that different alkene isomers
can be formed because the allyl-Pd species can lead to
double bond isomerization. The elimination of the mon-
oene is accompanied by the formation of Pd(0), which
upon reaction with H regenerates the active PdꢀH spe-
2
cies. In the second mechanism (Scheme 3), the diene co-
Nano Lett. 2002, 2, 999.
15] I. S. Cho, H. Alper, Tetrahedron Lett. 1995, 36, 5673.
steric bulk of the diene is a major factor in determining [16] A. Dahl e´ n, G. Hilmersson, Tetrahedron Lett. 2003, 44,
2
ordinates to the metal complex in an h -mode. Here the
[
which double bond should coordinate to the metal cen-
tre. Generally, the less substituted or least bulky unsatu-
2661.
Adv. Synth. Catal. 2006, 348, 725 – 731
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