Dendronized poly(Ru-BINAP) complexes: Highly effective and easily recyclable catalysts for asymmetric hydrogenation

Article abstract of DOI:10.1002/adsc.200404162

A new kind of dendronized polymeric chiral BINAP ligands has been synthesized and applied to the Ru-catalyzed asymmetric hydrogenation of simple aryl ketones and 2-arylacrylic acids. These dendronized poly(Ru-BINAP) catalysts exhibited high catalytic activity and enantioselectivity, very similar to those obtained with the corresponding parent Ru(BINAP) and the Ru(BINAP)-cored dendrimers. It was found that the pendant dendrons had a major impact on the solubility and the catalytic properties of the polymeric ligands. These polymeric catalysts could be easily recovered from the reaction solution by using solvent precipitation, and the reused catalyst showed no loss of activity or enantioselectivity.

Full text of DOI:10.1002/adsc.200404162

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Dendronized Poly(Ru-BINAP) Complexes: Highly Effective and
Easily Recyclable Catalysts For Asymmetric Hydrogenation
Guo-Jun Deng, Bing Yi, Yi-Yong Huang, Wei-Jun Tang, Yan-Mei He,
Qing-Hua Fan*
Laboratory of Chemical Biology, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences,
100080, Beijing, P. R. China
Fax: (þ86)-10-6255-4449, e-mail: fanqh@iccas.ac.cn
Received: June 7, 2004; Accepted: September 15, 2004
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de.
Abstract: A new kind of dendronized polymeric chi-
ral BINAP ligands has been synthesized and applied
to the Ru-catalyzed asymmetric hydrogenation of
simple aryl ketones and 2-arylacrylic acids. These
dendronized poly(Ru-BINAP) catalysts exhibited
high catalytic activity and enantioselectivity, very
similar to those obtained with the corresponding pa-
rent Ru(BINAP) and the Ru(BINAP)-cored den-
drimers. It was found that the pendant dendrons
had a major impact on the solubility and the catalytic
properties of the polymeric ligands. These polymeric
catalysts could be easily recovered from the reaction
solution by using solvent precipitation, and the
reused catalyst showed no loss of activity or enantio-
selectivity.
ried out in a homogeneous manner (one phase) under
appropriate reaction conditions. Meanwhile, the immo-
bilized catalysts can be separated from the reaction mix-
ture afterwards by using a solvent precipitation method
(two-phase).[5a] From a practicalpoint of view, however,
there are some drawbacks existing in these two systems.
For soluble polymer-supported catalysts, the flexibility
of linear polymeric catalysts may result in their mutual
entangling or self-twisting,[7,8a] shielding their active cen-
ters and lowering their catalytic properties. Moreover,
their catalytic sites are randomly oriented among the
support and the microenvironment around the active
site is difficult to fine tune. In the case of dendrimer cat-
alysts,[8] the syntheses of the dendrimer, particularly a
higher generation dendrimer which can be removed
easily from the reaction mixture by using solvent precip-
itation or nanofiltration, are difficult and time-consum-
ing processes. It is thus fundamentally interesting and
practically very useful to develop new kinds of soluble
chiral macromolecular catalysts that have the potential
to combine the advantages of both chiraldendrimer cat-
alysts and soluble linear chiral polymeric catalysts.
One of the new classes of macromolecules, namely
Keywords: asymmetric catalysis; dendrimers; hydro-
genation; immobilization; soluble polymer; phos-
phane ligands
Homogeneous asymmetric catalysis is one of the most dendronized polymers or dendrimers with a polymeric
important developments in modern chemistry over the core, has been recently developed.[9] These rather large
[1]
past severaldecades.
Transition metalcompelxes
molecules possess a high degree of structural rigidity
with diphosphine ligands, such as BINAP, have proved due to steric congestion around the linear polymeric
to be excellent homogeneous catalysts in various asym- core, which could turn into a molecular cylinder with de-
metric hydrogenation reactions.[2] However, a major fined dimensions. These dendritic rods can thus be used
problem associated with most homogeneous catalyst as ideal scaffolds for the immobilization of chiral cata-
systems is the separation and recycling of the expensive lysts, yielding structurally well-defined recyclable cata-
chiralcatalyst. Today, different approaches to the heter-
ogenization of chiralcataylsts have been reviewed.
lysts. They may bridge the gap between homogeneous
and heterogeneous catalysis. However, in sharp contrast
[3,4]
Unfortunately, most of the insoluble polymer-supported to random-coil shaped linear polymer catalysts[5] and
chiral catalysts often suffered from much low catalytic spherically shaped dendrimer catalysts,[10] so far very
activity and enantioselectivity. Alternatively, two new few examples of dendronized polymer catalysts have
strategies – soluble polymer-supported catalysts[5] and been reported.[11,12]
chiraldendrimer catayl sts [6] – have recently been high-
Recently, we have developed several kinds of soluble
lighted as new options for the immobilization of chiral linear chiral polymer catalysts and chiral dendritic cata-
diphosphine-containing catalysts. The unique feature lysts for asymmetric reactions.[13] As an extension of our
of both strategies is that the catalytic reactions are car- research program aiming at developing practical immo-
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bilized chiral catalysts, we reported here the synthesis of tations of (R)-4a and (R)-4b were found to be [a]D ¼ þ
dendronized polyamides with BINAP incorporation 72 (c 0.50, THF) and [a]D ¼ þ48 (c 0.50, THF), respec-
into the main chain and their application to asymmetric tively, indicating that the whole helical conformation
31
hydrogenation. To the best of our knowledge, this is the of the polymer did not form.[16] The P NMR spectra
first example of a dendrimer with a catalytically active of both ligands displayed singlet signals at about d¼ ꢀ
chiral polymer core. Such novel dendritic catalysts pro- 15, which were consistent with the BINAP. A strong ab-
vided severalkey advantages:
sorption of (R)-4a at n¼1664 cmꢀ1 and (R)-4b n¼
1678 cmꢀ1 in the IR spectra indicated the existence of
¼
a) The dendrons serve as excellent solubilizers due to C O bonds.
their highly branched structure.
In order to evaluate the efficiency of these dendron-
b) Unlike coil-shaped linear polymers, these rod-shap- ized poly-BINAP ligands, the asymmetric hydrogena-
ed polymers provide more reliable and easy separa- tion of simple aryl ketones was first chosen as the model
tion and recycling of the catalyst via solvent precip- reaction. The ternary catalyst system of Ru-chiral di-
itation or nano-filtration techniques.
phosphine-chiraldiamine-KOH, which was described
c) The pendant dendrons envelope and protect the by Noyori, has been found to be the most effective cata-
catalytically active polymer rods from aggregation lytic system for asymmetric hydrogenation of simple ke-
and degradation.
tones.[17] Recently, we have successfully described the
d) It is possible to fine-tune the catalytic properties of use of BINAP-cored dendrimers for the highly effective
the polymer core through the adjustment of the hydrogenation of simple ketones.[13g] According to our
size, structure and solubility of the pendant den- previous study,[13g] the dendronized polymer catalysts,
drons.
[RuCl2(R-4)-(1R, 2R)-dpen], were prepared in situ and
e) In comparison with the monodispersed dendrimers, applied to the asymmetric hydrogenation without fur-
dendronized polymers with polydispersity can be ther purification. A 2-propanol-THF mixture was chos-
prepared with relatively less difficulty.
en as the solvent because the polymeric catalysts are in-
soluble in neat 2-propanol. The preliminary results are
Consequently, these dendronized polymer catalysts summarized in Table 1. Both dendronized polymers
might combine the advantages of soluble polymeric were found to be active catalysts in the asymmetric hy-
and dendritic catalysts: high catalytic activity and enan- drogenation of simple ketones and gave high enantiose-
tioselectivity, tailor-made catalyst, facile separation and lectivity (up to 92% ee), which was only slightly lower
reuse of the catalyst.
than that of the Ru-BINAP catalyst. It was particularly
The dendronized poly(BINAP) ligands were synthe- noted that a higher enantioselectivity was obtained
sized as shown in Scheme 1. (R)-5,5’-Diamino-BINAP when the generation of the pendant dendron was in-
(R-1) was firstly synthesized according to the published creased from 2 to 3. For example, in the asymmetric hy-
method.[14] A polyether dendrimer was chosen due to its drogenation of 1-acetonaphthone, Ru(R-4b) gave 92%
inertness to catalytic reactions. The second- and third- ee, while Ru(R-4a) afforded 88% ee (Table 1, entries 5
generation dendritic bromides (2)[15] were reacted with and 6).
the methyl ester of 5-hydroxyisophthalic acid followed
The catalytic efficiency of the dendronized poly(Ru-
by hydrolysis to give the corresponding dendronized di- BINAP) catalytic system was further demonstrated in
carboxylic acid monomers (3). Subsequent direct con- the asymmetric hydrogenation of 2-arylacrylic acids.
densation of 3 (3a and 3b) with R-1 in the presence of tri- The Ru catalyst was prepared by mixing [Ru(benze-
phenylphosphite, pyridine, and caclium chol ride in N- ne)Cl2]2 and the appropriate polymeric ligand in situ in
methyl-2-pyrrolidone (NMP) at 1108C gave the hot DMF.[13f] The preliminary experimental data are
dendronized poly(BINAP)s (4a and 4b) in more than also collected in Table 1. High enantioselectivities
90% yield. Unlike common linear polyamides, these were obtained in the asymmetric hydrogenation of 2-
dendronized rigid polymers are well soluble in most [p-(2-methylpropyl)phenyl]acrylic acid and 2-phenyl-
common organic solvents such as THF, toluene, and di- acrylic acid (Table 1, entries 7–10), which were compa-
chloromethane, but they are insoluble in methanol. This rable to those with Ru(BINAP) under otherwise identi-
different solubility provides a convenient and reliable calreaction conditions. It was found that the size of the
method for the purification and characterization of the pendant dendrons also slightly influenced the enantiose-
polymeric ligands. These ligands were thus purified by lectivity of the polymeric catalysts.
a precipitation method and characterized by common
An important feature of the design of dendronized
spectraltechniques such as H, 31P NMR and IR spectro- polymeric catalysts is the easy and reliable separation
scopy, and gelpermeation chromatography (GPC). of the chiral catalyst based on its rather large molecular
1
The molecular weights of (R)-4a and (R)-4b were size with a rigid cylinder structure and different solubil-
found to be Mw ¼33449, Mn¼20500 (PDI¼1.63) and ities in various organic solvents. In this study, for exam-
Mw¼39014, Mn¼26070 (PDI¼1.49) by using polystyr- ple, Ru(R-4b) with the third generation pendant den-
ene as the standard, respectively. The specific optical ro- drons was used for the recycling experiments. The re-
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Guo-Jun Deng et al.
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Scheme 1. Synthesis of dendronized poly(BINAP)s.
Table 1. Dendronized Poly(Ru-BINAP)-catalyzed asymmetric hydrogenation of simple aryl ketones and 2-arylacrylic acids.
Entry
Substrate
Ligand
ee [%][a]
1[b]
2[b]
3[c]
acetophenone
acetophenone
2-methylacetophenone
2-methylacetophenone
1-acetonaphthone
R-4a
R-4b
R-4a
R-4b
R-4a
R-4b
R-4a
R-4b
R-4a
R-4b
73
75
88
92
88
92
82
83
80
82
4[c]
5[c]
6[c]
1-acetonaphthone
7[d]
8[d]
9[d]
10[d]
2-[p-(2-methylpropyl)phenyl]acrylic acid
2-[p-(2-methylpropyl)phenyl]acrylic acid
2-phenylacrylic acid
2-phenylacrylic acid
[a]
Determined by chiral GC analysis. The absolute configuration of the product of ketone is R, while hydrogenation of 2-ar-
ylacrylic acid gave S product. All catalytic reactions reached 100% conversion in given reaction time.
[b]
Reactions were carried out at room temperature using 0.6 mmolof substrate in 5 mL 2-propano/lTHF ¼1: 3 (v/v); sub-
strate:ligand:Ru:diamine:t-C4H9OK¼500: 1.1: 1: 1: 4 (molratio); H pressure¼40 atm; reaction time¼20 h.
2
[c]
[d]
Reaction conditions were the same as [b] except that the reaction temperature was 508C.
Reactions were carried out at room temperature using 0.1 mmolof substrate in 4 mL of methano/lTHF ¼1: 3 (v/v); sub-
strate/catalyst¼100 (molratio); NEt /substrate¼3: 2 (mol/mol); H2 pressure¼50 atm; reaction time¼12 h.
3
sults are shown in Table 2. Upon completion of the reac-
tion, methanolwas added to the reaction mixture and
In summary, we have developed the first optically ac-
tive dendronized polymeric BINAP ligands as a new
the catalyst was quantitatively precipitated and recov- type of macromolecular chiral catalyst for asymmetric
ered via filtration. The recovered catalyst was reused hydrogenation, which were easily synthesized by con-
for at least three cycles in the asymmetric hydrogenation densation of 5,5’-diamino-BINAP with dendritic dicar-
of 2-methylacetophenone with similar enantioselectivi- boxylic acid monomers. These polymeric Ru(BINAP)
ty (entries 3–6). These results are better than those ob- catalysts exhibited high catalytic activity and enantiose-
tained with the Ru(BINAP)-cored dendrimers under lectivity very similar to those obtained with the corre-
otherwise identicalreactions conditions. [13 g]
sponding parent Ru(BINAP) and the Ru(BINAP)-cor-
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Table 2. Dendronized poly(Ru-BINAP) R-4b-catalyzed asymmetric hydrogenation of 2-arylacrylic acid and aryl ketones: cat-
alyst recycling.
Entry
Substrates
ee [%][a]
1 (run 1)[b]
2 (run 2)[b]
3 (run 1)[c]
4 (run 2)[c]
5 (run 3)[c]
6 (run 4)[c]
2-[p-(2-methylpropyl)phenyl]arylic acid
2-[p-(2-methylpropyl)phenyl]arylic acid
2-methylacetophenone
2-methylacetophenone
2-methylacetophenone
82
82
92
92
92
91
2-methylacetophenone
[a]
Determined by chiral GC analysis. All catalytic reactions reached 100% conversion in the given reaction time;
[b]
Reactions were carried out at room temperature using 0.1 mmolof substrate in 4 mL of methano/lTHF ¼1: 3 (v/v); sub-
strate/catalyst¼100 (molratio); NEt 3/substrate¼3: 2 (mol/mol); H2 pressure¼50 atm; reaction time¼12 h. Upon comple-
tion of the reaction, most of the solvent was removed under reduced pressure, and dry methanol was then added to preci-
pitate the polymeric catalyst (1H NMR analysis indicated that there was not any polymeric catalyst in the methanol layer).
Reactions were carried out at 508C using 0.6 mmolof arylketones in 5 mL 2-propano/lTHF ¼1: 3 (v/v); substrate :li-
[c]
gand :Ru :diamine :t-C4H9OK¼500: 1.1: 1: 1: 4 (molratio); H pressure¼40 atm; reaction time¼20 h.
2
ꢀ
ꢀ
ꢀ
6.55 (m, 9H, Ar H), 4.99–4.87 (m, 14H, O CH2 Ph);
31P NMR (CDCl3, 125 MHz): d¼ ꢀ15.0 (s). GPC analysis
shows that its molecular weight is Mw ¼33449 and Mn¼
20500 (PDI¼1.63).
ed dendrimers. It was found that the pendant dendrons
have a major impact on the solubility and the catalytic
properties of the polymeric catalysts. These immobi-
lized catalysts could be easily recovered from the reac-
tion solution by using solvent precipitation. This study
opens up a new frontier for the development of highly
effective and easily recoverable chiral catalysts, which
will have potential not only for laboratory-scale re-
search but also for industrial applications. Current
work is aimed at other applications of this kind of
dendronized polymeric catalyst.
Compound 4b
Prepared according to the above procedure, the residue was
purified by solvent precipitation; yield: 95%. IR (KBr): n¼
3060, 2927, 2870, 1678, 1594, 1450, 1372, 1297, 1154, 1048,
832, 741, 696 cmꢀ1; [a]D25:þ48 (c 0.5, THF); 1H NMR (CDCl3,
ꢀ
ꢀ
ꢀ
500 MHz): d¼8.31–7.82 (m, 7H, CO NH Ar and Ar H),
ꢀ
ꢀ
7.54–6.88 (m, 68H, Ar H), 6.75–6.48 (m, 21H, Ar H),
5.00–4.74(m, 30H, O CH2 Ph); 31P NMR (CDCl3,
125 MHz): d¼ ꢀ15.3 (s). GPC analysis shows that its mo-
lecular weight is Mw ¼39014 and Mn¼26070 (PDI¼1.49).
ꢀ
ꢀ
Experimental Section
Materials
All moisture- or air-sensitive experiments were carried out un-
der a nitrogen atmosphere using standard Schlenk techniques.
Commercialreagents were used as received without further
purification unless otherwise noted. Methanol and 2-isopropa-
nol were distilled from magnesium and THF from sodium ben-
zophenone ketyl. All aryl ketones were distilled before use.
(R)-5,5’-Diamino-BINAP (R)-1 was prepared according to
the reported procedures.[14]
Acknowledgements
We thank National Natural Science Foundation of China (No
20132010, 203900507 and 20325209), Chinese Academy of Sci-
ences and the Major State Basic Research Development Pro-
gram of China (Grant No. 2002CCA03100) for financial sup-
port of this study.
Compound 4a
References and Notes
To a suspension of 3a (435 mg, 0.50 mmol), 5,5’-diamino-BI-
NAP (326 mg, 0.50 mmol) and anhydrous calcium chloride
(200 mg) in N-methyl-2-pyrrolidone (NMP, 3 mL) under nitro-
gen was added triphenylphosphite (0.20 mL). The reaction
mixture was stirred at 1008C for 24 h. After cooling to room
temperature, the reaction solution was poured into 20 mL of
methanol. The precipitate was collected by filtration, washed
with methanol, and dried under vacuum to give R-4a; yield:
700 mg (90%). IR (KBr): n¼3061, 2931, 2877, 1664, 1596,
1451, 1373, 1298, 1154, 1054, 743, 697 cmꢀ1; [a]D25:þ72 (c 0.5,
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