Dendritic catalysts for asymmetric transfer hydrogenation

Article abstract of DOI:10.1039/b104160f

The synthesis of chiral diamine based dendritic ligands and their ruthenium complex catalysed asymmetric transfer hydrogenation is described.

Full text of DOI:10.1039/b104160f

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Dendritic catalysts for asymmetric transfer hydrogenation†
Ying-Chun Chen,a Tong-Fei Wu,a Jin-Gen Deng,*a Hui Liu,a Yao-Zhong Jiang,a Michael C. K.
Choib and Albert S. C. Chanb
a Union Laboratory of Asymmetric Synthesis, Chengdu Institute of Organic Chemistry, the Chinese
Academy of Sciences, Chengdu 610041, China. E-mail: jgdeng@cioc.ac.cn
b Open Laboratory of Chirotechnology and Department of Applied Biology and Chemical Technology,
The Hong Kong Polytechnic University, Hong Kong, China
Received (in Cambridge, UK) 10th May 2001, Accepted 27th June 2001
First published as an Advance Article on the web 26th July 2001
The synthesis of chiral diamine based dendritic ligands and
their ruthenium complex catalysed asymmetric transfer
hydrogenation is described.
Since the pioneer works reported in 1994,1 dendritic catalysts
with well-defined nanostructures have triggered increasing
attention because in principle they have the potential to combine
the advantages of homogeneous and heterogeneous catalysts in
one system.2 Although a number of dendritic catalysts have
been described,3 so far, relatively few reports on catalytic
asymmetric synthesis are available.4 Also, chiral ligands and
metal catalysts are very expensive and the finding of the
recyclable catalysts becomes increasingly important.
Noyori et al. discovered an excellent ligand, (S,S)-N-(p-
tolylsulfonyl)-1,2-diphenylethylenediamine [(S,S)-TsDPEN],
for the ruthenium catalysed transfer hydrogenation reactions.5
The chiral DPEN is available in our laboratory on the kilogram
scale. Moreover, the polymeric catalysts, in which the chiral
catalyst (entries 5 and 4). Thus, we refer the relatively robust
TsDPEN was incorporated in a polymeric matrix or bound to
the insoluble polymers, were reported to be detrimental to the
catalytic activity and selectivity in varying degrees.6 Herein we
report the first use of soluble and recyclable dendritic catalysts
for the enantioselective transfer hydrogenation of prochiral
ketones. The amino-functionalised chiral ligand (S,S)-1 was
synthesized in three steps from (S,S)-DPEN, and subsequent
condensation with Fréchet’s polyether dendritic wedges7 and
the final deprotection of the Boc-group readily gave the
dendritic chiral ligands.‡
activity of the dendritic catalysts to the “dendrimer effects” on
stability of the catalytically active complex on the dendron,
which had been observed in bis(m-oxo)dicopper species toward
oxidative self-decomposition.8
In conclusion, various generations of chiral diamine based
dendritic catalysts encapsulated within the matrix have been
synthesized and demonstrated good recyclable catalytic activity
and enantioselectivity in transfer hydrogenation of an aromatic
ketone. Current work is aiming at a detailed insight of the nature
Asymmetric transfer hydrogenation reactions were studied
using acetophenone as the model substrate.§ Compared to the
monomer Ru[(S,S)-2] complex, a slightly enhanced reactivity
was observed for the dendritic catalysts with the high enantiose-
lectivity ( > 96% ee), in which the first and third generation
catalysts possess higher reactivity (Table 1).
Table 1 Dendritic TsDPEN–Ru(II) complex catalysed asymmetric transfer
hydrogenation of acetophenonea
Another unique feature of these dendritic catalysts was that
they completely maintained the enantioselectivity with only
slight loss of activity in successive use. Using the third and
fourth generation catalysts Ru[(S,S)-5] and Ru[(S,S)-6], (S)-
1-phenylethanol was formed after ~ 30 h for the fourth use with
88, 85% conversions and 96.4, 96.7% ee, respectively (entries 8
and 12), and high enantioseletivities ( > 95% ee) remained even
for the fifth use (entries 9 and 13). Further addition of
[RuCl2(cymene)]2 into the Ru[(S,S)-6] complex could not
regain the reactivity and selectivity (entry 14), and subsequent
TLC analysis of the recovered catalyst confirmed that this
mostly resulted from the decomposition of the dendritic ligand.
For the heterogeneous polymer immobilised catalysts,6 the
reactivity has been mostly lost after the third use. Moreover, the
fourth generation catalyst was more active than the third one for
the fifth use with 73 vs. 52% conversions, respectively (entries
13 and 9), although it is less active than the third generation
Entry
Ligand
t/h
Conv.b (%) TOFc/h21 Eed (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
2
20
20
20
20
20
20
25
31
40
20
25
30
40
40
95
> 99
98
8.6
11.9
9.5
11.0
9.6
—
96.5
96.6
96.5
96.5
96.5
96.7
96.7
96.4
95.0
96.6
96.8
96.7
96.3
87.0
3
4
5
> 99
98
93
6
5 (2nd use)e
5 (3rd use)e
5 (4th use)e
5 (5th use)e
6 (2nd use)e
6 (3rd use)e
6 (4th use)e
6 (5th use)e
6 (6th use)e,f
86
—
88
—
52
—
92
—
87
—
85
—
73
—
52
—
a (S)-Alcohol was obtained. b Based on GC and 1H NMR analysis.
c Average turn-over frequency calculated over the 5 h reaction time.
d Determined by GC with a Chrompack CP Chrasil-dex column (25 mm 3
0.25 mm). e Recovered catalyst was used. f Additional [RuCl2(cymene)]2
was supplemented.
† Electronic supplementary information (ESI) available: synthesis details,
recycling procedure and
a graph of time-dependent conversion of
acetophenone by the catalysts. See http://www.rsc.org/suppdata/cc/b1/
b104160f/
1488
Chem. Commun., 2001, 1488–1489
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104160f

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of the dendritic wedge stabilizing effect on the catalyst and the
exploration of these catalysts in other asymmetric transfer
hydrogenation reactions.
We are grateful for the financial support of the National
Science Fund for Distinguished Young Scholars of China (No.
20025205) and The Hong Kong Polytechnic University, ASD
Fund.
Notes and references
‡ ESI-HRMS data for the dendritic ligands: (S,S)-3 calcd for C41H37N3O5S
683.2454, found 684.2429 [M + H]+; (S,S)-4 calcd for C69H61N3O9S
1107.4129, found 1108.4179 [M + H]+; (S,S)-5 calcd for C125H109N3O17
S
1955.7478, found 1956.7929 [M + H]+; (S,S)-6 (MALDI-TOF-MS) calcd
for C237H205N3O33S + Na 3675.41, found 3676.03.
§ General procedure: 1 mol% catalyst was prepared in situ by mixing 2
equivalents of triethylamine, a dendritic ligand and [RuCl2(cymene)]2
(1.1+0.5 molar ratios) in a 2 M DCM solution for 1 h under argon at 28 °C.
Then an acetophenone and formic acid–triethylamine azeotrope (0.5 ml per
mmol ketone) was added and the mixture was stirred at 28 °C.
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