Novel dendritic ligands of chiral 1,2-diamine and their application in asymmetric hydrogenation of simple aryl ketones

Article abstract of DOI:10.1055/s-2005-869862

Novel dendritic chiral vicinal diamine ligands have been synthesized and a series of dendritic Ru(BINAP)(diamine) catalysts were developed for asymmetric hydrogenation of a variety of simple aryl ketones in good catalytic activity and high enantioselectiv

Full text of DOI:10.1055/s-2005-869862

LETTER
1591
Novel Dendritic Ligands of Chiral 1,2-Diamine and Their Application in
Asymmetric Hydrogenation of Simple Aryl Ketones
N
ovel Dendritic Ligand
e
of
C
hiral
1
i
,2-Dia
g
mine uo Liu,a,c Xin Cui,a Linfeng Cun,a Jun Wu,a Jin Zhu,a Jingen Deng,*a,c Qinghua Fanb
a
b
c
Key Laboratory of Asymmetric Synthesis & Chirotechnology of Sichuan Province and Union Laboratory of Asymmetric Synthesis,
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, P. R. China
Laboratory of Chemical Biology, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing,
P. R.China
Graduate School of the Academy of Sciences, Beijing, P. R. China
Fax +86(28)85223978; E-mail: jgdeng@cioc.ac.cn
Received 23 March 2005
As shown in Scheme 1, the synthesis of dendritic diamine
ligands, (S,S)-7a–d were achieved by direct deprotection
of (S,S)-6a–d in 80–96% yields, which were prepared in a
straightforward manner by the coupling of (S,S)-4 with
Abstract: Novel dendritic chiral vicinal diamine ligands have been
synthesized and a series of dendritic Ru(BINAP)(diamine) catalysts
were developed for asymmetric hydrogenation of a variety of
simple aryl ketones in good catalytic activity and high enantioselec-
tivity, as well as facile catalyst recycling. An increase of enantio- the corresponding dendritic bromides 5 in the presence of
selectivities was obtained by using the dendritic catalysts compared
with Noyori catalysts under the same conditions in the case of
several substrates. Meanwhile, a remarkable structural effect on
following Boc protection with 86% yield in two steps
catalytic activity and enantioselectivity, as well as reuse was
observed.
K2CO3 and 18-crown-6 in 56–88% yields. (S,S)-4 was
obtained by deprotection of methoxyl group and the
from (S,S)-2.11 All kinds of dendritic ligands, (S,S)-7a–d
were characterized by NMR (1H and 13C), IR, and HRMS
Key words: chiral 1,2-diamine, dendritic ligand, ruthenium com-
(ESI) or MALDI–TOF.12
plex, asymmetric hydrogenation
In the recent years, the design and synthesis of effective
enantioselective catalysts for hydrogenation of simple ke-
tones have been attracting much attention, and various
As a result of their unique architecture and structural as
strategies have been introduced.13 Among all the chiral
well as functional versatility, dendrimers have generated
catalysts reported today, the ternary catalyst system of
considerable interest in numerous areas of physical sci-
Ru–chiral diphosphine–chiral diamine–KOH,5a has been
found to be the most effective catalysts for the asymmetric
hydrogenation of simple ketones lacking a secondary co-
ordinating functional group. To investigate the efficiency
of 7a–d in the asymmetric hydrogenation, we first needed
to prepare the ternary system (Ru–chiral diphosphine–
chiral diamine–KOH). The ruthenium catalysts were pre-
pared in situ according to the Noyori protocol.5c For ex-
ample, the second generation dendritic pre-catalyst was
prepared by reacting the (S)-BINAP (8) with [RuCl2(ben-
zene)]2 at 100 °C in DMF for 10 minutes, followed by
treatment with 1.0 equivalent of dendritic diamine ligand,
(S,S)-7c at room temperature for 10 hours. The resulting
dendritic catalyst showed a predominant 31P NMR signal
at d = 47.3 ppm, which was very close to the small molec-
ular complex (d = 47.4 ppm).5c The ruthenium complexes
of other dendritic diamine ligands, 7a, 7b and 7d and the
mono ligand, (S,S)-2 were also prepared with the similar
method and used in the catalytic reactions without further
purification.
ences, engineering, as well as the biological sciences.1,2
One intensively studied application has been in the area of
catalysis where metallodendritic compounds appear as a
novel generation of catalysts that have catalytic properties
of homogeneous catalysts but may be easily separated af-
ter use.3 This feature is essential for high reaction efficien-
cy, economical and environmental reasons.
Optically active 1,2-diphenylethylenediamine (DPEN),
(R,R)-1 is one of the most important chiral ligands.4
Noyori and coworkers5,4d found that ruthenium(II) di-
chloride(diphosphine)(DPEN) complexes are effective
catalysts for asymmetric hydrogenation of ketones.6 Re-
cently, polymer-supported and heterogenized ligands of
Noyori catalysts have been reported for the asymmetric
hydrogenation of ketones.7,8 To the best of our knowl-
edge, however, dendrimer-supported DPEN had not been
reported before this research was launched. Herein we re-
port the synthesis of the dendritic ligands, (S,S)-7a–d
based on the functionalization of the benzene rings9 and
the application in the asymmetric hydrogenation of simple
aryl ketones. Moreover, the dendritic ligands, (S,S)-7a–d
can be served as a chiral scaffold on which many types of
dendritic chiral catalysts could be easily built.4,10
Our initial efforts were aimed at probing the asymmetric
induction of these catalysts using 1-acetonaphthone (9) as
a typical substrate. In most cases 2-propanol is the choice
of solvent for the asymmetric hydrogenation of ketones
with the ternary catalyst system. Because the dendritic
catalysts are insoluble in neat 2-propanol and more solu-
ble in toluene, mixed solvents with toluene were first se-
lected. Although slightly higher enantioselectivity was
SYNLETT 2005, No. 10, pp 1591–1595
1
7
.0
6
.2
0
0
5
Advanced online publication: 07.06.2005
DOI: 10.1055/s-2005-869862; Art ID: U08005ST
© Georg Thieme Verlag Stuttgart · New York

1592
W. Liu et al.
LETTER
O
O
O
O
n
RHN
RHN
PPh2
PPh2
O
O
H2N
NH2
8, (R)- or (S)-BINAP
1, (R,R)-DPEN
6–7
n
HO
OH
MeO
OMe
a, b
BocHN
NHBoc
H2N
NH2
2
4
c
O
(S,S)-6a, n = 0, 87%; (S,S)-6b, n = 1, 78%
(S,S)-6c, n = 2, 88%; (S,S)-6d, n = 3, 56%
6 R = Boc
Br
O
d
n
5 n = 0, 1, 2, 3
(S,S)-7a, n = 0, 96%; (S,S)-7b, n = 1, 86%
(S,S)-7c, n = 2, 83%; (S,S)-7d, n = 3, 80%
7 R = H
Scheme 1 Reagents and conditions: a) (i) 1 N BBr3, CH2Cl2, 48 h; (ii) 2 N NaOH, pH = 7–8; b) Boc2O (2.2 equiv), DIPEA (3.0 equiv), THF,
0 °C to r.t., 86% (two steps); c) 5, K2CO3 (2.5 equiv), 18-crown-6 (0.2 equiv), acetone, reflux; d) TFA, CH2Cl2, r.t.
achieved in the mixture of 2-propanol and THF for the hy- core of the dendritic catalyst. This phenomenon had been
drogenation of 9 with 7c as ligand (Table 1, entry 11 vs. observed by other groups.14 When the hydrogen pressure
entries 5 and 10), only 79% ee was obtained for the hydro- was increased to 70 atm, remained enantioselectivity
genation of acetophenone (11) compared with 82% ee in (94% ee) was obtained with complete conversion in 20
2-propanol–toluene (Table 2, entry 3). The 1:1 ratio of 2- hours (entry 14). This shows that higher pressure is favor-
propanol and toluene is the best choice, because increase able for the substrate easily to traverse the third dendritic
and decrease of the ratio lowed both conversion and enan- catalyst’s globular shell.
tioselectivity of the asymmetric hydrogenation due to the
solubility of the dendritic catalyst (Table 1, entry 5 vs. en-
tries 7 and 8). Pure toluene retarded the reaction (entry 9).
With these results in hand, the asymmetric hydrogenation
was extended to other ketone substrates (11–18) by using
the new dendrimer-based catalytic system. As shown in
As expected, the mismatching dendritic catalyst gave poor
Table 2, the dendritic catalysts gave remained even higher
enantioselectivity (9% ee, entry 6). The enantioselectivi-
enantioselectivities compared with both monomeric and
ties of the first and second generation dendritic catalysts
Noyori catalysts for the hydrogenation of aryl ketones un-
are comparable to the enantioselectivities of both mono-
der same conditions. For hydrogenation of 11 catalyzed
by dendritic catalysts, (S)-8–Ru–(S,S)-7c gave the best re-
sult (82% ee), which is comparable to the enantioselectiv-
meric and Noyori catalysts (entries 1–5). However, the
third generation catalyst gave lower enantioselectivity
(84%) and significantly decreased conversion (45%) un-
ities of both monomeric and Noyori catalysts (entries 1–
der the same conditions (entry 12) even at higher temper-
3). Although the monomeric catalyst afforded lower enan-
ature (50 °C, 51% conversion and 88% ee, entry 13). The
tioselectivities than Noyori catalyst for the hydrogenation
sudden loss of the reaction activity of the third generation
of 12 and 17 (entry 5 vs. 6 and entry 25 vs. 26), the same
or higher enantioselectivities were obtained with the den-
dendritic catalyst might be thus attributed to the change in
dendrimer conformation, i.e., from an extended to a more
dritic catalysts (entries 7, 27 and 28). It is notable that hy-
globular structure as the steric requirements of the den-
drogenation of ketones 14 and 18 gave higher
enantioselectivities with the dendritic ligands and the mo-
nomeric ligand (S,S)-2 than with (R,R)-1 (entries 13–16,
and 29–32). These results showed that the electronic ef-
dritic branches increase. Therefore, the globular structure
of the dendrimer resulted in encapsulation of the active
species by the dendrimer, which consequently influenced
the diffusion of the substrate into the catalytically active
Synlett 2005, No. 10, 1591–1595 © Thieme Stuttgart · New York

LETTER
Novel Dendritic Ligands of Chiral 1,2-Diamine
1593
Table 2 Hydrogenation of Aryl Ketones Using the in situ Dendritic
fect of the electron-donating methoxyl group at the para
site of the benzene ring might lead to higher enantioselec-
tivities.
Ru-BINAP Catalystsa
O
O
O
R
Table 1 Hydrogenation of 1-Acetonaphthone Using the in situ Den-
dritic Ru-BINAP Catalystsa
18
17
11: R = H
12: R = o-Me
13: R = o-OMe
14: R = m-OMe
15: R = p-OMe
16: R = p-Br
OH
O
(S)-BINAP RuCl2 + (S,S)-2 or (S,S)-7
H2, 2-PrOH–toluene, t-C4H9OK, 28 °C
Entry
1
Substrate
11
11
11
11
12
12
12
12
13
13
13
13
14
14
14
14
15
15
15
15
16
16
16
16
17
17
17
17
18
18
Ligand
ee (%)b
83c
83
9
10
(R)-8/(R,R)-1
(S)-8/(S,S)-2
(S)-8/(S,S)-7c
(S)-8/(S,S)-7dd
(R)-8/(R,R)-1e
(S)-8/(S,S)-2e
(S)-8/(S,S)-7ae
Entry Ligand
Solvents
(v/v)
Conversion ee (%)b
(%)b
2
1
2
3
4
5
6
7
8
(R)-8/(R,R)-1
2-PrOH–toluene
(1:1)
>99
>99
>99
>99
>99
>99
41
95c
95
96
94
95
9c
3
82
4
77
(S)-8/(S,S)-2
(S)-8/(S,S)-7a
(S)-8/(S,S)-7b
(S)-8/(S,S)-7c
(R)-8/(S,S)-7c
(S)-8/(S,S)-7c
(S)-8/(S,S)-7c
2-PrOH–toluene
(1:1)
5
94c
91
2-PrOH–toluene
(1:1)
6
7
94
2-PrOH–toluene
(1:1)
8
(S)-8/(S,S)-7dd,e
(R)-8/(R,R)-1f
(S)-8/(S,S)-2f
(S)-8/(S,S)-7bf
(S)-8/(S,S)-7dd,f
(R)-8/(R,R)-1
(S)-8/(S,S)-2
(S)-8/(S,S)-7a
(S)-8/(S,S)-7dd
(R)-8/(R,R)-1
(S)-8/(S,S)-2
(S)-8/(S,S)-7c
(S)-8/(S,S)-7dd
(R)-8/(R,R)-1
(S)-8/(S,S)-2
(S)-8/(S,S)-7b
(S)-8/(S,S)-7dd
(R)-8/(R,R)-1
(S)-8/(S,S)-2
(S)-8/(S,S)-7c
(S)-8/(S,S)-7dd
(R)-8/(R,R)-1
(S)-8/(S,S)-2
89
2-PrOH–toluene
(1:1)
9
73c
72
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
2-PrOH–toluene
(1:1)
76
2-PrOH–toluene
(2:1)
90
92
73
76c
87
2-PrOH–toluene
(1:2)
63
87
9
(S)-8/(S,S)-7c
(S)-8/(S,S)-7c
toluene
9
87
93
85
10
2-PrOH–CH2Cl2
(1:1)
>99
84c
83
11
12
(S)-8/(S,S)-7c
(S)-8/(S,S)-7d
2-PrOH–THF (1:1) >99
96
84
2-PrOH–toluene
(4:5)
45
84
13
14
(S)-8/(S,S)-7dd
(S)-8/(S,S)-7de
2-PrOH–toluene
(4:5)
51
88
94
83
59c
59
2-PrOH–toluene
(4:5)
>99
62
a Unless otherwise stated, reaction was carried out at 28 °C by using
0.6 mmol of ketones in 4 mL of solvents for 20 h; 1-acetonaphtho-
ne:BINAP:Ru:dendritic diamine:t-BuOK = 500:1.1:1:1:6 (molar ra-
tio); H2 pressure = 40 atm.
60
74c
72
b Determined by GC on CP-Cyclodex B-236 M column.
c The absolute configuration of the product is S-form.
d Reaction was carried out at 50 °C for 20 h.
e H2 pressure is 70 atm.
76
77
81c
88
Synlett 2005, No. 10, 1591–1595 © Thieme Stuttgart · New York

1594
W. Liu et al.
LETTER
Table 2 Hydrogenation of Aryl Ketones Using the in situ Dendritic
It is notable that an increase of enantioselectivities was
obtained for several substrates by using the dendritic cat-
alysts compared with Noyori catalyst under same condi-
tions, but a light decrease had been reported with the
polymer-supported catalyst8a as well as the dendritic
BINAP catalyst.7f These results demonstrated that the use
of soluble dendrimer-based catalysts might combine the
advantages of homogeneous and heterogeneous catalysis.
In this work, we also found that the electronic properties
of the benzene ring of DPEN greatly affect the enantiose-
lectivity and this will provide a guiding concept for us to
design novel catalytic system for the asymmetric hydro-
genation.17 A further interesting aspect of this study is that
such ligands can be served as a chiral platform on which
many types of chiral dendritic catalysts could be easily
built. Our attention will now turn to this chemistry and
results will be reported in due course.
Ru-BINAP Catalystsa (continued)
O
O
O
R
18
17
11: R = H
12: R = o-Me
13: R = o-OMe
14: R = m-OMe
15: R = p-OMe
16: R = p-Br
Entry
31
Substrate
Ligand
ee (%)b
86
18
18
(S)-8/(S,S)-7c
(S)-8/(S,S)-7dd
32
88
a Unless otherwise stated, reaction was carried out at 28 °C by using
0.6 mmol of ketones in 4 mL 2-PrOH–toluene (v/v, 1:1); aryl ke-
tone:BINAP:Ru:dendritic diamine:t-C4H9OK = 500:1.1:1:1:6 (molar
ratio); H2 pressure = 40 atm. All catalytic reactions reached 100%
conversions in 20 h.
Acknowledgment
b Determined by GC on CP-Cyclodex B-236 M column. Unless other-
wise stated, the absolute configuration of the product is R-form.
c The absolute configuration of the product is S-form.
d Reaction was completed with 2-PrOH–toluene (v/v, 4:5) as solvent
at 70 atm of H2 pressure.
We are grateful for the financial support of the National Natural
Science Foundation of China (No. 203900507, 20025205).
e Reaction was carried out at 50 °C.
References
f Reaction was completed in 48 h at 70 atm of H2 pressure.
(1) For the applications in the fields of nanoscience and
nanotechnology, see: (a) Fréchet, J. M. J. J. Polym. Sci., Part
A: Polym. Chem. 2003, 41, 3713. (b) Fréchet, J. M. J.
Macromol. Symp. 2003, 201, 11. (c) Caminade, A.-M.;
Majoral, J.-P. Acc. Chem. Res. 2004, 37, 341.
(2) For recent reviews on drug- and gene-delivery vectors, see:
(a) Aulenta, F.; Hayes, W.; Rannard, S. Eur. Polym. J. 2003,
39, 1741. (b) Boas, U.; Heegaard, P. M. H. Chem. Soc. Rev.
2004, 33, 43. (c) Haag, R. Angew. Chem. Int. Ed. 2004, 43,
278. (d) Gillies, E. R.; Fréchet, J. M. J. Drug Discov. Today
2005, 10, 35.
(3) For the comprehensive reviews on dendritic catalysts, see:
(a) Oosterom, G. E.; Reek, J. N. H.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M. Angew. Chem. Int. Ed. 2001, 40,
1828. (b) Astruc, D.; Chardac, F. Chem. Rev. 2001, 101,
2991. (c) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.;
Yueng, L. K. Acc. Chem. Res. 2001, 34, 181. (d) Reek, J. N.
H.; de Groot, D.; Oosterom, G. E.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M. Rev. Mol. Biotechnol. 2002, 90, 159.
(e) Twyman, L. J.; King, A. S. H.; Martin, I. K. Chem. Soc.
Rev. 2002, 31, 69. (f) van Heerbeek, R.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M.; Reek, J. N. H. Chem. Rev. 2002, 102,
3717. (g) Caminade, A.-M.; Maraval, V.; Laurent, R.;
Majoral, J.-P. Curr. Org. Chem. 2002, 6, 739. (h) Dahan,
A.; Portnoy, M. J. Polym. Sci., Part A: Polym. Chem. 2005,
43, 235.
(4) For the comprehensive reviews, see: (a) Lucet, D.; Gall, T.
L.; Mioskowski, C. Angew. Chem. Int. Ed. 1998, 37, 2580.
(b) Fehring, V.; Selke, R. Angew. Chem. Int. Ed. 1998, 37,
1827. (c) Fache, F.; Schulz, E.; Tommasino, M. L.; Lemaire,
M. Chem. Rev. 2000, 100, 2159. (d) Noyori, R.; Ohkuma, T.
Angew. Chem. Int. Ed. 2001, 40, 40. (e) Manabe, K.;
Kobayashi, S. Chem.–Eur. J. 2002, 8, 4095.
An important feature of the design of soluble dendrimer-
based catalyst is easy and reliable separation of the chiral
catalyst. The high generations of the dendritic catalysts
are expected to achieve quantitative recovery of catalyst
from the reaction mixtures based on large molecular size
and different solubility in various organic solvents. In this
study, the second and the third generation catalysts were
used to carry out the recycling experiment. Upon the com-
pletion of the reaction, methanol was added to the reaction
mixture and the catalyst was precipitated and recovered
via centrifugation. The recovered catalyst was reused for
at least two cycles with remained enantioselectivity. In
three consecutive runs, the following enantioselectivities
were observed: 95%, 94%, 94% by using the second gen-
eration catalyst and 94%, 93%, 94% by using the third
generation catalyst with complete conversions. ICP anal-
ysis showed that less than 3.6 mol%, 0.7 mol% of ruthe-
nium of the second and the third generation catalysts had
leached into the MeOH solution, respectively. Thus, we
refer that larger dendrimer has robust protection for the
stability of the catalyst complex, which had been observed
in bis (m-oxo)dicopper species15 and binuclear (m-O)(m-
OAc)2diiron(III) complexes16 toward oxidative self-de-
composition.
In conclusion, a series of chiral dendritic ligands based on
the phenyl-functionalized 1,2-diamine have been synthe-
sized for the first time and the dendritic Ru(BINAP)(di-
amine) catalysts showed to be a recoverable and highly
effective dendritic catalyst system for asymmetric hydro-
genation of simple aryl ketones. A remarkable structural
effect on catalytic activity as well as reuse was observed.
(5) (a) Ohkuma, T.; Ooka, H.; Hashiguchi, S.; Ikariya, T.;
Noyori, R. J. Am. Chem. Soc. 1995, 117, 2675.
(b) Ohkuma, T.; Doucet, H.; Pham, T.; Mikami, K.;
Korenaga, T.; Terada, M.; Noyori, R. J. Am. Chem. Soc.
1998, 120, 1086. (c) Doucet, H.; Ohkuma, T.; Murata, K.;
Yokozawa, T.; Kozawa, M.; Katayama, E.; England, A. F.;
Synlett 2005, No. 10, 1591–1595 © Thieme Stuttgart · New York

LETTER
Ikariya, T.; Noyori, R. Angew. Chem. Int. Ed. 1998, 37,
Novel Dendritic Ligands of Chiral 1,2-Diamine
1595
(9) Recently reported dendritic DPEN ligands based on the
functionalization of the nitrogen atom by us, see: (a) Chen,
Y.-C.; Wu, T.-F.; Deng, J.-G.; Liu, H.; Jiang, Y.-Z.; Choi,
M. C. K.; Chan, A. S. C. Chem. Commun. 2001, 1488.
(b) Chen, Y. C.; Wu, T. F.; Deng, J. G.; Liu, H.; Cui, X.; Zhu,
J.; Jiang, Y. Z.; Choi, M. C. K.; Chan, A. S. C. J. Org. Chem.
2002, 67, 5301. (c) Chen, Y. C.; Wu, T. F.; Jiang, L.; Deng,
J. G.; Liu, H.; Zhu, J.; Jiang, Y. Z. J. Org. Chem. 2005, 70,
1006.
(10) (a) Raja, R.; Thomas, J. M.; Jones, M. D.; Johnson, B. F. G.;
Vaughan, D. E. W. J. Am. Chem. Soc. 2003, 125, 14982.
(b) Jones, M. D.; Raja, R.; Thomas, J. M.; Johnson, B. F. G.;
Lewis, D. W.; Rouzaud, J.; Harris, K. D. M. Angew. Chem.
Int. Ed. 2003, 42, 4326.
1703.
(6) For other ruthenium(II) dichloride(diphosphine)(DPEN)
complexes as catalysts for hydrogenation of ketones, see:
(a) Burk, M. J.; Hems, W.; Herzberg, D.; Malan, C.; Zanotti-
Gerosa, A. Org. Lett. 2000, 2, 4173. (b) Wu, J.; Chen, H.;
Kwok, W.; Guo, R.; Zhou, Z.; Yeung, C.; Chan, A. S. C. J.
Org. Chem. 2002, 67, 7908. (c) Wu, J.; Ji, J.-X.; Guo, R.;
Yeung, C.-H.; Chan, A. S. C. Chem.–Eur. J. 2003, 9, 2963.
(d) Chaplin, D.; Harrison, P.; Henschke, J. P.; Lennon, I. C.;
Meek, G.; Moran, P.; Pikington, C. J.; Ramsden, J. A.;
Watkins, S.; Zanotti-Gerosa, A. Org. Process Res. Dev.
2003, 7, 89. (e) Aikawa, K.; Mikami, K. Angew. Chem. Int.
Ed. 2003, 42, 5455. (f) Xie, J.-H.; Wang, L.-X.; Fu, Y.; Zhu,
S.-F.; Fan, B.-M.; Duan, H.-F.; Zhou, Q.-L. J. Am. Chem.
Soc. 2003, 125, 4404.
(11) Corey, E. J.; Lee, D.-H.; Sarshar, S. Tetrahedron:
Asymmetry 1995, 6, 3.
(7) For the polymer-supported and organic or inorganic
heterogenized BINAPs, see: (a) ter Halle, R.; Schulz, E.;
Spagnol, M.; Lemaire, M. Synlett 2000, 680. (b) Yu, H.-B.;
Hu, Q.-S.; Pu, L. Tetrahedron Lett. 2000, 41, 1681. (c) Yu,
H.-B.; Hu, Q.-S.; Pu, L. J. Am. Chem. Soc. 2000, 122, 6500.
(d) Ohkuma, T.; Takeno, H.; Honda, Y.; Noyori, R. Adv.
Synth. Catal. 2001, 343, 369. (e) Hu, A.; Ngo, H. L.; Lin,
W. J. Am. Chem. Soc. 2003, 125, 11490. (f) Deng, G.-J.;
Fan, Q.-H.; Chen, X.-M.; Liu, G.-H. J. Mol. Catal. A: Chem.
2003, 193, 21.
(12) ESI-HRMS data for the dendritic ligands.
Ligand (S,S)-7a: m/z calcd for C28H28N2O2: 424.2151;
found: 424.1915.
Ligand (S,S)-7b: m/z calcd for C56H54KN2O6 [M + K + 2 H]:
889.3619; found: 889.4199; C56H50NO6 [M – NH2]:
832.3638; found: 832.3673.
Ligand (S,S)-7c: m/z calcd for C112H101N2NaO14 [M + Na +
H] (MALDI–TOF): 1719.7; found: 1720.1.
Ligand (S,S)-7d: m/z calcd for C224H203N2NaO30 [M + Na +
7 H] (MALDI–TOF): 3423.4: found: 3423.4.
(8) For polymer-supported DPENs, see: (a) Itsuno, S.; Tsuji,
A.; Takahashi, M. Tetrahedron Lett. 2003, 44, 3825. (b) Li,
X.; Chen, W.; Hems, W.; King, F.; Xiao, J. Org. Lett. 2003,
5, 4559.
(13) Fehring, V.; Selke, R. Angew. Chem. Int. Ed. 1998, 37, 1827.
(14) (a) Chow, H.-F.; Mak, C. C. J. Org. Chem. 1997, 62, 5116.
(b) Yi, B.; Fan, Q.-H.; Deng, G.-J.; Li, Y.-M.; Qiu, L.-Q.;
Chan, A. S. C. Org. Lett. 2004, 6, 1365.
(15) Enomoto, M.; Aida, T. J. Am. Chem. Soc. 1999, 121, 874.
(16) Enomoto, M.; Aida, T. J. Am. Chem. Soc. 2002, 124, 6099.
(17) Ohkuma, T.; Hattori, T.; Ooka, H.; Inoue, T.; Noyori, R.
Org. Lett. 2004, 6, 2681.
Synlett 2005, No. 10, 1591–1595 © Thieme Stuttgart · New York