Highly effective and recyclable dendritic ligands for the enantioselective aryl transfer reactions to aldehydes

Article abstract of DOI:10.1021/jo050994h

A series of chiral pyrrolidinylmethanol-based dendritic ligands were synthesized for application in enantioselective aryl transfer reactions to aldehydes with the (ArBO)₃ZnEt₂ system in up to 98% ee.

Full text of DOI:10.1021/jo050994h

these cases suffer from inherent drawbacks: For ex-
ample, the reduction method requires the presence of
electronically and/or stereochemically differentiated aryls
for optimum results. The method of aryl transfer reaction
seems more suitable for chiral induction because of the
large steric and electronic differences between an aryl
group and a hydrogen atom on an aldehyde substrate,
and great progress has been achieved in this field.4-6
Fu et al. first reported, in 1997, the addition of salt-
free Ph2Zn to p-chlorobenzaldehyde using a planar-chiral
azoferrocene ligand with moderate enantioselectivity.4a
Pu and co-workers reported that performing the addition
reaction at a low concentration of substrate improved the
enantioselectivity dramatically, using chiral 3,3′-diaryl
binaphthol as a ligand.4b Recently, Bolm et al. and, later,
Chan et al. have reported an excellent protocol wherein
the aryl transfer reagent was generated by mixing
arylboronic acids, a substantially more stable and less
expensive reagent than diphenylzinc, with diethylzinc.6,4g
With the intention to facilitate the workup as well as the
recovery and reuse of the catalyst in asymmetric syn-
thesis, we have now focused on the immobilization of a
chiral ligand by anchoring it onto a polymeric support.7
Recently, several catalytically active dendrimers have
been reported that perform with very high enantioselec-
tivities in the catalytic enantioselective addition of di-
ethylzinc to both aromatic and aliphatic aldehydes.8,9
However, in the field of asymmetric phenyl transfer from
organozinc reagents to aldehydes, to the best of our
knowledge, a dendritic system has not been widely
investigated except for a few examples.4b,5f Since previous
Highly Effective and Recyclable Dendritic
Ligands for the Enantioselective Aryl
Transfer Reactions to Aldehydes
Xin yuan Liu, Xiao yu Wu, Zhuo Chai, Yong yong Wu,
Gang Zhao,* and Shi zheng Zhu*
Laboratory of Modern Synthetic Organic Chemistry,
Shanghai Institute of Organic Chemistry, Chinese Academy
of Sciences, 354 Fenglin Lu, Shanghai 200032, P. R. China
zhaog@mail.sioc.ac.cn
Received May 19, 2005
A series of chiral pyrrolidinylmethanol-based dendritic
ligands were synthesized for application in enantioselective
aryl transfer reactions to aldehydes with the (ArBO)3/ZnEt2
system in up to 98% ee.
(5) (a) Bolm, C.; Mun˜iz, K. Chem. Commun. 1999, 1295. (b) Bolm,
C.; Hermanns, N.; Hildebrand, J. P.; Mun˜iz, K. Angew. Chem., Int.
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Kesselgruber, M.; Hermanns, N.; Hildebrand, J. P. Angew. Chem., Int.
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Kesselgruber, M.; Grenz, A.; Hermanns, N.; Hildebrand, J. P. New J.
Chem. 2001, 25, 13. (e) Bolm, C.; Hermanns, N.; Kesselgruber, M.;
Hildebrand, J. P. J. Organomet. Chem. 2001, 624, 157. (f) Bolm, C.;
Hermanns, N.; Claâen, A.; Mun˜iz, K. Bioorg. Med. Chem. Lett. 2002,
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Angew. Chem., Int. Ed. 2004, 42, 3002. (h) Rudolph, J.; Bolm, C.;
Norrby, P.-O. J. Am. Chem. Soc. 2005, 127, 1548. (i) For a review on
asymmetric arylation reactions, see: Bolm, C.; Hildebrand, J. P.;
Mun˜iz, K.; Hermanns, N. Angew. Chem., Int. Ed. 2001, 40, 3284;
Angew. Chem. 2001, 113, 3382.
Optically active diarylmethanols are versatile and
important chiral building blocks for therapeutically
important medicines such as (R)-neobenodine, (R)-or-
phenadrine, or (S)-carbinoxamine.1 Two general ap-
proaches aimed at the enantioselective synthesis of these
compounds have been reported: the enantioselective
reduction of prochiral ketones2,3 and asymmetric aryl
transfers onto aromatic aldehydes.4,5 However, some of
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(b) Rudolph, J.; Hermanns, N.; Bolm, C. J. Org. Chem. 2004, 69, 3997.
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(7) (a) DeVos, D. E., Vankelecom, I. F. J., Jacobs, P. A., Eds. Chiral
Catalyst Immobilization and Recycling; Wiley-VCH: Weinheim, 2000.
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(c) Bra¨se, S.; Dahmen, S. Synthesis 2001, 1431.
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Int. Ed. 1998, 37, 1986; Angew. Chem. 1998, 110, 2092. (f) General
review: Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40;
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Int. Ed. 2002, 41, 2008; Angew. Chem. 2002, 114, 2108.
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H.; Liu, G. H.; Chen, X. M.; Deng G. J.; Chan, A. S. C. Tetrahedron:
Asymmetry 2001, 42, 9047 (d) Seebach, D.; Marti, R. E.; Hintermann,
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Chem. Eur. J. 1999, 5, 3221. (f) Rheiner, P. B.; Seebach, D. Polym.
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D. Helv. Chim. Acta 1997, 80, 2027, (h) Sellner, H.; Seebach, D. Angew.
Chem., Int. Ed. 1999, 38, 1918. (i) Sellner, H.; Gaber, C.; Rheiner, P.
B.; Seebach, D. Chem. Eur. J. 2000, 6, 3692. (j) Sellner, H.; Karjalainen,
J. K.; Seebach, D. Chem. Eur. J. 2001, 7, 2873. (k) For a review on
recoverable catalysts, see: Fan, W.-H.; Li, Y.-M.; Chan, A. S. C. Chem.
Rev. 2002, 102, 3385.
(3) For enantioselective hydrogenations of aromatic and heteroaro-
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McWilliams, C. J. Org. Lett. 2003, 5, 5039.
(4) (a) Dosa, P. I.; Ruble, J. C.; Fu, G. C. J. Org. Chem. 1997, 62,
444. (b) Huang, W.-S.; Hu, Q.-S.; Pu, L. J. Org. Chem. 1999, 64, 7940.
(c) Huang, W.-S.; Pu, L. Tetrahedron Lett. 2000, 41, 145. (d) Zhao, G.;
Li, X.-G.; Wang, X.-R. Tetrahedron: Asymmetry 2001, 12, 399. (e) Ko,
D.-H.; Kim, K. H.; Ha, D.-C. Org. Lett. 2002, 21, 3759. (f) Fontes, M.;
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10.1021/jo050994h CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/02/2005
7432
J. Org. Chem. 2005, 70, 7432-7435

SCHEME 1. Synthesis of Pyrrolidinylmethanol Ligands with Polyether Dendritic Wedges.a
a
Reagents and Conditions: (a) TBAF, THF, rt, 5 h. (b) K2CO3, 18-crown-6, acetone. (c) LiAlH4, THF.
studies on the aryl transfer from diphenylzinc-derived
arylation reagents had shown that the presence of
polyethers could lead to a significant increase in enantio-
selectivity,4g,5,6 we decided to choose a polyether as a
supporting matrix as well. Herein, we wish to report the
synthesis of new polyether dendritic chiral pyrrolidinyl-
methanol derivatives and their application in catalytic,
highly enatioselective aryl transfer to aldehydes.
Pyrrolidinylmethanol derivative 1 was chosen as a
model ligand for this study because it is the most effective
chiral ligand among all the chiral nitrogen ligands that
have been studied for asymmetric catalysis in our group
and because it can also be easily prepared.4d,10 Consider-
ing that pyrrolidinylmethanol 1 itself cannot be easily
attached to a dendrimer, 2 was synthesized according to
the literature.9e Cleavage of the two protecting groups
with Bu4NF gave the compound 3 in good yield. Polyether
dendritic wedges 5-7 with bromo groups located at the
focal point were synthesized by the convergent growth
method introduced by Hawker and Fre´chet.11 The chiral
dendritic pyrrolidinylmethanol ligands 8-11 were syn-
thesized in >79% yields by the coupling of the wedges
4-7 with 3 in the presence of K2CO3 and 18-crown-6 in
N,N-dimethylformamide (DMF) or acetone at between
room temperature and ∼40 °C. Reduction of the ester
moieties with LiAlH4 afforded the corresponding alcohols
12-15 in high yields (Scheme 1). These ligands were
purified by flash column chromatography and character-
ized by 1H and 13C NMR spectroscopy, MALDI mass
spectrometry, and elemental analysis. All results were
in full agreement for the proposed structures.
A preliminary study was performed to test the catalytic
property of the known ligand 1 in the phenyl transfer
reaction to p-chlorobenzaldehyde. Indeed, the reaction of
the nucleophile, preformed from phenylboronic acid and
diethyl zinc in toluene according to Bolm’s protocol, with
p-chlorobenzaldehyde in the presence of 1 in various
catalyst loadings and temperatures afforded (S)-p-chlo-
robenzhydrol in 49-93% ee and in good yields (Table 1,
entries 1-6). The optimum yield and ee were obtained
when the reaction was performed at -15 °C, and the
optimum amount of catalyst loading was 20 mol % for 6
h (entry 3). To investigate the influence of the dendritic
branches on the catalytic activity and the stereoselectiv-
ity of the pyrrolidinylmethanol, we tested dendrimers
12-15 in the enantioselective phenyl transfer reaction
in the optimum reaction conditions. In general, an ex-
cellent conversion of p-chlorobenzaldehyde was achieved,
and the corresponding optically active alcohols were
obtained in excellent yields (>90%). The enantioselec-
tivities were high (88-93%), and the (S)-configured
products were formed predominantly (Table 1, entries
7-10). Among all the dendritic chiral catalysts evaluated
in this reaction, the second-generation ligand of 14 was
the best one in terms of yield and ee (Table 1, entry 9).
With these promising results, attention was then
turned to modify the above protocol in anticipation of
obtaining better results. Inspired by Hayashi and co-
workers’ report that the rhodium-catalyzed asymmetric
1,4-addition to 1-alkenylphosphonates was greatly im-
proved by carrying out the reaction with triphenylcyclo-
triboroxane [phenylboroxine, (PhBO)3]12 in place of phen-
(10) (a) Hu, J. B.; Zhao, G.; Yang, G. S.; Ding, Z. D. J. Org. Chem.
2001, 66, 303. (b) Hu, J. B.; Zhao, G.; Ding, Z. D. Angew. Chem., Int.
Ed. 2001, 40, 1109. (c) Zhao, G.; Hu, J. B.; Qian, Z. S.; Yin, W. X.
Tetrahedron: Asymmetry 2002, 13, 2095. (d) Wang, G. Y.; Hu, J. B.;
Zhao, G. Tetrahedron: Asymmetry 2004, 15, 807.
(11) Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990, 112,
7638.
J. Org. Chem, Vol. 70, No. 18, 2005 7433

TABLE 1. Catalyzed Phenyl Transfer from Boronic
TABLE 3. Catalyzed Asymmetric Phenyl Transfer to
Various Aldehydesa
Acid to p-Chlorobenzaldehydea
yield ee of 16
absolute
entry
R in RCHO
4-Cl-C6H4
4-Br-C6H4
2-Br-C6H4
4-F-C6H4
T (h)
(%)b
(%)c
configuration
catalyst loading
(mol %)
yield
(%)b
1
2
3
4
5
6
7
8
9
6
6
6
6
16a, 98
16b, 92
16c, 92
16d, 98
98
98
97
96
98
93
95
97
88
65
S
S
S
S
S
S
S
S
R
R
entry
ligand
T (°C)
ee (%)c,d
1
2
1
10
15
20
20
20
15
20
20
20
20
10
0
83
92
93
96
87
95
90
92
98
98
49
77
86
93
88
78
88
89
93
91
1
4-CH3O-C6H4
4-CH3-C6H4
4-CF3-C6H4
2-C10H9
10 16e, 98
10 16f, 98
3
1
0
4
1
-15
-30
-30
-15
-15
-15
-15
6
6
6
6
16g, 88
16h, 90
16i, 96
16k, 77
5
1
6
1
(E)-C6H5CHdCH
7
12
13
14
15
10 CH(CH3)2
8
9
a In toluene, 20 mol % 14, 4.5 equiv of ZnEt2, and 0.5 equiv of
(PhBO)3 with respect to RCHO. b After column chromatography.
c Determined by HPLC analysis.
10
a In toluene, 7.2 equiv of ZnEt2 and 2.4 equiv of PhB(OH)2 with
respect to 4-Cl-C6H4CHO. b After column chromatography. c De-
termined by HPLC using a chiral stationary phase. d Comparison
of the HPLC peak elution order of 16a with known data revealed
that the (S)-enantiomer was formed in excess.
pyrrolidinylmethanol ligands in various solvents provided
a convenient and reliable method for the separation and
reuse of the ligands. For example, upon completion of the
reaction, methanol was added to the reaction mixture,
and the ligand 14 was quantitatively precipitated and
recovered via filtration. The recovered ligand was reused
at least five times with little or no loss of activity and
enantioselectivity (Table 2, entries 6-9).
TABLE 2. Catalyzed Asymmetric Aryl Transfer
Reactions to p-Chlorobenzaldehyde with Phenylboroxine
as a Phenyl Source and Recycling of Dendritic Catalysta
With the purpose of determining the scope and utility
of the present system [(PhBO)3/Et2Zn and 14] for the
asymmetric arylation of aldehydes, a series of substrates
with different steric and electronic properties were
investigated (Table 3). It should be emphasized that the
level of enantiocontrol for all aromatic substrates was
outstanding. The corresponding diarylcarbinols were
obtained in optical purities in the range of 93 to 98%
enantiomeric excess. Furthermore, even substrate 2-bro-
mobenzaldehyde bearing an ortho substituent, which
previously proved to be difficult, afforded the correspond-
ing product in good yield and with high ee (97% ee) (Table
3, entry 3). trans-Cinnamaldehyde also reacted highly
selectively and gave the corresponding alcohol with 88%
ee (Table 3, entry 9). Unfortunately, aliphatic aldehyde
provided poorer selectivity. Addition to iso-butyraldehyde
yielded the product alcohol in only 65% ee (Table 3, entry
10).
Next, we investigated the possibility of varying the
structure of the aryl source and studied the asymmetric
aryl transfer from various substituted phenylboroxines
(17b-f) to benzaldehyde (Table 4). To our delight, we
found that substituted aryls were transferred very well,
affording the corresponding products in good yields and
with excellent ee. In all cases, no ethyl transfer product
was formed that was consistent with others’ results4g,6
and related theoretical study.5g,5h As for the mechanism
of this reaction, it appears that the phenylzinc species
was produced in situ by boron-to-zinc exchange, and the
reactive catalyst was formed from the dendritic ligand
and zinc species.
entry
ligand
T (h)
yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
9
1
6
6
6
6
6
6
6
6
6
94
98
92
98
97
92
98
97
91
94
94
97
98
96
95
96
97
96
12
13
14 (run 1)
15
14 (run 2)
14 (run 3)
14 (run 4)
14 (run 5)
a In toluene, 4.5 equiv of ZnEt2 and 0.5 equiv. of (PhBO)3 with
respect to 4-Cl-C6H4CHO (catalyst loading is 20 mol %). b After
column chromatography. c Determined by HPLC using a chiral
stationary phase.
yl boronic acid as the phenyl source, we found that after
optimizing the reaction conditions, using phenylboroxine
(PhBO)3 as the phenyl source, a slightly higher enantio-
selectivity could be achieved with a shorter reaction time
(from 12 to 4 h) and a reduced amount of diethyl zinc
(from 7.2 to 4.5 equiv) under the same conditions when
1 was used as the ligand (Table 2, entry 1). Then,
p-chlorobenzaldehyde was again used in the modified
protocol to investigate the catalytic activity and enantio-
selectivity of the dendritic ligands. The results are
summarized in Table 2, and in the best case, with 20 mol
% 14, the (S)-configured product 16a was obtained with
98% ee and 98% yield (Table 2, entry 4). These ligands
(12-15) showed higher ee values than 1, although the
highest-generation ligand 15 gave a slightly lower enan-
tioselectivity (Table 2, entry 5). Furthermore, the large
molecular size and different solubility of the dendritic
In summary, a series of new dendritic chiral pyrrolidi-
nylmethanol derivatives were synthesized and proved to
be highly effective ligands for the asymmetric catalytic
aryl transfer to aldehydes with the PhB(OH)2/ZnEt2 or
(ArBO)3/ZnEt2 systems. This study opens up a new
(12) Hayashi, T.; Senda, T.; Takaya, Y.; Ogasawara, M. J. Am.
Chem. Soc. 1999, 121, 11591.
7434 J. Org. Chem., Vol. 70, No. 18, 2005

TABLE 4. Catalyzed Aryl Transfer from 17 to
Benzaldehydea
were washed with H2O and then dried over Na2SO4, and the
solvent was evaporated to yield the crude product as a yellow
oil. Flash chromatography (EtOAc/petroleum ether 1:3) yielded
8 (0.588 g, 96%) as a white solid: mp 136-138 °C; [R]D20 -180.3
(c 1.10, CHCl3); IR (film, cm-1) υ 1750, 1609, 1513, 1259; 1H NMR
(300 MHz, CDCl3) δ 1.09-1.13 (m, 1H), 1.64-1.71 (m, 1H), 1.77-
1.96 (m, 2H), 3.21-3.29 (m, 1H), 3.69-3.78 (m, 1H), 4.15-4.52
(m, 1H), 5.04 (s, 4H), 6.96 (d, J ) 9.0 Hz, 4H), 7.26-7.43 (m,
14H); MS (EI) m/e 491 (M+, 8.31), 378 (M+ - 113, 15.75), 91
(M+ - 400, 100). Anal. Calcd for C32H29NO4: C, 78.19; H, 5.95;
N, 2.85. Found: C, 78.32; H, 5.99; N, 2.75.
Preparation of 12. To a cooled (0 °C) suspension of LiAlH4
(75 mg, 2 equiv) in freshly distilled THF (2 mL) was added
dropwise a THF (2 mL) solution of 8 (488 mg, 1 equiv). Upon
complete addition of 8, the ice-water bath was removed, and
the suspension was stirred until the reaction was complete, as
indicated by TLC (3 h). The reaction was quenched with
Baechstro¨ms reagent (Na2SO4‚10H2O), stirred for 15 min,
filtered, and concentrated in vacuo to afford a yellow oil. Flash
chromatography (CH2Cl2/acetone 1:1) yielded 12 (0.394 g, 83%)
as a white solid: mp 128-130 °C; [R]D20 15.6 (c 1.30, CHCl3); IR
(film, cm-1) υ 3292, 1604, 1506, 1379, 1236; 1H NMR (300 MHz,
CDCl3) δ 1.60-1.72 (m, 4H), 1.83 (s, 3H), 2.16-2.44 (m, 1H),
3.08-3.11 (m, 1H), 3.49-3.54 (m, 1H,), 4.98 (s, 4H), 6.84-6.94
(m, 4H), 7.24-7.50 (m, 14H); 13C NMR (75.0 MHz, CDCl3) δ 24.2,
30.0, 43.3, 59.3, 70.1, 72.1, 77.1, 114.4, 126.6, 127.6, 128.0, 128.6,
137.2, 139.8, 141.2, 157.2; ESI, MS m/e 480 ([M + H]+); HRMS
(ESI) m/e calcd for [C32H3NO3 + H]+ 480.2533, found 480.2533.
T
yield ee of 18
entry
(ArBO)3
product (h) (%)b
(%)c,d
1
2
3
4
5
Ar ) 4-chlorophenyl (17b)
Ar ) 4-bromophenyl (17c)
Ar ) 4-fluorophenyl (17d)
Ar ) 4-methylphenyl (17e)
Ar ) 4-methoxyphenyl (17f)
18b
18c
18d
18e
18f
6
6
6
8
8
92
95
92
86
80
98 (R)
95
92
91
89
a In toluene, 7.2 equiv of ZnEt2 and 2.4 equiv of PhB(OH)2 with
respect to 4-Cl-C6H4CHO. b After column chromatography. c De-
termined by HPLC using a chiral stationary phase. d Comparison
of the HPLC peak elution order of 18b with known data revealed
that the (R)-enantiomer was formed in excess. Assuming an
analogous mechanism for the aryl transfer and on the basis of the
HPLC elution order, we assume that the other products have the
(R)-configuration as well.
frontier for the development of highly effective and easily
separable chiral ligand. Current work is focused on
gaining detailed insight into the nature of the dendrictic
effect and the exploration of these catalysts in other
reactions.
Acknowledgment. We are grateful to the Ministry
of Sciences and Technology, the State Key Project of
Basic Research (Project 973, No. G 2000048007), and
the National Natural Science Foundation of China for
financial support.
Experimental Section
Preparation of 8. Benzyl bromide (0.428 g, 2.5 mmol) was
added to a solution of 3 (0.389 g, 1.25 mmol) in DMF (5 mL). To
this solution was added dried and finely powdered K2CO3 (0.345
g, 2.5 mmol), and the resulting suspension was stirred for 3 h
at room temperature. H2O (10 mL) and CH2Cl2 (20 mL) were
added; the two layers were separated, and the aqueous layer
was extracted twice with CH2Cl2. The combined organic layers
Supporting Information Available: Experimental pro-
cedure and characterization data, including 1H NMR spectral
data and HPLC analysis. This material is available free of
charge via the Internet at http://pubs.acs.org.
JO050994H
J. Org. Chem, Vol. 70, No. 18, 2005 7435