Chiral linear polymers bonded alternatively with salen and 1,4-dialkoxy-2,6-diethynylbenzene: synthesis and application to diethylzinc addition to aldehydes

Article abstract of DOI:10.1016/j.tetasy.2007.09.004

The synthesis of chiral polymers 1 bonded alternatively with salen and 1,4-dialkoxy-2,6-diethynylbenzene was accomplished. These polymers are recyclable and catalyze the Et₂Zn addition to aldehydes with good enantioselectivity.

Full text of DOI:10.1016/j.tetasy.2007.09.004

Tetrahedron: Asymmetry 18 (2007) 2016–2020
Chiral linear polymers bonded alternatively with salen and
1,4-dialkoxy-2,6-diethynylbenzene: synthesis and
application to diethylzinc addition to aldehydes
Suribabu Jammi, Laxmidhar Rout and Tharmalingam Punniyamurthy*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, India
Received 6 August 2007; accepted 5 September 2007
Abstract—The synthesis of chiral polymers 1 bonded alternatively with salen and 1,4-dialkoxy-2,6-diethynylbenzene was accomplished.
These polymers are recyclable and catalyze the Et2Zn addition to aldehydes with good enantioselectivity.
ꢀ 2007 Elsevier Ltd. All rights reserved.
1. Introduction
aldehyde 5 as a yellow solid.6 Aldehyde 5 was transformed
into silyl derivative 6 by coupling with trimethylsilylacetyl-
ene using Pd(PPh3)2Cl2 and CuI in the presence of Et3N in
THF. Desilylation of 6 with K2CO3 in MeOH gave ethynyl
derivative 7, which was coupled with 1,4-dibromo-2,5-dioct-
yloxybenzene 9 using Pd(PPh3)2Cl2 and CuI to afford di-
aldehyde 3 as a yellow solid (Schemes 1 and 2).
The application of optically active 1,2-diamine derivatives
in asymmetric synthesis has attracted extensive attention.1
These molecules have demonstrated excellent chiral induc-
tion in a number of organic transformations when used
either as a chiral auxiliary2 or a chiral ligand.3 Sub-
sequently, the synthesis of chiral 1,2-diamine based poly-
mers is developed for asymmetric catalysis to provide
simplified product isolation, easy recovery of the catalysts,
and potential use in continuous production.4 From an
environmental and economic standpoint, these studies are
attractive since the cost and environmental impact (E-
factor) of the process can be lowered.5 Herein, we report
the synthesis of new chiral 1,2-diamine based polymers 1
from (1R,2R)-diamines 2 and dialdehyde 3 by Schiff base
formation. These polymers 1 catalyze Et2Zn addition to
aldehydes with up to 70% ee, in a homogeneous process
with no additives involved. Polymers 1 can be recovered
from the reaction mixture by precipitation using MeOH
and recycled without any loss in activity.
CHO
OH
t-Bu
a
OH
t-Bu
Br
b
5
4
CHO
OH
t-Bu
CHO
OH
t-Bu
c
Me3Si
6
7
d
CHO
OC8H17
OHC
HO
OH
t-Bu
H17C8O
2. Results and discussion
t-Bu
3
The reaction of 2-tert-butylphenol 4 with paraformalde-
hyde in the presence of SnCl4 followed by bromination
with Br2 in CHCl3 afforded 5-bromo-3-tert-butylsalicyl-
Scheme 1. Reagents and conditions: (a) (i) (CH2O)n (2.2 equiv), SnCl4
(0.1 equiv), toluene, 100 ꢁC, 10 h, 85%; (ii) Br2, AcOH, rt, 1 h, 95%; (b)
Me3SiC2H (1.1 equiv), Pd(PPh3)2Cl2 (3 mol %), CuI (4 mol %), Et3N
(2 equiv), THF, rt, 12 h, 99%; (c) K2CO3 (1.5 equiv), MeOH, rt, 6 h, 60%;
(d) 1,4-dibromo-2,5-dioctyloxybenene 9 (0.5 equiv), Pd(PPh3)2Cl2 (3 mol %),
CuI (4 mol %), (iPr)2NH, THF, reflux, 12 h, 20%.
*
Corresponding author. Tel.: +91 361 2582309; fax: +91 361 2690762;
e-mail: tpunni@iitg.ernet.in
0957-4166/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2007.09.004

S. Jammi et al. / Tetrahedron: Asymmetry 18 (2007) 2016–2020
2017
OH
OC8H17
Br
OC8H17
1.0
a
b,c
Polymer 1a
Polymer 1b
0.8
0.6
0.4
0.2
0.0
Br
OH
OC8H17
H17C8O
10
8
9
Scheme 2. Reagents and conditions: (a) (i) C8H17Br (2.2 equiv), KOH,
EtOH, rt, 24 h, 80%; (ii) Br2, CHCl3, 0 ꢁC, 8 h, 95%; (b) Me3SiC2H
(2.1 equiv), Pd(PPh3)2Cl2 (5 mol %), CuI (5 mol %), (iPr)2NH, THF, rt,
12 h, 99%; (c) 20% aq KOH, MeOH, THF, rt, 2 h, 90%.
The synthesis of polymers 1 was then accomplished by
Schiff base formation between dialdehyde 3, and amines
2a and 2b (Schemes 3 and 4). The products were obtained
as yellow colored powder. The specific rotations of 1a7 and
250
300
350
400
Wavelength (nm)
Figure 1. UV–vis spectra of polymers 1 in CHCl3.
25
25
1b8 are ½aꢀD ¼ þ376 (c 0.1, CHCl3) and ½aꢀD ¼ þ166 (c 0.1,
CHCl3), respectively. GPC analysis employing polystyrene
as internal standard and THF as the eluent showed the
molecular weights corresponding to ca. 21 repeating units
for 1a (Mw = 18,512, Mn = 15,242 and PDI = 1.2) and
ca. five repeating units for 1b (Mw = 4864, Mn = 3441
and PDI = 1.4). The 1H NMR spectra recorded at
400 MHz are consistent with their structures. The elec-
tronic spectra of both the polymers exhibited very similar
absorption.7 In the UV spectrum of 1a, kmax were observed
at 248, 266, 274, 284, 302, and 333 (br) nm (Fig. 1). Like-
wise, polymer 1b showed absorption maximum at 249, 268,
279, 300, and 333 (br) nm. These polymers display very
similar cotton effects in circular dichroism spectra
(Fig. 2).9 The CD spectrum of 1a exhibited weak positive
cotton effect with splitting9b at 271, 291, 331, 355,
387 nm, and a negative cotton effect at 238 nm showing a
positive chirality. Polymer 1b showed weak positive cotton
effect with splitting9b at 272, 304, 332, 352 nm and a nega-
tive cotton effect at 241 nm showing a positive chirality. We
also attempted the synthesis of 1a by coupling chiral salen
15
10
5
0
-5
-10
Polymer 1a
Polymer 1b
-15
-20
-25
-30
-35
250
300
350
400
450
Wavelength (nm)
Figure 2. CD spectra of polymers 1 in CHCl3 (C = 1.99 · 10ꢁ5 M for 1a
and C = 8.45 · 10ꢁ5 M for 1b) at ambient temperature.
OC8H17
OC8H17
N
N
N
N
a
*
*
OH
OH
HO
HO
n
H2N
NH2
OC8H17
t-Bu
OC8H17
t-Bu
t-Bu
t-Bu
2b
1b
Scheme 3. Reagents and conditions: (a) dialdehyde 3 (1 equiv), CHCl3, 40 ꢁC, 3 h, 76%.
Ph
N
Ph
N
Ph
N
Ph
OC8H17
OC8H17
N
Ph
Ph
a
*
OH
OH
HO
*
HO
t-Bu
H2N
NH2
H17C8O
t-Bu
H17C8O
t-Bu
t-Bu
2b
1b
Scheme 4. Reagents and conditions: (a) dialdehyde 3 (1 equiv), CHCl3, 40 ꢁC, 3 h, 70%.

2018
S. Jammi et al. / Tetrahedron: Asymmetry 18 (2007) 2016–2020
Table 1. Polymer 1 catalyzed Et2Zn addition to aldehydes
11 with 1,4-dialkoxy-2,6-diethynylbenzene 10 using
Pd(PPh3)2Cl2 and Pd(PPh3)4 in combination with CuI in
OH
i
the presence of Et3N and Pr2NH in THF at 25–70 ꢁC.10
CHO
1 mol% 1
However, this approach was not successful and no cross-
coupled polymerized product was observed.
X
X
s
2 equiv Et2Zn
toluene
30 oC, 18 h
Entry
X
Polymer
Yielda,b (%)
ee (%)
N
N
1
2
3
4
5
3-NO2
3-NO2
3-NO2
H
1a
1b
1b
1b
1b
72
74
73
83
70
50
70
70c,d
50e
60e
R
R
OH
HO
t-Bu
t-Bu
4-OMe
11 R = Br
12 R = t-Bu
a Aldehyde (1 mmol), Et2Zn (2 mmol) and polymer 1 (1 mol % with
respect to monomeric unit) were stirred at 30 ꢁC in toluene under N2
atmosphere.
b Isolated yield.
Polymers 1 were then studied for Et2Zn addition to 3-nitro-
benzaldehyde (Table 1).11 We were pleased to find that the
reaction occurred to afford the corresponding alcohol with
50% ee, when the reaction was stirred in the presence of
1 mol % of 1a (with respect to monomeric unit) and 2 equiv
of Et2Zn in toluene at ambient temperature.12 The enantio-
selectivity was further increased to 70% ee when the poly-
mer 1b was employed in the place of polymer 1a. These
reactions did not involve any additive. Benzaldehyde and
4-methoxybenzaldehyde were next reacted with Et2Zn to
provide the corresponding alcohols with 50% and 60% ee,
respectively. A similar result has been reported with the
monomeric salen complex 12 for Et2Zn addition to alde-
hydes.12d The advantage of the present system is that poly-
mer 1b can be recovered from the reaction mixture by
adding MeOH and then recycled. The recovered 1b was
used for the fresh reaction of 3-nitrobenzaldehyde with
Et2Zn. As above, the reaction occurred to give the respec-
c Recovered 1b used.
d Determined by HPLC with chiralcel OD-H column using 98:2 hexane–
isopropanol.
e Determined by HPLC with chiralcel OD-H column using 97:3 hexane–
isopropanol.
tive alcohol with 70% ee. This study clearly suggests that
the polymer is recyclable without any loss in activity or
selectivity.
The proposed catalytic cycle is shown in Scheme 5.12 The
reaction of 1 with Et2Zn may provide complex a, which
may transform to intermediate b by reaction with Et2Zn
and aldehyde. The transfer of the ethyl group from the
coordinated Et2Zn to the coordinated aldehyde in b may
give intermediate c which on reaction with new Et2Zn
and aldehyde may complete the catalytic cycle.
R
R
N
N
O
R = Ph, -[CH2]4-
Et2Zn
Zn
1
O
a
Et2Zn, ArCHO
t-Bu
t-Bu
R'OZnEt
H
Ar
Et2Zn, ArCHO
R
O
O
R
OR' =
N
N
Ar
Et
R
R
N
Zn
N
O
O
Et
Zn
Zn
Et
O
O
OR'
Zn
t-Bu
t-Bu
t-Bu
t-Bu
b
Et
c
Scheme 5. Proposed catalytic cycle.

S. Jammi et al. / Tetrahedron: Asymmetry 18 (2007) 2016–2020
2019
5. (a) Velusamy, S.; Ahamed, M.; Punniyamurthy, T. Org. Lett.
2004, 6, 4821; (b) Punniyamurthy, T. Rout, L. Coord. Chem.
Rev., in press, doi:10.1016/j.ccr.2007.04.003; (c) Rout, L.;
Nath, P.; Punniyamurthy, T. Adv. Synth. Catal. 2007, 349,
846.
3. Conclusion
In conclusion, the synthesis of chiral polymers 1 was
accomplished. These polymers are soluble in common
organic solvents; are recyclable and catalyze the Et2Zn
addition to aldehydes with good enantioselectivity. Further
study of this reaction is currently underway in our
laboratory.
6. Sellner, H.; Karjalainen, J. K.; Seebach, D. Chem. Eur. J.
2001, 7, 2873.
7. Polymer 1a: To a stirred solution of aldehyde 3 (0.272 mmol,
200 mg) in CHCl3 (3 ml), (1R,2R)-diaminocyclohexane 2a
(0.272 mmol, 31.8 mg) was added. The resulting solution was
stirred at 40 ꢁC for 3 h and cooled to ambient temperature.
Methanol was added and the precipitate 1a collected as a
yellow powder with 76% (176.1 mg) yield. GPC:
Mw = 18,512, Mn = 15,242 (PDI 1.2); 1H NMR (400 MHz,
CDCl3) d 14.00–14.30 (m, 2H), 8.23 (br s, 2H), 7.39 (s, 2H),
7.18 (s, 2H), 6.94 (s, 2H), 3.95 (t, J = 6.4 Hz, 4H), 3.33 (br s,
2H), 1.20–1.99 (m, 50H), 0.83–1.0 (m, 6H); 13C NMR
(100 MHz, CDCl3) d 164.89, 160.98, 154.03, 149.53, 137.82,
134.17, 133.11, 132.85, 118.31, 118.31, 117.64, 117.08, 113.37,
112.90, 112.74, 94.42, 83.47, 72.47, 70.19, 69.91, 35.17, 33.30,
32.16, 29.99, 29.62, 29.55, 26.41, 26.35, 24.54, 23.00, 14.45;
Acknowledgments
We thank Professor Tsutomu Katsuki for CD analysis.
This was supported by the Department of Science and
Technology, New Delhi, and the Council of Scientific
and Industrial Research, New Delhi. We thank Dr. John
M. Brown for discussions.
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