Recyclable Helical Poly(phenyl isocyanide)-Supported l-Proline Catalyst for Direct Asymmetric Aldol Reaction in Brine

Article abstract of DOI:10.1007/s10562-020-03369-8

Abstract: A novel helical poly(phenyl isocyanide) bearing Boc protected l-proline pendants (poly-1_m) was designed and synthesized. Removed the protecting Boc groups on the l-proline pendants led to the formation of helical polymer poly-2_m, which showed high optical activity owing to the preferred right-handed helix of polyisocyanide main chain. Optically active helical poly-2_m showed excellent catalytic ability on asymmetric aldol reaction. Helical polymer catalysts exhibited enhanced stereoselectivity in aldol reaction compared to small molecule l-proline. Under the optimized aldol reaction condition, the enantiomeric excess (ee) and diastereomeric ratio (dr) values of the aldol reaction product were respectively up to 90% and > 20/1. Moreover, the helical polyisocyanide catalyst Poly-2_m can be easily recovered and reused in the aldol reaction for at least five cycles with maintained its activity and stereoselectivity. Graphic Abstract: Enantioselective aldol reaction catalyzed by poly-2_m.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03369-8

Catalysis Letters
https://doi.org/10.1007/s10562-020-03369-8
Recyclable Helical Poly(phenyl isocyanide)‑Supported l‑Proline
Catalyst for Direct Asymmetric Aldol Reaction in Brine
Chonglong Li1,2 · Jihai Wang · Huiyun Ding1
1
Received: 14 July 2020 / Accepted: 21 August 2020
©
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
A novel helical poly(phenyl isocyanide) bearing Boc protected l-proline pendants (poly-1 ) was designed and synthesized.
m
Removed the protecting Boc groups on the l-proline pendants led to the formation of helical polymer poly-2 , which showed
m
high optical activity owing to the preferred right-handed helix of polyisocyanide main chain. Optically active helical poly-2
m
showed excellent catalytic ability on asymmetric aldol reaction. Helical polymer catalysts exhibited enhanced stereoselectivity
in aldol reaction compared to small molecule l-proline. Under the optimized aldol reaction condition, the enantiomeric excess
(
ee) and diastereomeric ratio (dr) values of the aldol reaction product were respectively up to 90% and>20/1. Moreover, the
helical polyisocyanide catalyst Poly-2 can be easily recovered and reused in the aldol reaction for at least ꢀve cycles with
m
maintained its activity and stereoselectivity.
Graphic Abstract
Enantioselective aldol reaction catalyzed by poly-2m.
C
N
Ar
Ar
n
n
N
N
PEt3
Ar
Pd Cl
PEt3
TFA, CH Cl
2
2
o
HN
O
N
THF, 55 C, 15 h
HN
O
N
r.t. 12 h
HN
O
N
Boc
Boc
H
Ar = H CO
3
poly-1m
poly-2m
1
Right-handed Helix
Right-handed Helix
O
O
O
OH
H
Cat. 10 mol%
Brine, -10 C
R'
R'
o
3
4
1
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-020-03369-8) contains
supplementary material, which is available to authorized users.
Extended author information available on the last page of the article
Vol.:(0
1
123456789)
3

C. Li et al.
Keywords Helical polymer · Supported-catalyst · Asymmetric aldol reaction · Stereoselectivity
1
Introduction
chemistry and materials science, such as chiral packing mate-
rials for asymmetric catalysis [8–11], separating enantiomers
in chromatography [12–15], chiral selectors [16–18], and chi-
rality materials [19–22]. Now, many optically active helical
polymers have been synthesized by directed polymerizing
monomers with needed functional groups. Poly(phenyl iso-
cyanide) is a better helical polymer because of environmental
stability and wide applications [23–26]. A new type of asym-
metric chiral polymer-supported catalyst can be prepared by
introducing the catalytic group into the helical polyisocyanate.
In this contribution, we designed and synthesized of a novel
helical poly(phenyl isocyanide) bearing l-proline pendant
(poly-2m). The poly-2m had good stability and expanded the
scope than previously reported references [27, 28]. The pol-
ymer-supported catalyst exhibited signiꢀcant enhancement
on stereoselectivity in the aldol reaction compared to small
molecule l-proline. The polymer-supported catalyst has the
advantages of mild reaction conditions, easy separation from
the reaction system, high reuse rate, innocuity. It can be carried
out in environmentally friendly brine solvent, which meets the
needs of current green chemistry and sustainable chemistry. It
is expected to be used as a new type of green catalyst instead of
small molecule catalytic system, and has a very broad applica-
tion prospect in the ꢀeld of asymmetric synthesis.
The asymmetric organocatalysis reaction catalyzed by organic
small molecule catalyst has the advantages of simple operation,
mild reaction conditions, high yield and high selectivity [1–5].
However, there are still some defects in the organic small mol-
ecule catalyst, such as large amount of small molecule catalyst,
diꢁcult in the product puriꢀcation, and poor recycling per-
formance, which will cause environmental pollution and limit
the application of this kind of catalyst in industrial production.
With the concept of green chemistry and sustainable devel-
opment, it has become an urgent need to ꢀnd a catalyst that
can be recycled, eꢁcient, pollution-free and environmentally
friendly in the ꢀeld of drug synthesis. Therefore, how to recy-
cle catalyst is an urgent problem for chemists. The eꢂective
method to solve this problem is polymer-based catalyst which
can be easily separated from the reaction mixture and reused
[
6, 7]. Polymer-based catalyst meets the requirements of green
chemistry, and provides the possibility for large-scale indus-
trial production. Generally speaking, the preparation method
of polymer-catalyst is to assemble the existing small molecule
chiral catalytic unit on the polymer pendants, far away from
the polymer scaꢂold, so as to avoid adverse interference to the
chiral catalytic sites. Therefore, it is of great theoretical and
scientiꢀc signiꢀcance to design and synthesize the supported-
catalyst with high activity, high selectivity and excellent reus-
ability for asymmetric reaction.
2 Experimental
Helical polymers have attracted considerable research
attentions during the past few decades in the ꢀelds of polymer
See Scheme 1.
NH2
Boc O
2
O N
O N
2
NH
O
(
S)
2
N
H
COOH
COOH
N
Boc
(S)
Et N
3
EDCI/DCM
N
Boc
2
3
NHCHO
O
Boc
NH
(CH CO) O
3 2
Pd/C H2
H N
HCOOH
dry EA
2
N
N
H
THF
N
O
4
Boc
5
C–
N+
BTC Et N
O
3
Boc
o
N
dry DCM 0 C
N
H
1
Scheme 1 Synthesis of monomer 1
1
3

Recyclable Helical Poly(phenyl isocyanide)-Supported l-Proline Catalyst for Direct…
2
.1 Synthesis of Compound 2
(m, 1H, CH), 2.22–2.11 (m, 1H, CH), 1.75–1.96 (m, 3H,
CH CH), 1.43 (s, 3H, CH ), 1.32 (s, 6H, CH ).
2
3
3
1
3
l-proline (5.75 g, 0.05 mol) was dissolved in CH Cl
C NMR (150 MHz, DMSO-d 25 °C): δ 170.90, 153.68,
2
2
6,
(
50 mL) and triethylamine (10.4 mL, 0.075 mol). The
145.16, 128.64, 121.46, 114.23, 78.79, 60.67, 46.98, 31.46,
28.41, 23.81.
resulting mixture was cooled to 0 °C and a solution of
Boc O (13.8 mL, 0.06 mol, in 50 mL CH Cl ) was added
FT-IR (KBr, 25 °C): 3224, 2977, 2882, 1662, 1525,
2
2
2
−
1
drop-wise over 0.5 h at 0 °C. After the addition completed,
1407 cm . Anal. Calcd (%) for C H N O : C, 62.93; H,
16 23 3 3
the reaction mixture was stirred at room temperature for
7.59; N, 13.76. Found: C, 62.67; H, 7.68; N, 13.59.
1
2 h. When the reaction was over, the reaction mixture was
concentrated in vacuo. The resulting residue was cooled
to 0 °C and acidiꢀed with a 2 N aqueous solution of HCl
until pH = 2 was measured. The resulting solution was
extracted with 3 × 50 mL of CH Cl . The organic layer
2.4 Synthesis of Compound 5
After the mixture of acetic anhydride (1.0 mL, 12.5 mmol)
and formic acid (1.87 mL, 62.5 mmol) was stirred at room
temperature for 2 h, the mixed acid solution was added
drop-wise to compound 4 in ethyl acetate (50 mL) at 0 °C
for 30 min. The mixture was stirred at room temperature
for 2 h. The reaction mixture was poured into water and
extracted with ethyl acetate for three times. The combined
2
2
was washed with water and brine, dried over anhydrous
Na SO , ꢀltered, and concentrated in vacuo. The crude
2
4
solid was puriꢀed by recrystallization with hexane/EtOAc
(
6:1 v/v) to obtain white solid 2 (9.28 g, 96%).
organic phase was washed with H O (50 mL×3), saturated
2
2.2 Synthesis of Compound 3
NaHCO and brine (100 mL×2), and then dried over anhy-
3
drous Na SO . After ꢀltration, the solvent was removed by
2
4
To a 500 mL round-bottomed ꢃask containing a magnetic
stirring bar were added compound 2 (8.84 g, 41.1 mmol)
and EDC·HCl (9.44 g, 49.3 mmol) in CH Cl (300 mL)
evaporation under reduced pressure and the crude product
was puriꢀed by column chromatography (EA:PE=5:1) to
2
2
aꢂord white solid compound 5 (1.30 g, 69%).
under N . A solution of 4-nitroaniline (5.95 g, 43.1 mmol)
2
1
H NMR (600 MHz, DMSO-d ) δ 10.06 (s, 1H), 9.89 (s,
6
in dry CH Cl (50 mL) was added at 0 °C. The mixture was
2
2
1
4
1
H), 8.19 (s, 1H), 7.47–7.51 (m, 3H), 7.08–7.10 (m, 1H),
stirred at 0 °C for 15 min, and then at room temperature
.12–4.21 (m, 1H), 3.27–3.40 (m, 2H), 2.09–2.21 (m, 1H),
for 2 h. The reaction mixture was treated with 2 M HCl
.73–1.91 (m, 3H), 1.36 (s, 3H), 1.24 (s, 6H).
aqueous solution and saturated NH Cl aqueous solution,
4
13
C NMR (150 MHz, DMSO-d ): δ 171.71, 159.65,
6
and the organic layer was separated. The aqueous layer
1
6
53.60, 135.36, 134.08, 120.64, 119.90, 119.77, 78.84,
was extracted with CH Cl . Then, the combined organic
2
2
0.66, 46.99, 31.42, 28.37, 23.84.
layers were dried over Na SO , ꢀltered and concentrated
2
4
FT-IR (KBr, 25 °C): 3265, 3100,2976, 2871, 1665,
in vacuo. The crude product was puriꢀed by column chro-
−
1
1
603, 1616, 1521, 1411, 1366 cm . HRMS m/z: calcd for
matography on silica gel (eluent:hexane/EtOAc = 4:1) to
+
C H N O [M+H] : 334.1689; Found: 334.1182. Anal.
1
7
24
3
4
aꢂord compound 3 as a yellow solid (7.99 g, 58%).
Calcd (%) for C H N O : C, 61.25; H, 6.95; N, 12.60.
1
17 23
3
4
H NMR (600 MHz, CDCl , 25 °C): δ 10.22 (s, 1H,
3
Found: C, 61.47; H, 6.88; N, 12.53.
NH), 8.13 (d, J = 8.4 Hz, 2H, ArH), 7.64 (d, J = 8.4 Hz,
2
H, ArH), 4.63–4.37 (m, 1H, CH), 3.30–3.53 (m, 2H,
2.5 Synthesis of Compound 1
CH ), 2.45–2.60 (m, 1H, CH), 1.84–2.05 (m, 3H, CHCH ),
2
2
1
.36–1.57 (S, 9H, CH ).
3
Compound 5 (0.92 g, 2.76 mmol) was dissolved in dry
CH Cl (50 mL) and triethylamine (1.53 mL). The result-
2
2
2.3 Synthesis of Compound 4
ing mixture was cooled to 0 °C under N and a solution of
2
triphosgene (0.72 g, 2.43 mmol) in CH Cl (10 mL) was
2
2
Compound 3 (4.45 g, 13.3 mmol) was dissolved in THF
added drop-wise at 0 °C over 0.5 h. After the reaction mix-
ture was stirred at room temperature for 1 h, the reaction
mixture was poured into water and extracted with CH Cl
(
100 mL). A catalytic amount of 10% Pd/C (0.5 g) was
added to the solution and hydrogenation at H -overpressure
2
2
2
was carried out during 24 h. After the reaction was com-
pleted, it was ꢀltered through Celite and concentrated in
vacuo to obtain gray solid 4 (3.52 g, 87%).
for three times. The combined organic phase was washed
with H O (50 mL×3), saturated aqueous NaHCO and brine
2
3
(100 mL×2), and then dried over anhydrous Na SO . After
2
4
1
H NMR (600 MHz, DMSO-d 25 °C): δ 9.53 (s, 1H,
ꢀltration, the solvent was removed by evaporation under
reduced pressure and the crude product was puriꢀed by
column chromatography (PE:EA=5:1, v/v) to aꢂord white
solid compound 1 (0.80 g, 92%).
6
,
NH), 7.24 (d, J=8.6 Hz, 2H, ArH), 6.53 (d, J=8.6 Hz, 2H,
ArH), 4.86 (s, 2H, NH ), 4.37–4.11 (m, 1H, CH), 3.33–3.43
2
1
3

C. Li et al.
1
−1
H NMR (600 MHz, CDCl , 25 °C): δ 9.89 (s, 1H), 7.55
FT-IR (KBr, 25 °C, cm ): 3407 (νN–H), 2952 (νC–H),
1678 (ν ), 1602 (v ).
3
(
d, J=8.4 Hz, 2H), 7.30 (d, J=8.4 Hz, 2H), 4.41–4.52(m,
C=O
C=N
1
1
H, CH), 3.28–3.51 (m, 2H, CH2), 2.49–2.62 (m, 1H, CH),
.81–2.04 (m, 3H, CH CH), 1.50 (s, 9H, CH ).
2
3
2
.8 General Procedure for Aldol Reaction
1
3
C NMR (150 MHz, CDCl , 25 °C): δ 171.00, 163.21,
3
1
2
56.23, 139.63, 126.82, 119.81, 81.11, 60.70, 47.46, 28.56,
Polymer catalyst poly-2 with 20% loading (with respec-
1
50
4.69.
tive to repeating units) was added to expected solvent,
cyclohexanone (4.0 eq) was added to the above stirring solu-
tion. After 1 h, aromatic aldehyde (1.0 eq) was added to the
solution, and the resulting solution was stirred for the certain
time at a speciꢀc temperature. After the reaction was over,
ethyl ether was added to the solution, and then the mixture
was centrifuged to remove the catalyst. The solution was
concentrated under reduced pressure and the crude prod-
uct was puriꢀed by preparative thin layer chromatography
FT-IR (KBr, 25 °C): 3283, 3199,2978, 2876, 2123,1694,
−1
1
663, 1600, 1544, 1509, 1477, 1410,1368 cm . HRMS m/z:
+
calcd for C H N O [M−H] : 314.1583; Found: 314.1259.
17
20
3
3
Anal. Calcd (%) for C H N O : C, 64.74; H, 6.71; N,
1
7
21
3
3
1
3.32. Found: C, 64.57; H, 6.70; N, 13.41.
2
.6 Representative Polymerization Procedure
for Poly‑1ms
(
TLC) on silica gel using petroleum ether and ethyl acetate
Typical experimental procedures for the preparation of poly-
is given below as an example.
as eluent to aꢂord the product.
1
1
50
Taking the reaction of cyclohexanone with 4-nitrobenza-
ldehyde as an example.
A 10 mL oven-dried flask was charged with mono-
mer 1 (126.1 mg, 0.40 mmol) and a stirring bar. A solu-
tion of bis(triethylphosphine)-((4-methoxyphenyl)ethy-
nyl) palladium(II) chloride (2.03 mg, 4.0 µmol) in dry
Poly-2 (8.55 mg, 0.04 mmol, with respective to repeat-
150
ing units) was added to the stirring solution of cyclohex-
anone (78.52 mg, 0.80 mmol) in brine. Then 4-nitrobenzal-
dehyde (30.22 mg, 0.20 mmol) was added to this solution via
a microsyringe. The resulting solution was stirred at room
temperature for 48 h. After the reaction was accomplished,
diethyl ether was added to the solution which caused the
polymer catalyst precipitated. After centrifugation to remove
the solid, the solution was concentrated under reduced pres-
sure and the crude product was puriꢀed by preparative thin
layer chromatography (TLC) on silica gel using petroleum
ether and ethyl acetate as an eluent to aꢂord the product.
Other Aldol addition reactions were performed under the
similar procedure and the characterization data for the Aldol
reaction products are showed below.
THF (2.5 mL) was then added under dry nitrogen ([1] /
0
[
Pd] = 100, [1] = 0.16 M). The reaction ꢃask was then
0
0
immersed in an oil bath and stirred at 55 °C for 15 h. After
cooling to room temperature, the polymerization solution
was precipitated in a large amount of n–hexane, collected
by centrifugation, and dried in vacuum at room tempera-
ture overnight to aꢂord the desired poly-1 as a red solid
1
00
(
103.7 mg, 82%).
1
H NMR (600 MHz, CDCl , 25 °C): δ 9.11 (br, 1H, NH),
3
6
3
.50 (br, 2H, ArH), 5.60 (br, 2H, ArH), 4.23 (br, 1H, CH),
.46 (br, 2H, CH ), 2.13 (br, 1H, CH), 1.84 (br, 3H, CH
2
2
‒
1
CH), 1.32 (br, 9H, (CH ) ). FT-IR (KBr, 25°C, cm ): 2975
3
3
(
ν
), 1670 (ν ), 1597 (ν ).6.50, 5.60 ppm ascribed to
C-H C=O C=N
the phenyl ring, and also showed a wider resonance at 4.23,
.46, 2.13.
O
OH
3
2.7 Typical Procedure for Poly‑2ms
NO2
Typical experimental procedures for the preparation of
poly-2 is given below as an example. Poly-1 (0.20 g)
(2S,1′R)-2-(hydroxyl-(p-nitrophenyl)methyl)
1
50
150
1
was dissolved in dry CH Cl (8.0 mL), triꢃuoroacetic acid
cyclohexan-1-one [29]: H NMR (600 MHz, CDCl ,
2
2
3
(
TFA, 0.6 mL) was slowly added to the stirring solution at
25 °C): δ 8.20 (d, 2H, J = 8.4 Hz, ArH), 7.50 (d, 2H,
J=8.4 Hz, ArH), 5.48 (s, 0.44H, syn CH), 4.89‒4.91 (dd,
J = 8.4 Hz, J = 2.4 Hz, 0.63H, anti CH), 4.04 (d, 1H,
ice water bath. The mixture solution was stirred at room tem-
perature for 15 h, then poured into water, used ammonium
hydroxide adjusted to neutral, and extracted with CH Cl for
1
2
J=3.0, OH), 1.35‒2.64 (m, 9H, –CHCH CH CH CH –).
2
2
2
2
2
2
three times. After the solvent removed by evaporation under
reduced pressure, the crude product was dissolved in MeOH,
and then added to a large amount of ethyl ether, ꢀltered by
centrifugation, and dried in vacuum at 40 °C overnight to
aꢂord the desired poly-2 as a red solid (105.0 mg, 77%
Enantiomeric excess (ee): 84%, diastereomeric ratio (dr):
78/22, determined by HPLC analysis, Chiralpak AD-H;
n-hexane/i-PrOH = 85/15 (v/v); 0.5 mL/min; 254 nm;
25 °C; t (syn.) = 28.09 min, t (syn.) = 30.66 min, t
R1
R2
R1
(anti.)=33.66 min, t (anti.)=43.78 min.
1
50
R2
yield).
1
3

Recyclable Helical Poly(phenyl isocyanide)-Supported l-Proline Catalyst for Direct…
CDCl , 25 °C): 7.58‒7.60 (m, 2H, ArH), 7.41‒7.44
3
(
(
(
m, 2H, ArH), 5.43 (s, 0.30H, syn CH), 4.82‒4.84
O
OH
dd, J = 2.9 Hz, J = 8.6 Hz, 0.79H, trans CH), 4.02
NO2
1
2
d, 1H, J = 2.9 Hz, OH), 1.26‒2.59 (m, 9H, –CH CH-
2
CH CH –). ee: 72%, dr: 73/27, determined by HPLC
2
2
2
analysis, Chiralpak AD-H; n-hexane/i-PrOH = 95/5 (v/v);
−
1
0
.8 mL min ; 210 nm; 25 °C; t (syn.) = 12.80 min,
R1
(
2S,1′R)-2-(hydroxyl-(m-nitrophenyl)methyl)
t
(syn.) = 15.18 min, t (anti.) = 20.83 min, t
R1 R2
1
R2
cyclohexan-1-one [29]: H NMR (600 MHz, CDCl , 25 °C):
3
(
anti.) = 26.66 min.
δ 8.05‒8.27 (m, 2H, ArH), 7.66‒7.68 (d, J=7.8, 1H, ArH),
7
.50‒7.54 (m, 1H, ArH), 5.48 (s, 0.26H, syn CH), 4.89‒4.91
O
OH CF3
(
dd, J =8.4 Hz, J =2.5 Hz, 0.80H, anti CH), 4.08 (d, 1H,
1
2
J=3.1 Hz, OH), 1.36‒2.68 (m, 9H, –CHCH CH CH CH –).
2
2
2
2
ee: 81%, dr: 81/19, determined by HPLC analysis, Chiral-
pak AD-H; n-hexane/i-PrOH = 85/15 (v/v); 0.50 mL/min;
2
54 nm; 25 °C; t (syn.)=37.75 min, t (syn.)=41.45 min,
R1 R2
(
2S,1′R)-2-(hydroxyl-(o-trifluoromethylphenyl)
t
(anti.)=46.96 min, tR2 (anti.)=60.75 min.
R1
1
methyl)cyclohexan-1-one [31]: H NMR (600 MHz,
CDCl , 25 °C): 7.71 (d, J = 5.2 Hz, 1H, ArH), 7.58‒7.66
3
O
OH
(
m, 2H, ArH), 7.38‒7.44 (t, 1H, ArH), 5.30 (d,
J = 8.4 Hz, 1H), 4.01 (d, J = 2.1 Hz, 1H), 1.61‒2.75 (m,
H, -CH CH CH CH -). ee: 90%, determined by HPLC
9
CN
2
2
2
2
analysis, Chiralpak AD-H; n-hexane/i-PrOH=90/10 (v/v);
−
1
0
.5 mL min ; 254 nm; 25 °C; t (anti.) = 23.14 min, t
R1 R2
(
2S,1′R)-2-(hydroxyl-(p-cyanophenyl)methyl)
(anti.) = 24.59 min.
cyclohexan-1-one [29]: ee: 72%, dr: 80/20, determined by
HPLC analysis, Chiralpak AD-H; n-hexane/i-PrOH=95/5
−
1
(
t
v/v); 0.8 mL min ; 230 nm; 25 °C; t (syn.)=24.50 min,
3 Results and Discussion
R1
(syn.) = 27.81 min, t
(anti.) = 30.99 min, t
R2
R2
R1
(
anti.)=38.42 min.
As shown in Scheme 2, phenyl isocyanide monomer 1
bearing chiral l-proline pendant was designed and syn-
thesized (see Supporting Information for details). After the
isolated phenyl isocyanide monomer 1 was characterized
by FT-IR, NMR, mass spectrum and elemental analyses
(Figs. S9–S11, Supporting Information), it was polym-
erized in THF at 55 °C using bis(triethylphosphine)-((4-
methoxyphenyl)ethynyl) palladium(II) chloride as cata-
lyst under dry nitrogen atmosphere [23]. Due to the poor
O
OH
CF3
(
2S,1′R)-2-(hydroxyl-(p-trifluoromethylphenyl)
1
methyl)cyclohexan-1-one [30]: H NMR (600 MHz,
C
N
Ar
Ar
n
n
N
N
PEt3
Ar
Pd Cl
PEt3
TFA, CH Cl
2
2
o
HN
O
N
THF, 55 C, 15 h
HN
O
r.t. 12 h
HN
O
N
Boc
Boc
H
N
Ar = H CO
3
1m
poly-2m
1
poly-
Right-handed Helix
Right-handed Helix
Scheme 2 Synthesis of Helical Poly-2m
1
3

C. Li et al.
solubility, the molecular weight of the isolated poly-1 s
m
(
the footnote indicates the initial feed ratio of monomer to
catalyst, the same below) cannot be measured.
Monomer 1 exhibited the resonances signal at 7.55,
7
.30 ppm corresponding to the aryl ring, and the signal
at 4.48, 3.50, 3.39, 2.26, 2.00 attributable to the proton
signal of the pyrrole ring (Fig. S9, Supporting Informa-
1
tion). H NMR spectra of the obtained poly-1 is similar
1
50
after 1 was polymerized by the Pd(II) initiator (Fig. S12).
It showed a wider resonance at 6.50, 5.60 ppm ascribed
to the phenyl ring, and also showed a wider resonance at
4
.23, 3.46, 2.13 ppm of the pyrrole ring. In the same way,
the chemical shift of the proton on the Boc of poly-1
1
50
moved to a higher ꢀeld, and the NMR signal peak becomes
wider. The chemical shift of CH hydrogen atom on Boc of
3
poly-1 is 1.32, while that on monomer 1 is 1.47.
Fig. 1 CD and UV–vis spectra of poly-1 s with diꢂerent M meas-
1
50
m
n
ured in THF at 25 °C (c=0.20 mg/mL)
By comparing the FT-IR spectra of monomer 1 and
poly-1 (Fig. S13), it can be seen that the absorption
1
50
−
1
peak of monomer 1 at 2121 cm corresponds to the char-
acteristic absorption of vibration of C≡N bond, while
−
1
−1
the absorption peaks at 1693 cm and 1660 cm are
the characteristic absorption of vibration of amide C=O
bond. It can be seen from the FT-IR spectra of poly-1
1
50
that the biggest change is the disappearance of the vibra-
−
1
tion absorption peak of C≡N at 2121 cm , and a new
−
1
absorption peak appears at around 1597 cm , corre-
sponding to the characteristic vibration absorption peak
of C=N bond of poly-1 . All of these further indicated
1
50
that polymer-1 with C=N double bond was formed by
1
50
the complete polymerization of monomer 1 initiated by
palladium alkyne catalyst, and the structure of other parts
of the monomer did not change.
A series of polymerizations of monomer 1 bearing chi-
ral l-proline pendant using the Pd(II) initiator were accom-
plished with diꢂerent initial feeding ratios of monomer 1
to the initiator in THF at 55 °C. The chiroptical properties
Fig. 2 CD and UV–vis spectra of poly-2ms with diꢂerent Mn meas-
ured in THF at 25 °C (c=0.20 mg/mL)
of poly-1 s were investigated by circular dichroism (CD)
m
and UV–vis spectra. As shown in Fig. 1, the isolated poly-
In order to investigate the catalytic activity of l-proline
structural units on the side group of helical polymers poly-
1 s, the protective Boc group on the poly-1 pendants was
1
s showed intense positive CD signal at the absorption
m
region of the polyisocyanide backbone around 365 nm,
and UV absorption also occurs at about 365 nm, suggest-
ing the formation of a preferred right-handed helix [32].
From the circular dichroism and UV visible spectrum of
poly-1 with diꢂerent molecular weight in Fig. 1, it can
m
m
removed using 10% triꢃuoroacetic acid (TFA) in dichlo-
romethane at room temperature (Scheme 2). Although
poly-1 had good solubility in some organic solvents, the
m
solubility of poly-2 was poor. It can be clearly seen that
1
50
m
be seen that g of poly-1 , poly-1 , poly-1 , poly-1
,
100
there is an intense vibration absorption peak of C=N bond at
3
65
25
50
75
−
1
poly-1 , poly-1 are 59.8, 69.4, 81.9, 95.8, 108.6, 108.7
1602 cm , suggesting that polyisocyanide does not destroy
the main chain of polymer during the deprotection reaction
with triꢃuoroacetic acid. In the FT-IR spectra of poly-2 , a
1
50
150
and 108.7, respectively. The g values of polyisocya-
3
65
nide increased with the increase of the degree of polym-
erization. However, when the degree of polymerization of
poly-1 increases to 150, the g values of poly-1 do not
1
50
−
1
new vibration absorption peak appears at 3407 cm , which
is the vibration absorption peak of N–H bond in pyrrole
m
365
m
increase, and tend to a ꢀxed value 108.6, indicated poly-1
ring on the side chain of poly-2 , further indicating that
m
150
can form a stable helix.
the Boc protecting group on N atom of pyrrole ring on the
side chain of poly-1 has been completely removed (Fig.
1
50
1
3

Recyclable Helical Poly(phenyl isocyanide)-Supported l-Proline Catalyst for Direct…
S14). The CD and UV–vis spectra of poly-2 were outlined
details, the reaction conditions of aldol template reaction
m
in Fig. 2. Comparing to poly-1 , CD intensity of poly-2
were further optimized by using poly-2 as catalyst.
1
50
150
150
was decreased, while the Max of the CD moved left because
Further strengthen screening of solvents revealed acetoni-
trile and brine were good reaction medium for the asym-
metric aldol reaction. (Table 1, run 4, 10–18). In the same
reaction system, adding cocatalyst benzoic acid, ee value did
not improve (Table 1, run 6–9). When lowering the reaction
temperature from room temperature to 10°C, the ee value of
the major product was improved to 80% in saturated brine,
but 78% in acetonitrile (Table 1, run 19, 20). It can be seen
from the above data that the best reaction solvent for asym-
metric aldol reaction is saturated salt water. When lowering
the aldol reaction temperature from 25 to − 10 °C, the stere-
oselectivity of the product was greatly improved, and the ee
value of the product increased from 77 to 84% (Table 1, run
22). However, further lowering the aldol reaction tempera-
ture to − 20 °C, the ee value of the reaction did not increase,
but the yield of the reaction decreased a lot (Table 1, run 23).
It is further proved that the stereoselectivity of poly-2 is
of remove of the Boc group (Fig. S15).
As we all know, l-proline and its derivatives can eꢂec-
tively catalyze asymmetric aldol reaction. Therefore, we
studied the catalytic performance of poly-2 on asymmet-
m
ric aldol reaction, and selected the reaction of p-nitroben-
zaldehyde 4a and cyclohexanone 3a as template reaction.
The template reaction was catalyzed by l-proline or poly-
2
in saturated brine at 25 °C with 20% (with respective
m
to repeating units) poly-2 loading. The yield of product
1
50
1
a was 74% and 75%, the dr (diastereomeric ratio) value
was 62/38, 74/26, and the enantiomeric excess (ee) value
was 70%, 77% as determined by high performance liquid
chromatography (HPLC), respectively. It can be seen that
the yield of the product using poly-2 as catalyst was the
m
same as that using proline, but dr and ee values were better
than those using l-proline under the identical experimental
conditions. This result showed that adding l-proline into the
helical polyisocyanide could provide the extra chiral envi-
ronment to improve the enantioselectivity and diastereose-
lectivity of hydroxyaldehyde reaction. We also found that
oligomer poly-2 can catalyze asymmetric aldol reaction
1
50
aꢂected by the main chain helix structure of polymer. There-
fore, the optimal reaction temperature of template reaction
was − 10 °C. When the loading of poly-2 lowering from
1
50
20 to 10%, the enantioselectivity and product yield were not
signiꢀcantly decreased (run 22 and 24 in Table 1). However,
further lowering the catalyst poly-2 loading to 5%, the
1
0
in 72% yield, 75/25 dr, and 59% ee (Table 1, run 1). Both
1
50
ee and dr values were lower than those using poly-2 as
yield and the ee value of the reaction decreased signiꢀcantly
(Table 1, run 25). Thus, the optimized reaction conditions
are using 10 mol% of poly-2 loading in brine at −10 °C.
1
50
catalyst under the same experimental conditions (Table 1,
run 4). Due to its small degree of polymerization and small
molecular weight, it was not enough to provide a helix main
chain structure environment, so the stereoselectivity of the
product obtained was worse than that using poly-2 as cata-
1
50
After the asymmetric aldol reaction catalyzed by poly-
was established, the scope of the substrates was then
50
2
1
studied. As summarized in Table 2, the optimal reaction
condition was applied to cyclohexanone and different
substituted aromatic aldehydes. When the substrate was
p-nitrobenzaldehyde (4a), m-nitrobenzaldehyde (4b) and
4-formylbenzonitrile (4c), 4-(trifluoromethyl)benzalde-
hyde (4d), 2-(triꢃuoromethyl)benzaldehydethe (4e), yields
of the products 1a, 1b, 1c, 1d and 1e were 63%, 70%, 60%,
67%, 56% respectively; the dr values were 78/22, 81/19,
80/20, 73/27, > 20/1; the ee values were 84%, 81%, 72%,
72%, 90% (Table 2, run 1–5). It can also be seen from the
results that the position of substituents on the phenyl ring
and the substituents with diꢂerent electronic eꢂects had a
great inꢃuence on the activity and stereoselectivity of the
aldol reaction. For example, we have studied the diꢂerent
substrates 4a, 4c, 4e and 4f with NO , CN, CF and CH
150
lyst. The stereoselectivity of the asymmetric aldol reaction
catalyzed by l-proline and oligomer poly-2 was worse than
10
poly-2 , which indicated that the helix chiral environment
1
50
provided by helix polymer can promote the stereoselectivity
of asymmetric aldol reaction.
Next, the catalytic performance of poly-2 was further
m
studied. Poly-2 poly-2 , poly-2 and poly-2 with dif-
50,
100
150
200
ferent degrees of polymerization catalyzed the aldol template
reaction in saturated brine at room temperature with 20%
poly-2 loading respectively. As summarized in Table 1, it
m
was found that an increase in the degrees of polymerization
of poly-2 can further improve the product enantioselectiv-
m
ity. For example, the ee of the major aldol product 1a was
5
9% using poly-2 , which increased to 76% and 77% using
5
0
2
3
3
poly-2 and poly-2 as catalyst, respectively (Table 1,
respectively on the benzene ring. The results showed that
there were electron-withdrawing groups on the benzene ring,
the yields of 1a, 1c and 1e were 65%, 60%, 56% respectively;
the dr values were 78/22, 80/20,>20/1; the ee values were
84%, 72%, 90% respectively (Table 2, run 1, 3, 5). When the
substituent on the benzene ring was the electron-denoting
1
00
150
runs 2–4). Further increased the degree of polymerization
of the polymer cannot further enhance the enantioselectivity
of the aldol template reaction. The ee of 1a in the template
reaction using poly-1 as catalyst was almost the same that
200
of poly-2 (Table 1, runs 5). In order to further obtain more
150
1
3

C. Li et al.
Table 1 Optimization of the reaction conditions for cyclohexanone with 4-nitrobenzaldehyde catalyzed by poly-2m
O
O
Cat. (X mol%)
O
OH
additive(20 mol%)
+
H
Sol. Temp.
NO2
NO2
3
a
4a
1a
a
ee (%)b
b
Run
Cat.
Additive
Solvent
Temp. (°C)
X
Yield (%)
Anti./syn. (dr)
1
2
3
4
5
6
7
8
9
Poly-210
Poly-250
Poly-2100
Poly-2150
Poly-2200
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
Poly-2150
l-Proline
–
–
–
–
Brine
Brine
Brine
Brine
Brine
Water
DMF
DMSO
Brine
DMF
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
10
5
68
72
74
75
74
71
63
62
75
61
60
63
57
66
67
73
78
80
73
74
70
67
49
65
43
75
48
59
76
77
76
71
44
37
74
47
46
52
25
56
54
48
51
77
80
78
83
84
84
84
77
70
73/27
75/25
72/28
74/26
73/27
71/29
85/15
75/25
68/32
71/29
69/31
75/25
66/34
75/25
77/23
65/35
70/30
73/27
73/27
71/29
75/25
78/22
79/21
77/23
73/27
62/38
–
PhCOOH
PhCOOH
PhCOOH
PhCOOH
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
DMSO
THF
Toluene
EtOH
MeOH
CHCl3
CH Cl
r.t.
r.t.
10
10
2
2
MeCN
Brine
MeCN
Brine
Brine
Brine
Brine
Brine
Brine
0
− 10
− 20
− 10
− 10
r.t.
20
Unless otherwise denoted, all reactions were carried out with 4-nitrobenzaldehyde (0.2 mmol), cyclohexanone (0.8 mmol) in speciꢀc solvent
(
1.0 mL)
Yield of isolated products
The ee and dr values were determined by HPLC analysis using a chiral stationary phase
a
b
group CH , the catalytic activity of the polymer catalyst was
One of the advantages of polymer-supported organocata-
lyst is the possibility of recovery and utilization in the aldol
reactions. Due to the large molecular weight of the polymer
catalyst, it is very easy to separate from the products of small
molecules and raw materials after the reaction. After the
3
low, and the target product 1f could not be detected (Table 2,
run 6). These results showed the electron-withdrawing group
on the phenyl ring could improve the diastereoselectivity of
the aldol reactions.
1
3

Recyclable Helical Poly(phenyl isocyanide)-Supported l-Proline Catalyst for Direct…
Table 2 Enantioselective aldol
reaction catalyzed by poly-2
O
O
O
OH
1
50
H
Cat. 10 mol%
Brine, -10 o
R'
R'
C
3
4
1
a
b
b
run substrate
3)
substrate
product
(1)
time yield anti./syn. ee
(
(4)
(h)
(%)
(dr)
(%)
O
OH
O
OHC
4
a
1
3a
NO2
36
65
78/22
84
1
a
NO2
NO2
O
O
O
OH
OHC
NO2
2
3
3a
3a
4
b
36
36
70
60
81/19
80/20
81
72
1
b
O
OH
OHC
4
c
CN
1
c
CN
O
OH
O
O
O
OHC
4
d
4
5
6
3a
3a
3a
CF3
36
36
90
67
56
73/27
>20/1
--
72
90
--
1d
CF3
CF3
O
OH CF3
OHC
4e
1
e
O
OH
OHC
4
f
CH3
n.d.
1f
CH3
Unless otherwise denoted, all reactions were carried out with aldehyde (0.2 mmol), cyclohexanone
0.8 mmol) in brine (1.0 mL)
(
n.d. Not detected
a
Yield of isolated products
b
The dr and ee values are referred to the major isomer determined by HPLC analysis using a chiral sta-
tionary phase
reaction, poly-21 showed poor solubility in brine, therefore
the product using the ꢀfth cycle recovered poly-2 are 80%
50
150
the polymer can be easily separated from the reaction system
in almost quantitative yield by centrifugation and ꢀltration.
The recovered poly-2 was then reused in the aldol reac-
and 77/23, respectively. However, the reaction time of the
ꢀfth recycle was extended to 72 h, indicating the catalytic
activity was slightly decreased. Thus, it can be concluded
that the helical polyisocyanide organocatalyst can be reused
in the aldol reaction without signiꢀcantly lost its enantiose-
lectivity (run 1–4 in Table 3).
1
50
tion using cyclohexanone with p-nitrobenzaldehyde as the
substrates. It was found that the recovered poly-2 also
1
50
showed excellent catalytic activity. The ee and dr values of
1
3

C. Li et al.
Table 3 Yield and ee values of the Aldol reaction for cyclohexanone and 4-nitrobenzaldehyde catalyzed by recycled poly-2150
O
O
O
OH
Poly-1150(20 mol%)
+
H
0
brine, -10 C
NO2
NO2
a
drb
b
Cycle
Yield (%)
ee (%)
1
2
3
4
65
64
65
63
63
78/22
75/25
80/20
79/21
77/23
84
84
83
83
82
c
5
The reactions were carried out with 4-nitrobenzaldehyde (0.20 mmol), cyclohexanone (0.80 mmol) in brine (1.0 mL) at 0 °C
a
Yield of isolated products
The ee and dr values were determined by HPLC analysis using a chiral stationary phase
The reaction time of the ꢀfth recycle was 72 h
b
c
4
Conclusions
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Aꢀliations
Chonglong Li1,2 · Jihai Wang · Huiyun Ding1
1
2
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Chonglong Li
Key Laboratory for Chemical Engineering and Technology,
chlong@nun.edu.cn
State Ethnic Aꢂairs Commission, North Minzu University,
Yinchuan 750021, People’s Republic of China
1
College of Chemistry and Chemical Engineering, North
Minzu University, Yinchuan, China
1
3