3228 3234
Daicel Chemical Industries) employing n-hexane/propan-2-ol as eluents
(flow rate: 1.0 mLminÀ1) and UV detection (Prof. Dr. Bach, Organic
Chemistry I, TU M¸nchen). The fluorescence analysis for the determi-
nation of the critical micelle concentration (cmc) was performed on a Spex
FLUORLog spectrometer. Polymer solutions were prepared in a concen-
tration range from 10À3 to 10À10 molLÀ1 and 0.2mm solutions of the
fluorescence dye 6-p-toluidine-2-naphthylsulfonic acid in doubly distilled
water.
ported triphenylphosphine (PS-PEG polystyrene poly-
ethylene glycol)[12] or as insoluble catalyst network through
self-assembly of triphenylphosphine-functionalized amphi-
philic polymers and (NH4)2PdCl4.[13] An extension of the
utility of amphiphilic polymers is based on their unique self-
assembly behavior in a selective solvent, that is, a thermody-
namically good solvent for one block and a bad solvent for the
second one. Dependent on polymer architecture and compo-
sition, such as AB, ABA, or ABC block or graft copolymers,
these polymers form different types of micellar aggregates in
solution.[14] Whereas micellar catalysis in aqueous solution
have been known for long time,[15] there are only few reports
in literature in which the use of amphiphilic, water-soluble
block copolymers for the immobilization of transition-metal
catalysts has been studied, although polymers exhibit many
advantages over their low-molecular-weight counterparts
with respect to structural variability, different solubility, and
functionality. G. Oehme et al. used block copolymers based
on PEO-PPO (poly(ethylene oxide) poly(propylene oxide))
to solubilize different catalysts and proved the efficiency of
such micellar supported systems in asymmetric hydrogena-
tion.[16] In this case, the catalyst is only solubilized in the
micellar core and may be washed out after product/catalyst
separation. Clearly, a covalent linkage of the catalyst to the
polymer would offer advantages in terms of catalyst recycling.
Recently, we reported the successful synthesis of amphiphilic
block copolymers bearing triphenylphosphine[17] and bipyr-
idine units[18] in the hydrophobic block and their application
for hydroformylation of oct-1-ene[19] or atomic transfer radical
polymerisation (ATRP) of methyl methacrylate[18] in pure
aqueous solution. With the intention to expand the scope of
transition-metal-catalyzed reactions in polymer micelles, we
report in this paper the synthesis of new amphiphilic block
copolymers with (2S,4S)-4-diphenylphosphino-2-(diphenyl-
phosphinomethyl)pyrrolidine (PPM) units in the side chain
and their application in the asymmetric hydrogenation of two
prochiral enamides, a-acetamido cinnamic acid and its methyl
ester. Additionally, catalyst recovery and reuse is possible by
simple extraction of the substrate/product from the aqueous
polymer phase after each cycle.
Methyl 7-chloro-4-oxo-5-azaheptanoate (1a): Methyl succinyl chloride
(20.0 g, 0.13 mol) and 2-chloroethylammonium chloride (15.4 g, 0.13 mol)
were suspended in dry dichloromethane (150 mL). At 08C triethylamine
(30.0 g, 0.30 mol) was added dropwise over a period of 1 h. The reaction
mixture was allowed to warm up to room temperature and was stirred
overnight before water (40 mL) was added. The organic phase was washed
twice with water and once with brine, and dried over anhydrous sodium
sulfate. After removal of the solvent, a yellow oil remained. Yield: 19.82 g
(77%) of a yellow oil; 1H NMR (CDCl3): d 2.52 (t, J 6.49 Hz, 2H), 2.69
(t, J 6.49 Hz, 2H), 3.62 (m, 4H), 3.70 (s, 3H), 6.17 ppm (s, 1H; NH);
13C NMR (CDCl3): d 29.3, 30.9, 41.3, 43.9, 51.9, 171.7, 173.4 ppm.
Methyl 3-(oxazol-2-yl)propionate (1): Compound 1a (19.0 g, 0.098 mol)
and anhydrous sodium carbonate (7.6 g, 0.072 mol) were reacted under
stirring at a pressure of 0.1 mbar. A clear colorless liquid was obtained after
fractionated distillation Yield: 10.2 g (66%) of a colorless liquid; 1H NM R
(CDCl3): d 2.58 (t, J 7.44 Hz, 2H), 2.68 (t, J 7.44 Hz, 2H), 3.70 (s, 3H),
3.82 (t, J 9.16 Hz, 2H), 4.24 ppm (t, J 9.73 Hz, 2H); 13C NMR (CDCl3):
d 23.1, 30.1, 51.8, 54.4, 67.5, 167.0, 172.8 ppm; elemental analysis calcd
(%) for C7H11NO3 (157,17): C 53.49, H 7.05, N 8.91; found: C 53.39, H 7.11,
N 9.01.
Methyl 9-chloro-6-oxo-7-azanonoate (2a): Methyl adipinoyl chloride
(20.6 g, 0.115 mol) and 2-chloroethylammonium chloride (13.35 g,
0.115 mol) were used in a reaction similar to that described for 1a. Yield:
1
19.3 g (76%) of a yellow oil; H NMR (CDCl3): d 1.60 (m, 4H), 2.18 (t,
J 6.90 Hz, 2H), 2.28 (t, 3J 6.87 Hz, 2H), 3.55 (m, 4H), 3.60 (s, 3H),
6.58 ppm (s, 1H; NH); 13C NMR (CDCl3): d 24.7, 25.3, 34.0, 36.2, 41.6,
44.0, 51.8, 173.4, 174.2 ppm.
Methyl 3-(oxazol-2-yl)pentanoate (2): Compound 2a (17.6 g, 0.079 mol)
and anhydrous sodium carbonate (6.3 g, 0.059 mol) were heated together
with stirring at a pressure of 0.1 mbar. A clear colorless liquid with a boiling
point of 1108C (0.1 mbar) was obtained after fractionated distillation.
Yield: 9.2 g (63%) of a colorless liquid; 1H NMR (CDCl3): d 1.61 (m,
4H), 2.24 (m, 4H), 3.59 (s, 3H), 3.74 (t, J 9.16 Hz, 2H), 4.15 ppm (t, J
9.35 Hz, 2H); 13C NMR (CDCl3): d 25.7/24.7, 27.8, 33.9, 51.7, 54.6, 67.4,
168.3, 174.0 ppm; elemental analysis calcd (%) for C9H15NO3 (185.22): C
58.36, H 8.16, N 7.56, O 25.91; found: C 57.15, H 8.00, N 7.66.
Block copolymer synthesis: All polymerizations were carried out in a
Schlenk tube under inert atmosphere (N2) using freshly distilled and dried
solvents.
Polymer 3a: A typical procedure for 3a was as follows: 2-Methyl-2-
oxazoline (5.11 g, 60 mmol) was added to a solution of methyltriflate
(328 mg, 2.0 mmol) in acetonitrile (30 mL) and chlorobenzene (15 mL) at
08C. The mixture was heated up to 808C and stirred for 12 h. At 08C,
2-nonyl-2-oxazoline (1.58 g, 8.0 mmol) and 1 (1.48 g, 8.0 mmol) were added.
The solution was stirred at 908C for additional 24 h. The polymerization
was terminated with piperidine (0.5 g, 6 mmol) at room temperature for
4 h. After removal of the solvent, the residue was dissolved in chloroform
(40 mL) and stirred with potassium carbonate (2 g) overnight. After
filtration, the polymer was purified by precipitation in ice cold diethyl ether
and dried in a vacuum oven at 508C. Yield: 6.0 g (73%) of a white solid;
1H NMR (CDCl3): d 0.88 (t, J 6.58 Hz, 3H), 1.26 (m, 12H), 1.58 (m,
2H), 2.11 (m, 3H), 2.33 (m, 4H), 2.64 (m, 4H), 3.05/2.96 (3H), 3.46 (s, 4H),
Experimental Section
Materials: All chemicals were purchased from Aldrich and Merck Eurolab
and were used as received, unless otherwise noted. 2-Methyl-2-oxazoline,
2-nonyl-2-oxazoline, acetonitrile, and chlorobenzene were refluxed over
CaH2 and stored under dry nitrogen atmosphere and mole sieve 4 ä.
Instrumentation: 1H NMR (300,13 MHz), 13C NMR (75,5 MHz), and 31P
NMR (121,5 MHz) spectra were recorded on a Bruker ARX300 spec-
trometer. FTIR spectroscopy was carried out on
a Bruker IFS55
spectrometer. Elemental analyses were measured by the Microanalytical
Laboratory of the Inorganic Institute of the TU M¸nchen. Gel-permeation
chromatography (GPC) was carried out on a Waters (GPC) 510 equipped
with a UV and refractive index (RI) detector; poly(styrene) was used for
the calibration of the poly(2-oxazoline) samples in chloroform as solvent.
The rhodium content was determined by inductively coupled plasma
atomic emission (ICP-AES, Jobin Yvon JY 38 plus) at the Institute of
Analytical Chemistry and Radiochemistry (Prof. M. R. Buchmeiser),
University Innsbruck (Austria). Samples were mixed with 3 mL aqua regia,
heated in a microwave for several minutes and finally diluted with 10 mL
water. HPLC analyses were performed with chiral columns (Chiracel OD;
3.65 ppm (s, 3H); FT-IR (film in CHCl3): nÄ 1636 (C Oamide), 1729 cmÀ1
(C Oester).
Polymer 4a: Polymer 4a was prepared in a similar fashion to 3a by using
methyl triflate (0.164 g, 1 mmol), 2-methyl-2-oxazoline (2.55 g, 30.0 mmol),
2-nonyl-2-oxazoline (0.79 g, 4.0 mmol), 2 (0.74 g, 4.0 mmol), and piperidine
1
(0.22 g, 2.6 mmol). Yield: 2.6 g (62%); H NMR (CDCl3): d 0.86 (t, J
6.49 Hz, 3H), 1.24 (m, 12H), 1.62 (m, 6H), 2.10 (m, 5H), 2.33 (m, 4H), 3.03/
2.94 (3H), 3.45 (s, 4H), 3.63 ppm (s, 3H); FT-IR (film in CHCl3): nÄ 1636
(C Oamide), 1729 cmÀ1 (C Oester).
Chem. Eur. J. 2003, 9, 3228 3234
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3229