Enantioselective addition of dialkylzincs to N-diphenylphosphinylimines using polymer-supported N,N-dialkylnorephedrines as chiral ligands

Article abstract of DOI:10.1039/a702139i

Chiral N,N-dialkylnorephedrines supported on polystyrene resin catalyse the enantioselective addition of dialkylzincs to N-diphenylphosphinylimine, affording optically active N-diphenylphosphinylamines with high enantiomeric excess. The influence on the e

Full text of DOI:10.1039/a702139i

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Enantioselective addition of dialkylzincs to N-diphenylphosphinyl-
imines using polymer-supported N,N-dialkylnorephedrines as chiral
ligands
Takefumi Suzuki, Takanori Shibata and Kenso Soai*
Department of Applied Chemistry, Faculty of Science, Science University of Tokyo,
Kagurazaka, Shinjuku-ku, Tokyo, 162 Japan
Chiral N,N-dialkylnorephedrines supported on polystyrene resin catalyse the enantioselective addition of
dialkylzincs to N-diphenylphosphinylimine, affording optically active N-diphenylphosphinylamines with
high enantiomeric excess. The influence on the enantioselectivity of the carbon-chain spacer between the
chiral function and polymer resin has been examined. Reaction temperature- and solvent-dependency on
the chiral polymer-catalysed reaction has also been examined.
Introduction
CH2 CH
Me
Ph
m
Chiral polymers are attractive materials in their utility as chiral
catalysts in asymmetric synthesis.1 Polymer catalysts unite the
advantages of homogeneous and heterogeneous catalyst; they
possess high catalytic activity in organic solvents and are con-
veniently recovered and recycled. However, only a handful of
those show high enantioselectivity in asymmetric carbon–
carbon bond-forming reactions.2 We have reported that dialkyl-
zincs react enantioselectively with aromatic and aliphatic
aldehydes using polymeric chiral ligands, that is, chiral β-amino
alcohols supported on polystyrene resin, to afford the corre-
sponding optically active secondary alcohols in high enantio-
meric excesses up to 89% ee.3
On the other hand, the development of asymmetric syntheses
of optically active amines is of importance in the preparation
of compounds for therapeutic use, since partial structures of
physiologically active substances such as amino acids and
alkaloids frequently contain optically active amines.4 Although
one of the simpler methods for the synthesis of chiral amines is
by asymmetric alkylation of imines, there have been few reports
of enantioselective alkylation of imines.5
*
*
PhCH2N
R
OH
Me
Ph
*
*
CH
N
R
OH
2
R = Me (1R,2S)-1a
R = Bu (1S,2R)-1b
R = Me (1R,2S)-1ap
R = Bu (1S,2R)-1bp
CH2 CH
Me
Ph
m
*
*
PhCH2O(CH2)nN
R
OH
Me
Ph
*
*
CH2O(CH2)n
N
R
OH
R = Me n = 6 (1R,2S)-2a
R = Bu n = 4 (1S,2R)-2b
R = Me n = 6 (1R,2S)-2ap
R = Bu n = 4 (1S,2R)-2bp
Fig. 1
We have reported that dialkylzincs enantiomerically add to
N-diphenylphosphinylimines using chiral β-amino alcohols as
chiral ligands to afford the corresponding optically active
phosphinamides in high enantiomeric excess (up to 95% ee).6
Such optically active phosphinamides readily undergo acid
hydrolysis to give the corresponding optically active amines
without racemization.
We here disclose the full detail of an enantioselective add-
ition of dialkylzincs to N-diphenylphosphinylimines using
chiral β-amino alcohols supported on polystyrene resin as a
chiral ligand.7
polystyrene resin and the chiral entity are connected with a
carbon straight chain as a spacer. A chiral polymer 1ap*, con-
taining more of the chiral reactive site, was also prepared from
chloromethylated polystyrene resin (chlorine content 4.3 mmol
g
Ϫ1) and (1R,2S)-ephedrine.
In order to examine the catalytic activity, the corresponding
monomeric chiral amino alcohols 1a,2a 1b,3c and 2a,3c 2b3c were
also synthesized.
Enantioselective addition of diethylzinc to N-diphenyl-
phosphinylimine using monomeric or polymeric chiral ligands
As a preliminary experiment, asymmetric ethylation of N-
diphenylphosphinylimine 3a using diethylzinc using monomeric
chiral amino alcohols 1a, 1b, 2a and 2b as chiral ligands was
examined (Table 1). In each entry, optically active phosphin-
amide 4a was obtained in high enantiomeric excess. Among
them 1a was found to be the most effective ligand for the
enantioselective alkylation. Interestingly, the presence of a car-
bon spacer decreased the catalytic activity when the nitrogen
atom of the chiral ligand was substituted when a methyl group
(Entries 1 and 3), while it increased with a butyl substituent
(Entries 2 and 4).
Results and discussion
Synthesis of polymer-supported and the corresponding
monomeric N,N-dialkylnorephedrines as chiral ligands
In considering how the structure of the connection between the
polystyrene and the asymmetric auxiliary might influence the
chiral environment of the catalyst, we thought that the reactiv-
ity and enantioselectivity could possibly be altered by intro-
ducing spacers between the resin and the chiral amino alcohol.
We have, therefore, synthesized 3b,c the chiral polymers as shown
in Fig. 1; in 1ap3c and 1bp3c the chloromethylated polystyrene
resin (chlorine content 0.8 mmol gϪ1) and the chiral amino
alcohol are connected directly, whilst in 2ap3c and 2bp3c the
Next we examined the enantioselective addition of Et2Zn to
N-diphenylphosphinylimine using polymer-supported N,N-
dialkylnorephedrines 1ap, 1bp, 2ap and 2bp (Table 2).
J. Chem. Soc., Perkin Trans. 1, 1997
2757

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Table 1 Enantioselective ethylation of imine 3a using monomeric
Table 3 Effect of reaction temperature on the reaction using the
chiral ligands
polymeric ligand (1R,2S)-1ap
Polymeric
chiral ligand
H
Monomeric
Ph
H
N
Ph
chiral ligand
Ph
Ph
N
*
Ph
Ph
N
Ph
Ph
P
(1R,2S)-1ap
Ph
Et2Zn
Ph
+
P
Ph
N
P
toluene, RT, 1 d
Ph
Et2Zn
+
P
Et
O
toluene, RT
O
Et
(R)-4a
O
O
4a
3a
3a
Entry
Ligand
Yield (%)
Ee (%)
Config’n
Entry
Temp. (ЊC)
Time (h)
Yield (%)
Ee (%)
1
2
3
4
(1R,2S)-1a
(1S,2R)-1b
(1R,2S)-2a
(1S,2R)-2b
91
39
47
75
91
78
81
83
R
S
R
S
1
2
3
4
40
RT
0
12
48
48
12
40
70
19
9.5
57
84
79
59
Ϫ 10
active site interactions, which interfere with the formation of an
appropriate transition state.
Table 2 Enantioselective ethylation of the imine 3a using polymeric
chiral ligands
It was found that use of polymeric ligands gave rise to a
need for longer reaction times and resulted in generally lower
chemical yields than those found with monomeric ligands. This
reduced catalytic activity is a result of the partial insolubility of
polymeric chains that may fail to operate as a chiral ligand in
the asymmetric reaction, and thus result in a decrease of the
active sites for polymeric ligands.
H
Polymeric
Ph
Ph
chiral ligand
Ph
N
*
Ph
Ph
N
P
Ph
Et2Zn
+
P
toluene, RT, 2 d
Et
O
O
4a
3a
Ligand a
Yield (%)
Ee (%)
Config’n
Screening of the reaction conditions using the polymeric chiral
ligand 1ap
Entry
1
(1R,2S)-1ap
(1S,2R)-1bp
(1R,2S)-2ap
(1S,2R)-2bp
(1R,2S)-1ap
(1R,2S)-1ap*d
70
10
31
52
45
55
84
6
69
65
81
62
R
S
R
S
R
R
In order to improve the chemical and optical yield, we opti-
mized the reaction conditions of the enantioselective ethylation
of N-diphenylphosphinylimine 3a using the polystyrene-
supported norephedrine derivative (1R,2S)-1ap which had been
shown to give the best results of the compounds tested.
2b
3
4
5c
6
Effect of temperature. We examined the effect of temperature
on the reaction using the catalysts (1R,2S)-1ap (Table 3). At
room temperature the alkylated product (R)-4a was given with
the highest enantiomeric excess (84% ee) and chemical yield
(70%; Entry 2). At 0 ЊC, only a slight decrease in the enantio-
meric excess was observed, but there was a considerable decline
in the chemical yield (Entry 3). At 40 ЊC, the enantiomeric
excess decreased markedly (Entry 1). These results suggested
that although the catalytic reaction fails to proceed smoothly at
0 ЊC, direct ethylation by diethylzinc proceeds at 40 ЊC regard-
less of the use or not of a chiral catalyst. Moreover, it is notice-
able that temperature dependency is much increased in reac-
tions using a polymeric chiral ligand than those using a
monomeric chiral ligand (N,N-dialkylnorephedrine at RT, 0 ЊC
and Ϫ13 ЊC affords 4a with 86, 89 and 88% ees, respectively);8
this may indicate that the degree of the swelling of the polymers
has a potent influence on the catalytic activity.
Examination of solvent. On examining the effect of various
solvents (Table 4) on the enantioselective addition of diethyl-
zinc to 3a, we found that the product (R)-4a was given in high
enantiomeric excess (80–85% ee) in toluene, diethyl ether, o- or
m-xylene or isopropylbenzene (Entries 1–3, 6, 7, 11); it was
found that toluene was the best solvent. In hexane or benzene,
both enantiomeric excesses and chemical yields remained low
(Entries 4, 5). It is noted that a more significant solvent effect
was observed in the reactions with polymeric asymmetric
catalysts than with monomeric ones [monomeric N,N-dibutyl-
norephedrine (DBNE) shows the same enantioselectivity of
84% ee both in toluene and hexane].8 It is thought that the
degree of swelling of the polymeric ligand varied with different
solvents. Thus, it probably gives rise to a significant solvent
effect in the asymmetric alkylation, e.g. benzene was adsorbed
by the polymeric catalyst, and thus, for the reaction mixture to
be stirred smoothly double the volume of solvent was required
to that required of others; little swelling of the polymeric
asymmetric catalysts was observed in hexane, the optimum
amount of swelling was observed on use of toluene as solvent.
On the other hand, in the enantioselective addition of dialkyl-
a
Unless otherwise noted, polymeric chiral ligands were prepared using
chloromethylated polystyrene resin (1% divinylbenzene; chlorine con-
tent 0.8 mmol gϪ1; 100–200 mesh). b Reaction time is 5 d. c Recycled
chiral ligand 1ap was used. d Polymeric chiral ligand 1ap* was prepared
using chloromethylated polystyrene resin (2% divinylbenzene; chlorine
content 4.3 mmol gϪ1; 200–400 mesh).
The order of catalytic activity was in accordance with that
of the corresponding monomeric chiral catalyst and optically
active (R)-4a was obtained using the polymeric chiral ligand
(1R,2S)-1ap in good yield (70%) and with high enantiomeric
excess (84%; Entry 1). Further, when polymeric chiral ligands
are used, the influence of a carbon spacer on the catalytic activ-
ity varies with the substituent on the nitrogen atom; the differ-
ences noted were larger than those for monomeric ligands; 1bp
possesses almost no catalytic activity (10%, 6% ee), while 2bp,
which is different from 1bp only in the existence of carbon
spacer, has moderate catalytic activity (52%, 65% ee) (Entries 2
and 4). These results suggest that the bulky butyl group on the
nitrogen atom decreases the catalytic activity when the chiral
moiety is directly connected to the polymer resin but that the
spacer enables the substrate to approach the chiral moiety.
One of the advantages of a polymeric chiral ligand over a
monomeric chiral ligand is its easier separation from the reac-
tion mixture. Thus, filtration of the reaction mixture readily
separated insoluble polymeric chiral ligand, with subsequent
recycling of chiral ligand and purification of the product being
facilitated. In Entry 5, recycled chiral polymeric ligand which
had been used in Entry 1, recovered by filtration and washed
with acid and base, was examined. As a result, 4a was obtained
with almost the same ee as that in Entry 1, although the chem-
ical yield was diminished. These results imply that in this system
the facile recycling of the polymeric ligand is possible by the
appropriate treatment of the used ligand.
Use of 1ap* for the addition of diethylzinc to 3a gave a much
decreased ee (62% ee; Entry 6), a result probably explained in
terms of a higher degree of polymer loading of ephedrine in
1ap* than in 1ap. A high degree of loading may give rise to
2758
J. Chem. Soc., Perkin Trans. 1, 1997

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Table 4 Effect of solvent on the reaction using polymeric ligand
(1R,2S)-1ap
Table 6 Enantioselective ethylation of various imines 3a–d with
dialkylzincs using (1R,2S)-1ap
Polymeric
chiral ligand
Polymeric
chiral ligand
H
N
H
N
Ph
Ph
Ph
Ph
(1R,2S)-1ap
Ph
R1
(1R,2S)-1ap
Ph
Ph
R1
N
Ph
N
P
P
Ph
Et2Zn
Ph
+
R2
2Zn
+
P
P
solvent, RT, 2 d
toluene, RT, 1-2 d
R2
O
Et
(R)-4a
O
O
O
3a
4a–e
3a–d
Entrya
Solvent
Yield (%)
Ee (%)
Entry
R1
Phenyl
1-Naphthyl
2-Naphthyl
2-Furyl
R2
Yield (%)
Ee (%)
1
2b
3
Toluene
Toluene
Diethyl ether
Hexane
Benzene
o-Xylene
m-Xylene
p-Xylene
Ethylbenzene
Mesitylene
Isopropylbenzene
Acetonitrile
70
80
50
55
4
56
43
28
56
49
20
15
84
80
83
2
22
85
85
78
62
75
82
55
1
2
3
4
5
Et
Et
Et
Et
4a
4b
4c
4d
4e
70
65
61
31
46
84
62
85
69
86
4
5c
6
Phenyl
Me
7
8
9
10
11
12
4a–d were given in good or high enantiomeric excesses. In the
enantioselective addition of diethylzinc, phosphinylimine 3c
possessing a 2-naphthyl group gave the corresponding phos-
phinamide 4c in the optical yield of 85% ee (Entry 3). The
generality of the reaction for other dialkylzincs was exemplified
with dimethylzinc: enantioselective addition of dimethylzinc to
3a afforded the corresponding compound 4e with an ee as high
as 86% (Entry 5).
In conclusion, enantioselective addition of dialkylzincs to
N-diphenylphosphinylimine in the presence of a chiral amino
alcohol supported on polystyrene resin as a chiral ligand
affords optically active phosphinamides with high ee. The sol-
vent effect on the enantioselectivity is significant, and toluene is
the best solvent. The chiral polymer ligand easily separated
from the product by filtration, can be re-used without any
significant decline in its enantioselectivity.
a
Unless otherwise noted, a hexane solution of diethylzinc was used.
b A toluene solution of diethylzinc was used. c Reaction time is 5 d.
Table 5 Effect of the amount of polymeric ligand (1R,2S)-1ap
Polymeric
H
N
chiral ligand
(1R,2S)-1ap X (equiv.)
toluene, RT, 2 d
Ph
Ph
Ph
Ph
Ph
N
P
Ph
Et2Zn
+
P
Et
O
O
(R)-4a
3a
Entry
X (equiv.)
Yield (%)
Ee (%)
Experimental
1
2
3
4
5
0.4
0.6
0.8
1.0
2.0
50
55
60
70
54
61
71
80
84
88
General
1H NMR and IR spectra, and optical rotations (given in units
of 10Ϫ1 deg cm2 gϪ1) were recorded with an HITACHI R-1200
spectrometer, an HITACHI 260-10(30) spectrophotometer, and
a JASCO DIP-360 polarimeter, respectively. HPLC analyses
were performed using SHIMADZU LC-6A or 8A with a Daicel
chiral column.
zincs to aldehydes using polymeric chiral catalysts (1ap), hexane
as well as toluene affords the products (sec-alcohols) with high
ees.3
Because diphenylphosphinylimine 3a is bulkier than alde-
hydes, such as benzaldehyde, the enantioselectivities of the add-
ition to imine using polymeric ligand seems to be affected by the
environment of active site more drastically than those of the
addition to aldehydes. The appropriate swelling of polymeric
chiral ligand seems to be essential, i.e. too little swelling in
hexane and too much swelling in benzene.
Materials
Diethylzinc in hexane was purchased from Kanto Chemical
Co. Chloromethylated polystyrene resins (1% divinylbenzene;
chlorine content 0.8 mmol gϪ1; 100–200 mesh and 2% divinyl-
benzene; chlorine content 4.3 mmol gϪ1; 200–400 mesh) were
purchased from Kokusan Chemical Works and Fluka Chem-
ika, respectively. Compounds 1a,2a 1b,3c 1ap,3c 1bp,3c 2a,3c 2b,3c
2bp3c and 3a–c9 were prepared according to the literature pro-
cedures. Compound 3d was synthesized from 2-furaldehyde
and P,P-diphenylphosphinamide by a literature procedure.9
Examination of the amount of chiral catalyst
The optimal amount of the polymeric asymmetric catalyst
(1R,2S)-1ap was determined (Table 5). With the other
reaction conditions fixed (reaction time = 2 d, reaction tem-
perature = RT, solvent = toluene), the amount of 1ap was varied
in the range 0.4–2.0 equiv. compared with the phosphinylimine
3a. The enantiomeric excesses increased with increasing
amounts of 1ap. The chemical yield was at a maximum with
equivalent amounts of 1ap and substrate (Entry 4).
N-(2-Furylmethylidene)-P,P-diphenylphosphinamide
3d.
White solid, mp 151 ЊC; νmax(KBr disk)/cmϪ1 1625 and 1205;
δH 6.59 (m, 1H), 7.18–8.30 (m, 12H) and 9.10 (d, 1H, J 35.4)
[Found: m/z (HRMS) 295.0769. Calc. for C17H14NO2P: M,
295.0762].
General procedure for the enantioselective addition of diphenyl-
phosphinylimines 3 by using a polymeric chiral ligand (1R,2S)-
1ap
To a toluene solution (3 ml) of the polymeric asymmetric cata-
lyst (1R,2S)-1ap (0.2 mmol, 0.2597 g) and the imine 3 (0.2
mmol) at 0 ЊC under an argon atmosphere, diethylzinc was
added dropwise. After completion of the addition and removal
of the ice bath the mixture was stirred at room temperature for
2 days. After this, the reaction was quenched by the addition of
saturated aqueous ammonium chloride to the reaction mixture
Enantioselective addition of dialkylzincs to various N-diphenyl-
phosphinylimines
In order to evaluate the generality of this asymmetric reaction,
the enantioselective alkylation of various N-diphenylphos-
phinylimines 3a–d having phenyl, naphthyl or furyl groups on
the carbon atom of the imines were carried out in the presence
of (1R,2S)-1ap under the optimum reaction conditions as men-
tioned above (Table 6). The corresponding ethylated products
J. Chem. Soc., Perkin Trans. 1, 1997
2759

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which was then filtered under reduced pressure to remove the
precipitates and then extracted with dichloromethane (10
ml × 3). The combined extracts were dried (Na2SO4), filtered
and evaporated under reduced pressure. TLC purification
[hexane–acetone 3:2 (v/v), developed twice] of the residue gave
the corresponding optically active diphenylphosphinamide 4,
the enantiomeric excess of which was determined by HPLC
analysis using DAICEL Chiralcel OD (4.6 × 250 mm; 254 nm
UV detector; eluent: 3% propan-2-ol in hexane; flow rate: 1.0
ml minϪ1; column temperature: ca. 20 ЊC).
(R)-N-(1-Phenylpropyl)-P,P-diphenylphosphinamide 4a. Yield
70%, ee 84% (Rt /min 15 for R isomer, 24 for S isomer); νmax(KBr
disk)/cmϪ1 3150 and 1210; δH 0.80 (3H, t), 1.93 (2H, m), 3.48
(1H, m), 4.14 (1H, m) and 7.10–8.20 (15H, m) [Found: m/z
(HRMS) 335.1442. Calc. for C21H22NOP: M, 335.1439]; mp
129.0 ЊC (recrystallized from hexane–ethyl acetate, 98% ee); [α]D32
Ϫ42.3 (c 2.0, MeOH, 98% ee).
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research in Priority Areas of New Polymers and Their Nano-
Organized Systems from the Ministry of Education, Science,
Sports and Culture.
References
1 Reviews: Polymer-supported Reactions in Organic Synthesis, eds.,
P. Hodge and D. C. Sherrington, J. Wiley & Sons, Chichester, 1980,
p. 1; J. M. Maud, Solid Supports and Catalysts in Organic Synthesis,
ed. K. Smith, Horwood, New York, 1992, p. 40; S. Itsuno,
Macromolecules, 1992, ed. J. Kahovec, VSP, Utrecht, 1993, p. 413;
E. C. Blossey and W. T. Ford, Comprehensive Polymer Science, eds,
G. C. Eastmond, A. Ledwith, S. Russo and P. Sigwalt, Pergamon
Press, Oxford, 1989, vol. 6, p. 81; Synthesis and Separations Using
Functional Polymers, eds., D. C. Sherrington and P. Hodge, J. Wiley &
Sons, Chichester, 1988, p. 1; C. U. Pittman, Comprehensive
Organometallic Chemistry, eds., G. Wilkinson, A. G. Stone and
E. W. Abel, Pergamon Press, Oxford, 1982, vol. 8, p. 553; Chiral
Reactions in Heterogeneous Catalysis, eds., G. Jannes and V. Dubois,
Plenum, New York, 1995.
N-[1-(1-Naphthyl)propyl]-P,P-diphenylphosphinamide
4b.
Yield 65%, ee 62% (Rt /min 27 for R isomer, 41 for S isomer);
νmax(KBr disk)/cmϪ1 3150 and 1185; δH 0.84 (3H, t), 2.05 (2H,
m), 3.95 (1H, br s), 4.95 (1H, m) and 6.85–8.40 (17H, m)
[Found: m/z (HRMS) 385.1598. Calc. for C25H24NOP: M,
385.1596]; mp 133.5–134.0 ЊC (recrystallized from hexane–
dichloromethane, 65% ee); [α]D32 Ϫ21.5 (c 2.0, MeOH, 65% ee).
2 (a) Z. Zhengpu, P. Hodge and O. W. Stratford, React. Polym., 1991,
15, 71; (b) K. Kamahori, K. Ito and S. Itsuno, J. Org. Chem., 1996,
61, 8321; (c) D. Seebach, R. E. Marti and T. Hintermann, Helv. Chim.
Acta, 1996, 79, 1710.
3 (a) K. Soai, S. Niwa and M. Watanabe, J. Org. Chem., 1988, 53, 927;
(b) K. Soai, S. Niwa and M. Watanabe, J. Chem. Soc., Perkin Trans. 1,
1989, 109; (c) K. Soai and M. Watanabe, J. Chem. Soc., Perkin Trans.
1, 1994, 837.
4 R. A. Volkmann, Comprehensive Organic Synthesis, ed. B. M. Trost,
Pergamon Press, Oxford, 1991; vol. 1, ch. 1.12, p. 355; E. F. Kleinman
and R. A. Volkmann, Comprehensive Organic Synthesis, ed.
B. M. Trost, Pergamon Press, Oxford, 1991; vol. 2, ch. 4.3, p. 975;
J. Klein, in The Chemistry of Double-Bonded Functional Groups:
Supplement A, ed. S. Patai, J. Wiley & Sons, Chichester, 1989, vol. 2,
part 2, ch. 10.
5 I. Inoue, M. Shindo, K. Koga and K. Tomioka, Tetrahedron, 1994,
50, 4429; S. E. Denmark, N. Nakajima and O. J.-C. Nicaise, J. Am.
Chem. Soc., 1994, 116, 8797; I. Inoue, M. Shindo, K. Koga, M. Kanai
and K. Tomioka, Tetrahedron: Asymmetry, 1995, 6, 2527.
6 (a) K. Soai, T. Hatanaka and T. Miyazawa, J. Chem. Soc., Chem.
Commun., 1992, 1097; (b) T. Suzuki, N. Narisada, T. Shibata and
K. Soai, Tetrahedron: Asymmetry, 1996, 7, 2519.
N-[1-(2-Naphthyl)propyl]-P,P-diphenylphosphinamide
4c.
Yield 61%, ee 85% (Rt /min 27 for R isomer, 44 for S isomer);
νmax(KBr disk)/cmϪ1 3200 and 1190; δH 0.83 (3H, t), 2.01 (2H,
m), 3.37 (1H, br s), 4.25 (1H, m) and 7.05–8.15 (17H, m)
[Found: m/z (HRMS) 385.1596. Calc. for C25H24NOP: M,
385.1596]; mp 150.5 ЊC (recrystallized from hexane–dichloro-
methane, 91% ee); [α]D27 Ϫ50.6 (c 3.8, MeOH, 91% ee).
N-[1-(2-Furyl)propyl]-P,P-diphenylphosphinamide 4d. Yield
31%, ee 69% (Rt /min 38 for R isomer, 46 for S isomer); νmax(KBr
disk)/cmϪ1 3140 and 1210; δH 0.87 (3H, t), 1.94 (2H, q), 3.37
(1H, br), 4.18 (1H, m), 6.14 (2H, m) and 7.10–8.15 (11H, m)
[Found: m/z (HRMS) 325.1224. Calc. for C19H20NO2P: M,
325.1232]; mp 98 ЊC (recrystallized from hexane–dichloro-
methane, 76% ee); [α]D27 Ϫ40.0 (c 4.07, MeOH, 76% ee).
Recycling of the chiral polymer (1R,2S)-1ap
7 For a preliminary communication of a part of this work, see:
K. Soai, T. Suzuki and T. Shono, J. Chem. Soc., Chem. Commun.,
1994, 317.
8 Master’s degree dissertation of T. Hatanaka, Science University of
Tokyo, 1992.
The used polymer 1ap (2.2 g), recovered from the quenched
mixture by filtration, was stirred first in a mixture of THF (16
ml), 6  aq. HCl (2 ml) and water (4 ml) and then after being
filtered off was stirred again for 4 h in a mixture of THF (16 ml)
and 2  aqueous sodium hydroxide (4 ml). The polymer 1ap was
then filtered off and washed successively with 50 ml portions of
water, methanol, THF, aq. THF, THF and methanol. After
being dried at 40 ЊC in vacuo for 5 h, it was recycled and used for
enantioselective ethylation (Recovery: 1.6 g).
9 W. B. Jennings and C. J. Lovely, Tetrahedron, 1991, 47, 5561.
Paper 7/02139I
Received 27th March 1997
Accepted 11th June 1997
2760
J. Chem. Soc., Perkin Trans. 1, 1997