Easily accessible chiral amino-phosphinite ligands for highly enantioselective palladium-mediated allylic alkylation

Article abstract of DOI:10.1016/S0957-4166(02)00204-5

Palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with the dimethyl malonate-BSA-LiOAc system has been successfully carried out in the presence of easily prepared new chiral amino-phosphinite ligands such as 3b and 3c to

Full text of DOI:10.1016/S0957-4166(02)00204-5

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 13 (2002) 809–813
Easily accessible chiral amino-phosphinite ligands for highly
enantioselective palladium-mediated allylic alkylation
Guoshu Chen,a Xin Li,a Haile Zhang,a Liuzhu Gong,a,* Aiqiao Mi,a,* Xin Cui,a Yaozhong Jiang,
Michael C. K. Choib and Albert S. C. Chanb
aUnion Laboratory of Asymmetric Synthesis, Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences,
Chengdu 610041, China
bOpen Laboratory of Chirotechnology and Department of Applied Biology and Chemical Technology,
The Hong Kong Polytechnic University, Hong Kong, China
Received 4 February 2002; accepted 23 April 2002
Abstract—Palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with the dimethyl malonate–BSA–
LiOAc system has been successfully carried out in the presence of easily prepared new chiral amino-phosphinite ligands such as
3b and 3c to result in good yields and excellent enantioselectivities of up to 95% e.e. © 2002 Published by Elsevier Science Ltd.
1. Introduction
knowledge, only a few examples of phosphinite-amine
type ligands have been successfully involved in this
area.7 Herein we would like to describe the application
of phosphinite-amines 1–3 as chiral ligands in palla-
dium-catalyzed asymmetric allylic alkylation reactions
(Fig. 1).
Palladium-catalyzed allylic alkylation is known as a
very useful synthetic method for the formation of car-
bonꢀcarbon bonds.1,2 The selection of chiral ligands for
highly enantioselective allylic substitution has focused
on the use of mixed bidentate donor ligands such as
phosphorus–nitrogen2 or sulfur–nitrogen,3 and phos-
phorus–sulfur,4 which have shown excellent levels of
stereochemical control. Most of these chiral bidentate
ligands were equipped with strong and weak donor
heteroatom pairs (e.g. PR3/NR3 or PR3/SR2). The elec-
tronic effects have the potential to influence both the
stability and reactivity of the intervening diastereomeric
reaction intermediates in the catalytic cycle. Bidentate
aminophosphine ligands of this type have been applied
in the enantioselective palladium-catalyzed nucleophilic
alkylation of allylic esters, and shown similar stereocon-
trol properties to phosphine-oxazolines.5 Generally,
palladium complexes of mixed donor ligands with six-
membered rings deliver higher enantioselectivity than
those with five-membered rings.6 Upon coordination
chiral amino-phosphinite ligands 1–3 would form six-
membered ring complexes with palladium, thus it was
thought that they could induce high enantioselectivity
in the allylic alkylation reaction. To the best of our
2. Results and discussion
N,N-Dialkyl-1,2-diphenyl-2-aminoethanols,
derived
from (1R,2S)- and (1S,2S)-1,2-diphenyl-2-amino-
ethanol, have exhibited powerful enantioselective con-
trol properties in many reactions, and have been proven
to be good chirality sources for the derivation of new
chiral ligands.8 The amino-phosphinites 1–3 were syn-
thesized by reaction of the corresponding N,N-dialkyl-
1, 2-diphenyl-2-aminoethanols and N-alkyl-1,2-di-
phenyl-2-aminoethanols with 1 equiv. of chlorodi-
phenylphosphine in the presence of Et3N and
Ph
Ph
Ph
Ph
Ph
Ph
O
O
O
CH3N
R
CH3N
R
HN
R
PPh2
PPh2
PPh2
1a. R =Me
2. R =Bn
3a. R= Bn
1b. R = Et,
1c. R = i-Pr,
1d. R = Bn
3b. R= n-Bu
3c. R= iPr
* Corresponding authors. Fax: +86-28-5223978; e-mail: gonglz@
cioc.ac.cn; miaq@cioc.ac.cn
Figure 1. Chiral ligands studied in the titled reaction.
0957-4166/02/$ - see front matter © 2002 Published by Elsevier Science Ltd.
PII: S0957-4166(02)00204-5

810
G. Chen et al. / Tetrahedron: Asymmetry 13 (2002) 809–813
catalytic DMAP in toluene followed by filtration through
a plug of Al2O3 to remove the catalyst and triethylamine
hydrochloride.9 The free amine groups in 3a–c were
unreactive toward further phosphinylation due to the
bulky R group.
(1d) led to a change in the e.e. values from (R)-enantioen-
richments of 46 and 40% with ligands 1a and 1b, to
(S)-enantioenrichments of 8%, 22% e.e. with ligands 1c
and 1d, respectively. Thus, the configuration of the major
product could be inverted simply by changing the second
nitrogen substituent of the ligand. The ligands (1R,2S)-
1d and (1S,2S)-2 showed comparable enantioselectivities
and gave similar enantiomer-rich products (entries 4 and
5), which indicates that the chirality of the carbon
bonded to nitrogen plays an important role in the
stereochemical discrimination. This result is consistent
with the findings for palladium complexes in traditional
P,N-ligand-catalyzed allylic alkylation.5
The efficiency of N,N-disubstituted amino phosphinites
1a–d and 2 was first investigated using the standard
procedure for the palladium-catalyzed substitution of
1,3-diphenyl-2-propenyl acetate with dimethyl malonate.
The acetate 4 was reacted with the nucleophile in situ
generated from dimethyl malonate (3 equiv.) and N,O-
bis(trimethylsilyl)acetamide (BSA, 3 equiv.) in the
OAc
MeO2C
Ph
CO2Me
Ph
1 mol% Pd, 1mol%L*
+
CH2(COOCH3)2
LiOAc, BSA,
solvent
Ph
Ph
*
4
5
6
presence of 1 mol% catalyst, which was formed in situ
from allyl palladium chloride dimer with two molar
equivalents of the chiral ligands in the reaction solvent
for 20 min at room temperature. The results are summa-
rized in Table 1. The enantioselectivity was strongly
dependent on the nitrogen substituent: maintaining a
methyl group bonded to nitrogen while changing the
other nitrogen substituent in substrates 1a and 1c–e led
to a pronounced trend that increasing steric hindrance
from the N-substituent resulted in a decrease in the ratio
of the major (R)-enantiomeric products. Thus, changing
the N-R group from Me (1a) to Et (1b), iPr (1c) and Bn
The dramatic substituent effects on the stereochemical
outcome prompted us to fine tune the structure of the
ligands for the allylic alkylation reaction. It is encourag-
ing that the enantioselectivity of the allylic alkylation was
dramatically improved to 87% e.e. when 3a (Table 2,
entry 1) was used as a chiral ligand from the 22% e.e.
achieved with the N-methylated analogue 1d (Table 1,
entry 4). This result implies that N-monosubstituted
amino-phosphinites might give better enantioselectivity
than N,N-disubstituted ones. As the best chiral ligands
could be found by screening the nitrogen substituent on
the ligands, the N-butyl and N-isopropyl analogues 3b
Table 1. Allylic alkylation of 4 with dimethyl malonate catalyzed by the complexes of [Pd(allyl)Cl]2 with N,N-disubstituent
amino-phosphinites
Entry
Ligand
Temp. (°C)
Time (h)
Yielda (%)
E.e.b (%)
Config.c
1
2
3
4
5
1a
1b
1c
1d
2
25
25
25
25
30
6
10
10
10
5
95
98
98
98
90
46
40
8
22
28
R
R
S
S
S
a Isolated yield.
b Measured by HPLC (chiral OD, n-hexane:isopropanol=99:1).
c The absolute configuration was determined by comparing the specific rotation with the literature value.5c
Table 2. Allylic alkylation of 4 with dimethyl malonate catalyzed by palladium complexes of N-monosubstituent amino-
phosphinites
Entry
Palladium
Ligand
Solvent
Temp. (°C)
Time (h)
Yielda (%)
E.e.b (%)
1
2
3
4
5
6
7
8
9
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
Pd(dba)2
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
3a
3b
3c
3b
3b
3b
3b
3b
3c
3c
Toluene
Toluene
Toluene
Toluene
CH2Cl2
Toluene
Toluene
Toluene
Toluene
Toluene
20
30
20
30
25
12
6
10
3
15
24
36
72
36
48
88
98
98
98
47
96
90
84
88
75
87
73
91
53
82
75
84
95
93
95
0
−20
−78
0
10
−20
a Isolated yield.
b Measured by HPLC (chiral OD, n-hexane:isopropanol=99:1), and the absolute configuration is R.

G. Chen et al. / Tetrahedron: Asymmetry 13 (2002) 809–813
811
and 3c were prepared. Both of these ligands provided
4.3. N-Methyl-N-alkyl-1,2-diphenyl-2-aminoethanols
high enantioselectvities of 73% (entry 2) and 91% e.e.
(entry 3), respectively. A further improvement in the
enantioselectivity was realized by lowering the reaction
temperature. When the reaction was run at −78°C, 3b
gave 84% yield and 95% e.e. (entry 8). The same
enantioselectivity of 95% e.e. was provided using ligand
3c at −20°C (entry 10). These results demonstrated that
the amine proton in the ligand plays a dominant role in
the enantioselection. The reason that the N-monosub-
stituted amino-phosphinite ligands induced better enan-
tioselectivity is not really clear. We propose that such
a phenomenon probably results from the second inter-
action between the N–H proton in the ligand (for
example, in 3c) and the nucleophile.10
A typical procedure for the preparation of N-methyl-
N-alkyl-1,2-diphenyl-2-aminoethanols is as follows: the
1,2-diphenyl-2-aminoethanol (1.3 mmol) was treated
with the corresponding aldehyde or ketone (1.5 mmol)
in anhydrous ethanol (5 mL) for 3 h at room tempera-
ture, followed by reduction with NaBH4 (0.1 g, 2
mmol). The mixture was stirred for 1 h and adjusted to
pH 1 with aqueous HCl (2N). After removal of the
solvent, the residue was dissolved in water (5 mL) and
basified with aqueous NaOH (2N) to pH 10, the
aqueous layer was extracted with CH2Cl2 (3×10 mL)
and dried over anhydrous MgSO4. Removal of the
solvent gave N-alkyl-aminoethanol, which was stirred
with formic acid (5 mL) for 0.5 h and then treated with
formaldehyde (5 mL) and heated under reflux for about
8 h. The excess formaldehyde was removed. The residue
was cooled to room temperature, water (10 mL) was
added and the mixture was basified to pH 10 and
extracted with CH2Cl2 (3×10 mL). The combined
organic layer was washed with brine and dried over
anhydrous MgSO4. Removal of the solvent yielded a
white solid, which was recrystallized from petroleum
ether (60–90°C) to give the desired products.
3. Conclusion
In summary, we have investigated the applications of
N,N-disubstituted and N-monosubstituted amino phos-
phinites, which were readily derived from (1R,2S)- and
(1S,2S)-1,2-diphenyl-2-aminoethanol, in the palladium-
catalyzed allylic alkylation with excellent enantioselec-
tivities of up to 95% e.e. The results show that
N-monosubstituted amino phosphinites had better
asymmetric induction properties than the N,N-disubsti-
tuted ones. For N,N-disubstituted amino phosphinite 1,
the configuration of the product could be reversed by
variation of the substituent bonded to the nitrogen of
the ligands. Investigations into the uses of such facile
chiral ligands for other asymmetric transformations are
now in progress.
4.4. N,N-Dialkyl-O-diphenylphosphino-1,2-diphenyl-2-
aminoethanol
To a solution of N,N-dialkyl-1,2-diphenyl-2-amino-
ethanol or N-alkyl-1,2-diphenyl-2-aminoethanol (0.5
mmol), a catalytic amount of DMAP (10 mg) in dry
toluene (5 mL) under nitrogen was added chloro-
diphenylphosphine (0.5 mmol). After stirring at room
temperature for 24 h, the resulting mixture was filtered,
concentrated and purified by chromatography (toluene
as eluent) to afford the products.
4. Experimental
4.1. General
Compound 1a: White powder, yield: 87%, mp 116–
117°C. [h]2D0=−14.5 (c=0.2, CHCl3); 1H NMR (400
MHz, CDCl3): l (ppm) 2.14 (s, 6H), 3.68 (d, J=7.6 Hz,
1H), 5.37 (t, J=8.0 Hz, 1H), 7.08–7.29 (m, 20 H). 13C
NMR (100 MHz, CDCl3): l (ppm) 42.69, 75.89 (d,
J=7.5 Hz), 82.36 (d, J=20.0 Hz), 127.00, 127.39,
127.55, 127.60, 127.67, 127.75, 127.77, 127.83, 128.64,
128.79, 130.11, 130.26, 130.47, 130.57, 130.79. 31P
NMR (CDCl3): l (ppm) 112.03.
Palladium complexes are gifts from Professor Yoshinori
Yamamoto at Tohuku University in Japan. Melting
points were measured on a digital melting point
apparatus and were uncorrected. Mass spectra were
recorded on Finnigan LCQ DECA instrument. IR
spectra were obtained on a Nicolet 200SXV spectrome-
ter. 1H and 13C NMR spectra were recorded on a
Compound 1b: Yield: 75%, [h]2D0=+38.7 (c=0.7,
1
Bruker AC-E 300 and Bruker AC-E 400 instrument. 31
P
CHCl3). H NMR (400 MHz, CDCl3): l (ppm) 1.14 (d,
J=Hz, 3H), 2.22 (m, 5H), 2.94 (m, 1H), 4.99 (q, J=8.8
Hz, 5.2 Hz, 1H), 7.23–7.61 (m, 18H), 7.60–7.65 (m,
2H). 13C NMR (100 MHz, CDCl3): l (ppm) 9.28,
41.47, 65.38, 83.68 (d, 18.2 Hz), 127.12, 127.28, 127.92,
128.10, 128.16, 128.76, 129.29, 130.29, 130.51, 131.07,
131.29. 31P NMR (CDCl3): l (ppm) 110.08.
NMR spectra were recorded on Bruker AC-E 400. The
e.e. was determined by HPLC on a Beckman-110A
chromatography with a Beckman 165 variable wave-
length detector. The Chiralcel OD column was pur-
chased from Daicel Chemical Industries Ltd. Optical
rotations were recorded on a Perkin–Elmer 341 polar-
imeter. Toluene was distilled from sodium under a
nitrogen atmosphere.
Compound 1c: Viscous oil, yield: 87%, [h]2D0=−39.2
(c=0.36, CHCl3). 1H NMR (400 MHz, CDCl3): l
(ppm) 0.76 (d, J=6.4 Hz, 3H), 0.91 (d, J=6.4 Hz, 3H),
2.22 (s, 3H), 2.36 (q, J=9.2 Hz, 1H), 2.98 (m, 1H), 3.87
(d, J=5.6 Hz, 3H), 5.47 (q, J=6.0 Hz, 8.8 Hz, 1H),
6.99–7.40 (m, 20H). 13C NMR (100 MHz, CDCl3): l
(ppm) 17.30, 19.22, 32.00, 50.53, 73.63 (d, J=6.8 Hz),
4.2. N-Alkyl-1,2-diphenyl-2-aminoethanols
N-Alkyl-1,2-diphenyl-2-aminoethanols were prepared
according to the literature method.11

812
G. Chen et al. / Tetrahedron: Asymmetry 13 (2002) 809–813
82.42 (d, J=20.5 Hz), 126.76, 127.07, 127.28, 127.54,
127.82, 128.61, 128.90, 130.28, 130.76, 130.99, 131.61.
31P NMR (CDCl3): l (ppm) 111.60.
mixture was stirred at room temperature for about 20
min. 1,3-Diphenyl-2-allyl acetate (160 mg, 0.6 mmol),
dimethyl malonate (0.21 mL, 1.8 mmol), BSA (0.44
mL, 1.8 mmol) and LiOAc (5.0 mg, 0.072 mmol) were
added sequentially. After stirring the mixture at room
temperature for about 10 h (monitored by TLC), the
mixture was filtered and the filtrate was concentrated
and purified by chromatography (petroleum:ethyl ace-
tate, 8:1) to give the desired product (188 mg).
Compound 1d: Viscous oil, yield: 84%, [h]2D0=+3.0 (c=
3.1, CHCl3). 1H NMR (400 MHz, CDCl3): l (ppm)
2.28 (s, 3H), 3.22 (d, J=13.6 Hz, 1H), 3.52 (d, J=13.2
Hz, 1H), 4.01 (d, J=9.2 Hz, 1H), 5.45 (t, J=8.8 Hz,
1H), 6.84–7.39 (m, 25H). 13C NMR (100 MHz, CDCl3):
l (ppm) 38.10, 59.00, 72.84 (d, J=7.7 Hz), 82.78 (d,
J=20.0 Hz), 126.59, 127.10, 127.61, 127.72, 127.93,
127.99, 128.54, 128.83, 129.96, 130.19, 130.40, 130.64,
130.87. 31P NMR (CDCl3): l (ppm) 110.92.
Acknowledgements
Compound 2: Viscous oil, yield: 94%, [h]2D0=−24.9 (c=
We are grateful for the financial support from the
National Science Foundation of China (29972040), par-
tial support from NSFC project (20102005) and the
Hong Kong Polytechnic University ASD Fund. We
also thank Professor Yoshinori Yamamoto at Tohoku
University in Japan for his generous gift of palladium
complexes.
1
0.86, CHCl3). H NMR (400 MHz, CDCl3): l (ppm)
2.27 (s, 3H), 3.50 (d, J=13.2, 1H), 3.79 (d, J=5.6 Hz,
1H), 4.27 (dd, J=5.2 Hz, 4.0 Hz, 1H), 5.45 (m, 1H),
7.00–7.38 (m, 25H). 13C NMR (100 MHz, CDCl3): l
(ppm) 37.26, 59.70, 73.05 (d, J=5.5 Hz), 81.85 (d,
J=19.5 Hz), 125.25, 125.97, 126.59, 126.85, 127.39,
127.61, 127.78, 127.84, 127.99, 128.10, 128.18, 128.63,
128.88, 129.04, 129.77, 130.16, 130.38, 130.80, 131.02.
31P NMR (CDCl3): l (ppm) 110.62.
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1
(c=0.6, CHCl3). H NMR (400 MHz, CDCl3): l (ppm)
1.25 (s, 3H), 1.56 (s, 3H), 2.46 (d, J=6.0 Hz, 1H), 4.42
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Hz, 1H), 6.94–7.30 (m, 20H). 13C NMR (100 MHz,
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Hz), 86.30 (d, J=19.6 Hz), 127.19, 127.80, 127.87,
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