Application of phosphinous amide ligands in palladium complex-catalyzed asymmetric allylic alkylation: Influence of steric effects on enantioselectivity

Article abstract of DOI:10.1016/j.tetasy.2003.10.039

Phosphinous amide ligands 1-4 derived from 2,2′-diamino-1,1′- binaphthyl (BINAM) and 2,2′-diamino-5,5′,6,6′,7,7′,8, 8′-octahydro-1,1′-binaphthyl (H₈-BINAM) have been prepared and applied in palladium complex-catalyzed asymmetric allylic alkylat

Full text of DOI:10.1016/j.tetasy.2003.10.039

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 213–217
Application of phosphinous amide ligands in palladium complex-
catalyzed asymmetric allylic alkylation: influence of steric effects
on enantioselectivity
Xuanhua Chen,a,b Rongwei Guo,a Yueming Li,a Gang Chen,a Chi-Hung Yeunga,*
and Albert S. C. Chana,*
aOpen Laboratory of Chirotechnology of the Institute of Molecular Technology for Drug Discovery and Synthesis and
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hong Kong
bDepartment of Chemistry, Central China Normal University, Wuhan, Hubei 430079, China
Received 26 June 2003; revised 16 October 2003; accepted 18 October 2003
Abstract—Phosphinous amide ligands 1–4 derived from 2,20-diamino-1,10-binaphthyl (BINAM) and 2,20-diamino-5,50,6,60,7,70,8,80-
octahydro-1,10-binaphthyl (H8-BINAM) have been prepared and applied in palladium complex-catalyzed asymmetric allylic
alkylations. The alkylations of racemic 1,3-diphenyl-2-propenyl acetate with malonate, a-substituted malonates or acetylacetone
catalyzed by the palladium complexes bearing these ligands gave products in up to 96% ee.
ꢀ 2003 Published by Elsevier Ltd.
1. Introduction
Allylic substitution has become one of the most
important C–C bond formation reactions since its first
discovery in 1965;7 and palladium catalyzed asymmetric
allylic substitution has been actively studied due to its
potential application in asymmetric C–C bond forma-
tions.8–10 Oxazoline type ligands,11 ferrocene type
ligands,12 TrostÕs ligands,13 and several other ligands14
have shown excellent enantioselectivities in palladium-
catalyzed asymmetric allylic substitutions and several
applications to natural product synthesis have been
reported by Trost et al.15 On the other hand, readily
available phosphinous amide ligands have not been well
applied in this type of reaction. Spilling et al. prepared a
series of phosphinous amide ligands derived from 1,2-
cyclohexanediamine, and obtained moderate eeÕs in
applying these ligands in asymmetric allylic substitu-
tion.16 In this study we found the asymmetric allylic
alkylation catalyzed by palladium complexes containing
phosphinous amide ligands 1–4 derived from 2,20-di-
amino-5,50,6,60,7,70,8,80-octahydro-1,10-binaphthyl and
2,20-diamino-1,10-binaphthyl to be facile.
Chiral ligands bearing a 1,10-binaphthyl backbone have
been successfully applied in a variety of asymmetric
reactions.1 The highly skewed position of the naphthyl
rings in the binaphthyl backbone is believed to be the
determining factor for the ligands to be effective in
asymmetric catalytic reactions,2 and different dihedral
angles of the axial biaryl in the backbone might conse-
quently give better enantioselectivity of a catalyst.3
The dihedral angle of the axial biaryl in the binaphthyl-
type chiral catalysts can either be adjusted by partially
hydrogenating the naphthyl rings to H8-naphthyls,4 or
by the introduction of bulky groups onto the binaphthyl
ring adjacent of the hydroxyl group in the binaphthol
molecule,5 or by introducing bulky groups into the
diaryl phosphino group.6 Herein we report the effect of
steric hindrance on the enantioselectivity of asymmetric
allylic alkylation reactions catalyzed by phosphinous
amide–palladium catalysts.
2. Results and discussion
* Corresponding authors. Tel.: +852-27665607; fax: +852-23649932;
e-mail: bcachan@polyu.edu.hk
Phosphinous amide ligand 2,20-bis(diphenylphosphi-
noamino)-1,10-binaphthyl (BDPAB) has been used in
A University Grants Committee Areas of Excellence Scheme in Hong
Kong.
0957-4166/$ - see front matter ꢀ 2003 Published by Elsevier Ltd.
doi:10.1016/j.tetasy.2003.10.039

214
X. Chen et al. / Tetrahedron: Asymmetry 15 (2004) 213–217
the asymmetric hydrogenation of dehydroamino acids,
and up to 95% ee has been obtained.17 Increasing the
steric hindrance of the phosphinous amide ligand by
partially hydrogenating the naphthyl ring improved the
enantioselectivity of the catalyst in asymmetric hydro-
genation of enamides and their derivatives.18 Expanding
the scope of these findings, it is of interest to use this
class of phosphinous amide ligands in the asymmetric
allylic alkylation to investigate the steric hindrances
effect on the enantioselectivity of the reaction. Ligands
1–4 with different steric hindrance were prepared
through the reaction of 2,20-diamino-5,50,6,60,7,70,8,80-
octahydro-1,10-binaphthyl (H8-BINAM) and 2,20-dia-
mino-1,10-binaphthyl (BINAM) with chlorodiarylphos-
phine (Fig. 1).
was chosen for the examination of the efficiency of these
ligands in palladium complex-catalyzed asymmetric
allylic alkylations. The results shown in Table 1 revealed
that the most sterically hindered ligand 1 gave the least
satisfactory result in comparison to its less hindered
analog 2 (Table 1, entry 1 vs entry 2). The least hindered
ligand 4 (BDPAB) showed the highest efficiency among
the ligands tested in this reaction, giving quantitative
conversion and 75% ee at ambient temperature (Table 1,
entry 4). Lowering the reaction temperature gave im-
proved enantioselectivity; and the ee reached 87% when
the reaction was carried out at 0 ꢁC (Table 1, entry 5).
Since ligand 4 gave the best results, this ligand was
chosen for the rest of the study. When acetylacetone was
used as a nucleophile source in the BSA/LiOAc base
system, no substitution product was detected after
4 days (Table 2, entry 1). When KOAc was used instead
of LiOAc, 86% conversion and 81% ee was obtained
(Table 2, entry 2) after 7 h. When a combination of
LiOAc and KOAc, together with BSA, was employed,
the ee was improved to 85% at ambient temperature and
94% at 0 ꢁC (Table 2, entries 3 and 4). A variety of
combinations of bases were investigated and the results
are listed in Table 2. THF was found to be the best
choice of solvent, giving the highest ee in the reaction.
P
NH
P
NH
NH
P
NH
P
1
2
A series of malonates were tested with this catalyst
system and the results were summarized in Table 3. The
reaction using the substituted malonates as nucleophiles
also gave high enantioselectivities. For example, when
diethyl n-butylmalonate was used as the nucleophile
source, up to 96% ee was obtained (Table 3, entry 5).
Similarly the use of diethyl benzylmalonate and diethyl
formamidomalonate as nucleophile sources gave 94% ee
and 90% ee, respectively (Table 3, entries 8 and 13).
P
P
NH
N
H
NH
P
NH
P
3
4
Figure 1. Phosphinous amide ligands derived from BINAM.
3. Conclusion
Initially the reaction of racemic 1,3-diphenyl-2-propenyl
acetate with dimethyl malonate in the presence of base
[N,O-bis(trimethylsilyl)acetamide (BSA) and LiOAc]
In summary, a readily accessible chiral ligand derived
from BINAM shows high efficiency in asymmetric cat-
alytic alkylation. High enantioselectivities (90–96%)
Table 1. Asymmetric allylic alkylation of racemic 1,3-diphenyl-2-propenyl acetate with dimethyl malonate catalyzed by Pd0/phosphinite ligandsa
CH(CO2CH3 2
)
Pd/L
OAc
+
CH(CO2CH3 2
)
Ph
Ph
Ph
Ph
THF, BSA (3 eq.), LiOAc (cat.)
*
Entry
Ligand
Temperature (ꢁC)
Time (h)
Conversion (%)b
Ee (%)c
1
2
3
4
5
(S)-1
(R)-2
(S)-3
(R)-4
(R)-4
rt
rt
rt
rt
0
18
18
24
18
48
>99
>99
60
31(R)
46(R)
72(R)
75(S)
87(S)
>99
>99
a Reaction conditions: Substrate/BSA/dimethyl malonate/ligand/Pd ¼ 20/60/60/2/1 (molar ratio); LiOAc ¼ 1 mg; [Pd(g3-C3H5)Cl]2 ¼ 0.73 mg
(0.002 mmol); THF solvent ¼ 2 mL.
b Conversions were determined by NMR.
c The eeÕs were determined by HPLC analysis using a DAICEL Chiralpak AD-H column (hexane/2-propanol ¼ 90/10, 1.0 mL/min) and the absolute
configurations of the product were determined by comparison with literature values.11c

X. Chen et al. / Tetrahedron: Asymmetry 15 (2004) 213–217
215
Table 2. Asymmetric allylic alkylation of racemic 1,3-diphenyl-2-propenyl acetate with acetylacetone catalyzed by Pd0/ligand (R)-4a
CH(COCH3)2
Pd/(R)-4
OAc
+
CH2(COCH3)2
Ph
Ph
*
Ph
Ph
Solvent, BSA (3 eq.), Base (cat.)
Entry
Additive
Solvent
Temperature (ꢁC)
Time (h)
Conversion (%)b
Ee (%)c
1
2
LiOAc
KOAc
THF
THF
rt
rt
rt
0
96
7
––
––
86
81(S)
85(S)
94(S)
83(S)
82(S)
82(S)
69(S)
79(S)
39(S)
3
LiOAc + KOAc
LiOAc + KOAc
THF
THF
7
>99
93
4
30
18
18
18
18
18
18
5
Cs2CO3 + KOAc
SrCO3 + KOAc
NaOAc + KOAc
LiOAc + KOAc
LiOAc + KOAc
LiOAc + KOAc
THF
THF
rt
rt
rt
rt
rt
rt
>99
>99
>99
>99
>99
>99
6
7
THF
Et2O
8
9
10
Toluene
CH2Cl2
a Reaction conditions: Substrate/BSA/acetylacetone/ligand/Pd (molar ratio) ¼ 20/60/60/2/1; base ¼ 1 mg LiOAc or 0.5 mg LiOAc + 0.5 mg KOAc;
[Pd(g3-C3H5)Cl]2 ¼ 0.73 mg (0.002 mmol); THF solvent ¼ 2 mL.
b Conversions were determined by NMR.
c The eeÕs were determined by HPLC analysis using a DAICEL Chiralpak AD-H column (hexane/2-propanol ¼ 97/3, 1.0 mL/min) and the absolute
configurations of the product were determined by comparison with literature values.5
Table 3. Asymmetric allylic alkylation of racemic 1,3-diphenyl-2-propenyl acetate with various ester malonates catalyzed by Pd(0)-(R)-4a
CR(CO2Et)2
Pd/(R)-4
OAc
+
RCH(CO2Et)2
Ph
Ph
Ph
Ph
THF, BSA (3 eq.), Base (cat.)
*
Entry
R
Additive
Temperature (ꢁC) Time (h)
Conversion (%)b
Ee (%)c;d
1
H
H
LiOAc
LiOAc
LiOAc
rt
0
4
48
6
>99
>99
>99
>99
>99
>99
>99
>99
>99
80
68(S)
80(S)
2
3
n-Butyl
n-Butyl
n-Butyl
Benzyl
Benzyl
Benzyl
rt
rt
0
94(R)(+)
91(R)(+)
96(R)(+)
93(R)(+)
86(R)(+)
94(R)(+)
72(R)
4
LiOAc, KOAc
LiOAc
LiOAc
20
48
6
5
6
rt
rt
0
7
LiOAc, KOAc
LiOAc
LiOAc
10
48
6
8
9
Acetamido
Acetamido
Formamido
Formamido
Formamido
rt
0
10
11
12
13
LiOAc
LiOAc
48
6
83(R)
rt
rt
0
>99
>99
80
87(R)(+)
84(R)(+)
90(R)(+)
LiOAc, KOAc
LiOAc
6
48
a Reaction conditions: Substrate/BSA/various ester malonate/ligand/Pd (molar ratio) ¼ 20/60/60/2/1; base ¼ 1 mg LiOAc or 0.5 mg LiOAc + 0.5 mg
KOAc; [Pd(g3-C3H5)Cl]2 ¼ 0.73 mg (0.002 mmol); THF solvent ¼ 2 mL.
b Conversions were determined by NMR.
c The eeÕs were determined by HPLC analysis using a DAICEL Chiralpak AD-H column.
d Optical rotations were measured using a Perkin–Elmer polarimeter (Model 341); the absolute configurations were determined by comparison with
literature values.19;20
were achieved in the allylic substitution of racemic 1,3-
diphenylallylic acetate using a [Pd-(R)-4] catalyst. A
balanced rigidity/steric hindrance of the ligand is
important for the best results. This finding is consistent
with the results reported by Trost and Murphy21 and
RajanBabu and coworkers22 that binaphthyl phosphite
ligands bearing bulky groups give low enantioselectivity.
glove box. Flash column chromatography was per-
formed using silica gel as adsorbent. Hexane, toluene,
tetrahydrofuran, and diethyl ether were distilled from
sodium or sodium benzophenone ketyl before use.
1
Dichloromethane was distilled from CaH2. H, 31P, and
13C NMR were recorded on a Bruker AVANCE 400 or
Varian AS 500 at RT using CDCl3 as solvent. Enan-
tiomeric excesses were determined by HPLC conducted
on an Agilent 1100 series system. Optical rotations were
measured on a Perkin–Elmer polarimeter. Thin layer
chromatography (TLC) was carried out on aluminum
sheets precoated with silica gel 60 F254 (0.25 mm,
Merck).
4. Experimental section
All experiments were carried out under a nitrogen
atmosphere using standard Schlenk techniques or in a

216
X. Chen et al. / Tetrahedron: Asymmetry 15 (2004) 213–217
4.1. (R)-2,20-Bis[bis(3,5-dimethylphenyl)phosphinoamino]-
J ¼ 15:8 Hz, 1H), 7.17–7.33 (m, 15H). 13C NMR
(100 MHz, CDCl3): d (ppm) 13.72, 13.94, 41.04, 54.82,
61.09, 61.15, 64.20, 126.34, 126.72, 127.22, 127.26,
127.87, 128.38, 128.43, 129.51, 129.99, 130.37, 132.14,
136.87, 137.50, 139.39, 170.18, 170.26. Anal. Calcd for
C29H30O4: C, 78.71; H, 6.83; O, 14.46. Found: C, 78.34;
H, 6.93. Enantiomeric excess value was measured by
HPLC (Chiral AD-H, n-hexane/isopropanol ¼ 97/3):
8.83 min, 11.71 min (flow rate ¼ 1.0 mL/min).
5,50,6,60,7,70,8,80-octohydro-1,10-binaphthyl 1
(R)-2,20-diamino-5,50,6,60,7,70,8,80-octahydro-1,10-binaph-
thyl (292 mg, 1.0 mmol) and 4-(dimethylamino)pyridine
(15 mg) were placed in a 50 mL round-bottom Schlenk
flask with a stirring bar under N2 atmosphere and dry
Et3N (0.8 mL) and CH2Cl2 (30 mL) were added to the
mixture. The reaction mixture was cooled to 0 ꢁC with
an ice-water bath followed by dropwise addition of
bis(3,5-dimethylphenyl)phosphine chloride (2.5 mmol)
in 20 min. The solvent was removed under reduced
pressure after the reaction mixture was stirred at ambi-
ent temperature for 3 h. The residue was re-dissolved in
15 mL toluene and was purified through a flash silica gel
column (30 mL toluene as eluent). The solvent was re-
moved to give 645 mg white solid (85% yield). Colorless
crystals were obtained after recrystallization from di-
ethylether. 31P NMR (202 MHz, CDCl3): d 25.2 ppm. 1H
NMR (500 MHz, CDCl3, d, ppm): d 1.51–1.68 (m, 8H),
2.07 (s, 12H), 2.16–2.18 (m, 4H), 2.22 (s, 12H), 2.66–2.68
(m, 4H), 4.29 (d, 2H, J ¼ 9:0 Hz), 6.74–6.75 (m, 6H),
6.90–6.91 (m, 6H), 6.96–6.99 (m, 2H), 7.26–7.30 (m,
4.4. Diethyl 2-(n-butyl)-2-[(E)-1,3-diphenyl-2-propenyl]-
malonate
1
Viscous oil. H NMR (400 MHz, CDCl3): d (ppm) 0.82
(t, J ¼ 6:9 Hz, 3H), 1.17–1.31 (m, 10H), 1.69–1.72 (m,
1H), 1.82–1.85 (m, 1H), 4.09–4.24 (m, 5H), 6.34 (d,
J ¼ 15:7 Hz, 1H), 6.77 (dd, J ¼ 7:1 Hz, J ¼ 15:7 Hz,
1H), 7.18–7.34 (m, 10H). 13C NMR (100 MHz, CDCl3):
d (ppm) 13.85, 14.08, 22.99, 26.79, 28.48, 34.44, 53.48,
60.99, 61.08, 61.26, 126.30, 127.08, 127.15, 128.25,
128.41, 129.28, 130.07, 131.70, 137.56, 139.62, 170.66,
170.94. Anal. Calcd for C26H32O4: C, 76.44; H, 7.90.
Found: C, 76.63; H, 7.82. The enantiomeric excess was
measured by HPLC (Chiralpak AD-H, n-hexane/
2-propanol¼ 97/3):6.20min, 7.60min(flowrate¼ 1.0 mL/
min).
2H). 13C NMR (125 MHz, CDCl3): d 21.12, 21,27, 23.19,
20
D
23.29, 27.41, 29.31, 112.23–142.16 (Ph); ½aꢀ ¼ þ3:3 (c
1.0, THF). Anal. Calcd for C52H58N2P2: C, 80.80; H,
7.56; N, 3.62; P, 8.01. Found: C, 80.59; H, 7.53; N, 3.78;
P, 8.10.
4.5. Diethyl 2-formamido-2-[(E)-1,3-diphenyl-2-propenyl]-
malonate
4.2. A typical procedure for the asymmetric allylic
alkylation
1
Colorless needles. H NMR (400 MHz, CDCl3): (ratio
In a glove box, ligand 4 (52.2 mg, 0.08 mmol), [Pd(g3-
C3H5)Cl]2 (7.3 mg, 0.02 mmol) and 2 mL dried THF
were placed in a 4 mL glass bottle, the mixture was
stirred at room temperature for 1 h to prepare a stock
solution of the catalyst. In a typical experiment, 200 lL
(0.002 mmol) of the catalyst solution was added in a
4 mL glass bottle, to the solution were added succes-
sively racemic 1,3-diphenyl-2-propenyl acetate (0.08
mmol), N,O-bis(trimethylsilyl)acetamide (0.24 mmol),
catalytic amount of anhydrous lithium acetate (1 mg)
and malonate ester (0.24 mmol) and then THF was
added to form 2 mL solution. The reaction mixture was
stirred at appropriate temperature for a given period of
time. The reaction mixture was diluted with ether
(20 mL), and washed with saturated aqueous ammo-
nium chloride (3 · 20 mL). The organic phase was dried
over anhydrous MgSO4 and concentrated in vacuo. The
residue was purified by column chromatography and
enantiomeric composition was measured by HPLC
analysis using a DAICEL Chiralpak AD-H column.
of NH tautomer 6/1; major tautomer) d (ppm) 1.17 (t,
J ¼ 7:1 Hz, 3H), 1.26 (t, J ¼ 7:1 Hz, 3H), 3.60–4.16 (m,
2H), 4.27–4.32 (m, 2H), 4.77 (d, J ¼ 7:1 Hz, 1H), 6.32
(d, J ¼ 15:9 Hz, 1H), 6.76 (dd, J ¼ 7:1 Hz, J ¼ 15:9 Hz,
1H), 6.81 (br, 1H), 7.23–7.34 (m, 10H), 8.18 (d,
J ¼ 2:6 Hz, 1H). 13C NMR (100 MHz, CDCl3): d (ppm)
13.83, 14.02, 53.20, 62.64, 62.91, 68.40, 126.46, 127.25,
127.75, 128.37, 128.43, 128.47, 129.51, 132.68, 137.39,
137.88, 159.83, 166.52, 167.03. Anal. Calcd for
C23H25NO5: C, 69.86; H, 6.37; N, 3.54. Found: C, 69.63;
H, 6.46; N, 3.60. The enantiomeric excess was measured
by HPLC (Chiralpak AD-H, n-hexane/2-propanol ¼ 90/
10): 15.20 min, 21.84 min (flow rate ¼ 1.0 mL/min);
10.10 min, 14.23 min (flow rate ¼ 1.5 mL/min).
4.6. Determination of the configuration of new allylic
alkylation products
(R)-Diethyl-2-[(E)-1,3-diphenyl-2-propenyl] malonate
(82% ee, 70 mg, 0.2 mmol) and NaH (6 mg, 0.25 mmol)
in 3 mL dry THF stirred for 0.5 h, followed by added in
benzyl chloride (30 mg, 0.24 mmol). The mixture was
stirred overnight at ambient temperature. Crude prod-
uct was purified with silica gel column, (S)-diethyl
2-benzyl-2-[(E)-1,3-diphenyl-2-propenyl]malonate was
obtained with 81% ee. Similarly, (S)-diethyl 2-(n-butyl)-
2-[(E)-1,3-diphenyl-2-propenyl]malonate was obtained
with 80% ee by the reaction of (R)-diethyl-2-[(E)-1,3-
diphenyl-2-propenyl]malonate (82% ee) with n-butyl
4.3. Diethyl 2-benzyl-2-[(E)-1,3-diphenyl-2-propenyl]-
malonate
1
Viscous oil. H NMR (400 MHz, CDCl3): d (ppm) 1.01
(t, J ¼ 7:1 Hz, 3H), 1.18 (t, J ¼ 7:1 Hz, 3H), 3.09 (d,
J ¼ 13:8 Hz, 1H), 3.24 (d, J ¼ 13:8 Hz, 1H), 3.94–3.98
(m, 2H), 4.11–4.23 (m, 2H), 4.28 (d, J ¼ 8:4 Hz, 1H),
6.33 (d, J ¼ 15:8 Hz, 1H), 6.75 (dd, J ¼ 8:4 Hz,

X. Chen et al. / Tetrahedron: Asymmetry 15 (2004) 213–217
217
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Acknowledgements
We thank The Hong Kong Research Grants Council
Central Allocation Fund (Project number ERB003), The
University Grants Committee Area of Excellence
Scheme in Hong Kong (AoE P/10-01) and The Hong
Kong Polytechnic University ASD Fund for financial
support of this study.
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