Diastereomeric phosphite-pyridine ligands for enantioselective 1,4-conjugate additions

Article abstract of DOI:10.1016/j.tetasy.2005.02.002

Diastereomeric phosphite-pyridine ligands derived from racemic biphenyl units and homochiral BINOL, achieved high enantioselectivities (up to 96.1% ee) in the Cu(I)-catalysed conjugate additions of Et₂Zn to a variety of acyclic enones.

Full text of DOI:10.1016/j.tetasy.2005.02.002

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 1227–1231
Diastereomeric phosphite–pyridine ligands for
enantioselective 1,4-conjugate additions
Xiancai Luo, Yuanchun Hu and Xinquan Hu*
Dalian Institute of Chemical Physics, The Chinese Academy of Sciences, Dalian 116023, PR China
Received 7 January 2005; accepted 2 February 2005
Abstract—Diastereomeric phosphite–pyridine ligands derived from racemic biphenyl units and homochiral BINOL, achieved high
enantioselectivities (up to 96.1% ee) in the Cu(I)-catalysed conjugate additions of Et2Zn to a variety of acyclic enones.
Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction
in comparison with analogues 1,10-binaphthyl ligands.8
Similarly, Alexakis and Rosini gained success in Cu-
catalysed conjugate addition reactions.9
Since Mikami introduced the concept of asymmetric
activation, biphenyl-based compounds have been exten-
sively used to convey asymmetry in enantioselective
catalysis without their asymmetric synthesis and resolu-
tion.1,2 Generally, the asymmetric activation of biphen-
yl-based catalysts is mainly based on the catalyst
enantiomersbeing discriminated or the chirality of the
catalyst being controlled by a chiral activator. Accord-
ing to such a strategy, Mikami and Noroyi achieved
high enantioselectivities in hydrogenation by using a
BIPHEP/RuCl2 complex (BIPHEP = bis(phosphanyl)-
biphenyl) activated by a chiral diamine.3 Mikami acti-
vated the catalyst of (biphenoxide)Ti(O-i-Pr)2 with
(R,R)-TADDOL and obtained almost 100% ee in the
asymmetric addition of methyl group to aldehydes.4
Other good examplesof thistype of asymmetric induc-
tion were also gained in Baeyer–Villiger oxidation5
and carbon–carbon bond formation.6 A similar method-
ology to the asymmetric activation of biphenyl-based
catalyst has recently been highlighted, where a mixture
of diasteromeric ligands derived from an achiral biphe-
nyl subunit and a chiral moiety were successfully
employed in asymmetric catalysis.7–9 Reetz and
Neugebauer demonstrated that the ligands based on
the biphenol unit and a chiral diphosphite gave high
enantioselectivity in the Rh-catalysed hydrogenation.7
Gong and co-workersreported that the biphenol-based
diastereomeric oxavanadium(IV) complexes provided
comparable ee in the oxidative coupling of 2-naphthol
We recently designed the phosphite–pyridine ligands
(S,S)-1, derived from chiral biphenyl compound, (S)-2-
amino-20-hydroxy-4,40, 6,60-tetramethyl-1,10-biphenyl 2
and (S)-BINOL (BINOL = 2,20-dihydroxy-1,10-binaph-
thyl) (Fig. 1), for high enantioselective Cu(I)-catalysed
1,4-conjugate additionsof Et Zn to a variety of acyclic
2
enones.10 Encouraged by the above results, we became
interested in developing the phosphite–pyridine ligands
L from the racemic biphenyl backbone 2 and (S)-BI-
NOL and testing their application in transition metal
catalysed asymmetric reactions. Our more previous
studies had shown that the matched phosphite–pyridine
ligand (S,S)-3a, derived by (S)-NOBIN (NOBIN = 2-
amino-20-hydroxy-1,10-binaphthyl) and (S)-BINOL, ob-
tained much better result (91% ee with 92% of yield)
than itsdiastereomer ( R,S)-3b [derived by (R)-NOBIN
and (S)-BINOL, 54% ee with 15% of yield] in the
Cu(I)-catalysed asymmetric 1,4-conjugate addition of
O
N
N
R
NH2
OH
H
O
P
O
O
(S,S)-1a: R=H
(S,S)-1b: R=Me
2
*
Corresponding author. Tel.: +86 411 8437 9233; fax: +86 411 8468
4746; e-mail: xinquan@ms.dicp.ac.cn
Figure 1.
0957-4166/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2005.02.002

1228
X. Luo et al. / Tetrahedron: Asymmetry 16 (2005) 1227–1231
Table 1. Cu-catalysed enantioselective 1,4-conjugate addition of Et2Zn
to chalconea
O
O
[Cu(CH CN) ]BF / 3
3
4
4
*
Et Zn
2
+
o
O
O
toluene, -10 C, 12 h
[Cu(CH3CN)4]BF4 / La
Et2Zn
*
+
toluene
(S,S)-3a: 91% ee, 92% yield for 12 h
(R,S)-3b: 54% ee, 15% yield for 12 h
Entry Temp (°C) Time (h) Yield (%)b Ee (%)c Configd
1
2
3
4
5
6
7
8
10
0
6
6
80.5
77.3
77.8
74.2
58.0
67.7
75.4
73.1
79.4
87.4
92.1
94.5
91.0
91.5
92.3
93.8
S
S
S
S
S
S
S
S
O
O
À10
À20
À20
À20
À20
À20
6
6
NH
N
NH
N
O
1
O
P
O
P
O
2
O
O
4
12
a The reaction wascarried out in 3 mL toluene, chalcone (1.0 mmol)/
[Cu(CH3CN)4]BF4/ligand La = 1/0.01/0.025, Et2Zn:substrate = 1.5:1.
b Isolated yield.
c The ee valueswere determined by HPLC with a ChiralPak-AD
column.
d The absolute configuration was assigned by the comparison of the
specific rotation with reported data.
(S,S)-3a
(R,S)-3b
Scheme 1.
Et2Zn to acyclic enonesunder the asme conditions
(Scheme 1). It isnoteworthy that both ligands( S,S)-3a
and (R,S)-3b afforded the products with the same abso-
lute configuration.11 Therefore, we envisioned that the
strategy could obtain high stereoselectivity in Cu(I)-
catalysed 1,4-conjugate addition of Et2Zn to acyclic
enones.12,13 Moreover, the low efficient resolution of
biphenyl backbones 214 also drove us to develop a con-
venient and efficient ligandswith a new strategy.
To evaluate the catalytic performance of diasteromeric
ligands La and Lb, two typesof enones, para-substituted
chalconesand trans-aryl-3-buten-2-oneswere tetsed in
Cu(I)-catalysed 1,4-conjugate additions under the opti-
mal procedure. The results in Table 2 showed the
ligandsprepared from the racemic biphenyl backbone
were efficient in the Cu(I)-catalysed 1,4-conjugate addi-
tionsof Et 2Zn to various para-substituted chalcones.
Ligand La gave comparable enantioselectivities with
(S,S)-1 for chalcone and 4-substituted chalcones. When
4-Cl-chalcone was used as the substrate, up to 96.1% ee
2. Results and discussion
2.1. Synthesis of diastereomeric phosphite–pyridine
ligands La and Lb
0
wasachieved for the addition product. For 4 -substi-
tuted chalcones, an obvious electronic effect was
observed. Ligand La provided good enantioselectivity
and catalytic activity for 40-chloro-chalcone, whereasthe
results for electronic-rich substrates were much lower.
Surprisingly, sterically hindered ligand Lb did not
exhibit better stereoselectivity with low catalytic activity,
while itsenantiopure analogue ( S,S)-1b showed the best
results in the same reaction.10 Asthere are two dia-
stereomeric phosphite–pyridine Cu(I) complexes, which
showed different reaction rates in the reaction system,
there can be no doubt that these rate differences deter-
mine the product ee under thisistuation. To improve
the reaction conversion, which resulted from the steri-
cally hindered ligand, more catalyst was used to acceler-
ate the reaction. The results of the conjugate additions
of Et2Zn to chalconesin the preesnce of 2 mol % of
According to a previousreport, diatsereomeric P,N
ligands La and Lb were conveniently synthesised in high
yields, while 1H, 13C and 31P NMR spectra showed two-
fold parallel signals10,11 (Scheme 2).
2.2. Enantioselective 1,4-conjugate addition of Et2Zn to
enones
Chalcone was chosen as a typical substrate to optimise
the reaction conditionsfor the Cu(I)-catalysed 1,4-con-
jugate addition (Table 1). The addition of Et2Zn to chal-
cone wasconducted in the preesnce of 1 mol% of
[Cu(CH3CN)4]BF4 and 2.5 mol % of La in 3 mL of tolu-
ene. The results summarised in Table 1 showed that the
optimal reaction conditionswere À20 °C for 6 h.
O
O
N
NH
N
O
(S)-MonoPhos
HOOC
N
R
NH2
OH
R
H
N
OH
toluene, reflux
O
P
DMTMM, THF
R
O
4a: R=H
4b: R=Me
racemic 2
La: R=H
Lb: R=Me
Scheme 2. Synthesis of diastereomeric phosphite–pyridine ligands L.

X. Luo et al. / Tetrahedron: Asymmetry 16 (2005) 1227–1231
Table 2. Cu-catalysed enantioselective 1,4-conjugate addition of Et2Zn to chalconesa
1229
O
O
[Cu(CH CN) ]BF / L
3
4
4
*
Et Zn
2
+
2
1
2
1
o
R
R
R
R
toluene, 6h, -20
C
Entry
R1
R2
La
Lb
Config.d
Yield (%)b
Ee (%)c
Yield (%)b
Ee (%)c
1
2
3
4
5
6
7
H
Cl
H
72.5
70.5
75.7
56.0
76.3
57.4
26.2
93.9
96.1
94.3
91.2
91.7
87.2
70.5
49.3 (75.7)
42.5 (72.5)
57.8 (70.6)
45.0 (65.4)
39.5 (51.5)
43.3 (54.0)
15.7 (32.0)
89.3 (91.9)
90.7 (91.9)
91.8 (93.2)
93.2 (93.0)
81.5 (82.4)
87.1 (83.8)
73.3 (82.0)
S
+
+
S
e
e
H
Me
MeO
H
H
H
Cl
e
À
e
+
H
Me
MeO
e
H
À
a The reaction wascarried out at À20 °C for 6 h in 3 mL of toluene, substrate (1.0 mmol)/[Cu(CH3CN)4]BF4/L = 1/0.01/0.025, Et2Zn:substrate =
1.5:1, the data in parentheses were carried out at À20 °C for 6 h in 1.5 mL of toluene, substrate (0.5 mmol)/[Cu(CH3CN)4]BF4/L = 1/0.02/0.05,
Et2Zn:substrate = 1.5:1.
b Isolated yield.
c The ee valueswere determined by HPLC with a ChiralPak-AD column.
d The absolute configuration was assigned by the comparison of the specific rotation data.
e Sign of the specific rotation of addition product.
[Cu(CH3CN)4]BF4 and 5 mol % of the diastereomeric
ligand Lb were as listed in parentheses in Table 2. As
expected, using more catalyst could significantly improve
the chemical yields, but just provided equivalent eeÕs.
Cu(I)-catalysed enantioselective 1,4-conjugate additions
of Et2Zn to chalcones, and achieved up to 96.1% ee.
4. Experimental section
4.1. General
Under the same reaction conditions, ligands La were
also employed in the Cu(I)-catalysed 1,4-conjugate addi-
tionsof Et 2Zn to some trans-4-aryl-3-buten-2-ones, with
moderate catalytic activity and enantioselectivity being
achieved (Table 3).
Melting pointswere measured on a Yazawa micromelt-
ing point apparatus(uncorrected). 1H NMR and 13C
NMR spectra were recorded on a BRUKER DRX
400 system with TMS as an internal standard. 31P
NMR spectra were recorded with 85% phosphoric acid
as the external standard. The enantiomeric excesses
(ee) were determined by HPLC with a Daicel Chiral-
Pak-AD column or by GC with a capillary Supelco
c-DEX 225 column. All experimentswere carried out
under an argon atmosphere using standard Schlenk
techniques. All solvents were dried before use according
to standard procedures and stored under argon. (S)-BI-
NOL wascommercially available and directly used. The
amides 4 were prepared according to the literature
procedure.10,12e
Table 3. Cu-catalysed enantioselective 1,4-addition of Et2Zn to trans-
4-aryl-3-buten-2-onesa
O
O
[Cu(CH3CN)4]BF4 / La
*
Et2Zn
+
R1
R1
toluene, 6h, -20 oC
Entry
R1
La
Config.d
Yield (%)b
Ee (%)c
1
2
3
4
H
Cl
60.9
63.7
40.5
31.2
72.2
86.5
77.7
75.5
S
e
+
e
Me
MeO
+
e
+
4.2. Synthesis of diastereomeric phosphite–pyridine
ligands
a The reaction wascarried out at À20 °C for 6 h in 3 mL of toluene,
substrate (1.0 mmol)/[Cu(CH3CN)4BF4/La = 1/0.01/0.025, Et2Zn:
substrate = 1.5:1.
4.2.1. La. Racemic amide 4a (346.0 mg, 1.0 mmol), (S)-
MonoPhos(462.2 mg, 1.3 mmol) and 10 mL of toluene
were added to a 50 mL air-free Schlenk flask with a reflux
condenser under an argon atmosphere. The mixture was
heated to reflux. After the reaction wascomplete (de-
tected by TLC), the reaction solution was cooled to room
temperature and purified by flash chromatograghy on a
silica gel column [eluted with hexanes/CH2Cl2 (1/1)] to
b Isolated yield.
c The ee value wasdetermined by GC with a Chiral capillary gamma-
225 column.
d The absolute configuration was assigned by the comparison of the
specific rotation data.
e Sign of the specific rotation of addition product.
3. Conclusion
afford 605.0 mg (92%) of diastereomeric La asa white
13
D
foamy solid: mp 99–120 °C; ½aŠ ¼ þ164:0 (c 0.520,
1
We have successfully applied Cu(I) complexes coordi-
nated with diastereomeric ligands, derived from the
racemic biphenyl backbone 2 and (S)-BINOL, in
THF), H NMR (CD2Cl2) d 1.84–1.90 (m, 6H), 2.33–
2.35 (m, 3H), 2.43–2.45 (m, 3H), 6.60–6.80 (m, 1H),
6.91–6.98 (m, 3H), 7.11–7.31 (m, 8H), 7.65–7.70 (m,

1230
X. Luo et al. / Tetrahedron: Asymmetry 16 (2005) 1227–1231
2H), 7.78–7.80 (m, 3H), 8.00–8.20 (m, 2H), 8.33–8.35 (m,
1H), 9.62–9.68 (m, 1H); 13C NMR (CD2Cl2) d 22.25,
22.70, 22.86, 23.78, 24.36, 120.74, 120.85, 124.42,
124.50, 124.64, 124.78, 127.74, 127.97, 128.88, 129.11,
129.22, 129.36, 129.59, 130.35, 131.16, 132.36, 132.52,
133.11, 140.24, 142.24, 142.50, 150.66, 152.91, 164.37;
31P NMR (DMSO-d6) d +144.70, 145.04.
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4.2.2. Lb. Following a similar method for the synthesis
of La, ligand Lb (607.5 mg, 90%) wasprepared from
amide 4b (360.0 mg, 1.0 mmol) and (S)-MonoPhos
(465.1 mg, 1.3 mmol) asa white foamy oslid: mp 99–
14
125 °C; ½aŠ ¼ þ150:3 (c 0.508, THF), 1H NMR
(CD2Cl2) dD 1.86–1.94 (m, 86H), 2.08–2.11 (m, 3H),
2..24–2.45 (m, 6H), 6.50–6.61 (m, 1H), 6.89–7.30 (m,
11H), 7.50–7.80 (m, 6H), 8.39–8.44 (m, 1H), 9.88–9.94
(m, 1H); 13C NMR (CD2Cl2) d 21.99, 22.40, 22.58,
23.44, 24.11, 26.01, 119.67, 121.08, 121.69, 124.52,
127.44, 124.52, 127.68, 128.16, 128.60, 128.82, 129.05,
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4.3. General procedure for the asymmetric 1,4-conjugate
addition
4.3.1. Preparation of the catalyst. La (133.0 mg, 0.2
mmol), 25.2 mg of [Cu(CH3CN)4]BF4 (0.08 mmol) and
16 mL of toluene were added to a 50 mL air-free
Schlenk flask under an argon atmosphere. After
45 min of stirring at room temperature, the solvent
wasstripped off in vacuo, 8 mL of CH 2Cl2 then added
to the flask and the catalyst solution used for eight
separate conjugate addition reactions.
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4.3.2. Asymmetric 1,4-conjugate addition. Substrate
(1.0 mmol) and 1.0 mL of the above prepared catalyst
solution were added to a flame-dried Schlenk tube under
an argon atmosphere. After stripping off the solvent,
3 mL of toluene were added. The slurry was stirred at
room temperature for 10 min and then cooled to
À20 °C. After the slurry was stirred for 15 min, 1.4 mL
of Et2Zn (1.1 M in toluene, 1.5 mol equiv) wasadded
slowly. The resulting mixture was stirred at À20 °C for
6 h. HCl (5%, 4 mL) wasthen added to quench the reac-
tion. The mixture wasallowed to warm to room temper-
ature, and then 15 mL of diethyl ether added. The
organic layer waswashed with 5 mL of saturated NaH-
CO3 and 5 mL of brine and then dried over anhydrous
Na2SO4. The solvent was removed under reduced pres-
sure and the residue purified by column chromatogra-
phy on silica gel and eluted with EtOAc/hexane (1/40–
1/20) to afford the addition product. The eeÕsof the
addition productswere determined by chiral HPLC or
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Acknowledgements
Thiswork wassupported by the Knowledge Innovation
Program of the Chinese Academy of Sciences, Grant:
DICP K2004D05.

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