Asymmetric addition of diethylzinc to diphenylphosphinoyl-imines catalyzed by copper(II) trifluoromethanesulfonate-chiral (2′-ethylamino-[1,1′] binaphthalenyl-2-yl)-thiophosphoramidic acid O,O′-diaryl ester ligands

Article abstract of DOI:10.1002/adsc.200606128

The chiral binaphthylthiophosphoramide L1 prepared from the reaction of O,O-diphenyl chlorothiophosphate with (R)-(+)-N-ethyl-1,1′-binaphthyl-2, 2′-diamine was used as a catalytic chiral ligand in the copper(II) trifluoromethanesulfonate-promoted asymmetr

Full text of DOI:10.1002/adsc.200606128

UPDATES
DOI: 10.1002/adsc.200606128
Asymmetric Addition of Diethylzinc to Diphenylphosphinoyl-
Imines Catalyzed by Copper(II) Trifluoromethanesulfonate-Chiral
(2’-Ethylamino-[1,1’]binaphthalenyl-2-yl)-thiophosphoramidic
Acid O,O’-Diaryl Ester Ligands
Min Shi,a,* Zhi-Yu Lei,a and Qin Xua
a
School of Chemistry & Pharmaceutics, East China University of Science and Technology, 130 MeiLong Road, Shanghai
200237, Peopleꢀs Republic of China
Fax : (+86)-21-6416-6128; e-mail: Mshi@pub.sioc.ac.cn
Received: March 23, 2006; Revised: July 22, 2006; Accepted: August 3, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The chiral binaphthylthiophosphoramide
L1 prepared from the reaction of O,O-diphenyl
chlorothiophosphate with (R)-(+)-N-ethyl-1,1’-bi-
naphthyl-2,2’-diamine was used as a catalytic chiral
ligand in the copper(II) trifluoromethanesulfonate-
promoted asymmetric addition of diethylzinc to di-
phenylphosphinoyl-imines to give the correspond-
ing adducts in 90–98% ee and good yields under
mild conditions.
pects of this chemistry still need to be developed. Ad-
ditions to diphenylphosphinoyl-imine derived from
benzaldehyde have been studied almost exclusively,
and as a result of the poor electrophilic character of
diphenylphosphinoyl-imines, stoichiometric amounts
of amino alcohol ligand are normally required to
ensure high conversion and enantioselectivity.
We are interested in thesyntheses and applications
of novel chiral ligands based on axially chiral binaph-
thenediamine (BINAM), which has been much less
popular in comparison to the other widely used axial-
ly chiral binaphthyl structures such as BINOL and
NOBIN as well as chiral diamines such as 1,2-cyclo-
hexanediamine and 1,2-diphenylethenediamine.[8,9]
Previously, we reported that the catalytic asymmetric
addition of diethylzinc to diphenylphosphinoyl-imines
in 61–89% yields and 66–85% ee had been achieved
Keywords: asymmetric addition; asymmetric cataly-
sis; chiral binaphthylthiophosphoramides; copper
(II) trifluoromethanesulfonate; diethylzinc; diphe-
nylphosphinoyl-imines
using Cu(OTf)2 and easily available, stable and the re-
G
Introduction
coverable axially chiral binaphthylthiophosphoramide
ligand, (2’-ethylamino-[1,1’]binaphthalenyl-2-yl)-thio-
The efficient and asymmetric preparation of amines is phosphoramidic acid O,O’-diethyl ester (Scheme 1).[9g]
one of the most promising methodologies in organic On the basis of above results, we envisioned that
synthesis.[1] In addition to the asymmetric reduction of
imines, the enantioselective addition of alkylmetals to
imines is a convenient route to optically active
amines. Among these, the chiral amine ligand-cata-
lyzed addition of alkyllithium,[2] copper-amidophos-
phine,[3] Zr-peptide-based chiral ligand and copper(II)
triflate/diphosphine complex-catalyzed asymmetric
addition of organozinc,[4] chiral allylpalladium-cata-
lyzed allylation with allylstannane,[5] and rhodium-
monophosphine-catalyzed arylation with arylstan-
nane[6] showed very good enantioselectivities. Recent-
ly, the use of diphenylphosphinoyl-imines as activated
substrates in combination with diethylzinc as the nu-
cleophillic reagent and an amino alcohol promoter is
Scheme 1. Catalytic asymmetric addition with the chiral thio-
receiving increasing attention.[7] However, many as- phosphoramide ligand.
Adv. Synth. Catal. 2006, 348, 2237 – 2242
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2237

Min Shi et al.
UPDATES
enantioselectivity of this addition reaction can be im-
proved by replacement of the O,O’-diethyl group with
the sterically more bulky O,O’-aryl group in the bi-
naphthylthiophosphoramide ligand system. Herein,
we report the catalytic asymmetric addition of dieth-
ylzinc to diphenylphosphinoyl-imines using Cu(OTf)2
A
as the catalytic precursor and novel chiral binaph-
thylthiophosphoramides, (2’-ethylamino-[1,1’]binaph-
thalenyl-2-yl)-thiophosphoramidic acid O,O’-diaryl
esters, as ligands to give the corresponding adducts in
good yields and 90–98% ee (Figure 1).
Scheme 2. Preparation of (2’-ethylamino-[1,1’]binaphthalen-
yl-2-yl)-thiophosphoramidic acid O,Oꢀ-diaryl ester ligands.
Using diphenylphosphinoyl-imine as substrate and
diethylzinc as nucleophilic addition reagent, we exam-
ined this asymmetric addition in toluene using copper
salt Cu(OTf)2 (5–10 mol%) and ligand L1 (5–10
A
mol%) to develop the optimal reaction conditions.
The results are summarized in Table 1. Initially, sol-
vent effects in this asymmetric addition reaction were
examined at room temperature (208C). We found
that in toluene, the corresponding adduct 1a was ob-
tained in 85% yield and 67% ee, but in dichlorome-
thane, acetonitrile or tetrahydrofuran (THF), neither
reaction occurred or the corresponding adduct 1a was
Figure 1.
Results and Discussion
These chiral ligands L1–L5 are easily obtained from formed in trace (Table 1, entries 1–4). Next, an exami-
the reaction of N2’-ethyl-[1,1’]binaphthalenyl-2,2’-di- nation of the temperature profile of Cu
AHCTREUNG
amine, derived from (R)-(+)-1,1’-binaphthyl-2,2’-di- alyzed asymmetric addition of diethylzinc to diphenyl-
amine,[10] with the corresponding thiophosphorochlori- phosphinoyl-imines was performed. The yield and
dic acid O,Oꢀ-diaryl ester (Scheme 2 and Supporting enantioselectivity are both dependent on the temper-
Information).
ature. At 08C, the reaction was completed within 48 h
Table 1. Optimization of the reaction conditions of ZnEt2 to diphenylphosphinoyl-imine catalyzed by the copper salt Cu-
(OTf)2 and chiral ligand L1.
ACHTREUNG
Entry
Solvent
Temp. [8C]
L1 [mol%]
Cu
(OTf)2 [mol%]
Time [h]
Yield [%][a]
ee [%][b]
1
2
3
4
5
6
7
8
CH2Cl2
CH3CN
THF
20
20
20
20
0
5
5
5
5
5
5
7.5
10
5
10
15
5
5
5
5
5
5
5
5
48
48
48
36
48
96
48
48
48
48
48
trace
NR
trace
85
78
67
86
90
41
70
–
–
–
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
67
82
89
83
83
10
92
92
À20
0
0
0
9
10
11
10
10
10
À20
À20
87
[a]
Isolated yields.
Determined by chiral HPLC.
[b]
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ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 2237 – 2242

Asymmetric Addition of Diethylzinc to Diphenylphosphinoyl-Imines
UPDATES
to give the addition product 1a in 78% yield and been seen from Table 3, most of the reactions pro-
82% ee (Table 1, entry 5). Lowering the temperature ceeded smoothly to provide the corresponding chiral
to À208C, the yield of the addition product 1a was diphenylphosphinoyl-amides 1 in good yields (75–
67% with 89% ee (Table 1, entry 6). However, when 87%) and high enantioselectivities (90–98% ee)
the reaction was carried out at À208C, the reaction (Table 3, entries 1–8). The enantioselectivity was not
was sluggish and a prolonged reaction time is re- affected by introduction of a chlorine atom into the
quired (Table 1, entry 6). Increasing the amounts of ortho-position of benzaldehyde (Table 3, entries 7 and
chiral ligand, we found that the yield of the addition 8). Lower yields and ees were obtained for imines
product 1a can be improved at 08C (Table 1, en- having a strongly electron-donating group (Table 3,
tries 7–9). At À208C using 15 mol% of L1 and 10 entry 4). Based on this catalytic asymmetric addition,
mol% of Cu
A
adduct 1a can be obtained in 87% yield and 92% ee tained by acidic hydrolysis of the obtained diphenyl-
(Table 1, entry 11).
phosphinoyl-amides (Scheme 3).[11] Using the sulfinic
(OTf)2 (5 adduct of N-phosphinoylbutylimine as substrate to ex-
Using L2–L5 as ligands (5 mol%) and Cu
ACHTREUNG
mol%) as the catalytic precursor in the reaction, we amine aliphatic N-diphenylphosphinoyl-imines ac-
found that the corresponding adduct 1a was obtained cording to Charetteꢀs procedure,[4e] we found that the
in 83–93% yields and 67–79% ee at 08C in toluene corresponding adduct was formed in traces under the
(Table 2, entries 1–3 and 5). It seems to us that ligand standard conditions (Scheme 4).
L4 gave the better result under identical conditions.
Using L4 (15 mol%) and Cu(OTf)2 (10 mol%) at phorus is crucial for this catalytic asymmetric reaction
À208C in toluene, we found that the corresponding to be so effective because the corresponding axially
adduct 1a was formed in 80% yield and 88% ee chiral binaphthyldiphenylphosphoramide ligand
As we mentioned before, the heteroatom on phos-
AHCTREUNG
(Table 2, entry 4). Therefore, L1 is the best ligand in showed no enantioselectivity for this kind of addition
this reaction under similar conditions. Using copper reaction.[9] We believe that this family of binaphthyl-
salt Cu(CH3CN)4ClO4 as catalytic precursor and L1 diphenylthiophosphoramides L1–L5 are bidentate li-
N
as ligand, the adducts were formed in lower yields gands in this catalytic asymmetric reaction.[12]
and ee under our standard conditions (Table 2, en- 31P NMR spectroscopic studies of a 1:1 mixture of L1
tries 6 and 7). Cu
study due to its greater air stability and associated carried out. In the absence of Cu
convenience in handling. Thus, the best reaction con- rus signal of L1 appeared at d=+58.03 as a sharp sin-
ditions are using Cu(OTf)2 (10 mol%) and L1 (15 glet (see Supporting Information). In contrast, a
mol%) in toluene at À208C. The reaction can be slightly upfield shift of the signal of the phosphorus
completed within 48 h in 87% yield and 92% ee. atom connecting to the sulfur atom to d=+57.83 in
Encouraged by the result obtained for the diphenyl- L1 was observed in the presence of Cu(OTf)2, sug-
A
ACHTREUNG
AHCTREUNG
AHCTREUNG
AHCTREUNG
phosphinoyl-imine of benzaldehyde, we investigated a gesting coordination of the sulfur atom to the copper
variety of other imines to probe their behaviors under center and this signal broadened after 12 h, presuma-
these optimized reaction conditions in this catalytic bly due to the reduction of Cu(II) into Cu(I) (see
system. The results are summarized in Table 3. As can Supporting Information). Secondly, in order to obtain
Table 2. Screening of chiral ligands in the asymmetric addition reaction.
Entry
Copper salt [mol%]
Cu(OTf)2
Ligand [mol%]
Temp. [8C]
Time [h]
Yield [%][a]
ee [%][b]
1
2
3
4
5
6
7
G
5
5
5
10
5
L2
L3
L4
L4
L5
L1
L1
5
5
5
15
5
0
0
0
48
48
48
48
48
48
48
89
83
93
80
86
79
77
67
58
79
88
70
48
61
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(MeCN)4ClO4
Cu(MeCN)4ClO4
G
T
G
À20
ACHTREUNG
0
R
5
10
7.5
15
0
À20
E
[a]
Isolated yields.
Determined by chiral HPLC.
[b]
Adv. Synth. Catal. 2006, 348, 2237 – 2242
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.asc.wiley-vch.de
2239

Min Shi et al.
UPDATES
Table 3. Asymmetric addition reaction of ZnEt2 to diphenylphosphinoyl-imines in the presence of chiral ligand L1 and Cu-
(OTf)2 in toluene.
ACHTREUNG
Entry
Ar
Product
Yield [%][a]
ee [%][b]
Configuration
1
2
3
4
5
6
7
8
C6H5
1a
1b
1c
1d
1e
1f
1g
1h
87
83
87
75
87
82
82
82
92
98
91
90
93
95
98
95
R
R
R
R
R
R
R
R
p-MeC6H4
m-MeC6H4
p-MeOC6H4
p-ClC6H4
p-BrC6H4
o-ClC6H4
2,4-Cl2C6H3
[a]
Isolated yields.
Determined by chiral HPLC.
[b]
good yields and >90% enantioselectivities has been
achieved using Cu(OTf)2 and easily available, stable
AHCTREUNG
axially chiral binaphthylthiophosphoramide ligands
(2’-ethylamino-[1,1’]binaphthalenyl-2-yl)-thiophosphor-
amidic acid O,O’-diaryl esters. Efforts are underway
to elucidate the mechanistic details of this catalytic
system and to extend the scope and limitations of
Scheme 3. Preparation of optically active amines.
À
those novel chiral ligands in other asymmetric C C
evidence for the coordination of the nitrogen atom of bond forming processes.
the aniline moiety (ArNHCH2CH3) to the copper
compound, 13C NMR studies of the L1/Cu
ACHTREUNG
Experimental Section
AHCTREUNG
two carbons in the ethyl group of L1 appeared at d=
38.17 and d=14.95, but the two corresponding car-
bons signals appeared at d=38.88 and d=14.76 along
with a new signal at d=13.40 in the presence of Cu-
General Remarks
Melting points were obtained with a Yanagimoto micro
melting point apparatus and are uncorrected. H NMR spec-
tra were recorded on a Bruker AM-300 spectrometer for
solutions in CDCl3 with tetramethylsilane (TMS) as internal
standard; J values are in Hz. Mass spectra were recorded
1
ACHTREUNG
(OTf)2 can be potentially with an HP-5989 instrument. All of the solid compounds re-
ported in this paper gave satisfactory CHN microanalyses as
deetrmined with a Carlo–Erba 1106 analyzer or HR-MS.
Commercially obtained reagents were used without further
purification. All reactions were monitored by TLC with
Huanghai GF254 silica gel coated plates. Flash column chro-
matography was carried out using 200–300 mesh silica gel at
increased pressure. Enantiomeric ratios were determined by
chiral HPLC analysis. Racemic products were synthesized
coordinated by the S, N atoms in the L1 ligand.
Conclusions
In conclusion, the catalytic asymmetric addition of di-
ethylzinc to diphenylphosphinoyl-imines in generally
Scheme 4. Asymmetric addition reaction of ZnEt2 to the sulfinic adduct of N-phosphinoylimine in the presence of chiral
ligand L1 and Cu(OTf)2 in toluene.
ACHTREUNG
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Adv. Synth. Catal. 2006, 348, 2237 – 2242

Asymmetric Addition of Diethylzinc to Diphenylphosphinoyl-Imines
UPDATES
by addition of substrates with ethylmagnesium bromide
(1.0M in THF) at 08C.
Chichester, 1997; pp. 225–244. For a review on catalytic
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Ishitani, Chem. Rev. 1999, 99, 1069–1094.
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Representative Experimental Procedure for the
Synthesis of Ligands 1–5
To
a solution of (R)-(+)-N-ethyl-1,1’-binaphthyl-2,2’-di-
amine (200 mg, 0.64 mmol) in THF (10.0 mL) was added
dropwise n-butyllithium (1.12 mL, 1.8 mmol, 1.6M solution
in hexane) at À408C over 40 min, and the reaction mixture
was stirred for 1 h at the same temperature. Then, O,O-di-
phenyl chlorothiophosphate (365 mg, 1.28 mmol) in 5.0 mL
of THF was added dropwise and the reaction solution was
slowly warmed to room temperature. After 2 h, THF was re-
moved under vacuum. The residue was purified by alumina
column chromatography to give the corresponding ligand 1
(L1) as a colorless solid; yield: 316 mg (88%).
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General Procedure for the Cu(II)-Catalyzed
Asymmetric Addition of Diethylzinc to
Diphenylphosphinoyl-Imines
A solution of Cu(OTf)2 (5.4 mg, 0.015 mmol) and ligand L1
G
(12.6 mg, 0.0225 mmol) in dry toluene (3.0 mL) was stirred
for 1 h at room temperature under an argon atmosphere.
The diphenylphosphinoyl-imine of benzaldehyde (47 mg,
0.15 mmol) was added and the solution was stirred for a fur-
ther 10 min, then Et2Zn (0.45 mL, 0.45 mmol, 1.0M solution
in hexane) was added dropwise at À208C. The resulting
mixture was stirred for 48 h at the same temperature and sa-
turated aqueousNH4Cl solution (10.0 mL) was added. After
extraction with ethyl acetate (3”10.0 mL), the combined or-
ganic layers were dried over MgSO4. The residue obtained
upon removal of the volatiles under vacuum was purified by
column chromatography on silica gel (eluent: petroleum/
ethyl acetate=1/1) to afford the addition product N-(1-phe-
nylpropyl)-P,P-diphenylphosphinoylamide 1a as a colorless
solid; yield: 43.5 mg (87%).
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Supporting Information
The spectroscopic and analytical data for the compounds
shown in Tables 1, 2 and 3 and the detailed description of
experimental procedures are included in supporting infor-
mation for this article, which is also available from the
author.
Acknowledgements
[8] a) I. Ojima, (Ed.), CatalyticAsymmetricSynthesis
VCH, New York, 1993; b) E. N. Jacobsen, A. Pfaltz, H.
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[9] a) M. Shi, W.-S. Sui, Chiraliy 2000, 12, 574–580; b) M.
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We thank the State Key Project of Basic Research (Project
973) (No. G2000048007), Shanghai Municipal Committee of
Science and Technology (04 JC14083), and the National Nat-
ural Science Foundation of China for financial support
(20472096, 203900502 and 20272069).
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