Furanoside thioether-phosphinite ligands for Pd-catalyzed asymmetric allylic substitution reactions

Article abstract of DOI:10.1016/j.tetasy.2004.11.094

A new series of thioether-phosphinite ligands, easily prepared in a few steps from inexpensive d-(+)-xylose, was tested in the Pd-catalyzed allylic substitution of substrates with different steric properties. The results show that the enantiomeric excesse

Full text of DOI:10.1016/j.tetasy.2004.11.094

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 959–963
Furanoside thioether–phosphinite ligands for
Pd-catalyzed asymmetric allylic substitution reactions
*
Eugeni Guimet, Montserrat Dieguez, Aurora Ruiz and Carmen Claver
´
´
´
`
´
Departament de Quımica Fısica i Inorganica, Universitat Rovira i Virgili, C/MarcelÆlı Domingo, s/n, 43007 Tarragona, Spain
Received 22 October 2004; accepted 30 November 2004
Abstract—A new series of thioether–phosphinite ligands, easily prepared in a few steps from inexpensive D-(+)-xylose, was tested in
the Pd-catalyzed allylic substitution of substrates with different steric properties. The results show that the enantiomeric excesses are
strongly dependent on the steric properties of the substituent in the thioether moiety. Enantiomeric excesses of up to 93% with high
activities were obtained for the substrate rac-1,3-diphenyl-3-acetoxyprop-1-ene 7 with dimethyl malonate as the nucleophile.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
ligands, and encouraged by the success of Evansꢀ
thioether–phosphinite ligands, we have designed the
furanoside thioether–phosphinites ligands 1–3 (Fig. 1).
Palladium-catalyzed asymmetric allylic substitution is a
versatile, widely used process in organic synthesis for the
enantioselective formation of C–C and C–heteroatom
bonds.1 Many chiral ligands, bidentate nitrogen and
phosphorus donors (both homo- and heterodonors),
have been successfully applied.1 Mixed phosphorus–
nitrogen ligands have played a dominant role among
the heterodonor ligands. To a lesser extent, phospho-
rus–thioether ligands have also demonstrated their
potential utility in Pd-catalyzed asymmetric allylic
substitution.2 Evans et al. have developed a family of
thioether–phosphinite ligands that proved to be effec-
tive3 but, despite this success, the use of other thio-
ether–phosphinite ligands has not been reported. As
far as carbohydrate ligands are concerned, despite their
advantages such as availability at low price and easy
modular constructions, they have only very recently
shown their huge potential as a source of highly effective
chiral ligands in this process.4 In this context, several
combinations of S–P ligands such as thioether–phos-
phine,5 thioether–phospholane,6 thioether–phosphite7
have been studied and have proven to be effective. How-
ever, to our knowledge, carbohydrate-based thioether–
phosphinite ligands have not been tested in this process.
Ph
Ph
P
O
1
2
3
R = i-Pr
R = Me
R = Ph
RS
O
O
O
Figure 1. Thioether–phosphinite ligands 1–3.
Herein we report the synthesis of the thioether–phosphi-
nite ligands 1–3 and their use in Pd-catalyzed enantio-
selective asymmetric allylic substitution.
2. Results and discussion
2.1. Synthesis of the chiral thioether–phosphinite ligands
The new ligands 1–3 were synthesized very efficiently in
one step from the corresponding thioether-alcohols 4–6,
which are easily prepared on a large scale from D-(+)-
xylose using a standard procedure (Scheme 1).8 Reaction
of the corresponding thioethers 4–6 with one equivalent
of chlorodiphenylphosphine in dry THF, under nitrogen
and in the presence of pyridine and 4-(dimethylamino)-
pyridine (DMAP), then provided the desired ligands 1–3
in 71%, 87% and 89% yields, respectively.
Following our interest in carbohydrates as an inexpen-
sive and highly modular chiral source for preparing
*
Corresponding author. Tel./fax: +34 97 755 9563; e-mail:
montserrat.dieguez@urv.net
0957-4166/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.11.094

960
E. Guimet et al. / Tetrahedron: Asymmetry 16 (2005) 959–963
Table 1. Pd-catalyzed allylic alkylation of 1,3-diphenyl-3-acetoxyprop-
1-ene 7 using ligands 1–3a
Ph
Ph
P
O
RS
RS
% Conv (min)b % Eec
O
Entry Ligand Solvent
Base
Ph2PCl
O
a
OH
D-(+)-xylose
Py / DMAP
1
1
1
1
1
1
1
1
1
2
3
1
1
CH2Cl2 KOAc
DMF
THF
100 (30)
100 (5)
77 (30)
60 (30)
72 (30)
86 (S)
67 (S)
84 (S)
86 (S)
83 (S)
83 (S)
82 (S)
72 (S)
61 (S)
47 (S)
93 (S)
88 (S)
O
O
2
KOAc
KOAc
O
O
3
4
Toluene KOAc
CH2Cl2 K2CO3
4
5
6
1
2
3
R = i-Pr
R = Me
R = Ph
R = i-Pr
R = Me
R = Ph
5
6
CH2Cl2 NaOAc 95 (30)
CH2Cl2 Li2CO3 95 (30)
7
8d
9
CH2Cl2 KOAc
CH2Cl2 KOAc
CH2Cl2 KOAc
CH2Cl2 KOAc
CH2Cl2 KOAc
70 (30)
91 (5)
Scheme 1. Synthesis of ligands 1–3. a—Ref. 8.
10
11e
12f
90 (30)
100 (90)
100 (60)
All ligands were stable during purification on neutral
alumina under an atmosphere of argon and were iso-
lated as colourless oils. The 1H, 31P and 13C NMR spec-
tra were as expected for these C1 ligands (see
Experimental section).
a 0.5 mol % [Pd(p-C3H5)Cl]2, 1.1 mol % ligand, room temperature,
30 min; 3 equiv of CH2(COOMe)2 and N,O-bis(trimethylsilyl)acet-
amide (BSA), a pinch of the corresponding base.
b Measured by 1H NMR. Reaction time in minutes shown in
parentheses.
2.2. Asymmetric allylic substitution reactions
c Determined by HPLC (Chiralcel OD). Absolute configuration drawn
in parentheses.
We first investigated the Pd-catalyzed allylic substitution
of rac-1,3-diphenyl-3-acetoxyprop-1-ene 7, which is
widely used as a model substrate, with dimethyl malon-
ate using the chiral thioether–phosphinite ligands 1–3
(Scheme 2). The catalysts were generated in situ from
0.5 mol % of p-allyl-palladium chloride dimer
[PdCl(g3-C3H5)]2, the corresponding ligand and a cata-
lytic amount of the corresponding base.
d L/Pd = 2.
e Reaction carried out at 0 ꢁC.
f Substrate/Pd ratio of 1000.
Enantioselectivity can be further improved (eeꢀs up to
93% (S)) using ligand 1 by lowering the reaction temper-
ature to 0 ꢁC (entry 11). We also performed the reaction
at a low catalyst concentration (7/Pd = 1000) using
ligand 1. Good enantioselectivity [(S) 88% ee] and
excellent activity (100% conversion after 1 h at
room temperature, TOF > 1000 mol · (mol · h)ꢀ1) were
achieved.
OAc
Ph
CH(COOMe)2
Ph
CH2(COOMe)2 / BSA
Ph
Ph
[Pd(π-C3H5)Cl]2 / 1 - 3
8
7
To investigate further the catalytic efficiency of these Pd/
1–3 systems, we then tested these ligands in the Pd-cat-
alyzed allylic amination of 7 with benzylamine (Scheme
3). The results, which are summarized in Table 2, indi-
cate that the catalytic performance (activities and
enantioselectivities) followed the same trend as for the
allylic alkylation of 7, however, the activities were lower.
Scheme 2.
The effect of the solvent, base and the ligand-to-palla-
dium ratio were investigated using the catalyst precursor
containing ligand 1 (Table 1). Our results indicate that
both solvent and base affected catalytic performance.
Dichloromethane as solvent and potassium acetate as
base provided the best combination of activity and
enantioselectivity (entries 1–4 and 5–7). Good activity
[TOF > 200 mol · (mol · h)ꢀ1] and good enantioselec-
tivity [(S) 86% ee] were obtained. Varying the ligand-
to-palladium ratio showed that excess ligand was not
needed to obtain good enantioselectivities and activities
(entries 1 and 8). At higher ligand-to-palladium ratios,
activities and enantioselectivities were lower. This was
probably due to the fact that the thioether–phosphinite
ligand acts as a monodentate ligand.
Ph
Ph
OAc
Ph
HN
PhCH2NH2
Ph
Ph
[Pd(π-C3H5)Cl]2 / 1-3
7
9
Scheme 3.
Table 2. Pd-catalyzed allylic amination of 1,3-diphenyl-3-acetoxy-
prop-1-ene 7 using ligands 1–3a
Under the optimized conditions, we studied how the thio-
ether substituents affect the catalytic performance with
ligands 2 and 3 (Table 1). The use of ligand 2 with
methyl substituents in the thioether moieties showed
higher activities but lower asymmetric induction than
those obtained with the catalytic system Pd/1 (entry 9).
The use of ligand 3 with phenyl substituents in the thio-
ether moieties showed the lowest activity and asymmet-
ric induction (entry 10).
Entry
Ligand
% Conv (h)b
% Eec
1
2
3
1
2
3
100 (4)
100 (4)
100 (10)
88 (R)
63 (R)
40 (R)
a 0.5 mol % [Pd(p-C3H5)Cl]2, 1.1 mol % ligand, room temperature,
30 min; 3 equiv of benzylamine. Dichloromethane as solvent.
b Measured by 1H NMR. Reaction time in hours shown in parentheses.
c Determined by HPLC (Chiralcel OJ).

E. Guimet et al. / Tetrahedron: Asymmetry 16 (2005) 959–963
961
Encouraged by the good results obtained so far we also
tested ligands 1–3 in the Pd-catalyzed allylic alkylation
of rac-3-acetoxycyclohexene 10 (Scheme 4), which is
usually used as a model cyclic substrate. It is usually
more difficult to control enantioselectivity in cyclic sub-
strates, mainly because of the presence of less sterically
syn substituents, which are thought to play a crucial role
in the enantioselection observed with acyclic substrates
in the corresponding Pd-allyl intermediate.1
lyzed allylic substitution reactions of several substrates
with different steric properties. Our results show that
the enantiomeric excesses are strongly dependent on
the steric proprieties of substituent in the thioether moi-
ety of the carbohydrate backbone. Enantiomeric
excesses of up to 93% with high activities were obtained
using KOAc as base and CH2Cl2 as solvent.
Interestingly, these thioether–phosphinite ligands
showed a much higher degree of enantioselectivity and
higher reaction rates than their thioether–phosphite ana-
logues under similar reaction conditions.7
OAc
CH(COOMe)2
CH2(COOMe)2 / BSA
We are confident that further development of new
ligands, taking advantage of the high modularity of this
family of ligands, will improve enantioselectivities so far
obtained in cyclic substrates. Such studies and mecha-
nistic are currently under way.
[Pd(π-C3H5)Cl]2 / 1 - 3
10
11
Scheme 4.
Our preliminary investigations into the solvent effect,
the ligand-to-palladium ratio and base using ligand 1
provided the same trends as those observed in the previ-
ously tested linear substrate 7 (Table 3). The optimum
compromise between enantioselectivities and reaction
rates was therefore obtained when dichloromethane
was used as solvent, the ligand-to-palladium ratio was
1.1 and potassium acetate was used as base (entry 1).
Under optimized conditions, the results obtained with
ligands 1–3 indicate that the catalytic performance (activ-
ities and enantioselectivities) followed the same trend as
for the allylic alkylation of 1,3-diphenyl-3-acetoxyprop-
1-ene 7. However, the activities and enantiomeric ex-
cesses were lower. Lowering the temperature to a 0 ꢁC
increased enantioselectivity up to 51% (entry 11 vs 1).
4. Experimental section
4.1. General comments
All syntheses were performed using standard Schlenk
techniques under argon atmosphere. Solvents were puri-
fied by standard procedures. Compounds 4–6 were pre-
pared by previously described methods.8 All other
reagents were used as commercially available. 1H,
13C{1H} and 31P{1H} NMR spectra were recorded on
a Varian Gemini 400 MHz spectrometer. Chemical
shifts are relative to SiMe4 (1H and 13C) as internal stan-
dard or H3PO4 (31P) as external standard. All assign-
ments in NMR spectra were determined by H–1H and
1
13C–1H spectra. Racemics 1,3-diphenyl-3-acetoxyprop-
1-ene 79 and 3-acetoxycyclohexene 1010 were prepared
as previously reported.
Table 3. Pd-catalyzed allylic alkylation of 3-acetoxycyclohexene 10
with ligands 1–3a
Entry Ligand Solvent
Base
% Conv (min)b % Eec
4.2. Synthesis of the chiral thioether–phosphinite ligands
1
1
1
1
1
1
1
1
1
2
3
1
CH2Cl2 KOAc
DMF
THF
100 (30)
51 (30)
53 (30)
53 (30)
162 (30)
41 (R)
30 (R)
30 (R)
31 (R)
29 (R)
33 (R)
32 (R)
20 (R)
21 (R)
4 (R)
2
KOAc
KOAc
4.2.1. 1,2-O-Isopropylidene-3-diphenylphosphinite-5-iso-
propylsulfanyl-D-xylofuranose 1. A solution of 0.6 mL
(3.3 mmol) of chlorodiphenylphosphine in 12 mL of
THF was slowly added at 0 ꢁC to a solution of 744 mg
(3 mmol) of 4 and 18.3 mg (0.15 mmol) of DMAP in
3 mL of pyridine. The reaction mixture was stirred over-
night at room temperature. Ether ethylic was then added
and the pyridine salts were removed by filtration. The
residue was purified by flash chromatography (eluent:
toluene/NEt3 100:1, Rf 0.9) to produce 0.92 g (71%)
3
4
Toluene KOAc
CH2Cl2 K2CO3
5
6
CH2Cl2 NaOAc 88 (30)
CH2Cl2 Li2CO3 81 (30)
7
8d
CH2Cl2 KOAc
CH2Cl2 KOAc
CH2Cl2 KOAc
CH2Cl2 KOAc
24 (30)
100 (30)
98 (30)
98 (90)
9
10
11e
51 (R)
a 0.5 mol % [Pd(p-C3H5)Cl]2, 1.1 mol % ligand, room temperature,
30 min; 3 equiv of CH2(COOMe)2 and N,O-bis(trimethylsilyl)acet-
amide (BSA), a pinch of the corresponding base.
b Conversion percentage determined by GC. Reaction time in minutes
shown in parentheses.
1
of a colourless oil. 31P NMR, d: 116.1 (s). H NMR
3
(CDCl3), d: 1.16 (d, 3H, CH3, i-Pr, JMe–CH = 1.2 Hz),
1.18 (d, 3H, CH3, i-Pr, JMe–CH = 1.2 Hz), 1.26 (s, 3H,
3
CH3), 1.49 (s, 3H, CH3), 2.72 (m, 1H, H-50), 2.78 (m,
1H, CH–S), 2.82 (m, 1H, H-5), 4.33 (m, 1H, H-4),
4.48 (dd, 1H, H-3, J3-P = 9.2 Hz, J3-4 = 2.4 Hz), 4.56
c Enantiomeric excesses determined by GC (FS-b-Cyclodex). Absolute
configuration drawn in parentheses.
3
3
d L/Pd = 2.
e Reaction carried out at 0 ꢁC.
3
3
(d, 1H, H-2, J2-1 = 3.2 Hz), 5.91 (d, 1H, H-1, J1-2
=
3.2 Hz), 7.3–7.6 (m, 10H, CH@). 13C NMR (CDCl3),
d: 23.5 (CH3, i-Pr), 23.6 (CH3, i-Pr), 26.5 (CH3), 27.0
(CH3), 28.5 (C-5), 35.8 (CH–S), 80.8 (d, C-4,
3. Conclusions
3J4-P = 6.9 Hz), 82.7 (d, C-3, J3-P = 19.8 Hz), 84.0 (d,
2
3
C-2, J2-P = 6.1 Hz), 105.1 (C-1), 112.1 (CMe2), 128.5
We have designed and synthesized a new family of read-
ily available thioether–phosphinite ligands for Pd-cata-
(CH@), 128.6 (CH@), 128.8 (CH@), 129.8 (CH@),

962
E. Guimet et al. / Tetrahedron: Asymmetry 16 (2005) 959–963
129.9 (CH@), 130.1 (CH@), 130.3 (CH@), 130.9 (CH@),
131.1 (CH@), 141.2 (d, C, JC–P = 16.0 Hz), 142.2 (d, C,
JC–P = 17.6 Hz). Anal. Calcd (%) for C23H29O4PS: C,
63.87; H, 6.76; S, 7.41. Found: C, 63.78, H, 6.84, S 7.34.
added. The reaction mixture was stirred at room tem-
perature. After 5 min the reaction mixture was diluted
with Et2O (5 mL) and a saturated NH4Cl (aq) (25 mL)
was added. The mixture was extracted with Et2O
(3 · 10 mL) and the extract dried over MgSO4. Solvent
was removed and conversion was measured by 1H
NMR. To determine the ee by HPLC (Chiralcel OD,
0.5% 2-propanol/hexane, flow 0.5 mL/min), a sample
was filtered over basic alumina using dichloromethane
as the eluent.
4.2.2.
methylsulfanyl-D-xylofuranose 2. Treatment of
1,2-O-Isopropylidene-3-diphenylphosphinite-5-
5
(3 mmol) with chlorodiphenylphosphine (0.6 mL,
3.3 mmol) as described for compound 4 afforded thio-
ether–phosphinite 2, which was purified by flash chroma-
tography (eluent: toluene/NEt3 100:1, Rf 0.9) to produce
1.05 g (87%) of a colourless oil. 31P NMR, d: 115.9 (s).
4.4. Typical procedure of allylic alkylation of rac-3-
acetoxycyclohexene 10
1H NMR, d: 1.27 (s, 3H, CH3), 1.50 (s, 3H, CH3),
3
2.04 (s, 3H, CH3–S), 2.70 (dd, 1H, H-50, JH–H
8.0 Hz, JH–H = 13.2 Hz), 2.75 (dd, 1H, H-5, JH–H
=
2
3
=
6.0 Hz, JH–H = 13.2 Hz), 4.37 (m, 1H, H-4), 4.48 (dd,
A degassed solution of [PdCl(g3-C3H5)]2 (1.8 mg,
0.005 mmol) and the thioether–phosphinite ligand
(0.011 mmol) in dichloromethane (0.5 mL) was stirred
for 30 min. Subsequently, a solution of rac-10 (70 mg,
0.5 mmol) in dichloromethane (1.5 mL), dimethyl malo-
nate (171 lL, 1.5 mmol), N,O-bis(trimethylsilyl)-acet-
amide (370 lL, 1.5 mmol) and a pinch of KOAc were
added. The reaction mixture was stirred at room tem-
perature. After 30 min the reaction mixture was diluted
with Et2O (5 mL) and a saturated NH4Cl (aq) (25 mL)
was added. The mixture was extracted with Et2O (3 ·
10 mL) and the extract dried over MgSO4. Conversion
and enantiomeric excess were determined by GC using
2
3
1H, H-3, JH–H = 2.8 Hz, JH–P = 9.6 Hz), 4.58 (d, 1H,
3
3
H-2, JH–H = 3.2 Hz), 5.92 (d, 1H, H-1, JH–H
=
3.2 Hz), 7.3–7.6 (m, 10H, CH@). 13C NMR, d: 16.4
(CH3–S), 26.5 (CH3), 27.0 (CH3), 31.9 (C-5), 80.3 (d,
C-4, JC–P = 6.8 Hz), 82.7 (d, C-3, JC–P = 19.9 Hz), 84.0
(d, C-2, JC–P = 5.3 Hz), 105.1 (C-1), 112.1 (CMe2),
128.6 (CH@), 128.7 (CH@), 128.8 (CH@), 129.7
(CH@), 130.1 (CH@), 130.3 (CH@), 131.0 (CH@),
131.2 (CH@), 141.2 (d, C, JC–P = 16.8 Hz), 142.1 (d,
C, JC–P = 16.0 Hz). Anal. Calcd (%) for C21H25O4PS:
C, 62.36; H, 6.23; S, 7.93. Found: C, 62.43, H, 6.34, S,
7.82.
a FS-b-Cyclodex 25 m column, internal diameter
0.2 mm, film thickness 0.33 mm, carrier gas: 100 kPa
4.2.3. 1,2-O-Isopropylidene-3-diphenylphosphinite-5-
phenylsulfanyl-D-xylofuranose 3. Treatment of 6
He, FID detector).
(3.5 mmol) with chlorodiphenylphosphine (0.7 mL,
3.9 mmol) as described for compound 4 afforded thio-
ether–phosphinite 3, which was purified by flash chro-
matography (eluent: toluene/NEt3 100:1, Rf 0.9) to
produce 1.47 g (89%) of a colourless oil. 31P NMR, d:
4.5. Typical procedure of allylic amination of rac-1,3-
diphenyl-3-acetoxyprop-1-ene 7
A degassed solution of [PdCl(g3-C3H5)]2 (1.8 mg, 0.005
mmol) and the thioether–phosphinite ligand (0.011
mmol) in dichloromethane (0.5 mL) was stirred for
30 min. Subsequently, a solution of rac-7 (126 mg,
0.5 mmol) in dichloromethane (1.5 mL) and benzyl-
amine (131 lL, 1.5 mmol) was added. The reaction mix-
ture was stirred at room temperature. After 1 h the
reaction mixture was diluted with Et2O (5 mL) and a
saturated NH4Cl (aq) (25 mL) was added. The mixture
was extracted with Et2O (3 · 10 mL) and the extract
dried over MgSO4. Solvent was removed and conversion
was measured by 1H NMR. To determine the ee by
HPLC (Chiralcel OJ, 13% 2-propanol/hexane, flow 0.5
mL/min), a sample was filtered over silica using 10%
Et2O/hexane mixture as the eluent.
1
116.8 (s). H NMR, d: 1.25 (s, 3H, CH3), 1.39 (s, 3H,
3
2
CH3), 3.06 (dd, 1H, H-50, JH–H = 8.4 Hz, JH–H
12.8 Hz), 3.19 (dd, 1H, H-5, JH–H = 6.0 Hz, JH–H
=
3
2
=
12.8 Hz), 4.32 (m, 1H, H-4), 4.51 (dd, 1H, H-3,
3JH–H = 2.1 Hz, JH–P = 9.6 Hz), 4.59 (d, 1H, H-2,
3JH–H = 3.6 Hz), 5.93 (d, 1H, H-1, JH–H = 3.6 Hz),
3
7.1–7.6 (m, 15H, CH@). 13C NMR, d: 26.6 (CH3),
26.9 (CH3), 31.7 (C-5), 79.3 (d, C-4, JC–P = 6.9 Hz),
82.5 (d, C-3, JC–P = 20.0 Hz), 84.0 (d, C-2, JC–P
=
5.6 Hz), 105.1 (C-1), 112.1 (CMe2), 126.5 (CH@),
128.7 (CH@), 128.8 (CH@), 129.1 (CH@), 129.8
(CH@), 129.9 (CH@), 130.1 (CH@), 130.4 (CH@),
131.0 (CH@), 131.3 (CH@), 135.6 (C), 141.0 (d, C,
JC–P = 16.8 Hz), 142.0 (d, C, JC–P = 16.8 Hz) Anal.
Calcd (%) for C26H27O4PS: C, 66.94; H, 5.83; S 6.87.
Found: C, 70.01, H, 5.79, S, 6.85.
Acknowledgements
´
We thank the Spanish Ministerio de Educacion, Cultura
y Deporte for their financial support (BQU2001-0656).
4.3. Typical procedure of allylic alkylation of rac-1,3-
diphenyl-3-acetoxyprop-1-ene 7
A degassed solution of [PdCl(g3-C3H5)]2 (1.8 mg,
0.005 mmol) and the thioether–phosphinite ligand
(0.011 mmol) in dichloromethane (0.5 mL) was stirred
for 30 min. Subsequently, a solution of rac-7 (126 mg,
0.5 mmol) in dichloromethane (1.5 mL), dimethyl malo-
nate (171 lL, 1.5 mmol), N,O-bis(trimethylsilyl)-acet-
amide (370 lL, 1.5 mmol) and a pinch of KOAc were
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