Highly air- and water-stable fluorinated ferrocenylphosphine-aminophosphine ligands and their applications in asymmetric hydrogenations

Article abstract of DOI:10.1002/adsc.200505241

Air- and water-stable fluorinated ferrocenylphosphine-aminophosphine ligands have been synthesized and applied to the Rh-catalyzed asymmetric hydrogenation of enamides and α-dehydroamino acid derivatives. These reactions afforded the corresponding product

Full text of DOI:10.1002/adsc.200505241

COMMUNICATIONS
DOI: 10.1002/adsc.200505241
Highly Air- and Water-Stable Fluorinated
Ferrocenylphosphine-Aminophosphine Ligands and their
Applications in Asymmetric Hydrogenations
a, b
a, c
a
a
a,
Xingshu Li, Xian Jia, Lijin Xu, Stanton H. L. Kok, C. W. Yip, *
a,
Albert S. C. Chan *
Open Laboratoryof Chirotechnologyof the Institute of Molecular Technologyfor Drug Discoveryand S yn thesis and
a
Department of Applied Biologyand Chemical Technolog y, The Hong Kong Pol yt echnic Universit y, Hong Kong, P. R.
China
Fax: (þ852)-2364-9932, e-mail: bcachan@polyu.edu.hk
b
School of Pharmaceutical Sciences, Sun Yat-Sen University, Guangzhou, P. R. China
c
Shenyang Pharmaceutical University, Shenyang, Liaoning, P. R. China
Received: June 8, 2005; Accepted: September 19, 2005
Abstract: Air- and water-stable fluorinated ferroce-
nylphosphine-aminophosphine ligands have been
synthesized and applied to the Rh-catalyzed asym-
metric hydrogenation of enamides and a-dehydro-
amino acid derivatives. These reactions afforded
the corresponding products in good to excellent ees
Chiral ligands with a ferrocenyl backbone have found
useful applications in transition metal-catalyzed asym-
[
1,3,4]
metric hydrogenations.
Some well known examples
include Hayashiꢀs (aminoalkyl)ferrocenylphosphine li-
[
5]
[6]
gands, Togniꢀs Josiphos-type ligands, Burkꢀs Ferro-
[
7]
TANE-type ligands, Knochelꢀs ferrocenyl ligands
with two phosphanyl substituents, Zhangꢀs f-bina-
[9] [10]
[
8]
(
up to 99.7% ee) and nearlyquantitative iye lds.
The combination of the remarkable air- and water-
stabilitywith excellent catal yt ic performance makes
these catalysts excellent choices for practical applica-
tions.
phane
and Spindlerꢀs Walphos-type ligands. Re-
cently, Baoz et al. reported the preparation of phosphi-
noferrocenylaminophosphines and their application in
the rhodium-catalyzed asymmetric hydrogenation with
[
11]
high activities and enantioselectivities. In our quest
for highly efficient ligands in asymmetric hydrogena-
Keywords: air-stable ligands; asymmetric hydrogena-
tion; enamides; ferrocenylphosphine-aminophos-
phine ligands; rhodium
[
12]
tion, we became interested in ferrocene-based ligands
with high efficiencyand stabilit y. Herein we report the
interesting findings of some new fluorinated phosphino-
ferrocenylaminophosphines in rhodium-catalyzed
asymmetric hydrogenation. The distinctly high air- and
water-stabilities of the fluorinated ligands as well as
Asymmetric catalytic reactions have attracted much at- their Rh(I) complexes, combined with the high enantio-
tention in recent years because of their great potential in selectivities in asymmetric hydrogenation, make them
[
1]
making higher-value-added products. In this respect, good choices for practical applications.
the use of chiral ligands has played a very important We first examined ligand 1 (Scheme 1) in hydrogena-
tion of arylenamides, and relatively poor ee values were
and used for Rh- and Ru-catalyzed asymmetric hydro- observed in contrast to the results obtained from the hy-
[2]
role. So far manychiral ligands have been prepared
[3]
genation. Although moderate to excellent enantiose- drogenation of a-dehydroamino acid derivatives under
[
11]
lectivities have been achieved using these ligands, similar conditions (>99% ee). This is not surprising
most of the catalytic reactions needed to be performed because chiral ligands for asymmetric catalysis usually
with the strict exclusion of oxygen and moisture, thus perform well for specific types of substrates. It is well-
making them quite inconvenient for general organic known that effectiveness of ligands might be changed
[3,13]
synthesis and industrial applications. From a practical substantiallywhen their structures are modified.
In
standpoint, it is desirable to develop effective, easily this pursuit we have prepared a series of new ligands
prepared chiral ligands and catalysts with high air- and with electron-donating or electron-withdrawing groups
water-stability, so that the associated catalytic reactions on the phenyl rings of the phosphine or amino-phos-
can be easilycarried out in ordinarylaboratoryor indus-
trial settings.
phine. The synthetic route for the target ligands is illus-
trated in Scheme 1. The reaction of diarylphosphorous
1904
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2005, 347, 1904 – 1908

Air- and Water-Stable Fluorinated Ferrocenylphosphine-Aminophosphine Ligands
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Scheme 1.
chlorides 9–11 with ferrocene-amines 6–8 in the pres- Table 1. Asymmetric hydrogenation of aryl enamides 12a
ence of triethylamine gave the desired products 1–5 in and 12b with different Rh-phosphinoferrocenylaminophos-
[a]
phines catalysts.
80–95% yield. We were delighted to find that ligand 3
was air-stable in solution. A chloroform solution of li-
gand 3 exposed to air and in the presence of water
31
showed no change in the P NMR spectrum even after
two months. In contrast, ligands 1, 2 and 4 decomposed
within 8 h, as expected. Other ligands, such as amino-
phosphine, phosphinite, and monodentate phosphor-
amidite, decomposed quicklyunder similar conditions.
EntryLigand Sub-
strate
Ar
Ph
Solvents
t
Ee
[b]
[h] [%]
[
14]
1
2
3
4
5
6
7
8
9
1
1
2
2
3
4
5
3
3
5
12a
12b
12a
12b
12a
12a
12a
12a
12a
12a
CH Cl
10 70.0
10 73.1
For example, bisaminophosphine ligand BDPAB and
2
2
2
2
2
2
2
[
15]
p-F C-C H CH Cl
phosphinite ligand BINAPO decomposed completely
in one hour; the monodentate phosphoramidite ligands
3
6
4
2
Ph
p-F
Ph
Ph
Ph
Ph
Ph
Ph
CH Cl
8
8
80.6
79.6
2
[16]
[12b]
C-C
3
H
6
4
CH Cl
2
Monophos and H -Monophos
decomposed within
8
CH Cl
16 94.6
10 35.0
2
2
4 h.
CH Cl
2
The new ligands (2–5) and ligand 1 were tested in the
i-PrOH 16 94.4
Toluene 16 93.5
THF
THF
rhodium-catalyzed hydrogenation of arylenamide sub-
strates 12a and b. The three new ligands 2, 3 and 5
were found to be superior to ligand 1 (Table 1, entries 10
16 96.5
96.2
8
3
–5 vs. entries 1 and 2). These results demonstrated
[
a]
Reaction conditions: all the catalysts used in the reactions
were prepared in situ. Hydrogen pressure was 300 psi in all
reactions; substrate:catalyst¼100: 1. All the reactions
were carried out at ambient temperature; quantitative
yields were obtained in all cases.
[b]
the importance of the steric and electronic effects of
the ligand on the enantioselectivities. Ligand 4, which
contained a 3,5-bis(trifluoromethyl)phenylphosphino
group on the ferrocenyl ring, displayed a negative effect
(
entry6). A preliminaryscreening of the solvent effect
The ees were determined bychiral GC anal ys is using a
50 mꢀ0.25 mm Chrompack chiral fused silica chirasil-L-
VAL column. The S configuration was assigned bycom-
paring the experimental results with published data.
showed that there were no significant differences among
dichloromethane, 2-propanol, toluene and THF (Ta-
ble 1, entries 5, 7–10). THF was chosen as the solvent
of choice for the rest of the studybecause it was slightly
better than the other common organic solvents tested.
Based on this interesting lead, we further studied ligands ble is thus highlydesirable. In this regard we were de-
and 5 in more detail. lighted to find that not onlyligand 3, but also its rhodium
An inconvenience of using rhodium phosphine com- complex were air-stable, even in solution. Surprisingly,
3
plexes in catalytic hydrogenation is that most active rho- catalytic hydrogenation reactions in the presence of air
dium-phosphine catalysts are air-sensitive. Since only a using the Rh-3 complex prepared in situ in air showed es-
verytinyamount of catalyst is used in a laboratorystudy
sentiallythe same ee values for the chiral products as
of hydrogenation reactions, the presence of even a small compared with those from the air-tight system (Table 2,
amount of air in the system can ruin the experiments. For entries 1–3 vs. Table 1, entry9). The Rh- 3 complex pre-
this reason special equipment and tedious pretreat- pared and kept in air for 8 h and then used for the hydro-
ments are usuallyneeded to get consistent results. De-
veloping a highlyeffective chiral catal ys t that is air-sta-
genation of enamide 12a showed no change in enantio-
selectivity(95.2% ee, entry4). The stabilityof the cata-
Adv. Synth. Catal. 2005, 347, 1904 – 1908
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1905

COMMUNICATIONS
Xingshu Li et al.
Table 2. The investigation of the stabilityof Rh- 3 in water
and air.
Table 3. Asymmetric hydrogenation of various arylenamides
catalyzed by Rh-3.
[a]
[a]
[
b]
EntryS/C
C sa tt as loy lution
exposed to air [h]
H O
2
ee [%]
EntrySubstrate Ar
S/C
T
t
ee
[
1
2
3
4
5
6
100
500
100
100
100
100
–
–
0.5
8
0.5
0.5
–
–
–
96.1
95.8
95.5
95.2
95.1
77.2
b]
[
8C] [h] [%]
1
2
3
4
5
6
7
8
9
0
1
2
12a
12a
12a
12a
12b
12c
12d
12f
12a
12b
12c
12d
12f
Ph
Ph
Ph
Ph
100 rt
200 rt
500 rt
1000 rt
500 rt
500 rt
500 rt
500 rt
500
500
500
500
500
500
500
16 96.5
–
16 95.8
16 96.4
16 95.8
16 97.1
16 99.3
16 92.1
16 93.5
30 98.3
30 98.6
30 99.7
30 99.4
30 99.3
30 98.5
30 99.0
5%
30%
p-F C-C H
p-Br-C H
p-CH -C H
p-CH O-C H
Ph
p-F C-C H
4
p-Br-C H
p-CH -C H
p-CH
[
a]
3
6
4
The catalyst used in all the reactions was prepared in situ
in air; all the reactions were performed at room tempera-
ture and the hydrogen pressure was 300 psi.
The ee values were determined using the method described
in Table 1.
6
4
3
6
4
[
b]
3
6
4
4
5
5
5
5
5
5
5
1
1
1
3
6
6
4
3
6
4
lyst to water was also examined by adding 5% of water to 13
O-C
H
6
3
1
1
4
5
12g
12h
m-CH -C H
m-CH O-C H
4
the reaction system containing the Rh-3 complex and
the enamide 12a. The enantioselectivityremained un-
changed with quantitative yield of the product (entry
3 6 4
3
6
[a]
The preparation of catalyst and the reaction conditions
were the same as those in Table 2.
b]
5
). This propertyis desirable in industrial applications
[
as no laborious drying of solvents is needed when the
catalysts have enough moisture stability. When water
content of the solvent was increased to 30%, the conver-
sion was still nearlyquantitative, indicative of the high
activityof the catalyst even in the presence of a high con-
The ee values were determined using the same method as
described in Table1.
centration of water. However, the ee value decreased to Table 4. Applications of ligands 2–5 in the Rh-catalyzed
7
7.2% (entry6), which confirmed that water was not a
asymmetric hydrogenation of a-dehydroamino acid deriva-
[a]
suitable solvent for this enantioselective hydrogenation tives.
reaction.
The enantioselectivityof the Rh- 3 complex was also
examined with a varietyof arylenamides under the opti-
mized conditions at room temperature, and the results
are summarized in Table 3. All the substrates were
quantitativelyconverted to the desired chiral products
with ee values ranging from 92.1% to 99.7%. Changing
the reaction temperature to 5 8C increased the enantio-
[
b]
selectivitynoticeabl y, and some of the desired products
were almost opticallypure (entries 9–15).
EntryLigand
Substrate
ee [%]
1
2
3
4
5
6
2
2
3
3
4
5
13a
13b
13a
13b
13a
13a
99.0
99.5
99.2
99.7
96.1
98.5
The enantioselectivities of ligands 2–5 were also ex-
amined in the Rh-catalyzed hydrogenation of a-dehy-
droamino acid derivatives (Table 4). Complete conver-
sions and more than 90% enantioselectivities have
been achieved with ligands 2–5 at ambient temperature.
Among the ligands employed, both 2 and 3 gave the best
results (entries 1–4). The use of ligands 4 and 5 led to
lower ees (entries 5 and 6). These results are comparable
[a]
The catalyst used in all the reactions was prepared in situ.
Hydrogen pressure was 300 psi in all reactions; sub-
strate:catalyst¼200: 1 (molar ratio); All the reactions
were carried out at ambient temperature.
[b]
[
10]
to those achieved byusing ligand 1.
In summary, we have developed phosphinoferrocenyl-
aminophosphines ligands 2, 3, 4 and 5 and applied them
to asymmetric hydrogenations. Good to excellent re-
sults were obtained. The studyrevealed that Rh- 3 and
The ees were determined bychiral GC anal ys is using a
Chrompack chiral fused silica 25 mꢀ0.25 mm chirasil-L-
VAL column. The S configuration was assigned based on
comparison with published data.
1906
asc.wiley-vch.de
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2005, 347, 1904 – 1908

Air- and Water-Stable Fluorinated Ferrocenylphosphine-Aminophosphine Ligands
COMMUNICATIONS
Rh-5 complexes displayed remarkable enantioselectivi- 132.28, 132.10, 131.91, 131.73, 131.64, 131.58, 131.54, 131.51,
1
1
9
6
31.32, 131.27, 131.23, 131.01, 129.44, 128.18, 128.11, 127.49,
27.45, 127.13, 124.28, 124.26, 122.76, 122.44, 122.11, 122.08,
7.61, 97.55, 97.38, 97.31, 75.37, 75.27, 72.27, 72.24, 70.20,
ty(up to 99.7% ee). Their air- and water-stabilities, com-
bined with their excellent enantioselectivities, make
these catalysts good choices for practical applications.
3
1
9.94, 69.92, 69.31, 33.72, 33.63, 20.99, 20.93; P NMR
(
(
200 MHz, CDCl ): d¼50.26 (d, J¼9.80 Hz), ꢁ24.63
3
d, J¼9.80 Hz); HRMS (EI): m/z¼883.1094 [calcd for
þ
20
C H F FeNP (M ): 883.1089]; [a] : ꢁ2538 (c 0.8, toluene).
Experimental Section
41 31 12
2
D
General Methods
(
R)-N-Methyl-N-diphenylphosphino-1-[(S)-2-(di-3,5-
All solvents were used as received except where indicated. All trifluoromethylphenylphosphino) ferrocenyl]ethyl-
reagentswere used as received from Aldrich Chemical Compa- amine (4)
nyexcept where indicated. (R)- N,N-Dimethyl-1-[(S)-2-(di-
A similar procedure as for the preparation of 2 was used, af-
phenylphosphino)ferrocenyl]ethylamine, (R)-N,N-dimethyl-
1
fording the product 4; yield: 780 mg (89%). H NMR
1-[(S)-2-(di-3,5-trifluoromethylphenylphosphino)ferrocenyl]-
(
500 MHz, CDCl ): d¼8.08 (d, J¼7.5 Hz, 2H), 7.99 (s, 1H),
3
ethylamine and (R)-N,N-dimethyl-1-[(S)-2-[(di-3,5-dimethyl-
7
.67 (s, 1H), 7.36 (m, 2H), 7.29 (m, 4H), 7.18 (m,1H), 7.00 (m,
phenylphosphino)ferrocenyl]ethylamine were prepared ac-
[17]
1H), 6.82 (m, 4H), 5.04 (q, J¼6.5 Hz, 1H), 4.630 (s, 1H), 4.54
cording to methods reported in the literature. Compounds
[11]
(s, 1H), 3.92 (s, 1H), 3.80 (s, 5H), 2.24 (d, J¼2.5 Hz, 3H), 1.53
6
, 7 and 8 were prepared according to a literature procedure.
1
3
(
d, J¼6.5 Hz, 3H); C NMR (125 MHz, CDCl ): d¼144.54,
3
1
1
1
1
9
2
44.42, 142.04, 141.91, 140.09, 139.92, 138.65, 137.90, 135.31,
35.14, 132.98, 132.81, 132.06, 131.90, 131.37, 131.29, 131.00,
29.64, 129.07, 128.44, 128.25, 128.00, 127.90, 127.85, 127.44,
27.40, 126.36, 125.33, 124.19, 123.57, 122.02, 121.74, 119.84,
8.42, 98.06, 72.26, 72.17, 71.39, 71.11, 70.53, 69.88, 58.71,
(
[
R)-N-Methyl-N-[di-3,5-dimethylphenyl]phosphino-1-
(S)-2-(diphenylphosphino)ferrocenyl]ethylamine (2)
Amine 6 (427 mg, 1.0 mmol) was dissolved in 10 mL of toluene
under nitrogen atmosphere. Triethylamine (0.29 mL,
.1 mmol) and DMAP (20 mg) were added. The solution was
3
1
9.31, 29.22, 17.42; P NMR (200 MHz, CDCl ): d¼62.435,
3
a
ꢁ
19.756; HRMS (EI): m/z¼883.1082 [calcd. for
20
þ 0
2
C H F FeNP (M ): 883.1089]; [a] : ꢁ208 (c 0.48, toluene).
4
1
31 12
2
D
cooled in ice/water and chlorodi[3,5-dimethylphenyl]phos-
phine (1.0 mmol in toluene) was added dropwise. The reaction
mixture was allowed to warm to ambient temperature and stir-
red overnight and the resulting reactions mixture was filtered
through a flash silica gel column and washed with toluene.
The combined filtrate was concentrated under vacuum to af-
(
R)-N-Methyl-N-[di-3,5-trifluorodimethylphenyl]-
phosphino-1-[(S)-2-[(di-3,5-dimethylphenylphosphino)-
ferrocenyl]ethylamine (5)
1
ford 2 as a yellow foam; yield: 613 mg (92%). H NMR
(
500 MHz, CDCl ): d¼7.68–7.65 (m, 2H), 7.39–7.37 (m,
A similar procedure as for the preparation of 2 was used, af-
3
1
3
3
1
6
1
1
1
1
9
6
2
H), 7.26–7.16 (m. 2H), 7.05–6.89 (m, 6H), 6.77–6.76 (d,
H), 4.80–4.68 (m, 1H), 4.60 (s, 1H), 4.39 (s, 1H), 4.11 (s,
H), 3.76 (s, 5H), 2.27 (d, 3H, J¼3.5 Hz), 2.19 (s, 6H), 2.04 (s,
fording the product 5; yield: 873 mg (93%). H NMR
(500 MHz, CDCl
): d¼1.75 (d, J¼7.0 Hz, 3H), 1.94 (s, 6H),
3
2.36 (d, J¼5.0 Hz, 3H), 2.39 (s, 6H), 3.82 (s, 5H), 4.22 (s, 1H),
4.49 (s, 1H), 4.62 (s, 1H), 4.92–4.96 (m, 1H), 6.45 (d, J¼
7.0 Hz, 2H), 6.53 (s, 1H), 7.07 (s, 1H), 7.32 (m, 2H), 7.48 (m,
1
3
H), 1.59 (d, 3H, J¼7 Hz); C NMR (125 MHz, CDCl ): d¼
3
42.23, 142.16, 140.23, 140.15, 139.74, 139.59, 139.49, 139.41,
37.80, 136.94, 136.89, 136.87, 136.81, 135.87, 135.68, 131.83,
31.71, 131.12, 130.95, 129.90, 129.73, 129.58, 129.48, 129.14,
29.00, 128.20, 128.96, 127.90, 127.47, 127.43, 126.70, 125.27,
8.84, 98.75, 98.60, 98.52, 75.91, 75.80, 71.74, 71.71, 69.91,
9.60, 69.58, 69.44, 57.86, 57.58, 33.04, 32.96, 21.43, 21.31,
1
3
2H), 7.69–7.76 (m, 4H); C NMR (125 MHz, CDCl
): d¼
3
142.77, 142.62, 142,09,141.88, 140.49, 140.43, 139.18, 139.12,
137.60, 137.53, 136.76, 136.72, 133.68, 133.49, 133.28, 133.09,
131.39, 131.26, 131.13, 129.60, 129.47, 128.97, 124.47, 124,43,
123.07, 122.26, 122.08, 96.96, 96.88, 96.73, 96.66, 77.13, 72.86,
72.82, 70.11, 70.05, 69.53, 58.98, 58.90, 58.68, 58.61, 21.56,
3
1
1.21, 19.64, 19.57; P NMR (200 MHz, CDCl ): d¼54.52,
3
3
1
ꢁ
22.53; HRMS (EI): m/z¼667.2225 [calcd. for C H FeNP
21.10, 20.40, 20.35; P NMR (200 MHz, CDCl
3
): d¼51.648,
4
1
43
2
þ
20
(
M ): 667.2220]; [a] : ꢁ3018 (c 0.88, toluene).
ꢁ23.747; HRMS (EI): m/z
¼
939.1707 [calcd. for
D
þ
20
C H F FeNP (M ): 939.1715]; [a] : ꢁ2198 (c 0.56, toluene).
4
5
39 12
2
D
(
R)-N-Methyl-N-[di-3,5-trifluoromethylphenyl]-
phosphino-1-[(S)-2-(diphenylphosphino)ferrocenyl]-
ethylamine (3)
Typical Procedure for Asymmetric Hydrogenation
The catalyst was made in situ bymixing [Rh(COD) ]BF4
2
A similar procedure as for the preparation of 2 was used, af-
fording the product 3; yield: 821 mg (93%). H NMR
(2.0 mg, 0.005 mmol) and ligand 3 (4.9 mg, 0.0055 mmol) in
THF (1 mL) for 10 min. A small portion of the catalyst solution
(0.1 mL) was transferred into a 50-mL glass-lined stainless
steel autoclave, which contained the substrate (0.1 mmol)
and a magnetic stirring bar. The reactor was charged with hy-
drogen gas, and the solution was stirred at the required temper-
ature for a predetermined period of time. After the reaction
1
(
5
1
500 MHz, CDCl ): d¼7.76–7.37 (m, 11H), 6.89–6.77 (m,
3
H), 4.90–4.88 (m, 1H), 4.68 (s, 1H), 4.51 (s, 1H), 4.19 (s,
H), 3.82 (m, 5H), 2.24 (d, 3H, J¼2.0 Hz), 1.80–1.78 (d, 3H,
13
J¼7.0 Hz); C NMR (125 MHz, CDCl ): d¼142.20, 142.02,
3
1
41.62, 141.45, 140.43, 140.38, 138.89, 138.83, 135.70, 135.52,
Adv. Synth. Catal. 2005, 347, 1904 – 1908
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1907

COMMUNICATIONS
Xingshu Li et al.
was completed, the hydrogen gas was released. The ee value of
the product was determined byGC anal ys is with a chiral GC
column as indicated in the footnotes of the Tables.
[8] a) T. Ireland, G. Grossheimann, C. Wieser-Jeunesse, P.
Knochel, Angew. Chem. Int. Ed. 1999, 38, 3112; b) M.
Lotz, G. Kramer, P. Knochel, Chem. Commun. 2002,
2
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[
9] D. Xiao, X. Zhang, Angew. Chem. Int. Ed. 2001, 40, 3425.
Acknowledgements
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al. 2003, 345, 160.
We thank the University Grants Committee of Hong Kong
[11] a) N. W. Boaz, S. D. Debenham, E. B. Mackenzie, S. E.
Large, Org. Lett. 2002, 4, 2421; b) N. W. Boaz, S. D. De-
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(
Areas of Excellence Scheme, AOE P/10–01) and the Hong
Kong Polytechnic University ASD Fund for the financial sup-
port of this study.
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