Highly enantioselective palladium-catalyzed hydrosilylation of norbornene with trichlorosilane using ferrocenyl ligands

Article abstract of DOI:10.1016/S0957-4166(98)00409-1

The Pd-catalyzed hydrosilylation of norbornene with trichlorosilane using different chiral ferrocenyl ligands containing a phosphine and a pyrazole as donors was studied. Both steric and electronic factors affect stereoselectivity in this system. The combination of a sterically bulky pyrazole substituent with a Π-acidic phosphine leads to an enantioselectivity of >99.5% ee. Important substrate electronic effects on stereoselectivity were observed using para-substituted styrenes.

Full text of DOI:10.1016/S0957-4166(98)00409-1

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 3903–3910
Highly enantioselective palladium-catalyzed hydrosilylation of
norbornene with trichlorosilane using ferrocenyl ligands
∗
Giorgio Pioda and Antonio Togni
Laboratory of Inorganic Chemistry, ETH-Zentrum, Swiss Federal Institute of Technology, CH-8092 Zürich, Switzerland
Received 18 September 1998; accepted 9 October 1998
Abstract
The Pd-catalyzed hydrosilylation of norbornene with trichlorosilane using different chiral ferrocenyl ligands
containing a phosphine and a pyrazole as donors was studied. Both steric and electronic factors affect stereoselec-
tivity in this system. The combination of a sterically bulky pyrazole substituent with a π-acidic phosphine leads to
an enantioselectivity of >99.5% ee. Important substrate electronic effects on stereoselectivity were observed using
para-substituted styrenes. © 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction
We previously reported studies aimed at developing a ferrocene-based ligand system characterized by
1
a modular synthetic approach. Thus, e.g., diphosphine derivatives affording high enantioselectivities
2
in hydrogenation, hydroboration, and allylic substitution reactions were found. More recently, we
3
described the synthesis of mixed phosphorus/nitrogen donors, prepared by following the same strategy.
4
These are excellent auxiliaries in, e.g., Pd-catalyzed allylic amination. We have now extended the use
5
of such ligands to the asymmetric hydrosilylation of olefins.
6
Hydrosilylation with trichlorosilane offers a powerful tool for the one-pot conversion of an olefin
into an alcohol, via the oxidation of the alkyltrichlorosilane formed as the product of the catalytic
reaction, e.g., by the method developed by Tamao.7 In fact, complete retention of configuration in the
,8
9
oxidation step allows the stereoselective synthesis of alcohols. Hayashi, Kumada, and Ito first reported
the Pd-catalyzed hydrosilylation of olefins utilizing chiral ferrocenyl phosphineamine ligands, bearing,
among others, perfluoroalkyl substituents enhancing catalyst solubility in the reactants mixture.1
0–13
In
more recent years Hayashi and co-workers were able to achieve both high chemoselectivities (linear to
branched product ratios up to 1:100) and enantioselectivities (up to 97% ee) for many different olefins,
including terminal ones, using Pd catalysts containing MOP and related ligands.6
,14–17
∗
Corresponding author. E-mail: togni@inorg.chem.ethz.ch
0957-4166/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00409-1

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G. Pioda, A. Togni / Tetrahedron: Asymmetry 9 (1998) 3903–3910
2. Results and discussion
2.1. Hydrosilylation of norbornene
In order to assess the steric and electronic requirements of the ligands in the Pd-catalyzed hydrosilyl-
ation, we studied the reaction of norbornene 1 with trichlorosilane 2, producing after oxidative workup,
exo-norborneol 3 (Scheme 1). The reactions have been carried out at different temperatures, depending
on the observed activity of the corresponding catalyst. The latter was used in a concentration of 0.1
mol% in all experiments and was either generated in situ from equimolar amounts of Pd(COD)Cl and
2
the ferrocenyl P,N-ligand or from the preformed dichloropalladium complex (see Experimental). Table 1
collects the results obtained using ligands bearing different substituents at the phosphorus atom and at
positions 3 and 5 of the pyrazole, respectively. It is apparent that for diphenylphosphino derivatives 4–9,
3
the activity of the catalyst shows a pronounced dependence on the steric bulk of the substituent R of
the nitrogen donor fragment (position 3). With the catalyst containing ligand 4 (methyl substituent), the
reaction has to be run at 70°C in order to obtain a reasonable conversion within 15 h. Correspondingly,
the enantioselectivity is very low (10% ee). On the other hand, the two bulky 2,4,6-trisubstituted phenyl
groups (2,4,6-trimethoxy- and 2,4,6-trimethylphenyl, ligands 7 and 8, respectively), give catalysts of
much higher activity with which comparable or higher yields are obtained operating at 0°C for an equal
reaction time. This also leads to much higher enantioselectivities of 82% and 91% ee, respectively. That
the stereoselectivity of the reaction is influenced by the electronic nature of the phosphorus donor is
demonstrated by ligand 9, containing the electron-withdrawing 3,5-bis(trifluoromethyl)phenyl groups. In
this case, an ee of >99.5% is obtained. This is the highest value reported for this particular reaction so
far.
Scheme 1. The Pd-catalyzed hydrosilylation of norbornene using various ferrocenyl P,N-ligands
2.2. Hydrosilylation of para-substituted styrenes
The electronic effects on stereoselectivity due to the substrate were analyzed by performing the
hydrosilylation of a series of para-substituted styrenes, utilizing complex 10 as a catalyst precursor at
room temperature (Scheme 2). As the results in Table 2 indicate, the electron-withdrawing or releasing

G. Pioda, A. Togni / Tetrahedron: Asymmetry 9 (1998) 3903–3910
3905
Table 1
Hydrosilylation of norbornene using different ferrocenyl ligands.
nature of the para-substituent plays a crucial role in determining both the level and the sense of chiral
induction. Highest R-selectivity of 64% ee was obtained with 4-dimethylaminostyrene, whereas the
corresponding chloro derivative afforded the S-product in 67.2% ee. Thus, this is a rare example of
enantioselectivity reversal due to a remote substituent in the substrate.
Scheme 2. The Pd-catalyzed hydrosilylation of para-substituted styrenes
A clear demonstration that one is dealing with electronic effects is given by the linear correlations of

3906
G. Pioda, A. Togni / Tetrahedron: Asymmetry 9 (1998) 3903–3910
Table 2
Results of the hydrosilylation of styrene and its derivatives with trichlorosilane, with 10 as catalyst
precursor.
1
8
log(er) (er=enantiomer ratio) presented as Hammett plots in Fig. 1 (linear free-energy relationship).
The best correlation is obtained utilizing the parameter σp+ and not with the more commonly used σp.
However, the results for trifluoromethylstyrene 16 significantly deviate in both cases from the linear
correlation. In other words, the obtained 58.7% ee is much lower than expected, based upon the linear
free-energy relationship (range of 84–87% ee (S)). Furthermore, 16 reacts significantly slower than the
other substrates of the series. The exact reason for the deviating behavior of this substrate is unknown.
+
The correlation with σ , as originally introduced by Brown for reactions involving, e.g., arenium
p
1
9,20
or cationic benzyl species,
in our case is indicative of the development of positive charge in the
transition state of the enantioselectivity-determining step of the catalytic reaction. From this observation,
a possible mechanistic implication may be derived. In the course of the olefin insertion process (the
likely enantioselectivity-determining step) a significant build-up of positive charge occurs at the benzylic
3
center as a consequence of the ensuing η -coordination mode preferred in the product, as illustrated
in Scheme 3. This means that the benzylic center becomes more electrophilic, by virtue of its π-
allylic nature. An analogous situation is encountered in the Ni-catalyzed hydrocyanation of vinylarenes,
studied in detail by Rajanbabu and co-workers.21 However, for this particular reaction, no data regarding
substrate electronic effects were reported. It is important to note that both olefin coordination and the
3
η -coordination mode in the insertion product are likely to require a ligand dissociation reaction. The

G. Pioda, A. Togni / Tetrahedron: Asymmetry 9 (1998) 3903–3910
3907
Figure 1. Hammett plots for the hydrosilylation of para-substituted styrenes. Data for para-(trifluoromethyl)styrene have been
omitted (see text)
opening of the chelate ring, with the pyrazole nitrogen dissociating, could fulfill such a condition. The
above observation that ligands containing bulky pyrazole substituents give rise to more active catalysts
is consistent with this interpretation, i.e., they probably tend to dissociate more readily. A comparable
mechanistic description, involving olefin hydride insertion and a low coordinated metal center, based on
a theoretical study, has recently been reported for the analogous Pt-catalyzed hydrosilylation by Sakaki
22
and co-workers.
3
Scheme 3. The styrene insertion step forming an η -coordinated benzylic intermediate
3. Conclusion
We have shown that ferrocene-based P,N-ligands are suited chiral auxiliaries for the Pd-catalyzed
hydrosilylation of selected olefins with trichlorosilane. A fine-tuning of these ligands, both in terms
of steric and electronic properties is readily achieved. Thus, a ligand with optimal properties, leading
to a very high enantioselectivity, could be found for the hydrosilylation of norbornene (>99.5% ee).
Furthermore, very pronounced substrate electronic effects on stereoselectivity were found for a series of

3908
G. Pioda, A. Togni / Tetrahedron: Asymmetry 9 (1998) 3903–3910
para-substituted styrenes. Such effects have fundamental implications for a reaction that is not very well
known from a mechanistic point of view. We are currently extending the use of our catalyst system to
further olefins, as well as studying the mechanism in more detail. We shall report on this in due course.
4. Experimental
All reactions were carried out in an atmosphere of argon using standard Schlenk techniques. Benzene
and diethyl ether were distilled from sodium benzophenone ketyl. Dichloromethane was distilled from
calcium hydride. Acetic acid was degassed prior to use. All other chemicals were used without prior
purification. TLC was performed on Merck plates coated with silica gel 60 F254 (0.2 mm). Chromato-
graphy was performed with Fluka silica gel 60 (230–400 mesh). Melting points are uncorrected. Optical
rotations were determined on a Perkin–Elmer 341 polarimeter (10 cm cell, CHCl at 23°C, c in g/100
3
1
13
31
mL). Chemical shifts are reported in ppm relative to TMS ( H, C), to 85% H PO ( P) and to CFCl
3
3
4
19
(
F): δ 0.00 as the internal standard. NMR data were recorded on Bruker DPX-250, Bruker DPX-300,
or Bruker DRX-400 spectrometers. HPLC analyses were performed on a Hewlett–Packard series 1050
instrument with UV detector (254 nm); the chiral columns used are Daicel Chiracel OJ (4.6×250 mm, 5
µm mesh), Chiracel OD-H (4.6×250 mm, 5 µm mesh), Chiracel OB-H (4.6×250 mm, 5 µm mesh). GC
analyses were performed on a Fisons GC 8000 Series instrument; the chiral columns used are Supelco
α-DEX (0.25 mm×30 m, 0.25 µm film), β-DEX (0.25 mm×30 m, 0.25 µm film), γ-DEX (0.25 mm×30
23
m, 0.25 µm film). Pd(COD)Cl was prepared following a reported procedure.
2
4.1. A general procedure for the hydrosilylation of norbornene 1
Pd(COD)Cl (6.05 mg, 0.021 mmol), ligand (0.025 mmol), norbornene (2 g, 21 mmol), and benzene
2
(
8 mL) were stirred for 30 min until the solid Pd(COD)Cl dissolved and the solution turned from yellow
2
to red. By the addition of HSiCl (2.8 mL, 28 mmol) the reaction was started. After complete conversion,
3
the product was carefully poured into a suspension of KF (20 g, 0.34 mol) in methanol (150 mL) and
stirred for 30 minutes. The solvent was removed in vacuo, dimethylformamide (150 mL) and H O (30%
2
2
water solution, 20 mL) were added and the suspension heated for 1 hour at 70°C. Product 3 was then
isolated by aqueous workup, extraction and sublimation. The yields given in Table 1 refer to sublimed
product.
4.2. A general procedure for the hydrosilylation of vinylarenes 17–22
Complex 10 (13 mg, 0.017 mmol) and the vinylarene (8.5 mmol) were mixed in 4 mL of benzene.
The addition of trichlorosilane (1.1 mL, 11 mmol) started the reaction. The mixture was stirred until
disappereance of the NMR signals of the vinylarene. The crude product was carefully poured into a
suspension of KF (10 g, 0.17 mol) in 80 mL of methanol, and stirred for 30 minutes. The solvent was
removed in vacuo. The resulting solid was suspended in 100 mL of DMF, H O (30% water solution,
2
2
10 mL) was added and the mixture heated for 1 hour at 60–70°C. The products 17–22 were isolated by
aqueous workup, extraction and distillation. In the case of product 20, column chromatography (silica,
hexane:ethyl acetate=2:1+1% of NEt ) was preferred to distillation because of its thermal sensitivity. The
3
yields given in Table 2 refer to purified products.

G. Pioda, A. Togni / Tetrahedron: Asymmetry 9 (1998) 3903–3910
3909
4.3. 1-{(R)-1-[(S)-2-(Diphenylphosphino)ferrocenyl]ethyl}-3-(2,4,6-trimethoxyphenyl)-1H-pyrazole 7
0 0 0
R)-(S)-PPFA (1 g, 2.26 mmol) and 3-(2 ,4 ,6 -trimethoxyphenyl)pyrazole (690 mg, 2.94 mmol)
(
were dissolved in 8 mL of degassed AcOH and stirred for 3 hours at 80°C. The solvent was then
removed in vacuo. The product was isolated as a yellow foam by chromatography on silica (hexane:ethyl
acetate=2:1+1% of NEt ). Yield: 427 mg, 30%. R (silica, hexane:ethyl acetate=2:1+1% of NEt ): 0.18.
3
f
3
1
H NMR (250 MHz, CDCl , 20°C): δ 7.70–7.58 (m, 2H, Ph–H), 7.45–7.35 (m, 3H, Ph–H), 7.10–6.90
m, 4H, Ph–H, Py–H), 6.85–6.75 (m, 2H, Ph–H), 6.17 (s, 2H, trimethoxyphenyl–H), 5.78 (m, 1H,
Py–H), 5.74 (dq, 1H, CHMe, J(HH)=7, J(HP)=3), 4.78 (m, 1H, C H ), 4.38 (m, 1H, C H ), 4.01
3
(
3
4
5
3
5
3
(
s, 5H, C H ), 3.94 (m, 1H, C H ), 3.82 (s, 3H, OCH ), 3.67 (s, 6H, OCH ), 2.15 (d, 3H, CHMe,
5
5
31
5
3
3
3
3
1
13
1
J(HH)=7). P{ H} NMR (101.26 MHz, CDCl , 20°C): δ −23.8. C{ H} NMR (62.89 MHz, CDCl ,
3
3
2
0°C): δ 161–126 (Ph–C, Py–C, trimethoxyphenyl–C), 106.7 (Py–C), 94.8 (d, C H , J(CP)=26), 91.4
5
3
(trimethoxyphenyl–C), 75.5 (d, C H , J(CP)=11), 71.6 (d, C H , J(CP)=5), 70.1 (d, C H , J(CP)=4),
5 3 5 3 5 3
6
9.8 (CP–C), 69.7 (d, C H , J(CP)=5), 56.1 (OCH ), 55.6 (CHMe), 55.3 (OCH ), 21.8 (CHMe). MS
5 3 3 3
+
+
+
(
8
EI) m/z 630 (M , 40%), 564 (M −C H , 100%), 396 (M −3-(2,4,6-trimethoxyphenyl)-1H-pyrazole,
5
5
20
7.3%). [α]D −277 (c 1.18). Anal. calcd for C H FeN O P: C, 68.58; H, 5.59; N, 4.44. Found: C,
36
35
2
3
68.36; H, 5.68; N, 4.30.
4.4. 1-{(R)-1-[(S)-2-(Diphenylphosphino)ferrocenyl]ethyl}-3-(2,4,6-trimethylphenyl)-1H-pyrazole 8
From (R)-(S)-PPFA (2 g, 4.53 mmol) and 3-(2,4,6-trimethylphenyl)-1H-pyrazole (1.01 g, 5.44 mmol),
analogously to 7 (80°C, 2 hours). Chromatography (silica, hexane:ethyl acetate=10:1+1% of NEt ): yield
3
1
1
2
4
4
.572 g (60%). R (silica, hexane:ethyl acetate=4:1+1% of NEt ): 0.51. H NMR (250 MHz, CDCl ,
f
3
3
0°C): δ 7.66–7.58 (m, 2H, Ph–H), 7.45–7.35 (m, 3H, Ph–H), 7.05–6.95 (m, 4H, Ph–H), 6.92–6.80 (m,
3
4
3
H, Ph–H, Mes–H, Py–H), 6.87 (dq, 1H, CHMe, J(HH)=7, J(HP)=2), 5.65 (d, 1H, Py–H, J(HH)=2),
.82 (m, 1H, C H ), 4.45 (m, 1H, C H ), 4.04 (s, 5H, C H ), 4.01 (m, 1H, C H ), 2.29 (s, 3H,
5
3
5
3
5
1
5
5
3
31
Mes–CH ), 2.01 (d, 3H, CHMe), 1.96 (s, 6H, Mes–CH ). P{ H} NMR (101.26 MHz, CDCl , 20°C): δ
3
3
3
13
1
−
25.6. C{ H} NMR (62.89 MHz, CDCl , 20°C): δ 149.6–127.4 (Ph–C, Mes–C, Py–C), 105.1 (Py–C),
3
9
4.1 (d, C H , J(CP)=25), 75.6 (d, C H , J(CP)=10), 70.1 (C H ), 69.9 (C H , C H ), 55.5 (d, CHMe,
5 3 5 3 5 3 5 5 5 3
+
+
J(CP)=11), 21.8 (Mes–CH ), 21.1 (CHMe), 20.6 (Mes–CH ). MS (EI) m/z 582 (M , 65%), 396 (M −3-
3
3
20
(
7
2,4,6-trimethylphenyl)-1H-pyrazole, 100%). [α]D −299.9 (c 1.13). Anal. calcd for C H FeN P: C,
36
35
2
4.23; H, 6.06; N, 4.81. Found: C, 74.07; H, 6.01; N, 4.75.
4.5. 1-{(R)-1-[(S)-2-Bis(3,5-bis(trifluoromethyl)phenyl)phosphino)ferrocenyl]ethyl}-3-(2,4,6-
trimethylphenyl)-1H-pyrazole 9
From dimethyl-{(R)-1-[(S)-2-(bis(3,5-bis(trifluoromethyl)phenyl)phosphino)-ferrocenyl]ethyl}-
amine (835 mg, 1.17 mmol) and 3-(2,4,6-trimethylphenyl)-1H-pyrazole (435 mg, 2.34 mmol),
analogous to 7 (90°C, 15 hours). Chromatography (silica, hexane:diethyl ether=10:1+2% of NEt ): yield
3
1
8
1
1
93 mg (89%). R (silica, hexane:diethyl ether=3:1): 0.34. H NMR (400 MHz, CDCl , 20°C): δ 7.99 (s,
H, Ph–H), 7.94 (d, 2H, Ph–H, J(HP)=7), 7.65 (s, 1H, Ph–H), 7.28 (d, 2H, Ph–H, J(HP)=7), 7.06 (d,
H, Py–H, J(HH)=2), 6.80 (s, 2H, Mes–H), 5.91 (m, 1H, CHMe), 5.76 (d, 1H, Py–H, J(HH)=2), 4.94
f
3
3
3
3
3
(
1
m, 1H, C H ), 4.67 (m, 1H, C H ), 4.05 (s, 5H, C H ), 3.86 (m, 1H, C H ), 2.25 (s, 3H, Mes–CH ),
5 3 5 3 5 5 5 3 3
3
31
1
.90 (d, 3H, CHMe, J(HH)=7), 1.69 (s, 3H, Mes–CH ). P{ H} NMR (161.98 MHz, CDCl , 20°C): δ
3
3
13
1
−
21.7. C{ H} NMR (62.89 MHz, CDCl , 20°C): δ 151.3–119.2 (Ph–C, Py–C, Mes–C), 106.8 (Py–C),
4.3 (d, C H , J(CP)=71), 72.7 (d, C H , J(CP)=10), 72.0 (C H ), 71.8 (d, C H , J(CP)=5), 71.2
3
9
5
3
5
3
5
3
5
3

3910
G. Pioda, A. Togni / Tetrahedron: Asymmetry 9 (1998) 3903–3910
(
d, C H , J(CP)=5), 70.6 (C H ), 56.4 (d, CHMe, 3J(CP)=11), 22.3 (Mes–CH ), 21.4 (CHMe), 20.4
5 3 5 5 3
19
1
(
8
Mes–CH ). F{ H} NMR (376.46 MHz, CDCl , 20°C): δ −63.3 (CF ), −63.4 (CF ). MS (EI) m/z
3
3
20
3
3
+
+
54 (M , 100%), 789 (M −C H , 44.9%). [α]
−190.4 (c 0.57). Anal. calcd for C H F FeN P:
5
5
D
40 31 12
2
C, 56.22; H, 3.66; N, 3.28. Found: C, 56.30; H, 3.74; N, 3.28.
4
.6. (8)PdCl 10
2
Pd(COD)Cl (82 mg, 0.286 mmol) and 8 (160 mg, 0.289 mmol) were stirred for 15 min in 4 mL
2
of CH Cl under argon. Et O (5 mL) was added, and the complex precipitated. The red solid was
2
2
2
1
filtered off, washed twice with 5 mL of Et O and dried in vacuo. Yield 162 mg (75%). H NMR (250
MHz, CD Cl , 20°C): δ 8.11–7.93 (m, 4H, Ph–H), 7.81 (d, 1H, Py–H, J(HH)=3), 7.75–7.65 (m, 1H,
Ph–H), 7.62–7.48 (m, 5H, Ph–H), 7.05–6.82 (m, 2H, Mes–H, CHMe), 6.78 (s, 1H, Mes–H), 6.21 (d,
2
3
2
2
3
1
H, Py–H, J(HH)=3), 4.42 (m, 1H, C H ), 4.33 (m, 1H, C H ), 4.29 (m, 1H, C H ), 3.90 (s, 5H,
5 3 5 3 5 3
C H ), 2.37 (s, 3H, Mes–CH ), 2.33 (d, 3H, CHMe, 3J(HH)=7), 2.25 (s, 3H, Mes–CH ), 1.09 (s, 3H,
5
5
3
3
31
1
13
1
Mes–CH3). P{ H} NMR (101.26 MHz, CD Cl , 20°C): δ 11.2. C{ H} NMR (62.89 MHz, CD Cl ,
2
2
2
2
2
0°C): δ 156.4–125.5 (Ph–C, Mes–C, Py–C), 110.8 (Py–C), 91.7 (d, C H , J(CP)=21), 74.4 (d, C H ,
5
3
5
3
J(CP)=3), 73.4 (d, C H , J(CP)=55), 70.9 (C H ), 69.8 (d, C H , J(CP)=7), 66.2 (d, C H , J(CP)=7),
5
3
5
5
5
3
5
3
+
5
(
7.9 (CHMe), 21.6 (Mes–CH ), 21.0 (Mes–CH ), 20.7 (Mes–CH ), 17.1 (CHMe). MS (FAB ) m/z 725
3 3 3
+
+
M −Cl, 100%), 723 (M −Cl, 98%). Mp >186°C (dec.). Anal. calcd for C H Cl FeN PPd: C, 56.91;
3
6
35
2
2
H, 4.64; N, 3.69. Found: C, 56.63, H, 4.69, N, 3.52.
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