Preparation of new ferrocenylmonophosphine ligands containing two planar chiral ferrocenyl moieties and their use for palladium-catalyzed asymmetric hydrosilylation of 1,3-dienes

Article abstract of DOI:10.1002/1522-2675(200211)85:11<3848::AID-HLCA3848>3.0.CO;2-V

Bis{(R_p)-2-[(1S)-1-methoxyethyl]ferrocenyl arylphosphines (S,R_p)-9 (aryl=4-MeOC₆H₄ (9a), Ph (9b), 4- CF₃C₆Ha (9c), 3,5-(CF₃)₂C₆H₃ (9d)), which contain two planar chiral ferrocenyl moieties, were prepared via (R_p)-1-bromo-2-[(1S)-1-methoxyethyl]ferrocene (S,_Rp)-8). Asymmetric hydrosilylation of linear 1,3-dienes such as deca-1,3-diene (10a) with trichlorosilane in the presence of a palladium catalyst coordinated with 9d gave allylic silanes of up to 93% ee.

Full text of DOI:10.1002/1522-2675(200211)85:11<3848::AID-HLCA3848>3.0.CO;2-V

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Helvetica Chimica Acta Vol. 85(2002)
Preparation of New Ferrocenylmonophosphine Ligands Containing Two
Planar Chiral Ferrocenyl Moieties and Their Use for Palladium-Catalyzed
Asymmetric Hydrosilylation of 1,3-Dienes
by Jin Wook Han, Norihito Tokunaga, and Tamio Hayashi*
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan
Dedicated to Professor Dieter Seebach on the occasion of his sixty-fifth birthday
Bis{(Rp)-2-[(1S)-1-methoxyethyl]ferrocenyl}arylphosphines (S,Rp)-9 (aryl ˆ 4-MeOC6H4 (9a), Ph (9b), 4-
CF3C6H4 (9c), 3,5-(CF3)2C6H3 (9d)), which contain two planar chiral ferrocenyl moieties, were prepared via
(Rp)-1-bromo-2-[(1S)-1-methoxyethyl]ferrocene ((S,Rp)-8). Asymmetric hydrosilylation of linear 1,3-dienes
such as deca-1,3-diene (10a) with trichlorosilane in the presence of a palladium catalyst coordinated with 9d
gave allylic silanes of up to 93% ee.
1. Introduction. Palladium-catalyzed asymmetric hydrosilylation of CÀC multiple
bonds provides one of the most efficient routes to enantiomerically enriched
organosilicon compounds1). One important point in the palladium-catalyzed hydro-
silylation is that no chelating bisphospine ligands can be used because of the low
catalytic activity of their palladium complexes. We have reported that high enantio-
selectivity as well as high catalytic activity in the asymmetric hydrosilylation is achieved
by use of chiral monodentate phosphine ligands (MOP) whose chirality is due to the
binaphthyl axial chirality [2]. The MOP ligands have an advantage over others in that
their fine tuning is readily made by the introduction of a desired group at the 2' position.
In the asymmetric hydrosilylation of simple terminal alkenes [3] and cyclic alkenes [4],
high enantioselectivity (> 90% ee) has been observed by use of 2-(diphenylphosphi-
no)-2'-methoxy-1,1'-binaphthyl (MeO-MOP; 1). For styrene derivatives, 2-(diphenyl-
phosphino)-1,1'-binaphthyl (H-MOP; 2a) [5] and its analog 2b containing the bis[3,5-
bis(trifluoromethyl)phenyl]phosphino group [6] are more effective than MeO-MOP at
giving the hydrosilylation products of over 95% ee. For the asymmetric hydrosilylation
of 1,3-dienes, which is a very useful asymmetric transformation because it produces
enantiomerically enriched allylic silanes, we found that MOP ligand 2c substituted with
a 3,5-dimethyl-4-methoxyphenyl group at the 2' position [7] and its long-chain-
alkylated version 2d [8] are better than others. The ligand 2d showed 91% ee for the
hydrosilylation of cyclopentadiene. However, unfortunately, these MOP ligands are not
so effective for linear 1,3-dienes such as deca-1,3-diene. The highest enantioselectivity
so far observed for deca-1,3-diene is 77% ee [8]. On the other hand, we have also
developed the chiral ferrocenylphosphine ligands, which have ferrocene planar
chirality2)3). They possess high modularity, allowing us to prepare both mono-
1
)
)
)
For reviews on catalytic asymmetric hydrosilylation, see [1].
For reviews on chiral ferrocenylphosphines, see [9].
For recent examples of new types of chiral ferrocenylphosphines, see [10].
2
3

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Helvetica Chimica Acta Vol. 85(2002)
3849
phosphines and bisphosphines and to introduce various kinds of functional groups at
the side chain according to the demand of the reaction type. Nevertheless, there have
been few efforts on the modification of ferrocenylphosphines for the asymmetric
hydrosilylation of 1,3-dienes4)5) [12]. Ferrocenylmonophosphines, PPFA 3a, PPFOMe
3b, PPFOAc 3c, and perfluoroalkylated PPFA 3d, have been used for the catalytic
asymmetric hydrosilylation of both cyclic and linear 1,3-dienes, but the enantioselec-
tivity is not satisfactory. The highest was 77% ee, which was observed with 3c for
cyclohexa-1,3-diene [11e]. Here we wish to report the preparation of new ferroce-
nylmonophosphines containing two chiral ferrocenyl moieties6) and their use as highly
effective chiral ligands for the palladium-catalyzed asymmetric hydrosilylation of linear
1,3-dienes.
Fig. 1. MOP and Ferrocenylphosphine ligands for palladium-catalyzed asymmetric hydrosilylation of olefins
2. Results and Discussion. In 1980, we reported [14] the preparation of chiral
bis(ferrocenyl)monophosphine (S,Rp)-47), which contains two planar chiral ferrocene
moieties, by way of ferrocenylstannane derivative (S,Rp)-5 starting from (S)-N,N-
dimethyl-1-ferrocenylethylamine ((S)-6) [15] (Scheme 1). Attempts to obtain (S,Rp)-4
in a one-pot reaction by the lithiation of (S)-6 with BuLi and then addition of
dichlorophenylphosphine failed, probably due to the low conversion of (S)-6 to the
lithiated ferrocene, which causes the reaction of remaining BuLi with chlorophosphines
giving butylphosphines. By use of the route involving the ferrocenylstannane (S,Rp)-5,
we succeeded in the preparation of (S,Rp)-4, but the yields for the two steps in Scheme 1
are still not high enough.
4
)
)
For palladium-catalyzed hydrosilylation of 1,3-dienes with chiral ferrocenylphosphine ligands, see [11].
For palladium-catalyzed hydrosilylation of trinorbornene and styrene with chiral ferrocenylphosphine
ligands, see [12].
A C3-symmetric tri(ferrocenyl)monophosphine has been reported [13].
Subscript p means planar.
5
6
7
)
)

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Helvetica Chimica Acta Vol. 85(2002)
Scheme 1. Preparation of Chiral Bis(ferrocenyl)monophosphine 4 as Reported in [14]
We decided to take the synthetic route shown in Scheme 2, which involves
bromoferrocene (S,Rp)-7 as a key intermediate, because bromoferrocenes are known to
be converted to lithioferrocenes with high efficiency [16]. Considering that phosphine
ligands substituted with a dialkylamino group are not suitable for the catalytic
hydrosilylation with trichlorosilane due to their insolubility in the reaction media, the
dimethylamino group in (S,Rp)-4 was replaced by an alkoxy group in our target
ferrocenylphosphines (S,Rp)-9. Thus, first, (S)-6 was brominated in 85% yield by
lithiation with sec-BuLi followed by bromination with 1,2-dibromo-1,1,2,2-tetrafluoro-
ethane according to modified procedures based on the reported method [16] to give the
bromoferrocene (S,Rp)-7, which is diastereoisomerically pure. The dimethylamino
group of (S,Rp)-7 was replaced by a MeO group by methanolysis of its ammonium salt
in refluxing MeOH, which proceeded with retention of configuration at the stereogenic
ferrocenylmethyl position to give 94% yield of (S,Rp)-8. Lithiation of (S,Rp)-8 with
1 equiv. on BuLi at À788 for 1 h followed by addition of 0.5equiv. of aryldichloro-
phosphine gave high yields of the bis(ferrocenyl)monophosphines (bisPPFOMe)
(S,Rp)-9. During our previous studies on the palladium-catalyzed asymmetric hydro-
silylation by means of MOP ligands [6], we observed that substituents at the phenyl
rings at the P-atom have strong electronic effects on the catalytic activity and
enantioselectivity. To examine the electronic effects in the bis(ferrocenyl)monophos-
phines, we introduced both electron-donating and -withdrawing groups at the phenyl
ring. The monophosphine 9a is substituted with a 4-methoxyphenyl group, while 9c and
9d contain trifluoromethyl groups.
Scheme 2. Preparation of Chiral Bis(ferrocenyl)monophosphines (S,Rp)-9

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The bis(ferrocenyl)monophosphine ligands (S,Rp)-9 were examined for their
enantioselectivity in the palladium-catalyzed asymmetric hydrosilylation of deca-1,3-
diene (10a) with trichlorosilane (Scheme 3). The reaction was carried out without
solvents at 208 in the presence of 1 mol-% of the palladium catalyst generated in situ by
mixing [PdCl(p-C3H5)]2 with one of the bis(ferrocenyl)monophosphines (S,Rp)-9. The
hydrosilylation proceeded in a 1,4-fashion to give (Z)-4-(trichlorosilyl)-dec-2-ene (11a)
as a major product with over 80% selectivity, together with a minor amount of
regioisomers (E)-2-(trichlorosilyl)-dec-3-ene (12a) and (Z)-1-(trichlorosilyl)-dec-2-
ene (13a), the ratio being slightly dependent on the ligand employed (Entries 1 4 in
the Table 1). The enantiomer excess of 11a was measured by HPLC analysis (Chiralcel
Scheme 3. Palladium-Catalyzed Asymmetric Hydrosilylation of Linear 1,3-Dienes 10
Table. Palladium-Catalyzed Asymmetric Hydrosilylation of Dienes 10 with Trichlorosilanea)
Entry
Diene Ligand Temp. [8] Time [h] Yieldb) [%] of 11 ‡ 12 ‡ 13 Ratioc) 11/12/13
% eed) of 11 (config.)e)
1
2
3
4
5
6
7
10a
a
a
a
a
9a
b
c
d
d
d
d
20
20
20
20
96
29
8
2591
81
26
81
43
78
89
82 : 3 : 1568 (
90 : 9 : 1
90 : 8 : 2
S)
76 (S)
78 (S)
S)
S)
88 (S)
S)
87: 10 : 3
89 : 9 : 2
94 92 : 6 : 2
: 293 : 5 90 (
87 (
93 (
À 5168
b
b
20
À 5168
a) The hydrosilylation was carried out without solvent. The catalyst was generated in situ by mixing [PdCl(p-
C3H5)]2 and a chiral ligand (S,Rp)-9. The initial ratio diene/HSiCl3/Pd/(S,Rp)-9 was 1.0 : 1.2 : 0.01 : 0.02.
b) Isolated yield of a mixture of allyltrichlorosilanes 11, 12, and 13 by bulb-to-bulb distillation. c) Determined by
1H-NMR analysis of the allyltrichlorosilanes. d) Determined by HPLC analysis of homoallyl alcohols 14 with a
chiral-stationary-phase column (Chiralcel OD-H). e) Determined by the optical rotation of homoallyl alcohols
14 (see Exper. Part). For Entry 5 (14a), [a]D20 ˆ ‡16.5( c ˆ 1.0, CHCl3). For Entry 7 (14b), [a]2D0 ˆ ‡15.7 (c ˆ 1.0,
CHCl3).
OD-H) of the homoallyl alcohol 14a [7] obtained by the reaction with benzaldehyde in
DMF, which proceeds via a six-membered cyclic transition state [17], and the absolute
configuration was determined by correlation with the known hydroxy ester 15 [18]. The
highest enantioselectivity (87% ee) was observed with ligand 9d where Ar at the P-
atom is 3,5-(CF3)2C6H3 (Entry 4) an the enantioselectivity decreased in the order 9d
(Arˆ 3,5-(CF3)2C6H3) > 9c (Arˆ 4-CF3C6H4) > 9b (ArˆPh) > 9a (Arˆ 4-MeOC6H4).