Modification of chiral monodentate phosphine (MOP) ligands for palladium-catalyzed asymmetric hydrosilylation of styrenes

Article abstract of DOI:10.1246/cl.2000.1272

In the palladium-catalyzed asymmetric hydrosilylation of styrene with trichlorosilane, several chiral monophosphine ligands, (R)-2-diarylphosphino-1,1′-binaphthyls (2), were examined for their enantioselectivity. The highest enantioselectivity was observed in the reaction with (R)-2-bis[3,5-bis-(trifluoromethyl)phenyl]phosphino-1,1′-binaphthyl (2g) , which gave (S)-1-phenylethanol of 98% ee after oxidation of the hydrosilylation product.

Full text of DOI:10.1246/cl.2000.1272

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1272
Chemistry Letters 2000
Modification of Chiral Monodentate Phosphine (MOP) Ligands
for Palladium-Catalyzed Asymmetric Hydrosilylation of Styrenes
Tamio Hayashi,* Seiji Hirate, Kenji Kitayama, Hayato Tsuji, Akira Torii, and Yasuhiro Uozumi#
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502
(Received August 14, 2000; CL-000770)
In the palladium-catalyzed asymmetric hydrosilylation of
styrene with trichlorosilane, several chiral monophosphine lig-
ands, (R)-2-diarylphosphino-1,1'-binaphthyls (2), were exam-
ined for their enantioselectivity. The highest enantioselectivity
was observed in the reaction with (R)-2-bis[3,5-bis-
(trifluoromethyl)phenyl]phosphino-1,1'-binaphthyl (2g) , which
gave (S)-1-phenylethanol of 98% ee after oxidation of the
hydrosilylation product.
the palladium catalyst generated in situ by mixing [PdCl(π-
C3H5)]2 with two equiv (to palladium) of the H-MOP ligand.
The hydrosilylation product, 1-trichlorosilyl-1-phenylethane
(4a) was oxidized into 1-phenylethanol (5a) with hydrogen per-
oxide in the presence of potassium fluoride, which is known to
proceed with retention of configuration at the stereogenic car-
bon center.7 The enantiomeric purity of 5a was determined by
HPLC analysis with a chiral stationary phase column (Table 1).
It was found that the enantioselectivity is not strongly depend-
ent on the electron-withdrawing or electron-donating characters
of the para-substituents, H-MOP(p-OMe) (2b) and H-MOP(p-
CF3) (2c) giving (S)-5a of 92% ee (entry 2) and 93% ee (entry
3), respectively. These values are almost the same as that
observed with unsubstituted H-MOP (2a) (entry 1). On the
other hand, higher enantioselectivity (95% ee) was observed
with H-MOP(m-CF3) (2d) (entry 4). Based on the higher selec-
tivity observed with the meta-substitution, three H-MOP lig-
ands, H-MOP(m,m-2Me) (2e), H-MOP(m,m-2Cl) (2f), and H-
MOP(m,m-2CF3) (2g), were also prepared, all of which are di-
substituted at meta,meta-positions. Of the three ligands, bis-tri-
fluoromethylated ligand 2g was found to be most effective lig-
and giving (S)-5a of 97% ee (entry 7). In addition to the high
enantioselectivity, the reaction with 2g is much faster than that
with other ligands. The reaction in the presence of the palladi-
um/2g catalyst was so exothermic that the reaction temperature
is difficult to be kept 0 °C under the standard reaction condi-
tions where no solvents were used. The reaction diluted with
benzene (1.0 M solution) in the presence of 0.1 mol% of the
palladium/2g catalyst was completed in 1 h at 0 °C (entry 7).
Catalytic asymmetric hydrosilylation of alkenes has been
recognized to be one of the most useful methods for the asym-
metric transformation of prochiral alkenes into optically active
alcohols.1 We have previously reported that the asymmetric
hydrosilylation of simple terminal alkenes such as 1-octene2
and cyclic alkenes such as norbornene3 is efficiently catalyzed
by a palladium complex coordinated with 2-diphenylphosphi-
no-2'-methoxy-1,1'-binaphthyl (MeO-MOP (1))4 and that 2-
diphenylphosphino-1,1'-binaphthyl (H-MOP (2a)) is more
enantioselective than MeO-MOP for the hydrosilylation of
styrene derivatives.5,6 However, unfortunately, the enantiose-
lectivity with H-MOP was still not high enough, especially for
the styrene derivatives containing electron-donating groups on
the phenyl ring. Thus, for example, hydrosilylation of 4-methyl-
styrene and 4-methoxystyrene with trichlorosilane in the presence
of palladium/H-MOP catalyst gave 1-(4-methylphenyl)ethanol of
89% ee and 1-(4-methoxyphenyl)ethanol of 61% ee, respectively,
while the reaction of styrene gave 1-phenylethanol of 93% ee.6
We have further modified H-MOP ligand on the diphenylphosphi-
no group and found that introduction of bis[3,5-bis(trifluo-
romethyl)phenyl]phosphino group greatly enhances both the
enantioselectivity and catalytic activity in the palladium-catalyzed
asymmetric hydrosilylation of styrene derivatives. Here we report
the preparation of (R)-2-bis[3,5-bis(trifluoromethyl)-
phenyl]phosphino-1,1'-binaphthyl (2g) and its use for the hydrosi-
lylation of styrene derivatives.
First, H-MOP ligand (2a) was modified on the diphenyl-
phosphino group by introduction of methoxy group or trifluo-
romethyl group on the phenyl ring, and these substituted H-
MOP ligands were examined for their enantioselectivity in the
palladium-catalyzed asymmetric hydrosilylation of styrene (3a)
with trichlorosilane (Scheme 1). The hydrosilylation was car-
ried out at 0 °C without solvent in the presence of 0.1 mol% of
Copyright © 2000 The Chemical Society of Japan

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Chemistry Letters 2000
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of the electron-withdrawing or electron-donating characters of
the substituents on the phenyl, ranging between 95% and 98%
ee. Although the difference in enantioselectivity between
unsubstituted H-MOP (2a) and H-MOP(m,m-2CF3) (2g) is not
very large for the p-chlorostyrene (3b), the enantioselectivity
was greatly improved for the styrenes substituted with electron-
donating groups, methyl (3c) and methoxy (3d).
This work was supported by "Research for the Future" pro-
gram, the Japan Society for the Promotion of Science.
References and Notes
#
Present address: Institute for Molecular Science, Myodaiji,
Okazaki 444-8585, Japan.
1
For reviews: a) T. Hayashi, in "Comprehensive Asymmetric
Catalysis," ed. by E. N. Jscobsen, A. Pfaltz, and H. Yamamoto,
Springer, Berlin (1999), Vol. 1, Chap. 7. b) H. Nishiyama, K.
Itoh, in "Catalytic Asymmetric Synthesis," 2nd ed., ed. by I.
Ojima, Wiley-VCH, New York (2000), p. 111.
a) Y. Uozumi and T. Hayashi, J. Am. Chem. Soc., 113, 9887
(1991). b) Y. Uozumi, K. Kitayama, T. Hayashi, K. Yanagi,
and E. Fukuyo, Bull. Chem. Soc. Jpn., 68, 713 (1995).
a) Y. Uozumi, S.-Y. Lee, and T. Hayashi, Tetrahedron Lett., 33,
7185 (1992). b) Y. Uozumi and T. Hayashi, Tetrahedron Lett.,
34, 2335 (1993).
a) Y. Uozumi, A. Tanahashi, S.-Y. Lee, and T. Hayashi, J. Org.
Chem., 58, 1945 (1993). b) Y. Uozumi, N. Suzuki, A. Ogiwara,
and T. Hayashi, Tetrahedron, 50, 4293 (1994).
Y. Uozumi, K. Kitayama, and T. Hayashi, Tetrahedron:
Asymmetry, 4, 2419 (1993).
Higher enantioselectivity (98% ee) was observed in the reaction
carried out at –20 °C (entry 8). The 98% ee observed here is by
far the highest of the enantioselectivities reported for asymmet-
ric hydrosilylation of styrene.8
The H-MOP ligands 2 that contain substituted diaryl-
phosphino groups at 2-position on the 1,1'-binaphthyl skeleton
were prepared9 starting from (R)-2-trifluoromethanesulfonyl-
oxy-1,1'-binaphthyl (6)4 (Scheme 2). Diarylphosphinyl groups
were introduced at the 2-position by the palladium-catalyzed
cross-coupling type reaction2,4,10 and the phosphine oxide was
reduced with trichlorosilane and triethylamine according to the
reported procedures.4
2
3
4
5
6
7
K. Kitayama, Y. Uozumi, and T. Hayashi, J. Chem. Soc., Chem.
Commun., 1995, 1533.
a) K. Tamao, N. Ishida, T. Tanaka, and M. Kumada,
Organometallics, 2, 1694 (1983). b) K. Tamao and N. Ishida, J.
Organomet. Chem., 269, C37 (1984). c) K. Tamao, E. Nakajo,
and Y. Ito, J. Org. Chem., 52, 4412 (1987). d) K. Tamao, in
"Organosilicon and Bioorganosilicon Chemistry," ed. by H.
Sakurai, Ellis Horwood, Chichester (1985), p. 231.
8
For the palladium-catalyzed asymmetric hydrosilylation of
styrene with other chiral ligands: a) K. Yamamoto, Y. Kiso, R.
Ito, K. Tamao, and M. Kumada, J. Organomet. Chem., 210, 9
(1981). b) T. Hayashi, K. Tamao, Y. Katsuro, I. Nakae, and M.
Kumada, Tetrahedron Lett., 21, 1871 (1980). c) T. Okada, T.
Morimoto, and K. Achiwa, Chem. Lett., 1990, 999. d) A.
Marinetti, Tetrahedron Lett., 35, 5861 (1994). e) A. Marinetti
and L. Ricard, Organometallics, 13, 3956 (1994). f) S. Gladiali,
S. Pulacchini, D. Fabbri, M. Manassero, and M. Sansoni,
Tetrahedron: Asymmetry, 9, 391 (1998). g) G. Pioda and A.
Togni, Tetrahedron: Asymmetry, 9, 3903 (1998).
The high efficiency of (R)-H-MOP(m,m-2CF3) (2g) was
also observed in the asymmetric hydrosilylation of styrene
derivatives substituted on the phenyl ring 3b–3d (Scheme 3).
The regioselectivity in forming benzylic silanes was always
perfect, as is usually observed in the palladium-catalyzed
hydrosilylation of styrene derivatives.6,8 The enantioselectivity
is generally very high with H-MOP(m,m-2CF3) (2g) irrespective
9
Specific rotations ([α]D2 0 (c 0.8–1.0 in CHCl3)) of (R)-2 are as
follows: –91.2° (2b). –69.9° (2c). –53.6° (2d). –52.7° (2e).
–38.3° (2f). –22.2° (2g).
10 L. Kurz, G. Lee, D. Morgans, Jr., M. J. Waldyke, and T. Wars,
Tetrahedron Lett., 31, 6321 (1990).