Preparation of a new MOP ligand containing a long-chain alkyl group and its use for palladium-catalyzed asymmetric hydrosilylation of cyclic 1,3-dienes

Article abstract of DOI:10.1246/cl.2001.976

A new MOP ligand containing n-octyl group at 6 and 6′ positions of (R)-2-(diphenylphosphino)-2′-aryl-1,1′-binaphthyl skeleton was prepared and used for palladium-catalyzed asymmetric hydrosilylation of cyclic 1,3-dienes with trichlorosilane. By introducti

Full text of DOI:10.1246/cl.2001.976

976
Chemistry Letters 2001
Preparation of a New MOP Ligand Containing a Long-Chain Alkyl Group
and Its Use for Palladium-Catalyzed Asymmetric Hydrosilylation of Cyclic 1,3-Dienes
Jin Wook Han and Tamio Hayashi*
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502
(Received August 2, 2001; CL-010741)
A new MOP ligand containing n-octyl group at 6 and 6'
catalyst at 130 °C gave a high yield of the monophosphinylation
product 6. The reduction of phosphine oxide in 6 with
trichlorosilane followed by replacement of the remaining triflate
in 7 by 3,5-dimethyl-4-methoxyphenyl group by the nickel-cat-
alyzed Grignard cross-coupling gave the Ar-MOP ligand 8.9
positions of (R)-2-(diphenylphosphino)-2'-aryl-1,1'-binaphthyl
skeleton was prepared and used for palladium-catalyzed asym-
metric hydrosilylation of cyclic 1,3-dienes with trichlorosilane.
By introduction of the n-octyl group, the palladium-phosphine
catalyst became soluble in the reaction system, realizing high
catalytic activity at a low reaction temperature.
Palladium-catalyzed asymmetric hydrosilylation of 1,3-
dienes is one of the important synthetic methods for optically
active allylsilanes,1 which are readily transformed to a wide
variety of enantiomerically enriched compounds by the reaction
with electrophiles in an SE' fashion.2 We have recently reported
that the palladium-catalyzed asymmetric hydrosilylation of
cyclic 1,3-dienes such as cyclohexadiene and cyclopentadiene
is efficiently catalyzed by palladium complexes coordinated
with (R)-2-diphenylphosphino-2'-aryl-1,1'-binaphthyls.3 Thus,
for example, the most effective Ar-MOP ligand for this
hydrosilylation is (R)-2-(diphenylphosphino)-2'-(3,5-dimethyl-
4-methoxyphenyl)-1,1'-binaphthyl (1), which gave 90% ee of
the corresponding allylsilanes. However, the catalytic reactivi-
ty is not high enough due to the low solubility of a palladium
complex coordinated with the MOP ligand in the reaction
media. Although the concept of introducing a long-chain alkyl
group to enhance solubility has often appeared in polymer
chemistry,4 there have been few reports in the field of homoge-
neous asymmetric catalysis.5 Here we wish to report that
appropriate modification of the Ar-MOP ligand by introducing
a long-chain alkyl group at 6 and 6' positions leads to the higher
catalytic reactivity and enantioselectivity for cyclic 1,3-dienes.
The Ar-MOP containing two n-octyl groups at 6 and 6'
positions of binaphthyl skeleton 8 thus obtained was examined
for its catalytic reactivity and enantioselectivity in the palladi-
um-catalyzed asymmetric hydrosilylation of cyclic 1,3-dienes 9
with trichlorosilane (Scheme 2). The hydrosilylation was car-
ried out without solvent in the presence of 0.25 mol% of the
palladium catalyst generated in situ by mixing [PdCl(π-C3H5)]2
with 2 equiv (to palladium) of chiral ligand 8. The resulting
allyl(trichloro)silane was allowed to react with benzaldehyde in
DMF according to Kobayashi's procedure10 to give the homoal-
lylic alcohol, which was subjected to GLC or HPLC analysis
with a chiral stationary phase column for the determination of
the enantioselectivity. The results are summarized in Table 1,
which also contains the data obtained with (R)-2-diphenylphos-
phino-2'-(3,5-dimethyl-4-methoxyphenyl)-1,1'-binaphthyl (1)
for comparison. In the reaction of 1,3-cyclohexadiene (9a), the
highest enantioselectivity so far recorded was 79% ee, which
was observed with Ar-MOP ligand 1 at 0 °C (entry 1).3 At a
lower temperature, the hydrosilylation of 9a did not proceed at
all even if the reaction time was prolonged, which is due to the
insolubility of the palladium catalyst coordinated with ligand 1
in the reaction media consisting of 1,3-cyclohexadiene (9a) and
trichlorosilane (entry 3). Actually, a considerable amount of
off-white precipitates were observed in the reaction flask.11 On
The Ar-MOP ligand which contains n-octyl group at 6 and
6' positions of binaphthyl was prepared starting from (R)-6,6'-
dibromo-2,2'-diethoxy-1,1-binaphthyl (2)6 (Scheme 1). The n-
octyl group was introduced into binaphthyl skeleton by the pal-
ladium-catalyzed cross-coupling reaction of 2 with the n-octyl
Grignard reagent in the presence of PdCl2(dppf) catalyst7 in high
yield. The n-octyl substituted binaphthol 4 was prepared by
deprotection of ethyl ether 3 with BBr3. The binaphthol 4 was
converted into (R)-2-diphenylphosphino-2'-(3,5-dimethyl-4-
methoxyphenyl)-6,6'-dioctyl-1,1'-binaphthyl (8) according to the
reported procedures.8 Thus, the selective monophosphinylation
of the ditriflate 5, readily obtained by treatment of 4 with Tf2O,
with diphenylphosphine oxide in the presence of a palladium
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
977
This work was supported by “Research for the Future”
Program, the Japan Society for the Promotion of Science, and a
Grant-in-Aid for Scientific Research, the Ministry of Education,
Science, Sports and Culture, Japan.
Dedicated to Prof. Hideki Sakurai on the occasion of his
70th birthday.
References and Notes
1
For reviews: a) T. Hayashi, Acc. Chem. Res., 33, 354 (2000). b) T.
Hayashi, in “Comprehensive Asymmetric Catalysis,” ed. by E. N.
Jacobsen, A. Pfaltz, and H. Yamamoto, Springer, Berlin (1999), Vol. 1,
Chap. 7. c) H. Nishiyama and K. Itoh, in “Catalytic Asymmetric
Synthesis,” 2nd ed., ed. by I. Ojima, Wiley-VCH, New York (2000), p
111.
2
a) C. E. Masse and J. S. Panek, Chem. Rev., 95, 1293 (1995). b) Y.
Yamamoto and A. Asao, Chem. Rev., 93, 2207 (1993). c) I. Fleming,
in “Comprehensive Organic Synthesis,” ed. by C. H. Heathcock,
Permagon, Oxford (1991), Vol. 2, p 563.
3
4
5
T. Hayashi, J. W. Han, A. Takeda, J. Tang, K. Nohmi, K. Mukaide, H.
Tsuji, and Y. Uozumi, Adv. Synth. Catal., 343, 279 (2001).
P. A. Toy and K. D. Janda, Acc. Chem. Res., 33, 546 (2000), and refer-
ences cited therein.
a) “Comprehensive Asymmetric Catalysis,” ed. by E. N. Jacobsen, A.
Pfaltz, and H. Yamamoto, Springer, Berlin (1999). b) “Catalytic
Asymmetric Synthesis,” 2nd ed., ed. by I. Ojima, Wiley-VCH, New
York (2000).
6
7
8
C. Dong, J. Zhang, W. Zheng, L. Zhang, Z. Yu, M. C. K. Choi, and A.
S. C. Chan, Tetrahedron Asym., 11, 2449 (2000).
T. Hayashi, M. Konishi, Y. Kobori, M. Kumada, T. Higuchi, and K.
Hirotsu, J. Am. Chem. Soc., 106, 158 (1984).
a) S.-Y. Cho and M. Shibasaki, Tetrahedron Lett., 39, 1773 (1998). b)
S. Gladiali, S. Pulacchini, D. Fabbri, M. Manassero, and M. Sansoni,
Tetrahedron Asym., 9, 391 (1998). c) Y. Uozumi, N. Suzuki, A.
Ogiwara, and T. Hayashi, Tetrahedron, 50, 4293 (1994). d) Y. Uozumi,
A. Tanahashi, S.-Y. Lee, and T. Hayashi, J. Org. Chem., 58, 1945
(1993).
9
Characterization of compounds: (3) colorless oil. 1H NMR (CDCl3) δ
0.87 (t, J = 6.1 Hz, 6H), 1.04 (t, J = 7.0 Hz, 6H), 1.20–1.43 (m, 20H),
1.62–1.68 (m, 4H), 2.69 (t, J = 7.8 Hz, 4H), 3.98–4.03 (m, 4H), 7.04 (s,
4H), 7.37 (d, J = 8.9 Hz, 2H), 7.60 (s, 2H), 7.84 (d, J = 8.9 Hz, 2H).
[α]20D +0.4 (c 1.3, CHCl3). (4) colorless oil. 1H NMR (CDCl3) δ 0.85
(t, J = 7.1 Hz, 6H), 1.21–1.38 (m, 20H), 1.63–1.69 (m, 4H), 2.71 (t, J =
7.8 Hz, 4H), 4.97 (s, 2H), 7.07 (d, J = 8.7 Hz, 2H), 7.15 (d, J = 8.6 Hz,
2H), 7.33 (d, J = 9.0 Hz, 2H), 7.65 (s, 2H), 7.88 (d, J = 9.0 Hz, 2H).
[α]20D –66 °(c 1.4, CHCl3). (5) colorless oil. 1H NMR (CDCl3) δ 0.87
(t, J = 6.9 Hz, 6H), 1.21–1.39 (m, 20H), 1.54–1.72 (m, 4H), 2.76 (t, J =
7.6 Hz, 4H), 7.16 (d, J = 8.8 Hz, 2H), 7.24 (d, J = 8.7 Hz, 2H), 7.56 (d,
J = 9.1 Hz, 2H), 7.75 (s, 2H), 8.04 (d, J = 9.1 Hz, 2H). [α]20D –108 °(c
1.3, CHCl3). (6) colorless oil. 1H NMR (CDCl3) δ 0.85–0.89 (m, 6H),
1.19–1.43 (m, 20H), 1.62–1.73 (m, 4H), 2.69 (t, J = 8.0 Hz, 2H), 2.74
(t, J = 8.0 Hz, 2H), 6.89 (d, J = 8.6 Hz, 1H), 6.99 (d, J = 8.7 Hz, 1H),
7.07 (d, J = 8.8 Hz, 1H), 7.15 (d, J = 8.9 Hz, 1H), 7.20–7.27 (m, 5H),
7.33–7.38 (m, 2H), 7.40–7.48 (m, 4H), 7.57 (s, 1H), 7.62 (dd, J = 11.5,
8.6 Hz, 1H), 7.69 (s, 1H), 7.79 (d, J = 9.1 Hz, 1H), 7.92 (d, J = 8.7 Hz,
the other hand, the palladium catalyst of the new ligand 8 became
soluble in the reaction media, forming a clear solution even at
–10 °C. As a result, the hydrosilylation of 9a was catalyzed by
the palladium/8 at –10 °C to give (S)-3-trichlorosilylcyclohexene
(10a) of 83% ee, which is the highest value for the reaction of 9a
(entry 4). The enantioselecctivity of the Ar-MOP ligands was
not influenced greatly by introduction of the n-octyl group, which
is shown by the same stereochemical outcome obtained in the
hydrosilylation at 0 °C (entries 1 and 2). The highest enantiose-
lectivity was also observed in the hydrosilylation of cyclopenta-
diene (9b) by use of the n-octylated MOP ligand 8. Thus, the
reaction of 9b proceeded at –30 °C in the presence of the palladi-
um/8 as a catalyst, to give (S)-3-trichlorosilylcyclopentene (10b)
of 91% ee (entry 8). Here again, the reaction with ligand 1 did
not take place at the same temperature (entry 7).
1H); 31P{1H} NMR (CDCl3) δ 28.9 (s). [α]20 +30 °(c 1.3, CHCl3).
D
(7) colorless oil. 1H NMR (CDCl3) δ 0.84–0.88 (m, 6H), 1.23–1.32
(m, 20H), 1.62–1.73 (m, 4H), 2.68 (t, J = 7.8 Hz, 2H), 2.71 (t, J = 7.9
Hz, 2H), 6.81 (d, J = 8.8 Hz, 1H), 6.90 (d, J = 8.8 Hz, 1H), 6.98–7.01
(m, 2H), 7.05–7.17 (m, 5H), 7.22–7.29 (m, 5H), 7.39 (dd, J = 8.8, 3.0
Hz, 1H), 7.47 (d, J = 9.3 Hz, 1H), 7.64 (s, 1H), 7.67 (s, 1H), 7.84 (d, J
= 8.3 Hz, 1H), 7.95 (d, J = 9.3 Hz, 1H); 31P{1H} NMR (CDCl3) δ
–12.07 (s). [α]20 –11 °(c 1.3, CHCl3). (8) colorless oil. 1H NMR
(CDCl3) δ 0.85–0D.90 (m, 6H), 1.20–1.33 (m, 20H), 1.64–1.69 (m, 4H),
1.83 (s, 6H), 2.69 (t, J = 8.0 Hz, 2H), 2.73 (t, J = 8.0 Hz, 2H), 3.60 (s,
3H), 6.61 (s, 2H), 6.66 (t, J = 7.1 Hz, 2H), 6.74 (d, J = 8.7 Hz, 1H),
6.82 (dd, J = 8.7, 1.6 Hz, 1H), 6.90 (t, J = 7.4 Hz, 2H), 7.01–7.23 (m,
8H), 7.37 (d, J = 8.7 Hz, 1H), 7.57 (d, J = 8.5 Hz, 1H), 7.60 (s, 1H),
7.64 (d, J = 8.6 Hz, 1H), 7.66 (s, 1H), 7.93 (d, J = 8.6 Hz, 1H); 31P{1H}
NMR (CDCl3) δ –14.20 (s). FAB MS m/z (M+ + H) Calcd for
C57H66OP: 797.48 Found: 797.51. [α]20D +130 ° (c 0.3, CHCl3).
In summary, the introduction of two n-octyl groups into the
Ar-MOP ligand made the palladium catalyst soluble in the
hydrosilylation media. The high solubility of the chiral palladi-
um catalyst realized the hydrosilylation at a lower reaction tem-
perature, resulting in the higher enantioselectivity in the asym-
metric hydrosilyaltion of 1,3-dienes.
10 S. Kobayashi and K. Nishio, J. Org. Chem., 59, 6620 (1994).
11 Use of polar solvents such as dichloromethane makes the reaction mix-
ture homogeneous, but the hydrosilylation is slow in the polar solvents.