Rhodium-catalyzed asymmetric synthesis of spirosilabifluorene derivatives

Article abstract of DOI:10.1002/anie.201207723

Si goes chiral: Treatment of a bis(biphenyl)silane with a catalytic amount of a rhodium complex gave a spirosilabifluorene bearing a quaternary silicon atom. By using a rhodium catalyst with a chiral phosphine ligand (see scheme), asymmetric dehydrogenative cyclization proceeded to give chiral derivatives in good yields and enantioselectivities. Copyright

Full text of DOI:10.1002/anie.201207723

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Angewandte
Communications
DOI: 10.1002/anie.201207723
Chirality
Rhodium-Catalyzed Asymmetric Synthesis of Spirosilabifluorene
Derivatives**
Yoichiro Kuninobu,* Kanae Yamauchi, Naoya Tamura, Takayuki Seiki, and Kazuhiko Takai*
Chiral compounds occupy a large and important area of
organic molecules, such as natural products, bioactive com-
pounds, drugs, and functional materials. However, examples
of the synthesis of chiral organosilicon compounds are still
rare among organic compounds.[1] It is important to construct
a quaternary silicon atom to synthesize chiral organosilicon
compounds. Our strategy for the construction of a quaternary
silicon atom is a rhodium-catalyzed double dehydrogenative
cyclization of bis(biphenyl)silanes through carbon–silicon
bond formation (Scheme 1).[2,3] We report herein the asym-
metric synthesis of chiral spirosilabifluorene derivatives from
bis(biphenyl)silanes, bearing substituents on the aromatic
rings, using a rhodium catalyst with a chiral phosphine
products are generated as a mixture of enantiomers (racemic
mixture). Therefore, chiral spirocompounds must be obtained
using rhodium catalysts with a chiral phosphine ligand.
Indeed, as a result of the treatment of the bis(methoxybiphe-
nyl)silane 1b with catalytic amounts of [{RhCl(cod)]2}] and
(R)-binap, the spirosilabifluorene 2b was afforded in 95%
yield and 81% ee (Table 1, entry 1).[6–8] The enantiomers were
ligand.[4]
Table 1: Synthesis of chiral spirosilabifluorenes 2.
Scheme 1. Strategy for the construction of a quaternary silicon atom.
Treatment of the bis(biphenyl)silane 1a with a catalytic
amount of either the rhodium complex [RhCl(PPh3)3] or
a mixture of [{RhCl(cod)}2] and rac-binap, in 1,4-dioxane at
1358C for 3 hours gave 9,9’-spiro-9-silabifluorene (2a) in 96
and 92% yield, respectively [Eq. (1), binap = 2,2’-bis(diphe-
nylphosphanyl)-1,1’-binaphthyl, cod = cyclo-1,5-octadiene].
In this reaction, dehydrogenative intramolecular cyclization
proceeded twice, and a spirocompound with a quaternary
silicon atom was produced.[4,5]
Entry
R
Yield [%][a]
ee [%][b]
Major
enantiomer
1
2
4-MeO (1b)
4-tBu (1c)
4-CF3 (1d)
4-Ph (1e)
95 (2b)
94 (2c)
90 (2d)
90 (2e)
73 (2 f)
81
78
75
70
77
(+)
(+)
(À)
(+)
(À)
S
[c]
–
–
3[d]
4
[c]
S
[c]
If the reaction is carried out using bis(biphenyl)silanes
with two or more substituents on different aromatic rings, the
5
2-MeO (1 f)
–
[a] Yield of isolated product. [b] The ee values for the products 2 were
determined by HPLC analysis using a chiral stationary phase. [c] Not
determined. [d] 1158C, 2 h.
[*] Prof. Dr. Y. Kuninobu, K. Yamauchi, N. Tamura, T. Seiki,
Prof. Dr. K. Takai
Division of Chemistry and Biotechnology, Graduate School of
Natural Science and Technology, Okayama University
3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530 (Japan)
E-mail: kuninobu@mol.f.u-tokyo.ac.jp
separated by HPLC methods using a chiral stationary phase,
and the structure of the major enantiomer of 2b was
determined by single-crystal X-ray structure analysis to be
the S form (Figure 1 and the Supporting Information).[9] The
reaction also proceeded well when the bis(biphenyl)silane
with tert-butyl groups (1c) was employed as a substrate
(Table 1, entry 2). The corresponding spirosilabifluorene 2d
was afforded in 90% yield and 75% ee when using the
bis(biphenyl)silane having electron-withdrawing groups (1d;
Table 1, entry 3).[10] The bis(biphenyl)silane bearing phenyl
groups (1e) provided the spirosilabifluorene 2e (Table 1,
entry 4).[11,12] The absolute configuration of the major enan-
ktakai@cc.okayama-u.ac.jp
Prof. Dr. Y. Kuninobu
Present Address: Graduate School of Pharmaceutical Sciences, The
University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Japan)
[**] This work was partially supported by the Ministry of Education,
Culture, Sports, Science, and Technology of Japan.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207723.
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Chemie
tiomer of 2e was determined by
preparation of 2e from (S)-2b (See
the Supporting Information). The
retention time of the major enan-
tiomer of 2e is consistent with the
compound derived from (S)-2b on
a chiral HPLC column. The chiral
spirosilabifluorene 2 f was produced
by double dehydrogenative cyclization using either the
rhodium catalyst [RhCl(PPh3)3] or a mixture of [{RhCl-
(cod)}2] and rac-binap. This reaction is a rare example of the
formation of a quaternary silicon atom. This reaction was
applied to the synthesis of chiral spirosilabifluorene deriva-
tives using rhodium catalysts with chiral phosphine ligands
([{RhCl(cod)}2] + (R)-binap). The stereochemistry of the
major enantiomer of the product was determined by single-
crystal X-ray structure analysis, thus confirming this rare
example of the synthesis of chiral organosilicon compounds
with a quaternary silicon atom. In addition, spirosilabifluor-
enes are interesting compounds as organic materials.[4b–e] We
hope that this reaction will become a useful method to
synthesize chiral spirosilabifluorene derivatives.
Figure 1. Molecular struc-
ture of chiral spirosilabi-
fluorene (S)-2b. Thermal
ellipsoids set at 50%
probability. Hydrogen
atoms are omitted for
clarity.[13]
when
bis(biphenyl)silane
with
methoxy groups at the 2-position
of the phenyl groups (1 f) was used
as a substrate (Table 1, entry 5).
The mechanistic details of the
À
Si C bond formation reaction are
proposed as follows (Scheme 2):[2]
Experimental Section
Typical procedure for the synthesis of 2,2’-dimethoxy-9,9’-spiro-9-
silabifluorene (2b): A mixture of [{RhCl(cod)}2] (2.5 mg, 0.50 mmol),
(R)-binap (0.75 mg, 1.2 mmol), and 1,4-dioxane (0.10 mL) was stirred
at 258C for 30 min. The mixture was added to bis(4’-methoxybi-
phenyl-2-yl)silane (1b, 39.6 mg, 0.100 mmol), and the mixture was
heated at 1358C for 3 h. After the reaction, the solvent was removed
in vacuo. The product was isolated after column chromatography on
silica gel (n-hexane/ethyl acetate = 50:1) to give 2,2’-dimethoxy-9,9’-
spiro-9-silabifluorene (2b, 37.4 mg, 0.0952 mmol, 95% yield).
Received: September 25, 2012
Published online: December 13, 2012
À
Keywords: asymmetric synthesis · C H activation · rhodium ·
silicon · synthetic methods
.
[1] For several examples of the synthesis of chiral silicon com-
pounds, see: a) K. Tamao, K. Nakamura, H. Ishii, S. Yamaguchi,
M. Shiro, J. Am. Chem. Soc. 1996, 118, 12469; b) D. R. Schmidt,
S. J. OꢀMalley, J. L. Leighton, J. Am. Chem. Soc. 2003, 125, 1190;
c) K. Igawa, J. Takada, T. Shimono, K. Tomooka, J. Am. Chem.
Soc. 2008, 130, 16132; d) Y. Yasutomi, H. Suematsu, T. Katsuki,
J. Am. Chem. Soc. 2010, 132, 4510; e) R. Shintani, H. Otomo, K.
Ota, T. Hayashi, J. Am. Chem. Soc. 2012, 134, 7305.
[2] T. Ureshino, T. Yoshida, Y. Kuninobu, K. Takai, J. Am. Chem.
Soc. 2010, 132, 14324.
[3] For a review on transition-metal-catalyzed silylation of aromatic
compounds, see: F. Kakiuchi, Handbook of C-H Transforma-
tions, Vol. 1 (Ed.: G. Dyker), Wiley-VCH, Weinheim, 2005,
pp. 131 – 137.
[4] For examples of previous methods to synthesize achiral spiro-
silabifluorene derivatives, see: a) A. G. Russell, N. S. Spencer, D.
Philp, B. M. Kariuki, J. S. Snaith, Organometallics 2003, 22, 5589;
b) H. Xiao, B. Leng, H. Tian, Polymer 2005, 46, 5707; c) T.
Matsuda, S. Kadowaki, T. Goya, M. Murakami, Org. Lett. 2007,
9, 133; d) H. Xiao, H. Shen, Y. Lin, J. Su, Dyes Pigm. 2007, 73,
224; e) T. Agou, M. D. Hossain, T. Kawashima, Chem. Eur. J.
2010, 16, 368; f) S. Furukawa, J. Kobayashi, T. Kawashima,
Dalton Trans. 2010, 39, 9329.
[5] There have been several reports on transition-metal-catalyzed
asymmetric synthesis of spirocompounds. See: a) M. Takahashi,
M. Tanaka, E. Sakamoto, M. Imai, A. Matsui, K. Funakoshi, K.
Sakai, H. Suemune, Tetrahedron Lett. 2000, 41, 7879; b) M.
Tanaka, M. Takahashi, E. Sakamoto, M. Imai, A. Matsui, M.
Fujio, K. Funakoshi, K. Sakai, H. Suemune, Tetrahedron 2001,
57, 1197; c) T. Takahashi, H. Tsutsui, M. Tamura, S. Kitagaki, M.
Scheme 2. Proposed mechanism for the formation of spirosilabifluor-
ene frameworks.
1) oxidative addition of a bis(biphenyl)silane (hydrosilane)
À
to a metal atom (Si H bond activation), in which the metal
À
atom is oriented close to an aromatic C H bond; 2) sequen-
À
tial oxidative addition of the aromatic C H bond to a metal
atom (C H bond activation); 3) elimination of H2 to give the
À
intermediate A. Another possible pathway for the formation
of A is by step 4: s-bond metathesis. After the generation of
A, there is 5) reductive elimination, and then 6) steps 1, 2, 3,
and 5 (or 1, 4, and 5) are repeated one more time to give
a chiral spirosilabifluorene.
The chirality of the spirosilabifluorenes is determined at
the first dehydrogenative cyclization. The conformation of
this intermediate is such that steric hindrance between
biphenyl groups of the bis(biphenyl)silane and the chiral
ligand of the catalyst is avoided. Therefore, the Rh-H species
reacts enantioselectively with the biphenyl group closer to the
metal atom. After determination of the chirality, the second
dehydrogenative cyclization occurs between the remaining
À
Si H and biphenyl group.
In summary, we have succeeded in the synthesis of
a spirosilabifluorene derivative from a bis(biphenyl)silane
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Angewandte
Communications
Nakajima, S. Hashimoto, Chem. Commun. 2001, 1604; d) A.
Wada, K. Noguchi, M. Hirano, K. Tanaka, Org. Lett. 2007, 9,
1295; e) K. Takenaka, N. Itoh, H. Sasai, Org. Lett. 2009, 11, 1483.
[6] There has been a report on highly fluorescent solid-state chiral
spirosilabifluorene derivatives. See: S. H. Lee, B.-B. Jang, Z. H.
Kafafi, J. Am. Chem. Soc. 2005, 127, 9071.
[7] Investigation of several chiral phosphine ligands: (R)-DM-
binap, 72%, 67% ee [major: (+)-2b]; (R)-H8-binap, 95%,
74% ee [major: (+)-2b]; (R)-Tol-binap, 66%, 27% ee [major:
(+)-2b]; (À)-1,2-bis[(2R,5R)-2,5-diphenylphospholano]ethane,
72%, 35% ee [major: (À)-2b]; (R)-DM-segphos, 95%, 73%
ee [major: (+)-2b]; (À)-1,2-bis[(2R,5R)-2,5-diethylphosphina-
no]benzene [(R,R)-ethyl-duphos], 61%, 62% ee [major:
(+)-2b]; (2S,3S)-(À)-bis(diphenylphosphino)butane, 0%, 0%
ee.
DM-binap, 31%, 54% ee [major: (À)-2d]; [{RhCl(cod)}2] and
(R)-H8-binap, 79%, 68% ee [major: (À)-2d]; [{RhCl(cod)}2]
and (À)-2,3-bis[(2R,5R)-2,5-dimethylphospholano]maleic anhy-
dride, 18%, 19% ee [major: (À)-2d]; [{RhCl(cod)}2] and (À)-
1,2-bis[(2R,5R)-2,5-diphenylphospholano]ethane, 53%, 80% ee
[major: (+)-2d]; [{RhCl(cod)}2] and (R)-(+)-7,7’-bis(diphenyl-
phosphino)-2,2’,3,3’-tetrahydro-1,1’-spirobiindene [(R)-SDP],
6%, 3% ee [major: (À)-2d]; (4S)-2-[2-(diphenylphosphino)-
phenyl]-4,5-dihydro-5,5-dimethyl-4-(1-methylethyl)oxazole,
38%, 50% ee [major: (À)-2d]; (À)-1,2-bis[(2R,5R)-2,5-diethyl-
phosphinano]benzene [(R,R)-ethyl-duphos], 11%, 27% ee
[major: (À)-2d].
[11] One of the purposes of the introduction of the phenyl groups is
to expand the p-conjugated system. In fact, a stronger purple
fluorescence was observed compared with other spirosilabifluor-
ene derivatives when irradiating a either the solid or an ethyl
acetate solution of spirosilabifluorene 2e with l = 254 nm (UV)
light.
[8] A reaction of 1b using (S)-binap instead of (R)-binap gave (À)-
(R)-2b in 95% yield, 81% ee.
[9] The enantiomers of 2b could be separated using a chiral HPLC
column {Chiralpak IB [LTD., 2.0 cm I.D ꢁ 25 cmL, Daicel
Chemical Industries; eluent:methanol:H2O = 95:5; flow rate:
3.2 mLminÀ1; temp = 258C; det. 300 nm (UV); injection: 5.0 mL
(ca. 400 mgLÀ1 in eluent)]; time: R form, 23 min; S form,
24.5 min}.
[10] Investigation of several combinations of rhodium complexes and
phosphine ligands (1158C, 15 min): [{RhCl(cod)}2] and (R)-
binap, 78%, 70% ee [major: (À)-2d]; [{RhCl(cod)}2] and (R)-
[12] The absolute configuration of 2e was determined by preparation
of (S)-2e from (S)-2b. The details of the preparation is described
in the Supporting Information.
[13] CCDC 914745 [(S)-2b] contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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