Palladium-catalyzed asymmetric synthesis of axially chiral allenylsilanes and their application to SE2' chirality transfer reactions

Article abstract of DOI:10.1021/ol102554a

Stepwise application of the Pd-catalyzed S_N2' reaction and the desilylative S_E2' reaction to the ambivalent 2-bromo-1-silyl-1,3- dienes provides a novel route to the highly enantioselective construction of tertiary and quaternary pro

Full text of DOI:10.1021/ol102554a

ORGANIC
LETTERS
2010
Vol. 12, No. 24
5736-5739
Palladium-Catalyzed Asymmetric
Synthesis of Axially Chiral
Allenylsilanes and Their Application to
SE2′ Chirality Transfer Reactions
Masamichi Ogasawara,* Atsushi Okada, Velusamy Subbarayan, Sebastian So¨rgel,
and Tamotsu Takahashi*
Catalysis Research Center and Graduate School of Life Science, Hokkaido UniVersity,
Kita-ku, Sapporo 001-0021, Japan
ogasawar@cat.hokudai.ac.jp; tamotsu@cat.hokudai.ac.jp
Received October 21, 2010
ABSTRACT
Stepwise application of the Pd-catalyzed SN2′ reaction and the desilylative SE2′ reaction to the ambivalent 2-bromo-1-silyl-1,3-dienes provides
a novel route to the highly enantioselective construction of tertiary and quaternary propargylic stereogenic centers via axially chiral allenylsilanes.
Allenylsilanes are versatile intermediates for organic synthesis,1,2
and their axially chiral counterparts are capable of transfer-
ring the allenic axial chirality into newly formed propargylic
stereogenic centers by a regio- and stereospecific SE2′
reaction with an appropriate electrophile.1,3 Although various
protocols of preparing allenylsilanes have been reported,2
methods to prepare enantiomerically enriched axially chiral
allenylsilanes have not been well developed.3 To the best of
our knowledge, only three methods have been reported on
the catalytic asymmetric synthesis of axially chiral allenyl-
silanes.4 This can be attributed to the lack of general routes
to scalemic axially chiral allenes by asymmetric catalysis.4-7
Recently, we developed the Pd-catalyzed reaction of prepar-
ing various multisubstituted allenes,8a-h and the use of a
(3) (a) Borzilleri, R. M.; Weinreb, S. M.; Parvez, M. J. Am. Chem. Soc.
1995, 117, 10905. (b) Suginome, M.; Matsumoto, A.; Ito, Y. J. Org. Chem.
1996, 61, 4884. (c) Marshall, J. A.; Adams, N. D. J. Org. Chem. 1997, 62,
8976. (d) Jin, J.; Weinreb, S. M. J. Am. Chem. Soc. 1997, 119, 5773. (e)
Marshall, J. A.; Maxson, K. J. Org. Chem. 2000, 65, 630. (f) Guintchin,
B. K.; Bienz, S. Organometallics 2004, 23, 4944. (g) Gonzalez, A. Z.;
Soderquist, J. A. Org. Lett. 2007, 9, 1081. (h) Brawn, R. A.; Panek, J. S.
Org. Lett. 2007, 9, 2689. (i) Reginato, G.; Mordini, A.; Tenti, A.; Valacchi,
M.; Broguiere, J. Tetrahedron: Asymmetry 2008, 19, 2882. (j) Brawn, R. A.;
Panek, J. S. Org. Lett. 2009, 11, 473. (k) Ohmiya, H.; Ito, H.; Sawamura,
M. Org. Lett. 2009, 11, 5618. (l) Brawn, R. A.; Welzel, M.; Lowe, J. T.;
Panek, J. S. Org. Lett. 2010, 12, 336.
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Fleming, I.; Barbero, A.; Walter, D. Chem. ReV. 1997, 97, 2063. (c)
Marshall, J. A. Chem. ReV. 2000, 100, 3163. (d) Pornet, J. In Science of
Synthesis; Fleming, I., Ed.; Georg Thieme Verlag: Stuttgart, 2002; Vol. 4,
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Chem. Soc. 2001, 123, 12915. (c) Nishimura, T.; Makino, H.; Nagaosa,
M.; Hayashi, T. J. Am. Chem. Soc. 2010, 132, 12865.
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R. L.; Carini, D. J.; Basak, A. J. Am. Chem. Soc. 1981, 103, 1604. (c)
Danheiser, R. L.; Carini, D. J.; Fink, D. M.; Basak, A. Tetrahedron 1983,
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10.1021/ol102554a  2010 American Chemical Society
Published on Web 11/19/2010

proper chiral Pd catalyst furnished axially chiral allenes with
high enantioselectivity.8i-m In this communication, we report
the application of the Pd-catalyzed reaction to the asymmetric
synthesis of axially chiral allenylsilanes. The newly devel-
oped substrates for the present study, 2-bromo-1-silyl-1,3-
dienes, possess both electrophilic and nucleophilic sites
within single molecules. Stepwise application of the Pd-
catalyzed SN2′ reaction and the desilylative SE2′ reaction to
the ambivalent compounds provides a novel route to the
asymmetric construction of chiral propargyl compounds in
up to 94% ee (Scheme 1). Whereas the Pd-catalyzed reaction
methylenation method in the final step is important for the
high yields of 1. While Peterson’s protocol was highly
effective for the transformation giving 1 in up to 94% yield,
the Wittig reaction of 3a with Ph3PdCH2 gave a complex
mixture with less than 20% of 1a. The bromosilyldienes 1
are fairly stable and purified by silica gel chromatography
and/or vacuum distillation.
The bromosilyldienes 1 are excellent substrates for the Pd-
catalyzed reaction with various carbon soft nucleophiles 4,
and various allenylsilanes 5, including 1,3-disubstituted
(Table 1, entries 1-9) and 1,3,3-trisubstituted (entries
10-12) allenylsilanes, were obtained in good to excellent
yields in the presence of 2 mol % of a palladium catalyst
generated in situ from [PdCl(π-allyl)]2 and dpbp.8a,10 The
substrates 1 were consumed completely within 18 h, and the
NMR and GC analyses of the crude reaction mixtures
revealed concomitant formation of the dehydrobrominated
enynes [Si]sCtCsCRdCH2 611 as byproducts. Similar
dehydrobromination was not apparent in the analogous
palladium-catalyzed reaction of 1-hydrocarbyl-2-bromo-1,3-
dienes.8 Apart from the formation of 6, the reaction was
generally very clean and the allenylsilanes 1 were isolated
in up to 93% yield by silica gel chromatography.
Whereas the synthetic usefulness of 1 in the Pd-catalyzed
reaction was proven as above, enantioselective synthesis of the
axially chiral allenylsilanes was examined. Under the reaction
conditions similar to those in our previous report,8i i.e., with a
palladium catalyst (10 mol %) generated from Pd2(dba)4 and (R)-
binap, (R)-5am of 54% ee was obtained in 61% yield by a reaction
of 1a with 4m at 23 °C (entry 13). The enantioselectivity was
improved by the use of (R)-segphos8j,k,12 in place of (R)-binap,
and (R)-5am was obtained in 86% ee (entry 14). Despite
the higher enantioselectivity, the Pd/(R)-segphos species was
prone to give 5 in lower yields. To gain reasonable yields
of 5 with the Pd/(R)-segphos, higher temperatures (up to 80
°C) were required, but the loss of the enantioselectivity was
minimal to negligible. A variety of axially chiral allenylsi-
lanes of excellent enantiopurity (up to 94% ee in 87% yield
in 5an; entry 15) were obtained under the optimized
Scheme 1
is tolerant of various functional groups, a variety of substit-
uents could be introduced in the allenylsilanes. The 3,3-
dialkylallenylsilanes thus obtained can be converted to the
corresponding propargylic compounds with an “all-carbon”
quaternary stereogenic center by the SE2′ chirality transfer
process.
To realize the Pd-catalyzed synthesis of allenylsilanes, first,
the preparation of yet unknown 2-bromo-1-silyl-1,3-dienes
was examined. A series of bromosilyldienes 1a-f were
obtained in good overall yields by the reaction sequence
depicted in Scheme 2. Readily accessible ꢀ-silylenals/enones
Scheme 2
(8) (a) Ogasawara, M.; Ikeda, H.; Hayashi, T. Angew. Chem., Int. Ed.
2000, 39, 1042. (b) Ogasawara, M.; Ikeda, H.; Nagano, T.; Hayashi, T.
Org. Lett. 2001, 3, 2615. (c) Ogasawara, M.; Ge, Y.; Uetake, K.; Fan, L.;
Takahashi, T. J. Org. Chem. 2005, 70, 3871. (d) Ogasawara, M.; Fan, L.;
Ge, Y.; Takahashi, T. Org. Lett. 2006, 8, 5409. (e) Ogasawara, M.; Okada,
A.; Watanabe, S.; Fan, L.; Uetake, K.; Nakajima, K.; Takahashi, T.
Organometallics 2007, 26, 5025. (f) Ogasawara, M.; Okada, A.; Nakajima,
K.; Takahashi, T. Org. Lett. 2009, 11, 177. (g) Ogasawara, M.; Okada, A.;
Murakami, H.; Watanabe, S.; Ge, Y.; Takahashi, T. Org. Lett. 2009, 11,
4240. (h) Ogasawara, M.; Murakami, H.; Furukawa, T.; Takahashi, T.;
Shibata, N. Chem. Commun. 2009, 7366. (i) Ogasawara, M.; Ikeda, H.;
Nagano, T.; Hayashi, T. J. Am. Chem. Soc. 2001, 123, 2089. (j) Ogasawara,
M.; Ueyama, K.; Nagano, T.; Mizuhata, Y.; Hayashi, T. Org. Lett. 2003,
5, 217. (k) Ogasawara, M.; Nagano, T.; Hayashi, T. J. Org. Chem. 2005,
70, 5764. (l) Ogasawara, M.; Ngo, H. L.; Sakamoto, T.; Takahashi, T. Org.
Lett. 2005, 7, 2881. (m) Ogasawara, M.; Fan, L.; Ge, Y.; Takahashi, T.
Org. Lett. 2006, 8, 5409.
2 were converted into the corresponding R-bromo-ꢀ-silyle-
nals/enones 3 by a successive Br2 and NEt3 treatment.9
Conversion of 3 into 1 was achieved by Peterson’s protocol:
i.e., reaction of 3 with Me3SiCH2MgCl followed by an acidic
treatment of the generated alcohols at 60 °C afforded 1
predominantly in (Z)-forms (>96%). The choice of the
(7) For transition-metal-catalyzed kinetic resolutions of racemic allenes,
see: (a) Noguchi, Y.; Takiyama, H.; Katsuki, T. Synlett 1998, 543. (b)
Sweeney, Z. K.; Salsman, J. L.; Andersen, R. A.; Bergman, R. G. Angew.
Chem., Int. Ed. 2000, 39, 2339. For transition-metal-catalyzed dynamic
kinetic resolutions of racemic allenes, see: (c) Imada, Y.; Ueno, K.; Kutsuwa,
K.; Murahashi, S. Chem. Lett. 2002, 140. (d) Trost, B. M.; Fandrick, D. R.;
Dinh, D. C. J. Am. Chem. Soc. 2005, 127, 14186. (e) Imada, Y.; Nishida,
M.; Kutsuwa, K.; Murahashi, S.; Naota, T. Org. Lett. 2005, 7, 5837. (f)
Imada, Y.; Nishida, M.; Naota, T. Tetrahedron Lett. 2008, 49, 4915. (g)
Nemoto, T.; Kanematsu, M.; Tamura, S.; Hamada, Y. AdV. Synth. Catal.
2009, 351, 1773.
(9) (a) Borrelly, S.; Paquette, L. A. J. Org. Chem. 1993, 58, 2714. (b)
Fleming, I.; Marangon, E.; Roni, C.; Russell, M. G.; Chamudis, S. T. Can.
J. Chem. 2004, 82, 325.
(10) dpbp ) 2,2′-bis(diphenylphosphino)-1,1′-biphenyl. See: Ogasawara,
M.; Yoshida, K.; Hayashi, T. Organometallics 2000, 19, 1567, and
references cited therein
.
(11) Trost, B. M.; Tour, J. M. J. Org. Chem. 1989, 54, 484.
(12) Saito, T.; Yokozawa, T.; Ishizaki, T.; Moroi, T.; Sayo, N.; Miura,
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Org. Lett., Vol. 12, No. 24, 2010
5737

Table 1. Palladium-Catalyzed Synthesis of Axially Chiral Allenylsilanes 5a
entry substrate 1 Nu-H 4
base
NaH
NaH
NaH
KH
NaH
NaH
NaH
NaH
NaH
NaH
NaH
CsOtBu
NaH
Pd-precursor (mol %)
P-P
temp (°C) yield of 5 (%)b % eec of 5 (config)d
1
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1f
4m
4n
4o
4p
4q
4r
4s
4m
4r
[PdCl(π-allyl)]2 (1.0)
[PdCl(π-allyl)]2 (1.0)
[PdCl(π-allyl)]2 (1.0)
[PdCl(π-allyl)]2 (1.0)
[PdCl(π-allyl)]2 (1.0)
[PdCl(π-allyl)]2 (1.0)
[PdCl(π-allyl)]2 (1.0)
[PdCl(π-allyl)]2 (1.0)
[PdCl(π-allyl)]2 (1.0)
[PdCl(π-allyl)]2 (1.0)
[PdCl(π-allyl)]2 (1.0)
[PdCl(π-allyl)]2 (1.0)
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
dpbp
dpbp
dpbp
dpbp
dpbp
dpbp
dpbp
dpbp
dpbp
dpbp
dpbp
dpbp
23
40
40
50
40
40
40
50
40
40
40
40
23
40
40
40
80
40
80
80
80
40
60
80
40
80
80
60
88 (5am)
73 (5an)
87 (5ao)
72 (5ap)
81 (5aq)
72 (5ar)
92 (5as)
93 (5bm)
76 (5cr)
81 (5dm)
88 (5em)
75 (5fr)
61 (5am)
62 (5am)
87 (5an)
82 (5ao)
64 (5ap)
67 (5aq)
63 (5ar)
63 (5as)
76 (5bm)
86 (5bn)
38 (5cr)
71 (5dm)
24 (5dr)
73 (5dr)
58 (5em)
73 (5fr)
s
2
s
3e
4
s
s
5
6
7
8
s
s
s
s
9
s
10
11
12
13
14
15
16e
17
18
19
20
21
22
23
24
25
26
27
28
4m
4m
4r
s
s
s
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1c
1d
1d
1d
1e
1f
4m
4m
4n
4o
4p
4q
4r
(R)-binap
54 (R)
86 (R)
94 (R)
80 (R)
85 (R)
91 (R)
88 (R)
83 (R)
86 (R)
91 (R)
87 (R)
76 (R)
79 (R)
76 (R)
60 (R)
60 (R)
NaH
(R)-segphos
(R)-segphos
(R)-segphos
(R)-segphos
(R)-segphos
(R)-segphos
(R)-segphos
(R)-segphos
(R)-segphos
(R)-segphos
(R)-segphos
(R)-segphos
(R)-segphos
(R)-segphos
(R)-tBu-segphos
CsOtBu
CsOtBu
CsOtBu
CsOtBu
CsOtBu
CsOtBu
NaH
4s
4m
4n
4r
4m
4r
4r
4m
4r
CsOtBu
CsOtBu
NaH
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
Pd2(dba)4 (5.0)
CsOtBu
CsOtBu
NaH
CsOtBu
a The reaction was carried out in THF or dioxane (5 mL) for 18 h with 1 (0.50 mmol), 4 (0.65 mmol), and base (0.60 mmol) in the presence of
Pd-precursor (5.0 µmol or 25 µmol) and bisphosphine (1.1 equiv to Pd). b Isolated yield by column chromatography. c Determined by chiral HPLC analysis
(see Supporting Information for details). d The absolute configurations were deduced by the Lowe-Brewster rule (ref 12). e With 2.50 mmol of 4o (ref 8k).
conditions. The bulkiness of the silyl substituents in 1 showed
virtually no influence on the enantioselectivity (entries
14-15, 19 vs 21-23). The Me3Si-substrate 1a generally gave
the allenylsilanes in higher yields than 1b and 1c. The present
Pd-catalyzed reaction is tolerant of additional functional
groups, and a proelectrophile moiety (dimethyl acetal) could
be installed in the allenylsilanes by the use of 4r and 4s
(entries 19, 20, 23, 25, 26, and 28). The Pd-catalyzed
reactions of the 3-alkyl-2-bromo-1-trimethylsilyldienes 1d-f
were less enantioselective than those of 1a, and the corre-
sponding 3,3-dialkylallenylsilanes were obtained in up to
79% ee (entries 24-28). Note that catalytic asymmetric
synthesis of 3,3-disubstituted allenylsilanes is realized for
the first time in this study. All the allenylsilanes obtained
here are levorotatory, and their absolute configurations are
deduced to be (R) by the Lowe-Brewster rule.13,14
The obtained (R)-5an of 94% ee was allowed to react with
propanal dimethyl acetal at -78 °C in the presence of TiCl4 to
give 73% of homopropargyl methyl ethers 7an, which consists
of (S,S)- and (S,R)-diastereomers in a ratio of 4:1 (Scheme 3,
Scheme 3
(13) (a) Lowe, G. Chem. Commun. 1965, 411. (b) Brewster, J. H. Top.
Stereochem. 1967, 2, 1.
5738
Org. Lett., Vol. 12, No. 24, 2010

top).15-17 The absolute configurations of the propargyl carbons
in 7an were deduced to be (S) based on the known anti
stereochemistry in the SE2′ reactions of allenylsilanes.15,16 Their
enantiopurities, determined by chiral HPLC analysis, were
found to be 94% ee for both diastereomers, demonstrating
that the axial chirality of the allenylsilane was completely
transferred to the propargylic stereogenic centers in 7an in
the SE2′ reaction with the acetal. In the same way, a treatment
of (R)-5an (94% ee) with Selectfluor, an electrophilic
fluorinating reagent, in acetonitrile at 23 °C afforded the
levorotatory chiral propargylic fluoride 8an in 94% ee in
77% yield with retention of the original enantiomeric purity
in 5an (Scheme 3, bottom). The absolute configuration of
(-)-8an was deduced to be (S) based on the stereospecific
anti SE2′ mechanism of the desilylative fluorination.17
Protodesilylation of the 3,3-dialkylallenylsilanes took place
via the anti SE2′ pathway and resulted in the formation of a
propargylic tertiary stereogenic center in the products with
axial-to-central chirality transfer (Scheme 4).18 The reactions
Scheme 5
10as (82% ee) and 11as (83% ee) were prepared in 86%
yield in a 1:6 molar ratio from (R)-5as (83% ee) in the same
way with the nearly complete transfer of chirality. The
absolute configurations of the propargyl carbons in 10 and
11 were assigned to be (S) as in the case of 7an.13 The
relative configurations of the adjacent stereogenic centers
1
were determined by H NMR NOE experiments, and thus
the major enantiomers in the cyclized products were (S,S)-
10 and (S,R)-11 as depicted in Scheme 5.
Scheme 4
The reaction sequence could be applied to the asymmetric
construction of quaternary chiral centers bonded to four carbon
substituents.19 The axial chirality of the trisubstituted allenyl-
silanes (R)-5dr (76% ee) and (R)-5fr (60% ee), which were
prepared by the Pd-catalyzed asymmetric reaction of 1d and
1f, was transferred to the quaternary carbons in 10dr, 11dr,
10fr, and 11fr by the TiCl4-promoted intramolecular SE2′
reaction with complete retention of the enantiomeric purity in
(R)-5dr and (R)-5fr (Scheme 5). These results demonstrated
that, with a proper R substituent in 1, the asymmetric reaction
shown in Table 1 could be an alternative to catalytic asymmetric
synthesis of all-carbon quaternary centers.
In summary, we have developed a novel route to axially
chiral allenylsilanes by asymmetric catalysis. The axial
chirality in the allenylsilanes, which were prepared with high
enantioselectivity (up to 94% ee), was transferred to tertiary
and quaternary stereogenic centers without loss of their
enantiomeric purity.
of the optically active allenylsilanes (R)-5dm (76% ee) or
(R)-5em (60% ee) with BF3·CH3CO2H in dichloromethane
gave the corresponding propargyl compounds (R)-9dm (74%
ee, 61% yield) or (R)-9em (59% ee, 60% yield), respectively.
Although a slight decrease in enantiopurity was detected in
the protodesilylation, the axial-to-central transfer of chirality
was still greater than 97%. The chirality transfer in protode-
silylation has been unique and could be realized by the use
of the 3,3-disubstituted allenylsilanes.
Treatment of the acetal-tethered allenylsilane (R)-5ar of
88% ee with TiCl4 promoted cyclization via the SE2′
pathway. The five-membered carbocycles were obtained in
71% yield as a mixture of two diastereomers 10ar and 11ar
in a 1:2 molar ratio. Both diastereomers retained the
enantiopurity of (R)-5ar within experimental error (Scheme
5). The highly enantioenriched six-membered carbocycles
Acknowledgment. A part of this work was supported by
a Grant-in-Aid for Scientific Research on Priority Areas
“Advanced Molecular Transformations of Carbon Resources”
from MEXT, Japan, and NOASTEC (to M.O.).
(14) Previously, the validity of this deduction was confirmed by the
asymmetric total synthesis of a naturally occurring allene of known
configuration utilizing the Pd-catalyzed reaction as a key step (ref 8k).
(15) (a) Buckle, M. J. C.; Fleming, I. Tetrahedron Lett. 1993, 34, 2383.
(b) Buckle, M. J. C.; Fleming, I.; Gil, S.; Pang, K. L. C. Org. Biomol.
Supporting Information Available: Detailed experimen-
tal procedures and compound characterization data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Chem. 2004, 2, 749
.
(16) The relative configurations of the homopropargyl carbons in 7an
were tentatively assigned as in Scheme 3 by comparison of their 1H NMR
data with those of analogous compounds of which configurations are known;
OL102554A
see: Favre, E.; Gaudemar, M. J. Organomet. Chem. 1975, 92, 17
(17) (a) Carroll, L.; Pacheco, M. C.; Garcia, L.; Gouverneur, V. Chem.
Commun. 2006, 4113. (b) Carroll, L.; McCullough, S.; Rees, T.; Claridge,
.
(19) (a) Corey, E. J.; Guzman-Perez, A. Angew. Chem., Int. Ed. 1998,
37, 388. (b) Douglas, C. J.; Overman, L. E. Proc. Natl. Acad. Sci. U.S.A.
2004, 101, 5363. (c) Trost, B. M.; Jiang, C. Synthesis 2006, 369. (d)
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