Enantioface-selective palladium-catalyzed silaboration of allenes via double asymmetric induction

Article abstract of DOI:10.1021/ja0368958

Enantioenriched β-borylallylsilanes were synthesized by palladium-catalyzed enantioface-selective addition of the silicon-boron bond to terminal allenes using a palladium catalyst possessing a chiral monodentate phosphine ligand. Use of a silylborane bear

Full text of DOI:10.1021/ja0368958

Published on Web 08/20/2003
Enantioface-Selective Palladium-Catalyzed Silaboration of Allenes via Double
Asymmetric Induction
Michinori Suginome,* Toshimichi Ohmura, Yoshihiro Miyake, Shin’ichirou Mitani, Yoshihiko Ito, and
Masahiro Murakami*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto UniVersity,
and PRESTO, Japan Science and Technology Corporation (JST), Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
Received June 25, 2003; E-mail: suginome@sbchem.kyoto-u.ac.jp
Table 1. Reaction of 7a with Silylboranes Bearing Chiral
Transition metal-catalyzed additions of metal-containing σ-bonds
across carbon-carbon multiple bonds have attracted much interest
Auxiliaries in the Presence of a Palladium-PPh3 Catalysta
in synthetic organic chemistry, because these reactions provide syn-
thetically useful organometallic compounds, which are otherwise
difficult to prepare. In particular, enantioselective versions of the
σ-bond addition processes have been gaining considerable attention
entry
silylborane
product (% yield)b
drc
%de
with the increasing demand for chiral organometallic reagents for
asymmetric organic synthesis. Studies on asymmetric hydrometa-
lation reactions have been motivated not only by their synthetic
potential, but also by the ease of M-H bond activation, which
allows the extensive survey of a variety of catalyst systems.1 On
the other hand, only a few asymmetric bis-metalations have been
reported to date.2-5 Although the highly enantioselective bis-silyl-
ation of R,â-unsaturated ketones using a Pd-BINAP catalyst has
been reported,2 asymmetric bis-metalations of unactivated CdC
bonds have only achieved moderate enantioface selectivities.3,4 It
is likely that the difficulty associated with the activation of the
intermetallic σ-bonds has hampered the development of asymmetric
bis-metalations.
1
2
3
4
5
(R,R)-2
(S,S)-3
(S,S)-4
(R,R)-5
(1R)-6
8 (86)
9 (94)
10 (90)
11 (85)
12a (96)
51:49
52:48
68:32
58:42
81:19
2
4
36
16
62
a A mixture of silylborane 2-6 (0.5 mmol) and 7a (0.6 mmol) in toluene
(0.25 mL) was stirred at room temperature in the presence of Cp(allyl)Pd
(2 mol %) with PPh3 (2.4 mol %). b NMR yield (anisole as an internal
standard). c Diastereomeric ratio determined by HPLC or 1H NMR (400
MHz).
diastereoselectivities (entries 1-4), silylborane 6, derived from
pinanediol, gave an isomer ratio greater than 4:1 (entry 5).
Our recent efforts have focused on transition metal-catalyzed
reactions of silylboranes with unsaturated organic molecules.1,6 We
were particularly interested in silaboration of terminal allenes to
provide synthetically useful â-borylallylsilanes via regioselective
addition of the Si-B bond to the internal CdC bond.7,8 Herein,
we describe enantioface-selective silaboration of terminal allenes
as the first asymmetric addition of a metal-containing σ-bond across
an allene CdC bond (eq 1).
We then examined chiral monodentate phosphine ligands 13-
15 in the reaction of 6 with 7a (Table 2). To evaluate matched and
mismatched chiral ligand-auxiliary combinations, we carried out
the reactions using both enantiomers of 6. When the phosphora-
midite ligand 13 was used with either enantiomer (1S)-6 or (1R)-
6,13 the diastereoselectivity was considerably lower than that
occurring with achiral PPh3. The diastereoselectivity was signifi-
cantly improved when the ligand was switched to the ferrocene
derivatives 14.14 The matched pairs (14a/(1R)-6 and 14b/(1R)-6)
afforded 12a with 81% de. Although 14a and 14b showed the same
diastereoselectivity in the matched cases, the results for the
mismatched pairs (-1% for 14a/(1S)-6 vs -34% for 14b/(1S)-6)
indicated that the 3,5-bis(trifluoromethyl)phenyl derivative (14b)
effects larger stereochemical induction than does 14a. Finally, MOP
ligands 15a and 15b were tested.15 The 2′-methoxy derivative 15a
gave almost the same diastereoselectivity as that obtained by the
PPh3 catalyst system, indicating that the ligands had no strong
influence on the enantioface selection. To our surprise, however, a
remarkably high enantioface selectivity (89% de) was recorded with
The original conditions for the silaboration of allenes used a
ligand/palladium ratio greater than 2:1. These require high temper-
atures to drive the reactions to proceed.7,8 We found that the use
of (η5-cyclopentadienyl)(π-allyl)palladium [Cp(allyl)Pd] with ter-
tiary phosphines in a 1:1 ratio allowed us to perform the silaboration
of allenes at room temperature.9,10 Using the new monophosphine-
palladium (1:1) catalyst system, we initially examined the reactions
of silylboranes bearing a series of chiral auxiliaries on the boron
atom.
Silylboranes 2-6 bearing diol-derived chiral auxiliaries were
prepared11 and subjected to reactions with 5-phenyl-1,2-pentadiene
(7a) in the presence of a catalyst prepared from Cp(allyl)Pd with
PPh3 (1:1.2).12 All of the reactions proceeded at room temperature
to give high yields, affording silaboration products 8-12 with
perfect regioselectivity (Table 1). The diastereomeric ratios,
however, varied significantly with the chiral auxiliaries. Although
silylboranes 2-5, derived from acyclic diols, resulted in poor
9
11174
J. AM. CHEM. SOC. 2003, 125, 11174-11175
10.1021/ja0368958 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Screening of Chiral Ligands for Palladium-Catalyzed
Silaboration of 7a with (1R)-6 and (1S)-6
The enantioenriched â-borylallylsilane 12c (88% de), obtained
from allene 7c, bearing a terminal siloxy group, was subjected to
a Marko´-type cyclization (eq 2).7c,16 The seven-membered ring
formation with cyclohexanecarboxaldehyde took place effectively,
giving an enantioenriched oxacyclic alkenylborane 16. Oxidation
of 16 afforded cyclic ketone 17 with 88% ee, indicating that the
stereochemical course of the reaction of 12c relies solely on the
silicon-bound chiral center via flawless chirality transfer, with no
influence of the boron-bound chiral auxiliary.
reaction of (1S)-6
(1S,R):(1S,S)c
reaction of (1R)-6
(1R,S):(1R,R)c
c
c
ligand
% yieldb
%de
% yieldb
%de
13
95
98
99
73
96
71:29
50:50
33:67
78:22
37:63
43
-1
88
91
95
88
99d
62:38
90:10
90:10
83:17
94:6
24
81
81
65
89
14a
14b
15a
15b
-34
55
-27
a Silylborane 6, 7a (1.2 equiv), Cp(allyl)Pd (1 mol %), and 13-15 (1.2
mol %) were reacted in toluene at room temperature. b NMR yield unless
otherwise noted. c Diastereomeric ratio determined by HPLC. d Isolated
yield.
In summary, we have demonstrated enantioface selective addition
of silylboranes across an internal CdC bond of allenes, using a
new palladium catalyst system. Further improvement of the catalytic
system, as well as the synthetic application of the new chiral
allylsilanes, is now being undertaken in this laboratory.
Table 3. Silaboration of Allenes with (1R)-6 in the Presence of a
15b-Palladium Catalysta
Supporting Information Available: Detailed experimental pro-
cedures and spectral data for the new compounds (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
c
entry
allene
7b (R ) CH3)
% yieldb
ratio
% de
1
2
3
4
5
6
92
91
95
95
96
92
93:7
94:6
98:2
96:4
95:5
96:4
86
88
96
92
91
92
7c (R ) CH2CH2OSiMe2Ph)
7d (R ) c-Hex)
References
(1) Nishiyama, H.; Itoh, K. In Catalytic Asymmetric Synthesis, 2nd ed.; Ojima,
I., Ed.; Wiley-VCH: New York, 2000; p 111.
7e (R ) Ph)
7f (R ) p-MeOC6H4)
7g (R ) p-CF3C6H4)
(2) Hayashi, T.; Matsumoto, Y.; Ito, Y. J. Am. Chem. Soc. 1988, 110, 3692.
(3) Suginome, M.; Nakamura, H.; Ito, Y. Tetrahedron Lett. 1997, 38, 555.
(4) Marder, T. B.; Norman, N. C.; Rice, C. R. Tetrahedron Lett. 1998, 39,
155. Clegg, W.; Johann, T. R. F.; Marder, T. B.; Norman, N. C.; Orpen,
A. G.; Peakman, T. M.; Quayle, M. J.; Rice, C. R.; Scott, A. J. J. Chem.
Soc., Dalton Trans. 1998, 1431.
(5) For reviews on bis-metalations, see: (a) Beletskaya, I.; Moberg, C. Chem.
ReV. 1999, 99, 3435. (b) Suginome, M.; Ito, Y. Chem. ReV. 2000, 100,
3221.
a Silylborane (1R)-6, allenes 7b-g (1.2 equiv), Cp(allyl)Pd (1.0 mol %),
and 15b (1.2 mol %) were reacted in toluene at room temperature. b Isolated
yield. c Diastereomeric ratio determined by H NMR (500 MHz).
1
15b lacking a substituent at the 2′ position.
(6) For recent examples, see: (a) Suginome, M.; Matsuda, T.; Ito, Y. J. Am.
Chem. Soc. 2000, 122, 11015. (b) Suginome, M.; Matsuda, T.; Yoshimoto,
T.; Ito, Y. Organometallics 2002, 21, 1537.
(7) (a) Suginome, M.; Ohmori, Y.; Ito, Y. Synlett 1999, 1567. (b) Suginome,
M.; Ohmori, Y.; Ito, Y. J. Organomet. Chem. 2000, 611, 403. (c)
Suginome, M.; Ohmori, Y.; Ito, Y. J. Am. Chem. Soc. 2001, 123, 4601.
(d) Suginome, M.; Ohmori, Y.; Ito, Y. Chem. Commun. 2001, 1090.
(8) Onozawa, S.-y.; Hatanaka, Y.; Tanaka, M. Chem. Commun. 1999, 1863.
(9) Bidentate phosphine ligands such as BINAP completely failed to promote
the reaction at room temperature.
(10) For the generation of bis(phosphine)palladium complexes from Cp(allyl)-
Pd, see: Bennet, M. A.; Chiraratvatana, C.; Robertson, G. B.; Tooptakong,
U. Organometallics 1988, 7, 1403. Wallow, T. I.; Goodson, F. E.; Novak,
B. M. Organometallics 1996, 15, 3708 and references therein.
(11) Silylboranes 2-6 were prepared by reactions of PhMe2SiBCl(NEt2) with
the corresponding diols (1 equiv) in hexane at room temperature. See:
Buynak, J. D.; Geng, B. Organometallics 1995, 14, 3112.
(12) General procedure for the silaboration: To a mixture of a phosphine ligand
(6 µmol) and Cp(allyl)Pd (5 µmol) in toluene was added an allene (0.6
mmol) at room temperature. Silylborane (0.5 mmol) was then added, and
the resultant mixture was stirred at room temperature for 8-24 h. The
product was isolated by bulb-to-bulb distillation under reduced pressure.
(13) de Vries, A. H. M.; Meetsma, A.; Feringa, B. L. Angew. Chem., Int. Ed.
Engl. 1996, 35, 2374.
(14) Murakami, M.; Minamida, R.; Itami, K.; Sawamura, M.; Ito, Y. Chem.
Commun. 2000, 2293.
(15) (a) Hayashi, T.; Hirate, S.; Kitayama, K.; Tsuji, H.; Torii, A.; Uozumi,
Y. J. Org. Chem. 2001, 66, 1441. (b) Hayashi, T. Acc. Chem. Res. 2000,
33, 354.
(16) (a) Marko´, I. E.; Mekhalia, A. Tetrahedron Lett. 1992, 33, 1799. For related
cyclizations using enantioenriched allylsilanes, see: (b) Huang, H.; Panek,
J. S. J. Am. Chem. Soc. 2000, 122, 9836. (c) Suginome, M.; Iwanami, T.;
Ito, Y. J. Am. Chem. Soc. 2001, 123, 4356.
The optimized reaction conditions using 15b were applied to
the asymmetric silaboration of a series of terminal allenes (Table
3). Diastereomeric excesses comparable to that of 7a were obtained
in the silaboration of terminal allenes 7b and 7c bearing non-
branched alkyl groups (entries 1 and 2). The stereoselectivity
reached 96% in the reaction of allene 7d, bearing a sterically more
demanding alkyl group (entry 3). Under these reaction conditions,
arylallenes 7e-7g also provided the corresponding â-borylallylsi-
lanes in high stereoselectivities (entries 4-6). No marked effect of
the p-substituents of the arylallenes on the diastereoselectivity and
the reaction efficiency was observed. Application of this catalyst
system to the reaction of achiral silylborane 1 with 7a, however,
led to only a moderate enantioselectivity (68% ee), indicating that
the chiral pinanedioxy group on the boron atom plays an important
role in the enantioface discrimination.
JA0368958
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J. AM. CHEM. SOC. VOL. 125, NO. 37, 2003 11175