Palladium-catalyzed asymmetric silaboration of allenes

Article abstract of DOI:10.1021/ja063934h

An enantioselective silaboration of allenes was achieved using an achiral silylborane in the presence of a palladium catalyst bearing a chiral monodentate phosphine ligand. (R)-2-Bis(3,5-dimethylphenyl)phosphino-1,1-binaphthyl gave the highest enantiosele

Full text of DOI:10.1021/ja063934h

Published on Web 09/29/2006
Palladium-Catalyzed Asymmetric Silaboration of Allenes
Toshimichi Ohmura, Hiroki Taniguchi, and Michinori Suginome*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Katsura, Kyoto 615-8510, Japan
Received June 5, 2006; E-mail: suginome@sbchem.kyoto-u.ac.jp
Table 1. Screening of Chiral Ligands for Palladium-Catalyzed
Silaboration of 2aa
The synthesis of stereodefined organometallic compounds has
attracted much attention because of the increasing demand for
stereoselective construction of complex organic molecules. Allyl-
silanes are recognized as one of the most important classes of
organometallic reagents in synthetic organic chemistry.1 Their
unique features involve stability, reactivity, and functional group
tolerability, which make these particular compounds synthetically
attractive. Further interest has been focused on the synthetic use
of optically active allylsilanes, which enable asymmetric allylations
via highly efficient chirality transfer.2 To take advantage of these
features of allylsilanes, much effort has been devoted to catalytic
asymmetric synthesis of highly enantioenriched allylsilanes via
asymmetric Grignard cross-coupling,3 hydrosilylation of 1,3-dienes,4
and allylic substitution.5 However, these reactions are limited to
the synthesis of simple allylsilanes that do not carry functional
groups at their CdC bonds.6
We have developed Pd-catalyzed silaboration of terminal allenes
for the efficient synthesis of the allylsilanes that bear boryl groups
at their â-positions.7,8 We have reported that highly diastereose-
lective formation of optically active â-borylallylsilanes was achieved
under double asymmetric induction conditions using an optically
active ligand and a chiral auxiliary on the boron atom of the
silylborane.9 The use of the pinanedioxy group as the chiral auxiliary
was critical in obtaining high stereoselectivity. Our efforts were
then focused on exploring a catalytic asymmetric synthesis that does
not rely on the stoichiometric use of a chiral auxiliary. Herein, we
describe an asymmetric silaboration of terminal allenes using an
achiral silylborane with a Pd catalyst having an optically active
monodentate phosphine ligand.
entry
ligand
yield (%)b
ee (%)c
1
2
3
(+)-NMDPP
84
85
99
84
96
98
88
99
93
98
67f
99
99
98
85
99
83
37d
16
63d
51
12
74
77
53
77
78
13
84
84
81
53
59
47
(S)-MONOPHOS
(R,R)-4
(S)-QUINAP
(S)-MOP
(R)-5a (Ar ) Ph)
4e
5e
6e
7
(R)-5b (Ar ) 4-MeOC6H4)
(R)-5c (Ar ) 4-CF3C6H4)
(R)-5d (Ar ) 4-MeC6H4)
(R)-5e (Ar ) 3-MeC6H4)
(R)-5f (Ar ) 2-MeC6H4)
(R)-5g (Ar ) 3,5-Me2C6H3)
(R)-5h [Ar ) 3,5-(MeOCH2)2C6H3]
(R)-5i (Ar ) 3,5-Me2-4-MeOC6H2)
(R)-5j [Ar ) 3,5-(CF3)2C6H3]
(R)-5k (Ar ) 3,5-tBu2-4-MeOC6H2)
(R)-5m [Ar ) 3,5-(Me3Si)2C6H3]
8
9
10
11
12
13
14
15
16
17
a 1 (0.40 mmol), 2a (0.48 mmol), Pd(dba)2 (8.0 µmol), and ligand (9.6
µmol) were stirred in toluene (0.2 mL) at room temperature for 3-15 h
unless otherwise noted. b Isolated yield. c Determined by HPLC analysis
with a chiral stationary phase column. d (R)-3a was formed as major isomer.
e CpPd(η3-allyl) (1.0 mol %) and ligand (1.1 mol %) were used. f Yield
after 140 h.
Silaboration of cyclohexylallene (2a) with Me2PhSiB(pin) (1a)
was examined in the presence of Pd catalysts bearing chiral
monodentate phosphorus ligands (entries 1-6 in Table 1). The
reactions proceeded smoothly at room temperature, giving 3a in
84-99% yields with varying enantiomeric excesses (ees) (12-74%
ee). Whereas (+)-NMDPP, (S)-MONOPHOS, and (S)-MOP gave
poor selectivities (entries 1, 2, and 5), (R,R)-410 and (S)-QUINAP
showed moderate selectivities (63 and 51% ee, entries 3 and 4).
The highest selectivity was attained with (R)-5a (Ar ) Ph, 74%
ee, entry 6), suggesting that 1,1′-binaphth-2-yl was the chiral group
of choice in the asymmetric silaboration of allenes.
59% when phosphines having CF3, t-Bu, and Me3Si groups at their
3,5-positions on the phenyl group were used (entries 15-17).
We then modified the diarylphosphino group on the 1,1′-
binaphthyl skeleton (entries 7-17 in Table 1).11 A derivative with
a p-MeO group showed an appreciable increase in ee (entry 7),
whereas a p-CF3 derivative resulted in a significant drop in ee (entry
8). Among the tolyl derivatives examined, p- and m-tolyl derivatives
(R)-5d and (R)-5e showed higher ees than the parent (R)-5a ligand
(entries 9 and 10), although almost no enantioselectivity was
attained with the o-tolyl derivative (R)-5f (entry 11). We observed
the highest selectivity with the 3,5-dimethylphenyl derivative (R)-
5g, which afforded 3a with 84% ee at room temperature (entry
12). The same levels of enantioselectivity were attained with analo-
gous phosphines (R)-5h and (R)-5i having 3,5-substituents (entries
13 and 14). It is interesting to note that the ees dropped to 47-
Figure 1. Silylboranes used in the asymmetric silaboration.
Silylboranes bearing different substituents on the boron and
silicon atoms were applied to the asymmetric silaboration of 2a
with the Pd/(R)-5g catalyst under the same conditions as those
described in Table 1 (Figure 1). Silylboranes derived from catechol
(1b, 27% ee) and ethylene glycol (1c, 40% ee) gave much lower
selectivity than 1a. Enantioselectivity was found to be improved
significantly by use of the more sterically hindered diphenylmethyl-
silyl derivative (1d), affording the corresponding â-borylallylsilane
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10.1021/ja063934h CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Asymmetric Silaboration of Various Allenesa
The ees of 8 and 10 were found to be 93 and 92% ee, respectively,
indicating the cyclization steps proceeded with perfect chirality
transfer.
entry
R
product
yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
cyclo-C6H11 (2a)
Me2PhSiOCH2C(Me)2 (2b)
OHCC(Me)2 (2c)
Ph (2d)
4-MeC6H4 (2e)
2-MeC6H4 (2f)
1-naphthyl (2g)
Me (2h)
6a
6b
6c
6d
6e
6f
6g
6h
6i
90
97
95
93
94
93
86
96
91
91
91d
92d
93d
88
89
90
90
80d
82d
90
9
Ph(CH2)2 (2i)
AcOCH2C(Me)2 (2j)
10e
3j
a 1d (0.40 mmol), 2 (0.48 mmol), Pd(dba)2 (8.0 µmol), and (R)-5g (9.6
µmol) were stirred in toluene (0.2 mL) at 0 °C for 22-90 h unless otherwise
noted. b Isolated yield. c Determined by HPLC analysis with a chiral
stationary phase column. d Determined after transforming 6 into corre-
sponding â-hydroxysilane. See Supporting Information. e Reaction with 1a
at -10 °C in the presence of 4.0 mol % of catalyst.
In conclusion, we have demonstrated Pd-catalyzed asymmetric
silaboration of terminal allenes with an achiral silylborane, affording
synthetically useful â-borylallylsilanes with high ees.
Acknowledgment. This work is supported in part by Grant-
in-Aid for Young Scientists (B) from MEXT and the Sasagawa
Scientific Research Grant from The Japan Science Society (to T.O.).
Supporting Information Available: Experimental details and
characterization data of the products. This material is available free of
charge via Internet at http://pubs.acs.org.
6a with 89% ee. The more sterically demanding triphenylsilyl de-
rivative (1e) did not react at all under the same reaction conditions.
Under the optimized conditions using the Pd/(R)-5g catalyst and
the silylborane 1d, various allenes were subjected to enantioselective
silaboration at 0 °C (Table 2). The enantioselectivity was improved
to 91% ee in the reaction of 2a (entry 1). The silaboration of allenes
having sec- and tert-alkyl groups gave the corresponding products
6 with high ees (91-93% ee, entries 1-3). The same level of
enantioinduction was observed in the reaction of phenylallene (2d)
and its derivatives 2e-g (88-90% ee, entries 4-7). On the other
hand, methyl or primary alkyl-substituted allenes gave the corre-
sponding â-borylallylsilanes with lower ees (entries 8 and 9). These
results indicate that the enantioselectivity largely depends on the
bulkiness of the substituents of the allenes. An additional example
is shown by the reaction of 1a with bulky 2j, providing 3j with
90% ee (entry 10).
To examine the chemoselectivity of the silaboration, a pair of
asymmetric silaborations of enantiomeric allenynes (R)- and (S)-
2k was carried out at 20 °C (eqs 1 and 2). The additions of 1d
took place exclusively at the allene moiety rather than the carbon-
carbon triple bond to give 6k in high yields.12 Diastereomeric ratios
of the products were slightly affected by the stereochemistry of
the original stereogenic carbon centers (94:6 for the matched pair,
88:12 for the mismatched pair).
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The allylsilane aldehyde 6c (93% ee) cyclized via oxonium
intermediate formation in the presence of Me3SiOTf with Me3-
SiOBn, giving boryl-substituted cyclopentene 7 (eq 3).13 On the
other hand, reaction of 6b (92% ee) with PhCHO afforded seven-
membered cyclic alkenylborane 9 (eq 4).14 Suzuki-Miyaura cross-
coupling of the alkenylborane products (7 and 9) with ethyl
4-bromobenzoate under the conditions using 2-dicyclohexylphos-
phino-2′,6′-dimethoxybiphenyl (S-PHOS)15 gave the corresponding
alkenylarenes (8 and 10) in 58 and 71% total yield, respectively.
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