Iridium(III)-catalyzed enantioselective Si-H bond insertion and formation of an enantioenriched silicon center

Article abstract of DOI:10.1021/ja100833h

Iridium(III)-salen complexes were found to efficiently catalyze enantioselective carbene Si-H bond insertion. Highly enantioselective Si-H insertion with α-alkyl- α-diazoacetates (≥97% ee) was achieved for the first time by using the iridium complex 4 {(aR,S), Ar = 4-TBDPSC ₆H₄} bearing a concave-shaped salen ligand as the catalyst. Formation of a chiral silicon center was also achieved for the first time by the Si-H insertion into prochiral silanes: the reactions between prochiral silanes and tert-butyl α-diazopropionate in the presence of complex 5 {(aR,S), Ar = Ph} proceeded with high stereoselectivity (84-99% de, 94→99% ee). The Si-H insertion into trisubstitued silanes with α-aryl- α-diazoacetates proceeded with almost complete enantioselectivity (≥99% ee) by using complex 1 {(aR,R), Ar = Ph} as catalyst.

Full text of DOI:10.1021/ja100833h

Published on Web 03/16/2010
Iridium(III)-Catalyzed Enantioselective Si-H Bond Insertion and Formation of
an Enantioenriched Silicon Center
Yoichi Yasutomi, Hidehiro Suematsu, and Tsutomu Katsuki*
Department of Chemistry, Faculty of Science, Graduate School, Kyushu UniVersity, Hakozaki, Higashi-ku,
Fukuoka 812-8581, Japan
Received October 14, 2009; E-mail: katsuscc@chem.kyushu-univ.jp
Recently, synthetic application of the compounds possessing silyl
group(s) has been rapidly increasing,1 and the development of their
efficient synthesis is a topic of interest. Although various methods
are available for the preparation of silyl compounds, asymmetric
Si-H functionalization is a direct method for the preparation of
chiral compounds possessing silyl group(s). In particular, asym-
metric Si-H insertion of a metal carbenoid derived from R-dia-
zocarbonyl compounds is a potentially attractive route to produce
chiral R-silylcarbonyl compounds.2 To date, copper(I) and rhod-
ium(II) complexes have been widely used as the catalyst for this
type of reaction. In 1996, Doyle and Moody reported a seminal
study on enantioselective Si-H bond insertion with chiral dirhod-
ium(II) catalysts in the presence of R-phenyl-R-diazoacetate, albeit
with moderate selectivity.3,4 Subsequent to this, Davies and co-
workers reported that Si-H insertion of R-alkenyl-R-diazoacetate
catalyzed by the rhodium(II) (S)-N-[p-(dodecylphenyl)sulfonyl-
]prolinate complex proceeded with good to high enantioselectivity
(77-95% ee).5 Panek and Jacobsen reported that a chiral copper-
Schiff base complex catalyzed Si-H insertion using R-phenyl-R-
diazoacetate with moderate to good yield and enantioselectivity (up
to 88% ee).6 Ge and Corey reported that a N-nonafluorobutane-
sulfonylproline rhodium(II) complex was a good catalyst for Si-H
insertion using R-diazocyclohexenone derivatives.7 Moreover, Zhou
and co-workers reported that a copper/chiral spiro-diimine catalyst
showed high enantioselectivity (93-99% ee) in the Si-H insertion
using ArC(N2)CO2Me, while the reaction with MeC(N2)CO2Et had
modest enantioselectivity and a moderate yield.8 Still, the reaction
using RC(N2)CO2Et (R ) acyclic alkyl group) that readily
undergoes ꢀ-hydride elimination2a,9 is a challenging research
target.8,10
in bond length and bond energy between C-H and Si-H bonds,1d
we expected that the complexes having a higher molecular
recognition ability would serve as an efficient catalyst for this
reaction. Thus, we further examined the reaction at -78 °C with
iridium complexes (3 and 4) that bear a concave-shaped salen
ligand.13 While the enantioselectivity was moderately increased by
using 3 (entry 4), significant improvement to 97% ee was achieved
by using 4 (entry 5). The reaction of tert-butyl R-diazopropionate
proceeded with high enantioselectivity in a good yield (entry 6).
The insertion to o-tolyldimethylsilane also proceeded equally
efficiently (entry 7). Moreover, the reaction of tert-butyl R-diaz-
obutanoate proceeded with a high enantioselectivity of 97%, though
the yield was moderate because of the formation of tert-butyl (Z)-
2-butenoate (entry 8).14
Table 1. Asymmetric Si-H Insertion with R-Alkyl-R-Diazoacetatea
We have recently discovered that iridium(III)-salen complexes
(Figure 1) show potent carbene-transfer catalysis. The iridium-salen
complex 1 catalyzes cyclopropanation of a variety of olefins using
an R-diazoacetate with high cis- and enantioselectivity,11a,b while
the reactions using a vinyldiazolactone show high trans- and
enantioselectivity.11c Moreover, complexes 1 and 2 catalyze highly
enantioselective C-H insertion using either R-aryl- or R-methyl-
substituted R-diazoacetates.12 These results suggested that an
appropriate iridium-salen complex could regulate the conformation
of the carbenoid intermediate, so as to prevent the competitive
ꢀ-hydride elimination. Herein, we communicate the first iridium
catalyzed asymmetric carbenoid insertion into a Si-H bond and
the formation of a highly enantioenriched silicon center.
entry
cat.
prod.
Ar
R1
R2
% yieldb
% eec
1d
2f
3
4
5
6
7
8
1
1
1
3
4
4
4
4
6a
6a
6a
6a
6a
6b
6c
6d
Ph
Ph
Ph
Ph
Ph
Ph
o-tol
Ph
Me
Me
Me
Me
Me
Me
Me
Et
Et
Et
Et
Et
Et
tBu
Et
71(86)
83(96)
89(98)
85
85
88
41e
37e
51e
73e
97
97
98
97
85
47
tBu
a Reaction was carried out on a 0.3 mmol scale. b Isolated yield. The
value in the parentheses is the yield determined by 1H NMR analysis
with 1-bromonaphthalene as an internal standard. c Determined by
HPLC analysis. d Run at 0 °C. e The major product is an enantiomer of
the major product obtained with 4 as catalyst. f Run at -30 °C.
We initially examined the reaction of dimethylphenylsilane and
ethyl R-diazopropionate in the presence of complex 1 at 0 °C in
dichloromethane (Table 1). The desired Si-H insertion occurred
preferentially, and elimination proceeded slowly under the condi-
tions (entry 1). The undesired ꢀ-hydride elimination was signifi-
cantly suppressed at lower temperature, but the enantioselectivity
was not much improved (entries 2 and 3). Considering the difference
Although Si-H insertion reactions shown in Table 1 proceeded
with excellent enantioselectivity, chirality was induced only on the
carbon atom of the resultant Si-C bond. However, if the silane
compound is prochiral, chirality should be induced also on the
silicon atom15 and provide a pair of diastereomers. To explore this
possibility, we examined the reaction between prochiral (1-
9
4510 J. AM. CHEM. SOC. 2010, 132, 4510–4511
10.1021/ja100833h  2010 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Construction of Chiral Silicon Atoma
In conclusion, we were able to develop a highly enantioselective
Si-H carbenoid insertion reaction into trisubstituted silanes using
R-alkyl-R-diazoacetates (g97% ee) or R-aryl-R-diazoacetates (g99%)
in the presence of an appropriate iridium complex as catalyst. To
our knowledge, this is the first example of highly enantioselective
Si-H insertion reactions using R-alkyl-R-diazoacetates. Moreover,
Si-H insertion into a prochiral silane that provides a new direct
method for forming a stereogenic silicon center was for the first
time achieved with high enantioselectivity.
entry
cat.
prod.
R
% yieldb
% dec
% eed
1
2
3
4
5
4
5
5
5
5
6e
6e
6f
6g
6h
1-Np
1-Np
2,6-Xy
iPr
73
86
69
83
74
85
92
99
84
86
94
99
>99
94
Cy
95
a Reaction was carried on
a
0.3 mmol scale. b Isolated yield.
c Determined by 1H NMR analysis. d Determined by HPLC analysis.
1-Np ) 1-Naphthyl; 2,6-Xy ) 2,6-Xylyl; Cy ) Cyclohexyl.
Acknowledgment. Financial support from Specially Promoted
Research 18002011 and the Global COE Program, ‘Science for
Future Molecular Systems’ from MEXT, Japan is gratefully
acknowledged. H.S. is grateful for the JSPS Research Fellowships
for Young Scientists.
Table 3. Asymmetric Si-H Insertion with R-Aryl-R-Diazoacetatesa
Supporting Information Available: Experimental procedures,
spectral data for Si-H insertion products, and HPLC conditions. This
material is available free of charge via the Internet at http://pubs.acs.org.
% yieldb c
% eed
,
entry
cat.
prod.
R1
R2
1
2
3
4
5
6
7f
8
1
5
1
1
1
1
1
1
6i
6i
6j
6k
6l
6m
6n
6o
PhMe2
PhMe2
Et3
Et3
Et3
Et3
Et3
Et3
Ph
Ph
93
91
92
95
95
97
94
97
>99
50e
99
>99
>99
>99
>99
>99
2-MeOC6H4
2-ClC6H4
3-MeOC6H4
3-ClC6H4
4-MeOC6H4
4-ClC6H4
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a Reaction was carried on
a
0.3 mmol scale. b Isolated yield.
c Formation of a trace amount carbene dimer (<5%) was detected by 1H
NMR analysis. d Determined by HPLC analysis. e The major product
obtained with 5 is an enantiomer of the major product obtained with 1
as catalyst. f 3 equiv of silane were used.
naphthyl)phenylsilane and tert-butyl R-diazopropionate (Table 2,
entry 1). The reaction using 4 as catalyst proceeded with good
diastereo- and enantioselectivity.16 To our delight and surprise,
superior selectivity was obtained when the complex 5 was used as
the catalyst, and one of four possible diastereomers was produced
exclusively (entry 2).17 The reactions between other prochiral silanes
including alkyl aryl silanes also proceeded with high diastereo- and
enantioselectivity (entries 3-5).18
As discussed in the beginning of this article, several highly
enantioselective Si-H insertions into trisubstituted silanes with
R-aryl-R-diazoacetates have been reported.5–8 We also examined
the insertion reactions between trisubstituted silanes and R-aryl-
R-diazoacetates using complex 1 or 5 as catalyst (Table 3). Since
ꢀ-hydride elimination cannot occur in these reactions, the reactions
were carried out at -30 °C with 1.2 equiv of the silanes. While
the reaction with 5 as the catalyst was modestly enantioselective
(entry 2), all the reactions with 1 were found to proceed with almost
complete enantioselectivity (g99%) and good yields. The formation
of a trace amount of the carbene dimer (<5%) was observed under
the conditions.
The kinetic isotope effect for the present reaction between
triethylsilane and methyl R-phenyl-R-diazoacetate was determined
to be 1.6 in a competitive experiment. This value is consistent with
the reported value (kH/kD ) 1.5) for rhodium-carbenoid Si-H
insertion of dimethylphenylsilane19 and supports the notion that
the present reaction is a carbenoid Si-H insertion.
(11) (a) Kanchiku, S.; Suematsu, H.; Matsumoto, K.; Uchida, T.; Katsuki, T.
Angew. Chem., Int. Ed. 2007, 46, 3889. (b) Suematsu, H.; Kanchiku, S.;
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(13) Hamada, T.; Irie, R.; Mihara, J.; Hamachi, K.; Katsuki, T. Tetrahedron
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(14) The yield of tert-butyl (Z)-2-butenoate was determined to be 35% by 1H
NMR analysis with 1-bromonaphthalene as an internal standard, and its
stereochemistry was assigned to be Z based on the coupling constant (J )
11.5 Hz) [the coupling constant of the E-isomer (J ) 15.5 Hz)].
(15) For an example of enantiotopic selective alcoholysis of prochiral dihy-
drosilane with a chiral alcohol, see:Schmidt, D. R.; O’Malley, S. J.;
Leighton, J. L. J. Am. Chem. Soc. 2003, 125, 1190.
(16) For the determination of stereoselectivity of the reaction, see the Supporting
Information.
(17) The reaction with 1 was modestly stereoselective (10% de, 79% ee).
(18) The reaction with PhC(N2)CO2Me did not give the desired product because
it decomposed on chromatography to give PhCH2CO2Me.
(19) A concerted electrophilic metal-carbenoid insertion mechanism has been
proposed based on kinetic studies:Landais, Y.; Parra-Rapado, L.; Planchenault,
D.; Weber, V. Tetrahedron Lett. 1997, 38, 229. (b) A similar value (kH/kD
) 1.29, at 0 °C) has been reported: Dakin, L. A.; Ong, J. S.; Panek, P. C.;
Staples, R. J.; Stavropoulos, P. Organometallics 2000, 19, 2896.
JA100833H
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