Palladium-catalyzed asymmetric synthesis of silicon-stereogenic dibenzosiloles via enantioselective C-H bond functionalization

Article abstract of DOI:10.1021/ja302278s

A Pd-catalyzed asymmetric synthesis of Si-stereogenic dibenzosiloles is developed through enantioselective C-H bond functionalization of prochiral 2-(arylsilyl)aryl triflates. High chemo- and enantioselectivities are achieved by employing a Josiphos-type ligand under mild conditions.

Full text of DOI:10.1021/ja302278s

Communication
pubs.acs.org/JACS
Palladium-Catalyzed Asymmetric Synthesis of Silicon-Stereogenic
Dibenzosiloles via Enantioselective C−H Bond Functionalization
Ryo Shintani,* Haruka Otomo, Kensuke Ota, and Tamio Hayashi*
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan
S
* Supporting Information
Table 1. Pd-Catalyzed Synthesis of Dibenzosiloles from 1a:
Optimization
ABSTRACT: A Pd-catalyzed asymmetric synthesis of Si-
stereogenic dibenzosiloles is developed through enantio-
selective C−H bond functionalization of prochiral 2-
(arylsilyl)aryl triflates. High chemo- and enantioselectiv-
ities are achieved by employing a Josiphos-type ligand
under mild conditions.
ligand
yield of 2a yield of 3a ee of 2a
a
a
b
entry
1
(mol%)
conditions
(%)
(%)
(%)
ibenzosiloles constitute a class of compounds that can be
PCy3 (10)
DMA, 100 °C,
24 h
44
37

D
applied to various useful materials1 such as light-emitting
diodes,2 thin-film transistors,3 solar cells,4 and detectors for
explosives,5 due to their unique optoelectronic properties based
on the π-conjugation system. Considering this wide utility of
the dibenzosilole structural motif in materials science, the
preparation of enantioenriched chiral dibenzosiloles would be
of high significance in view of their potential future
applications. Unfortunately, however, no synthetic methods
are available to date for enantioenriched dibenzosiloles as far as
we are aware. In this context, we envisioned that the
preparation of Si-stereogenic dibenzosiloles would be highly
attractive because the requisite biaryl moiety of the
dibenzosilole can be flexibly tuned without the necessity of
introducing extra C stereocenters, but available methods for the
construction of Si stereocenters, particularly in a catalytic
asymmetric manner, are very limited compared to those for the
construction of C stereocenters.6,7 Here we describe a new way
of synthesizing such Si-stereogenic dibenzosiloles with high
enantioselectivity through Pd-catalyzed enantioselective C−H
bond functionalization of 2-(arylsilyl)aryl triflates.8,9
2
3
4
5
6
(R,Sp)-L1
DMA, 60 °C,
57
84
95
19
25
4
4
82 (S)
87 (S)
94 (S)
19 (R)
54 (R)
(5.5)
48 h
(R,Sp)-L1
(5.5)
toluene, 60 °C,
48 h
(R,Sp)-L2
(5.5)
toluene, 60 °C,
48 h
3
(S,S)-L3
(5.5)
toluene, 60 °C,
48 h
<1
3
(R,R)-L4
(5.5)
toluene, 60 °C,
48 h
a
Determined by 1H NMR against an internal standard (MeNO2).
Determined by chiral HPLC on a Chiralcel OD-H column with
b
hexane/2-propanol = 90/10.
Our investigation was initiated by employing prochiral aryl
triflate 1a having a (tert-butyl)diphenylsilyl group as a model
substrate for the Pd-catalyzed intramolecular C−H bond
arylation reaction, a non-asymmetric variant of which was
recently reported by Shimizu et al.10 Under their reported
conditions that were highly effective for yielding various achiral
dibenzosiloles (PCy3 as the ligand in DMA at 100 °C), an
almost 1:1 mixture of desired product 2a and its isomeric
achiral dibenzosilole 3a was obtained in 81% combined yield
(Table 1, entry 1). In contrast, the formation of undesired 3a
was significantly suppressed by using (R,Sp)-L1 (Josiphos),11 an
electron-rich bisphosphine ligand, and product 2a was obtained
in 57% yield with 82% ee at 60 °C (entry 2). Higher yield and
ee were achieved by conducting the reaction in toluene instead
of DMA, giving 2a in 84% yield with 87% ee (entry 3). The
change of diphenylphosphino group of ligand (R,Sp)-L1 to
bis(3,5-dimethyl-4-methoxyphenyl)phosphino group ((R,Sp)-
L2)11c led to further improvement of both yield and
enantioselectivity (95% yield, 94% ee; entry 4). In comparison,
other chiral electron-rich bisphosphine ligands such as (S,S)-L3
(Me-DuPhos)12 and (R,R)-L4 (QuinoxP*)13 also gave 2a
selectively over 3a, but the yields and ee values were
significantly lower than those with Josiphos-type ligands
(entries 5 and 6).14
Under the optimized conditions described in Table 1, entry
4, 4-substituted 5-(tert-butyl)-5-phenyl-5H-dibenzosiloles 2a−
2c were effectively prepared with high enantioselectivity (91−
98% ee; Table 2, entries 1−3). Dibenzosiloles 2d−2f having
substituents at the 2- or 3-position were also synthesized in
high yield, albeit with somewhat reduced ee values (75−80%
ee; entries 4−6). In addition, dibenzosiloles 2g and 2h derived
from 2-naphthyl and 5-indolyl triflates, respectively, were
Received: March 7, 2012
Published: April 16, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
Table 2. Pd-Catalyzed Asymmetric Synthesis of Si-
Stereogenic Dibenzosiloles 2: Scope 1
Table 3. Pd-Catalyzed Asymmetric Synthesis of Si-
a
Stereogenic Dibenzosiloles 2: Scope 2
a
The reaction conditions are the same as those in Table 2 unless
b
c
otherwise noted. Combined isolated yield of 2 and 3. Determined by
d
1H NMR. Determined by chiral HPLC with hexane/2-propanol.
e
f
Contaminated with small impurity (<5%). Conducted in the
presence of 10 mol% of Pd(OAc)2 and 10.5 mol% of ligand.
absolute configuration of chiral 5 was determined to be (S,S) by
a
Combined isolated yield of 2 and 3 unless otherwise noted.
X-ray crystallographic analysis.15
b
c
Determined by 1H NMR. Determined by chiral HPLC with
d
e
hexane/2-propanol. Isolated yield of 2a. Conducted in the presence
of 10 mol% of Pd(OAc)2 and 10.5 mol% of ligand. (R,Sp)-L1 was
f
used as the ligand.
obtained with high enantioselectivity (94−96% ee; entries 7
and 8). The absolute configuration of product 2g in entry 7 was
determined to be (S) by X-ray crystallographic analysis with
Cu Kα radiation.15
We have also begun to explore derivatizations of
enantioenriched dibenzosiloles 2 obtained in these reactions.
In our preliminary experiments, bromination of 2a (94% ee)
with NBS in MeCN proceeds regioselectively at the 1-position
to give 6 in 87% yield with 94% ee (eq 2, left).19 On the other
In addition to aryl triflates having a (tert-butyl)diphenylsilyl
group at the 2-position as described in Table 2, several other 2-
silylated aryl triflates can also be employed for the present
catalysis. For example, substrates 1, having two of the same
substituted phenyl groups on the Si atom, were converted to
the corresponding dibenzosiloles 2 with good to excellent ee
(81−97% ee; Table 3, entries 1−4). It is worth noting that 1l,
having a (tert-butyl)di(3-phenylphenyl)silyl group, selectively
undergoes C−H bond functionalization at the less hindered
site, giving 2l with extended π-conjugation of the p-terphenyl
moiety (entry 4).16 Other dibenzosiloles such as 2m and 2n,
having bulky 1,1,2-trimethylpropyl and 3,5-di-tert-butylphenyl
groups, respectively, were also synthesized with relatively high
enantioselectivity (79−89% ee; entries 5 and 6).17 The result in
entry 6 represents an interesting example that effectively
differentiates the two phenyl groups on the Si atom in the
presence of another substituted phenyl group. Furthermore, the
reaction of aryl ditriflates 4 gives doubly cyclized product 5 in
74% combined yield of chiral and meso isomers (ratio = 72/28)
with 97% ee for the chiral isomer through the two-fold
enantioselective C−H bond functionalization (eq 1).18 The
hand, lithiation of 2a with n-BuLi/TMEDA, followed by
bromination, results in the formation of 3-bromo compound 7
in 64% yield (eq 2, right). These bromination products could
be of further use with rich chemistry of aryl halides (cross-
coupling, homocoupling, metal−halogen exchange, etc.).
To gain some mechanistic insight into the present catalysis,
we conducted several competition experiments. Electron-
deficient aryl triflate 1d, having a trifluoromethyl group at the
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Journal of the American Chemical Society
Communication
4-position, reacted 21 times faster than structurally similar but
more electron-rich 4-methyl substrate 1e (Scheme 1a). In
Scheme 1. Reactivity Difference between 1d and 1e and
Observed Inter- and Intramolecular Kinetic Isotope Effects
Figure 1. Possible transition states for (a) A→B and (b) A→C.
base, as proposed by Maseras and Echavarren.20c On the other
hand, based on the report by Dedieu and Suffert for the
reaction involving a 1,5-Pd migration,21 the undesired step from
A to C could proceed through a four-centered H atom transfer
of a Pd(II)L1 species (Figure 1b),23,24 and this pathway seems
effectively suppressed by using bisphosphine ligands such as
L1−L4 in the present catalysis (Table 1).
In summary, we have developed a Pd-catalyzed asymmetric
synthesis of Si-stereogenic dibenzosiloles through enantiose-
lective C−H bond functionalization of prochiral 2-(arylsilyl)aryl
triflates. High chemo- and enantioselectivities have been
achieved by employing Josiphos-type ligand (R,Sp)-L2 under
mild conditions. This method also represents the first example
of catalytic asymmetric construction of Si stereocenters by
enantioselective C−H bond activation. Future studies will
explore further expansion of the reaction scope, and we hope
this methodology will open the door for various applications
using enantioenriched chiral dibenzosiloles.
addition, an intermolecular competition reaction between 2-
(tert-butyl)diphenylsilyl aryl triflate 1a and 2-(tert-butyl)di-
(pentadeuteriophenyl)silyl aryl triflate 1a-d10 showed kH/kD =
1.5 (Scheme 1b). On the other hand, kH/kD = 4.9 was observed
for an intramolecular competition reaction of 2-(tert-butyl)di(2-
deuteriophenyl)silyl aryl triflate 1a-d2 (Scheme 1c).
A proposed reaction pathway for the present reaction of 1a is
illustrated in Scheme 2. Thus, oxidative addition of the C−O
ASSOCIATED CONTENT
* Supporting Information
Scheme 2. Proposed Reaction Pathways to Dibenzosilole 2a
and Its Isomer 3a
■
S
Experimental procedures and compound characterization data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
shintani@kuchem.kyoto-u.ac.jp; thayashi@kuchem.kyoto-u.ac.
jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support has been provided in part by a Grant-in-Aid for
Scientific Research, MEXT, Japan (the Global COE Program
“Integrated Materials Science”, Kyoto University).
bond of aryl triflate 1a to Pd(0) gives arylpalladium triflate A. A
base-assisted intramolecular C−H bond activation then gives
diarylpalladium species B,20 reductive elimination of which
leads to the formation of chiral dibenzosilole 2a along with
regeneration of Pd(0). Formation of undesired achiral isomer
3a could be explained by 1,5-Pd migration21,22 from A to
arylpalladium triflate C, which then undergoes C−H bond
activation toward diarylpalladium D followed by reductive
elimination. On the basis of the competition experiment in
Scheme 1a (k1d/k1e = 21) and the result of intermolecular KIE
in Scheme 1b (kH/kD = 1.5), oxidative addition of aryl triflate to
Pd(0) (formation of A) is most likely the turnover-limiting step
of the catalytic cycle, and the subsequent C−H bond activation
(A→B) occurs faster.20i,j The intramolecular KIE in Scheme 1c
(kH/kD = 4.9), which is consistent with the one for related
intramolecular arylation reactions of aryl bromides,20c may
indicate that this C−H bond activation step proceeds through a
transition state (Figure 1a) with intermolecular assistance of the
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