Palladium-catalyzed Ci-H fluorosilylation of 2-phenylpyridines: Synthesis of silafluorene equivalents

Article abstract of DOI:10.1002/anie.201310293

Treatment of 2-phenylpyridines with amino(1,3,2-dioxaborolan-2-yl) diphenylsilane produced fluorosilylated 2-phenylpyridines in good to excellent yields under palladium catalysis. This reaction is the first example of Ci-H fluorosilylation. Single-crystal X-ray structure analysis revealed a Lewis acid-base interaction between the silicon and nitrogen atoms, and the obtained fluorosilylated products are silafluorene equivalents. The fluorosilylated products showed stronger fluorescence than the corresponding silafluorene derivative. Treatment of 2-phenylpyridines with amino(1,3,2-dioxaborolan-2-yl) diphenylsilane produced fluorosilylated 2-phenylpyridines in good to excellent yields under palladium catalysis. This reaction is the first example of Ci-H fluorosilylation. A Lewis acid-base interaction exists between the silicon and nitrogen atoms, and the obtained fluorosilylated products can be regarded as silafluorene equivalents.

Full text of DOI:10.1002/anie.201310293

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Communications
DOI: 10.1002/anie.201310293
Fluorescent Heterocycles
À
Palladium-Catalyzed C H Fluorosilylation of 2-Phenylpyridines:
Synthesis of Silafluorene Equivalents**
Qing Xiao, Xiangtai Meng, Motomu Kanai,* and Yoichiro Kuninobu*
À
Abstract: Treatment of 2-phenylpyridines with amino(1,3,2-
dioxaborolan-2-yl)diphenylsilane produced fluorosilylated 2-
phenylpyridines in good to excellent yields under palladium
transition-metal-catalyzed direct C H silylation. Such trans-
formations are highly efficient and atom economical com-
pared with previous methods such as the lithiation/trans-
metallation sequence. Previously, one of the authors suc-
ceeded in synthesizing silafluorenes by rhodium-catalyzed
À
catalysis. This reaction is the first example of C H fluorosi-
lylation. Single-crystal X-ray structure analysis revealed
a Lewis acid–base interaction between the silicon and nitrogen
atoms, and the obtained fluorosilylated products are silafluor-
ene equivalents. The fluorosilylated products showed stronger
fluorescence than the corresponding silafluorene derivative.
[6,7]
À
direct intramolecular C H silylation.
groups can be used as an effective directing group of
transition-metal catalysts, several methods for intermolecular
Because pyridyl
À
direct C H silylation at the ortho position of 2-phenylpyr-
idines have been reported,[8] however, coordination of the
directing groups to the introduced silicon atom was not
detected in the products, and is presumably a result of the low
Lewis acidity of the silicon atoms introduced by the previous
p-Conjugated molecules with silicon atom(s) such as
silafluorenes (dibenzosiloles) are important organic func-
tional materials, as are their basic structures.[1] Several
methods have been developed to synthesize silafluorenes.
The most general method is dilithiation of 2,2’-dihalobiaryls
and successive treatment of the formed 2,2’-dilithiobiaryls
with dichlorosilanes.[2] In silafluorene structures, p-conju-
gated biaryl moieties and a silicon atom are connected by two
[8–11]
À
C H silylations (trialkyl- or triarylsilyl groups).
If elec-
tron-deficient silyl groups can be introduced, the designed
silafluorene equivalents (Figure 1b) comprising intramolecu-
lar Si···N interactions can be obtained from 2-phenylpyridines
in only one step. Herein we describe the successful develop-
À
À
Si C covalent bonds (Figure 1a). Our design for a new
ment of the first aromatic C H fluorosilylationm and the
products are fluorescent in solution, similar to silafluorenes.
Based on recent studies by the group of Suginome, we
selected a palladium complex and N,N-diethyl-1,1-diphenyl-
1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)silanamine
(2)[12] as the catalyst and silicon source, respectively. Treat-
ment of 2-phenylpyridine (1a) with 2 in the presence of
a palladium catalyst [Pd(MeCN)4](BF4)2 (6.0 mol%) in
toluene at 1008C for 24 hours did not lead to formation of
the assumed hydrosilane, but rather, the ortho-fluorosilylated
product 3a was obtained in 10% yield [for structure see
Eq. (1)]. The origin of the fluorine atom of 3a must be the BF4
anion of the palladium catalyst, thus KF was added as
a fluorine source and the reaction conditions were opti-
mized.[13–20] As a result, the yield of 3a increased to 74%. In
addition, the order of the addition of 1a, 2, [Pd(MeCN)4]-
(BF4)2, KF, and K2S2O8 was altered, that is, a mixture of 1a
and 2 was treated with KF before addition of the palladium
catalyst and oxidant, and thus improved the yield of 3a to
84% [Eq. (1)].
Figure 1. Comparison of the structures of silafluorenes and their
proposed equivalents. EWG=electron-withdrawing group.
category of original p-conjugated molecules is a silafluorene
equivalent in which one of the two Si C covalent bonds is
À
replaced with a Si···N Lewis acid–base interaction (Fig-
ure 1b).[3] This molecular design is based on the character-
istics of silicon atoms bearing electron-withdrawing group(s),
so that hypervalent structures can be formed as a result of the
high Lewis acidity of silicon atoms.[4,5]
To synthesize the designed structures (Figure 1b), we
planned to use directing-group-assisted (pyridyl group),
[*] Dr. Q. Xiao, Dr. X. Meng, Prof. Dr. M. Kanai, Prof. Dr. Y. Kuninobu
Graduate School of Pharmaceutical Sciences, The University of
Tokyo, ERATO (Japan) Science and Technology Agency (JST)
Kanai Life Science Catalysis Project
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
E-mail: kanai@mol.f.u-tokyo.ac.jp
kuninobu@mol.f.u-tokyo.ac.jp
[**] This work was supported in part by ERATO from JST.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310293.
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Table 1: Investigation of several 2-phenylpyridines and their derivatives
(1).[a]
Figure 2. Single-crystal X-ray structure of 3a. The ellipsoids are shown
at 50% probability. Hydrogen atoms are omitted for clarity. Selected
Entry
1
3
Yield [%]
À
À
distances [ꢀ]: Si-N 2.2068(15), Si F 1.6674(12), Si C(1a) 1.8748(19),
1
2
À
À
Si C(Ph ) 1.8779(19), Si C(Ph ) 1.871(2). Selected angles [8]:
C(1a)-Si-C(Ph1) 118.16(9), C(Ph1)-Si-C(Ph2) 123.28(8), C(Ph2)-Si-C(1a)
115.17(8), C(1a)-Si-N 79.00(7), C(Ph1)-Si-N 84.72(7), C(Ph2)-Si-N
87.64(7), C(1a)-Si-F 98.05(7), C(Ph1)-Si-F 95.18(7), C(Ph2)-Si-F
95.29(8).
1
2
3
4
5
6
7
8
9
1b (R1 =4-MeO)
1c (R1 =4-Me)
1d (R1 =4-CF3)
1e (R1 =4-CO2Et)
1 f (R1 =4-F)
3b
3c
3d
3e
3 f
3g
3h
3i
74
82
58
57
84
75
88
75
78
66
77
The structure of 3a was determined by single-crystal X-
ray structure analysis (Figure 2). The findings indicated that
the nitrogen atom of the pyridyl group coordinated to the
silicon atom and 3a was a pentacoordinated silicon compound
(trigonal-bipyramidal geometry). In fact, the length between
the nitrogen and silicon atoms was 2.2068(15) ꢀ, thus
supporting the existence of a Lewis acid–base interactio-
n.[4a,b,21] The arrangement of N-Si-F was almost linear (176.58).
The dihedral angle of N-C(py)-C(benzene)-C(1a) was 4.88, and the
sum of the three angles of C(1a)-Si-C(Ph1), C(Ph1)-Si-C(Ph2), and
1g (R1 =4-Cl)
1h (R1 =3-MeO)
1i (R1 =3-Ph)
1j (R1 =2-MeO)
1k (R1 =2-Me)
1l (R1 =2-Ph)
3j
3k
3l
10
11
C
(Ph2)-Si-C(1a) was 356.68. These findings indicate that the
pyridine and benzene rings, and Si, C(1a), C(Ph1), and C(Ph2)
atoms existed in almost the same planes, perpendicular to
each other.
12
13
14
1m
1n
1o
3m
3n
3o
62
64
88
Next, we investigated the substrate scope and limitations
(Table 1). Fluorosilylation proceeded in good to excellent
yields using the 2-phenylpyridines 1b–g, having an electron-
donating or electron-withdrawing group, a chlorine, or an
ethoxycarbonyl group at the 4-position, without loss of the
functional groups (entries 1–6). Although two possible
regioisomers might be formed when a substituent exists at
the 3-position, only a single isomer, with the silicon atom
introduced at the less hindered site, was formed (entries 7 and
8). The silylation reaction was not inhibited by steric
hindrance from a substituent at the 2-position (entries 9–
11). The corresponding silylated products, such as the
pyridylnaphthalenes 3m and 3n, and benzo[h]quinoline 3o,
were produced in 62–88% yields (entries 12–14). An elec-
tron-donating or electron-withdrawing substituent on the
pyridine ring was also tolerated in this fluorosilylation
reaction (entries 15 and 16). A quinoline group also worked
as a directing group, and the corresponding fluorosilylated
product 3r was obtained in 76% yield (entry 17). Other
directing groups, however, such as pyrazolyl and N-methyl-
imidazolyl groups, were not effective for the fluorosilylation.
The proposed mechanism for the fluorosilylation is shown
in Scheme 1. The reaction would proceed through the
following four key steps: 1) formation of an ortho-palladated
15
16
1p (R2 =OMe)
1q (R2 =CF3)
3p
3p
75
68
À
intermediate A (C H bond activation); 2) formation of the
fluorosilyl palladium intermediate C from A and B; 3) reduc-
tive elimination of C to give the fluorosilylated product 3 and
17
1r
3r
76
[a] 2 (1.5 equiv).
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1,2-bis(2-hexyloxy-5-(2-pyridinyl)phenyl)ethyne (4) and 2
with KF, and successive treatment with a palladium catalyst
and K2S2O8 afforded the double-fluorosilylated product 5 in
68% yield.
As we expected, the fluorosilylated 2-phenylpyridines 3
were fluorescent. For expression of the fluorescence, both
nitrogen and fluorine atoms are indispensable because the
quantum yields of 2-(2-(methyldiphenylsilyl)phenyl)pyridine
(6) and 2-fluorosilylbiphenyl 7 are quite low (Figure 3).
Interestingly, the quantum yield of 3a was much higher than
that of the corresponding silafluorene 8 (dibenzosilole).[22]
The double-fluorosilylated product 5 showed blue fluores-
cence (lem = 435 nm) and had a high quantum yield (FF =
0.60).
Scheme 1. Proposed mechanism for the formation of 2-fluorosilylated
2-phenylpyridines and their derivatives.
a palladium(0) species; and 4) oxidation of the palladium(0)
species by K2S2O8 to regenerate the palladium(II) catalyst.
The intermediate B could be formed by the formation of the
silicate D from 2 and KF (5), and elimination of Et2N-Bpin
from D (6).
The reaction can be performed on a gram scale. The
reaction of 0.62 g of 1a with a fluorine source, KF, and
successive treatment with
a catalytic amount of [Pd-
(NCCH3)4](BF4)2 and the oxidant K2S2O8 produced 1.03 g of
3a in 72% yield [Eq. (2)].
Figure 3. Comparison of fluorescent quantum yields in THF.
According to the density functional theory (DFT) calcu-
lations at the B3LYP/6-31 + G* level of theory, the HOMO
orbitals of 3a and 8 differed considerably, whereas the
LUMO orbitals were almost the same (Figure 4). The
HOMO orbital of 3a is located mainly on one of the phenyl
To expand the p-conjugated system, we investigated
double fluorosilylation [Eq. (3)]. Treatment of a mixture of
Figure 4. LUMO and HOMO orbitals of 2-(2-fluorosilylated)phenylpyr-
idine (3a) and the silafluorene 8.
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À
a directing group to promote C H fluorosilylation and
a component of the silafluorene equivalents. This reaction is
useful and practical because the fluorosilylation reaction has
high functional-group tolerance and proceeds in good to
excellent yields, even on gram scale. The fluorosilylated 2-
phenylpyridines and their derivatives are fluorescent, and the
Lewis acid–base interaction between the silicon and nitrogen
atoms is critical for the fluorescence emission because of the
expansion of the p-conjugated system through the interac-
tion. In addition, the quantum yield of ortho-fluorosilylated 2-
phenylpyridine was much higher than that of the correspond-
ing silafluorene. The time-dependent DFT calculations indi-
cate that the emission of fluorosilylated products occurs from
an excited state generated by the intramolecular charge-
transfer transition from the HOMO to the LUMO. We hope
that this result will contribute to a novel category of func-
tional materials based on noncovalent bond(s).
Experimental Section
Typical procedure for the reaction of 2-phenylpyridine (1a) with
amino(1,3,2-dioxaborolan-2-yl)diphenylsilane (2): Under an argon
atmosphere (or in a glove box), 1a (621 mg, 4.00 mmol), 2 (2.29 g,
6.00 mmol), and KF (697 mg, 12.0 mmol) were added to xylene
(12 mL). The mixture was then stirred at 1358C for 15 min. At the
same time, K2S2O8 (3.24 g, 12.0 mmol) and [Pd(MeCN)4](BF4)2
(88.8 mg, 0.200 mmol) was added into the reaction system. The
mixture was then stirred at 1358C for 24 h. Upon completion of the
reaction, the crude product was purified by column chromatography
(n-hexane/EtOAc 10:1) to afford pure 3a as a white solid (1.03 g,
72%).
Received: November 27, 2013
Published online: February 12, 2014
À
Keywords: C H activation · fluorescence · heterocycles ·
palladium · silicon
.
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À
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À
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1008C: 36%, 1358C: 60%, 1508C: 59%.
[19] Second investigation of several solvents ([Pd(NCCH3)4](BF4)2
(5.0 mol%), KF (3.0 equiv), K2S2O8 (2.0 equiv), 1358C): octane:
55%, 1,4-dioxane: 22%, xylene: 60%, chlorobenzene: 32%,
THF: 12%, 1,2-dichloroethane: 21%, N,N-dimethylformamide:
0%, acetonitrile: trace.
[20] Investigation of several concentrations ([Pd(NCCH3)4](BF4)2
(5.0 mol%), KF (3.0 equiv), K2S2O8 (2.0 equiv), 1358C): 0.20m:
61%, 0.33m: 74%, 0.50m: 68%.
[21] There have been several examples of 2-phenylpyridine deriva-
tives with a Lewis acid–base interaction between boron and
nitrogen atoms. See: a) Y.-L. Rao, H. Amarne, S.-B. Zhao, T. M.
McCormick, S. Martic, Y. Sun, R.-Y. Wang, S. Wang, J. Am.
Chem. Soc. 2008, 130, 12898; b) S. K. Murphy, C. Baik, J.-S. Lu, S.
Wang, Org. Lett. 2010, 12, 5266; c) C. Baik, S. K. Murphy, S.
Wang, Angew. Chem. 2010, 122, 8400; Angew. Chem. Int. Ed.
2010, 49, 8224; d) M. R. Buchmeiser, S. Camadanli, D. Wang, Y.
Zou, U. Decker, C. Kuhnel, I. Reinhardt, Angew. Chem. 2011,
123, 3628; Angew. Chem. Int. Ed. 2011, 50, 3566; see also
Refs. [3b] and [3f].
[13] Investigation of several catalysts (KF (3.0 equiv), K2S2O8
(2.0 equiv), toluene, 1008C): [Pd(NCCH3)4](BF4)2: 36%, Pd-
(O2CtBu)2: 28%, [PdCl2(PCy3)2]: 7%, Pd(OAc)2: 27%, Pd(tfa)2:
30%.
[14] Investigation of several FÀ sources ([Pd(NCCH3)4](BF4)2
(5.0 mol%), K2S2O8 (2.0 equiv), toluene, 1008C): CsF: trace,
KHF2: trace, KBF4: 11%, KF: 36%, Bu4NF: trace, Et4NF·3HF:
0%.
[22] Y. Yabusaki, N. Ohshima, H. Kondo, T. Kusamoto, Y. Yamanoi,
H. Nishihara, Chem. Eur. J. 2010, 16, 5581.
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