.
Angewandte
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
[**] This work was supported in part by ERATO from JST.
Supporting information for this article is available on the WWW
3168
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 3168 –3172