Angewandte
Chemie
triarylboroxines were tested and gave the corresponding
products 3d–3 f in 76%–80% yields. In all the examples
discussed up till now, the presence of additional alkyl
substituents at the benzylic position of the pyridine substrates
1
could be accelerating the catalysis by steric crowding which
III
Scheme 2. Optimization of the Rh -catalyzed arylation of 2-alkylpyri-
favors the orientation of the primary CÀH bond towards the
dine 1a. Various Ph-[B] 2 and the resulting yields are shown in the box
coordinated metal and the formation of the rhodacyclic
on the right.
[12]
intermediate (Thorpe–Ingold effect). Next, we tried more
challenging substrates that had less substitution at the
benzylic position. Pleasingly, using a substrate which features
only one butyl chain at the benzylic site of the pyridine ring
could also afford the arylated product 3g in 61% yield.
Furthermore, the catalysis is also compatible for simple 2-
ethylpyridine, while a derivative with a methoxy substituent
on the pyridine was also successfully applied (3h,i). In
addition, 2-ethylpyridine can react smoothly with several
differently substituted triarylboroxines, such as methyl and
fluorine-substituted triarylboroxine and b-naphthylboroxine,
indicating that this catalytic system could be widely applicable
for the arylation of substrates containing only b hydrogen
atoms. In some cases, some amounts of diarylated products
were observed in the reactions using 2-ethylpyridine, pre-
sumably a result of the relatively small steric hindrance of the
monoarylated products. The reaction between substrate 1a
and trimethylboroxine was also tried under the standard
conditions, however, the desired product was not observed.
Owing to the attractive biological properties of function-
alents led to higher yield (83%; see the Supporting Informa-
tion). In all the investigations, only the arylated product 3a
was formed, which showed extremely high selectivity for the
3
functionalization of the primary C(sp )ÀH bond over the
3
secondary C(sp )ÀH bonds and all other CÀH bonds present
in the molecule.
Subsequently, we investigated the substrate scope of the
catalysis using 2-ethylpyridine derivatives as the substrates
and several kinds of triarylboroxine as the aryl source. As
shown in Scheme 3, a series of arylated products was
[13]
3
alized quinolines, direct arylation of the C(sp )ÀH bond of
-benzylquinoline was also examined. Although there are
a few examples of primary CÀH bond activation on 8-
8
[
4c,d]
methylquinolines,
to our knowledge, there is no report on
III
3
Rh -catalyzed secondary C(sp )ÀH activation with quinoline
as a directing group. Triarylmethanes and their derivatives
have found widespread applications in material sciences and
[
14]
[15,16]
medicinal chemistry; however, general methods
for the
synthesis of unsymmetrical triarylmethanes by secondary CÀ
[17]
H bond arylation are less developed.
Therefore, we
wondered if this transformation could be utilized to straight-
III
forwardly synthesize triarylmethanes through Rh -catalyzed
3
C(sp )ÀH arylation of diarylmethanes 4. We were delighted to
see that the reaction afforded the desired product 5a in 82%
yield at a lower temperature (808C) with a lower loading of
the triarylboroxine and the oxidant. As shown in Scheme 4a,
the reactions proceeded well for the substrates with a variety
of important functional groups on the phenyl ring, such as
Scheme 3. The reactions of diverse 2-alkylpyridine derivatives with
(
(
(
ArBO) . Reactions were carried out using [Cp*Rh(CH CN) ](SbF )
3 3 3 6 2
5.0 mol%), Ag O (2.5 equiv), 1 (0.2 or 0.4 mmol), and (ArBO)
halogens (fluoro and chloro), ester, CF and OMe groups, and
2
3
3
2.0 equiv) in DMF for 24 h at 1008C. [a] The combined yield of
gave the corresponding products 5a–5g in 64%–82% yields.
These results show that the substrates with electron-deficient
aryl groups are more favorable for the triarylmethane syn-
isolated mono- and diarylated products. The ratio of mono:diarylation
is shown in parentheses.
3
thesis, possibly owing to their C(sp )ÀH bond being more
synthesized in moderate to good yields. This method was
compatible with some important functional groups on the
phenyl ring of the triarylboroxine or pyridine, such as fluorine
and methoxy substituents, which could be subjected to further
synthetic transformations. For the benzylic dialkyl-substituted
substrates, the reactions led to the desired product (3a–3c) in
good yields (83–85%), indicating that the catalysis can be
compatible with different alkyl chains at the benzylic site.
para- and meta- mono-substituted as well as disubstituted
acidic and prone to undergo CÀH activation. Subsequently,
the scope of triarylboroxines was investigated and all the
reactions worked well (Scheme 4b). Especially the introduc-
tion of useful substituents in the substrates or triarylboroxines
allows the direct construction of functionalized products, thus
providing an excellent opportunity for further modification of
the triarylmethanes.
Based on the results and previous studies, we propose the
following mechanism using 2-ethyl pyridine as the example
Angew. Chem. Int. Ed. 2015, 54, 10280 –10283
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