Yoshihiro Sato et al.
À
Table 2. Optimization of Rh-catalyzed C H silylations.
suppress the introduction of an ethyl group (see the Sup-
porting Information).
Next, we examined the scope of the Ir-catalyzed silylation
(Scheme 1). The reaction system could also work for 2-alkyl-
Entry Catalyst
Additive
(x equiv)
Solvent
t [h] Yield [%][a]
AHCTUNGTRENNUNG
2a 3a 1a
1
2
3
4
5
6
7
8
[Rh
[Rh
[Rh
[Rh
[Rh
[Rh
[Rh
[Rh
N
–
–
–
–
–
–
–
–
toluene
hexane
THF
6
12
12
64
45
56
27
3
50
51
18
0
9
5
7
2
0
4
5
0
0
3
0
0
27
32
25
32
58
23
24
46
79
43
72
42
1,2-DCE 12
MeCN
THF
THF
THF
THF
THF
12
12
12
12
12
12
12
12
12
15
12
12
20
12
Scheme 1. Silylation of 2-alkyl pyridines with Ir-catalysis.
pyridines (R=Et, iPr), albeit in low yields. However, the
sterically bulky 2-tert-butylpyridine shut down the reaction.
Although silylated products were obtained in good yields
by the Ir-catalysis, the generation of byproducts (less than
ca. 25% in total) could not be suppressed. Thus, we next ex-
amined other potential metal catalysts and eventually found
that Rh-complexes were also highly catalytically active for
9
A
–
10
11
12
13
14[b]
15[c]
16[c]
17[d]
18[e]
[Rh
[Rh
E
PPh3 (0.1)
Me4phen (0.1) THF
–
COD (0.2)
COD (1)
COD (1)
NBE (1)
NBE (5)
40
7
13
U
THF
THF
THF
THF
THF
toluene
THF
64 17 18
39 50
63 22 12
45
5
G
3
[Rh
[Rh
Ru3(CO)12
RuH2(CO)
6
0
0
33
35
58
À
this dehydrogenative C H bond silylation.
Unlike Ir-catalysis, the use of Rh-catalysts gave only sily-
lated products without the generation of any side-products
such as 4a and 5a (Table 2). The reaction of 1a with HSiEt3
(PPh3)3 NBE (5)
6
[a] Yields were determined by 1H NMR spectroscopic analysis using
1,1,2,2-tetrachloroethane as an internal standard. A small amount of 1a
was evaporated during the work-up procedure due to its volatile charac-
ter, and the total amount of each component (2a+3a+rec. 1a) could
therefore not reach 100%, especially when a large amount of 1a re-
mained (entries 4, 5, 8, 12, 17 and 18). [b] 36% of 2a and 48% of 3a
were isolated after silica gel column chromatography. [c] The reaction
was carried out in a sealed tube at 908C. [d] 6 mol% of Ru3(CO)12 was
catalyzed by 5 mol% of [RhACHTNUTRGNENG(U cod)Cl]2 in toluene under
reflux conditions afforded mono-silylated product 2a and
bis-silylated product 3a in 64% and 9% yields, respectively
(entry 1). We then examined various solvents (entries 2–5)
and found that the use of THF could also promote the sily-
lation with a similar level at a lower temperature (bath
temp: 908C). We next tested different RhI catalysts, but
employed. [e] 5 mol% of RuH2(CO)
nene.
ACHTUNGTREN(NNUG PPh3)3 was used. NBE=norbor-
[Rh
activities (entries 6 and 7). A cationic RhI catalyst, [Rh-
(cod)2]OTf, had much lower activity (entry 8), and the RhIII
ACHTUNGTRENNUNG(cod)OMe]2 and [RhACHTUTGNREN(NGUN cod)OH]2 exhibited slightly lower
a specific Rh complex during the catalytic cycle) rather than
trapping of dihydrogen. Other metal catalysts such as
ACHTUNGTRENNUNG
species [Cp*RhCl2]2 gave no product (entry 9). As in the
case of Ir-catalysis, neither the addition of phosphine and
phenanthroline ligands nor the use of an NHC complex was
effective (entries 10–12). By contrast, the addition of olefin
ligands significantly enhanced the reaction; the yields of
both 2a and 3a increased in the presence of 20 mol% of
COD (entry 13). Finally, the conversion was dramatically
improved when 1 equivalent of COD was added, with 39%
of 2a and 50% of 3a being obtained (total yield of 89%;
entry 14). Prolonging the reaction time and increasing the
amount of HSiEt3 or COD added did not substantially im-
prove the yield of bis-silylated product 3a. Since we as-
sumed that hydrogen gas is generated in this dehydrogena-
tive silylation, we next tested the reaction in a closed system
using a sealed tube to examine the role of COD. The reac-
tion carried out in the presence of 1 equivalent of COD led
to a decrease in conversion (entry 15). We also examined
the addition of norbornene, which is a common hydrogen
acceptor; however, the conversion was not improved
(entry 16). According to these data, hydrogen gas can be re-
leased efficiently under these conditions and an additional
COD ligand might have some effects (e.g., stabilization of
Ru3(CO)12 and RuH2(CO)ACTHUNGETR(UNNG PPh3)3 that exhibit high activity
[5b,e]
À
for C H bond silylations
were not effective even in the
presence of norbornene (entries 17 and 18). Unfortunately,
this rhodium catalysis could not promote the silylations of 2-
alkyl pyridine derivatives.
With the optimal conditions in hand, we next investigated
the substrate scope including the effects of the electronic
property on the pyridine ring (Scheme 2).[11] Electron-rich
pyridine substrates bearing 4-Me (1e), 4-OMe (1g), and 4-
OBn (1h) groups resulted in higher conversion to afford
larger amounts of bis-silylated products. On the other hand,
electron-withdrawing substituents such as 5-F (1i) and 4-
CONH2 (1j) caused decreases in the amounts of bis-silylat-
ed products. However, the reaction proceeded in the pres-
ence of a primary amide without affecting the NH2 moiety.
In addition, the conversion was significantly decreased when
a 6-Me-substituted pyridine derivative (1 f) was used. These
results suggest that the coordination ability of the pyridine
nitrogen to the Rh center is a key factor for inducing high
conversion. When a substrate bearing both primary and sec-
À
ondary C H bonds adjacent to the nitrogen atom (5a) was
À
used, the primary C H bond reacted selectively to afford 4a
&
&
2
Chem. Asian J. 2013, 00, 0 – 0
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