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termediate. For example, the thiazolium carbenes have been
developed for N-formylation with PMHS (polymethylhydrosilox-
ane) at 508C. Nevertheless, the N-methylation product, that is,
methylated amine can be obtained in 81% yield under slightly
Table 1. Catalyst and solvent screening for the formylation of 1a with
CO and triethoxysilane.
2
[a]
[
21]
more forcing conditions (1008C). It is well known that hydro-
silanes with different substituents show discriminatory reduc-
[
22]
[
b]
[b]
ing ability. We envisioned that divergent reducing ability to-
Entry
Catalyst
Solvent
CH CN
t [h] Conv. [%]
Yield [%]
wards CO or the formamide intermediate may be realized by
2
1
2
3
4
5
6
7
8
9
–
CsF
KF
3
12
12
12
12
12
12
12
12
12
12
12
0
6
0
5
choosing hydrosilanes with different substituents, and thus dif-
ferent products, that is, formamide or methylamine can be ob-
tained with high selectivity.
3
CH CN
CH
3
3
3
3
CN
CN
CN
CN
trace
98
0
99
trace
8
trace
96
0
99
trace
5
KF/[18]crown-6 CH
[18]crown-6
TBAF
TBAB
TBAC
TBAF
TBAF
TBAF
TBAF
TBAF
CH
CH
Fluoride has been usually employed for the reduction of al-
[23]
dehydes and ketones with a hydrosilane as a reductant.
3
CH CN
Fluoride anion interacts with the Si atom of hydrosilane to
generate the penta- or hexa-coordinated silicon intermediate,
thus elevating the hydrosilane reducibility. Baba et al. used CsF
3
CH CN
DMF
toluene
THF
1,4-dioxane 12
99
98
10
trace
trace
8
trace
trace
6
84
74
11
as catalyst for the hydrosilylation of CO to silyl formate for the
12
13
2
[
[
c]
first time, resulting in successful synthesis of the formamides
CH
CH
CH
CH
3
3
3
3
CN
CN
CN
CN
12
12
8
85
76
99
d]
1
1
1
4
5
6
TBAF
TBAF
TBAF
from piperidine and N-methylaniline with dimethylphenylsi-
99
90
[24a]
lane, though with relative high temperature and long time.
4
92
Herein, we would like to report TBAF (tetrabutylammonium
[
(
a] Conditions: 1a (108 mL, 1 mmol), catalyst (5 mol%, relative to 1a),
EtO) SiH (0.74 mL, 4 mmol), 1 bar CO , 308C, solvent (2 mL). [b] Deter-
fluoride) as an excellent organocatalyst for fixing CO with vari-
2
3
2
ous amines using hydrosilanes as reductants. Excellently, for-
mamides and further reduction products methylamines could
be selectivity controlled by simply tuning hydrosilane types,
thus realizing chemoselective two- and six-electron reduction
mined by GC using 1,3,5-trimethyoxybenzene as an internal standard.
[c] (EtO) SiH (0.56 mL, 3 mmol). [d] TBAF (3 mol%).
3
of CO coupled to CÀN bond construction. A tentative mecha-
hypervalent silicon intermediate (entry 6 vs. 9–12, Table 1). Less
polar solvents such as toluene, THF, and 1,4-dioxane were in-
compatible with the catalytic system, leading to poor results
2
nism involving the fluoride-promoted hydride transfer from
the hydrosilane to CO was proposed based on the identified
2
intermediate silyl formate.
(entries 10–12), presumably due to the decreased stability of
À
The reductive functionalization of CO in the presence of N-
the hypervalent silicon intermediate [HSiF(OEt) (solvent)] in
2
3
[23b]
methylaniline (1a) with triethoxysilane as a reductant was in-
vestigated to find suitable catalysts as summarized in Table 1.
No reaction occurred in the absence of any catalyst (entry 1,
Table 1). CsF and KF were almost inactive (entries 2 and 3). In-
terestingly, combination of KF and [18]crown-6 gave an excel-
lent result with 96% yield of N-methylformanilide (1b)
those solvents.
Lowering the amount of triethoxysilane or
TBAF led to a decrease in the yield of 1b to some extent (en-
tries 13 and 14 vs. 6). Four hours was enough to complete the
reaction (entry 16).
Various hydrosilanes were investigated for the N-formylation
of 1a with CO2 (Table 2). PMHS (polymethylhydrosiloxane),
TMDS (1,1,3,3-tetramethyldisiloxane), triethylsilane, and triphe-
nylsilane were found to be inactive (entries 1–4, Table 2). Dime-
thylphenylsilane, diethoxymethylsilane, and triethoxysilane
were effective for the N-formylation (entries 5–7), and triethox-
ysilane showed the highest reactivity among the monohydrosi-
lanes in this study (entry 7 vs. 1–6). The discriminatory activity
of various hydrosilanes is presumably due to electronic and
(entry 4), although [18]crown-6 itself was ineffective (entry 5).
This may be because [18]crown-6 is able to coordinate with
potassium cation, thus enhancing the nucleophilicity of fluo-
ride anion and the catalyst solubility. Inspired by this, we envis-
aged that a bulky group, that is, the tetrabutylammonium ion
presumably works as the potassium fluoride/[18]crown-6
system, thus enhancing the nucleophilicity of the fluoride
anion and improving the catalyst solubility in organic solvent
[20]
steric effects of the substituted groups.
Interestingly, when a stronger reductant diphenylsilane in
[
25]
compared with potassium or cesium cation. As such, TBAF
allowed the reaction to afford a quantitative yield of 1b suc-
cessfully (entry 6). On the other hand, TBAB (tetrabutylammo-
nium bromide) and TBAC (tetrabutylammonium chloride) ex-
hibited barely any activity (entries 7 and 8 vs. 6), suggesting
that the anion is crucial to the reaction. Accordingly, the tetra-
butylammonium cation could improve the nucleophilicity of
the fluoride anion and the catalyst solubility, thus improving
the catalytic reactivity of TBAF by activating the SiÀH bond of
triethoxysilane.
[26]
comparison with triethoxysilane
was used as a reductant,
further reduction of the formamide 1b, namely, methylation
product N,N-dimethylaniline (1c) was detected (entry 8 vs. 7,
[12h]
Table 2).
The N-methylation reaction involves two steps,
that is, formylation and the further reduction of the formamide
intermediate. The latter step is more difficult than the for-
[12c]
mer,
thus providing an opportunity for selective reduction
by tuning the reducing ability of hydrosilanes. Phenylsilane,
[27]
which has a stronger reducing ability, was found to be more
favorable for the methylation step than diphenylsilane (entry 9
vs. 8). At last, the methylation product 1c was successfully ach-
In addition, polar solvents were more favorable for this for-
mylation, probably due to the coordination capability with the
&
&
Chem. Eur. J. 2016, 22, 1 – 6
2
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