A Versatile NHC-Parent Silyliumylidene Cation for Catalytic Chemo- And Regioselective Hydroboration

Article abstract of DOI:10.1021/jacs.9b06714

This study describes the first use of a silicon(II) complex, NHC-parent silyliumylidene cation complex [(I_Me)₂SiH]I (1, I_Me =:C{N(Me)C(Me)}₂) as a versatile catalyst in organic synthesis. Complex 1 (loading: 10 mol %) was shown to act as an efficient catalyst (reaction time: 0.08 h, yield: 94%, TOF = 113.2 h^-1 reaction time: 0.17 h, yield: 98%, TOF = 58.7 h^-1) for the selective reduction of CO₂ with pinacolborane (HBpin) to form the primarily reduced formoxyborane [pinBOC(-O)H]. The activity is better than the currently available base-metal catalysts used for this reaction. It also catalyzed the chemo- and regioselective hydroboration of carbonyl compounds and pyridine derivatives to form borate esters and N-boryl-1,4-dihydropyridine derivatives with quantitative conversions, respectively. Mechanistic studies show that the silicon(II) center in complex 1 activated the substrates and then mediated the catalytic hydroboration. In addition, complex 1 was slightly converted into the NHC-borylsilyliumylidene complex [(I_Me)₂SiBpin]I (3) in the catalysis, which was also able to mediate the catalytic hydroboration.

Full text of DOI:10.1021/jacs.9b06714

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Article
A Versatile NHC-Parent Silyliumylidene Cation for
Catalytic Chemo- and Regioselective Hydroboration
Bi Xiang Leong, Jiawen Lee, Yan Li, Ming-Chung Yang, Chi-Kit Andy Siu, Ming-Der Su, and Cheuk-Wai So
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06714 • Publication Date (Web): 10 Oct 2019
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A Versatile NHC-Parent Silyliumylidene Cation for Catalytic
Chemo- and Regioselective Hydroboration.
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a
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a
b
d
b,c
Bi-Xiang Leong, Jiawen Lee, Yan Li, Ming-Chung Yang, Chi-Kit Siu, * Ming-Der Su, * and
a
Cheuk-Wai So *
a
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang
b
Technological University, Singapore 637371. Department of Applied Chemistry, National Chiayi University, Chiayi
c
6
0004, Taiwan. Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 80708,
d
Taiwan. Department of Chemistry, City University of Hong Kong, Hong Kong SAR, China.
ABSTRACT: This study describes the first use of a silicon(II) complex, NHC-parent silyliumylidene cation complex
[(IMe
)
2
SiH]I (1, IMe = :C{N(Me)C(Me)}
2
) as a versatile catalyst in organic synthesis. Complex 1 (loading: 10 mol %) was shown
-1
to act as an efficient catalyst (reaction time: 0.08 h, yield: 94 %, TOF = 113.2 h ; reaction time: 0.17 h, yield: 98 %, TOF = 58.7
-1
2
h ) for the selective reduction of CO with pinacolborane HBpin to form the primarily reduced formoxyborane
[pinBOC(=O)H]. The activity is better than the currently available base-metal catalysts used for this reaction. It also
catalyzed the chemo- and regioselective hydroboration of carbonyl compounds and pyridine derivatives to form borate
esters and N-boryl-1,4-dihydropyridine derivatives with quantitative conversions, respectively. Mechanistic studies show
that the silicon(II) center in complex 1 activated the substrates and then mediated the catalytic hydroboration. In addition,
complex 1 was slightly converted into the NHC-borylsilyliumylidene complex [(IMe) SiBpin]I (3) in the catalysis, which was
2
also able to mediate the catalytic hydroboration.
Introduction
Mes = 2,4,6-Me
amine-borane adducts.
3
C
6
H
2
) to catalytically dehydrocouple
4c
Heavier group 14(II) complexes with a formal oxidation
state of +2 often consist of a vacant orbital and a lone pair
of electrons on the group 14 centers. As a result, they
display both electrophilic and nucleophilic characters,
leading to Lewis ambiphilicity. These electronic properties
should enable heavier group 14(II) complexes to display
reactivity that closely resembles that of transition metal
Similarly, silicon(II) compounds with a formal oxidation
state of +2 such as silyliumylidene cations (RSi: , R =
supporting ligand) and hydridosilylenes (R(H)Si:) can
show transition-metal-like reactivity in the area of small
molecules activation. For example, Inoue et al. showed
+
5
that the NHCarylsilyliumylidene cation complex
1
+
complexes in the area of catalysis. This was evidenced by
[ArMesSi(I ) ] (I = :C{N(Me)C(Me)} ) can reduce CO to
Me
2
Me
2
2
6
Jones
amido)(hydrido)germylene
Ar*(iPr Si)NËH] (E
iPr{C(H)Ph are efficient transition-metal-like
catalysts in mediating the hydroboration of unsaturated
et
al.
that
the
and
Sn;
two-coordinate
form CO and activate the CH bond of phenylacetylene.
(
[
-stannylene
Moreover, Kato et al. illustrated that the base-stabilized
(amido)(hydrido)silylenes underwent the reversible
3
=
Ge,
Ar*
=2,4,6-
2
}
2
C
6
H
2
)
oxidative addition of Si H and P H bonds at room
IV
III
temperature, in addition to the uncatalyzed insertion of
II
compounds, due to their far more reactive E H bonds in
II
the Si H bonds with unsaturated CX bonds (X = C, N, O,
IV
2,3
7
comparison with classical E H bonds. In addition, the
etc.).
However, catalytic organic transformations
II
II
two-coordinate Ge and Sn centers preserve their Lewis
ambiphilic characters, which enable the σ-bond
metathesis, oxidative addition and reductive elimination
processes in the catalyses. Moreover, research groups of
Wesemann and Zhao showed that the base-stabilized
mediated by silicon(II) compounds surprisingly remain
unexplored. It could be possibly due to the supporting
ligands in a silicon(II) compound, which impart kinetic
and thermodynamic stabilization effects on the highly
8
reactive silicon center. As a consequence, its Lewis
germylene
[AriPrËC(H)(Ph)PPh
iPr
HC{C(Me)N(Ar)}{C(=CH
can catalyze the hydroboration of carbonyl compounds
and
stannylene
compounds
ambiphilic character is not pronounced and the high
catalytic potential of a silicon(II) compound is suppressed.
To the best of our knowledge, only one example of
silicon(II) compounds shows catalytic capability, whereby
2
] (E = Ge, Sn; AriPr = 2,6-(2,4,6-
3
C
6
H
2 2
) C
6
H
3
)
and
[
2 2 6 3
)N(Ar)}Ge:] (Ar = 2,6-iPr C H )
Jutzi et al. used the cyclopentadienyl silyliumylidene
4
a-b
+
with the aid of the non-innocent ligands, respectively.
Furthermore, Power et al. used the methoxystannylene
2 6 3
pre-catalyst [{ArMesSn(µ-OMe)} ] (ArMes = 2,6-Mes C H ,
cation [Cp*Si] (Cp* = C Me ) to catalyze the controlled
5
5
9
degradation of oligo(ethyleneglycol) diethers. In this
context, it is imperative to develop a strategy to activate
2
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the catalytic ability of stable silicon(II) compounds, which
would greatly advance sustainable catalysis due to the high
abundance and non-toxicity of silicon.
system that selectively delivers the primarily reduced
formoxyborane 2a. Complex 1 is one of the very few
1
2
3
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9
1
1
1
1
1
1
1
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1
1
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2
3
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examples that catalyze the selective reduction of CO
including the main-group and transition metal catalysts,
namely an amine-lithium borohydride [(L)Li][HBPh ] (L =
NHC-copper
2
,
Recently, we described the synthesis of an NHC-parent
silyliumylidene cation complex [(IMe) SiH]I (1, Figure 1) and
2
3
-1 11
N(CH
2
CH
2
NMe
2 3
) ,
TOF
=
10 h ),
a
its transition-metal-like reactivity in functionalizing the
-1 12
10
[IArCu(OtBu)] (yield: 85 %, TOF = 0.35 h ) and a PSiP-
pincer-palladium complex [{ PSiP}PdOTf] [ PSiP
Si(Me)(2-PPh -C H ) ] (yield: 93 %, TOF = 1550 h ).
2 6 4 2
ortho-CH bond of fluorobenzene. Considering the
Ph
Ph
=
chemistry of the above-mentioned base-stabilized
silyliumylidene cations and hydridosilylenes, it is
anticipated that 1 could be a promising candidate to
-1
13
Moreover,
1
exceeds the base-metal catalysts,
[(L)Li][HBPh ] and [IArCu(OtBu)], in terms of both
3
0
1
2
3
4
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0
II
catalyze organic reactions due to its dual functionality: Si
II
reaction time and TOF. Furthermore, the TOF of the
hydroboration of CO with HBpin far surpasses the TOF
values of the activated non-transition metal catalysts (TOF
cation and reactive Si H bond. In this context, we were
2
highly interested in investigating its catalytic capability.
Herein, we report the NHC-parent silyliumylidene cation-
catalyzed chemo- and regioselective hydroboration of
carbon dioxide, carbonyl compounds and pyridine
derivatives (Figure 1).
-
1
=
0
0.07 - 2.5 h ) and even the non-metal catalysts (TOF =
-
1 14
.40 - 14.5 h ).
When more potent [BH
HBpin, 10 mol % of complex 1 catalyzed the reduction of
CO with >99 % conversion to form a mixture of the borate
ethers [B(OMe) ] (2d) and [BO(OMe)] (2e) in a ratio of 1:4
3 2
.SMe ] was used instead of
Catalyst
Substrates
2
N
N
3
3
CO2
-1
in 0.5 h (TOF = 19.8 h , Scheme 1, Table S3), but selectivity
cannot be achieved.
R
Si
O
N
N
H
I
2
On the other hand, the 1-catalyzed reduction of CO with
N
R
R'
catecholborane HBcat was slowly proceeded (24 h, >99 %
1
-1
conversion, TOF = 0.34 h , Table S4) to afford a mixture of
Figure 1. The NHC-parent silyliumylidene cation complex
1 and substrates.
2
diborate ether [(catB) O] (major product, yield: 70 %) and
methoxyborane [catBOMe] (minor product, yield: 12 %). In
comparison with other examples, the above-mentioned
NHC-copper complex [IArCu(OtBu)] did not catalyze the
Results and Discussion
1
2
selective reduction of CO
amido)(hydrido)stannylene
outperformed 1 using HBcat to non-selectively reduce CO
2
with HBcat, whereas the
The catalytic ability of the NHC-parent silyliumylidene
cation complex 1 toward hydroboration of CO with
2
(
[Ar*(iPr Si)NSnH]
3
2
pinacolborane HBpin was first examined, considering that
non-metal compounds have been rarely used as
homogeneous catalysts for such reaction, the obtained
turnover frequencies (TOF) and selectivity are often low,
leading to a mixture of methoxyborane [pinBOMe] (2b)
-1
2
into [(catB)OMe] and [(catB) O] (TOF = 1188 h , yield of
3
each compound was not reported).
O
B
H
O
O
O
O
B
B
O
B
O
O
+
1
(10 mol %)
O
O
O
H
and diborate ether [(pinB)
was no reaction between CO
BH .SMe , HBcat) in C at 90 C. However, in the
presence of 1 (10 mol %), the reduction of CO with HBpin
at 90 C was extremely clean, resulting in the
formation of the formoxyborane [pinBOC(O)H] (2a,
2
O] (2c). To begin with, there
C6D6, 90 oC, 10 min
and borane (HBpin,
2a
yield: 98 %
2c
2
yield: 2 %
o
3
2
6 6
D
TOF: 60 h-1
2
o
6 6
in C D
MeO
O
OMe
B
MeO
OMe
B
B
+
BH3.SMe2
-1
O
O
reaction time: 0.08 h, yield: 94 %, TOF = 113.2 h ; reaction
1 (10 mol %)
C6D6, 90 oC, 30 min
B
CO2
OMe
-1
time: 0.17 h, yield: 98 %, TOF = 58.7 h ; Scheme 1; see the
Supporting Information, Table S1), along with trace
amount of the diborate ether 2c (Yield: <2 %). No other
identifiable boron-containing products such as
methoxyborane 2b were found in the reaction mixture
when increasing the reaction time or temperature.
Moreover, complex 1 (10 mol %) was able to catalyze the
OMe
2d
2e
yield: 18 %
yield: 81 %
TOF: 19.8 h-1
O
B
H
O
B
O
O
O
O
B
B
O
O
+
1 (10 mol %)
C6D6, 90 oC, 24 h
O
O
reduction of CO2 under air and/or in wet C
D
6 6
to afford 2a
2f
2g
and 2c, but the yield of 2a decreased (Table S2). It is
because HBpin decomposed in these reaction conditions
to give 2c and hence the yield of the latter increased. When
the amount of complex 1 decreased (5 mol %), the catalytic
hydroboration was incomplete (90 % conversion, Table S1)
in 0.5 h, but the selectivity was still observed. It is
noteworthy that complex 1 is the first non-metal catalytic
yield: 12 %
yield: 70 %
TOF: 0.34 h-1
2
Scheme 1. 1-catalyzed reduction of CO . (All the catalytic
trials were repeated in triplicate)
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Upon completing the catalytic hydroboration of CO
2
downfield shifted in comparison with that of 1, indicating
that the Si lone pair electrons interact with CO to form
1
2
3
4
5
6
7
8
9
with HBpin, a weak singlet at ca. δ 0.9 ppm, in addition to
2
11
11
1
the signal of 2a, was observed in B (Figure S12b) and B{ H}
NMR spectroscopy. These indicate that a new boron
compound, which does not have any H atom on the boron
atom, was formed in the catalysis. To clarify these
phenomena, complex 1 was treated with excess HBpin in
Int01 (Scheme 3). The Si–H coupling constant, as well as
1
the absence of signal for formate –C(O)H moiety in the H
NMR spectrum, indicate that CO
2
did not insert into the
1
9a
Si–H bond in 1 during the catalysis. When stoichiometric
amount of HBpin was subsequently added to the reaction
mixture, compound 2a was afforded, along with
regeneration of complex 1. Notably, the reaction of
o
11
6 6
C D at 60 C for five minutes, whereby same B NMR
signal at δ 0.92 ppm (singlet) was observed, indicating
formation of the new boron compound. The reaction was
complex 1 and CO
2
was found to be reversible upon the
1
29
1
further analyzed by H and Si NMR spectroscopy. The H
NMR spectrum shows a set of signals due to methyl
removal of CO and volatiles of the reaction mixture in
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
vacuo at room temperature. As a result, complex 1 does not
resemble the (amido)(hydrido)germylene and -stannylene
to catalyze hydroboration through σ-E–H bond metathesis
2
9
protons of IMe and Bpin. The Si NMR spectrum displays a
singlet at δ -93.0 ppm ( Si{ H} NMR: δ -95.6 ppm, singlet;
29
1
2,3
Figures S4 - S5), which is an intermediate value between
mechanism (E = Ge, Sn).
1
that of 1 (δ -77.9 ppm, JSi-H = 283 Hz) and the NHC-
DFT calculations were then performed (Scheme 3). It
was found out that complex 1 is capable of mediating the
catalysis, whereby the Si lone pair electrons interact with
hydridosilylene complex [IMe:SiH(SitBu
3
)] (δ -137.8
1
0,15
II
ppm).
The upfield Si NMR signal corresponds to a Si
cationic center and there is no hydrogen atom on the Si
center. On the basis of NMR spectroscopic data, the new
boron compound formed in the reaction is an NHC-
CO
Int01 shows that the Si lone pair orbital interacts with the
π* orbital of CO (Figure S91). Accordingly, the NBO
analysis shows that the Si–CCO2 bond results from the
2
to form Int01 (ΔG = -3.1 kcal/mol). The HOMO-3 of
2
borylsilyliumylidene complex [(IMe
Its composition is also supported by the theoretical Si
NMR value (δ -94.0 ppm, B3LYP/6-
11 G(2df,2pd)//B3LYP/6-31G**, Table S12) and HRMS.
After work-up, complex 3 was isolated as colorless solid
yield: ca 30 % for 5-min reaction time). Compound 3 is
2
) SiBpin]I (3, Scheme 2).
29
overlap of a sp hybrid on Si with a sp2.59 hybrid on C
Table S13). In the formation of Int01, the natural charge of
the Si atom increases from 0.44 (compound 1) to 1.02
Int01), while that of the CCO2 atom decreases from 1.03 (1)
2.14
(
+
+
3
(
(
to 0.54 (Int01). Subsequently, the H–B bond of HBpin
inserts into the C=O bond via a low kinetic barrier (TS01;
highly unstable in solution and hence obtaining suitable
single crystals for X-ray crystallography is still in progress.
In this context, its structure was simulated by DFT
calculations (Figure S89). Complex 3 easily decomposed in
toluene and ethereal solvents to form a white insoluble
precipitate, which comprises a mixture of an NHC-
o
ΔGInt01⟶TS01 = 10.6 kcal/mol at 24 C, Scheme S2), which
results in the formation of the formoxyborane 2a and
regeneration of 1 (ΔG = -8.5 kcal/mol, Scheme S2). In other
words, the Si lone pair of electrons in complex 1 is Lewis
basic enough to activate CO
hydroboration, whereas the Si–H bond is not sufficiently
hydridic to activate CO . The mechanism is in contrast to
σ-E–H bond metathesis mechanism (E = main-group
element) in main-group element catalyzed
2
for subsequent
11
2
boronium cation [(IMe) Bpin]I ( B NMR: δ 2.37 ppm), an
imidazolium salt [IMeH]I (see Figure S88) and
unidentified products. However, they are inactive in any
catalytic hydroboration.
2
14c
hydroboration. Moreover, the interaction of the Si lone
N
N
N
N
pair in 1 with 2a is endergonic (ΔG = 7.0 kcal/mol, Scheme
S5), whereas that with CO is exergonic (Scheme 3). This
2
suggests that complex 1 does not prefer to further react
with 2a after a catalytic cycle, while it chooses to react with
2
CO again and achieves the selective reduction.
HBpin, C6D6, 60 oC, 5 minutes
Si
Si
H
I
N
Bpin
I
N
N
N
1
3
Scheme 2. Synthesis of compound 3.
H
O
B
N
O
N
O
In supporting complex 3 that involves in the catalysis, it
was used to catalyze the hydroboration of carbon dioxide
O
N
N
C
C
O
O
HBpin
1
+
CO2
Si
Si
with HBpin, whereby selective reduction of CO
2
was
I
H
N
H
I
N
achieved to obtain 2a (2.5 mol %; time: 0.17 h, yield: 94 %,
N
N
-1
TOF = 227 h , Table S1).
TS01
Grel = 7.5 kcal/mol
Int01

Grel = -3.1 kcal/mol
Grel = 0.0 kcal/mol
These results brought up a question of whether complex
1
or 3 is a genuine catalyst in the hydroboration of CO
Considering that the conversion of complex 1 into
complex 3 is slight (see above), it is suggested that complex
could prefer to react with CO instead of HBpin in the
first step of catalysis. To gain insight, complex 1 was used
2
.
O
1
+
O
H
B
O
O
2a
1
2
Grel = -8.5 kcal/mol
o
29
to react with CO
2
in C
5
D
5
N at -40 C, whereby the Si NMR
Scheme 3. Calculated Gibbs free energies (ΔGrel) for the
1
o
o
signal (-68.9 ppm, JSiH = 202 Hz, -40 C, Figure S10) is
reaction of 1, CO
2
and HBpin at 24 C (M06-2X/def2-
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TZVP). Int and TS stands for intermediate and transition
state. For the detailed mechanism with arrow pushing, see
Scheme S3.
S1) to afford a mixture of [pinBOC(O)H] (2a), [pinBOMe]
(2b) and [(pinB) O] (2c). In comparison with the catalytic
results mediated by 1, it is suggested that IMe did not
dissociate and involve in the hydroboration of CO
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
2
.
Theoretical studies and experimental results show that
complex 1 is the first stable silicon(II) species undergoing
catalytic selective hydroboration with CO . Interestingly,
2
the proposed mechanism is very similar to a recent report
of the PNP pincer ligand-iron(II) hydride complex-
catalyzed hydroboration of alkynes, whereby the iron(II)
N
O
C
N

Grel = 15.2 kcal/mol
O
O
CO2
1
+ HBpin
3
Si
I
-H2
B
N
O
Grel = 7.4 kcal/mol
N
Grel = 0.0 kcal/mol
Int04 Grel = 11.6 kcal/mol
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
hydride
complex
[LFeH]
(L
=
2,5-bis-
HBpin
H
(phosphinomethyl)pyrrolide) can convert into the
corresponding iron(II) boryl complex [LFeBpin] in the
catalysis, along with both complexes were capable of
O
B
N
O
C
O
B
O
O
N
3
+
O
H
O
16
O
I
Si
catalyzing the hydroboration.
2a
B O
O
N

Grel = -1.1 kcal/mol
N
a
Table 1. Scope of Aldehyde Substrates
TS04 Grel = 12.4 kcal/mol
O
O
Cat 1 (10 mol %)
+
B
H
R
OBpin
OBpin
C6D6, 24 o
C
Scheme 4. Calculated Gibbs free energies (ΔGrel) via 3 at 24
R
H
O
o
C (M06-2X/def2-TZVP).
HBpin
4
5
DFT calculations also support that the catalytic
O
OBpin
OBpin
hydroboration of CO
kcal/mol; M06-2X/def2-TZVP, Scheme 4, Scheme S1),
especially complex 3 catalyzes the hydroboration of CO
2
via complex 3 is feasible (ΔG = -1.1
5a
5b
>99 % (91 %) 115.8 h
5c
>99 % (87 %), 115.8 h
-1
-1
-1
>
99 % (89 %), 115.8 h
2
via a lower kinetic barrier (ΔG3⟶TS04 = 5.0 kcal/mol) in
comparison with the mechanism via TS01. It is consistent
with experimental results that the activity of 3 in terms of
TOF is much higher than that of 1 (Table S1). However, the
kinetic barrier for the formation of 3 is relatively high
OBpin
OBpin
S
OBpin
O
O
5d
5e
5f
>99 % (82 %), 115.8 h
-1
-1
-1
>
99 % (87 %), 115.8 h
>99 % (83 %), 115.8 h
(
ΔG1⟶3
=
15.2 kcal/mol), which suggests that the
OBpin
Fe
OBpin
O
mechanism via TS01 should be more dominant than that
via 3 and TS04 in the catalysis.
OBpin
O
O
5g
5h
5j
Inoue et al. reported that the NHC-arylsilyliumylidene
>99 % (86 %), 118.8 h-1
>99 % (86 %), 115.8 h-1
>99 % (85 %), 115.8 h-1
cation [RSi(IMe
)
2
]Cl with steric hindered substituent 2,6-
at room
Mes (R = ArMes) reacted with CO
2
C
6
H
3
2
O
OBpin
5l
OBpin
OBpin
temperature to afford the NHC-arylsilaacylium
6
5k
5m
[RSi(=O)(IMe
)
2
]Cl. In contrast, when the ArMes substituent
-1
-1
-1
>
99 % (83 %), 19.8 h
>99 % (89 %), 23.8 h
>99 % (89 %), 115.8 h
was replaced by a lesser steric hindered substituent 2,4,6-
iPr (R = Tipp), only insoluble amorphous precipitates
3
C
6
H
2
OBpin
OBpin
OBpin
were formed in the reaction at room temperature.
Considering the steric environment of complex 1 and the
catalytic conditions, it is anticipated that the 1-catalyzed
F
Br
5n
5p
5q
-1
-1
-1
>
99 % (85 %), 115.8 h
>99 % (89 %), 17.0 h
>99 % (85 %), 29.7 h
reduction of CO
silaacylium intermediate is not possible. To support this
hypothesis, the reaction of 1 with CO was studied by DFT
2
with HBpin via an NHC-parent
BpinO
OBpin
OBpin
NC
O
2
N
5r
5s
5tb
calculations. The kinetic barrier and free energy for the
formation of an NHC-parent silaacylium intermediate
-1
-1
-1
>
99 % (91 %), 39.6 h
>99 % (89 %), 39.6 h
>99 % (91 %), 14.9 h
[(IMe
2
) Si(=O)H]I (kinetic barrier: ΔG = 13.4 kcal/mol,
a
6 6
Reaction conditions: aldehyde substrates (0.10 mmol), HBpin (0.11 mmol), C D
b
o
Scheme S4) are energetically less favorable in comparison
with those for the formation of formoxyborane 2a (Scheme
(0.50 mL), catalyst 1 (10 mol %). Reaction performed at 40 C. NMR yields are
determined by H NMR spectroscopy on the basis of the consumption of the
1
2
aldehyde and the identity of the product was confirmed by RCH OBpin or
resonances. Isolated yields are shown in parentheses. All the catalytic trials were
repeated in triplicate.
3
).
The presence of IMe in complex 1 also brought up a
question of whether IMe dissociates from complex 1 and
then catalyzes the hydroboration of CO . As such, 0.1 mol
2
Following the hydroboration of CO , the catalytic ability
2
% of IMe, which presumes small amount of the NHC ligand
of complex
1
towards hydroboration of carbonyl
being dissociated during the catalysis, was used to mediate
compounds was further examined. First, there were no
reaction between carbonyl compounds with HBpin in C D
o
the reduction of CO
2
with HBpin in C
D
6 6
at 90 C for 0.25h,
6
6
resulting in non-selective catalysis (19 % conversion, Table
at room temperature. Second, complex 1 (10 mol %) was
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Journal of the American Chemical Society
the product was confirmed by RC(R’)(H)OBpin or resonances. Isolated yields
are shown in parentheses. All the catalytic trials were repeated in triplicate.
found to be capable of catalyzing hydroboration of
aromatic aldehyde ArC(O)H (Ar = Ph, 4a, Table 1, Table S5)
as well as its derivatives with electron donating (Ar =
1
2
3
4
5
6
7
8
9
MeC
(MeCO
6
H
4
4b, MeOC
6
H
4
4c) and withdrawing substituents
4n) at different positions in 10
The catalytic ability of complex 1 toward the
hydroboration of pyridine derivatives was also studied
(Table 3, Table S7). First, there was no hydroboration
2
C
6
H
4
4e, FC
6
H
4
minutes, which quantitatively afforded the corresponding
borate esters. In these reactions, the activity of 1 in terms
reaction between HBpin and pyridine derivatives in C
at 90 C. Second, 10 mol % of 1 catalyzed the reaction of
6
D
6
-1
o
of TOF (17 - 115.8 h ) is intermediate between the heavier
group 14 element(II) compounds, namely
(amido)(hydrido)germylene (17 - 67 h ) and -stannylene
o
pyridine 8a with one equivalent of HBpin in C
D
6 6
at 90 C
-1
to quantitatively form N-boryl-1,4-dihydropyridine 9a as
-1
2
-1
(
400 - 800 h ). Third, >99 % yield was achieved for the
the only regioisomer (TOF = 2.48 h ), whereas the 1,2-
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
hydroboration of non-aromatic aldehydes, namely
cyclohexanecarboxaldehyde 4d and 2,2-dimethylpropanal
hydroborated product was not formed. Such
regioselectivity is comparable with metal-free B(C F )
6 5 3
1
7
4
k. Complex 1 is less active in these reactions (TOF = 19.8 -
catalyst (yield of 9a: >99 %)
and the 1,3,2-
-1
18
1
15.8 h ) in comparison with those catalyzed by the
diazaphosphenium triflate catalyst (yield of 9a: 96 %). In
addition, such catalysis has not been reported for heavier
group 14 element(II) compounds. Third, both chemo- and
regioselectivity were observed in the 1-catalyzed
hydroboration of functionalized pyridines 8b-c and ring-
fused pyridine 8d. Third, the scope of substrates can
further be extended to 1,2- and 1,3-pyrazines 8e-f.
(
>
amido)(hydrido)germylene and -stannylene (TOF =
2000 h ). Fourth, the olefinic functionality in 3-
-1 2
cyclohexene-1-carboxaldehyde 4h, cinnamaldehyde 4m
and 2-methyl-3-phenylprop-2-enal 4q remains intact in the
catalyses, showing that the chemoselective hydroboration
of aldehydes is possible. Such selectivity is also observed in
the hydroboration of thiophene-2-carbaldehyde 4f,
ferrocenecarboxaldehyde 4g, furan-2-carbaldehyde 4l, 4-
formylbenzonitrile 4r and isoquinoline-5-carboxaldehyde
Table 3. Scope of Pyridine Substratesa
O
O
R
Cat 1 (10 mol %)
R
+
B H
4s in which the functional groups such as nitrile and
C6D6, 90 o
N
C
N
Bpin
pyridine were not hydroborated. Such chemoselective
8
HBpin
9
catalyses have not been reported before for heavier group
2
O
O
1
4
element(II) compounds. In addition, excellent
CN
chemoselectivity of aldehydes over ketones was observed
N
Bpin
N
Bpin
N
Bpin
in the catalytic hydroboration of 4-acetylbenzaldehyde 4t.
o
Fifth, as expected, a higher reaction temperature (90 C)
9a
9b
9c
and longer reaction time were required for the
chemoselective hydroboration of ketones when compared
to aldehydes due to their steric nature (Table 2, Table S6).
Various functional groups in aromatic and aliphatic
ketones were well tolerated in these reactions and the
corresponding borate esters were afforded in high yields.
>99 % (90 %), 2.48 h-1
>99 % (89 %), 39.6 h-1
89 % (80 %), 4.45 h-1
N
N
N
N
N
Bpin
Bpin
Bpin
9d
9e
9f
>99 % (91 %), 1.1 h-1
>99 % (84 %), 2.48 h-1
67 % (59 %), 1.98 h-1
a
Reaction conditions: pyridine substrates (0.10 mmol), HBpin (0.11 mmol), C
6 6
D
a
(0.50 mL), catalyst 1 (10 mol %). NMR yields are determined by H NMR
1
Table 2. Scope of Ketone Substrates
spectroscopy on the basis of the consumption of the pyridine and the identity
of the product was confirmed by C=C or resonances. Isolated yields are shown
in parentheses. All the catalytic trials were repeated in triplicate.
O
O
R'
H
+
B
H
Cat 1 (10 mol %)
R
R'
O
o
R
OBpin
C6D6, 90 C
6
HBpin
7
Upon completing the above-mentioned catalysis (Tables
OBpin
BpinO
OBpin
1
– 3), complex 3 was observed (Figures S18 - 20, S33a, S43,
S45, S47, S49, S53, S65, S79, S81, S83a and S85a). These
indicate that both complexes 1 and 3 are involved in the
7
a
7b
7c
-1
-1
-1
catalyses. Similar to the case of CO , it is suggested that the
>
99 % (84 %), 1.65 h
>99 % (90 %), 1.98 h
>99 % (86 %), 1.65 h
2
Si lone pair of electrons in complexes 1 and 3 activate the
carbonyl compounds, which are then reacted with HBpin
O
OBpin
S
OBpin
OBpin
O
to form the corresponding hydroborated products, along
7d
7e
7f
19b,21
with the regeneration of the catalysts.
In case of
-1
-1
-1
>
99 % (85 %), 9.90 h
>99 % (89 %), 0.46 h
75 % (66 %), 0.31 h
pyridine derivatives, it is proposed that the catalysis
1
7
OBpin
OBpin
proceeds through coordination of 8 with HBpin first,
which induces nucleophilic attack of complexes 1 and 3 at
N
OBpin
20,21
Br
NC
the para-position of 8 due to lesser steric congestion.
7
g
7h
7j
>99 % (90 %), 9.90 h
Subsequent dearomatization of 8 results in displacing the
hydride from the borane moiety, which then attacks at the
para-position to afford 9, along with the regeneration of
the catalysts. In supporting complex 3 that involves in
-1
-1
-1
>
99 % (91 %), 1.41 h
84 % (75 %), 0.35 h
a
6 6
Reaction conditions: ketone substrates (0.10 mmol), HBpin (0.11 mmol), C D
(
0.50 mL), catalyst 1 (10 mol %). NMR yields are determined by 1H NMR
spectroscopy on the basis of the consumption of the ketone and the identity of
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Journal of the American Chemical Society
Page 6 of 9
these catalyses, it was used to catalyze the hydroboration
immersed in liquid nitrogen under vacuum to obtain a
frozen solution. The J-Young NMR tube was lifted from
liquid nitrogen and 1 bar of CO gas was then added. The
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
of benzaldehyde 4a (Table S8), 1,4-dioxaspiro[4.5]decan-8-
one 6d (Table S9) and pyridine 8a (Table S10), whereby
complex 3 shows better activity in terms of TOF in
comparison with complex 1.
reaction mixture was warmed to room temperature and
o
then heated at 90 C. The reaction was monitored by NMR
spectroscopy. The yields of products were reported
1
according to the integration of H NMR signals of
Conclusion
pinBOC(=O)H (2a) at 0.93 ppm and (pinB)
ppm with reference to the –OMe and CAr–H protons (3.36,
6.20 ppm) of the internal standard, 1,3,5-
2
O (2c) at 1.02
The NHC-parent silyliumylidene cation complex 1 is a
versatile catalyst to catalyze the metal-free chemo- and
regioselective hydroboration of carbon dioxide, carbonyl
compounds and pyridine derivatives with HBpin to form
trimethoxybenzene. The NMR data of 2a and 2c can be
found in the Supporting Information.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
formoxyborane,
borate
esters
and
N-boryl-1,4-
Synthesis of 2d – 2e. Catalyst 1 (4.0 mg, 0.01 mmol),
internal standard methyltriphenylsilane (27.4 mg, 0.10
dihydropyridine derivatives, respectively. In particular,
complex 1 is the first non-metal catalytic system that
efficiently and selectively delivers the primarily reduced
formoxyborane. Its activity is better than that of currently
available base-metal catalysts used for such reaction.
mmol, 10 equiv.) and 0.5 mL of C
Young NMR tube. Borane dimethylsulfide, BH
6
D
6
were mixed in a J-
.SMe (9.5
3
2
µL, 0.10 mmol, 10 equiv.) was then added. The NMR tube
was immersed in liquid nitrogen under vacuum to obtain a
frozen solution. The J-Young NMR tube was lifted from
Mechanistic studies show that complex
1 exhibits
transition-metal-like catalysis, whereby the silicon(II)
center in complex 1 activates the substrates and then
mediates the catalytic hydroboration. In addition, complex
2
liquid nitrogen and 1 bar of CO gas was then added. The
reaction mixture was warmed to room temperature and
o
then heated at 90 C. The reaction was monitored by NMR
1
was slightly converted into the NHC-borylsilyliumylidene
spectroscopy. The yields of products were reported
complex [(IMe SiBpin]I (3) in the catalysis, which was also
)
2
1
according to the integration of H NMR signals of B(OMe)
3
able to mediate the catalytic hydroboration. It seems
reasonable that complex 1 will find a range of other
catalytic applications (e.g. C–C bond formation, C–H bond
(
3
2d) at 3.42 ppm and (MeOBO) (2e) at 3.34 ppm with
reference to the –Me protons (0.71 ppm) of the internal
standard, methyltriphenylsilane. The NMR data of 2d and
1
0
functionalization ). We are currently investigating this
possibility and will report on our findings in due course.
The trapping of reactive intermediates with various Lewis
bases and acids will also be reported in the course of time.
2
e can be found in the Supporting Information.
Synthesis of 2f – 2g. Catalyst 1 (4.0 mg, 0.01 mmol),
internal standard 1,3,5-tri-tert-butylbenzene (2.5 mg, 0.01
mmol, 1 equiv.) and 0.5 mL of C were mixed in a J-Young
6 6
D
NMR tube. Catecholborane, HBcat (10.7 µL, 0.10 mmol, 10
equiv.) was then added. The NMR tube was immersed in
liquid nitrogen under vacuum to obtain a frozen solution.
The J-Young NMR tube was lifted from liquid nitrogen and
Experimental Procedure
General procedures. All manipulations were carried
out under an inert atmosphere of argon gas by standard
Schlenk techniques. Compound 1 was prepared according
1
bar of CO
2
gas was then added. The reaction mixture was
10
to the literature procedure. Toluene was dried over Na/K
alloy and distilled prior to use. C and d -pyridine were
dried over K metal and distilled prior to use. CH Cl and
CDCl were dried over CaH and distilled prior to use.
Chemicals were purchased and used directly without
o
warmed to room temperature and then heated at 90 C.
The reaction was monitored by NMR spectroscopy. The
yields of products were reported according to the
D
6 6
5
2
2
3
2
1
integration of H NMR signals of catBOMe (2f) at 3.39 ppm
and (catB)
2
O (2g) at 6.72 – 6.75 ppm with reference to the
1
11
11
1
13
1
29
further purification. The H, B, B{ H}, C{ H}, Si and
-Me and CAr-H protons (1.35, 7.42 ppm) of the internal
29
1
Si{ H} NMR spectra were recorded on a JEOL ECA 400
6
spectrometer. The NMR spectra were recorded in C D ,
standard, 1,3,5-tri-tert-butylbenzene. The NMR data of 2f
and 2g can be found in the Supporting Information.
6
CDCl
SiMe
3
or d
5
-pyridine, and the chemical shifts are relative to
1
13
29
.
11
Synthesis of 3. Compound 1 (4.0 mg, 0.01 mmol) and
pinacolborane, HBpin (14.5 µL, 10 equiv, 0.10 mmol) were
4
for H, C and Si; BF
3
2
Et O for B, respectively. The
following abbreviations are used to describe signal
multiplicities: s = singlet, d = doublet, t = triplet, q = quartet
and m = multiplet. Coupling constants J are given in Hertz
mixed with 0.5 mL of C
temperature. The resulting mixture was stirred at 60 C for
6
D
6
in a J-Young NMR tube at room
o
o
5
min. Volatiles were immediately removed at 60 C in
(
Hz). Melting points were measured in sealed glass tubes
vacuo. The residue was extracted with benzene (1 mL).
After filtration, the filtrate was removed in vacuo to afford
compound 3 as a colorless solid (1.6 mg, Yield: 30 %). When
the reaction was performed for 16 hours, the yield of
and were not corrected. Electrospray ionization (ESI) mass
spectra were obtained at the Mass Spectrometry
Laboratory at the Division of Chemistry and Biological
Chemistry, Nanyang Technological University.
compound 3 is 63 % (3.3 mg). Mp: 167 °C (dec.). HRMS
Synthesis of 2a – 2c. Catalyst 1 (4.0 mg, 0.01 mmol),
internal standard 1,3,5-trimethoxybenzene (8.4 mg, 0.05
mmol, 5 equiv.) and 0.5 mL of C D were mixed in a J-
6 6
Young NMR tube. Pinacolborane, HBpin (14.5 µL, 0.10
mmol, 10 equiv.) was then added. The NMR tube was
+
(
ESI): m/z calcd for C20
H
37BIN
4
O
2
Si [M-H] : 531.18236;
found: 531.18240. Satisfactory elemental analysis data could
not be obtained due to compound 3 being highly sensitive
1
to moisture and air. H NMR (395.9 MHz, 24 °C, C
6 6
D , ppm):
3 3
δ = 3.69 (s, 12H, N-CH ), 1.75 (s, 12H, C-CH ), 1.08 (s, 12H,
ACS Paragon Plus Environment

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Journal of the American Chemical Society
1
1
Bpin-CH
3
1
). B NMR (128.41 MHz, 24 °C, C
D
6 6
, ppm): δ =
*midesu@mail.ncyu.edu.tw
*chiksiu@cityu.edu.hk
1
2
3
4
5
6
7
8
9
11
0
.92. B{ H} NMR (128.41 MHz, 24 °C, C D , ppm): δ = 0.95.
Si NMR (78.65 MHz, 24 °C, C
NMR (78.65 MHz, 24 °C, C , ppm): δ = -95.6. The Si NMR
signal is relatively weak due to quadrupolar broadening
with the boron nucleus (I = 3/2).
6 6
29
29
1
6 6
D , ppm): δ = -93.0. Si{ H}
Notes
6 6
D
The authors declare no competing financial interests.
Author Contributions
The catalytic hydroboration of carbon dioxide using
The manuscript was written through contributions of all
authors.
3
as the catalyst. Catalyst 3 (1.3 mg, 0.0025 mmol), internal
standard 1,3,5-trimethoxybenzene (8.4 mg, 0.05 mmol, 20
equiv.) and 0.5 mL of C were mixed in a J-Young NMR
D
6 6
ACKNOWLEDGMENT
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
tube. Pinacolborane, HBpin (14.5 µL, 0.10 mmol, 40 equiv.)
was then added. The NMR tube was immersed in liquid
nitrogen under vacuum to obtain a frozen solution. The J-
Young NMR tube was lifted from liquid nitrogen and 1 bar
This work is supported by ASTAR SERC PSF grant and AcRF
Tier 1 grant (C.-W. So.). M.-C. Yang and M.-D. Su are grateful
to the National Center for High-Performance Computing of
Taiwan for generous amounts of computing time, and the
Ministry of Science and Technology of Taiwan for the financial
support.
of CO
2
gas was then added. The reaction mixture was
o
warmed to room temperature and then heated at 90 C.
The reaction was monitored by NMR spectroscopy. The
yields of products were reported according to the
REFERENCES
1
(1) (a) Hadlington, T. J.; Driess, M.; Jones, C. Low-Valent Group
4 Element Hydride Chemistry: Towards Catalysis. Chem. Soc.
integration of H NMR signals of pinBOC(=O)H (2a) at 0.93
1
2
ppm and (pinB) O (2c) at 1.02 ppm with reference to the –
Rev. 2018, 47, 4176-4197. (b) Yadav, S.; Saha, S.; Sen, S. S.
Compounds with Low-Valent p-Block Elements for Small
Molecule Activation and Catalysis. ChemCatChem 2016, 8, 486-
501. (c) Power, P. P. Main-group Elements as Transition Metals.
Nature 2010, 463, 171-177.
OMe and CAr–H protons (3.36, 6.20 ppm) of the internal
standard, 1,3,5-trimethoxybenzene. The NMR data of 2a
and 2c can be found in the Supporting Information.
General procedures for the catalytic hydroboration
of carbonyl compounds and pyridine derivatives
using 1 as the catalyst. Catalyst 1 (4.0 mg, 0.01 mmol) and
(
2) Hadlington, T. J.; Hermann, M.; Frenking, G.; Jones, C. Low
Coordinate Germanium(II) and Tin(II) Hydride Complexes:
Efficient Catalysts for the Hydroboration of Carbonyl
Compounds. J. Am. Chem. Soc. 2014, 136, 3028-3031.
0
6 6
.5 mL of C D were added into a J-Young NMR tube.
Pinacolborane, HBpin (16.0 µL, 0.11 mmol, 10.1 equiv.) and
substrates (0.10 mmol, 10 equiv.) were then added. The
reaction conditions are indicated in Tables S5 – S7 in the
Supporting Information and the reactions were followed
by NMR spectroscopy to determine their yields. The NMR
data of substrates can be found in the Supporting
Information.
(
3) Hadlington, T. J.; Kefalidis, C. E.; Maron, L.; Jones, C.
Efficient Reduction of Carbon Dioxide to Methanol Equivalents
Catalyzed by Two-Coordinate Amido-Germanium(II) and -Tin(II)
Hydride Complexes. ACS Catal. 2017, 7, 1853-1859.
(4) (a) Schneider, J.; Sindlinger, C. P.; Freitag, S. M.; Schubert,
H.; Wesemann, L. Diverse Activation Modes in the Hydroboration
of Aldehydes and Ketones with Germanium, Tin, and Lead Lewis
Pairs. Angew. Chem., Int. Ed. 2017, 56, 333-337. (b) Wu, Y.; Shan,
C.; Sun, Y.; Chen, P.; Ying, J.; Zhu, J.; Liu, L.; Zhao, Y. Main Group
Metal-Ligand Cooperation of N-heterocyclic Germylene: an
Efficient Catalyst for Hydroboration of Carbonyl Compounds.
Chem. Commun. 2016, 52, 13799-13802. (c) Erickson, J. D.; Lai, T.
Y.; Liptrot, D. J.; Olmstead, M. M.; Power, P. P. Catalytic
Dehydrocoupling of Amines and Boranes by an Incipient Tin(II)
Hydride. Chem. Commun. 2016, 52, 13656-13659.
General procedures for the catalytic hydroboration
of carbonyl compounds and pyridine derivatives
using 3 as the catalyst. Catalyst 3 (5.3 mg, 0.01 mmol),
HBpin (16.0 µL, 0.11 mmol, 1.1 equiv.) and substrates (0.10
6 6
mmol, 1 equiv.) were mixed with 0.5 mL of C D in a J-
Young NMR tube at ambient temperature. The reaction
mixture was stirred with the reaction conditions stated in
Tables S8 – S10. The reactions were monitored by NMR
spectroscopy to determine their yields. The NMR data of
substrates can be found in the Supporting Information.
(
5) Selected recent articles: (a) Driess, M.; Yao, S.; Brym, M.; van
Wüllen, C.; Lentz, D. A New Type of N-Heterocyclic Silylene with
Ambivalent Reactivity. J. Am. Chem. Soc. 2006, 128, 9628-9629.
(
b) Yao, S.; vanꢀWüllen, C.; Sun, X.-Y.; Driess, M. Dichotomic
Reactivity of a Stable Silylene toward Terminal Alkynes: Facile C-
Bond Insertion versus Autocatalytic Formation of
Silacycloprop-3-ene. Angew. Chem. Int. Ed. 2008, 47, 3250-3253.
c) Xiong, Y.; Yao, S.; Driess, M. Striking Reactivity of a Stable,
A full description of experimental methods and
theoretical studies can be found in the Supporting
Information.
H
(
Zwitterionic Silylene Towards Substituted Diazomethanes,
Azides, and Isocyanides. Chem. –Eur. J. 2009, 15, 8542-8547. (d)
Xiong, Y.; Yao, S.; Driess, M. Versatile Reactivity of a Zwitterionic
Isolable Silylene toward Ketones: Silicon-Mediated, Regio- and
Stereoselective C-H Activation. Chem. – Eur. J. 2009, 15, 5545-
5551. (e) Yao, S.; Brym, M.; vanꢀWüllen, C.; Driess, M. From a
Stable Silylene to a Mixed-Valent Disiloxane and an Isolable
Silaformamide–Borane Complex with Considerable Silicon–
Oxygen Double-Bond Character. Angew. Chem. Int. Ed. 2007, 46,
ASSOCIATED CONTENT
Supporting Information. The Supporting Information is
available free of charge on the ACS Publications website.
Experimental procedures and theoretical studies (PDF)
X-ray crystallographic data for [(IMe) Bpin]I and [IMe–H]I (CIF)
2
AUTHOR INFORMATION
Corresponding Author
4
159-4162. (f) Yao, S.; vanꢀWüllen, C.; Sun, X.-Y.; Driess, M.
Dichotomic Reactivity of a Stable Silylene toward Terminal
Alkynes: Facile C-H Bond Insertion versus Autocatalytic
*
CWSo@ntu.edu.sg
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 9
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1
1
1
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1
1
1
1
1
2
2
2
2
2
2
2
2
2
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3
3
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Neumann,
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Stammler,
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Jutzi,
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The
Pentamethylcyclopentadienylsilicon(II) Cation as a Catalyst for
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a Silicon(I) Radical with Carbenes: A Cationic cAAC-Silicon(I)
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31, 7562-7563. (i) Xiong, Y.; Yao, S.; Müller, R.; Kaupp, M.; Driess,
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Elusive Dioxasiliranes and Sila-ureas Stable at Room
Temperature. Nat. Chem. 2010, 2, 577. (j) Xiong, Y.; Yao, S.; Driess,
M. Unusual [3+1] Cycloaddition of a Stable Silylene with a 2,3-
Diazabuta-1,3-diene versus [4+1] Cycloaddition toward a Buta-1,3-
diene. Organometallics 2010, 29, 987-990. (k) Meltzer, A.; Inoue,
S.; Präsang, C.; Driess, M. Steering S−H and N−H Bond Activation
0
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(11) Mukherjee, D.; Osseili, H.; Spaniol, T. P.; Okuda, J. Alkali
by a Stable N-Heterocyclic Silylene: Different Addition of H
NH , and Organoamines on a Silicon(II) Ligand versus Its
Si(II)→Ni(CO) Complex. J. Am. Chem. Soc. 2010, 132, 3038-3046.
(l) Präsang, C.; Stoelzel, M.; Inoue, S.; Meltzer, A.; Driess, M.
Metal-Free Activation of EH (E=P, As) by an Ylide-like Silylene
and Formation of a Donor-Stabilized Arsasilene with a HSi=AsH
Subunit. Angew. Chem. Int. Ed. 2010, 49, 10002-10005. (m) Asay,
M.; Jones, C.; Driess, M. N-Heterocyclic Carbene Analogues with
Low-Valent Group 13 and Group 14 Elements: Syntheses,
Structures, and Reactivities of a New Generation of Multitalented
Ligands. Chem. Rev. 2011, 111, 354-396. (n) Yao, S.; Xiong, Y.; Driess,
M. Zwitterionic and Donor-Stabilized N-Heterocyclic Silylenes
2
S,
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] (M = Li, Na, K):
2
Chemoselective Catalysts for Carbonyl and CO Hydroboration. J.
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(13) Suh, H.-W.; Guard, L. M.; Hazari, N. A Mechanistic Study
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of a Pd(II) Pincer Complex for the Catalytic Hydroboration of
CO . Chem. Sci. 2014, 5, 3859-3872.
2
Resulting in the Development
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NHSis) for Metal-Free Activation of Small Molecules.
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Driess, M. New Vistas in N-Heterocyclic Silylene (NHSi)
Transition-Metal Coordination Chemistry: Syntheses, Structures
and Reactivity towards Activation of Small Molecules. Chem. –
M. F.; Weetman, C. Selective Reduction of CO
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to a Methanol
Equivalent by B(C -Activated Alkaline Earth Catalysis. Chem.
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Soc. 2018, 140, 16962-16966. (q) Wang, Y.; Kostenko, A.;
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828-5836. (b) Ahmad, S. U.; Szilvási, T.; Inoue, S. A Facile Access
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7) (a) Rodriguez, R.; Gau, D.; Kato, T.; Saffon-Merceron, N.; De
a
1
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Rodriguez, R.; Gau, D.; Contie, Y.; Kato, T.; Saffon-Merceron, N.;
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2
insertion of CO into the Si–H bond of 1 is not possible. (b)
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6
D
6
, whereby
1
the H NMR spectroscopy did not show any signal for –CH
2
O
moiety, indicating that PhC(O)H did not insert into the Si–H
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Hydride and Its Addition to Olefins:
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A
Catalyst-Free
‡
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= 48.4 kcal/mol, M06-2X/def2-SVP). It is suggested that the
catalysis should proceed through the activation of PhC(O)H by
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(20) Bertuzzi, G.; Bernardi, L.; Fochi, M., Nucleophilic
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and Si -Cl σ-Bonds at Room Temperature. Angew. Chem., Int. Ed.
II
II
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015, 54, 15276-15279. (d) Rodriguez, R.; Contie, Y.; Nougue, R.;
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6
6
o
at 90 C for 6 h.
1
4358.
(8) Nakagaki, M.; Baceiredo, A.; Kato, T.; Sakaki, S. Reversible
Oxidative Addition/Reductive Elimination of a Si-H Bond with
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Journal of the American Chemical Society
SYNOPSIS TOC
1
2
3
4
5
6
7
8
9
Catalyst
N
N
Product
R'
Substrate
H
Si
O
H
NHC-Parent Silyliumylidene Cation
N
I
R
OBpin
N
Metal-free catalysis
Green chemistry
R
R'
O
O
O
B
O
O
1st silicon(II) species in catalytic CO2
CO2
+
B
H
O
H
hydroboration
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
High selectivity
R
R
N
N
Bpin
ACS Paragon Plus Environment