?-Silicon-effect-promoted intermolecular site-selective C(sp3)-H amination with dirhodium nitrenes

Article abstract of DOI:10.1039/d0cc00959h

A dirhodium-catalyzed, ?-selective C-H amination of organosilicon compounds has been developed. Primary C(sp³)-H bonds of silylethyl groups and secondary C(sp³)-H bonds of silacycloalkanes can be selectively converted to C-N bonds at the ?-position of the silicon atoms. The experimental data and theoretical calculations indicate that the strong s-donor ability of the carbon-silicon bonds is responsible for the ?-selectivity. Kinetic isotope effects clearly demonstrate that the C-H bond cleavage step is not turnover-limiting, but selectivity-determining.

Full text of DOI:10.1039/d0cc00959h

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DOI: 10.1039/D0CC00959H
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β-Silicon-Effect-Promoted Intermolecular Site-Selective C(sp3)-H
Amination with Dirhodium Nitrenes
Ryo Ninomiya,‡ Kenta Arai,‡ Gong Chen, Kazuhiro Morisaki, Takeo Kawabata and Yoshihiro Ueda*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A dirhodium-catalyzed, β-selective C-H amination of organosilicon
compounds has been developed. Primary C(sp3)-H bonds of
silylethyl groups and secondary C(sp3)-H bonds of silacycloalkanes
can be selectively converted to C-N bonds at the β-position of the
silicon atoms. The experimental data and theoretical calculations
indicate that the strong σ-donor ability of the carbon-silicon bonds
is responsible for the β-selectivity. Kinetic isotope effects clearly
demonstrate that the C-H bond cleavage step is not turnover-
limiting, but selectivity-determining.
H
Rh2 cat.
NHR’’
nitrene source
R
R’
R
R’
(b) This work: β-Position of Si atom
(a) Reported site selectivity
H
H
R3Si
!
C-Si → !*C-H
&
1°
%
R3Si
%
%
R
R
RO
R’
H
H
%
3°
&
2°
H
R2Si
H
R’’
β-silicon effect
R’
n
Fig. 1 Site selectivity in the intermolecular C-H amination of substrates
Metal-catalyzed nitrogen-group-transfer via C-H bond
cleavage has become an important method for the construction
of C-N bonds.1 While the cleavage C-H bonds is position-limited
in intramolecular reactions, control of site selectivity becomes
an issue in intermolecular reactions.2 Dirhodium nitrenes have
emerged as powerful intermediates for C(sp3)-H amination
reactions.3,4 At present, electron rich C(sp3)-H bonds, such as
C(sp3)-H bonds α to C-C multiple bonds and oxygen atoms, and
tertiary C(sp3)-H bonds can be selectively converted to C-N
bonds, even in intermolecular reactions (Fig. 1a).3–5 Here we
report dirhodium-catalyzed intermolecular site-selective C(sp3)-
H amination of organosilicon compounds promoted by the β-
silicon effect6 (Fig. 1b). The strong σ-donor ability of carbon-
silicon bonds activates C(sp3)-H bonds at the β-position of
silicon atoms to be selectively converted to C-N bonds. Because
silicon can be considered a bioisostere of carbon, the present
protocol is applicable to synthesizing multi-functionalized
organosilicon compounds with the potential to be powerful
tools for drug discovery.7
without coordinating directing groups.
Recently, Davies reported an enantioselective C-H alkylation
catalyzed by their original chiral dirhodium complexes.8g,h
However, to the best of our knowledge, there are no examples
of intermolecular β-selective C-H amination of organosilicon
compounds.9 In 2018, we reported a chemo- and regioselective
aromatic C(sp2)-H amination of alkoxyarenes promoted by
dirhodium nitrenes.10 During the study, we unexpectedly found
that competitive C(sp3)-H amination of the silyl- substituted
compounds took place at the β-position of the silicon atoms.
Inspired by this, we investigated the β-selective C(sp3)-H
amination of organosilicon compounds to find that primary
C(sp3)-H bonds of silylethyl groups and secondary C(sp3)-H
bonds of silacycloalkanes have sufficient reactivity to be
selectively converted to C-N bonds (Fig. 1b).
Various reaction parameters, including dirhodium
complexes, aminating agents, solvents, and bases were
screened to optimize the reaction conditions for the C-H
amination of triethylphenylsilane (1a) (see ESI for details).
C(sp3)-H aminated product 2a was obtained in 72% isolated
yield when 10 equivalents of 1a was treated with Rh2(tpa)4 (5
Prior investigations into the β-silicon effect in
intermolecular C-H functionalization reaction have mainly
focused on carbene insertion reactions.8 As for dirhodium-
catalyzed reactions, Hatanaka and Tanaka reported the first
example of a β-selective C-H alkylation of silacycloalkanes.8f
mol%),
O-tosyl-N-trichloroethoxycarbonylhydroxylamine11
(Troc-NHOTs, 1.0 equiv), and K2CO3 (1.5 equiv) in chlorobenzene
at 20 °C for 12 h (Scheme 1). The combination of a catalyst and
a solvent is critical for obtaining 2a in high yield; the use of
Rh2(octanoate)4, Rh2(pivalate)4, and Rh2(n-C3F7CO2)4 gave
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x
‡ These authors contribute equally to this work.
3e
negligible C(sp3)-H amination, but Rh2(esp)2 and Rh2(tpa)4,
used in PhCl or AcOEt, provided 2a in moderate to good yield
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4. InViecwoAnrttircalesOt,nlin5e
underwent the C(sp )-H amination selectively at β-position of
the germane atom to provide in 46% yield. These results
clearly demonstrate that the β-selective C(sp3)-H amination is
responsible for β-effect with the heavier group 14 elements.6,13
It is worth noting that this is the first reported example of a β-
selective C-H functionalization of organogermane compounds.
We then explored the substrate scope of the β-selective
C(sp3)-H amination under the optimum conditions (Table 1).
any C-H aminated products, including
Rh2(tpa)4 (5 mol%)
TrocNH-OTs (1.0 equiv)
K2CO3 (1.5 equiv)
3
DOI: 10.1039/D0CC00959H
H
NHTroc
X
X
6
PhCl, 20 ºC, 12 h
(10 equiv)
2a: X=Si, 72%
4: X=C, 0%
1a: X=Si
3: X=C
tpa = triphenylacetate
6: X=Ge, 46%
5: X=Ge
Scheme 1 β-Position activation by group 14 elements.
Triethyl(4-trifluoromethylphenyl)silane
(
1b
)
and
(Table S1 and S2). In addition, investigation of aminating agents
indicated that TrocNHOTs was the most effective for the
present C-H amination (Table S3). Notably the reaction using
iminoiodinane (PhI=NTs) did not afford any products. Under the
optimum condition, Troc-NHOTs was almost completely
consumed and formation of Troc-NH2 was observed, which
indicated partial decomposition of Troc-NHOTs under the
condition.11b,12
diethyldiphenylsilane (1c) underwent C(sp3)-H amination at the
β-position of the C-Si bond in moderate yield. Amination of t-
butyldimethylphenylsilane (1d) proceeded selectively at the
sterically congested neopentyl position to provide 2d. C-H
amination of triethylalkylsilanes 1e
H bonds took place exclusively at the primary C(sp3)-H bonds to
provide 2e
2h in good yields. C(sp3)-H amination of 2-
–1h with various benzylic C-
–
triethylsilylindane (1i) proceeded at two β-positions to give the
primary C(sp3)-H aminated product 2ia and the benzylic C(sp3)-
H aminated product 2ib in 31% and 34% yield, respectively. The
relative configuration of 2ib was assigned from the observed
NOEs (see ESI). β-Selective C(sp3)-H aminations of 1j with a
C(sp3)-H α to an ethereal oxygen and 1k with a tertiary C(sp3)-H
also took place selectively to provide 2j and 2k in 44% and 77%
yield, respectively, as sole products. Diethylsilafluorene (1l), a
useful building block for functional materials,14 was also
aminated by the present protocol to provide 2l selectively.
In addition to the primary C(sp3)-H amination, the reactivity
of secondary and tertiary C(sp3)-H bonds at the β-position of
silicon atoms were investigated. Although C-H amination of
To investigate the β-activating effect of the silicon atom, the
optimized reaction conditions were then applied to 1,1,1-
diethylphenylpropane (
carbon and germane congener of 1a, respectively (Scheme 1).
Treatment of with the optimum conditions did not provide
3) and triethylphenylgermane (5), the
3
Table 1 Substrate scope
R
4 R5
R
4 R5
Rh2(tpa)4 (5 mol%)
TrocNH-OTs (1.0 equiv)
K2CO3 (1.5 equiv)
R1
R2
R1
R2
H
NHTroc
Si
R3
Si
R3
R
6 R7
R
6 R7
PhCl, 20 ºC, 12 h
1 (10 equiv)
2
Me
Si
tripropylphenylsilane (1m) and triisobutylphenylsilane (1n
)
NHTroc
NHTroc
NHTroc
Si
Si
Ar
Ph
exclusively proceeded at the β-position, the efficiency was
significantly decreased. Conversely, the endocyclic secondary
C(sp3)-H bonds of silacycloalkanes are highly reactive.
Treatment of silacyclobutane 1o (5 equiv) under the optimum
condition provided the β-aminated product in good yield. In the
Ph
Ph
Me
2a: Ar = Ph, 72%
2b: Ar = 4-CF3C6H4, 57%
2c 45%
2d 25%
H
H
Me
NHTroc
Ph
NHTroc
Ph
Si
Si
Si
NHTroc Ph
H
case of silacyclopentane 1p and silacyclohexane 1q
, 2
2e 53%
2f 63%
2g 68%
NHTroc
SiEt3
equivalents of the substrates are enough to obtain the β-
aminated products 2p and 2q in excellent yield. C(sp3)-H bonds
of germacyclopentane 1r have comparable reactivity to
silacyclopentane to afford 2r in good yield.
Ph
NHTroc
Si
H
Si
+
NHTroc
A rationale for the reactivity difference between 1m and 1p
is shown in Fig. 2. When a C-Si bond and a C-H bond are anti-
periplanar, maximum hyperconjugation between the σC-Si and
the σ*C-H is achieved,15 and the C-H bond is most reactive. In the
2h 58%
2ia 31%
2ib 34% (d.r. >20:1)
H
NHTroc
NHTroc
Si
Si
Ph
O
H
Si
NHTroc
2k 77%
2j 44%
(a)
(b)
Si = -Si(n-Pr)2Ph
2l 37%
Si
Si
n-Pr
H
H
H
H
Me
H
H
H
>
NHTroc
Si
Ph
Ph
Si
Si
Ph
Me
H
Si
NHTroc
2o 58%
with 5 equiv of 1n
n-Pr
Ph
Ph
NHTroc
stable reactive
conformer conformer
1m
2m 18%
2n 10%
Ph
Ph
Ph
Si
Si
1p
H
Ph
Ph
Ph
stable and reactive conformer
Ph
Ph
"
Ph
Si
Si
Ge
!
NHTroc
Ph
NHTroc
NHTroc
2q 89% (β/γ = 25/1)
with 2 equiv of 1q
2p 88%
with 2 equiv of 1p
2r 71%
with 2 equiv of 1r
Fig. 2 (a) Stable and reactive conformers of 1m and 1p. (b) HOMO of 1p
calculated at the B3PW91/6-31g(d,p) level of theory.
2 | J. Name., 2012, 00, 1-3
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produced by DFT calculations, using (U)M06 functiVoienwsA(rStieclee OEnSliIn)e,
with 1,1-dimethylsilacyclopentane asDOtIh: 1e0.10s3u9b/Dst0rCaCte009a5n9Hd
Rh2(OAc)4 as a catalyst model (Fig. 3b). We calculated both of
the singlet and triplet pathways for the C-H insertion reaction to
find the activation free energy of the singlet pathway (ΔG‡ =
10.9 kcal/mol) was significantly lower than that of the triplet
pathway (ΔG‡ = 13.3 kcal/mol). The trigonal N-H-C angle of
152.6 ° in the TS indicated that both hydride transfer and C-N
bond formation take place in an asynchronous concerted
manner. Indeed, calculations did not provide any intermediates
between the TS structure and the product structure. In natural
bond orbital (NBO) calculations, the second order perturbation
H
NHTroc
Si
Si
Ph
Ph
Rh2(tpa)4 (5 mol%)
TrocNH-OTs (1.0 equiv)
K2CO3 (1.5 equiv)
2a 18%
1a (10 eq.)
+
+
PhCl, 20 ºC, 12 h
H
NHTroc
7 (10 eq.)
8 43%
Scheme 2 Intermolecular competitive reaction between 1a and 7.
stable conformer of 1m, C-Si bonds and C-H bonds are not anti-
periplanar and the reactive conformer of 1m is disfavored by
the gauche interaction between the silyl group and the methyl
(a)
Rh2(tpa)4 (5 mol%)
TrocNH-OTs (1.0 equiv)
K2CO3 (1.5 equiv)
C2H5
Si
C2H5
C2H5
Si
C2H5
2b
NHTroc
C2H5
group (Fig. 2a). However, in the stable conformer of 1p
,
pseudoequatorial C-H bonds and endocyclic C-Si bonds are
naturally anti-periplanar (Fig. 2a), which is responsible for the
higher reactivity of the C-H bonds in 1p. These considerations
were supported by frontier orbital calculations of 1p by density
functional theory (DFT) (Fig. 2b): the calculated HOMO
extended over the pseudoequatorial C-H bonds at the β-
position as well as the C-Si bonds and the aromatic rings. While
the σ-σ* orbital interaction is effective in the case of 5- and 6-
membered silacycloalkanes, it is less effective in the case of
silacyclobutanes, which is consistent with the experimental
results in Table 1 (2o vs 2p and 2q).
PhCl, 20 ºC
rate constant = kH
CF3
CF3
1b (10 eq.)
kH/kD = 1.0 ± 0.1
Rh2(tpa)4 (5 mol%)
TrocNH-OTs (1.0 equiv)
K2CO3 (1.5 equiv)
D
D
C2D5
C2D5
Si
C2D5
NHTroc
C2D5
Si
D
D
C2D5
PhCl, 20 ºC
rate constant = kD
CF3
CF3
CF3
CF3
1b-d15 (10 eq.)
2b-d14
(b)
C2H5
C2H5
Si
C2H5
2b
NHTroc
NHTroc
C2H5
Si
Rh2(tpa)4 (5 mol%)
TrocNH-OTs (1.0 equiv)
K2CO3 (1.5 equiv)
C2H5
CF3
1b (5.0 eq.)
+
+
PhCl, 20 ºC, 12 h
D
D
To investigate the activation ability of C-H bonds by C-Si
bonds, competitive C-H amination reaction between 1a and
mesitylene (7
), which has the same number of primary C(sp3)-H
C2D5
C2D5
Si
C2D5
C2D5
Si
kH/kD = 5.3 ± 0.4
D
D
C2D5
CF3
bonds with 1a at the benzylic positions, was examined (Scheme
2). The reaction provided a mixture of β-aminated product 2a
1b-d15 (5.0 eq.)
2b-d14
Scheme
3 Kinetic isotope effects. (a) Two parallel experiments. (b)
(18%) and benzylic aminated product
suggest that activation ability by C-Si bonds is slightly inferior to
that by phenyl ring. Based on this result, substrate 1e and 1j
underwent selective amination at β-position of the silicon
8 (43%). This result
Intermolecular competition experiment.
-
h
(a)
atoms to give only 2e-h and 2j probably due to their sterically
Rh Rh
N
H
Rh2(tpa)4
=
Rh Rh
demanding benzylic C(sp3)-H bonds (Table 1).
Troc
TrocNH-OTs
K2CO3
KOTs
KHCO3
To gain insights into the mechanism, the kinetic isotope
effect of the reaction was measured (Scheme 3). Two
independent deuterium labeling studies for the reactions of
Si
Si
TrocHN
selectivity-
determining step
Si
Si
trifluoromethylphenylsilane derivatives 1b and 1b-d15 showed
H-N bond formation
the intermolecular kinetic isotope effect (KIE) value as kH/kD =
1.0 ± 0.1 (Scheme 3a). In contrast, the intermolecular
competitive reaction using 1b and 1b-d15 afforded the
H
Rh Rh
N
C-N bond formation
Rh Rh
N
Troc
H
not turnover-
limiting step
Troc
TS
intermolecular competitive KIE value as kH/kD = 5.3 ± 0.3
(Scheme 3b). These kinetic studies indicated that the C-H bond
cleavage is not involved in the turnover-limiting step, but in the
selectivity-determining step.16
A proposed catalytic cycle is illustrated in Fig. 3a. The
dirhodium nitrene is generated from Rh2(tpa)4, TrocNHOTs, and
K2CO3 as described in Lebel’s report for C(sp3)-H
amination.11,12b,17 The dirhodium nitrene species is thought to
be responsible for the C-H insertion reaction (N-H bond
formation and C-N bond formation). Based on the kinetic
studies shown in Scheme 3, the C-H insertion step is not the
turnover-limiting step, but the selectivity-determining step. The
transition state (TS) structure of the C-H bond cleavage step was
!
C-Si → !*C-H
(b)
Me
4.07 kcal/mol
Si
Me
O
Me
Me
O
O
O
H
Rh
Rh
N
O
O
O
O
O
OCH2Cl3
Me
Me
Me
C
vs
!
C-C → !*C-H
Me
∠N-H-C=152.6 °
ΔG‡ = 10.9 kcal/mol
2.12 kcal/mol
H
(AcO)4Rh2
N
Troc
Fig. 3 (a) Proposed catalytic cycle. (b) Calculated structure of the TS of the C-
H cleavage step and NBO analysis.
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J. Name., 2013, 00, 1-3 | 3
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Lett., 2002, 43, 9561; (e) C. G. Espino, K. W. Fiori, M. Kim and
of the TS structure based on the donation of the endocyclic C-Si
σ bond towards the reacting C-H σ* orbital was 4.07 kcal/mol.18
From the NBO analysis of the corresponding TS structure
derived from 1,1-dimethylcyclopentane, donation of the
endocyclic C-C σ bond towards the reacting C-H σ* orbital was
calculated to be 2.12 kcal/mol, which indicated that the strong
σ-donor ability of the carbon-silicon bonds is responsible for site
selectivity in the present protocol.
In conclusion, we have reported the first example of an
intermolecular β-selective C-H amination of organosilicon and
organogermane compounds. Primary C(sp3)-H bonds of
silylethyl groups and secondary C(sp3)-H bonds of
silacycloalkanes have sufficient reactivity to be selectively
transformed into C-N bonds. Kinetic isotope effects clearly
showed that the C-H bond cleavage step is not the turnover-
limiting step, but the selectivity-determining step. The
experimental data and theoretical calculations indicated that
the β-selectivity reflected the hyperconjugation between C-Si σ
bonds and C-H σ* orbitals. Because organosilicon compounds
represent an important area of chemical space in diverse fields
such as material science19 and pharmaceutical chemistry,7 the
present protocol could provide useful tools for exploring new
functional molecules. Investigation of further substrate scope
and stereoselective reaction is currently underway.
View Article Online
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8
This work was financially supported by JSPS KAKENHI
(Grants Number JP26221301, JP18H04405 in Middle Molecular
Strategy, JP18K14866, and JP19K16314) and the Uehara
Memorial Foundation. R. N. acknowledges the financial support
through JSPS Research Fellowships for Young Scientists
(JP18J22026). We thank Alison McGonagle, PhD, from Edanz
Group (www.edanzediting.com/ac) for editing a draft of this
manuscript.
9
Only one example of intramolecular C-H amination of a silyl-
substituted aminobutane derivative forming a 6-membered
ring has been reported. D. E. Olson, A. Roberts and J. Du Bois,
Org. Lett., 2012, 14, 6174.
10 K. Arai, Y. Ueda, K. Morisaki, T. Furuta, T. Sasamori, N. Tokitoh
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Conflicts of interest
The authors declare no conflict of interest.
Notes and references
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