In Situ Generation of Silyl Anion Species through Si?B Bond Activation for the Concerted Nucleophilic Aromatic Substitution of Fluoroarenes

Article abstract of DOI:10.1002/cplu.201900069

In situ generated silyl anion species enable the concerted nucleophilic aromatic substitution of fluoroarenes. Model DFT calculations indicated that addition of a base to a silylborane would thermodynamically form a silyl borate complex and then kinetically release a silyl anion species through Si?B bond cleavage, and that the in situ generated silyl anion equivalent would further react with a fluoroarene through a concerted nucleophilic aromatic substitution pathway with an activation barrier of ca. 20 kcal/mol to afford the silylated product with a large energy gain. Experiments confirmed that the defluorosilylation reaction took place smoothly at room temperature simply upon mixing fluoroarenes with commercially available silylborane and NaO^tBu. Radical scavenger and radical clock reaction experiments provide further evidence for the in situ generation of the silyl anion.

Full text of DOI:10.1002/cplu.201900069

Accepted Article
Title: In Situ Generation of Silyl Anion Species through Si–B Bond
Activation for the Concerted Nucleophilic Aromatic Substitution of
Fluoroarenes
Authors: Masanobu Uchiyama, Kumiko Kojima, Yuki Nagashima, and
Chao Wang
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10.1002/cplu.201900069
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In Situ Generation of Silyl Anion Species through Si–B Bond
Activation for the Concerted Nucleophilic Aromatic Substitution
of Fluoroarenes
Kumiko Kojima,[a] Yuki Nagashima,[a] Chao Wang,*[ab] and Masanobu Uchiyama.*[ab]
Dedication ((optional))
Abstract: In situ generated silyl anion species enable the concerted
nucleophilic aromatic substitution of fluoroarenes. Model DFT
calculations indicated that addition of a base to a silylborane would
thermodynamically form a silyl-borate complex and then kinetically
release a silyl anion species thorough Si–B bond cleavage, and that
the in situ generated silyl anion equivalent would further react with a
fluoroarene through a concerted nucleophilic aromatic substitution
pathway with an activation barrier of ca. 20 kcal/mol to afford the
silylated product with a large energy gain. Experiments confirmed
that the defluorosilylation reaction took place smoothly at room
temperature simply upon mixing fluoroarenes with commercially
available silylborane and NaOtBu. Radical scavenger and radical
clock reaction experiments provide further evidence for the in situ
generation of the silyl anion.
Previously, in 2013, one of our group reported a B–B bond
activation of diborons with metal alkoxide, but the in situ
generation of boryl anion is thermodynamically difficult, even
though it is kinetically favorable (Scheme 1-1).[5] On the other
hand, we very recently established a protocol for in situ direct
preparation of silylzinc species (Si–Zn) via activation of the Si–B
bond of stable silylboranes with dialkylzinc and phosphine,
enabling stereoselective silylzincation of terminal alkynes
(Scheme 1-2).[6] The facile Si–B bond cleavage and quantitative
formation of Si–Zn species prompted us to consider whether this
protocol could be also used for the in situ generation of [R3Si–M]
or silyl anion equivalent.
RO
RO
R
OR
OR
OR
OR
RO
RO
OR
OR
OR
OR
(1)
(2)
B
B
B
+
RO
M
B
M
+
RO
Me
B
B
Trialkylsilyl anion species (denoted as silyl anion or R3Si–) have
long been used as highly reactive silylation reagents.[1] However,
they have serious limitations. Firstly, conventional silyl anion
reagents ([R3Si–M]; M = Li, Na, Mg, K) are thermally labile,
leading to difficulty in storage and safety issues. Thus, most silyl
anions are not suitable for reactions that require heating; indeed,
they sometimes decompose even at room temperature. To
overcome this problem, in situ transmetalation of the highly
instable [R3Si–M] species into more stable R3Si–metal
complexes has been developed. For example, silylzincates,
which are easily prepared from [R3Si–M] and dialkylzinc, show
satisfactory reactivity and good stability.[2-3] However, the
preparation and generation of [R3Si–M] generally involve the use
of highly inflammable alkali metals such as Li, Na, and K, with
strict requirements for the reaction conditions,[1] such as
extremely low temperature. Here, we present a protocol for in
situ generation of silyl anion equivalent from readily available,
storable, easy-to-handle silylboranes via Si–B bond activation[4]
with alkoxide anion under mild conditions. This in turn enables
concerted nucleophilic aromatic substitution reaction of
fluoroarenes simply by mixing fluoroarenes with commercially
available silylborane and NaOtBu at room temperature.
R
R
PR3
Me2Zn
PR3
R
Si
+
+
Si Zn
+
R
R
Me
Scheme 1. In-situ generation of boryl and silyl anion.
To examine the feasibility of this idea, we performed DFT
calculations[7-10] using PhMe2Si–B(OCH2CH2O) and NaOMe as
chemical models of silylboranes and metal alkoxides (Scheme
2). It has been reported that R3Si–B(pin) can efficiently form a
silyl-borate complex with alkoxides, but it was not clear whether
a highly reactive silyl anion equivalent could be generated from
the silyl-borate species and practically utilized. We selected the
defluorosilylation reaction[11-12] of Ph–F as a model reaction,
based on the experimental observation by Würthwein and
Studer that the SNAr (nucleophilic aromatic substitution) reaction
of unactivated fluoroarenes with silyllithiums proceeds smoothly
without any transition metal (TM) catalyst.[13] SNAr reaction has
long been considered to proceed in a stepwise manner via the
Meisenheimer intermediate,[14] though more recently an in-depth
mechanistic study by Jacobsen[15] demonstrated that SNAr
reaction also occurs via a concerted pathway.[16]
[a]
[b]
K. Kojima, Y. Nagashima, Dr. C. Wang, Prof. Dr. M. Uchiyama
Graduate School of Pharmaceutical Sciences, The University of
Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
E-mail: chaowang@mol.f.u-tokyo.ac.jp (C.W.);
E-mail: uchiyama@mol.f.u-tokyo.ac.jp (M.U.)
1) in-situ generation of silyl anion species
2) concerted SNAr
Me
Si Me
Ph
Me
Si Me
Ph
RO
RO
MeO
RO
NaOMe
B
B
O
RO
P
Na
Me
Si Me
Ph
borate type
Dr. C. Wang, Prof. Dr. M. Uchiyama
F
P
Me
Cluster for Pioneering Research (CPR), Advanced Elements
Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama
351-0198, Japan
DFT estimation
and
experimental study
Na
Si Me
P
Si–Na type
Ph
Supporting information for this article is given via a link at the end of
the document.
Scheme 2. Features of this defluorosilylation reaction.
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10.1002/cplu.201900069
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O
O
O
O
Me
Me
Me
Me
Si
Me
B
B
O
O
Me
O
Si
F
B
O
B
+75.1
–116.3
–20.1
Na
Na
O
F
O
O
O
Me
Na
Si
Si
Me
F
F
Me
Me
Na
Me
Me
IM2
TS1
IM3
Me
PD
–1.4
Me
+ Ph–F
Me
Si
Me
+ Ph–F
Me
Me
Me
Si
Si
O
B
+15.1
–6.9
B
O
O
+ Ph–F
O
–89.0
O
B
O
O
Na
IM4
Na
O
Na
TS2
Me
Me
IM1
O
–3.1
ΔG (kcal/mol)
TS1
Me
Si
Me
Si
TS3
TS2
IM2
Me
Me
Na
Me
Me
+21.1
O
O
B
IM4
Na
IM5
IM1
B
F
F
O
O
O
O
PD
IM3
IM5
TS3
Figure 1. DFT calculation at the M06L/6-31+G* level. ΔG: Gibbs free energy in kcal/mol.
The possible SNAr reaction pathways of Ph–F are shown in
Figure 1. Although we must keep in mind the possibility that the
simplification inherent in these models may lead to
underestimation of the steric/electronic interactions, the model
calculation provides a valuable starting point for considering 1)
whether the silyl anion equivalent can be generated from a
mixture of silylborane and alkoxide[17] and 2) whether the in-situ-
generated silyl anion equivalent has sufficient reactivity for the
defluorosilylation of fluoroarene. The reactants (silylborane and
NaOMe) first form an association borate complex IM1 with a
large energy gain (–34.8 kcal/mol). The Si-borate formation
causes the O–B–O angle (104.6°) to be deformed by 8.3° and
the Si–B bond length (2.10 Å) to be elongated by more than 3%
as compared with those in the starting silylborane (see
Supporting Information). Two types of silyl anions are a priori
possible for the potential active species, the Si-borate (itself) and
other silyl anion equivalents. We will discuss the SNAr reactions
of Ph–F with borate-type species first.
because of the steric bulkiness of the Si-borate moiety and its
low nucleophlicity due to delocalization of negative charge over
the borate structure. Obviously, such a transformation would not
take place under normal conditions.
Of several possible TSs for the generation of silyl anion
equivalent from IM1, we could identify one, TS2. The activation
energies are reasonably low (15.1 kcal/mol), and then a rather
endothermic process leads to IM4, which can be further
stabilized through the formation of IM5 with Ph–F. This is in
good accordance with the experimental findings. Then,
concerted SNAr reaction via C–F bond cleavage occurs via TS3
with an activation barrier of 21.1 kcal/mol to give PD. Overall,
this route is energetically more favorable than the Si-borate
pathway. These results suggested that 1) in situ generation of
the silyl anion reagent would be kinetically favorable, and 2)
defluorosilylation by the in-situ-generated silyl anion via the
concerted substitution route is feasible even at room
temperature.
Initial electrostatic coordination of a lone pair of Ph–F to Na
cation results in the formation of a complex (IM2) with a
stabilization energy of 1.4 kcal/mol. To reach the TS of the
defluorosilylation (SNAr) reaction, the Si-borate structure
changes drastically, so that the Si atom can approach the ipso-
carbon of Ph–F. The C–Si bond formation and C–F bond
cleavage take place as a single event at TS1 to produce IM3.
The stabilization energy is very large (–116.3 kcal/mol) because
of the formation of stable C–Si, B–O and Na–F bonds. However,
the activation energy is extremely high (+75.1 kcal/mol), mainly
Inspired by these theoretical results, we carried out “proof-of-
concept” experiments. We found that the use of NaOtBu as a
base in THF solution with commercially available PhMe2Si–
B(pin) resulted in a smooth reaction with various aryl fluorides,
such as Ph–F 1a, para-, meta-, and ortho-fluorotoluenes 1b-d,
para-, meta-, and ortho-fluorobiphenyls 1e-g to afford the
corresponding defluorosilylation products 2a-g in moderate to
good yields. It is noteworthy that no regioisomers of these
silylated products were detected by GC–MS or NMR analysis,
indicating that aryne intermediates was not involved in these
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COMMUNICATION
NaOtBu (3.0 eq.)
Me
Me
Me
Me
Si Me
Ph
Me
Si Me
Ph
Me
Si Me
Ph
(pin)B SiMe2Ph (2.0 eq.)
THF, r.t., 12 h
Si Me
Ar
1
F
Me
Ph
2c (46%)
2d (50%)
2a (58%)
2b (34%)
Ph
Ph
Me
Si Me
Ph
Me
Si Me
Ph
Me
Me
Ph
Si Me Ph2N
Si Me
Ph
Ph
2e (85%)
2f (89%)
2g (74%)
2h (41%)
Scheme 3. Substrate scope of the reaction.
reactions.[18] Remarkably, even 4-fluoroaniline derivative 1h,
which has a very electron-rich aryl group, could be used in this
protocol, albeit with a reduced yield (2h, 41%). These results are
consistent with a concerted SNAr reaction.
indicating that the silyl anion attacks the allyl ether moiety rather
than the C(aryl)–F bond. Overall, these experimental
observations rule out a radical mechanism for this reaction.
To confirm the reaction mechanism, we carried out a series of
control experiments. First, to examine whether radical route was
involved, the current reaction was performed with adding 9,10-
dihydroanthracene. We found that addition of 9,10-
dihydroanthracene blocked the reaction of 1e, though no
formation of anthracene was detected (Scheme 4a). Next, when
9,10-dihydroanthracene was treated under the standard reaction
conditions in the absence of 1e followed by quenching with D2O,
deuterated anthracene was obtained (Scheme 4b). This means
that deprotonation reaction of 9,10-dihydroanthracene proceeds
under the standard reaction conditions. However, deprotonation
of 9,10-dihydroanthracene with NaOtBu was not successful in
the absence of silylborane (Scheme 4c). These results support
the involvement of in situ generation of silyl anion, which would
act as a strong base to abstract the active benzyl proton of 9,10-
dihydroanthracene.[19]
NaOtBu (3.0 eq.)
O
O
O
Me
Si Me
Ph
Me
Si Me
Ph
(pin)B SiMe2Ph (2.0 eq.)
THF, r.t., 12 h
F
+
+
1i (1.0 eq.)
2i (n.d.)
3 (n.d.)
4 (49%)
Scheme 5. Control experiment with 1i (n.d.: not detected).
Very recently, Martin reported an elegant protocol for
defluorosilylation with Et3Si–B(pin) and LiHDMS, enabling both
C(sp2)–F and C(sp3)–F bond cleavage.[20] This protocol was
successfully applied for late-stage derivatization of several
functional fluoroarenes, demonstrating potential synthetic utility.
Martin also suggested a mechanism involving a concerted SNAr
reaction route. We compared the reactivities of Et3Si– and
PhMe2Si– by means of DFT calculation (see Supporting
Information). As expected, the calculated pathways were very
similar, and we identified the TS for the concerted SNAr process
with Et3Si–, TS3(Et3Si–), which is analogous to PhMe2Si–. The
calculations indicate that TS3(Et3Si–) is energetically more
favorable than TS3 by 2.8 kcal/mol, probably due to the more
electron-donating character of alkyl (Et) than aryl (Ph), in good
agreement with Martin’s results.[21]
In summary, based on DFT calculations, we designed an
efficient protocol for in-situ generation of highly active silyl anion
species, and predicted its applicability for inert C–F bond
activation. We experimentally confirmed that the expected
defluorosilylation reaction took place smoothly at room
temperature simply upon mixing fluoroarenes with commercially
available silylborane and NaOtBu. Radical scavenger and radical
clock reaction experiments supported in situ generation of the
silyl anion. Such an in situ preparation method for silyl anion
equivalent from readily available silylboranes under mild
conditions opens up new possibilities for the practical and
synthetic use of silyl anions. Investigations to expand the scope
of this protocol, as well as to establish synthetic applications, are
in progress.
anthracene
(2.0 eq.)
(n.d.)
Me
NaOtBu (3.0 eq.)
(a)
(b)
Ph
F
+
(pin)B SiMe2Ph
(2.0 eq.)
Ph
Si Me
Ph
THF, r.t., 12 h
1e (1.0 eq.)
2e (n.d.)
D
NaOtBu (1.5 eq.)
(pin)B SiMe2Ph
(1.0 eq.)
+
THF, r.t., 12 h
then D2O
(1.0 eq.)
D > 99%
D
NaOtBu (1.5 eq.)
(c)
THF, r.t., 12 h
then D2O
(1.0 eq.)
D < 1%
Scheme 4. Control experiments with 9,10-dihydroanthracene (n.d.: not
detected).
We also tested the reaction of fluoroarene 1i as a radical-clock
substrate under the standard conditions (Scheme 5). Neither
defluorosilylation product 2i nor cyclization product 3 was
detected, but instead, allylation product
4 was observed,
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Experimental Section
To a THF solution (0.4 M) of fluoroarene 1 (0.2 mmol) were added
PhMe2Si–B(pin) (0.4 mmol) and NaOtBu (0.6 mmol). The mixture was
stirred in an argon atmosphere for 12 h, then water was added, and the
aqueous layer was extracted with Et2O (3 x 10 mL). The combined
organic layer was dried over Na2SO4 and concentrated under vacuum.
The crude product was purified by chromatography (silica gel) to give
silylated product 2.
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Acknowledgements
This work was supported by a JSPS Grant-in-Aid for Scientific
Research on Innovative Areas (No. 17H05430), JSPS KAKENHI
(S) (No. 17H06173) (to M. U.), and Grant-in-Aid for Scientific
Research (C) (No. 18K06544, C.W.).
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silylation • silyl anions • nucleophilic aromatic substitution
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Organosodium compounds exhibit stronger carbanionic nature than
organolithium due to the lower electronegativity of sodium than lithium
and hence have extremely high reactivity and low stability. On the
other hand, organosodium compounds are less commercially available
compared with organolithium. These issues limited the application of
organosodium chemistry. Similar differences/problems also occur in
the silyl anion chemistry. As reflected in the structure of Si-Na in
current DFT calculation, Si and Na are more separated (compared
with the computation results in ref. 13), and Na+ shows a strong π-
cation interaction with Ph ring, indicating a strong anionic nature of Si.
Hence, it is known that Si-Na is less stable than Si-Li species, which
[4]
For representative reviews involving Si–B bond activation of
silylborane, see: a) M. B. Ansell, O. Navarro, J. Spencer, Coord. Chem.
Rev. 2017, 336, 54-77. b) A. B. Cuenca, R. Shishido, H. Ito, E.
Fernández, Chem. Soc. Rev. 2017, 46, 415 - 430. c) M. Oestreich, E.
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10.1002/cplu.201900069
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cause some difficulty for preparation and storage. By using the current
in situ protocol, Si-Na could be smoothly generated from commercially
available and bench-stable base (NaOtBu), and thus facilitate further
synthetic utilization of highly active Si-Na species. Further, according
to our current results, when R3Si-SiR3 was used as the Si-Na
precursor, no silylated product was observed, indicating that R3Si–
B(pin) has higher reactivity under the current reaction condition.
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10.1002/cplu.201900069
ChemPlusChem
COMMUNICATION
COMMUNICATION
Kumiko Kojima, Yuki Nagashima, Dong-
Yu Wang, Chao Wang, and Masanobu
Uchiyama.
O
DFT estimation and experimental study
Ar
F
NaOR
Me
Si Me
Ph
Me
Si Me
Ph
Na Me
Si Me
Ph
Me
Si Me
Ph
RO
RO
RO
MeO
RO
B
B
Ar
P
P
RO B(OR)2
P
concerted SNAr
Na
Page No. – Page No.
in-situ generation of silyl anion species
In Situ Generation of Silyl Anion
Species with Si–B Bond Activation
Allowing Concerted Nucleophilic
Aromatic Substitution of
DFT calculations indicate that addition of base to silylborane can kinetically release
silyl anion species thorough Si–B bond cleavage, and the anion can further react
with fluoroarene through a concerted SNAr pathway with an activation barrier of ca.
20 kcal/mol, with a large energy gain. Experimentally, this defluorosilylation reaction
took place smoothly at room temperature simply upon mixing fluoroarenes with
commercially available silylborane and NaOtBu.
Fluoroarenes
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