Regioselective Sequential Silylation and Borylation of Aromatic Aldimines as a Strategy for Programming Synthesis of Multifunctionalized Benzene Derivatives

Article abstract of DOI:10.1021/acs.orglett.9b04338

Regioselective difunctionalization of two different C-H bonds in one pot using a three-component coupling reaction is described. The reaction order is important for controlling the reactivity and regioselectivity, and the first silylation promotes the sec

Full text of DOI:10.1021/acs.orglett.9b04338

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Regioselective Sequential Silylation and Borylation of Aromatic
Aldimines as a Strategy for Programming Synthesis of
Multifunctionalized Benzene Derivatives
Masahito Murai,*,†,‡ Naoki Nishinaka,† Takahiro Enoki,† and Kazuhiko Takai*,†
†Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka,
Kita-ku, Okayama 700-8530, Japan
‡Department of Chemistry, Graduate School of Science, Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan
S
* Supporting Information
ABSTRACT: Regioselective difunctionalization of two different
C−H bonds in one pot using a three-component coupling
reaction is described. The reaction order is important for
controlling the reactivity and regioselectivity, and the first
silylation promotes the second borylation. The introduced
formyl, silyl, and boryl functional groups could be independently
converted to other functional groups, and the substitution
pattern for the resulting benzenes is difficult to access by
conventional methods.
ynthetic methods that are based on multiple trans-
formations in a single reaction vessel without the need
the intermediate in the initial functionalization often hampers
the efficiency of the second functionalization. Therefore, most
unsymmetrical difunctionalization reactions have been
achieved by the introduction of similar functional groups at
the ortho position of directing groups, and incorporation of two
different functional groups in a one-pot operation is typically
difficult.2 The previous approach to overcoming these
difficulties is a one-pot sequential reaction that involves rapid
intramolecular hydroarylation followed by an intermolecular
coupling reaction (Scheme 1a).3 Difunctionalization of C−H
bonds by a three-component coupling reaction, i.e., two
successive intermolecular functionalization reactions, was
achieved by only Ackerman et al. (Scheme 1b)4 and us
(Scheme 1c)5 in 2018.
S
for workup or product isolation between successive steps have
received considerable attention, especially in the field of
process chemistry.1 Such techniques reduce the amount of
solvent required for the workup and purification of
intermediates, avoid the formation of associated chemical
waste, and significantly reduce the overall working time and
effort involved in the synthesis process.
In this work, a programmed one-pot synthesis of multi-
functionalized benzene derivatives from simple and readily
available aromatic compounds is examined. Considerable
attention has been paid to difunctionalization via inter- or
intramolecular addition to unsaturated π-bonds for the
regioselective installation of two different functional groups
(Figure 1a), while difunctionalization via the substitution of
two different C−H bonds present in one molecule remains
challenging (Figure 1b).2 This is because catalysts and
directing groups, which are indispensable for controlling the
regioselectivity of most C−H bond functionalization reactions,
are highly specific for one reaction. Another difficulty is that
On the basis of our previous report,5 we envisioned that the
programmed synthesis of highly functionalized benzenes could
be achieved by the sequential silylation and borylation in one
pot. We are particularly interested in the synthesis of
multifunctionalized arylaldehydes, considering their high utility
as building blocks. Due to the high reactivity of the formyl
group, an efficient approach toward highly functionalized
arylaldehydes in a short step remains challenging. Our
approach is to utilize arylimines, which can be easily prepared
from generally cheap and commercially available monofunc-
tionalized arylaldehydes as platforms for sequential silylation
and borylation. This is a novel example of the difunctionaliza-
tion of C−H bonds by a three-component coupling reaction.
The regioselectivity of the initial silylation could be controlled
by chelation with an imino group, and that of the second
Received: December 3, 2019
Figure 1. Regioselective difunctionalization.
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b04338
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Letter
presented here. While dehydrogenative silylation of aldimines,
including N-(tert-butyl)- and N-(4-methoxyphenyl)aldimines,
has been reported,8,9 the silylation protocol proposed here with
easily hydrolyzed N-(sec-alkyl)aldimines is practicable when
considering the ease of functionalization of the products.
(E)-N-Cyclohexyl-1-(2-tolyl)methanimine 1a was next
chosen as a model substrate for the study of sequential
silylation and borylation of two different C−H bonds in a one-
pot process (Table 1). The preliminary study of the second
Scheme 1. Previous Difunctionalization with Incorporation
of Two Different Functional Groups in a One-Pot
Operation (see Table 1 for the structure of L3)
Table 1. Optimization of Reaction Conditions for One-Pot
Sequential Silylation and Borylation of Aldimine 1a
a
a
yield of 2a
yield of 3a′
entry
IrLn
ligand
(%)
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
[Ir(OMe)(cod)]2 dtbpy
12
14
35
74
31
33
12
14
32
61
75
20
8
73
72
46
0
61
53
77
80
41
8
[IrCl(cod)]2
[Ir(cod)2]BF4
[Cp*IrCl2]2
dtbpy
dtbpy
dtbpy
borylation could be controlled by steric repulsion from the
initially incorporated silyl group (Figure 2). The substitution
pattern for the resultant functionalized benzenes is difficult to
access by the conventional electrophilic functionalization of
arenes.
[Ir(OMe)(cod)]2 bipy
[Ir(OMe)(cod)]2 phen
[Ir(OMe)(cod)]2 tmphen
[Ir(OMe)(cod)]2 L1
[Ir(OMe)(cod)]2 L2
[Ir(OMe)(cod)]2 L3
[Ir(OMe)(cod)]2 L4
[Ir(OMe)(cod)]2 Ll
[Ir(OMe)(cod)]2 L1
[Ir(OMe)(cod)]2 L1
3
b
63
88
90
c
c,d
14
<1
Figure 2. Programmed synthesis of tetrafunctionalized benzenes (Si =
SiR3, and B = Bpin).
The study presented here began with iridium-catalyzed
regioselective dehydrogenative silylation of aldimines derived
from 2-tolualdehyde to obtain insight into the effect of the
substituents on the nitrogen atom (Scheme 2).6 Note that the
Scheme 2. Effect of Substituents on the Nitrogen Atom in
a
Determined by 1H NMR of the crude products in C6D6.
the Chelation-Assisted Dehydrogenative Silylation of
b
c
Norbornene was used in place of 3,3-dimethyl-1-butene. HSiEt3
and 3,3-dimethyl-1-butene (1.8 equiv each) at 100 °C. Excess HSiEt3
and 3,3-dimethyl-1-butene were removed in vacuo before borylation.
a
Aldimines
d
B2pin2 (1.1 equiv).
borylation step using ortho-silylated N-cyclohexylaldimines
suggested that borylation requires 1,10-phenanthroline-based
bidentate ligands. 3a′ was not obtained with phosphine-based
bidentates or in the absence of ligands. Therefore, the one-pot
silylation and borylation reaction was tested by the addition of
B2pin2 and diamine ligands after the completion of the initial
dehydrogenative silylation. Among the iridium precatalysts and
diamine ligands that were screened, the combination of
[Ir(OMe)(cod)]2 and 3,4,7,8-tetramethyl-1,10-phenanthroline
(tmphen) or 4,7-dimethyl-1,10-phenanthroline (L1) was most
effective (entries 7 and 8). The site selectivity of the second
borylation step was completely controlled by steric effects, and
no formation of regioisomers was observed. The use of 2-
norbornene in place of 3,3-dimethyl-1-butene as a hydrogen
acceptor for the dehydrogenative silylation decreased the yield
a
Yields were determined by 1H NMR of the crude products in C6D6.
ortho-selective dehydrogenative silylation of 2-arylpyridines has
been well-investigated,7 while the corresponding reaction of
arylimines is rare probably due to the competitive hydro-
silylation reaction. The results revealed that branched alkyl
groups that possess moderate steric hindrance, such as
isopropyl and cyclohexyl groups, were optimal, and the
formation of benzylsilyl ethers by hydrosilylation of aldimines
was not observed in these reactions. Silylation did not proceed
at 80 °C with bulkier N-tert-butylimine, whereas N-
methylimine was decomposed under the reaction conditions
B
DOI: 10.1021/acs.orglett.9b04338
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Letter
of 3a′ (entry 12), which implied that the olefins employed in
the initial silylation step affected the efficiency of the second
borylation step. As expected, a decrease in the amount of
HSiEt3 and 3,3-dimethyl-1-butene and evacuation of the latter
after completion of the initial silylation step increased the
efficiency of the overall reaction (entry 13). Finally, the yield of
3a′ reached 90% with an increase in the amount of B2pin2 to
completely consume 2a (entry 14). The corresponding
tetrasubstituted benzene 3a with formyl, silyl, and boryl
functionalities was isolated in 87% yield after hydrolysis with
hydrochloric acid (Figure 3).
selectivity of the second borylation was affected by the
electronic factor of the fluoride group as well as by steric
factors. As expected, 1g with the oxytrifluoromethyl group,
which is sterically more hindered and electron-withdrawing,
was converted to the expected 3g as a single isomer. The
current sequential silylation and borylation can be applied to
five-membered ring heterocycles. To suppress the formation of
disilylated products, the amount of HSiEt3 and 3,3-dimethyl-1-
butene was decreased, and the corresponding 3h was obtained
in 65% yield from 2-thiophenecarboxaldehyde. The sequential
silylation and borylation of m-tolualdehyde (1i) were sluggish
and required a higher temperature (120 °C) for the second
borylation step. The expected 3i was obtained as a single
regioisomer albeit in low yield. Sequential silylation and
borylation of aldimine 1j derived from benzaldehyde provided
disilylated 3j via the regioselective activation of three different
C−H bonds (eq 1).
The current sequential silylation and borylation can be
conducted even on a large scale (eq 2). 544 mg of o-
Figure 3. Optimization of reaction conditions for one-pot sequential
silylation and borylation of aldimine 1a. The reaction was conducted
on a 0.20 mmol scale in a test tube with a screw cap. a3,4,7,8-
Tetramethyl-1,10-phenanthroline (tmphen) was used in place of L1
b
as a ligand. With 1.4 equiv of HSiEt3 and 3,3-dimethyl-1-butene.
cBolylation at 120 °C.
Under the optimized reaction conditions listed in entry 14 of
Table 1, the corresponding tetrasubstituted benzene deriva-
tives 3 were obtained by the regioselective silylation and
borylation of C−H bonds of 1 in a one-pot synthesis followed
by hydrolysis of aldimines (Figure 3). The overall reaction
efficiency was generally dependent on the efficiency of the first
dehydrogenative silylation step.10 Electron-rich aldimines, such
as 1c, gave the expected products in higher yields because they
were inherently more reactive toward chelation-assisted
silylation, which reflects the coordination ability of their
nitrogen atoms. Borylation was sluggish with the aldimines 1b,
1d, and 1e, which have relatively bulky substituents on the
benzene ring, and the use of more electron-rich tmphen as a
ligand gave better results. Note that the reaction proceeded
regioselectively even with 1d and 1e that have potentially
coordinating methoxymethoxy and amino groups. On the
other hand, 3f was obtained as a mixture of two regioisomers
from the sequential silylation and borylation of arylimine 1f
derived from 2-fluorobenzaldehyde. In this case, the site
anisaldehyde with cyclohexylamine afforded aldimine 1c
quantitatively. Purification with chromatography, distillation,
or recrystallization was not required, and simple filtration to
remove MgSO4 followed by concentration was sufficinet to
obtain 1c in adequately pure form for the following
transformation. 1c was then sequentially treated with HSiEt3
and B2pin2 in a 15 mL test tube with a screw cap under
standard reaction conditions to give 1.05 g of 3c in 70% yield
(the yield was based on the amount of o-anisaldehyde
employed).
Although silylation and borylation were reported to proceed
under similar conditions using iridium catalysts,11 the reaction
order is important for controlling the reactivity and selectivity
during the current sequential C−H difunctionalization. For
example, the treatment of 1a with B2pin2 provided an
inseparable mixture of borylated compounds with or without
L1 and 3,3-dimethyl-1-butene. Due to the high reactivity of
C
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generated boryliridium species, assistance of chelation by an
imino group was not sufficient to control the regioselectivity of
the reaction (eq 3).
steps, and the regioselectivity of both reactions was controlled
by steric factors. Adducts derived from the competitive
reductive dechlorination were not observed, and 3k was
obtained in 63% yield. This is a very rare example of the
sequential difunctionalization of two C−H bonds that involves
C(sp3)−H bond activation.12
Derivatization of the resulting tetrasubstituted benzenes with
formyl, silyl, and boryl functionalities was examined to
illustrate their synthetic utility (Scheme 4). Treatment with
The presence of a silyl group on the benzene ring promoted
the borylation of C−H bonds. The initial rate of borylation for
1,3-bis(trimethylsilyl)benzene and m-xylene was compared and
revealed that the former proceeded more than 6 times faster
than the latter (eq 4). Silyl groups increase the electron density
Scheme 4. Derivatization of the Product (see the
Supporting Information for details)
in benzene rings, which could promote the approach of the
reactive electron-deficient boryliridium species to the C−H
bonds of an aromatic ring.
Other than the imino group, 2-pyridyl or 4,5-dihydro-2-
oxazolyl groups could be used as effective directing groups for
sequential silylation and borylation (Scheme 3). As expected,
Scheme 3. Sequential Silylation and Borylation of Aromatic
Compounds with Various Directing Groups
CuCl2 or NaN3 in the presence of CuSO4 converted the boryl
group of 3a into chloride or azide groups to yield 5a or 5b,
respectively, with no effect on the formyl or silyl groups.13
Selective conversion of silyl groups by iododesilylation with ICl
proceeded smoothly to afford 5c in 86% yield.14 The formyl
groups could be utilized for C−N bond formation via reductive
amination to yield 5d.15 The formyl groups could be also
selectively removed by simple heating with a Wilkinson catalyst
to yield 5e.16 Selective installation of boryl and silyl groups at
these positions is difficult without the current difunctionaliza-
tion protocol.
In conclusion, simple programmed synthesis of tetrasub-
stituted benzenes was demonstrated by sequential silylation,
borylation, and hydrolysis of functionalized aldimines. The
reaction order is important for controlling the reactivity and
regioselectivity, and the first silylation reaction promotes the
second borylation reaction. The regioselectivity was well-
controlled by a single iridium catalyst, and silylation proceeded
under chelation control with an imino group, whereas
borylation was accomplished under steric control by the
initially incorporated silyl group. The formyl, silyl, and boryl
functional groups could be independently converted to other
functional groups, and the proposed method opens up new
perspectives for the development of pot-economical rapid
approaches to synthetically useful building blocks.
silylation proceeded under chelation control, and borylation
was accomplished under steric control to afford the expected
4a and 4b in moderate yields. The use of tmphen as a ligand
again gave a better result in these reactions.
The use of 3-chlorotoluene as a substrate resulted in the
introduction of a silyl group onto the benzene ring and a boryl
group into the benzylic C(sp3)−H bond (eq 5). The addition
of L1 was required for the first silylation and second borylation
D
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04338.
■
S
Experimental procedures, spectroscopic data for all new
compounds, and copies of H and 13C NMR spectra
1
(PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: masahito.murai@chem.nagoya-u.ac.jp.
*E-mail: ktakai@cc.okayama-u.ac.jp.
ORCID
Masahito Murai: 0000-0002-9694-123X
Kazuhiko Takai: 0000-0002-2572-0851
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by a Grant-in-Aid for
Scientific Research (B; 18H03911 to M.M.) from MEXT,
Japan.
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