Ruthenium-Catalyzed Isomerization of ortho-Silylanilines to Their para Isomers

Article abstract of DOI:10.1021/acs.orglett.1c02280

The catalytic ortho to para transposition of a silyl group in aniline derivatives is described. [RuCl2(p-cymene)]2/BINAP in conjunction with a Cu(OAc)2 additive serves as a potent catalytic system. This method is also applicable to the isomerization of 2-silylpyrrole derivatives to the corresponding 3-silyl isomers.

Full text of DOI:10.1021/acs.orglett.1c02280

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Letter
Ruthenium-Catalyzed Isomerization of ortho-Silylanilines to Their
para Isomers
Wataru Ishiga, Masaya Ohta, Takuya Kodama, and Mamoru Tobisu*
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ABSTRACT: The catalytic ortho to para transposition of a silyl group in aniline
derivatives is described. [RuCl2(p-cymene)]2/BINAP in conjunction with a Cu(OAc)2
additive serves as a potent catalytic system. This method is also applicable to the
isomerization of 2-silylpyrrole derivatives to the corresponding 3-silyl isomers.
he substitution pattern of polysubstituted arenes has a
Tcritical impact on their chemical, physical, and biological
properties, and therefore, the development of methods for the
selective synthesis of polysubstituted arenes with the desired
arrangement of substituents is now one of the most important
subjects in the field of synthetic organic chemistry. In this
context, a number of methods that allow for the site-selective
introduction of a functional group to an aromatic compound
have been reported.1 Another approach to this end would be
the selective interconversion of positional isomers of
polysubstituted arenes, such as that between ortho, meta, and
para isomers of disubstituted arenes. This approach is
potentially powerful, because it would permit a positional
isomer that is difficult to access by direct introduction methods
to be synthesized. Reported transposition reactions of
substituents on a benzene ring are summarized in Scheme 1.
It would be possible to cause an alkyl group to migrate around
an aromatic ring with the aid of Brønsted or Lewis acids via a
reversible Friedel−Crafts-type reaction, which would result in
the formation of a mixture of positional isomers (Scheme 1a).2
The isomerization of ortho- or para-bromophenol to a meta
isomer was reported to be mediated by a superacid (SbF5−
HF) via an arenium cation intermediate (Scheme 1b, top).3
The 1,2-transposition of a halide group could also occur via an
anionic mechanism, a reaction that is known as the halogen
dance reaction (Scheme 1b, bottom).4 A silyl group of 1,2-
bis(silyl)arenes can migrate in a 1,2-fashion to form a meta
isomer with the aid of CF3CO2H (Scheme 1c).5 The reversible
transposition of an ester group is catalyzed by a palladium
complex, presumably via the oxidative addition of a C(acyl)−
O bond and decarbonylation (Scheme 1d).6 Herein, we report
on the ruthenium-catalyzed isomerization of ortho-silylaniline
derivatives to the corresponding para isomers (Scheme 1e).
Unlike the acid-catalyzed reaction (Scheme 1c),5 the exclusive
1,3-migration of a silyl group is possible by the use of a
transition metal catalyst.
using the o-silylaniline derivative 1a in the presence of a series
of transition metal catalysts. Although the expected product
was not observed, the corresponding para isomer 2a was
unexpectedly formed. Given that no catalytic 1,3-silicon
migration reactions have been reported to date, we focused
our attention on optimizing this catalytic isomerization
reaction. Finally, compound 2a was obtained in 73% isolated
yield when compound 1a was reacted in the presence of
[RuCl2(p-cymene)]2 (10 mol %), ( )-BINAP (20 mol %),
and Cu(OAc)2 (1.2 equiv) in toluene at 180 °C for 15 h (entry
1 in Table 1). Although no product was formed in the absence
of a ruthenium catalyst (entry 2), several other transition metal
complexes exhibited some levels of catalytic activity: [RhCl-
(cod)]2 (75%), [Cp*RhCl2]2 (18%), [Cp*IrCl2]2 (45%),
Ni(OAc)2 (7%), and Pd(OAc)2 (44%). The yield of
compound 2a was significantly lowered in the absence of an
added ligand (32%, entry 3), and ( )-BINAP proved to be an
optimal ligand among the ligands that were examined: dppm
(47%), dppe (57%), dppp (59%), dppb (69%), Xantphos
(28%), PCy3 (65%), and P(p-tolyl)3 (41%).8 The addition of
Cu(OAc)2 is required for the formation of compound 2a
(entry 4), and the yield was diminished to 34%, with 13% of
compound 1a being recovered and 24% of a protodesilylated
product being formed when the amount of Cu(OAc)2 was
reduced to 20 mol % (entry 5). Other copper salts, such as
CuCl (13%), CuI (10%), and CuBr2 (0%), were much less
effective. It is important to note that, when compound 1a was
exposed to CF3CO2H-catalyzed conditions,5 which were used
for the isomerization of bis(silyl)arenes (Scheme 1c),
compound 2a was obtained in only 14% yield, with a
Received: July 8, 2021
During the course of our studies on catalytic carbon−silicon
bond activation reactions,7 we examined annulation reactions
https://doi.org/10.1021/acs.orglett.1c02280
© XXXX American Chemical Society
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A

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Letter
sequential double 1,2-silicon migration is unlikely to be
involved in this 1,3-silicon transposition reaction.
Scheme 1. Transposition of Functional Groups on a
Benzene Ring
Under the optimized conditions identified above, we
subsequently explored the scope of this catalytic silicon
transposition reaction (Scheme 2). With regard to the silyl
Scheme 2. Scope of the Ru-Catalyzed Isomerization of
a
ortho-Silylanilines to Their para Isomers
a
Reaction conditions: compound 1 (0.50 mmol), [RuCl2(p-
cymene)]2 (0.050 mmol), ( )-BINAP (0.10 mmol), Cu(OAc)2
Table 1. Ru-Catalyzed Isomerization of ortho-Silylaniline 1a
to Its para Isomer 2a
(0.60 mmol), and toluene (2.5 mL) in a screw-capped pressure-
a
b
proof vial under N2 at 180 °C for 15 h. [RhCl(cod)]2 (0.050 mmol)
was used instead of [RuCl2(p-cymene)]2. The yield was determined
c
d
by GC analysis. Run for 9 h. [RuCl2(p-cymene)]2 (0.10 mmol) was
used.
t
i
group, both BuMe2Si (i.e., compound 1a) and Pr3Si (i.e.,
compound 1b) groups can be translocated from the ortho to
the para position of an aniline framework in an efficient
manner, but Et3Si and Me3Si groups were not suitable as a
result of the formation of a protodesilylation product as the
major product (48% for compound 1d and 99% for compound
1e). In the case of a substrate bearing a Ph3Si group (i.e.,
compound 1c), the yield of compound 2c was increased from
8 to 23% by changing the catalyst to [RhCl(cod)]2. These
results indicate that the use of bulky tBuMe2Si or iPr3Si groups
is essential for maximizing the yield of the silicon transposition
product by suppressing a competing protodesilylation pathway.
In addition to simple 2-silylaniline derivatives, substrates
bearing an additional substituent, including Me, F, Cl, and
OMe groups, at the 5 position also underwent this 1,3-silicon
migration to form the corresponding 4-silylanilines 2f−2j,
despite the increased steric congestion that is generated in the
products. This isomerization reaction was successfully applied
to a 1-aminonaphthalene-based substrate 1k, of which a silyl
b
entry
deviations from “standard conditions”
yield (%)
c
1
2
3
4
5
none
71 (73)
without [RuCl2(p-cymene)]2
without ( )-BINAP
without Cu(OAc)2
0
32
0
with 20 mol % of Cu(OAc)2, instead of 1.0 equiv
34
a
Reaction conditions: compound 1a (0.20 mmol), [RuCl2(p-
cymene)]2 (0.020 mmol), ( )-BINAP (0.040 mmol), Cu(OAc)2
(0.20 mmol), and toluene (1.0 mL) in a screw-capped pressure-proof
vial under an atmosphere of N2 for 15 h. GC yield. Isolated yield
with 1.2 equiv of Cu(OAc)2.
b
c
protodesilylated compound (i.e., N,N-dimethylaniline) being
the major product (48%). The corresponding m-silylaniline
derivative did not isomerize into compound 2a, indicating that
B
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group was translocated exclusively to the 4 position. A silyl
group in the 2-aminopyridine derivative 1l also underwent this
catalytic 1,3-silicon migration to form the corresponding
product 2l in 41% yield. A NMe2 group in the substrate is
currently essential for this silicon transposition reaction, and
substrates bearing NEt2, OMe, and CH2NMe2 groups instead
were completely unreactive under these conditions.
Scheme 4. Mechanistic Studies
This catalytic system also allows for the transposition of a
silyl group on five-membered heteroarenes (Scheme 3). For
Scheme 3. Ru-Catalyzed Translocation of a Silyl Group on
Heteroarenes
example, the 2-silylindoles 1m and 1n could be isomerized to
the corresponding 3-silylindoles 2m and 2n, respectively, by
applying the standard conditions. Unlike the six-membered
arene substrates shown in Scheme 2, a neighboring NMe2
group is not needed for the indole substrates to be isomerized,
and even an electron-withdrawing sulfamoyl group can be used
as a N-protecting group. Similarly, a silyl group at the 2
position of a pyrrole ring was translocated to the 3 position,
which indicates that the site selectivity is different from that
observed for typical SEAr reactions of pyrroles.1,9 This site
selectivity allows for readily accessible 2,5-bis(silyl)pyrrole
1o10 to be isomerized to the 2,4-bis(silyl) isomer 2o, which is
otherwise difficult to synthesize from the parent pyrrole.
Several preliminary mechanistic experiments were con-
ducted to shed light on the mechanism of this catalytic
isomerization reaction (Scheme 4). A crossover experiment
using compounds 1b and 1h revealed that the silicon
transposition occurs in an intramolecular manner (Scheme
4a). When 2-silylaniline 1a-d, which contains deuterium at the
para position, was reacted under the standard conditions,
compound 2a was formed in 67% yield, with no deuterium
incorporation (Scheme 4b), suggesting that an intermolecular
proton exchange process is involved. A kinetic isotope effect of
1.17 was observed in parallel kinetic experiments using
compounds 1a and 1a-d, indicating that the cleavage of a
C−H bond is not the turnover-limiting step in this catalytic
reaction. Although we identified a Ru(II) complex as the most
effective catalyst precursor, a Ru(III) complex (i.e., [Cp*Ru-
(III)Cl2]n) can also be used to mediate the silicon trans-
position reaction (Scheme 4c). Importantly, with this Ru(III)
precatalyst, the effect of Cu(OAc)2 additive is not quite as
profound [45% yield with Cu(OAc)2 and 39% yield without
Cu(OAc)2], which is in sharp contrast to the use of a
[RuCl2(p-cymene)]2 catalyst, where no reaction occurs in the
absence of Cu(OAc)2 (entry 4 in Table 1). These observations
can be rationalized by assuming that one of the roles of
Cu(OAc)2 in this reaction is to oxidize Ru(II) to Ru(III),
which presumably serves as the catalytically active species,
although the role of Cu(OAc)2 is not completely understood
at this stage. With regard to the silicon migration step, Li and
co-workers recently reported on an intriguing reaction between
3-silylarynes and allyl solfoxides that involves 1,3-silicon
migration.11 In their study, the electrophilic bromination of
2-silylphenol was reported to afford 2-bromophenol with a silyl
group at the 4 position. Prompted by this report, we examined
electrophilic bromination of compound 1a to investigate
whether 1,3-silicon migration is induced in the case of the 2-
silylanilne derivatives. As a result, 2-bromoaniline with the silyl
group having migrated to the 4 position (i.e., compound 3)
was formed in 21% yield, along with the 4-brominated product
4 (25%) (Scheme 4d). Similarly, the bromination of 2,5-
bis(silyl)pyrrole 1o underwent 2-bromination with a 1,2-
silicon shift to form compound 5 as a sole product. These
results indicate that an attack of an electrophile at the ipso
position of silyl(hetero)arenes appears to induce the migration
of a silyl group.
On the basis of the mechanistic studies described above, a
proposed mechanism is depicted in Scheme 5. RuX3, which is
generated in situ by the oxidation of RuX2 with Cu(II), serves
as the catalytically active species. Coordination of a NMe2
group in compound 1a to RuX3 forms intermediate A, which
C
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Scheme 5. Proposed Mechanism
Scheme 6. Transformation of a NMe2 Group in the
Isomerized Product
difficult to access by direct aromatic functionalization. Studies
along this line are currently underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c02280.
facilitates the electrophilic attack of RuX3 at the ipso position of
a silyl group, providing the cationic intermediate B. The silyl
group in intermediate B subsequently migrates to the para
position, likely driven by the relief of steric congestion, to form
the intermediate C, as was observed in the reaction of
compound 1a with a bromonium cation (Scheme 4d).12 The
deprotonation of intermediate C13 affords the aromatized
intermediate D, which undergoes protodemetalation to furnish
compound 2a with the regeneration of RuX3. Although the
formation of compound 2a directly from intermediate C via
the 1,3-migration of a proton could be an alternative pathway,
we currently exclude this possibility based on the results of a
labeling experiment (Scheme 4b). Arenium cation intermedi-
ates B and C are prone to desilylation by attack of an external
nucleophile, such as AcO−, when the size of a silyl group is
small, leading to the formation of the protodesilylation
product.14 A NMe2 group plays three roles in the catalytic
cycle: as an ortho directing group to facilitate the electrophilic
addition, imposition of steric repulsion on the ortho silyl group
to promote its migration, and as an electron-donating group
that stabilizes the cationic intermediates B and C.
Detailed experimental procedures and characterization
of new compounds (PDF)
Accession Codes
CCDC 2090589 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Mamoru Tobisu − Department of Applied Chemistry,
Graduate School of Engineering and Innovative Catalysis
Science Division, Institute for Open and Transdisciplinary
Research Initiatives (ICS−OTRI), Osaka University, Suita,
Osaka 565-0871, Japan; orcid.org/0000-0002-8415-
2225; Email: tobisu@chem.eng.osaka-u.ac.jp
Authors
Although the use of a NMe2 group is essential for this silicon
transposition reaction, this group can be transformed into
other substituents by cross-coupling via the cleavage of
C(aryl)−N bonds. For example, the NMe2 group in compound
2k can be converted into an alkyl or other amino groups by
iron-catalyzed cross-coupling with RMgX15a or nickel-catalyzed
amination15b through the conversion to the corresponding
ammonium salt (Scheme 6).
In summary, we report on the development of a catalytic
transposition reaction of a silyl group on an aromatic ring,
which allowed ortho-silylaniline derivatives to isomerize to the
corresponding para isomers with the aid of a ruthenium
catalyst. This transposition reaction is also applicable to 2-
silylpyrroles and 2-silylindoles, which undergo 1,2-silicon
migration. The findings reported herein will stimulate further
development of transition-metal-catalyzed transposition reac-
tions, which serve as potential methods for the synthesis of
positional isomers of multi-substituted (hetero)arenes that are
Wataru Ishiga − Department of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-
0871, Japan
Masaya Ohta − Department of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-
0871, Japan
Takuya Kodama − Department of Applied Chemistry,
Graduate School of Engineering and Innovative Catalysis
Science Division, Institute for Open and Transdisciplinary
Research Initiatives (ICS−OTRI), Osaka University, Suita,
Osaka 565-0871, Japan; orcid.org/0000-0001-8275-
2393
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c02280
Notes
The authors declare no competing financial interest.
D
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ment of 2,5-Bis(silylated)-N-Boc Pyrroles into the Corresponding 2,4-
Species. J. Org. Chem. 2009, 74, 8890−8892.
ACKNOWLEDGMENTS
■
This work was supported by the Japan Society for the
Promotion of Science (JSPS) Grants-in-Aid for Scientific
Research (KAKENHI) (JP18H01978) and Scientific Research
on Innovative Area “Hybrid Catalysis” (JP20H04818) from
the Ministry of Education, Culture, Sports, Science and
Technology (MEXT), Japan. The authors also thank the
Instrumental Analysis Center, Faculty of Engineering, Osaka
University, for the assistance with HRMS.
(10) Yu, Q.; Li, X.; Wang, X.; Liu, J. Regioselective Synthesis of 2,5-
Disubstituted Pyrroles via Stepwise Iododesilylation and Coupling
Reactions. Aust. J. Chem. 2018, 71, 95−101.
(11) Shi, J.; Li, L.; Shan, C.; Wang, J.; Chen, Z.; Gu, R.; He, J.; Tan,
M.; Lan, Y.; Li, Y. Aryne 1,2,3,5-Tetrasubstitution Enabled by 3-
Silylaryne and Allyl Sulfoxide via an Aromatic 1,3-Silyl Migration. J.
Am. Chem. Soc. 2021, 143, 2178−2184.
(12) Although the classical 1,3-silicon migration of allylsilanes is a
thermally forbidden process, this type of reaction was reported to
occur thermally via a highly charge-separated transition state when π-
extended substrates were used. See ref 11.
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