1,5-Migration of rhodium via C-H bond activation in catalytic decyanative silylation of nitriles

Article abstract of DOI:10.1039/c2cc36601k

Unprecedented aryl-to-aryl 1,5-rhodium migration is involved in decyanative silylation of aryl cyanides bearing a tethered arene. The 1,5-migration proceeds through remote C-H bond activation. 1,5-Migration also occurs in other rhodium-catalyzed reactions, including borylation and oxidative Mizoroki-Heck reactions.

Full text of DOI:10.1039/c2cc36601k

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COMMUNICATION
1,5-Migration of rhodium via C–H bond activation in catalytic
decyanative silylation of nitrilesw
Mamoru Tobisu,*a Junya Hasegawa,b Yusuke Kita,b Hirotaka Kinutab and Naoto Chatani*b
Received 11th September 2012, Accepted 10th October 2012
DOI: 10.1039/c2cc36601k
Unprecedented aryl-to-aryl 1,5-rhodium migration is involved in
decyanative silylation of aryl cyanides bearing a tethered arene.
The 1,5-migration proceeds through remote C–H bond activation.
1,5-Migration also occurs in other rhodium-catalyzed reactions,
including borylation and oxidative Mizoroki–Heck reactions.
In the course of our investigation of rhodium-catalyzed
decyanative silylation of aryl cyanides,10 we tested 2-phenoxy-
benzonitrile derivatives as possible substrates. The reaction of
2-(4-methoxyphenoxy)benzonitrile (1a) with hexamethyldisilane
(2) in the presence of a catalytic amount of [RhCl(cod)]2 afforded
two silylated products in a ratio of 73/27. 2D NMR spectroscopy,
including NOESY, COSY, HMQC and HMBC, as well as
independent synthesis of authentic compounds unambiguously
established that the major product was the expected silylated
compound 3a, and the minor one was its positional isomer 4a
(entry 1, Table 1).11 The formation of 4a clearly indicates that
a putative arylrhodium intermediate generated by the cleavage
of a C–CN bond12 underwent unprecedented 1,5-migration.
This intriguing result prompted us to investigate the rhodium-
catalyzed decyanative silylation of biaryl ether-based nitriles in
more detail. Addition of ligands and bases significantly decreased
the conversion in the silylation reaction, rather than affecting the
3/4 ratio.11 The product distribution proved to be highly dependent
on the electronic environment of the C–H bond to be activated, as
in the case of 1,4-palladium migration reactions.3 With substrates
bearing electron-donating groups, such as 1a (R2 = OMe) and 1b
1,4-Migration of transition metals has emerged as a unique and
powerful organometallic process applicable to catalytic reactions.1
The migration process proceeds through formal intramolecular
C–H activation, enabling site-specific generation of organometallic
species that are otherwise difficult to access. To date, a number of
catalytic reactions involving 1,4-migration of a metal center
have been developed with palladium2–6 and rhodium7,8 catalysts
(Scheme 1a). Extension of the 1,4-migration process to one-atom-
extended systems would enable remote functionalization via C–H
activation, thus significantly increasing the utility of the process
(Scheme 1b). However, the greater distance between the metal
center and the C–H bond and the relative difficulty in forming a six-
membered metallacyclic intermediate (or transition state) render
such a 1,5-migration process far less favorable than 1,4-migration.
Indeed, there have been only two catalytic reactions involving
1,5-palladium migration. One requires a well-designed and highly
elaborate tricyclic system,9a,b and the other uses triphenylsilane
derivatives.9c In the case of rhodium, no 1,5-migration has been
reported, to the best of our knowledge. In this communication,
we report the first catalytic reactions involving 1,5-migration
of rhodium, using relatively simple substrates.
Table 1 Rh-catalyzed decyanative silylation of 2-aryloxybenzonitrilesa
Entry
R1
R2
Yieldb (%)
3/4c
1
2
3
4
5
6
7
H
H
H
H
H
OMe
CF3
4-OMe
4-Me
4-CO2Me
4-CF3
3,4,5-F3
3,4,5-F3
3,4,5-F3
78
70
80
58
89
35
85
73/27
80/20d
64/36
50/50
23/77
23/77
12/88
Scheme 1 (a) 1,4- and (b) 1,5-migration of palladium and rhodium
via C–H bond activation.
a Center for Atomic and Molecular Technologies, Graduate School of
Engineering, Osaka University, Suita, Osaka 565-0871, Japan.
E-mail: tobisu@chem.eng.osaka-u.ac.jp
a
b Department of Applied Chemistry, Faculty of Engineering,
Osaka University, Suita, Osaka 565-0871, Japan.
E-mail: chatani@chem.eng.osaka-u.ac.jp
Reaction conditions: 1 (0.50 mmol), 2 (1.0 mmol), [RhCl(cod)]2
(0.025 mmol), and ethylcyclohexane (0.50 mL) in a screw-capped vial
b
under N2 at 160 1C for 15 h unless otherwise noted. Isolated
combined yield of 3 and 4 based on 1. Ratio of 3/4 determined by
1H NMR unless otherwise noted. Determined by GC.
c
w Electronic supplementary information (ESI) available: Preparative
and spectroscopic data and tables for optimization studies. See DOI:
10.1039/c2cc36601k
d
c
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Chem. Commun., 2012, 48, 11437–11439 11437

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(R2 = Me), a normal decyanative silylation product 3 was formed
preferentially (entries 1 and 2). Conversely, the more electron-
withdrawing the substituent R2 becomes, the more 1,5-migration
product 4 is formed (entries 3–5). Predominant formation of the
1,5-migration product was achieved by using a highly electron-
deficient 3,4,5-trifluorophenyl group (entry 5). The electronic
properties of the benzonitrile moiety of biaryl ether 1 also
influenced the reactivity and selectivity. Introduction of a trifluoro-
methyl group para to the cyano group further promoted
1,5-migration to furnish 3g/4g in a ratio of 12/88 (entry 7).
The nature of the linker between the two arenes exerted a
profound effect. The rhodium-catalyzed reaction of the methylene-
linked substrate 1h with 2 produced decyanative silylation
products with decreased selectivity for the 1,5-migration
product 4h (eqn (1)) compared with those obtained with 1e
as the substrate (entry 5, Table 1).13
Scheme 2 Mechanistic consideration.
metal/hydride migration,18 in both of which more electron-
deficient (or acidic) C–H bonds are expected to react faster.19–21
The 1,5-migration of rhodium was also observed in a related
decyanative borylation reaction (eqn (3)),22 in which the migrated
product 10 was formed to a greater extent than in the silylation
reaction.
ð1Þ
(2)
The many examples of facile alkyl to aryl 1,4-metal migration4,7
led us to examine alkyl cyanide 5 (Fig. 1), but neither normal nor
migrated silylation occurred under these conditions.14 Nitriles 6
and 7 simply afforded normal silylation products, and no 1,6- or
1,7-migration15 took place. Interestingly, when biaryl cyanide
8 was used under rhodium-catalyzed conditions, no silylation
via 1,4-migration was observed.16
(3)
There are several scenarios that could dictate the product
ratios (Scheme 2). One is the case in which 1,5-migration (A - B,
step a) is in a rapid equilibrium, and the rate difference between the
silylation of arylrhodium A (step b) and that of the migrated
arylrhodium B (step c) determines the ratio of 3/4 (Curtin–
Hammett scenario); if this were the case, isomeric aryl cyanides 1a
and 1i would result in a similar product distribution. However,
these two substrates afforded silylated products in a completely
different ratio (eqn (2) vs. entry 1 in Table 1). These observations
indicate that 1,5-migration is irreversible, and the migration rate is
one of the factors controlling the product distribution, in addition
to the rates of steps b and c. Although the exact mechanism of the
1,5-migration remains elusive, plausible processes are oxidative
addition/reductive elimination of a C–H bond17 and concerted
Importantly, the 1,5-rhodium migration process is not
limited to an arylrhodium intermediate generated from aryl
cyanides. We also observed a 1,5-migration product in
a rhodium-catalyzed oxidative Mizoroki–Heck reaction of
arylboronic acid 11 with alkene 12 (eqn (4)).23
ð4Þ
Fig. 1 Unsuitable substrates.
c
11438 Chem. Commun., 2012, 48, 11437–11439
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In summary, we have demonstrated that unprecedented
1,5-rhodium migration occurs using substrates with a simple
skeleton. The process enables generation of difficult-to-access
arylrhodium species via remote C–H activation. We anticipate
that 1,5-rhodium migration will be used as extensively as
1,4-rhodium migration in designing catalytic reaction after
further optimization.24
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This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas ‘‘Molecular Activation Directed
toward Straightforward Synthesis’’ from MEXT, Japan. We
also thank the Instrumental Analysis Center, Faculty of
Engineering, Osaka University, for assistance with the HRMS
analyses. We thank the reviewers for their insightful comments
on the electronic effects.
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´
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11437–11439 11439