Rh(I)-catalyzed silylation of aryl and alkenyl cyanides involving the cleavage of C-C and Si-Si bonds

Article abstract of DOI:10.1021/ja062745w

The Rh(I)-catalyzed silylation of nitriles with disilanes is described. The cleavage of inert carbon-cyano and silicon-silicon bonds occurs in this catalysis. Copyright

Full text of DOI:10.1021/ja062745w

Published on Web 06/07/2006
Rh(I)-Catalyzed Silylation of Aryl and Alkenyl Cyanides Involving the
Cleavage of C-C and Si-Si Bonds
Mamoru Tobisu, Yusuke Kita, and Naoto Chatani*
Department of Applied Chemistry, Faculty of Engineering, Osaka UniVersity, Suita, Osaka 565-0871, Japan
Received April 19, 2006; E-mail: chatani@chem.eng.osaka-u.ac.jp
Scheme 1. Design of a Catalytic Cycle Involving Silicon-Assisted
The cleavage of C-C σ bonds by transition-metal complexes
C-CN Bond Cleavage
represents a fundamental challenge in the field of organometallic
chemistry. Success has primarily been limited to systems that rely
on relatively specific driving forces, such as relief of ring strain
and aromatization or on chelation assistance.1 A notable exception
to this is the cleavage of C-C σ bonds of nitriles, in which
unstrained C-CN bonds can be cleaved in the absence of
neighboring coordinating groups.2-8 Mechanistic studies of such
transition-metal-mediated C-CN bond-cleavage reactions have
revealed two distinct pathways (eqs 1 and 2). One is the oxidative
addition of a C-CN bond to a low-valent metal center to afford
an alkyl(aryl)-cyano complex (eq 1). The majority of C-CN bond
cleavage reactions appear to proceed through this mechanism, and
several alkyl(aryl)-cyano complexes have been synthesized from
alkyl(aryl) cyanides.1-5 In addition, the oxidative addition of a
C-CN bond has also been postulated as a key step in some catalytic
Table 1. Rh(I)-Catalyzed Silylation Reaction of Nitriles: Catalyst
reactions.9,10 On the other hand, it has been reported that silyl-
and Solvent Screeninga
metal species mediate C-CN bond cleavage via an alternate
mechanism, which consists of the formal silylmetalation of a cyano
group to form η2-iminoacyl complex A and the subsequent
deinsertion of silyl isocyanide (eq 2). This silicon-assisted mech-
anism was first reported by Bergman and Brookhart by employing
a silyl-rhodium complex.11 Nakazawa reported that a similar
mechanism operates in the photochemically induced C-CN bond
cleavage reaction mediated by iron complexes, representing, to the
best of our knowledge, the only catalytic reaction that proceeds
through the pathway shown in eq 2.12 In this Communication, we
describe a new silylation reaction of nitriles based on eq 2, in which
C-CN and Si-Si bonds are cleaved nonphotochemically in a
catalytic manner.
entry
catalyst
solvent
yieldb (%)
1
2
[Cp*RhCl2]2/2PPh3
[RhCl(cod)]2
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
dioxane
DMA
ethylcyclohexane
ethylcyclohexane
12
53
62
80
65
71
36
87
49
3
[RhCl(coe)2]2
[Rh(OMe)(cod)]2
[Rh(cod)2]BF4
[RhCl(cod)]2
4
5c
6
7
[RhCl(cod)]2
[RhCl(cod)]2
[RhCl(cod)]2
8
9d
a Reaction conditions: 1 (2.0 mmol), 2 (4.0 mmol), catalyst (0.10 mmol)
in solvent (1.0 mL) at 130 °C, 15 h. b Isolated yields. c 10 mol % of
[Rh(cod)2]BF4 was used. d Run at 100 °C.
complexes could be employed as catalyst precursors in place of
silyl-metal complexes.
To investigate the feasibility of our working hypothesis, we
examined the reaction of 2-cyanonaphthalene (1) with hexameth-
yldisilane (2) in detail. In light of the pioneering work by Bergman
and Brookhart,11 we initially used [Cp*RhCl2]2/PPh3 as a catalyst
precursor and found that the reaction did indeed proceed, furnishing
2-trimethysilylnaphthalene (3) in 12% yield (Table 1, entry 1).
Further screening of catalysts indicated that several Rh(I) com-
plexes, such as [RhCl(cod)]2, [RhCl(coe)2]2, [Rh(OMe)(cod)]2, and
[Rh(cod)2]BF4, are more effective catalysts (entries 2-5).16 [RhCl-
(CO)2]2, [IrCl(cod)]2, [Cp*RuCl]n, and Ni(cod)2/PBu3 were com-
pletely inactive in this reaction under otherwise identical conditions.
The effect of solvents was also investigated using [RhCl(cod)]2 as
a catalyst. The use of ethylcyclohexane led to a dramatic increase
in the yield of 3 (entry 8). The reaction can be conducted at a lower
temperature (100 °C) at the expense of reaction rate (entry 9).
To incorporate the silicon-assisted C-CN bond-cleavage process
into a catalytic cycle, the critical issue is establishing the pathway
for regenerating the silyl-metal complex from the alkyl(aryl)-
metal complex B. The potential reactivity of a Si-Si bond toward
metal-carbon bonds13,14 led us to consider the catalytic cycle out-
lined in Scheme 1, in which complex B reacts with a disilane to
regenerate a silyl-metal complex with the concomitant formation
of an alkyl(aryl)silane and silyl isocyanide, which should isomerize
to thermodynamically stable silyl cyanide.12,15 In addition, because
disilanes would also be expected to exhibit a similar reactivity
toward metal halide complexes to form a silyl-metal complex, we
surmised that some of the more common metal halide (or related)
9
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J. AM. CHEM. SOC. 2006, 128, 8152-8153
10.1021/ja062745w CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Rh(I)-Catalyzed Silylation Reaction of Nitrilesa
cyanoferrocene (entry 16) were found to be good substrates for
this silylation reaction. It is noteworthy that the method can be
applied not only to aryl cyanides, but also to alkenyl cyanides,
affording disubstituted (entry 17) and trisubstituted (entry 18) al-
kenylsilanes. To the best of our knowledge, this is the first example
of the catalytic cleavage of the C-CN bond of alkenyl cyanides.
In summary, we report herein on the Rh(I)-catalyzed silylation
reaction of aryl and alkenyl cyanides involving the cleavage of
unreactive C-CN and Si-Si bonds.17 Expanding the scope of the
reaction and applying this concept to other functional group
transformations are currently in progress.
Acknowledgment. We thank the Instrumental Analysis Center,
Faculty of Engineering, Osaka University, for assistance in obtaining
MS, HRMS, and elemental analyses. We also acknowledge Prof.
Baba and Prof. Yasuda (Osaka University) for obtaining 29Si NMR
spectra.
Supporting Information Available: Detailed experimental pro-
cedures and the characterization of products. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
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a Reaction conditions: nitrile (2.0 mmol), hexamethyldisilane (4.0 mmol),
[RhCl(cod)]2 (0.10 mmol) in ethylcyclohexane (1.0 mL) at 130 °C, 15 h.
b Isolated yields. c Run on a 1-mmol scale. d 1,1,2,2-Tetramethyl-1,2-
diphenyldisilane was used in place of 2. e 1,2-Dibenzyl-1,1,2,2-tetrameth-
yldisilane was used in place of 2. f 10 mol % of the catalyst was used.
g GC yield. h [Rh(OMe)(cod)]2 was used as a catalyst. i Run for 40 h.
j [Rh(cod)2]BF4 was used as a catalyst. k Run for 96 h.
(13) For reviews on Si-Si bond activation, see: (a) Suginome, M.; Ito, Y.
Chem. ReV. 2000, 100, 3221. (b) Sharma, H. K.; Pannell, K. H. Chem.
ReV. 1995, 95, 1351.
We next turned our attention to the scope of this catalytic
silylation reaction (Table 2). With respect to disilanes, it was also
possible to introduce dimethylphenylsilyl and benzyldimethylsilyl
groups in modest yields by employing the corresponding disilanes
(entries 2 and 3). We were pleased to find that a diverse array of
aryl cyanides can be silylated. Thus, the catalytic process tolerates
a number of functional groups, including fluorides (entries 5 and
9), esters (entries 6-8), ethers (entry 10), tertiary amines (entry
11), and, notably, boronic esters (entry 13). The reaction is sensitive
to sterics surrounding the nitrile, as demonstrated by the reduced
yields for 1-naphthalenecarbonitrile (entry 4) and 2-methylben-
zonitrile (entry 12). Heteroaryl cyanides (entries 14 and 15) and
(14) Two mechanisms may be possible for this step: (1) via σ-bond metathesis
and (2) via oxidative addition of disilane, followed by reductive elimination
of alkyl(aryl)silane.
(15) (a) Thayer, J. S. Inorg. Chem. 1968, 7, 2599. (b) Secker, J. A.; Thayer,
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(16) The formation of trimethylsilyl cyanide was confirmed by 29Si NMR
measurements of the crude reaction mixture run in toluene-d8. Resonances
observed were in agreement with those of an authentic sample (-12.73
ppm).
(17) For recent examples of the catalytic synthesis of arylsilanes from aryl
halides and hexamethyldisilane, see: (a) Shirakawa, E.; Kurahashi, T.;
Yoshida, H.; Hiyama, T. Chem. Commun. 2000, 1895. (b) Gooâen, L. J.;
Ferwanah, A.-R. Synlett 2000, 1801. These reactions require bases, such
as KF, for the activation of a Si-Si bond.
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J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006 8153