880
J. Am. Chem. Soc. 1999, 121, 880-881
this reaction could be applied to the general exchange reaction
of an alkyl group to other alkyl groups in ketones through C-C
bond activation.
Catalytic Carbon-Carbon Bond Activation of
Unstrained Ketone by Soluble Transition-Metal
Complex
Our experiment was conducted under the following reaction
conditions: Benzylacetone (1a) reacted with 1-hexene at 150 °C
for 48 h in a mixture of [chlorotris(triphenylphosphine)rhodium-
(I)] (2, 5 mol % based upon 1) and 2-amino-3-picoline (3, 100
mol %) to give 2-octanone (4a) in 84% yield along with a trace
amount of styrene (eq 1).9
Chul-Ho Jun* and Hyuk Lee
Department of Chemistry, Yonsei UniVersity
Seoul 120-749, Korea
ReceiVed September 8, 1998
The activation of carbon-carbon bonds by soluble transition-
metal complexes has been one of the most prominent challenges
in recent years.1 Although C-C single bonds are generally inert
to transition metals under homogeneous conditions, some possible
ways to cleave C-C bonds have been devised: relieving ring
energy,2 inducing aromatic stabilization,3 forming stable metal-
lacyclic complexes (cyclometalation),4 etc.5 These methods all
involve stoichiometric reactions, not catalytic ones. Only a few
examples of homogeneous catalytic activation of C-C bonds have
been known,6-8 and these are limited to strained molecules,6 or
to model compounds.7 In this report, we describe the unprec-
edented catalytic C-C bond activation of unstrained ketone
compounds. This catalytic C-C bond activation can be achieved
by using 2-amino-3-picoline, which solves the problem of the
accessibility of metal complexes to C-C bonds. We suggest that
A possible mechanism for the transformation of ketone 1a to
ketone 4a is illustrated in Scheme 1. The ketimine 5 must be
formed in situ by condensation of ketone 1a and 3. A C-C bond
in 5 might be cleaved by rhodium(I) in 2 to give an (iminoacyl)-
rhodium(III) phenethyl 6. A â-hydrogen elimination of phenethyl
group in 6 provides (iminoacyl)rhodium(III) hydride 7 and styrene.
A hydride insertion of 7 into 1-hexene leads to an (iminoacyl)-
rhodium(III) hexyl 8, and reductive elimination in 8 produces
ketimine 9 with regeneration of catalyst 2. Hydrolysis of 9 with
H2O, previously formed from condensation of 3 and ketone 1a,
affords ketone 4a. In this reaction, 3 as well as metal complex 2
acts as a catalyst. When the reaction was carried out under
identical conditions in the absence of 3 to clarify its effect, no
reaction occurred, and the starting ketone 1a was completely
recovered. This implies that the C-C bond R to carbonyl group
in 1a is not directly cleaved by complex 2. Recently, this type of
chelation-assisted carbon-hydrogen bond activation has been
applied to the direct synthesis of ketone from aldehyde and
1-alkene, or primary alcohol and 1-alkene.10
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To identify the involvement of C-C bond cleavage of ketimine
5, 5 was allowed to react with 1-hexene at 130 °C for 6 h under
catalyst 2 (5 mol %), which resulted in a mixture of ketimine 9
and styrene in 50 and 36% yield (based upon 5) (eq 2).
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Formation of styrene and ketimine 9 confirms that the carbon-
carbon bond of 5 is clearly cleaved and that 1-hexene is exchanged
with the phenethyl group in 5. Since â-hydrogen elimination is a
necessary step proceeding catalytic C-C bond cleavage, the
starting ketone should have a â-hydrogen.
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145-147.
The catalytic reactions of various ketones and olefins with a
catalyst system of 2 (10 mol %) and 3 (20 and 100 mol %) were
examined (Table 1). The sterically hindered olefin, 3,3-dimethyl-
1-butene, also provided the corresponding ketone 4b in fairly good
10.1021/ja983197s CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/14/1999