Communications
DOI: 10.1002/anie.201104704
À
C C Cleavage
Manganese-Catalyzed Cleavage of a Carbon–Carbon Single Bond
between Carbonyl Carbon and a-Carbon Atoms of Ketones**
Yoichiro Kuninobu,* Tadamasa Uesugi, Atsushi Kawata, and Kazuhiko Takai*
Olefin metathesis, which proceeds through a carbon–carbon
À
(C C) double bond cleavage, is a well-known and useful
method in synthetic organic chemistry.[1] In contrast, cleavage
À
of a C C single bond is still one of the most difficult and
challenging reactions in organic synthesis. Recently, there
have been several reports on transition-metal-catalyzed
transformations.[2] For example, reactions of strained mole-
cules, such as three- and four-membered rings, have been
reported.[3] In these reactions, release of the ring strain is the
À
driving force for C C single bond cleavage. As for reactions
not involving ring strain, transformations using a directing
group,[4] cleavage of a carbon–nitrile bond,[5] and transforma-
tions by retro-reactions, including retro-allylations,[6] retro-
arylations,[7] retro-alkynylation,[8] retro-aldol reactions,[9,10]
group on the aromatic skeleton provided the corresponding
amides 3b, 3c, and 3d in yields in the range of 96–98%
(entries 1–3). Chlorine and bromine atoms on the aromatic
ring were not lost under the reaction conditions, and amides
3e and 3 f were obtained in 96% and 62% yields, respectively
(entries 4 and 5). In the case of using acetophenone (1g) or
the dialkyl ketone 1h, amides 3a and 3g were provided in
60% and 63% yields, respectively (entries 6 and 7). Cyclo-
hexyl ethyl ketone (1i) also produced amide 3h in 50% yield
(entry 8). The amide 3h was formed selectively without
formation of the regioisomer, probably because of the steric
and deallylation[11] are also well known. To promote C C
À
single bond cleavage, we employed a manganese catalyst and
carbodiimides. We report herein the cleavage of a unstrained
À
C C single bond between the carbonyl carbon and a-carbon
atoms of ketones, and its application to the synthesis of
amides.
Treatment of propiophenone (1a) with 1.0 equivalent of
1,3-di-p-tolylcarbodiimide (2a) in the presence of a catalytic
amount of a manganese complex, [Mn2(CO)10], in 1,4-dioxane
at 1508C for 24 hours gave amide 3a in 50% yield.[12–14] This
reaction also proceeds using either the iron complex
[Fe2(CO)9] or the cobalt complex [Co2(CO)8] as a catalyst.[15]
By increasing the amount of 2a to 3.0 equivalents, the yield of
amide 3a was improved to 97% [Eq. (1)]. In this reaction,
quinoline 4a was also formed in 73% yield. The catalytic
amount and reaction temperature could be reduced when the
trinuclear manganese complex [{HMn(CO)4}3] was used as
À
hindrance of the cyclohexyl group of 1i. A C C single bond
was cleaved using a ketone bearing a longer alkyl chain, 1j
Table 1: Reactions between several ketones 1 and carbodiimide 2a.[a]
À
the catalyst [Eq. (1)]. The C C single bond of 1a was cleaved
Entry
R1
R2
Yield [%][b]
regioselectively in this reaction. In the cleavage of unreactive
bonds, novel transition-metal catalysts are usually employed;
however, such transformations proceed efficiently with first-
row transition metal catalysts (manganese, iron, or cobalt
catalysts).
1[c]
2
4-(MeO)C6H4
4-MeC6H4
4-(CF3)C6H4
4-ClC6H4
4-BrC6H4
Ph
Me
Me
Me
Me
Me
H
1b
1c
1d
1e
1 f
3b
3c
3d
3e
3 f
3a
3g
98 (>99)
96 (>99)
96 (>99)
96 (>99)
62 (65)
3[c]
4
First, we investigated the scope of the ketones (Table 1).
Ketones with an electron-donating or electron-withdrawing
5
6[c]
7
1g
1h
60 (63)
63 (69)
n-C5H11
n-C4H9
8
Me
1i
3h
50 (–)
[*] Dr. Y. Kuninobu, T. Uesugi, Dr. A. Kawata, Prof. Dr. K. Takai
Division of Chemistry and Biochemistry, Graduate School of Natural
Science and Technology, Okayama University
Tsushima, Kita-ku, Okayama 700-8530 (Japan)
E-mail: kuninobu@cc.okayama-u.ac.jp
9
10
Ph
Ph
n-C5H11
Ph
1j
1k
3a
3a
72 (74)
95 (96)
11
1l
3a
62 (65)
[**] This work was partially supported by the Ministry of Education,
Culture, Sports, Science, and Technology of Japan.
[a] 2a (3.0 equiv). [b] Yield of isolated product. The yield determined by
1H NMR spectroscopy is reported within parentheses. [c] [Mn2(CO)10]
(5.0 mol%) was used as the catalyst, and the reaction temperature was
1508C.
Supporting information for this article is available on the WWW
10406
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 10406 –10408