Activation of a cyclobutanone carbon-carbon bond over an aldehyde carbon-hydrogen bond in the rhodium-catalyzed decarbonylation

Article abstract of DOI:10.1246/cl.2006.288

A rhodium(I)-N-heterocyclic carbene complex achieved a high-yield decarbonylation reaction of cyclobutanones to selectively afford cyclopropanes. With this catalyst, a cyclobutanone having an aldehyde moiety underwent chemoselective decarbonylation of the ketonic carbonyl group with the aldehydic carbonyl group left intact. Copyright

Full text of DOI:10.1246/cl.2006.288

288
Chemistry Letters Vol.35, No.3 (2006)
Activation of a Cyclobutanone Carbon–Carbon Bond over
an Aldehyde Carbon–Hydrogen Bond in the Rhodium-catalyzed Decarbonylation
Takanori Matsuda, Masanori Shigeno, and Masahiro Murakamiꢀ
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University,
Katsura, Kyoto 615-8510
(Received December 6, 2005; CL-051505; E-mail: murakami@sbchem.kyoto-u.ac.jp)
A rhodium(I)–N-heterocyclic carbene complex achieved a
O
O
high-yield decarbonylation reaction of cyclobutanones to selec-
tively afford cyclopropanes. With this catalyst, a cyclobutanone
having an aldehyde moiety underwent chemoselective decarbon-
ylation of the ketonic carbonyl group with the aldehydic carbon-
yl group left intact.
2 (5 mol %)
Rh
m-xylene, reflux, 4 h
Ar
3a
reductive
CO
CO
extrusion
elimination
Ar
Rh
4a 91%
Selective activation of low polarity ꢀ-bonds, such as car-
bon–hydrogen1 and carbon–carbon bonds,2 by transition metals
presents a challenge for the development of new chemical trans-
formations. Kinetically, a carbon–hydrogen bond generally en-
joys better reactivity, and thus, adds oxidatively to a transition
metal in preference to a carbon–carbon bond.3 Decarbonylation
reactions of carbonyl compounds provide a typical illustration of
preferential activation of C–H bonds over C–C bonds. Whereas
numerous examples with aldehydes have been reported in both
stoichiometric4 and catalytic reactions,4a,5 decarbonylation of
ketones has limited precedence.6 Herein, we report that a neutral
rhodium–N-heterocyclic carbene complex7 is a peculiar decar-
bonylation catalyst which can specifically activate the carbon–
carbon bond of a cyclobutanone in the presence of an aldehydic
carbon–hydrogen bond.
Scheme 2.
Control experiments between cyclobutanone 3a, aromatic
aldehyde 5a, and aliphatic aldehyde 5b were carried out to test
the chemoselectivity of the decarbonylation catalyst 2 (Eqs 1
and 2). Whereas 3a was decarbonylated by 2 to afford 4a in high
yield, both aldehydes, 5a and 5b, remained unchanged through-
out the reaction.
4a (95%)
3a
+
2 (5 mol %)
+
(1)
(2)
m-xylene, reflux, 4 h
4-PhC6H4CHO (5a)
5a (94% recovered)
4a (92%)
3a
+
2 (5 mol %)
+
m-xylene, reflux, 4 h
BnO(CH2)3CHO (5b)
5b (97% recovered)
Neutral monomeric complex [RhCl(cod)(NHC)]8 (2, NHC
= 1,3,4,5-tetramethylimidazol-2-ylidene) was prepared by a
procedure analogous to that for the imidazolin-2-ylidene com-
plexes:9 treatment of the complex [RhCl(cod)]2 with 1,3,4,5-
tetramethylimidazol-2-ylidene (1, NHC), which was generated
by reductive desulfurization of the corresponding thione with
potassium in THF,10 gave 2 (Scheme 1). The complex thus
produced was quite stable toward air and moisture and could
be isolated by column chromatography in 70% yield.
The catalytic activity of the complex 2 for decarbonylation
was examined. Both aliphatic and aromatic aldehydes were un-
reactive toward 2 up to 150 ꢁC. On the contrary, cyclobutanone
3a was decarbonylated to produce cyclopropane 4a in 91% yield
when heated at 150 ꢁC (bath temp) in the presence of a catalytic
amount of 2 (5 mol %) in m-xylene (Scheme 2).11 The insertion
of rhodium between the carbonyl carbon and the ꢁ-carbon
was followed by carbonyl extrusion and reductive elimination.
Linear ketones and ordinary less-strained cycloalkanones like
4-phenylcyclohexanone failed to undergo decarbonylation at
150 ꢁC even with a stoichiometric amount of 2.
The specific reactivity of cyclobutanones toward 2 was most
explicitly exemplified by the reaction of keto-aldehyde 6. Cyclo-
propane 7 was exclusively formed from 6 (82% isolated yield)
when 2 was used as catalyst (Eq 3). Only the ketonic carbonyl
group of the cyclobutanone moiety was removed with the alde-
hydic carbonyl group remaining intact.
O
2 (5 mol %)
(3)
O
O
m-xylene, reflux, 7 h
O
O
H
H
6
7 82%
The anomalous behavior of [RhCl(cod)(NHC)] (2) as the
decarbonylation catalyst led us to compare 2 with other rhodium
complexes. A neutral rhodium(I)–phosphine complex formed in
O
[RhCl(cod)]2 (2.5 mol %)
DPPP (10 mol %)
β
-hydride
elimination
3a
Rh
m-xylene, reflux, 4 h
Ar
H
Me
Me
Me
Me
CO
O
[RhCl(cod)]2
K
reductive
elimination
RhH
[RhCl(cod)(NHC)]
N
N
Me
Me
N
N
Me
THF
reflux
55 °C
Me
RhH
2 70%
S
Ar
Ar
1 (NHC)
8 88%
Scheme 1. Preparation of RhCl(cod)(NHC) (2).
Scheme 3.
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.3 (2006)
289
situ from [RhCl(cod)]2 and DPPP8 furnished not 4a but olefin 8
from 3a through a decarbonylation process involving ꢂ-hydride
elimination (Scheme 3).6d,12
Furthermore, no chemoselectivity was observed in the de-
carbonylation reaction of 6 in the presence of the Rh(I)–DPPP
complex; both the aldehydic and ketonic carbonyl groups were
decarbonylated to afford 1-isopropenyl-4-propoxymethylben-
zene (9) in 84% yield (Eq 4).
much more rapidly than 3d (Entry 4). During decarbonylation
of 3e with 2, ꢂ-carbon elimination did not follow the insertion
step of rhodium, unlike the case of a ring-expansion reaction
of an analogous spiro compound catalyzed by [Rh(dppp)2]Cl.14
In summary, the present study provides an intriguing exam-
ple of preferential activation of a C–C bond over a C–H bond.
The unique potential of NHC complexes for synthetic purposes
is inferred from the contrasting chemoselectivies observed in
decarbonylation. Mechanistic explanation of the marked con-
trast and application to other catalytic process are the subjects
of further studies in our laboratory.
[RhCl(cod)]2 (2.5 mol %)
DPPP (10 mol %)
6
(4)
m-xylene, reflux, 6 h
O
H
References and Notes
9 84%
1
a) A. E. Shilov, G. B. Shul’pin, Chem. Rev. 1997, 97, 2879.
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A chemoselectivity opposite to that provided by 2 was
observed when 6 was treated with a stoichiometric amount of
the Wilkinson’s complex at room temperature. Only the aldehy-
dic carbonyl group was decarbonylated with the cyclobutanone
carbonyl remaining intact in the product 10 (Eq 5).13 Thus, it
proved that aldehydes and cyclobutanones possess similar reac-
tivities toward rhodium-mediated decarbonylation and that an
appropriate choice of the ligand system can result in opposite
chemoselectivity.
2
a) M. Murakami, Y. Ito, in Topics in Organometallic Chemistry,
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3
4
For thermodynamic preference for C–C bond activation, see:
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O
[RhCl(PPh3)3] (1.1 equiv.)
6
(5)
CH2Cl2, rt, 10 days
O
H
10 83%
Other examples of the decarbonylation of cyclobutanones
using the Rh–NHC complex 2 are listed in Table 1. An active hy-
drogen of 3b remained intact during decarbonylation (Entry 1).
In the case of 2-(2-naphthyl)cyclobutanone (3c), 9% of (E)-1-(2-
naphthyl)propene (11), formed through ꢂ-hydride elimination,
was obtained as another decarbonylation product together with
the major product, cyclopropane 4a (83%) (Entry 2). 3,3-Disub-
stituted cyclobutanone 3d requires a longer reaction time to
reach full conversion, probably due to steric reasons (Entry 3).
On the other hand, spiro[3.3]heptan-2-one 3e, with constrained
geminal disubstituents at the 3-position, was decarbonylated
5
6
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ˇ
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Table 1. Decarbonylation of cyclobutanones 3b–3e using 2a
Enrty
3
Time/h
4 (%yieldb)
7
For reviews on transition metal–NHC complexes, see: a) W. A.
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Abbreviations: cod = cycloocta-1,5-diene, DPPP = 1,3-bis(di-
phenylphosphino)propane.
O
EtO2C
CO2Et
EtO2C
1
6
CO2Et
4b (92)
3b
8
9
F. E. Hahn, M. Paas, D. Le Van, T. Lugger, Angew. Chem., Int.
¨
O
2
8
4ac (83)
Ed. 2003, 42, 5243.
10 N. Kuhn, T. Kratz, Synthesis 1993, 561.
11 Decarbonylation of 3a with [RhCl(cod)(1,3-dimethylimidazol-2-
ylidene)] under otherwise identical conditions gave 4a in 82%
yield.
3c
3d
3e
Ph
Ph
Ph
O
3
4
84
2
4Pdh (90)
12 Catalyst with 1/2 ratio of Rh/DPPP exhibited a higher activity
than that with 1/1 ratio.
O
13 Decarbonylation of 3a in the presence of 1.1 equiv. of [RhCl-
(PPh3)3], which failed to occur at rt, proceeded in refluxing
m-xylene (4 days) to afford a mixture of 4a (7%), 8 (25%), and
11 (23%, E=Z ¼ 90=10).
4e (86)
aCyclobutanone 3 (0.60 mmol) and Rh–NHC complex 2 (0.03
mmol, 5 mol %) were heated in refluxing m-xylene (3.0 mL).
bIsolated yield. cObtained as a mixture with (E)-1-(2-naphthyl)-
propene (11) (9%).
14 M. Murakami, K. Takahashi, H. Amii, Y. Ito, J. Am. Chem. Soc.
1997, 119, 9307.
Published on the web (Advance View) February 11, 2006; DOI 10.1246/cl.2006.288