Rhodium(I)-Catalyzed successive double cleavage of Carbon-Carbon bonds of strained spiro cyclobutanones

Full text of DOI:10.1021/ja971949x

J. Am. Chem. Soc. 1997, 119, 9307-9308
9307
Although found rarely in either stoichiometric or catalytic
reactions,8 this process has recently received growing attention
especially in the field of polymer chemistry.9 In terms of
organic synthesis, however, both elementary steps of C-C bond
cleavage have received much less attention than those of C-C
bond formation, despite their great synthetic potential.
The oxidative addition of a C-C bond onto a transition metal
results in the formation of a σ alkyl-metal complex. Hence,
the two elementary steps of C-C bond cleavage, i.e., oxidative
addition and â-carbon elimination, possibly operate in sequence,
making an interesting synthetic reaction (eq 2). A leading
Rhodium(I)-Catalyzed Successive Double Cleavage
of Carbon-Carbon Bonds of Strained Spiro
Cyclobutanones
Masahiro Murakami,* Kunio Takahashi, Hideki Amii, and
Yoshihiko Ito*
Department of Synthetic Chemistry and Biological Chemistry
Kyoto UniVersity, Yoshida, Kyoto 606-01, Japan
ReceiVed June 13, 1997
Transition metals have been successfully exploited in organic
synthesis, in particular, for the formation of C-C bonds.1 One
of the most important elementary steps constituting C-C bond
formation is the reductive elimination of a di(organyl)metal
complex, which leads to the extrusion of the coupled product
from the metal (eq 1). On the contrary, the reverse process,
example of this sort has been identified by Liebeskind et al.10
We have recently found that the R C-C bond of a cyclobu-
tanone is catalytically cleaved by a rhodium(I) complex, wherein
the carbonyl group directs insertion of the metal.7 As a
continuation of our approach to the problem of C-C bond
activation from the viewpoint of synthetic chemistry, we report
herein a new example of organic transformations in which two
C-C bonds are successively cleaved by a transition metal in a
way shown in eq 2.
A preliminary experiment was carried out by using spiro
cyclobutanone equipped with another four-membered ring (1),11
which was expected to favor â-carbon elimination by relief of
the ring strain.8b,f,g,9d Heating a xylene solution of 1 at reflux
in the presence of [Rh(cod)(dppe)]BF4 (5 mol %)12 for 18 h
gave rise to 2-cyclohexenone (5) in 28% isolated yield along
with decarbonylated products. The formation of 5 can be
explained by assuming the pathway pictured in eq 3. Initially,
rhodium(I) undergoes an insertion into the bond between the
carbonyl carbon and the R-carbon of 1. The resultant five-
membered cyclic acylrhodium (2) undergoes â-carbon elimina-
tion to open the appended cyclobutane ring, forming seven-
membered cyclic acylrhodium (3). Subsequent reductive
elimination gives rise to the methylenecyclohexanone (4), which
finally isomerizes to the conjugated cyclohexenone (5). In the
net transformation of 1 to 5, rhodium(I) successively cleaved
oxidative addition of a C-C bond onto a transition metal,
provides a direct approach to C-C bond cleavage. Although
stoichiometric reactions involving this elementary step have been
the subject of a number of investigations,2-5 catalytic reactions
are still few in number6,7 due to the inertness of C-C σ-bonds
toward transition metals. An alternative method of breaking
C-C bonds is accessible by using σ alkyl-metal complexes.
The bond between the â- and γ-carbon atoms can be cleaved
via â-carbon elimination, the reverse of olefin insertion.
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4895. The authors reported a rhodium-catalyzed useful ring fusion reaction
of cyclobutenone derivatives, which are more labile thermally13 as well as
catalytically14 than cyclobutanone. Although the exact mechanism was not
mentioned, the reaction seemed to involve a sequence similar to that of the
present work.
(11) Spiro cyclobutanone (1) was readily prepared by [2 + 2] cycload-
dition of methylenecyclobutane with dichloroketene and the subsequent
dechlorination with zinc.
(12) The complex (cod ) cycloocta-1,5-diene, dppe ) Ph2PCH2CH2-
PPh2) was prepared in situ according to a literature procedure: Green, A.;
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S0002-7863(97)01949-5 CCC: $14.00 © 1997 American Chemical Society

9308 J. Am. Chem. Soc., Vol. 119, No. 39, 1997
Communications to the Editor
bond cleavage by â-carbon elimination would follow the second
C-C bond cleavage. However, the rhodium-catalyzed C-C
bond cleavage occurred only twice to afford spiro cyclohexenone
(14) (eq 5). In this particular case, [Rh(dppe)BPh4] gave the
best isolated yield (84%) of 14.
Successive double cleavage of C-C bonds also took place
with spiro cyclobutanone (15) appended with a cyclopropane
ring (eq 6). The reaction proceeded in an analogous way to
the two C-C bonds of 1, the first by oxidative addition and
the second by â-carbon elimination. Further examination of
the rhodium catalyst in detail revealed that complexes having
two bidentate diphosphines per rhodium were excellent catalysts
for the selective formation of 5. In particular, [Rh(dppp)2]Cl
produced 5 in 89% isolated yield.
Other examples of successive double cleavage of C-C bonds
of spiro cyclobutanones are shown below. Alkyl-substituted
spiro cyclobutanone (6) yielded the corresponding 3,5-disub-
stituted 2-cyclohexenone (7). Trisubstituted cyclohexenone (9)
was obtained in 59% yield from disubstituted spiro cyclo-
butanone (8). Spiro cyclobutanone (10) possesses two C-C
bonds (A and B) possibly susceptible to cleavage by â-carbon
elimination. Different products are predicted, depending on
which bond is cleaved; cleavage of the C-C bond A, connected
to a primary alkyl carbon atom, would lead to the formation of
cyclohexenone (11), whereas cleavage of bond B, connected
to a tertiary carbon atom, would lead to cyclohexenone (12).
The fact that the tetrasubstituted 2-cyclohexenone (11) was
obtained selectively from 10 suggests that in the process of
â-carbon elimination, cleavage of bond A is much favored over
that of bond B. A probable explanation might be that a sterically
less-hindered carbon atom is likely to be in closer proximity to
the rhodium.
afford cyclopentenone derivatives (16 and 17). The spiro
cyclobutanone (15) also has two C-C bonds possibly cleaved
by â-carbon elimination, and the two regioisomers were formed
as a result of the cleavage of each of them. Formed predomi-
nantly was isomer 16, which resulted from the second cleavage
of the less-substituted C-C bond.
With the other cyclobutanones, for example a spiro substrate
joined by a seven-membered ring (18) or a non-spiro compound
(19), analogous successive double cleavage of C-C bonds failed
to occur. Instead, attempted reactions with various rhodium(I)
complexes led to the predominant formation of a decarbonylated
product, i.e., cyclopropane. Relief of the structural strain of
the two small rings, therefore, may make a large contribution
to the driving force of the catalytic reactions mentioned above.
In conclusion, a new tandem sequence was developed in
which two C-C bonds are cleaved, the first through the insertion
of rhodium and the second by subsequent â-carbon elimination.
The present example clearly demonstrates the versatility of
transition metal-mediated C-C bond cleavage and its promising
application in synthetic organic chemistry.
Acknowledgment. This paper is dedicated to Prof. Dr. Dieter
Seebach on the occasion of his 60th birthday. Support from Ciba-Geigy
Foundation (Japan) for the Promotion of Science is gratefully acknowl-
edged.
Supporting Information Available: Experimental details and
characterization for 5, 7, 9, 11, 14, 16, and 17 (2 pages). See any
current masthead page for ordering and Internet access instructions.
Next, the reaction of cyclobutanone equipped with double
spiro structures (13) was examined in the hope that a third C-C
JA971949X