Synthesis of Cp-Re complexes via olefinic C-H activation and successive formation of cyclopentadienes

Article abstract of DOI:10.1021/ja805921f

Treatment of an α,β-unsaturated ketimine with an α,β-unsaturated carbonyl compound in the presence of a rhenium complex, Re₂(CO)₁₀, gave a cyclopentadienyl-rhenium complex. This reaction proceeds via rhenium-catalyzed C-H bond activation of an olefinic C-H bond, insertion of an α,β-unsaturated carbonyl compound into a Re-C bond of the alkenylrhenium intermediate, intramolecular nucleophilic cyclization, reductive elimination, elimination of aniline to give a cyclopentadiene derivative, followed by the formation of a cyclopentadienyl-rhenium complex from the cyclopentadiene derivative and the rhenium complex. Copyright

Full text of DOI:10.1021/ja805921f

Published on Web 10/01/2008
Synthesis of Cp-Re Complexes via Olefinic C-H Activation and Successive
Formation of Cyclopentadienes
Yoichiro Kuninobu,* Yuta Nishina, Takashi Matsuki, and Kazuhiko Takai*
DiVision of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama UniVersity,
Tsushima, Okayama 700-8530, Japan
Received July 29, 2008; E-mail: kuninobu@cc.okayama-u.ac.jp; ktakai@cc.okayama-u.ac.jp
Scheme 1. Strategy for the Retrosynthesis of
Cyclopentadienyl-Transition Metal Complexes
One of the most important categories of transition metal
complexes is cyclopentadienyl (Cp) complexes.1 Because the
reactivity of Cp complexes can usually be controlled by changing
the substituents of the Cp ring, many methods have been developed
to introduce appropriate substituents at desired positions.2 We
disclose here a novel method for [3 + 2] construction of substituted
cyclopentadienes from R,ꢀ-unsaturated ketimines and R,ꢀ-unsatur-
ated carbonyl compounds initiated by olefinic C-H activation of
the ketimines. The C-H activation, which is the key step in the
reaction, is accomplished with a rhenium catalyst.
In addition, we found that cyclopentadienyl-rhenium (Cp-Re)
complexes3 can be prepared in a one step domino reaction involving
the preparation of cyclopentadiene derivatives followed by the
complexation with Re2(CO)10. Because substituted cyclopentadienyl
complexes of group 7 metals have attracted much attention in the
field of biochemistry and pharmaceuticals,4 the method provides a
new entry for the substituted Cp complexes.
Thus, we examined the reaction between the isolated 4 and
Re2(CO)10 (3b, 0.50 equiv), and found that Cp-Re complex 5a
was generated in 85% yield (eq 2). Although [ReBr(CO)3(thf)]2
(3a) also catalyzed the formation reaction of 4, the Cp-Re complex
5a was not formed with 3a.
The strategy for the synthesis of Cp-transition metal complexes
is shown in Scheme 1. For the construction of substituted
cyclopentadienes, the transition metal-catalyzed activation of an
olefinic C-H bond and successive cyclization are key steps. The
C-H bond activation at olefinic positions has been achieved with
several transition metal complexes such as ruthenium and rhodium
complexes;5 however, the reactions are limited to the insertion of
an unsaturated molecule into the generated M-H bond. For the
successive cyclization, we thought that rhenium complexes may
be suitable because the insertion of an R,ꢀ-unsaturated carbonyl
compound occurs into a rhenium-carbon bond after the C-H
activation.6 The main problem is that it is not clear whether the
olefinic C-H bond can be activated with the rhenium complexes.7
By the reaction between ketimine 1a and 2-ethylhexyl acrylate (2a)
in the presence of a rhenium catalyst, [ReBr(CO)3(thf)]2 (3a), both
insertion of acrylate 2a into an olefinic C-H bond of 1a, and
intramolecular cyclization proceeded, and cyclopentadiene derivative
4 was formed in 25% yield. By using Re2(CO)10 (3b) as a catalyst,
the yield of 4 increased dramatically, and 4 was produced in 65%
yield (eq 1). Interestingly, Cp-Re complex 5a was also obtained in
4% yield as a side product.8 This result indicates that the rhenium
complex 5a could be formed by a stoichiometric reaction between
cyclopentadiene derivative 4 and the rhenium complex 3b.
From the result in eq 2, we were encouraged to investigate the
domino synthesis of a Cp-Re complex 5a from substrates to give
a ligand, and the rhenium complex 3b. Treatment of ketimine 1a
having an olefin moiety and 2-ethylhexyl acrylate (2a) with
Re2(CO)10 (3b) in xylenes at 150 °C for 72 h gave the Cp-Re
complex 5a in 94% yield as a colorless oil (Table 1, entry 1).
n-Butyl-substituted Cp-Re complex 5b was produced by the
reaction between ketimine 1b and 2a (Table 1, entry 2). Acyclic
R,ꢀ-unsaturated ketimine 1c also provided the corresponding
Cp-Re complex 5c in 82% yield (Table 1, entry 3). Cp-Re
complex 5d was afforded from an R,ꢀ-unsaturated ketimine having
an ether ring, 1d (Table 1, entry 4). A vinyl ketone 2b gave a
mixture of Cp-Re complexes 5e and 5e′ in 47% yield (Table 1,
entry 5). Formation of 5e′ indicates that 2b inserted into a
rhenium-carbon bond in the opposite direction to acrylic ester 2a
and amide 2c.7c,9 Amide-substituted Cp-Re complex 5f was also
formed in 69% yield using acrylamide 2c (Table 1, entry 6).
As a result of the above experiments, the proposed mechanism
is as follows (Scheme 2): (1) coordination of a nitrogen atom of a
ketimine to a rhenium center; (2) oxidative addition of a C-H bond
of the ketimine to the rhenium center (C-H bond activation);10
(3) insertion of an R,ꢀ-unsaturated carbonyl compound into a
rhenium-carbon bond;11 (4) intramolecular nucleophilic cycliza-
tion; (5) reductive elimination and the elimination of aniline to give
a cyclopentadiene derivative; (6) the formation of a Cp-Re complex
from the cyclopentadiene derivative and rhenium complex.
9
14062 J. AM. CHEM. SOC. 2008, 130, 14062–14063
10.1021/ja805921f CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Synthesis of Cyclopentadienyl-Rhenium Complexes 5
from Ketimines 1, R,ꢀ-Unsaturated Carbonyl Compounds 2, and a
Rhenium Complex 3ba
both the catalyst for the formation of substituted Cp rings and a
component of the desired complexes.13 We hope this versatile and
efficient method for the preparation of Cp complexes will find new
applications.
Acknowledgment. Financial support from the Ministry of
Education, Culture, Sports, Science, and Technology of Japan. Also,
Y.K. appreciates the Sumitomo Foundation, Meiji Seika Co., and
Okayama Foundation for Science and Technology for financial
support.
Supporting Information Available: General experimental proce-
dure, characterization data for a cyclopentadiene derivative and
cyclopentadienyl-rhenium complexes. This material is available free
of charge via the Internet at http://pubs.acs.org.
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Lett. 1995, 679. (b) Trost, B. M.; Imi, K.; Davies, I. W. J. Am. Chem. Soc.
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H.; Lee, J. H. Tetrahedron Lett. 2002, 43, 4233.
(6) For a coordinatively unsaturated Cp-Re complex that catalyzes aliphatic
C-H bond activation, see: Chen, H.; Hartwig, J. F. Angew. Chem., Int.
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Y.; Kawata, A.; Takai, K. J. Am. Chem. Soc. 2005, 127, 13498. (b)
Kuninobu, Y.; Tokunaga, Y.; Kawata, A.; Takai, K. J. Am. Chem. Soc.
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a 1 (1.0 equiv), 2 (1.0 equiv), 3b (0.50 equiv). b Isolated yield. Yield
determined by 1HNMR is reported in parentheses. c R ) CH2CHEtnBu.
d Run at 180 °C. e 5e/5e′ ) 1.0:1.8.
Scheme 2. Proposed Mechanism for the Formation of Cp-Re
Complexes
(8) The Cp-Re complex 5a did not work as a catalyst for olefinic C-H bond
activation. This result shows that cyclopentadiene derivative 4 was formed
by rhenium catalyst 3b in the first step.
(9) In the case of an arylimine, the regioselectivity of the insertion of an R,ꢀ-
unsaturated ketone is the same as acrylic esters.7c We examined the
existence of the interconversion between 5e and 5e′;14 however, both 5e
and 5e′ remained unchanged under the reaction conditions with Re2(CO)10
.
It is still unclear why the insertion occurred in the opposite direction in
the case of the R,ꢀ-unsaturated ketone.
(10) Rhenium-catalyzed C-H bond activation occurs only at the ortho-position
of R,ꢀ-unsaturated ketimines. This is in sharp contrast to C-H bond
activation with rhodium or ruthenium catalysts, where carbonyl oxygen
atoms can also act as directing groups. This feature of the rhenium-catalyzed
C-H activation enables R,ꢀ-unsaturated ketimines and carbonyl compounds
to serve different roles.
(11) The insertion step did not occur with either R- or ꢀ-substituted acrylates
(see ref 7c).
(12) (a) Janiak, C.; Schumann, H. AdV. Organomet. Chem. 1991, 33, 291. (b)
Hermann, E. A., Ed. Synthetic Methods of Organometallic and Inorganic
Chemistry; Thieme: New York, 1997; Vol. 8.
(13) One example using this concept is supramolecular chemistry. See: Lehn,
J.-M. Supramolecular Chemistry. Concepts and PerspectiVes; VCH Ver-
lagsgesellschaft: Weinheim, Germany, 1995.
(14) Kuninobu, Y.; Ueda, H.; Kawata, A.; Takai, K. J. Org. Chem. 2007, 72,
6749.
In summary, we have succeeded in the activation of an olefinic
C-H bond with rhenium complexes and utilized it in the domino
synthesis of Cp-Re complexes from R,ꢀ-unsaturated ketimines,
R,ꢀ-unsaturated carbonyl compounds, and Re2(CO)10. Although
there have been many methods for the synthesis of Cp-transition
metal complexes,12 it is usually necessary to synthesize the Cp
ligands in advance. In this reaction, the rhenium complex acts as
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