Rhenium-catalyzed formation of indene frameworks via C-H bond activation: [3+2] Annulation of aromatic aldimines and acetylenes

Article abstract of DOI:10.1021/ja0528174

A rhenium complex, [ReBr(CO)₃(thf)]₂, catalyzes the reaction of an aromatic aldimine with an acetylene to give an indene derivative in a quantitative yield. The reaction proceeds via C-H bond activation, insertion of the acetylene, intramolecular nucleophilic cyclization, and reductive elimination. In contrast to ruthenium and rhodium catalysts, which are usually employed in this type of reaction, the rhenium catalyst promotes the intramolecular nucleophilic cyclization of the alkenylmetal species generated by insertion of the acetylene. Copyright

Full text of DOI:10.1021/ja0528174

Published on Web 09/08/2005
Rhenium-Catalyzed Formation of Indene Frameworks via C-H Bond
Activation: [3+2] Annulation of Aromatic Aldimines and Acetylenes
Yoichiro Kuninobu,* Atsushi Kawata, and Kazuhiko Takai*
DiVision of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama UniVersity,
Tsushima, Okayama 700-8530, Japan
Received April 29, 2005; E-mail: kuninobu@cc.okayama-u.ac.jp; ktakai@cc.okayama-u.ac.jp
Table 1. Rhenium-Catalyzed Reaction of an Aromatic Aldimine
with an Acetylene
Indene derivatives are useful compounds serving as building
blocks for many functional materials,1 medicines,2 and organic
compounds.3 They can also be used as ligands for transition metals
by deprotonation.4 There have been many reports on the synthesis
of indene derivatives using transition metal compounds.5 Among
them, indene derivatives are obtained stoichiometrically by reactions
of ortho-manganated aryl ketones with acetylenes.6 If the starting
ortho-metalated aryl ketones can be prepared by catalytic C-H
bond activation,7 and then successive insertion, intramolecular
cyclization, and reductive elimination can occur, this indene
formation will become a catalytic process. In this paper, by using
a rhenium complex as the catalyst and changing the substrate from
aromatic ketones to aldimines, we succeed in the first catalytic
synthesis of indene derivatives via C-H bond activation followed
by insertion and intramolecular addition.
The formation of indene derivatives was examined by the reaction
of aromatic aldimine 1a (0.50 mmol) with acetylene 2a (0.50 mmol)
in toluene (1.0 mL) in the presence of a possible catalyst (5.0 mol
%) under reflux conditions for 24 h (eq 1).8-10 First, we examined
a popular rhodium(I) complex, RhCl(PPh3)3, and a ruthenium
complex, Ru3(CO)12, but the reactions did not occur and aldimine
1a remained unchanged in both cases. Although the ruthenium
complex has been reported to catalyze C-H bond activation and
successive insertion of the 1-trimethylsilyl-1-propyne,11 the process
stops at this stage, and further intramolecular cyclization does not
occur. Among the catalysts examined, a rhenium(I) complex, ReCl-
(CO)5, was found to catalyze the desired reaction and gave two
isomeric indene derivatives, 3a and 4a, though the yields were low
(both 17% yields). The result suggested that the rhenium complex
has a similar ability for C-H bond activation as Ru3(CO)12, and
the formed alkenyl-rhenium species has sufficient nucleophilicity
to add to the aldimine. The yields and selectivity were improved
by changing the ligand of the rhenium complex from chloride to
bromide and changing two carbon monoxides to THFs. In this case,
the indene derivative 3a was obtained quantitatively with [ReBr-
(CO)3(thf)]2 under the same reaction conditions.12,13
a Isolated yield. The yield determined by 1H NMR is reported in
parentheses. b The ratios of indene derivatives 3 and 4. c 1-Phenyl-1-propyne
(1.1 equiv), 135 °C. d At 135 °C.
obtained selectively (entry 2). The reaction did not proceed with
the oxime or hydrazone of an aldehyde instead of the aldimine.
Treatment of diphenylacetylene (2b) and aldimine 1a with [ReBr-
(CO)3(thf)]2 afforded the indene derivative quantitatively (entry 3).
Aldimines bearing electron-donating groups, such as methoxy and
methyl groups at the para-position of the aldimine, gave the
corresponding indene derivatives in excellent yields (entries 4 and
5). A reaction of an aldimine possessing a para-phenyl group with
2b gave the indene derivative 3b in good yield (entry 6). However,
an aldimine bearing an electron-withdrawing trifluoromethyl group
at the para-position gave the corresponding indene derivative in
low yield (entry 7). In contrast to the para-substituents, the yields
decreased when substituents were attached to the ortho-position of
the aldimines. For example, the indene derivative was obtained in
40% yield by the reaction of an aldimine bearing ortho-methyl
group with 2b (entry 8), and an aldimine with an ortho-methoxy
Then, we explored the substituents of aldimines (Table 1). By
the reaction of 1-phenyl-1-propyne (2a) with aldimine 1b instead
of 1a, the selectivity reversed and the indene derivative 4b was
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10.1021/ja0528174 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Proposed Mechanism of the Formation of Indene
Derivatives
Acknowledgment. Financial support was provided by a Grant-
in-Aid for Scientific Research on Priority Areas (No. 14078219,
“Reaction Control of Dynamic Complexes”) from the Ministry of
Education, Culture, Sports, Science, and Technology of Japan.
Supporting Information Available: General experimental proce-
dure, characterization data for indene derivatives, and X-ray crystal-
lographic data (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
References
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group did not afford the corresponding indene derivative (entry 9).
The reaction of 2b with benzaldehyde or acetophenone did not
proceed at all. The imine nitrogen atom could coordinate more
strongly to the rhenium center than the oxygen atoms and promoted
the C-H bond activation. In the case of the reaction of 2b with a
hydrazone derived from acetophenone, indene was not formed
because the internal cyclization did not proceed after C-H bond
activation and insertion of the acetylene.
From our experiments, it was found that acetylenes bearing at
least one aryl group gave the corresponding indene derivatives
(entries 10 and 11). Indene derivatives could not be obtained by
the reaction of aldimine with 6-dodecyne, 1-trimethylsilyl-1-
propyne, bis(trimethylsilyl)acetylene.
When the proposed reaction was carried out with ReBr(CO)5
under an atmosphere of carbon monoxide (1.0 atm), an indene
derivative was not obtained and the starting materials 1a and 2a
were recovered quantitatively. This result and the fact that the
catalytic activity of [ReBr(CO)3(thf)]2 is higher than that of ReBr-
(CO)5 indicated that the formation step of one or more vacant
coordination sites as a result of the carbonyl ligand(s) leaving the
rhenium center is important to promote the reaction. The proposed
reaction mechanism is as follows (Scheme 1): (1) coordination of
a nitrogen atom of an imine to a rhenium center; (2) C-H bond
activation (formation of ortho-metalated imine);14-16 (3) insertion
of an acetylene to the rhenium-carbon bond of the aryl-rhenium
intermediate; (4) intramolecular nucleophilic attack of the formed
alkenyl-rhenium moiety to a carbon atom of the imine; and (5)
reductive elimination and 1,3-rearrangement of hydrogen atoms (or
vice versa).
In summary, we have succeeded in the catalytic synthesis of
indene derivatives via C-H bond activation by the reactions of
aromatic aldimines with phenyl acetylenes. There are only a few
precedents for the rhenium-catalyzed C(sp2)-H bond activation.17,18
In addition, to our knowledge, the intramolecular nucleophilic
addition of organometallic species derived from C-H bond
activation to an imine moiety has not been reported. Since this
reaction needs only a catalytic amount of the metal reagent and a
few reaction steps compared with the reported synthetic methods
of indene derivatives, it will become a useful method to synthesize
indene frameworks. Further reactions initiated by the C-H bond
activation with rhenium catalysts are now in progress.
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1
(8) The H NMR yields were determined by using 1,1,2,2-tetrachloroethane
as an internal standard.
(9) A detailed structure of the indene framework was determined by X-ray
crystal structure analysis. See the Supporting Information.
(10) Heat and/or a rhenium catalyst promote interconversion of substituents
at the 1,2-position of the indene framework, though the mechanism is
not clear yet.
(11) Kakiuchi, F.; Sato, T.; Tsujimoto, T.; Yamauchi, M.; Chatani, N.; Murai,
S. Chem. Lett. 1998, 1053.
(12) Methyl-rhenium complex Re(CH3)(CO)5 promoted the reaction, but the
yield of 3a was very low. The formation reaction of indene derivatives
did not occur with the following catalysts: Re2(CO)10, ReBr(CO)3(CH3-
CN)2, ReCp*(CO)3, ReCl3(PMe2Ph)3, MnBr(CO)5, and RhCl(PPh3)3.
(13) The highest yield and selectivity was obtained by using toluene as a solvent
in the reaction of aromatic aldimine 1a with acetylene 2a. Results for the
other solvents are as follows: hexane, >99% (3a:4a ) 47:53); CH2Cl2,
35% (17:83); THF, 17% (<1:>99); DMI, 17% (41:59); DMF, <1%.
(14) There have been some reports on the synthesis of ortho-rhenated
complexes. For ortho-rhenation of aromatic imines with Re(CH2Ph)(CO)5,
see: (a) Bo¨hm, A.; Su¨nkel, K.; Polborn, K.; Beck, W. J. Organomet.
Chem. 1998, 552, 237. For ortho-rhenation of phenylpyridine derivatives
with Re(CH2Ph)(CO)5, see: (b) Djukic, J.-P.; Maisse, A.; Pfeffer, M.;
Do¨tz, K. H.; Nieger, M. Organometallics 1999, 18, 2786.
(15) Although we tried to prepare or isolate the rhenium-hydride intermediate
by the quantitative reaction of [ReBr(CO)3(thf)]2 with aldimine, we could
not observe the formation of the intermediate.
(16) A referee has proposed that electrophilic metalation is another possible
mechanism for the formation of the aryl-rhenium complex and H+. We
examined the rhenium-catalyzed reaction by addition of a base (K2CO3
or tributylamine) and noticed that the reaction with the base proceeded
without decreasing the yield. Thus, we are tempted to assume that the
rhenium-catalyzed reaction proceeds via the C-H bond activation.
(17) Jacob, J.; Espenson, J. H. Inorg. Chim. Acta 1998, 270, 55.
(18) An example of rhenium-catalyzed C(sp3)-H bond activation has been
reported: Chen, H.; Hartwig, J. F. Angew. Chem., Int. Ed. 1999, 38, 3391.
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