Rhodium/copper-catalyzed annulation of benzimides with internal alkynes: Indenone synthesis through sequential C-H and C-N cleavage

Article abstract of DOI:10.1002/anie.201200271

Doubled up: A rhodium(III)/copper(II) system co-catalyzes the annulation of benzimides with internal alkynes for the synthesis of indenones (see scheme; Cp=C₅Me₅). The reaction involves an uncommon nucleophilic addition of a transition-metal-carbon bond to an imide moiety. This novel reaction provides a facile route to synthesize indenones from readily available benzimides and internal alkynes. Copyright

Full text of DOI:10.1002/anie.201200271

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Angewandte
Communications
DOI: 10.1002/anie.201200271
À
C H Activation
Rhodium/Copper-Catalyzed Annulation of Benzimides with Internal
À
À
Alkynes: Indenone Synthesis through Sequential C H and C N
Cleavage**
Bi-Jie Li, Hao-Yuan Wang, Qi-Lei Zhu, and Zhang-Jie Shi*
Indenones are valuable synthetic intermediates for natural
products, ligand scaffolds, and material science.[1] Traditional
synthetic routes to indenones usually require multiple steps or
have limited scope.[2] Transition metal catalyzed approaches
have begun to address some of these issues by providing
various direct routes for their synthesis.[3,4] Among these
methods, several direct annulation reactions between alkyne
and ortho-functionalized aldehydes, esters, or nitriles have
developed as important synthetic entries,[4] and some of them
have been applied to organic synthesis.[1a,f] However, these
approaches require preactivation of the substrates, and this is
both time and cost consuming in a synthetic sequence.
À
À
Scheme 1. a) Addition of C M bond to ketone/imine through C H
Therefore, the development of a facile synthetic pathway to
indenone from easily available starting materials is highly
desirable.
À
À
À
cleavage. b) Addition of C M bond to imide through C H and C N
cleavage.
À
The underlying synthetic logic of C H functionalization
would meet these requirements.[5] In particular, catalytic
direct annulations of phenyl imine and phenone with alkynes
have been developed for the synthesis of indenyl amine and
indenol, respectively ( Scheme 1a).[6] Inspired by these
developments, we envisioned that by using a directing group
at the ester or amide oxidation level, annulation with an
alkyne would directly provide the indenone product (Sche-
me 1b). However, the choice of a proper directing group for
the designed annulation is challenging. First, the ester or
amide should have sufficient electron density to coordinate
with the metal center to facilitate ortho metallation and, at
the same time, exhibit substantial electrophilicity to react with
cleavage in amides or imides is particularly rare.[9] Finally, the
reaction of an organometallic reagent with ester or amide
generates a ketone product which is typically more reactive
than the starting material, thus further complicating such an
addition process. Nonetheless, if the desired annulation is
successfully implemented, this process would allow the
straightforward synthesis of diverse indenones from abundant
and essentially unfunctionalized benzoic acid derivatives.
À
Given our continuing interest in the addition of C H bonds to
polar functionalities,[10,11] we report herein a rhodium(III)-
catalyzed annulation of benzimides with alkynes for the
synthesis of indenones which involves an uncommon acyla-
tion of organorhodium(III) with an imide motif.
À
the C M bond obtained by alkyne insertion. Second,
although the addition of transition-metal organometallic
species to a carbonyl group is well known,[7] its reaction
with less reactive eletrophiles such as ester or amide groups is
At the outset of our study, we realized that the choice of
a proper directing group would be crucial for the reaction
development (Table 1). A cationic rhodium complex was
selected as the catalyst because of its established diverse
reactivity.[12–14] As expected, simple benzamides did not
undergo the desired annulation reaction since they are good
more difficult.[8] In fact, transition metal catalyzed C N bond
À
[*] B.-J. Li, H.-Y. Wang, Q.-L. Zhu, Prof. Dr. Z.-J. Shi
Beijing National Laboratory of Molecular Sciences (BNLMS) and
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education, College of Chemistry and Molecular
Engineering and Green Chemistry Center
À
À
substrates for either C H/N H annulation or hydroarylation
(entries 1 and 2).[12,13b] The use of N-acylbenzamides as the
substrates, however, provided some indenone product albeit
in low to moderate yields (entries 3 and 4). A higher yield was
obtained by using N-benzoyloxazolidinone as the substrate,
thus indicating that it has the appropriate balance between
directing ability and electrophilicity (entry 5). Interestingly,
Peking University, Beijing, 100871 (China)
E-mail: zshi@pku.edu.cn
Prof. Dr. Z.-J. Shi
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences, Shanghai 200032 (China)
N-acyloxazolidinone has rarely been used as a directing group
[5]
À
for C H functionalization. In addition, the oxazolidinone
[**] Support of this work by the “973” Project from the MOST of China
(2009CB825300) and NSFC (Nos. 20925207, and 21002001) is
gratefully acknowledged.
auxiliary is quite stable. It can be carried through a multistep
synthesis,[15] and generally requires the addition of organo-
À
À
lithium or Grignard reagents to cleave the C N bond for C C
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200271.
formation.[16] A catalytic amount of copper acetate as an
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Table 1: Reaction development.
Table 2: Substrate scope for benzimides.[a]
Entry
NRR’
Additive
Solvent
Yield [%][b]
1
2
3
NHMe
NMe2
NMe(Ac)
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
decalin
decalin
decalin
0
0
5
4
Cu(OAc)2
decalin
32
5
6
7
8
Cu(OAc)2
AgOAc
HOAc
decalin
decalin
decalin
decalin
toluene
PhCF3
80 (76)
20
10
<5
<5
74
–
9
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
10
11
12
dioxane
t-AmylOH
42
<5
[a] Reaction conditions: 0.2 mmol of 1, 0.3 mmol of 2a, 0.01 mmol of Rh
catalyst, 0.04 mmol of Cu(OAc)2, 2 mL of decalin. Yields are reported for
the isolated products.
[a] Reaction conditions: 0.2 mmol of 1, 0.3 mmol of 2a, 0.01 mmol Rh
catalyst, 0.04 mmol of additive, 2 mL of solvent, 12 h. [b] Yields
determined by GC analysis using n-dodecane as an internal standard.
Yield of isolated product given within parentheses. Decalin=deca-
hydronaphthalene, Cp*=C5Me5.
the yield (3q). In comparison, an improved yield was obtained
by using a less bulky methoxy group at the ortho position (3r).
Next we investigated the scope of the alkynes (Table 3).
Diarylalkynes having various electron-donating and electron-
withdrawing groups reacted without incident to give high
yields, except ortho-substituted diphenylacetylene, wherein
the steric hindrance largely inhibited the reaction (3s–ab).
Functional groups such as methoxy and halide groups were
additive is sufficient for the reaction, but its absence or
a change to other additives provided either no product or low
yields (entries 6–8). We have also tested several other Lewis
acids such as Zn(OTf)2, Cu(OTf)2, Sc(OTf)3, but none of
them gave the desired product, presumably because of the
À
lack of an OAc anion, which is essential for C H cleavage.
The reaction worked better in nonpolar solvents than in polar
ones (entries 9–12). No reaction occurred in the absence of
a rhodium catalyst.
Table 3: Substrate scope for alkynes.[a]
Having the optimized reaction conditions, we tested
various benzimides (Table 2). Electron-donating groups at
the para position of the aryl ring slightly decreased the yield,
presumably because of the decreased electrophilicity of the
À
imide which slows the addition of the C Rh intermediate to
the carbonyl group (3a–f). Electron-withdrawing groups and
various halides including fluoro, chloro, and bromo groups
were well tolerated (3g–j). Acetate and benzoic ester groups,
which are at the same oxidation level as imide, were also
accommodated (3k,l). The reaction of meta-methyl- or
bromo-substituted benzimide gave a single regioisomer
(3m,n). The annulation selectively occurred at the less
sterically hindered position. Multisubstituted indenones
were also easily accessible by the annulation process, regard-
less of the presence of an electron-donating or electron-
withdrawing groups (3o–p’). Interestingly, 3,5-difluorobenzi-
mide reacted faster than the nonsubstituted one, thus
suggesting a potential concerted metallation/deprotonation
[17]
À
mechanism for C H cleavage.
The synthesis of the
[a] Reaction conditions: 0.2 mmol of 1a, 0.3 mmol of 2, 0.01 mmol of Rh
catalyst, 0.04 mmol of Cu(OAc)2, 2 mL of decalin. Yields are reported for
the isolated products. Ratios of regioisomers are given within paren-
theses and were determined by NMR analysis. Major isomers are shown.
[b] 10 mol% catalyst.
functionalized indenone product 3p’ is of particular interest
because it closely resembles the synthetic intermediate for
pauciflorol F,[1a] thus illustrating the synthetic potential of the
reaction. An ortho-methyl substituent significantly lowered
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tolerated. An aryl alkyl alkyne, however, exhibited lower
reactivity (3ac). By using the more reactive 3,5-difluoroben-
zimide substrate, the reaction with 1-phenylpropyne and 1-
phenylpentyne proceeded with high yields and moderate
selectivity (3ad, ae). Presumably as a result of its increased
bulkiness, the reaction of 1-phenylcyclopropylacetylene gave
decreased yield but with exclusive regioselectivity (3af). A
conjugated enyne also participated in the annulation, thus
providing a 2:1 ratio of regioisomers (3ag). The ratio could be
attributed to the similarity in the electronic effects of phenyl
and vinyl groups. In the reactions of these unsymmetrical
alkynes, the phenyl group is selectively placed in proximity to
the carbonyl group, which is in agreement with previous
annulation reactions.[12] A dialkyl alkyne, which lacks a phenyl
substituent as the activating group through conjugation, failed
to undergo the desired reaction (3ah). Trimethylsilyl-substi-
tuted phenylacetylene did not react, either.
Scheme 2. Mechanistic proposal.
seven-membered intermediate 7, which undergoes an intra-
molecular insertion of the carbonyl group into the vinyl Rh
À
bond. Transmetallation between copper acetate and the
rhodium alkoxide 8 takes place to regenerate the catalyst
with formation of the copper alkoxide 9, which decomposes to
form the product and release the copper species. The
beneficial effect of copper salt may also originate from the
increased electrophilicity of imide moiety upon coordination
with copper,[18] thus facilitating the insertion of the carbonyl
Although benzoic ester was tolerated in the current
catalytic system (3l), a switch in the copper salt and solvent
also allows it to engage in the annulation reaction, albeit in
moderate yield [Eq. (1)]. This is the first example of direct
annulation of benzoic ester with an alkyne through directed
À
À
C H functionalization.
group into the C Rh bond.
In conclusion, we have developed a cationic rhodium-
catalyzed annulation between benzimides and alkynes for the
synthesis of indenones. The proper directing ability and
electrophilicity of the imide group provides a handle for the
À
À
annulation with concomitant C H and C N cleavage. Start-
ing from easily available benzimide derivatives, a range of
functionalized indenone products are obtained. Since the
reaction is redox neutral, only catalytic amounts of rhodium
and copper salts are required. The reaction involves a chal-
lenging nucleophilic addition of an organorhodium species to
an imide. We expect that more synthetically useful and
mechanistically novel transformations can be developed
through exploitation of the nucleophilicity of organometallic
To gain insight into the mechanism, we performed
deuterium-labeling experiments [Eqs. (2) and (3)]. The intra-
molecular and intermolecular kinetic isotopic effects were
determined to be 1.9 and 2.2, respectively, thus suggesting that
À
C H cleavage was involved in the rate-determining step.
À
species generated by C H activation.
Experimental Section
General procedures: N-benzoyloxazolidin-2-one 1a (0.20 mmol,
38.2 mg), diphenylacetylene 2a (0.30 mmol, 53.4 mg), [Cp*Rh-
(MeCN)3(SbF6)2] (5 mol%, 0.01 mmol, 8.3 mg), Cu(OAc)2
(20 mol%, 0.04 mmol, 7.3 mg) were weighed in the air and charged
into an oven-dried 25 mL Schlenk tube. The tube was evacuated and
refilled with N2, which was repeated for three times. After addition of
2 mL of decalin under a positive pressure of N2 via syringe, the sealed
tube was placed in a 1208C oil bath and stirred for 12 h. After
completion, the reaction mixture was cooled down to room temper-
ature and directly purified by flash column chromatography eluting
with CH2Cl2 and petroleum ether (1:2).
Received: January 11, 2012
Published online: March 6, 2012
À
Keywords: annulation · C H activation · heterocycles ·
rhodium · synthetic methods
.
Although the reaction mechanism remains to be eluci-
dated, we tentatively propose the following reaction pathway
(Scheme 2). Coordination of benzimide to the cationic
À
rhodium species 5 and subsequent C H cleavage forms the
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