Promoting effect of imidazolium ionic liquid in the reaction of chromium carbene complexes with alkynes: A most simple and convenient way to get demetallated-product!

Article abstract of DOI:10.1002/adsc.200700268

Imidazolium ionic liquid serves as an interesting alternative solvent for performing the reaction of chromium carbene complexes with alkynes, with an advantage of enhancing the activity, selectivity and in situ generation of demetallated organic products.

Full text of DOI:10.1002/adsc.200700268

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DOI: 10.1002/adsc.200700268
Promoting Effect of Imidazolium Ionic Liquid in the Reaction of
Chromium Carbene Complexes with Alkynes: A Most Simple
and Convenient Way to Get Demetallated-Product!
Amarnath Chakrabortya and Tarun K. Maishala,*
a
Department of Organic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S C Mullick Road,
Jadavpur, Kolkata 700 032, India
Phone: (+91)-33-2473-4971; fax: (+91)-33-2473-2805; e-mail: maishal@lcoms.cpe.fr
Received: May 29, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Imidazolium ionic liquid serves as an in-
teresting alternative solvent for performing the re-
action of chromium carbene complexes with al-
kynes, with an advantage of enhancing the activity,
selectivity and in situ generation of demetallated or-
ganic products.
of these reactions has inspired us to study the reac-
tions of Fischer carbene complexes with acetylenes in
an ionic liquid.
Recently, ionic liquids have been reported as attrac-
tive alternative solvents to replace conventional or-
ganic solvents for organic transformations, particular-
ly transition metal-catalyzed transformations of or-
ganic and organometallic reactions.[3] Due to their low
melting range, non-volatility, non-flammability, good
solvating ability, tunable miscibility, good hydrolytic
and thermal stability, and good physicochemical prop-
erties, they are particularly promising as solvents for
catalysis also. They can be recovered by careful wash-
ing in ether or pentane (or products can easily be ob-
Keywords: alkynes; benzannulations; demetalla-
tion; Fischer carbene complexes; ionic liquids; or-
ganometallic reagents
Since 1964, Fischer carbene complexes have emerged tained by distillation) after the reaction is over. More-
as valuable building blocks in the synthesis of organic over, their utilities as reactants and solvents for cata-
molecules. A variety of useful reactions of Fischer car- lyst recycling are becoming more significant for or-
bene complexes and their analogues are known. Par- ganic reactions and catalysis. Their use can enhance
ticularly, reactions of Fischer carbenes with alkynes activity, selectivity, and stability of catalysts. Since
have been well established. Since its discovery in these liquids are salts, the cations and anions may in-
1975, the benzannulation reaction has been the sub- teract with solutes and therefore can have dramatic
ject of a great deal of methodological and mechanistic effects on numerous reactions.[4] Since the coordinat-
study. Notably, it has significant utility as a tool for ing abilities of ordinary ionic liquids are often very
the total synthesis of a variety of natural products.[1] poor, the selection of an ionic liquid that can serve
Mostly, these reactions are carried out at 40–758C in for both immobilization and as coordinating ligand
tetrahydrofuran, dioxane, ether, benzene or toluene for the purpose of our study, is also very important.
and afford a number of products including air-sensi-
In our initial work, we have studied the reactions of
tive metallated compounds within a long time range Fischer carbenes with alkynes in the most frequently
of 2–60 h and, therefore, lead to poor to moderate used imidazole-based ionic liquid, 1-butyl-3-methyl-
yield of the products. Moreover, in most cases, these imidazolium hexafluorophosphate 2, [bmim]
A
reactions are followed by oxidative cleavage using which is prepared from 1-butyl-3-methylimidazolium
iron(III), cerium(IV), etc. to furnish the demetallated bromide 1, [bmim][Br], and KPF6 by anion metathe-
E
product. Due to the mechanistic complexity of the re- sis.[5] The major reason behind selecting the imidazoli-
action, the product distribution of these reactions
varies on slightly modifying reaction conditions such
as the solvent, Fischer carbene, and alkyne concentra-
tions and the nature of the alkyne.[2] Particularly, the
efficiency of the reaction depends on the coordinating
ability and size of the solvent. Hence, the importance
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Amarnath Chakraborty and Tarun K. Maishal
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um ionic liquid 2 is that it may coordinate to the ported previously (Table 2). Although there are some
metal center and which may lead to a dramatic effect reports on the Dçtz benzannulation reaction of chro-
on the reaction.[3,6]
mium carbene, which show enhancement of yields
Initially, the reactions of carbenes with alkynes under microwave,[2b,7] sonochemical and dry-state ad-
were carried out in ionic liquid 2 at room tempera- sorption conditions[8] and high intensity photolytic ir-
ture. However, no reaction occurred after 24 h. After radiation,[9] to date it is very rare to find an example
an extensive screening of various reaction conditions, in which demetallation occurs spontaneously in such
particularly temperature, we found that the reactions reactions yielding the decomplexed product very se-
were complete within 0.5–1.5 h after heating at 50– lectively.[10] The reason for the demetallation in ionic
808C (Table 1). In fact, it is really very interesting to liquid is indeed very difficult to understand at this
notice that in situ formation of demetallated product moment, but it is believed that the coordinating abili-
was observed with high selectivity, which is very rare ty of the ionic liquid with the chromium center facili-
to the best of our knowledge. It was observed that the tates the demetallation step at the end. We envisage
reactions were remarkably faster, more selective, and that, for the same reason, demetallation occurs in the
the yields of isolated products were consistently good. case of chalcogen-tethered Fischer carbene complexes
Moreover, complete conversion of starting materials even when the reactions are done in the absence of
were observed under these conditions unlike those re- ionic liquid. In the later case, demetallation occurs
Table 1. Reactions of Fischer carbene complexes of chromium with alkynes in ionic liquid.
Entry Carbene complex[a]
Alkyne
Time [min] Temp. [8C] Product
Yield in IL [%][b]
1
R=Ph
40
30
55
55
98
2
3
R=H
95
98
R=Ph
60
75
4
5
R=Ph
R=H
80
80
50
50
85
87
6
R=Ph
R=H
90
40
80
70
80 (X=S); 85 (X=Se)
87 (X=S); 90 (X=Se)
7
[a]
All the reactions were performed in 0.2–0.5 mmol scale in ionic liquid (IL) under argon and 1 mL of IL was taken per
50 mg of the carbene complex.
Isolated yields after passage through silica gel.
[b]
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Adv. Synth. Catal. 2007, 349, 2435 – 2438

Promoting Effect of Imidazolium Ionic Liquid
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Table 2. Reactions of Fischer carbene complexes of chromium with alkynes in organic solvent as reported in the literature.
Entry
Temp. [8C]
Solvent
Time [h]
Reported Yield [%]
Ref.
1
2
3
4
5
6
7
45
45
70
40–60
40–60
80
n-Bu2O
THF; hexane
n-Bu2O
t-BuOMe
t-BuOMe
benzene
3
62[a]
ACHTREUNG
23; 60
1
2–4
2–4
2
67[a]; 57[a]
27[a]
[2a]
[13, 2a]
ACHTREUNG
90[a]
[14, 10, 1h]
[14, 10, 1h]
[10]
90[a]
61[b]
80
benzene
2
72[b]
[10]
[a]
Yields of the metallated products.
Yields of the demetallated products.
[b]
due to the coordination of the SPh/SePh group to the tives (6a and 6b) in ionic liquid compared to benzene
chromium metal.[10] (entries 6 and 7).[10] Here, in both the cases, demetal-
In our initial studies, we have mainly focused on lated product was obtained.
the reactions of Fischer carbene complexes of chromi-
After performing the reaction in ionic liquid, the
um with alkynes (Table 1). Here, we have observed product was extracted in diethyl ether. Then, the
that phenylethoxychromium carbene 3 and methylme- ether extract was concentrated under reduced pres-
thoxychromium carbene 4, take part in the reactions sure and the resulting crude product was purified by
with alkynes efficiently in the ionic liquid to afford column chromatography using silica gel to obtain the
the demetallated naphthylene derivatives (3a and 3b) pure product. After extraction with diethyl ether, the
and cyclobutenone derivative (4a), respectively. On resulting ionic liquid layer was washed with ethyl ace-
the other hand, a,b-unsaturated carbene 5 and chalco- tate. Removing the ethyl acetate and drying under
gen-tethered Fischer carbene 6 show similar enhanced vacuum at 608C for 8 h was sufficient to recover the
activity in the benzannulation reaction in ionic liquid ionic liquid in its original state. The recycled imidazo-
medium to afford the polyphenyl derivatives (5a, 5b, lium salt was then used for further reactions. There
6a and 6b). Products were characterized by IR and was no significant loss of efficiency of the ionic liquid
1H NMR.[11] All the spectral data clearly support the after 4 cycles. Finally, it was dissolved in dichlorome-
formation of demetallated products.
thane after 4 cycles, and filtered through a pad of
For the purpose of comparing the reactions per- celite to remove insoluble materials. Then it was used
formed in ionic liquid and in conventional organic sol- for several times again. Similar high conversions were
vent, we have introduced Table 2 below. The reaction obtained which indicates that the imidazolium salt re-
of arylalkoxycarbene of chromium 3 with diphenyl- mains active after several cycles.
acetylene and phenylacetylene (entries 1 and 2) in
In summary, this work has demonstrated that the
ionic liquid afford the demetallated naphthaol deriv- imidazolium ionic liquid can serve as an interesting
atives (3a and 3b) in 95–98% yield at 558C in 30– alternative solvent for performing reactions of Fischer
40 min, whereas, the same reactions performed in carbene complexes with alkynes with an advantage of
heptane at 808C for 30 min followed by the CAN oxi- enhancing the activity, selectivity and yield. More-
dation afforded four different products (the different over, to the best of our knowledge it is the simplest
reaction conditions have been mentioned in and most convenient method to obtain the demetal-
Table 2).[2a] The reaction of alkylalkoxycarbene of lated product directly. More general and detailed
chromium 4 with diphenylacetylene in ionic liquid af- studies are underway.
fords cyclobutenone derivative 4a in 98% yield,
whereas the same reaction performed in ether afford-
ed the cyclobutenone chromium tricarbonyl complex
in 27% yield under very similar reaction conditions
Experimental Section
(entry 3).
Typical Procedure for the Reaction of Chromium
Carbene with Alkyne in Imidazolium Ionic Liquid
The reaction of alkenyl carbene 5 with alkynes in
ionic liquid yielded demetallated polyphenyl phenolic
products (5a and 5b) in 85–87% yield which is very
similar to the result reported when the reaction was
performed in ether (entry 4 and 5).[14,10,1h] On the
other hand, chalcogen-tethered Fischer carbene com-
plexes 6 showed enhancement of yield from 60% to
To a round-bottomed flask equipped with a magnetic stirrer,
ionic liquid 2 (1 mL) and carbene complex 4 (50 mg,
0.2 mmol) were added. To this solution, 1.2 equivalents of
diphenylacetylene (43 mg, 0.24 mmol) were added and stir-
ring was continued at room temperature under argon until
the solution became homogeneous. It was then heated at
80–90% of demetallated polyphenyl phenolic deriva- 758C until a colour change occurred (solution became
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Amarnath Chakraborty and Tarun K. Maishal
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almost colourless). After completion of the reaction as indi-
cated by the colour change and TLC, the reaction mixture
was triturated with diethyl ether (3”5 mL), followed by de-
canting off the ether layer. The combined ether extracts
were concentrated under reduced pressure and purified by
column chromatography using silica gel to obtain the pure
colourless semi-solid product 4a; yield: 52 mg (98%). All
other reactions were carried out in the similar way.
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Acknowledgements
Financial support from Syngene Int. Pvt. Ltd. and grant of a
research fellowship (AC) from CSIR, India, are gratefully ac-
knowledged. The authors thank Prof. Amitabha Sarkar for
his valuable suggestions.
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