Direct ionic liquid promoted organocatalyzed diazo-transfer reactions: diversity-oriented synthesis of diazo-compounds

Article abstract of DOI:10.1016/j.tetlet.2008.02.159

A practical and novel ionic liquid promoted organocatalytic selective diazo-transfer process for the synthesis of highly substituted diazo-compounds in high yields is reported. The ionic liquid can be reused without affecting the reaction rates or yields over five runs.

Full text of DOI:10.1016/j.tetlet.2008.02.159

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 2704–2709
Direct ionic liquid promoted organocatalyzed diazo-transfer
reactions: diversity-oriented synthesis of diazo-compounds
Dhevalapally B. Ramachary *, Vidadala V. Narayana, Kinthada Ramakumar
School of Chemistry, University of Hyderabad, Central University (PO), Hyderabad 500 046, India
Received 2 December 2007; revised 26 February 2008; accepted 29 February 2008
Available online 4 March 2008
Abstract
A practical and novel ionic liquid promoted organocatalytic selective diazo-transfer process for the synthesis of highly substituted
diazo-compounds in high yields is reported. The ionic liquid can be reused without affecting the reaction rates or yields over five runs.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Amines; Cascade reactions; a-Diazo-compounds; Ionic liquids; Organocatalysis
1. Introduction
Diazo-compounds are of considerable importance in a
variety of industries and research laboratories. As such,
the development of new and more general green methods
for their preparation is of significant interest.1 Recently,
ionic liquids have emerged as novel green reaction media
for many organic transformations.2 Surprisingly, to the
best of our knowledge; there is no report on diazo-transfer
reactions in ionic liquid. In this Letter, we present the syn-
Scheme 1. Direct ionic liquid promoted organocatalytic cascade diazo-
transfer reactions.
thesis of diazo-compounds in high yields via ionic liquid
promoted organo-catalyzed diazo-transfer reactions as
shown in Scheme 1.
Diazo-compounds 5 and 8 have been prepared by ionic
liquid promoted organocatalytic diazo-transfer (DT) reac-
tions using commercially available CH-acids 1, azides 2,
and ionic liquids 3 (Scheme 1). Direct combination of an
amine-catalyzed cascade amination/debenzoylation (A/
In continuation of our interest in the development of
DB) of highly substituted CH-acids 7 with azides 2 in ionic
liquid 3 has been developed for the generation of diazo-
compounds (a-diazo esters and a-diazo ketones) 8 as
of known ionic liquids 3 for the diazotization of dimedone
shown in Scheme 1. The diazo function is potentially a
more valuable synthetic tool than the presently recognized
because of its high reactivity, but its utility is limited by the
scarcity of good methods for its production.
green cascade reactions through organocatalysis in ionic
liquids,3 we initiated our studies by screening a number
1a with 1.0 equiv of p-toluenesulfonyl azide (2a) at room
temperature as shown in Table 1. Interestingly, the ionic
liquid [bmim]BF4 resulted in good conversions and moder-
ate yields of 5a as shown in Table 1, entries 1 and 2. The
*
Corresponding author. Tel.: +91 40 23134816; fax: +91 40 23012460.
E-mail addresses: buchiramachary@hotmail.com, ramsc@uohyd.
ionic liquid, [bmim]OH also promoted the formation of
ernet.in (D. B. Ramachary).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.02.159

D. B. Ramachary et al. / Tetrahedron Letters 49 (2008) 2704–2709
2705
Table 1
Preliminary studies on the diazo-transfer reaction of CH-acid 1a with 2a in ionic liquidsa
a
Reactions were carried out in ionic liquid with 0.5 mmol each of 1a and 2a.
b
Conversion is based on both TLC and 1H NMR analysis.
c
Yield refers to the column purified product.
Diazo-compound 5a was extracted from the ionic liquid with 30 mL of diethyl ether.
Diazo-compound 5a was extracted from the ionic liquid with 30 mL of ethyl acetate.
Diazo-compound 5a was extracted from the ionic liquid with 60 mL of ethyl acetate.
d
e
f
2-diazo-5,5-dimethyl-cyclohexane-1,3-dione 5a with good
conversions but poor to moderate yields as shown in Table
1, entries 3–5. Interestingly, [bmim]Br let to the formation
of 5a with excellent conversion and high yield (Table 1,
entry 6). Both [bmim]BF4 and [bmim]OH are good ionic
liquids for the diazotization of CH-acid 1a with 2a; how-
ever, the isolation of diazo product 5a from the ionic liq-
uids was tedious possibly due to the strong interactions
between 5a and the ionic liquids, especially [bmim]OH.
Interestingly, we did not encounter such isolation problems
from [bmim]Br. We also reproduced the same results
(Table 1, entry 6) with five different batches of [bmim]Br
prepared from N-methylimidazole and n-butylbromide.4
To increase the reaction rate and to investigate the effects
of the azides 2a–e and amines 4 on the diazotization reac-
tion, we used the ionic liquids [bmim]Br and [bmim]BF4
and screened a number of amines 4 as catalysts for the
diazotization of CH-acid 1a with 1.0 equiv of azides 2a–e
as shown in Table 2.
The diazo-transfer reaction of dimedone 1a with TsN3
2a in [bmim]BF4 using 1.0 equiv of K2CO3 as a catalyst
furnished the expected diazo product 5a in 72% yield after
3 h (Table 2, entry 1). Interestingly, addition of 5 mol % of
simple amines such as Et3N, DBU, DABCO, DMAP,
4-pyrrolidin-1-yl-pyridine (PP), pyrrolidine, pyridine and
N-methylimidazole (NMI) as catalyst in [bmim]BF4 fur-
nished the expected diazo product 5a in 75–95% yields after
1.5–18 h as shown in Table 2, entries 2–9. Amongst these
amines, DMAP and NMI showed promise as catalysts in
[bmim]BF4. To the best of our knowledge, there is no
report on the diazo-transfer reactions catalyzed by DMAP
and NMI. DMAP-catalyzed diazotization of 1a with
1.0 equiv of MsN3 2b in [bmim]BF4 furnished the expected
diazo product 5a in 99% yield after 1 h as shown in Table 2,
entry 11. The same reaction under DMAP-catalysis in
[bmim]Br as a solvent furnished the diazo product 5a in
99% yield after 0.5 h as shown in Table 2, entry 13. We also
investigated the DMAP-catalyzed reaction of 1a with the
azides 4-NO2C6H4SO2N3 (pNBSA) 2c, 2-NO2C6H4SO2N3
(oNBSA) 2d and N3CO2Et 2e, but the diazotization of
the CH-acid was inferior compared to TsN3 2a or MsN3
2b as shown in Table 2, entries 14–16.
We also tested the DMAP-catalyzed diazo-transfer reac-
tion of 1a with TsN3 2a in the conventional solvents
CH3CN and CH2Cl2; however, the diazotization of CH-
acid was slightly less efficient compared to the reaction in
[bmim]Br (Table 2, entries 17 and 18). The optimized con-
ditions involved the addition of 1.0 equiv of MsN3 2b to a
mixture of CH-acid 1a and 5 mol % of DMAP in 1.0 mL of
[bmim]Br at 25 °C to furnish dione 5a in 99% yield after
0.5 h (entry 13).
After successful demonstration of ionic liquid promoted
DMAP-catalyzed diazo-transfer reaction of CH-acid 1a
with MsN3 2b, we next focused on recycling the ionic
liquid, as shown in Table 3. The ionic liquids, [bmim]BF4
and [bmim]Br, could be recycled over five runs to produce
diazo-compound 5a without affecting significantly the reac-
tion rate and yield (Table 3).
With an efficient method for the diazotization in hand,
the scope of the self- and DMAP-catalyzed diazo-transfer
reactions was investigated with various CH-acids 1a–q.
CH-acids 1a–q were reacted with 1.0 equiv of MsN3 2b
catalyzed with or without 5 mol % of DMAP at 25 °C in
[bmim]Br (Table 4). The diazo-compounds 5a–q were
obtained as single products in excellent yields, being even
better than those from previous diazotization reactions
performed in organic solvents under base catalysis.5 Diazo
products 5o and 5p are used for the preparation of diazabi-
cyclo compounds6 for the treatment of ischemia and
hypoxia, emphasizing the value of this diazo-transfer
approach in ionic liquids. Mechanistically, the rates of
the diazotization reactions were accelerated by the ionic

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D. B. Ramachary et al. / Tetrahedron Letters 49 (2008) 2704–2709
Table 2
Effect of the amine and azide on the ionic liquid promoted diazo-transfer reaction of dimedone 1aa
a
Reactions were carried out in ionic liquid 3 (1.0 mL) with 1.0 equiv of 2 relative to 1a (0.5 mmol) in the presence of 5 mol % of amine 4.
Yield refers to the column purified product.
Diazo-compound 5a was extracted from the ionic liquid with 30 mL of diethyl ether.
Diazo-compound 5a was extracted from the ionic liquid with 20 mL of diethyl ether and 16 mL of ethyl acetate.
1 equiv of K2CO3 was used.
4-Pyrrolidin-1-yl-pyridine.
N-Methylimidazole.
b
c
d
e
f
g
Table 3
Recycling of ionic liquid solvent in the direct DMAP-catalyzed diazo-transfer reaction of CH-acid 1a with methanesulfonyl azide 2ba
Run
[bmim]BF4
[bmim]Br
Time (h)
Conversionb (%)
Yield 5ac,d (%)
Time (h)
Conversionb (%)
Yield 5ac,d (%)
1
2
3
4
5
a
1
2
3
3
3
>99
>99
>99
>99
>99
98
96
95
95
95
1
>99
>99
>99
>99
>99
99
97
97
96
95
1.5
1.5
1.5
1.5
Reactions were carried out in ionic liquid with 0.5 mmol each of 1a, 2b and 5 mol % of DMAP.
Conversion is based on both TLC and 1H NMR analysis.
Yield refers to the column purified product.
b
c
d
Diazo-compound 5a was extracted from the ionic liquid with 30 mL of diethyl ether.
liquid [bmim]Br, perhaps due to the basic nature of counter
anion BrÀ.
Next, we continued our investigation for the generation
of highly functionalized diazo-compounds 8 under DBU-
catalysis through the diazotization of CH-acids 7 via
debenzoylation7 (see Tables 5 and 6). Recently, Taber
et al. reported that the DBU-catalyzed diazo-transfer reac-
tion of highly functionalized CH-acids 7 with pNBSA 2c
proceeded with selective debenzoylation to provide the
desired unsymmetrical a-diazo ketones and a-diazo esters

D. B. Ramachary et al. / Tetrahedron Letters 49 (2008) 2704–2709
2707
Table 4
Synthesis of diazo-compounds via cascade ionic liquid promoted DMAP-catalyzed diazo-transfer reactions of CH-acids 1a–q with MsN3 2ba,b
a
Yield refers to the column purified product.
Yields represent both DMAP- and non-DMAP-catalyzed reactions.
b
Table 5
Direct ionic liquid promoted DBU-catalyzed diazo-transfer reaction of highly substituted CH-acid 7a with pNBSA 2c via debenzoylationa,b
Entry
pNBSA 2c (equiv)
DBU (equiv)
Yield 8ab (%)
1
2
3
a
1
1
2
1
2
2
50
75
80
See Section 2.
b
Yield refers to the column purified product.
8 in CH2Cl2 with moderate to good yields.7 Herein, we
at room temperature to furnish the unsymmetrical diazo-
demonstrated the same reaction in [bmim]Br as a solvent
compounds 8 in high yields. Optimization in [bmim]Br

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D. B. Ramachary et al. / Tetrahedron Letters 49 (2008) 2704–2709
Table 6
The synthesis of diazo-compounds through cascade ionic liquid promoted DBU-catalyzed diazo-transfer reactions of highly functionalized CH-acids 7
with pNBSA 2c via debenzoylationa,b
a
See Section 2.
Yield refers to the column purified product.
b
revealed that 2.0 equiv each of DBU and pNBSA were
required for the diazotization of CH-acid 7a to furnish
the unsymmetrical a-diazo ester 8a in 80% yield after 2 h
at 25 °C (Table 5, entry 3).
The results in Table 6 demonstrate the broad scope of
this novel methodology covering a structurally diverse
group of CH-acids 7a–e. Cascade diazotization reaction
of CH-acids 7a–e, pNBSA 2c and DBU furnished the a-
diazo esters 8a–d and a-diazo ketone 8e in 70–85% yields
(Table 6).
In conclusion, we have developed a metal-free, ionic
liquid promoted synthesis of highly substituted symmetri-
cal and unsymmetrical a-diazo ketones and a-diazo esters
5 and 8 from simple starting materials via diazotization
reactions under amine catalysis. The ionic liquid promoted
diazotization reaction proceeds in good yields with high
selectivity using DMAP or DBU as the catalyst. Further-
more, we have demonstrated the recycling of the ionic
liquids [bmim]BF4 and [bmim]Br in the DMAP-catalyzed
diazotization reactions. Further work is in progress to
utilize [bmim]Br promoted diazo-transfer reactions in syn-
thetic chemistry.
(15 mL). The combined organic layers were washed with
water, dried over anhydrous Na2SO4, filtered and concen-
trated under reduced pressure. Pure products 5 were
obtained by flash column chromatography (silica gel,
mixture of hexane/ethyl acetate).
2.2. DMAP-catalyzed ionic liquid promoted diazotization
reactions
Mesyl azide 2b (0.5 mmol) was added to a solution of
the corresponding CH-acid 1a–q (0.5 mmol) and DMAP
(5 mol %) in [bmim]Br (1.0 mL). The resulting reaction
mixture was stirred at room temperature for the time indi-
cated in Tables 2–4. The diazo-compounds 5a–q were
extracted from the ionic liquid with diethyl ether (25 mL)
and ethyl acetate (15 mL). The combined organic layers
were washed with water, dried over anhydrous Na2SO4,
filtered and concentrated under reduced pressure. Pure
products 5 were obtained by flash column chromatography
(silica gel, mixture of hexane/ethyl acetate).
2.3. DBU-catalyzed ionic liquid promoted cascade
diazotization reactions via debenzoylation
2. General experimental procedures for the ionic liquid
promoted diazotization reactions
To 0.5 mmol of CH-acids 7a–e and 1.0 mmol of pNBSA
2c in an ordinary glass vial equipped with a magnetic stir-
ring bar was added 1.0 mL of [bmim]Br, and then 1.0 mmol
of DBU was added dropwise over 0.25 h and the reaction
mixture was stirred at 25 °C for the time indicated in
Tables 5 and 6. The a-diazo-compound 8 was extracted
from ionic liquid with diethyl ether (25 mL) and ethyl ace-
tate (15 mL). The combined organic layers were washed
with water, dried over anhydrous Na2SO4, filtered, and
concentrated under reduced pressure. Pure products 8 were
obtained by flash column chromatography (silica gel, mix-
ture of hexane/ethyl acetate).
2.1. Self-catalyzed ionic liquid promoted diazotization
reactions
Mesyl azide 2b (0.5 mmol) was added to a solution of
the corresponding CH-acids 1a–q (0.5 mmol) in [bmim]Br
(1.0 mL). The resulting reaction mixture was stirred at
room temperature for the time indicated in Tables 1 and
4. The diazo-compounds 5a–q were extracted from the
ionic liquid with diethyl ether (25 mL) and ethyl acetate

D. B. Ramachary et al. / Tetrahedron Letters 49 (2008) 2704–2709
2709
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Acknowledgements
This work was made possible by a grant from The
Department of Science and Technology (DST), New Delhi.
V.V.N. and K.R. thank the Council of Scientific and
Industrial Research (CSIR), New Delhi for their research
fellowships.
Supplementary data
General experimental procedures, compound characteri-
1
zation and analytical data (IR, H NMR and 13C NMR)
for all new compounds. Copies of the IR and 13C NMR
spectra of all new compounds. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2008.02.159.
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[bmim]Br by column chromatography using silica gel with ethyl
acetate/hexanes and dichloromethane as a mobile solvent. The ionic
liquid promoted diazotization reaction of 1a and 2a with these five
different batches of ionic liquid [bmim]Br furnished the expected diazo
product 5a with very good yields within 1–2 h of reaction time.
5. See Supplementary data for information on diazo-compounds 5a–q
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