An efficient tertiary azidation of 1,3-dicarbonyl compounds in water catalyzed by tetrabutylammonium iodide

Article abstract of DOI:10.1002/ejoc.201501374

An efficient azidation of 1,3-dicarbonyl compounds led to tertiary azides in the presence of tetrabutylammonium iodide (TBAI). TBAI is used as a pre-catalyst along with aq. tert-butyl hydroperoxide (TBHP) as an oxidant in aqueous medium. This operationally simple, practical, mild and green method provides an opportunity to synthesize a variety of azidated β-keto esters, amides, and ketones in good yields. 1,3-Dicarbonyl compounds undergo a facile azidation to furnish the corresponding tertiary azides. This method employs tetrabutylammonium iodide (TBAI) as a catalyst, aq. tert-butyl hydroperoxide (TBHP) as an oxidant and TMSN₃ as an azide source. This strategy is general and works well for β-keto esters, β-diketones, and β-keto amides and furnish their corresponding tertiary azides in good yields.

Full text of DOI:10.1002/ejoc.201501374

DOI: 10.1002/ejoc.201501374
Communication
Tertiary Azidation
An Efficient Tertiary Azidation of 1,3-Dicarbonyl Compounds in
Water Catalyzed by Tetrabutylammonium Iodide
[a]
[a]
Jayaraman Dhineshkumar and Kandikere Ramaiah Prabhu*
Abstract: An efficient azidation of 1,3-dicarbonyl compounds This operationally simple, practical, mild and green method pro-
led to tertiary azides in the presence of tetrabutylammonium vides an opportunity to synthesize a variety of azidated β-keto
iodide (TBAI). TBAI is used as a pre-catalyst along with aq. tert- esters, amides, and ketones in good yields.
butyl hydroperoxide (TBHP) as an oxidant in aqueous medium.
Introduction
ment cannot be employed for synthesizing secondary and terti-
[
3]
Azidation has emerged as one of the methods for incorporating ary azides. Azidation of 1,3-dicarbonyl compounds is an im-
nitrogen in to organic molecules, which also provides an op- portant reaction as the azides formed in this reaction can be
portunity for late-stage modification of complex molecules like easily transformed into their α-amino derivatives. But, the azid-
biomolecules, drug candidates, polymers etc. The wide appli- ation of 1,3-dicarbonyl compounds is challenging, as both az-
cability of click reactions and the stability of azides in water ides and 1,3-dicarbonyl compounds are nucleophilic in na-
[
1]
[4]
[
1]
[
5]
have provided additional impetus for exploring novel methods ture. To address this issue, electrophilic azide sources such as
of synthesizing a variety of azides and their reactions.
[
1,2]
[6]
The sulfonyl azides have been developed, which can be used for
traditional methods of synthesizing alkyl azides by S 2 displace- the azidation of strong nucleophiles. However, these azidation
N
Scheme 1. Development of the reaction.
reactions with 1,3-dicarbonyl compounds led to the competing
diazo transfer reactions and deacylative rearrangements. To
[
a] Department of Organic Chemistry, Indian Institute of Science,
Bangalore 560 012, Karnataka, India
[6]
circumvent this problem, Moriarty and others employed in situ
generated electrophilic 1,3-dicarbonyl compounds in the pres-
E-mail: prabhu@orgchem.iisc.ernet.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201501374.
[
7]
ence of iodosobenzene and a Lewis acid. Later, Kirsch's group
Eur. J. Org. Chem. 2016, 447–452
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Communication
reported azidation of 1,3-dicarbonyl compounds by using a
stoichiometric amount of molecular iodine or 2-iodoxybenzoic
Table 1. Screening studies.[a]
[
8]
acid (IBX), NaI, and NaN3 (Scheme 1). Gade and Waser re-
ported azidation of β-keto esters and silyl enol ethers with a
[
9]
benziodoxole reagent and a Lewis acid (Scheme 1). Ibrahim
et al. reported the α-azidation of 1,3-dicarbonyl compounds by
using a stoichiometric amount of diacetoxy iodobenzene and
Entry Halogen source
(10 mol-%)
Oxidant
(2 equiv.)
Solvent
(2 mL)
Yield
[%][
b]
[
10]
tetrabutylammonium azide. The hypervalent iodine reagents
are environmentally benign, and hence the utility of these rea-
gents is very attractive. However, most of these methods use
1
2
3
4
5
6
7
8
9
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
NIS
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
DCE
EtOAc
toluene
1,4-dioxane
51
98
94
90
93
84
87
62
[
11]
large amounts of hypervalent iodine reagents
and a few
methods also require metal catalysts, such as Zn(OTf) , Fe-com-
2
CH
3
CN
plexes etc., for activating the 1,3-dicarbonyls. Therefore, the de-
velopment of a mild reaction that is more atom-efficient and
simple with low catalyst loading and that produces less waste is
desirable. To address these issues, in his seminal work, Ishihara
MeOH
DMSO
DMF
water
water
water
water
water
water
water
water
water
water
water
water
water
water
water
water
water
98
traces
10
1
0
developed a tetraalkylammonium iodide (catalytic) with H O
2
2
11
12
I2
KI
or TBHP system for an in situ generation of hypoiodite/iodate
species that mimics the hypervalent iodine mediated reactions
to carry out enantioselective oxidative cycloetherification of
22
1
1
1
1
17
18
3
4
5
6
TBAB
TBAC
NCS
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
none
TBAI
ND
ND
traces
62
ND
21
ND
ND
92
90
88
ND
ND
[
7,12]
ketophenols.
In pursuit of our efforts directed toward devel-
aq. H
DTBP
BPO
2 2
O (30 %)
[
13]
oping iodine-based catalytic systems,
herein we report our
recent findings on TBAI/TBHP-catalyzed azidation of 1,3-di-
carbonyl compounds to obtain tertiary azidated 1,3-dicarbonyl
compounds in aqueous medium.
19
20
21
22
23
24
25
O
2
[
[
[
[
c]
d]
e]
f]
TBHP
TBHP
TBHP
TBHP
TBHP
none
Results and Discussion
We started the optimization study with acyclic β-keto ester,
methyl 2-methyl-3-oxo-3-phenylpropanoate (1a), TMSN as az-
[a] Reaction conditions: 1a (0.26 mmol), TMSN3 (0.52 mmol), aq. TBHP
(2 equiv., 70 % in water) at 80 °C. [b] Yield determined by H NMR spectro-
3
1
ide source, TBAI as a catalyst (10 mol-%) and TBHP as an oxidant
in 1,2-dichloroethane (DCE). This reaction led to the azidated
product 2a in 51 % yield (determined by NMR spectroscopy,
Table 1, entry 1). The solvent screening studies revealed that
EtOAc is the most suitable solvent (entry 2) and most of the
other solvents are useful (entries 3–8). Further, using water as a
solvent, we were pleased to note that the reaction was facile
scopy (by using terephthalaldehyde as an internal standard). [c] NaN
3
(
3
2 equiv.) was used instead of TMSN . [d] TBAI (5 mol-%) was used.
[
e] 1.2 equiv. of TMSN was used. [f] 1 equiv. of aq. TBHP was used. ND =
3
not determined, DTBP = di-tert-butylperoxide, BPO = benzoyl peroxide.
Finally, the conditions using 10 mol-% of TBAI, 2 equiv. of
and furnished the product 2a in a quantitative yield (entry 9). TBHP and 2 equiv. of TMSN
As water is a desirable solvent for chemical reactions for reasons as the optimal conditions. Next, the substrate scope and gener-
in water (entry 9) were established
3
[
2a]
of safety, cost, and environmental concerns, we have carried ality of the reaction was explored under the optimized condi-
out all further studies in water. Other iodine sources, such as N- tions. Employing methyl and tert-butyl β-keto esters under the
iodosuccinimide (NIS), molecular iodine, and KI, were not found optimal reaction conditions afforded the azidated products 2a
to be suitable (entries 10–12). While other halogen sources, and 2b in excellent to good yields (98 and 60 %, respectively,
such as tetrabutylammonium bromide (TBAB), tetrabutylammo- Scheme 2). Substrates with electron-releasing and electron-
nium chloride (TBAC), and N-chlorosuccinimide (NCS) as cata- withdrawing groups on the phenyl ring of β-keto esters gave
lysts did not furnish the corresponding product (entries 13–15). the azidated products 2c, 2d, 2e, 2f, and 2g in moderate to
Among the oxidants, aq. H O gave the product 2a in 62 % excellent yields (98, 69, 83, 86, and 98 %, respectively,
2
2
(
Table 1, entry 16), whereas other oxidants produced either low Scheme 2).
yield or no product (entries 17–19). The use of sodium azide An excellent chemoselective reaction was observed in the
was not successful under the given reaction conditions (en- presence of oxidizable functional groups on the phenyl ring of
try 20). Lowering the amounts of TBAI and TMSN to 5 mol-% β-keto esters. Thus, an O-benzyl group in the phenyl ring sur-
3
and 1.2 equiv., respectively, resulted in the formation of the vived the reaction conditions to afford the azidated product 2h
product 2a in slightly lower yields (92 and 90 %, respectively, in 79 % yield. The easily oxidizable sulfenyl group was intact
entries 21 and 22). Lowering the amount of TBHP to 1 equiv. and afforded azidated product 2i in 73 % yield. Similarly, a
decreased the yield (88 %, entry 23). In the absence of either benzhydryl group was found to be moderately tolerated in the
TBAI or TBHP, the reaction did not proceed (entries 24 and 25) reaction conditions and the corresponding azidated product 2j
indicating that both TBAI and TBHP are essential for the reac- was obtained in moderate yield (41 %). 1-Tetralone and 1-indan-
tion.
one derived cyclic β-keto esters, are more reactive than acyclic
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Scheme 2. Substrate scope for β-keto esters. Reaction conditions: 1 (0.26 mmol), TMSN
0 °C, 3–12 h. Yields given are of the isolated products. [a] Reaction in EtOAc (2 mL).
3 2
(0.52 mmol), aq. TBHP (2 equiv.), TBAI (10 mol-%), H O (2 mL), at
8
esters[9a] and were found to give the corresponding products ides (Scheme 3). Thus, β-keto amides of 1-tetralone and 1-in-
in excellent yields. Notably, the similar reaction of 1-tetralone danone underwent a smooth reaction to furnish the corre-
[
9a]
with hypervalent iodine reagent such as azidobenziodoxole
sponding azidated products 2t and 2u in 97 and 72 % yields,
led to complete aromatization, whereas under the present reac- respectively. The azidation of acyclic amide in EtOAc (as a sol-
tion conditions, we obtained the corresponding tertiary azides vent) afforded the azidated product 2v in 51 % yield. The azid-
(
2k and 2l) in excellent yields.
Thus, methyl, ethyl, and tert-butyl ester derivatives under- esters and amides, was not successful under optimal reaction
went a smooth reaction to give the azidated products 2k, 2l, conditions. Nevertheless, the reaction in ethyl acetate as solvent
m, and 2n in good to excellent yields (98, 86, 98, and 65 %, instead of water proceeded well to yield the azidated products
ation of 1,3-diketones, which are highly reactive compared to
2
respectively, Scheme 2). The azidation of heterocyclic com- 2w, 2x, and 2y in moderate to good yields (67, 82, and 82 %,
pounds such as, indole, thiophene, and furan derived β-keto respectively).
esters was facile and afforded the azidated products 2o, 2p, and
q in moderate to excellent yields (65, 98, and 57 %, Scheme 2). experiments were performed on a larger scale as presented in
Substitutions such as an O-allyl group on the phenyl ring Scheme 4. Employing 5.2 mmol (1 g) of methyl 6-methoxy-1-
To demonstrate the utility of the azidation reaction, a few
2
and aliphatic β-keto esters failed to provide the azidated pro- oxo-1,2,3,4-tetrahydronaphthalene-2-carboxylate under the
ducts under the optimal reaction conditions. However, by standard reaction conditions the azidated product 2k was ob-
changing the solvent from water to EtOAc, azidated product 2r tained in almost quantitative yield (98 %). Similarly, the azid-
is obtained in 72 % yield. Similarly, a β-keto ester with an allyl ation reaction of β-ketoamides and 1,3-diketones on a 500 mg
group at the α-position furnished the expected product 2s by scale furnished the products 2t and 2x in good yields (60 and
[
14]
performing the reaction in EtOAc in moderate yield (47 %).
After successful azidation of β-keto esters, it was found that
the same catalytic system is useful for azidation of β-keto am- sized by subjecting 2a to “click” chemistry conditions to obtain
62 %).
The utility of tertiary azidated products was further empha-
Eur. J. Org. Chem. 2016, 447–452
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Communication
Scheme 3. Substrate scope for β-keto amides and 1,3-diketones. Reaction conditions: 1 (0.26 mmol), 2, TMSN
3
(0.52 mmol), aq. TBHP (2 equiv.), TBAI (10 mol-
%
), H O (2 mL), at 80 °C, 3–12 h. Yields given are of the isolated products. [a] Reaction in EtOAc (2 mL). [b] Reaction at room temp.
2
Scheme 4. Larger-scale reactions.
Scheme 5. Synthetic transformations.
the triazoles 3 and 4 in good yields (82 and 60 %, Scheme 5). is analogous to drug candidates such as ampyrone and metami-
The azido-β-keto ester 2a on further reaction with phenyl hy- zole. These compounds are otherwise difficult to access and are
[
15]
drazine furnished pyrazolone derivative 5 in 73 % yield, which synthesized using long synthetic sequences.
Scheme 6. Control experiments.
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Communication
For probing the reaction mechanism, a few control experi- acetate or water (2 mL) under open air. The reaction vessel was
capped and the mixture was allowed to stir at 80 °C (for 1,3-di-
ketones at room temperature) for 3–12 h. After the completion of
the reaction (monitored by TLC), the reaction mixture was
ments were performed (Scheme 6). Using a stoichiometric
amount of TBAI in the absence of TMSN , which furnished α-
3
iodo-β-keto ester 6 in 32 % yield, suggested an iodo derivative
as a possible intermediate. However, employing pre-made α-
quenched using saturated solution of Na S O and extracted with
2
2 3
ethyl acetate (3 × 15 mL). The combined organic layer was dried
with Na SO and concentrated under reduced pressure. The crude
iodo-β-keto ester 6, with TMSN failed to furnish the expected
3
2
4
product in either the presence or the absence of aq. TBHP. Reac-
tion in the presence of a radical inhibitor such as butylated
hydroxytoluene (BHT) or a radical quencher such as TEMPO un-
der the standard reaction conditions gave the azidated product
product was purified by silica gel flash column chromatography
using hexane/EtOAc to get the pure product.
Acknowledgments
2
a in lower yields of 33 and 38 %, respectively, suggesting a
radical pathway.
This work was supported by SERB (No.SB/S1/OC-56/2013), New
Based on these control experiments and the literature prece- Delhi, the Indian Institute of Science, and RL Fine Chem. We
[
12a,12b]
dence,
a plausible mechanism is proposed in Scheme 7. thank Dr. A. R. Ramesha (RL Fine Chem) for useful discussions.
An in situ reaction of TBAI and TBHP generates hypoiodite (III), J. D. thanks Council of Scientific and Industrial Research (CSIR),
which upon reaction with active methylene compounds forms New Delhi, for a fellowship.
the intermediate (IV). Further, the intermediate (IV) reacts with
TMSN to furnish the azidated product (2a). We believe that,
the hypernucleofugal ability of the intermediate (IV) formed
3
Keywords: Azides · Iodine · Keto esters · 1,3-Dicarbonyl
compounds · Synthetic methods
during the reaction drives the reaction for displacement by
[
16,17]
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2
1
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2
Received: October 29, 2015
Published Online: December 15, 2015
Eur. J. Org. Chem. 2016, 447–452
www.eurjoc.org
452
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim