Synthesis of α-diazo carbonyl compounds with the shelf-stable diazo transfer reagent nonafluorobutanesulfonyl azide

Article abstract of DOI:10.1002/adsc.201000846

Nonafluorobutanesulfonyl azide is a shelf-stable, cost-effective and general diazo transfer reagent for the efficient synthesis of a-diazo carbonyl compounds in excellent yields and in very short reaction times, under mild conditions. The diazo products can be readily isolated in pure form after a simple aqueous extractive work-up that avoids chromatographic purification in most cases. Because of its high efficiency and wide substrate scope, shelf-stability, relatively low cost, and ease of product purification, nonafluorobutanesulfonyl azide offers an advantageous alternative to other commonly used diazo transfer reagents.

Full text of DOI:10.1002/adsc.201000846

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DOI: 10.1002/adsc.201000846
Synthesis of a-Diazo Carbonyl Compounds with the Shelf-Stable
Diazo Transfer Reagent Nonafluorobutanesulfonyl Azide
Jose Luis Chiaraa,* and Josꢀ Ramꢁn Suꢂreza,*
a
Instituto de Quꢃmica Orgꢂnica General, C.S.I.C., Juan de la Cierva 3, E-28006 Madrid, Spain
Fax : (+34)-915644853; e-mail: jl.chiara@iqog.csic.esjota@iqog.csic.es
Received: November 11, 2010; Published online: March 2, 2011
Dedicated to the memory of Prof. Josꢀ Manuel Concellꢁn.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000846.
Abstract: Nonafluorobutanesulfonyl azide is
a
years.[8–23] The most recent additions to this arsenal in-
clude trifluoromethanesulfonyl azide (triflyl azide:
TfN3),[20] imidazole-1-sulfonyl azide,[21] 2-azido-1,3-di-
methylimidazolium chloride,[22] and benzotriazole-1-
yl-sulfonyl azide.[23] The beneficial use of ionic liquids
in diazo transfer reactions has also been described re-
cently.[24] Nevertheless, there is still a need for alterna-
tive diazo transfer reagents with high shelf-stability
and wide substrate applicability that also facilitate
product isolation.
We have recently shown that nonafluorobutanesul-
fonyl azide (nonaflyl azide: NfN3) is a shelf-stable and
cost-effective diazo transfer reagent for the efficient
synthesis of azides from primary amines.[25] An addi-
tional advantage of this reagent is that the nonafluor-
obutanesulfonamide formed as by-product in the
diazo transfer reaction can be easily removed from
the crude reaction mixture by a simple aqueous ex-
tractive work-up under mildly basic conditions, facili-
tating the purification of the azide product that often
shelf-stable, cost-effective and general diazo trans-
fer reagent for the efficient synthesis of a-diazo car-
bonyl compounds in excellent yields and in very
short reaction times, under mild conditions. The
diazo products can be readily isolated in pure form
after a simple aqueous extractive work-up that
avoids chromatographic purification in most cases.
Because of its high efficiency and wide substrate
scope, shelf-stability, relatively low cost, and ease of
product purification, nonafluorobutanesulfonyl
azide offers an advantageous alternative to other
commonly used diazo transfer reagents.
Keywords: azides; diazo compounds; diazo transfer;
ketones; sulfonyl azides
Diazo compounds are important substrates for the does not require chromatography. Moreover, NfN3
generation of carbenes, metal-complexed carbenoids showed a better reactivity profile in these diazo trans-
and diazonium cations, which participate in useful fer processes than its lower molecular weight ana-
chemical transformations including 1,3-dipolar cyclo- logue TfN3, while not presenting the serious draw-
additions,[1] cyclopropanations,[1b,c,2] X H insertion re- backs of the latter.[25] Thus, TfN3 is an unstable and
À
actions (X=C, O, N, P, etc.),[1b,c,3] phosphorus or explosive compound that has to be prepared prior to
sulfur ylide formation,[4] and Pd-catalyzed cross-cou- use and should be handled always in solution.[26] In
pling reactions.[5] One of the major routes for the syn- addition, the reagent is obtained from the rather
thesis of diazo compounds involves the base-assisted costly trifluoromethanesulfonic anhydride in variable
reaction of an active methylene group with a diazo yields and thus, in the absence of a cumbersome
transfer reagent.[6] Many electron-deficient organo standardization of the crude reagent solution, it re-
azide derivatives have been developed as diazo trans- quires to be used in a large excess to guarantee a
fer reagents, most of them sulfonyl azides. Readily ac- complete transformation of the substrate. By contrast,
cessible p-toluenesulfonyl azide has been the reagent NfN3 is readily prepared from the much cheaper and
employed most frequently.[7] With the objective to im- bulk reagent nonafluorobutanesulfonyl fluoride[27] by
prove efficiency and safety and to ease product purifi- reaction with NaN3 in MeOH and is stable at room
cation from the sulfonamide by-product, several alter- temperature as a neat compound, with a reported de-
native reagents have been investigated over the composition temperature of around 1208C.[27b] Zhu
Adv. Synth. Catal. 2011, 353, 575 – 579
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575

Jose Luis Chiara and Josꢀ Ramꢁn Suꢂrez
COMMUNICATIONS
Table 2. Synthesis of a-diazocarbonyl compounds using non-
aflyl azide.
et al. have previously studied the reactions of NfN3
with carbocyclic b-keto esters to give ring-opened
diazo amido esters or ring-contracted amido esters,
depending on the structure of the starting keto
ester.[28] More recently, while studying the reactivity
of NfN3 with enamines, the same group has found
that ethyl acetoacetate gives ethyl diazoacetoacetate
as the major product when treated with NfN3 in the
presence of morpholine.[29] As a continuation of our
previous work on the use of NfN3 as a safe and effi-
cient diazo transfer reagent,[25a,b] we now describe its
application to the synthesis of a-diazo carbonyl com-
pounds under mild reaction conditions.
We first studied the effect of the base in the diazo
transfer reaction using 1,3-diphenyl-1,3-propanedione
(1a) as a model substrate. Thus, 1a was reacted with a
small excess of NfN3 (1.2 equiv.) and base (1.1 equiv.)
in CH2Cl2 at room temperature (Table 1) to afford
the expected diazo product 2a. As previously ob-
served by Charette et al. for the case of TfN3,[20e]
yields and reaction rates roughly correlate with base
strength (Table 1). Gratifyingly, complete conversion
of 1a (TLC analysis) was achieved in just 15 min with
the stronger bases (Table 1, entries 3–5). However,
longer reaction times and lower yields were obtained
when we used weaker bases such pyridine or 2,6-luti-
dine (Table 1, entries 1 and 2).
Entry
1
Substrate
Product
Yield [%][a]
94
2
98
3
89
4
90
5
88 (90)[b]
6
91
95
93
92
81
86
We then investigated the scope of the diazo transfer
reaction using a diverse set of a-substituted carbonyl
compounds (Table 2), selecting Et3N as the optimum
base for its observed efficiency, lower cost, and easy
removal by simple evaporation. 1,3-Diketones, 3-keto
esters and malonates (entries 1–9) reacted smoothly
within 15 min at room temperature to give the corre-
sponding diazo compounds in very high yields in all
cases examined. Diazo compounds 2 were isolated
after repeated liquid-liquid extraction of the crude re-
7
8
9
Table 1. Effect of base on the diazo group transfer reaction
of 1a with NfN3.
10
11
Entry Base
pKa (MeCN)[a] Time
Yield [%][b]
12
13
87
89
1
2
3
4
5
pyridine
2,6-lutidine 14.13
12.53
6 h
2 h
58
71
Et3N
(i-Pr)2NEt
DBU
18.82
18.1[c]
24.34
15 min 94
15 min 93
15 min 91
ACHTUNGTRENNUNG
[a]
Refers to the pKa of the conjugated acid of the base
measured in MeCN (ref.[30]).
Yield of isolated product.
[b]
[c]
Ref.[31]
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Synthesis of a-Diazo Carbonyl Compounds with the Shelf-Stable Diazo Transfer Reagent
Table 2. (Continued)
Entry
Substrate
Product
Yield [%][a]
14
32
Scheme 1. Synthesis of a-nitro-a-diazocarbonyl compounds.
15
44
nitro-a-diazo carbonyl derivatives were obtained
(Table 2, entries 14 and 15). Since complete transfor-
mation of the starting nitro compound was clearly ap-
parent by TLC analysis of the crude reaction mixture,
we ascribed these low yields to a probable degrada-
[a]
Yield of isolated product.
[b]
Yield reported using morpholine instead of Et3N (ref.[29]).
action mixture in CH2Cl2 with a saturated aqueous so- tion of the highly electrophilic diazo product in the
lution of NaHCO3 to remove the nonafluorobutane- presence of the base. In line with our hypothesis,
sulfonamide by-product. Residual nonafluorobutane- better results were obtained when we employed a
sulfonamide was completely eliminated by sublima- weaker and more hindered base such 2,6-lutidine
tion during the concentration of the crude material at (Scheme 1). A similar negative effect of the base was
reduced pressure. This simple work-up afforded in previously observed by Charette et al.[20e] in the syn-
most cases analytically pure diazo products without thesis of a-cyano-a-diazo carbonyl compounds with
chromatography.
TfN3 when using Et3N or Hꢅnigꢆs base [(i-Pr)2NEt].
To further examine the scope and limitations of this These authors have also shown that the yield of a-
method, we next investigated the diazotation of other nitro-a-diazo carbonyl products obtained with TfN3
a-substituted carbonyl compounds. a-Cyano-a-diazo could be further improved by adding the base at 08C
carbonyl compounds are very versatile substrates that instead of room temperature[20c] and the same would
have been used for the synthesis of bisACTHNUTRGNEUNG
(oxazoles),[32] be expected to occur with NfN3, although we have
5-halo-,[33] 5-hydroxy-[34] and 5-amino-1,2,3-triazoles,[35] not tested these conditions with our substrates.
[36]
À
in O H insertion reactions
and in cyclopropana-
In conclusion, we have shown that nonafluorobuta-
tions.[20f,37] Under the standard reaction conditions, nesulfonyl azide is a shelf-stable, cost-effective and
ethyl cyanoacetate gave the corresponding a-cyano-a- general diazo transfer reagent for the efficient synthe-
diazo product 2j in 81% yield (Table 2, entry 10). sis of a-diazo carbonyl compounds in excellent yields
Nonaflyl azide performed much better in this trans- and in very short reaction times. The reaction takes
formation than other recently described diazo transfer place under mild conditions and the desired diazo
reagents (65% in 12 h with benzotriazole-1-yl-sulfonyl products can be readily isolated in pure form and in
azide;[23] 61% in 9 h with imidazole-1-sulfonyl high yield after a simple extractive work-up that
azide;[21] 79% in 14 h with TfN3[20e]). a-(Phenylsul- avoids chromatographic purification in most cases.
fonyl) carbonyl compounds 1k and 1l (Table 2, en- For its high efficiency, wide substrate scope, shelf-sta-
tries 11 and 12) gave also the corresponding diazo bility, relatively low cost, and ease of product purifica-
products 2k and 2l, respectively, under the same con- tion, nonaflyl azide is an advantageous alternative to
ditions in good yield and, again, in much shorter reac- other commonly used diazo transfer reagents, includ-
tion times than with other diazo transfer reagents re- ing the most recently described.[20–23]
cently developed. Thus, the synthesis of 2k using
[20e]
TfN3
and of 2l using benzotriazol-1-ylsulfonyl
azide[23] required in both cases a reaction time of 14 h.
Imidazole-1-sulfonyl azide, which is considered as a
stable and inexpensive alternative to TfN3, failed to
give any diazo product with (phenylsulfonyl)ace-
tates.[21] The method is also applicable to the prepara-
tion of the Ohira–Bestmann reagent[38] 2m in very
high yield (Table 2, entry 13) and again in shorter re-
action times than with other diazo transfer reagents
such as p-toluenesulfonyl azide (2 h),[39] 4-acetamido-
benzenesulfonyl azide (48 h),[40] or 2-azido-1,3-dime-
thylimidazolinium chloride (1 hour).[22]
Experimental Section
Diazo Transfer Reactions
With Et3N: To a solution of the corresponding carbonyl
compound 1a–m (0.80 mmol) in dry CH2Cl2 (1.5 mL) was
added in sequence Et3N (0.123 mL, 0.88 mmol), and per-
fluorobutanesulfonyl azide (0.312 g, 0.96 mmol). After stir-
ring the reaction mixture at room temperature for 15 min, a
saturated aqueous solution of NaHCO3 (4 mL) was added.
The organic layer was separated and extracted with a satu-
rated aqueous solution of NaHCO3 (5ꢇ4 mL) to remove the
nonafluorobutanesulfonamide by-product. After drying over
However, when we carried out the reaction with a-
nitro carbonyl compounds low yields of the desired a- anhydrous Na2SO4, the organic phase was concentrated at
Adv. Synth. Catal. 2011, 353, 575 – 579
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577

Jose Luis Chiara and Josꢀ Ramꢁn Suꢂrez
COMMUNICATIONS
reduced pressure. The corresponding diazo products 2a–m
were obtained in pure form in most cases without any fur-
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With 2,6-lutidine: To a solution of the corresponding a-
nitro carbonyl compound 1n/1o (0.80 mmol) in dry CH2Cl2
(1.5 mL) was added in sequence 2,6-lutidine (0.13 mL,
1.12 mmol) and perfluorobutanesulfonyl azide (0.390 g,
1.20 mmol). The reaction mixture was stirred at room tem-
perature for 30 min. The reaction was quenched with a satu-
rated aqueous solution of NaHCO3 (4 mL) and extracted
with CH2Cl2 (3ꢇ8 mL). The organic layers were separated,
dried over anhydrous Na2SO4, and concentrated at reduced
pressure. The residue was purified by column chromatogra-
phy on silica gel (hexane/EtOAc 5:1) to give the corre-
sponding a-nitro-a-diazocarbonyl compound 2n/2o.
Acknowledgements
We thank the Spanish Ministerio de Ciencia e Innovaciꢀn
(projects CTQ-2006-15515-C02-02/BQU and CTQ2009-
14551-C02-02) and Comunidad de Madrid (project S2009/
PPQ-1634 “AVANCAT”) for financial support. We also
thank C.S.I.C. for a JAE-Doc contract to J.R.S.
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