An Improved Synthesis of Polyfunctional Acyl Azides in PEG 400

Full text of DOI:10.1080/00304948.2020.1862029

Organic Preparations and Procedures International
The New Journal for Organic Synthesis
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An Improved Synthesis of Polyfunctional Acyl
Azides in PEG 400
Khalid Widyan
To cite this article: Khalid Widyan (2021) An Improved Synthesis of Polyfunctional Acyl
Azides in PEG 400, Organic Preparations and Procedures International, 53:2, 120-126, DOI:
10.1080/00304948.2020.1862029
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ORGANIC PREPARATIONS AND PROCEDURES INTERNATIONAL
2021, VOL. 53, NO. 2, 120–126
https://doi.org/10.1080/00304948.2020.1862029
EXPERIMENTAL PAPER
An Improved Synthesis of Polyfunctional Acyl Azides in
PEG 400
Khalid Widyan
Department of Chemistry, Tafila Technical University, Tafila, Jordan
ARTICLE HISTORY Received 19 February 2020; Accepted 6 August 2020
Acyl azides (R-CO-N3) belong to a synthetically important class of organic azides which
have been used as highly reactive reagents in organic chemistry. These versatile com-
pounds have been used in cycloadditions, for the preparation of amides, biologically
active molecules, natural products and isocyanates, which in turn undergo easy conver-
sion into ureas and urethanes.1–5
Acyl azides can be prepared directly from aldehydes using Dess-Martin periodinane
(DMP),6 tert-butyl hypochlorite,7 (diacetoxyiodo)benzene (DIB)8 and sodium azide, but
the reaction has to be conducted below 0 ꢀC to suppress thermal rearrangement to
isocyanates.
Nitrosyl precursors (NOþ) had given direct conversion of acylhydrazines to acyl
azides but this requires availability of the hydrazide.9–16 Direct conversion of carboxylic
acids to acyl azides is achieved using diphenylphosphoryl azide (DPPA) in the presence
of base,17–26 and the use of DPPA reduces Z- to E-isomerization in a,b-unsaturated
azides.21 Other protocols for direct conversion of carboxylic acids to acyl azides use
“acid activators” such as SOCl2/DMF,27–31 cyanuric chloride/N-methylmorpholine,32 tri-
chloroisocyanuric acid/triphenylphosphine (TCCA-PPh3),33 and hypervalent iodine
reagents.34 Triphosgene/triethylamine,35 or propylphosphonic anhydride (T3P)/triethyl-
amine36 are also used to convert aryl and alkyl carboxylic acids into the corresponding
acyl azides; however, this method is inapplicable to N-Boc-amino acids due to the for-
mation of oxazolidin-2,5-diones (Leuchs’ anhydrides).37
For the preparation of protected amino acid azides, it is necessary to use such coupling
reagents as benzotriazol-1-yl-oxy-tris-(dimethylamino)-phosphonium hexafluorophosphate
(BOP),38 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (TBTU)39 or
1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and 2-(1H-benzotriazol-11-yl)-
1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU).40 In such cases, DMSO is gener-
ally used as solvent and the reaction temperature should be controlled at 0 ꢀC.
The common methods to achieve acyl azides are by reaction of sodium azide with
acid chlorides,41–44 or mixed anhydrides.21,22,45–52 Acid chlorides are frequently not eas-
ily accessible, unstable or difficult to handle. Mixed anhydrides must be generated from
a carboxylic acid and alkyl chloroformate. Only a single successful preparation of a
Fmoc-protected amino acid azide from the corresponding acid chlorides or mixed anhy-
drides is reported.49 Attempts to prepare Z-a,b-unsaturated azides from the
CONTACT Khalid Widyan
k.widyan@ttu.edu.jo
Department of Chemistry, Tafila Technical University, Tafila, Jordan
ß 2021 Taylor & Francis Group, LLC

ORGANIC PREPARATIONS AND PROCEDURES INTERNATIONAL
121
i
ii
2a-r
1a-r
3a-r
i:BtH, SOCl2,RT; ii: PEG 400, ZnI2, NaN3, RT
R = (a) C6H5, (b) 2-CH3C6H4, (c) 4-CH3C6H4, (d) 4-ClC6H4, (e) 4-NO2C6H4, (f) 3-Hydroxy-2-naphthyl
(g) 4-C6H5C6H4, (h) C6H5CH2, (i) 2-Thienyl, (j) 2-Furanyl, (k) 2-Indolyl, (l) 2-Pyridyl, (m) 3-Pyridyl,
(n) C6H5CH=CH, (o) 2-Furanyl-CH=CH, (p) DL-Cbz-Phe, (q) L-Cbz-Phe, (r) L-Cbz-Tryp
Scheme 1. Synthesis of acyl azides.
corresponding Z-a,b-unsaturated acids via the mixed anhydride with ethyl chloroformate
followed by the reaction with sodium azide led to isomerization to E-a,b-unsaturated acyl
azides.21,22 Recent protocols employed cross-linked poly(N-methylpyridinium)azide ion,53
but only one example was reported using polyethylene glycol (PEG) 400 promoted nucleo-
philic substitution of acyl chlorides into azides.54
Katritzky and co-workers described the synthesis of a wide range of N-acylbenzotria-
zoles 2 as stable alternatives to acid chlorides.55,56 These have been widely adopted as
acylation agents for the preparation of amides, Weinreb amides, cinnamoyl hydrazides,
N-acylsulfonamides, and substituted 2-azinyl-1-ethanones.57–63 N-Acylbenzotriazoles 2
have also been used for the acylation of sodium azide in the preparation of a wide range
of acyl azides under mild conditions; however the reaction time was long at room tem-
perature.64 The use of ferric chloride hexahydrate as a catalyst (5-20 mol%) accelerated
the conversion of 2 to acyl azides in acetone-water.65
In continuation of our investigation of N-acylbenzotriazoles 2, we now report an
improved and efficient method for preparation of acyl azides using PEG 400 as a phase
transfer catalyst. We also introduced catalytic amounts of zinc iodide to increase the
reaction rate. N-Acylbenzotriazoles 2 were prepared by reaction of carboxylic acids 1
with benzotriazole (BtH) and thionyl chloride in almost quantitative yields according to
an established procedure (Scheme 1).55
Then, as a model reaction, we investigated the acylation of sodium azide with N-benzoyl-
benzotriazole (2a) with different equivalents of PEG 400 (Table 1). Reaction of 2a (0.1 mol)
with 0.12 mol of sodium azide in 4 equivalents of PEG 400 in the presence of zinc iodide
(1.5 mol%) for 20 min at room temperature afforded 3a in excellent yield of 95% (Table 1,
entry 4). These were the best results. When 2a was treated with sodium azide (1.2 equiva-
lents) in PEG 400 (4 equivalents) without using zinc iodide, 3a was obtained in lower yield
of 80% with a longer reaction time (30 min) at room temperature. Using lower equivalents
of PEG 400 in the absence of zinc iodide provided 3a in lower yields and longer reaction
times (Table 1, entries 1 and 2). When PEG 400 was used in larger amounts (8 equivalents)
3a was obtained in lower yields (Table 1, entry 5).
The recovery and reuse of PEG 400 was studied. The recovery procedure was simple,
as detailed in the Experimental section, and was based on the solubility differences in
ether of PEG 400 and the acyl azides. After the product was extracted with dry ether it
was found that the recovered PEG 400 could be used for three runs without a signifi-
cant decrease in yield (Table 4, entry 4). The PEG 400 was nearly quantitatively recov-
ered, and this allowed the recycling to be efficient.
Next the scope and generality of the method was examined. A number of N-acylben-
zotriazoles with alkenyl, arylalkyl, aryl and heteroaryl groups were converted to the

122
K. WIDYAN
Table 1. Optimization of the conditions for the conversion of 2a into benzoyl azide 3a.
Entry
PEG 400 mol%
ZnI2 mol%
moles NaN3
Time (min)
Estd Conva
Isolated Yield%
1
2
3
4
5
1
2
4
4
8
–
–
–
1.5
1.5
0.12
0.12
0.12
0.12
0.12
40
40
30
20
20
75
80
85
100
100
73
75
80
95, 92b, 90c
90
aFor 1 mmol scale. Approximately estimated by TLC.
bThe second run.
cThe third run.
corresponding acyl azides in high yields (85-95%) in short times (20 min) under mild
conditions (Scheme 1, Table 2). Isomerization of a,b-unsaturated derivatives was not
observed for compounds 3n and 3o. Using chiral N-(Cbz-aminoacyl)benzotriazoles pro-
vided the corresponding azides without racemization, as demonstrated by the L-form
and DL-forms of phenylalanine 3p and 3q. The chirality control was confirmed by
HPLC chromatography.
In summary, this approach conveniently affords a variety of acyl azides 3a-r. Our
method uses the low cost and non-toxic phase transfer catalyst PEG 400. Reaction times
are short. Yields are comparable or improved to those reported in the literature.
Isomerization of a,b-unsaturated derivatives and racemization of chiral derivatives were
not observed. The reaction was carried out under mild conditions without any Curtius
rearrangement. The PEG 400 can be recovered and used for several runs without loss
in efficiency.
Experimental section
N-Acylbenzotriazoles were prepared according to an established procedure.55 Melting
points were determined on a capillary melting point apparatus equipped with a digital
thermometer and were corrected. NMR spectra were recorded in CDCl3 or DMSO-d6
1
with TMS for H (300 MHz) and 13C (75 MHz) as the internal reference. Column chro-
matography was performed on 200–425 mesh silica gel. Thin layer chromatography was
carried out on silica gel plates with hexane-diethylether (2:1) as the eluting solvent.
Where appropriate, chirality control was confirmed by HPLC chromatography using a
Shimadzu instrument with a Daicel chiralcel OD-H, 250 x 4.6 mm column. Safety
Notes: CAUTION! Sodium azide is highly toxic. Sodium azide and all organic azides
are potentially explosive, both in use and in preparation. Note that the reaction proce-
dures were performed on a small scale. Experiments should be carried out in a well-
ventilated hood using a safety shield and personal protective gear. All workers should
be thoroughly trained in appropriate safety procedures before handling these materials.
Typical procedure for preparation of acyl azides 3a-r
Sodium azide (see Safety Notes above, 1.2 mmol, 77.6 mg) was added to a solution of
the appropriate N-acylbenzotriazole (1.0 mmol) and zinc iodide (0.478 g, 1.5 mmol) in
PEG 400 (1.5 g). The mixture was stirred at room temperature for 20 min. PEG 400 is
insoluble in diethyl ether. Thus, to isolate the acyl azides, the reaction mixture was

ORGANIC PREPARATIONS AND PROCEDURES INTERNATIONAL
123
Table 2. Preparation of acyl azides 3.
mp (ꢀC)
Entry
1
Product
Yield (%)
95
(lit mpref
33-35
)
1H NMR (d)
13C NMR (d)
128.5, 129.5,130.7,
134.3, 172.4
22.1, 126.8, 131.1,
132.3, 133.4,
3a
7.46-7.50 (m, 2H), 7.61-7.66 (m, 1H),
8.03 (d, J ¼ 7.8 Hz, 2H)
(32-3464
91-93
)
2
3b
90
2.60 (s, 3H), 7.20-7.25 (m, 2H), 7.40-
7.46 (m, 1H), 7.91 (d, J ¼ 7.9 Hz, 1H)
(90-9235
)
)
141.6, 173.1
3
4
5
6
3c
3d
3e
3f
92
90
88
89
32-33
(32-3464
39-40
2.40 (s, 3H), 7.24 (d, J ¼ 8.1 Hz, 2H),
7.90 (d, J ¼ 8.1 Hz, 2H)
21.6, 128.2, 129.3,
129.6, 145.3, 172.2
124.0, 129.0,131.1,
132.7, 135.2, 171.0
124.3, 130.7, 136.2,
151.7, 172.0
112.2, 115.3, 124.1,
126.2, 127.2, 129.7,
130.3, 133.1, 138.9,
156.4, 176.3.
7.7 (d, J ¼ 8.1 Hz, 2H), 7.93 (d,
J ¼ 8.1 Hz, 2H)
(3853
)
66-67
8.20 (d, J ¼ 7.00 Hz, 2H), 8.37 (d,
J ¼ 7.00 Hz, 2H)
53
(68-69
)
110-112
7.30-7.38 (m, 2H), 7.50 (t, J ¼ 7.3 Hz,
1H), 7.67 (d, J ¼ 8.2 Hz, 1H), 7.81 (d,
J ¼ 8.2 Hz, 1H), 8.40 (s, 1H), 10.30
(s, 1H)
(109-11164
)
7
3g
91
77-79
(-)a,b
7.45-7.53 (m, 5H), 7.84-7.92 (m, 2H),
8.10-8.45 (m, 2H);
126.6, 127.7, 129.0,
129.5, 130.3, 139.7,
147.3, 172.1
8
3h
3i
94
95
89
87-89
3.85 (s, 2H), 7.15 (m, 1H), 7.20 (d,
J ¼ 6.8 Hz, 2H), 7.38-7.41 (m, 2H)
7.12-7.21(m, 1H), 7.60-7.72 (m, 1H),
7.79-7.89 (m, 1H)
38.9, 127.8, 129.2,
130.1, 141.2, 173.1.
128.0, 134.8, 135.1,
135.4, 167.0
(8939
)
9
36-38
(34-3664
62-63
)
)
10
3j
6.60 (dd, J ¼ 3.6, 1.6 Hz, 1H), 7.30
(d, J ¼ 2.9 Hz, 1H), 7.70 (d,
J ¼ 0.8 Hz, 1H)
112.9, 120.4, 145.8,
(60-6264
148.1, 162.7.
11
12
13
14
15
3k
3l
93
85
88
92
91
140-141
7.10-7.20 (m, 1H), 7.25-7.32 (m, 1H),
7.37-7.49 (m, 2H), 7.65-7.74 (m, 1H),
9.10 (s, 1H)
110.3, 112.2, 121.5,
123.8, 126.5, 127.4,
128.6, 138.1, 166.3
125.6, 128.4, 137.5,
148.2, 150.3, 172.1
(142-14364
)
52-53
7.32-7.38 (m, 1H), 7.70-7.82 (m, 1H),
7.92 (d, J ¼ 7.1 Hz, 1H), 8.50 (d,
J ¼ 6.5 Hz, 1H)
(51-5264
)
)
)
)
3m
3n
3o
47-48
d 7.60 (dd, J ¼ 4.8, 8.0 Hz, 1H), 8.30
(dt, J ¼ 2.0 Hz, 1H), 8.85 (dd, J ¼ 4.8,
1.6 Hz, 1H), 9.3 (d, J ¼ 1.8 Hz, 1H)
6.50 (d, J ¼ 15.8 Hz, 1H), 7.45-7.55
(m, 3H), 7.60-7.76 (m, 2H), 7.86 (d,
J ¼ 15.8 Hz, 1H)
124.2, 126.8, 137.4,
150.8, 153.7, 166.7
(48-5064
83-85
119.3, 128.8, 129.1,
130.9, 133.8,
146.9, 172.3
113.0, 116.7, 117.1,
132.6, 145.9,
(82-8364
71-73
6.32 (d, J ¼ 15.5 Hz, 1H), 6.50 (dd,
J ¼ 3.4, 1.8 Hz, 1H), 6.73 (d,
J ¼ 3.4 Hz, 1H), 7.50 (d, J ¼ 15.5 Hz,
1H), 7.54 (d, J ¼ 1.8 Hz, 1H)
2.76-2.86 (m, 1H), 3.06-3.15 (m, 1H),
4.17-4.25 (m, 1H), 5.10 (s, 2H), 7.20-
7.36 (m, 10H), 7.68 (d,
(70-7264
150.6, 172.2.
16
17
18
3p
3qc
3rd
95
93
86
131-133
36.6, 55.7, 65.4, 126.3,
127.7, 127.9, 128.1,
128.3, 129.2, 137.1,
138.2, 156.3, 173.6.
36.5, 55.8, 65.5, 126.2,
127.7, 127.9, 128.2,
128.3, 129.1, 137.2,
128.0, 156.1, 173.5.
27.1, 55.3, 65.5, 110.1,
111.6, 118.3, 118.7,
121.2, 123.8, 127.1,
127.6, 127.8, 128.5,
136.3, 137.2,
(130-13264
)
)
)
J ¼ 8.5 Hz, 1H)
149-151
2.79-2.86 (m, 1H), 3.05-3.12 (m, 1H),
4.16-4.20 (m, 1H), 520 (s, 2H), 7.18-
7.37 (m, 10H), 7.67 (d,
(148-14964
J ¼ 8.5 Hz, 1H)
167-169
2.81-3.06 (m, 1H), 3.16-3.26 (m, 1H),
4.19-4.31 (m, 1H), 4.98 (q,
(166-16864
J ¼ 12.7 Hz, 2H), 7.04 (t, J ¼ 7.2 Hz,
1H), 7.07-7.12 (m, 1H), 7.20 (s, 1H),
7.30-7.41 (m, 6H), 7.58 (d,
J ¼ 8.1 Hz, 1H), 7.65 (d, J ¼ 8.1 Hz,
156.1, 174.1.
1H), 10.88 (s, 1H)
aNot previously reported.
bAnal. Calcd for C13H9N3O: C, 69.95; H, 4.06; N, 18.82. Found: C, 70.11; H, 4.15; N, 18.72.
c[a]23 ¼ –2.94ꢀ (c 1.39, DMF).
D
d[a]23 ¼ –8.81ꢀ (c 1.39, DMF).
D

124
K. WIDYAN
extracted with diethyl ether (dry ethyl ether is used if the PEG 400 will be recovered),
leaving behind the PEG 400, which was then recovered, nearly quantitatively. The ether
extract was washed with dilute aqueous sodium carbonate and water, then dried over
MgSO4 and filtered. The solvent was evaporated under reduced pressure and the residue
was passed through a short column of silica gel using hexane-diethyl ether (2:1) as elut-
ing solvent to give the acyl azide. The properties of the acyl azides prepared in this
study are tabulated in Table 2. Provided that the ether used in the extraction is dry as
noted above, the PEG 400 could be reused for as many as three runs without significant
loss of activity (Table 1, entry 4).
Acknowledgment
Dedicated to the memory of the late Prof. Alan R Katritzky for his excellent contributions to
benzotriazole chemistry.
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