A waste-minimized protocol for the preparation of 1,2-azido alcohols and 1,2-amino alcohols

Article abstract of DOI:10.1039/c3gc40988k

Under solvent-free conditions the reaction of epoxides 1a-i with trimethylsilylazide (2) catalyzed by polystiryl-supported fluoride (PS-DABCOF₂) has led to the efficient preparation of the corresponding O-TMS protected 1,2-azido alcohols 3a-i that, by treatment with Dowex-H, gave the related 1,2-azido alcohols 4a-i in excellent yields (83-99% and 82-96%, respectively). The use of a flow procedure has allowed us to significantly minimize waste in the preparation of representative 1,2-azido alcohols 4a, 4c and 4i that have been obtained with E-factors of 1.6, 2.1, and 1.9, respectively. The 1,2-amino alcohols 5a, 5c and 5f have been also prepared, in quantitative yields, by reduction of the corresponding O-TMS protected 1,2-azido alcohols 3a, 3c, and 3f by Pd on the Al₂O₃/HCOOH system.

Full text of DOI:10.1039/c3gc40988k

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Volume 15 | Number 9 | September 2013 | Pages 2305–2592
ISSN 1463-9262
PAPER
Luigi Vaccaro et al.
A waste-minimized protocol for the preparation of 1,2-azido alcohols and
1,2-amino alcohols

Green Chemistry
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A waste-minimized protocol for the preparation of
1,2-azido alcohols and 1,2-amino alcohols†
Cite this: Green Chem., 2013, 15, 2394
Eleonora Ballerini,a Paolo Crotti,b Ileana Frau,b Daniela Lanari,c Ferdinando Pizzoa
and Luigi Vaccaro*a
Under solvent-free conditions the reaction of epoxides 1a–i with trimethylsilylazide (2) catalyzed by poly-
stiryl-supported fluoride (PS-DABCOF2) has led to the efficient preparation of the corresponding O-TMS
protected 1,2-azido alcohols 3a–i that, by treatment with Dowex-H, gave the related 1,2-azido alcohols
4a–i in excellent yields (83–99% and 82–96%, respectively). The use of a flow procedure has allowed us
to significantly minimize waste in the preparation of representative 1,2-azido alcohols 4a, 4c and 4i that
have been obtained with E-factors of 1.6, 2.1, and 1.9, respectively. The 1,2-amino alcohols 5a, 5c and
5f have been also prepared, in quantitative yields, by reduction of the corresponding O-TMS protected
1,2-azido alcohols 3a, 3c, and 3f by Pd on the Al2O3/HCOOH system.
Received 30th May 2013,
Accepted 21st June 2013
DOI: 10.1039/c3gc40988k
www.rsc.org/greenchem
employing recoverable and reusable catalytic systems and by
using water as a reaction medium or solvent-free conditions
Introduction
1,2-Azido alcohols have been widely employed in organic syn- (SolFC).6
thesis for the regioselective preparation of 1,2-amino alcohols
Within our research, in several contributions we have paid
and highly oxygenated compounds such as carbohydrates and attention to the chemistry of epoxides.7
nucleosides.1 They are also useful intermediates2 for the
Starting from our expertise in this field, we have further
preparation of several target compounds such as triazoles,2a,c,d,g investigated the preparation of 1,2-azido alcohols and 1,2-
triazole-fused
dihydrooxazinones,2b
2-oxazolidinones,2e amino alcohols via ring-opening of epoxides in order to define
1,4-oxazepines and 1,3-oxazines,2f oxazaborolidines,2j 1,3-ox- a sustainable protocol featuring the minimal production of
azolidines,2k and in the chemistry of peptidomimetics2h and waste. As a trivial but effective measure of this parameter we
pseudopeptides.2k
have considered the environmental factor8 (E-factor, defined
Among the transformations used for the preparation of 1,2- as the ratio between the mass of waste produced/mass of
azido alcohols, the epoxide ring-opening is by far the most desired product).7
widely studied and sodium azide has been generally employed
We have individuated as the most efficient strategy towards
under different reaction conditions.3 In some cases, trimethyl- this end the definition of a flow procedure that combines the
silyl azide (TMSN3) has been also used for the introduction of use of solvent-free or highly concentrated conditions and a
the azido group,4 while other metal azides have been used very recoverable and reusable heterogeneous organocatalyst.9,10
rarely.3s,5 The widest variety of reaction conditions has been
In our previous work, we have specifically designed and pre-
applied including the use of different organic solvents, water pared several novel heterogeneous sources of fluoride ions
and a wide range of different catalysts.
based on the 1,4-diazabicyclo[2.2.2]octane (DABCO) nucleus
We have been contributing for several years to the develop- and on different solid polystyrene supports.10 These catalysts
ment of an environmentally-friendly organic synthesis by have been used to activate the Si–N bond of TMSN3 for the
β-azidation of α,β-unsaturated carboxylic acids under SolFC8b
and of α,β-unsaturated ketones in water.10a In these previous
aLaboratory of Green Synthetic Organic Chemistry, CEMIN – Dipartimento di
works, it was shown how the nature of the polystyrene support
Chimica, Università di Perugia Via Elce di Sotto, 8, Perugia, Italia.
may influence the efficiency of the immobilized catalysts
whether used under SolFC or in water.
E-mail: luigi@unipg.it; Fax: +39 075 5855560; Tel: +39 075 5855541
bDipartimento di Farmacia, Università di Pisa, Via Bonanno 33, 56126 Pisa, Italy
cDipartimento di Chimica e Tecnologia del Farmaco, Università di Perugia, Via del
Considering the synthetic utility of 1,2-azido alcohols, we
have selected the previously prepared10a bis-fluoride catalyst
polystyrene-DABCOF2 (PS-DABCOF2) gel-type (2% DVB cl)
200–400 mesh (Fig. 1) as most adequate for the definition of
an efficient azidolysis of epoxides under SolFC.
Liceo, 06123 Perugia, Italy
†Electronic supplementary information (ESI) available: Characterization data for
O-TMS protected 1,2-azido alcohols 3a, 3ba, 3bb, 3c, 3da, 3db, 3e, 3f, 3ga, 3gb,
3ha, 3hb, 3ia, 3ib that have not been previously reported. E-factor calculation
details for all compounds. See DOI: 10.1039/c3gc40988k
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The E-factor values range from 6.7 to 9.8.
In the representative case of epoxide 1a, the catalyst
PS-DABCOF2 has been recovered and reused for three consecu-
tive runs without observing any decrease of its catalytic
efficiency.
In addition, after the azidolysis of 1a–i we proceeded with
desilylation of the resulting products 3a–i by using a hetero-
geneous acidic catalyst. Dowex-H, Amberlyst-15 wet or dry, and
Amberlite IRC-86 were used in 20 mol% amounts to promote
the hydrolysis of 3a–i to 4a–i.12 Under these conditions, only
Dowex-H proved to be effective, in accordance with the results
previously obtained in a related process.11a
Fig. 1 The PS-DABCOF2 catalyst.
In the presence of 20 mol% Dowex-H in AcOEt (20 M),12 at
30 °C and in 5 h, O-TMS protected compounds 3a–i smoothly
convert to the corresponding 1,2-azido alcohols 4a–i in excel-
lent yields (see Table 1).
In order to minimize waste production and to highlight the
efficiency of the use of a heterogeneous recoverable catalyst
under SolFC, the representative azidolysis of epoxides 1a, 1c,
and 1i was also performed using a flow reactor operating in
the cyclic mode.
Scheme 1 Preparation of O-TMS protected 1,2-azido alcohols 3a–i and 1,2-
azido alcohols 4a–i.
In this paper we have therefore studied the preparation of
1,2-azido alcohols 4a–i by azidolysis of epoxides 1a–i and sub-
sequent deprotection of the corresponding β-azido-O-tri-
methylsilyl ethers 3a–i catalyzed by the acidic resin Dowex
50Wx8 (Dowex-H) (Scheme 1).
We have preliminarily optimized a small-scale (1–10 mmol)
batch protocol and then defined a waste-minimized protocol
using a flow procedure based on the combined use of the two
selected heterogeneous catalysts. In addition, we have also rea-
lized and combined with the previous protocol a reduction
step in flow which allows the direct preparation of 1,2-amino
alcohols 5a, 5c, 5f with the minimal waste production.
The cyclic-mode flow reactor operates basically in an auto-
mated batch-like fashion where stirring of the reaction mixture
through the solid catalyst is performed by using a pump
instead of a magnetic stirrer.10 Therefore, to set the cyclic-
mode flow protocol, the same optimized conditions as those
reported in Table 1 were used.
The reactor system consists of three 5 mL glass columns
labelled as PS-DABCOF2, Dowex-H, dry (drying/neutralizing
solid containing a 2 : 1 mixture of dry Na2SO4–K2CO3) and one
20 mL glass column labeled as the reservoir (Fig. 1). The
reactor system is installed in a thermostated box not shown in
Results and discussion
The catalyst PS-DABCOF2 has been prepared according to our Fig. 2 for clarity.
previous procedure obtaining in this case a 4.80 mmol g−1
Epoxide (50 mmol, 1a, 1c, or 1i) and TMSN3 (2, 55 mmol)
loading of fluoride. According to our previous results,9 when a were charged into a glass column functioning as a reservoir,
heterogeneous catalyst is used under SolFC, a higher loading and PS-DABCOF2 (10 mol% measured on the epoxide) was
is appreciated to minimize the mass of the solid catalyst used charged into a glass column (labelled as PS-DABCOF2). The
and facilitate reactants mixing. In this case our catalyst reaction mixture was continuously pumped through the cata-
loading is 60% higher than commercially available Amberlyst-F lysts at a flow rate of 0.5 mL min−1 and at the temperature and
(ca. 3 mmol g−1).
for the time reported in Table 1 (entries 1, 3, or 9) until com-
Based on our previous studies on several processes cata- plete conversion to 3a, 3c, or 3i was achieved.
lyzed by ammonium fluorides in SolFC11 and after some
The temperature was then set to 30 °C, the air/solvent valve
preliminary experiments, we have adopted the following was opened and the pump was set to work at 3 mL min−1 in
optimized conditions: epoxide 1 (1.0 mmol), TMSN3 (2) order to transfer the reaction mixture into the reservoir and
(1.10 mol eq−1) and PS-DABCOF2 (0.1 mol eq−1) under SolFC empty the PS-DABCOF2 column. To wash the PS-DABCOF2
and using 60 or 80 °C, as the reaction temperature.
The results obtained in the azidolysis of epoxides 1a–i, by inserted from the air/solvent valve into the system and cycli-
using these conditions, are reported in Table 1. cally pumped for 10 min after which it was collected in the
column, AcOEt (0.05 mL mmol−1 × 2 at 0.2 mL min−1) was
Azidolyses of epoxides 1a–i proceed in all cases with excel- reservoir. At this stage the concentrated reaction mixture was
lent yields (83–99%) and with the formation of the only O-TMS continuously pumped through a Dowex-H column (heated at
protected 1,2-azido alcohols 3a–i. The reaction times are 30 °C), and after 5 h, the final reaction mixture was collected
reasonable and, as for the regioselectivity, are expectedly and in the reservoir and AcOEt (0.05 mL mmol−1 × 2 at 0.40 mL
closely related to the substitution pattern of the oxirane ring. min−1), as previously done, was used to wash the Dowex-H
The α/β regioisomeric ratios (Table 1) are in agreement with an column. Finally, the reaction mixture was pumped through the
expected SN2-type nucleophilic attack of the azido ion to the third column (dry) for the final drying/neutralizing step to
oxirane ring.
eliminate traces of residual acidity coming from the hydrolysis
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Table 1 Preparation of O-TMS protected 1,2-azido alcohols 3a–i and 1,2-azido alcohols 4a–i under batch conditions
Entry
1
Epoxide
T (°C)
t (h)
α/βa ratio
Yield of 3b (%)
E-factor for 3
Yield of 4b (%)
E-factor for 4
60
18
1/99
99
7.0
90
16.8
2
80
96
77/23
95
8.2
95
18.8
3
4
80
80
18
18
—
96
96
8.9
8.4
96
96
21.6
19.6
50/50
5
80
120
99/1
95
6.7
95
14.6
6
7
80
80
96
24
1/99
95
92
8.0
8.2
94
90
18.1
19.0
30/70
8
9
60
60
96
18
20/80
77/23
83
95
9.8
8.4
82
94
23.1
19.5
a Nucleophilic attack at the more substituted and/or benzylic oxirane carbon; β: nucleophilic attack at the less substituted oxirane carbon. In the
case of epoxide 1i, α and β correspond to nucleophilic attack at oxirane carbon and α and β to the carbonyl group, respectively. b Isolated yield of
the pure product without further purification.
step (formation of trimethylsilycic acid). At the end, the flask reduction of O-TMS-protected 1,2-azido alcohols 3a, 3c and 3f
containing the recovered product solution was warmed at to the corresponding 1,2-amino alcohols 5a,13 5c,14 and 5f,14
50 °C and vacuum was applied to remove the organic solvent thus confirming its substantial chemical efficiency. 1,2-Amino
and recover the pure products 4a, 4c, or 4i (4a: 95% yield, alcohols 5a, 5c, and 5f have been obtained in all cases in quan-
E-factor = 1.6; 4c: 97% yield, E-factor = 2.1; and 4i: 95% yield, titative yields (Scheme 2).
E-factor = 1.9). In addition, any catalyst recovery issue has been
Accordingly, we have also defined a flow protocol to mini-
eliminated since the catalysts are safely charged into the glass mize waste production in the preparation of 1,2-amino alco-
column and have been directly reused for consecutive runs hols 5a, 5c, and 5f (Fig. 3).
showing identical efficiency.
Similar to the previously described flow procedure used for
As planned, we have also studied the reduction of the azido the preparation of 4a, 4c, and 4i (Fig. 2) the epoxide (50 mmol,
group of azido alcohols 4 to the amino group, in order to have 1a, 1c, or 1f) and TMSN3 (2, 55 mmol) were charged into a
access to the corresponding 1,2-amino-alcohols.
glass column functioning as a reservoir and the reaction
We have recently reported that the system Pd/Al2O3/HCOOH mixture was continuously pumped through the catalyst
in AcOEt–H2O efficiently reduces the azido group to the amino PS-DABCOF2 (10 mol% measured on the epoxide) at a flow rate
group.9b We have also used the same system for the direct of 0.5 mL min−1 and at the temperature and for the time
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Fig. 2 Preparation of azido alcohols 4a, 4c, or 4i in flow*. * A thermostated
chamber was used but not shown for clarity.
Fig. 3 Preparation of amino alcohols 5a, 5c or 5f using in flow*. * A thermo-
stated chamber was used but not shown for clarity.
= 2.9; 5c: 96% yield, E-factor = 4.2; and 5f 95% yield, E-factor =
3.1).
Conclusions
In conclusion, the azidolysis of epoxides 1a–i has been realized
under SolFC by using TMSN3 as the reagent and PS-DABCOF2
(10 mol%) as the catalyst. The consecutive desilylation of 3a–i
in the presence of 20 mol% Dowex-H furnished the corres-
ponding 1,2-azido alcohols 4a–i in excellent yields (82–96%).
The E-factor values associated with these procedures are also
satisfactory (14.6–23.1). In order to define a highly sustainable
synthetic procedure, a flow protocol has been also set allowing
the preparation of the representative 1,2-azido alcohols 4a, 4c
and 4i with very low E-factor values (1.6, 2.1 and 1.9, respecti-
vely) corresponding to a further ca. 90% of the waste pro-
duction compared to our already green batch protocol. As a
further application of our approach we have also defined a
one-pot protocol including the epoxide azidolysis and consecu-
tive reduction of the azido group to prepare the corresponding
representative 1,2-amino alcohols 5a, 5c, and 5f. The flow pro-
cedure defined for this two-step process allowed us to prepare
the final products 5a, 5c, and 5f with very low E-factor values
(2.9, 4.2 and 3.1 respectively).
Scheme 2 Pd/Al2O3-catalyzed reduction of O-TMS protected 1,2-azido alco-
hols 3a, 3c and 3f.
reported in Table 1 (entries 1, 3, or 6) until complete conver-
sion to 3a, 3c, or 3f was reached. The temperature was then set
to 30 °C, the air/solvent valve was opened and the pump was
set to work at 3 mL min−1 in order to transfer the reaction
mixture into the reservoir and empty the PS-DABCOF2 column.
To wash the PS-DABCOF2 column, AcOEt (0.05 mL mmol−1 × 2
at 0.2 mL min−1) was inserted from the air/solvent valve into
the system and cyclically pumped for 10 min after which it was
collected in the reservoir. At this stage, by setting appropriately
the valves, HCOOH (1.0 equiv.) was added and then the
reaction mixture was cyclically flowed through the catalyst
Pd/Al2O3 for 18 h at 30 °C until the azido group reduction was
complete.
The final reaction mixture was collected in the reservoir
and AcOEt (0.05 mL mmol−1 × 2 at 0.40 mL min−1), as pre-
viously done, was used to wash a Pd/Al2O3 column. Finally, the
reaction mixture was pumped through the third column (dry)
for the final drying/neutralizing step and then the flask con-
taining the recovered product solution was warmed at 50 °C
and vacuum was applied to remove the organic solvent and
recover the pure products 5a, 5c, or 5f (5a: 97% yield, E-factor
Experimental section
Unless otherwise stated, all chemicals were purchased and
used without any further purification. GC analyses were per-
formed by using Hewlett-Packard HP 5890A equipped with a
capillary column DB-35MS (30 m, 0.53 mm), a FID detector
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and helium as a gas carrier. GC-EIMS analyses were carried Representative experimental procedure for the desilylation
out by using a Hewlett-Packard HP 6890N Network GC system/ step
5975 Mass Selective Detector equipped with an electron
A solution of O-TMS-protected 1,2-azido alcohol 3a (0.200 g,
0.75 mmol) in 3 : 1 : 1 AcOH–THF–H2O (10.0 mL) was stirred at
room temperature for 2 h. Dilution with Et2O and evaporation
impact ionizer at 70 eV.
All 1H NMR and 13C NMR spectra were recorded at
200 MHz or 400 MHz, and at 50.3 or 100.6 MHz, respectively,
of the washed (sat. aq. NaHCO3 and brine) organic solution
using a Bruker DRX-ADVANCE 200 MHz and a Bruker DRX-
afforded a crude product (0.130 g, 90% yield) consisting of
azido alcohol 4a as the only reaction product.
ADVANCE 400 MHz spectrometer. The deuterated solvent used
was CDCl3, and TMS was employed as the internal standard.
Chemical shifts were reported in ppm and coupling constants
in hertz. Elemental analyses were realized by using a FISONS
Instrument, EA 1108 CHN.
Representative batch experimental procedure for the reduction
step
Compounds 3a–i are unknown compounds and 4a,3r 4b,3r In a screw capped vial equipped with a magnetic stirrer,
4c,3r 4d,3r 4e,3r 4f,3r 4g,3q 4h,3r,t 4i,3s 5a,13 5c14 and 5f14 are O-TMS-protected 1,2-azido alcohol 3a (0.265 g, 1.0 mmol),
1
known compounds. The H NMR spectra for compounds 3a–i AcOEt (0.50 mL), Pd/Al2O3 (0.106 g, 0.050 mmol, 5 wt% Pd),
are reported in the ESI.†
and formic acid (0.038 mL, 1.0 mmol) were consecutively
added. The resulting reaction mixture was left under stirring at
30 °C.
Representative batch experimental procedure for the azidolysis
After 18 h the catalyst was filtered off and washed with
AcOEt (1.0 mL). The collected organic phases were evaporated
under vacuum to give (0.164 g, 97% yield) pure 1-amino-3-
phenoxypropan-2-ol (5a) as a colourless oil.
of 1a–i
In a screw-capped vial equipped with a magnetic stirrer,
epoxide 1a (Table 1, entry 1) (0.150 g, 1.00 mmol), TMSN3
(0.1265 g, 1.10 mmol, 1.10 equiv.) and PS-DABCOF2 (0.020 g,
0.100 mmol, 0.10 equiv.) were consecutively added. The result-
ing reaction mixture was stirred at 60 °C and monitored using
TLC analysis (hexane–AcOEt, 8 : 2). After 18 h, AcOEt (1.0 mL)
was added and the catalyst was filtered off and washed with
AcOEt (1.0 mL). The collected organic phases were evaporated
under vacuum to give (0.262 g, 99% yield) pure 3-azido-
1-phenoxy-2(trimethylsilyloxy)-propane (3a) as a colourless oil.
Rf = 0.61, E-factor = 7.0.
Representative E-factor calculation
Batch preparation of 3a (1.0 mmol scale). 2-(Phenoxy-
methyl)oxirane (150 mg) + TMSN3 (126.5 mg) + PS-DABCOF2
(20 mg) + washing AcOEt (1788 mg) − product mass (262 mg)/
product mass (262 mg) = 7.0.
Batch preparation of 4a (1.0 mmol scale). 2-(Phenoxy-
methyl)oxirane (150 mg) + TMSN3 (126.5 mg) + PS-DABCOF2
(20 mg) + DOWEX-H wet (25 mg) + sodium sulphate–potass-
ium carbonate (2 : 1 mixture 100 mg) + washing AcOEt
(2682 mg) − product mass (174 mg)/product mass (174 mg) =
16.8.
3a 1H NMR (CDCl3) δ: 7.24–7.34 (m, 2H), 6.83–7.00 (m, 3H),
4.18 (dd, 1H, J = 11.9, 6.0, 4.0 Hz), 3.92 (d, 2H, J = 8.0 Hz), 3.42
(dd, 1H, J = 12.6, 4.0 Hz), 3.34 (dd, 1H, J = 12.6, 6.0 Hz), 0.20
(s, 9H).
Anal. Calcd for C12H19SiN3O2: C, 54.31; H, 7.22; N, 15.83.
Found: C, 54.09; H, 6.98; N, 15.64.
Batch preparation of 5a (1.0 mmol scale). O-TMS-protected
1,2-azido alcohol 3a (265 mg) + Pd/Al2O3 (106 mg) + formic
acid (46.36 mg) + washing AcOEt (1341 mg) − product mass
(164 mg)/product mass (164 mg) = 9.8.
Representative batch experimental procedure for the
preparation of 1,2-azido alcohols 4a–i
Preparation of 4a in flow (50 mmol). 2-(Phenoxymethyl)-
oxirane (7.50 g) + TMSN3 (6.325 g) + PS-DABCOF2 (1 g) +
washing AcOEt (8.94 g) − product mass (9.16)/product mass
(9.16 g) = 1.6.
Preparation of 5a in flow (50 mmol). 2-(Phenoxymethyl)-
oxirane (7.50 g) + TMSN3 (6.325 g) + PS-DABCOF2 (1 g) + Pd/
Al2O3 (5.3 g) + formic acid (2.3 g) washing AcOEt (8.94 g) −
product mass (8.1)/product mass (8.1 g) = 2.9.
In a screw-capped vial equipped with a magnetic stirrer,
epoxide 1a (Table 1 entry 1) (0.150 g, 1.00 mmol), TMSN3
(0.1265 g, 1.10 mmol, 1.10 equiv.) and PS-DABCOF2 (0.020 g,
0.100 mmol, 0.10 equiv.) were consecutively added. The result-
ing reaction mixture was stirred at 60 °C and monitored using
TLC analysis (hexane–AcOEt 8 : 2). After 18 h, AcOEt (2 mL)
was added and the catalyst was filtered off. 20% DOWEX-H wet
(25 mg) was added and the resulting mixture was stirred at
30 °C for 5 h.
The solid catalyst was then separated from the organic
phase by filtration and washed with AcOEt (1 mL). The com-
Acknowledgements
bined organic phases were dried over sodium sulphate–potass- We gratefully acknowledge the Ministero dell’Istruzione,
ium carbonate (2 : 1 mixture). The excess of TMSN3 (2) and dell’Università e della Ricerca (MIUR) within the project
solvent were removed under high vacuum at 50 °C to give pure “Firb-Futuro in Ricerca” ref. N. RBFR08JKHI, and PRIN 2008
1-azido-3-phenoxypropan-2-ol (4a) as a colourless oil (0.174 g, of the Università di Perugia and the Università di Pisa for
90% yield), E-factor = 16.8.
financial support.
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