General solvent-free highly selective N-tert-butyloxycarbonylation strategy using protic ionic liquid as an efficient catalyst

Article abstract of DOI:10.1039/c4ra02670e

A simple, rapid and solvent-free protocol is described for the chemo-selective transformation of amines to tert-butyloxycarbonyl protected derivatives (NHBoc) using Boc₂O and imidazolium trifluoroacetate protic ionic liquid (5-20 mol%). Unwanted side products such as isocyanate, urea or N,N-di-Boc were not detected. The scope of the protection strategy was successfully explored for substrate alcohols, phenols and thiol at elevated temperatures. Optically pure amino acids, amino acid esters and amino alcohols were efficiently converted to the corresponding N-Boc protected derivatives in excellent yields without racemization at the chiral center. The distinct advantages of this method are: operational simplicity, cleaner reaction, high selectivity, excellent yield, rapid reaction convergence, easy preparation and recyclability of the catalyst.

Full text of DOI:10.1039/c4ra02670e

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General solvent-free highly selective N-tert-
butyloxycarbonylation strategy using protic ionic
Cite this: RSC Adv., 2014, 4, 24544
liquid as an efficient catalyst†
a
Swapan Majumdar, Jhinuk De,a Ankita Chakrabortya and Dilip K. Maitib
Received 26th March 2014
Accepted 23rd May 2014
*
DOI: 10.1039/c4ra02670e
www.rsc.org/advances
A simple, rapid and solvent-free protocol is described for the chemo- above mentioned drawbacks, several Lewis acid or solid acid
selective transformation of amines to tert-butyloxycarbonyl protected catalysts6 are available to effect the conversion of amines to the
derivatives (NHBoc) using Boc2O and imidazolium trifluoroacetate corresponding N-Boc protected compounds. In spite of their
protic ionic liquid (5–20 mol%). Unwanted side products such as potentiality they are also plagued by a number of other draw-
isocyanate, urea or N,N-di-Boc were not detected. The scope of the backs such as difficulties associated with the preparation such
protection strategy was successfully explored for substrate alcohols, as use of solid acid catalysts (long reaction time, tedious work-
phenols and thiol at elevated temperatures. Optically pure amino up), moisture sensitivity, many of them is corrosive and more-
acids, amino acid esters and amino alcohols were efficiently converted over some of them can be deactivated by the amines. Therefore,
to the corresponding N-Boc protected derivatives in excellent yields new, mild and eco-friendly protocols for N-Boc protection are
without racemization at the chiral center. The distinct advantages of still in demand. In recent times, ionic liquids have received
this method are: operational simplicity, cleaner reaction, high selec- considerable interest as alternative to the traditional environ-
tivity, excellent yield, rapid reaction convergence, easy preparation mentally harmful volatile organic solvent. Ionic liquids can be
and recyclability of the catalyst.
used simultaneously as eco-friendly solvents, catalysts and
reagents in the context of green synthesis7 because of their
unique properties, such as low volatility, non-ammability,
high thermal stability, negligible vapour pressure and ability
to dissolve a wide range of compounds. Moreover, imidazolium
based ionic liquids particularly protic ionic liquids has been
selected as novel reaction media for promoting various organic
transformations due to their water miscibility and other useful
characteristics of solid acids and mineral liquid acids.7 There-
fore, replacement of conventional harmful acid catalyst to
protic ionic liquid promoted solvent free reactions not only
simplies organic synthesis but also improve process condi-
tions for large scale synthesis. Recently S. Sunitha et al.8 has
reported 1-methyl imidazolium tetrauoroborate [(HMIm)BF4]
as a catalyst for N-tert-butyloxycarbonylation of amines. The
catalyst [(HMIm)BF4] was prepared by mixing of 1-methyl
imidazole with 40–50% aqueous HBF4 (pure HBF4 never
prepared) at room temperature.9 Removal of excess HBF4 and
water from the ionic liquid is laborious, time and energy
consuming process and has limited substrate scope. During the
preparation of our manuscript, N-tert-butyloxycarbonylation of
amines particularly aromatic amines and some benzylic amines
were reported10 using pyridine based protic ionic liquid.
However pyridine based ionic liquid is more toxic and
volatile nature in contrast to its imidazole analogues and it
could not nd diverse application. To the best of our knowledge
The protection and deprotection of a functional group can be
essential in the chemistry of multi-functionalized molecules in
target oriented synthesis.1 The choice of a suitable protecting
group is oen crucial in the context of simplicity of the proce-
dure, highest yield of the desired product, easy workup and
separation. The protection of amines plays an important role in
organic synthesis especially in peptide chemistry. Among the
wide variety of available amine protecting group, the tert-buty-
loxycarbonyl (Boc) protecting group has demonstrated to be
most useful because it has all the fundamental properties for an
ideal protecting group such as easy availability of the reagents
for its installation, inertness in various reaction conditions and
simple deprotection.1–3 There are various conventional base
mediated strategies are available for the N-Boc protection of
amines in the literature.4 However, the base catalyzed reactions
oen lead to formation of undesirable side products such as
isocyanate, urea and N,N-di-Boc derivatives.5 To circumvent the
aDepartment of Chemistry, Tripura University, Suryamaninagar, 799 022, India.
E-mail: smajumdar@tripurauniv.in; Fax: +91-381-2374802; Tel: +91-381-237-9070
bDepartment of Chemistry, University of Calcutta, 92, A. P. C. Road, Kolkata 700 009,
India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra02670e
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N-tert-butyloxycarbonyl protection of amino acids and other mixture there was immediate exothermic evolution of gas with
chiral compounds using imidazolium triuoroacetate protic concomitant formation of the corresponding N-Boc derivative
1
ionic liquid is unexplored which is very much simple to in 98% yield which was conrmed by monitoring in TLC, H
prepare.11 It can be achieved just by mixing of equimolar NMR and GC-MS analyses. Incomplete conversion (ꢁ50%) was
amount of any N-1 substituted imidazole with pure triuoro- observed in absence of the catalyst that afforded 4a in 40% yield
acetic acid without the use of any additional solvents. Any trace aer 48 h (entry 8).
ꢀ
of TFA (bp 72 C) can be removed during vacuum drying. The
With this optimized process in hand (Table 1, entry 6), the
ease preparation, handling and high water solubility of imida- efficacy of our protocol was evaluated using various amines (1)
zolium triuoroacetate based ionic liquids prompted us12 to with Boc2O under the catalysis of 1-butyl-2-methyl imidazolium-
investigate their uses as solvents and/or catalyst in the tert- triuoroacetate (5 mol%) under the solvent-free reaction
butyloxycarbonylation. Herein, we wish to disclose a highly conditions to furnish corresponding N-Boc protected products
efficient and chemo-selective protocol for the tert-butyloxy- (4). The scope of the benign approach is illustrated in Table 2,
carbonylation of amines, amino acids, amino acid esters, amino Table 3 and Table 4. With aryl amines the reaction rate depends
alcohols, alcohols, phenols and thiols using catalytic amount of on the nature of substitution as well as their relative positions in
protic ionic liquid under solvent-free reaction conditions.
the aromatic rings. p-Toluidine reacted in a rst reaction rate
During our initial investigation, aniline was treated with (20 min, Table 2, entry 1) at room temperature but o-toluidine
Boc2O in presence of different imidazolium based protic ionic underwent slowly (150 min) at room temperature to afford
liquid in various reaction conditions. The results were (85%, entry 2) N-Boc protected compound where as it became
ꢀ
summarized in Table 1. Reaction of aniline (1a) with Boc2O was faster (45 min) at 70 C to obtain much improved yield (98%,
completed
in
presence
of
1-methyl
imidazolium- entry 3). This might be due to the steric hindrance of the o-
triuoroacetate (20 mol%) in dichloromethane (DCM) at room substituent present in the aromatic ring which prevented easy
temperature in 20 min and the desired product 4a was obtained attack to the one of the carbonyl of Boc2O. Other aromatic
in excellent yield (92%, Table 1, entry 1). Yield was improved to amines such as, 1-amino naphthalene, 1,2-diamino benzene, 2-
94–95% on use of 1-butyl imidazolium or 1-butyl-2-methyl amino pyridine, imidazole and benzimidazole (1d–h) all are
imidazolium triuoroacetate as a promoter (entries 2 and 3). converted to the corresponding N-Boc derivatives (4d–h) in
Change of solvent such as DCM to acetonitrile (MeCN) did not excellent yield (98–99%, entries 5–8). In the case of 1,2-diamino
improve the yield (entry 4). Gratifyingly, the reaction time benzene (1e), it was very difficult to control the reaction for
(5 min) and yield (99%) were drastically improved under mono-N-tert-butyloxycarbonylation. However, both the amino
solvent-free reaction conditions using 20 mol% of 1-butyl-2- groups can be efficiently protected with 2.2 moles of Boc2O
methyl imidazolium triuoro-acetate as a catalyst (entry 5). (entry 5). The reaction is somewhat sluggish at room tempera-
However, the reaction rate was slowed down (10 min) on ture for aromatic amines bearing electron withdrawing group
decreasing the catalyst loading (5 mol%, entry 6). No reaction such as 4-nitro aniline (1i) and 4-amino benzoic acid (1j) but at
ꢀ
took place in DCM (3 h) without protic PIL (entry 7). The role of 70 C the amines reacted with Boc2O to furnish corresponding
PIL was ascertained as follows: equimolar mixture of aniline N-Boc product with good yield (72–80%, entries 9 and 10). This
and Boc2O was stirred vigorously at room temperature for 30 protocol was also equally efficient for aliphatic primary,
min but commencement of effervescence was not observed. secondary or cyclic amines which afforded corresponding Boc
Upon addition of the protic ionic liquid (5 mol%) to the reaction protected derivatives (entries 11–15) at room temperature in
Table 1 Optimization of PIL catalyzed Boc protection reaction of amine functionality
Entry
PIL catalyst (mol%)
Solvent
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
1-Methyl imidazolium-triuoroacetate (20)
1-Butyl imidazolium-triuoroacetate (20)
1-Butyl-2-methyl imidazolium-triuoroacetate (20)
1-Butyl-2-methyl imidazolium-triuoroacetate (20)
1-Butyl-2-methyl imidazolium-triuoroacetate (20)
1-Butyl-2-methyl imidazolium-triuoroacetate (5)
None
DCM
DCM
DCM
MeCN
None
None
DCM
None
None
20
20
20
20
5
10
3 h
48 h
5
92
94
95
94
99
98
NR
40
None
1-Butyl-2-methyl imidazolium-triuoroacetate (5)
99, 96, 94, 93, 89
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Table 2 Protic ionic liquid catalysed N-tert-butyloxycarbonyl (Boc) protection of aminesa
Entry
1
Substrate (1)
Product (4)
Temp. (ꢀC)
Time (min)
20
Yieldb,c (%)
98
rt
2
3
rt
150
45
86
98
70
4
rt
30
78
5
6
rt
rt
30
60
98d
98
7
8
rt
rt
10
45
99
98
9
70
70
60
90
80
72
10
11
rt
10
99
12
13
rt
rt
20
60
98
89
14
rt
rt
30
15
95
98
15
a
1 mmol substrate was used in each entry. b Isolated yield. c All the products were characterized by IR, 1H & 13C NMR data. d 2.2 equivalent Boc2O
was used.
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Table 3 Chemoselective Boc protection of phenols and amino alcohols
Temperature
(ꢀC)
Entry
1
Substrate (2)
Product (5)
Time (min)
25
Yield (%)
94
70
70
70
2
3
45
45
91
98
4
5
6
70
70
rt
45
60
50
60a
90a
210
7
8
rt
240
92a
rt
5
99a,b
9
70
15
98c
1.1 eq. Boc2O was used. b Use of excess Boc2O did not produced any other product. c 2.2 eq. of Boc2O was used.
a
excellent yield (89–99%). In our experiments we observed that Table 3, entries 1–4) in good to excellent yield (78–98%). Next,
this protection approach is very much selective for primary we examined the chemo-selectivity of the present methodology.
amines. For instance, in our experiment to protect an equiva- 4-Hydroxy benzyl alcohol (2e) was treated with 1.1 moles of
lent mixture of aniline (primary amine) and secondary amines Boc2O in presence of 5 mol% of PIL. Aer 1 h, phenolic –OH
diphenyl amine we found only the Boc protected aniline deriv- protected compound (5e) (entry 5) was isolated in 60%
ative even at elevated temperature. However, on use of N-benzyl yield with the formation of traces of di-Boc derivative thereby
aniline only a trace of Boc-protected N-benzyl aniline was indicating that the reaction is very much chemo-selective
produce at higher temperature (70 ꢀC). Both steric and elec- for phenolic OH over alcohol. We were also very much
tronic repulsion played important role for the inertness of these pleased to observe that the reaction was highly chemo-
amines towards the reagent.
selective at room temperature since 3 or 4 amino phenol
Aer successful development of N-Boc protection of various (entries 6 and 7) and 3-amino 1-propanol (entry 8) afforded
aromatic, hetero-aromatic as well as aliphatic amines by only the corresponding N-Boc protected derivatives (5f–h) as
catalysis of the imidazolium triuoroacetate based protic ionic the sole product. Interestingly, on carrying out the reaction at
liquid we paid our attention to the protection of other func- 70 ꢀC using 2.2 equivalent of Boc2O, both the amino and
tionalities such as alcohols, phenols and thiol compounds (2). alcohol groups of 3-amino propanol were reacted and fur-
Phenols, alcohols or thiols do not reacted with Boc2O at room nished N-Boc and O-Boc protected product 5i in 98% yield
temperature. However, at 70 ꢀC these compounds underwent (entry 9) in 15 min without generation of N,N-di-Boc or other
smoothly to the desired O-Boc or S-Boc derivatives (5a–d, side products.
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Table 4 tert-Butyloxycarbonyl protection of functionalized chiral amine
Entry
Substrate (3)
Product (6)
Temp. (ꢀC)
Time (min)
20
Yielda (%)
97
1
rt
2
3
4
70
70
70
15
45
45
99b
81c
86c
5
70
75
86c
6
7
70
rt
60
60
80c
89
8
rt
45
98
9
70
rt
30
5
87d
98
10
11
rt
5
70
12
rt
180
89
a
Isolated yield. b Same yield was obtained when reaction was carried out in 100 mmol (11.7 g) scale. c Two drops of water was added. d Reaction was
very slow at room temperature.
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Enormous importance of Boc protected amino group (6, Table
4) in peptide and c (pseudo)-peptide13 chemistry led us to pursue
protecting amine functionality of chiral amino acids, amino acid
esters and amino alcohols because the protected precursors are
frequently used during synthesis of peptides. L-Leucinol (3a) was
treated with Boc2O (1.1 eq.) and 1-butyl-2-methyl imidazolium
triuoroacetate (5 mol%) catalyst which selectively afforded cor-
responding N-Boc protected amino alcohol in 97% yield aer 20
min (Table 4, entry 1).
Free amino acids did not react with Boc2O at room temper-
ature, but reacted efficiently at 70 ꢀC in presence of 20 mol% of
protic ionic liquid and a little water (two drops) to afford the
corresponding N-Boc amino acids in very good to excellent yield
(80–99%, entries 2–6). Herein, role of water is to generate non-
protonated amine functionality which is essential for initiation
of N-Boc protection. In case of secondary amino acid proline,
the protection reaction worked well even in 100 mmol scale
(yield: 99%) in a very short period of time (entry 2). The amino
acids esters were also efficiently converted to their corre-
sponding N-Boc products without effecting ester functionality
(entries 7–9). Unfortunately, Boc protection of amino acids
those contains side chain functional group such as serine,
histidine and tyrosine under the same reaction conditions were
unsuccessful due to completely insoluble nature of the precur-
sors under the reaction conditions. However, use of methyl
ester of these amino acids greatly improves their solubility and
the reaction underwent smoothly with Boc2O at room temper-
ature under the catalysis of 2-methyl-1-butyl imidazolium tri-
uoroacetate to afford the corresponding Boc protected amino
acid esters (6j–l) in good to excellent yield and selectivity
(entries 10–12). As expected phenolic –OH group was not
participated Boc protection reaction at room temperature. In
the case of methyl ester of histidine (3k) both a-amino group
and NH of imidazole ring undergoes tert-butyloxycarbonylation
reaction. Racemization of the chiral centers was not observed in
all cases (Table 4) which was determined by optical rotation
measurement of the products with comparison of the available
literature data.14
Scheme 1 Possible catalytic cycle for N-tert-butyloxycarbonylation.
General procedure for N-tert-
butyloxycarbonylation of amines
A mixture of Boc2O (5.0 mmol), protic ionic liquid (5 mol%) and
an amine (5.0 mmol) was stirred at room temperature or 70 ꢀC for
the time indicated in the Tables. Aer completion of reaction as
revealed by TLC, the mixture was diluted with water (5 ml) and
extracted with diethyl ether or ethyl acetate (3 Â 10 ml). The
aqueous layer was recycled. The combined organic layer was
washed with water, dried over anhydrous sodium sulphate and
concentrated under reduced pressure. The crude products were
essentially pure but for analytical data, the samples were passed
through short pad silica-gel (60–120 mesh) to remove any trace
impurities.
In conclusion, we have demonstrated herein a robust and
environmentally benign methodology for N-tert-butyloxy-
carbonylation of structurally diverse aliphatic and aromatic
amines, phenols, alcohols, thiol, chiral amino acids, amino acid
esters and amino alcohols without generation of any unwanted
side product such as isocyanate, urea, N,N-di-Boc derivative and
the optical antipodes. The reaction was developed under
solvent-free reaction conditions utilizing easily affordable imi-
dazolium triuoroacetate protic ionic liquid as an efficient
catalyst. This strategy is high efficient, inexpensive, operational
simple, chemo-selective, catalyst recyclable and green which is
an outstanding alternative to the commonly used base catalyzed
methods using objectionable and toxic organic bases (TEA,
DIPEA, DMAP, etc.) and solvents.
Since the practical utility of any catalytic process depend on
reusability of the catalyst, therefore recyclability of protic ionic
liquid was investigated and assessed by conducting N-tert-
butyloxycarbonylation of aniline with 20 mol% of 1-butyl-2-
methyl imidazolium triuoroacetate. We observed that the
catalytic activity of protic ionic liquid was retained over ve
successive cycles within yield of 99–89% (Table 1, entry 8). The
proposed catalytic cycle by the powerful PIL (I) is depicted in
Scheme 1. Mechanism of the transformation is expected to be
initiated through activation of carbonyl of Boc2O by protic
ionic liquid which nds extra stabilization from neighboring
À
CF3CO2 (II). This electrophilic activation greatly facilitates
nucleophilic attack by the –XH functionality (III) of the
substrates (1–3) to release Boc protected desired compounds
(4–6). Subsequent removal (IV) of carbon dioxide and tert-
Acknowledgements
butanol leads to generation of the catalyst for the next cycle We are pleased to acknowledge the nancial support from
(Scheme 1).
Department of Science & Technology (DST), Govt. of India (grant
no. SR/S1/OC-49/2010) for this investigation. J. D. is thankful to
Tripura University, India for providing RET fellowship.
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