First report on bio-catalytic N-formylation of amines using ethyl formate

Article abstract of DOI:10.1039/c6cc07679c

A bio-catalyzed N-formylation reaction of different amines has been developed using ethyl formate as a formylating agent. This protocol provides a facile and convenient strategy featuring mild reaction conditions, high efficacy, a broad substrate scope and recyclability of lipase. This method also works on a large scale in high yield.

Full text of DOI:10.1039/c6cc07679c

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DOI: 10.1039/C6CC07679C
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First Report of Bio-catalytic N-formylation of Amines Using
Ethyl formate
Rupesh E. Patre,*a Sanjib Mal,a Pankaj R. Nilkanth,a Sujit K. Ghorai,a Sudhindra H. Deshpande,a
Myriem El Qacemi,b Tomas Smejkal,b Sitaram Pal,a Bhanu N. Manjunath,*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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A Bio-catalyzed N-formylation reaction of different amines has remains active even under unfavorable conditions. The most
been developed using ethyl formate as formylating agent. This significant use of lipase in organic chemistry is kinetic resolution
protocol provides a facile and convenient strategy featuring mild of racemic amines or alcohols.19 In addition, they also catalyze
reaction condition, high efficacy, broad substrate scope and a variety of common organic reactions such as esterification,
recyclability of lipase. This method also works on a large scale in inter-esterification, transesterification, alcoholysis, acidolysis,
high yield.
aminolysis and hydrolysis of organic carbonates.20 Though
lipase catalyzed N-acetylation of amines are known in the
literature,19 to the best of our knowledge there is no report for
N-formylation catalyzed by lipase. Use of ethyl formate for N-
formylation of amines is known but it requires higher
temperature and longer reaction time (Scheme-1).21 This
prompted us to explore lipase catalyzed N-formylation of
amines using ethyl formate under mild reaction conditions.
N-formyl derivatives of amines are compound of considerable
interest due to their wide application in the synthesis of
important biologically active compounds.1 They have also been
employed as key reagents for the Vilsmeier formylation
reaction,2 amino acid protecting group in peptide synthesis,3
and as Lewis base catalysts for allylation4 or hydrosilylation5 of
carbonyl compounds. The most commonly used method for the
preparation of these compounds relies on formylation of
amines using a formylating agent such as chloral,6 acetic formic
anhydride,7 formic acid,8 ammonium formats,9 cyanide,10
formate esters,11 N,N-dimethylformamide,12 formyl fluoride,13
aldehydes14 and methanol.15 Use of carbon monoxide and
carbon dioxide is also reported in literature.16 However, these
methods suffer from several limitations such as harsh
conditions, use of expensive metal- or organo-catalysts, tedious
procedures or prolonged reaction time.17 Therefore, the
development of mild and environmentally friendly methods for
N-formylation of amines remains highly desirable for organic
transformations. Herein, we describe a simple and convenient
method for N-formylation of amines using ethylformate
catalyzed by lipase acrylic resin from Candida antarctica;
Novozyme 435® CALB.
Previous Work:
o
HCOOEt (Neat), 60 C
R1, R2 = Alkyl, Aryl, Heteroaryl, Benzyl
Scheme 1 N-formylation of amines with ethyl formate at 60oC.
This Work:
HCOOEt (1.5 - 3.0 equiv),
Novozyme 435 CALB (20% w/w),
THF, RT
In the recent years, use of biocatalysts in organic synthesis has
gained huge attention due to their efficacy and green nature.18
Lipases are highly stable and readily available enzymes, which
1
2
R1, R2 = Alkyl, Aryl, Heteroaryl, Benzyl
Scheme 2 N-formylation of amines with ethyl formate catalysed
by lipase; Novozyme 435® CALB at RT.
a.Syngenta Biosciences Pvt. Ltd., Santa Monica Works, Corlim, Ilhas, Goa, India-
403110, email: bhanu.manjunath@syngenta.com
b.Syngenta Crop Protection Muenchwilen AG., Schaffhauserstrasse, CH-4332 Stein,
Switzerland-CH3RD42
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Table 1 Formylation of amines with ethyl formate catalysed by
View Article Online
DOI: 10.1039/C6CC07679C
80b,f
lipasea
16
2p
Entry
Amine
Product
Yieldc
1p
99
29b,f
1
1a
17
18
2a
1q
2q
99
99
2
3
78b,f
1b
2b
1r
2r
91
19
20
2c
1c
2s
1s
95
94
4
5
2d
1d
90b
2t
1t
1e
2e
80b,g
21
22
23
24
98
6
1u
2u
1f
2f
94
94
91
7
8
1g
1v
2v
2g
95
2w
1w
2h
1h
92
90
9
2i
2j
1i
2x
1x
99
(8d)
10
11
95
90
25
26
1j
1y
2y
2z
>99e
1k
2k
2l
1z
99
99
12
13
14
aReaction condition (Method A): amine (5 mmol), ethyl formate (7.5 mmol), Novozyme®
435 CALB (20%w/w), THF, RT, 1-8 h.
1l
bReaction condition (Method B): amine (5 mmol), ethyl formate (neat), Novozyme® 435
CALB (20%w/w), RT, 1-8 h.
cIsolated yield.
1m
2m
dMethod A in absence of lipase (HPLC conversion).
eDone on 60 mmol scale.
fReaction time 24 h.
87b
g1.0 equiv of ethyl formate used.
1n
2n
Initially, we attempted N-formylation of benzyl amine using
excess ethyl formate and 50% w/w of lipase, similar to the
condition known for N-acetylation catalyzed by lipase.19 To our
gratification, the reaction went to completion within 1 h to
provide desired N-formyl benzyl amine in 99% of isolated yield.
With this exciting result in hand, we tried other formylating
97b
15
1o
2o
2 | J. Name., 2012, 00, 1-3
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agents such as formic acid, N,N-dimethylformamide, though it provided corresponding N-formylatedViepwroArdticulectOsnliinne
DOI: 10.1039/C6CC07679C
ammonium formate and trimethyl orthoformate. However, very good yields without any change in the enantiomeric ratio.
none of these formylating agents provided the desired product Preliminary steady-state kinetic parameters were measured for
in considerable yield except formic acid, which took more than the Novozyme 435® CALB catalyzed N-formylation reaction of
48 h for complete conversion. We next explored the impact of one of the representative substrates 4-methoxybenzylamine.
lipase stoichiometry on the outcome of the reaction. We The values of Km (Michaelis-Menten constant) and Kcat of the
observed that excess lipase was beneficial for faster conversion. lipase in the reaction were calculated from the Lineweaver-Burk
Conversely, a lower amount of lipase led to substantially slower plot (Fig. 1) and presented in Table 2 (for details of the kinetic
conversion. As a control, we also measured the background experiments see supporting information).
reaction in the absence of lipase confirming its vital role (Table
1, entry 10). To evaluate the effect of solvent, we screened Table 2 Kinetic constants for N-formylation reaction of 4-
solvents of different polarity, such as dichloromethane, 1,2- methoxybenzylamine with ethyl formate using Novozyme 435®
dichloroethane,
1,4-dioxane,
N,N-dimethylformamide, CALB.
tetrahydrofuran, water and cyclohexane. Tetrahydrofuran
found to be the most suitable solvent for the reaction. Our
subsequent aim was to reduce the amount of ethyl formate.
The best result was obtained 1.5 equivalent of ethylformate and
20% w/w of lipase in THF at room temperature (Scheme 2).22
Vmax
Km (M)
1.8
Kcat (min-1)
0.42X102
Kcat/Km
(mole min-1)
(mole-1 min-1)
0.100
0.23X102
To demonstrate the versatility of the method a wide number of
aliphatic, benzylic, aromatic and hetero-aromatic amines were
subjected to the novel reaction conditions (Table 1). In general,
the reaction proceeds efficiently with various amines to provide
the corresponding N-formylated product in good to excellent
yield within very short period of time. Aliphatic primary and
secondary amines underwent smooth N-formylation and gave
the product in 90-99% yields (Table 1, entry 1-9). We next
investigated the scope of N-formylation of benzyl amines. The
reaction with benzyl amines proceeded even more smoothly to
afford the desired product in 99% yield in a shorter reaction
time (Table 1, entry 10-13). We also studied the reaction on a
large scale (60 mmol) synthesis of N-[(4-methoxyphenyl)
methyl]formamide (2k), which afforded the N-formylated
product in quantitative yield (Table 1, entry 11). Anilines were
found to be low yielding under the optimized conditions.
However, when aniline was treated with neat ethyl formate
using 20% w/w lipase, gave the N-formyl product (2n) in 87%
yield (Table 1, entry 14). Aniline with electron donating group
provided excellent yield (Table 1, entry 15) whereas anilines
having electron withdrawing groups such 3-fluoroaniline or 3-
nitroaniline afforded product in moderate yield (Table 1, entry
16-17). In the same way, heteroarylamine 2-aminopyridine
provided N-(2-pyridyl)formamide (2r) in 78% yield (Table 1,
entry 18). Excellent chemo-selectivity of amino groups over
aryl-hydroxyl groups was observed with only N-formylated
product was obtained for both the molecules examined.23
(Table 1, entry 19-20). Similarly, controlled chemo-selectivity
was observed for aliphatic amines over aromatic amines in the
reaction of 4-amino benzylamine (Table 1, entry 21) to afford
Fig. 1 Lineweaver-Burk plot for N-formylation reaction of 4-
methoxybenzylamine with ethyl formate using Novozyme 435
CALB
®
Next we studied the reusability of the lipase for the N-
formylation reaction. After completion of the reaction the
lipase was recovered by filtration, washed with tetrahydrofuran
and dried under high vacuum at room temperature. Reusability
of lipase was examined up to 10 consecutive cycles, and
minimal loss of activity was observed. In all the 10 batches
starting material was fully consumed to afford desired N-
formylated product in quantitative yield, though longer reaction
times were required (Fig. 2).
Noteworthy, most of the reaction masses were purified by
simple filtration and evaporation of the volatiles to provide the
desired product in very high purity making the methodology
even more attractive.
the corresponding N-[(4-aminophenyl)methyl]formamide (2u
)
in 80% yield.
In conclusion, we have successfully demonstrated the novel
application of lipase for N-formylation of different amines using
ethyl formate, which is mild, robust, high yielding and
environmentally safe. The conditions were proven to be chemo-
selective and applicable to chiral substrates without
epimerization. The protocol is particularly advantageous for
The methodology was further extended to optically pure chiral
amines without epimerization at the stereo-genic center as
determined by chiral HPLC (Table 1, entry 24-26). However,
attempts24 for kinetic resolution of racemic amines (Table 1,
entry 22-23) under the same condition was not successful
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Preliminary kinetic parameters of one of the representative
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be useful for the large scale synthesis of N-formyl amines.
Efforts are currently underway in our research group to apply
this method to the construction of other potentially valuable
organic molecules.
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Acknowledgment
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22 General procedure for the N-formylation of amines:
Method-A: To a mixture of amine (5.0 mmol) and ethyl
formate (7.5 mmol) in THF (5.0 ml) was added Novozyme
435® CALB (Lipase acrylic resin from Candida antarctica
≥5,000 U/g, recombinant, expressed in Aspergillus niger)
(20% w/w), and the mixture was stirred at room temperature.
After complete consumption of starting material (1- 8 h), the
reaction mixture was filtered off, solid was washed with THF
and the combined organic layers was evaporated to afford the
N-formylated product in pure form.
1
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the product was isolated following the same procedure as
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