Formamide Synthesis through Borinic Acid Catalysed Transamidation under Mild Conditions

Article abstract of DOI:10.1002/chem.201600234

A highly efficient and mild transamidation of amides with amines co-catalysed by borinic acid and acetic acid has been reported. A wide range of functionalised formamides was synthesized in excellent yields, including important chiral α-amino acid derivatives, with minor racemisation being observed. Experiments suggested that the reaction rely on a cooperative catalysis involving an enhanced boron-derived Lewis acidity rather than an improved Br?nsted acidity of acetic acid. Amide bonds are reputedly difficult to activate due to their high resonance stabilization. An unusual mild activation of dimethylformamide and formamide by borinic acid 1 (see scheme), illustrated by a general formylation of a wide range of amines, including chiral α-amino esters, has been reported.

Full text of DOI:10.1002/chem.201600234

DOI: 10.1002/chem.201600234
Communication
&
Amide Synthesis
Formamide Synthesis through Borinic Acid Catalysed
Transamidation under Mild Conditions
Tharwat Mohy El Dine, David Evans, Jacques Rouden, and JØrôme Blanchet*[a]
reactions[4] and other transformations.[5] Moreover, the formyl
Abstract: A highly efficient and mild transamidation of
amides with amines co-catalysed by borinic acid and
acetic acid has been reported. A wide range of functional-
ised formamides was synthesized in excellent yields, in-
cluding important chiral a-amino acid derivatives, with
minor racemisation being observed. Experiments suggest-
ed that the reaction rely on a cooperative catalysis involv-
ing an enhanced boron-derived Lewis acidity rather than
an improved Brønsted acidity of acetic acid.
group is a useful protecting group of the amine functionality.[6]
Beside the use of formic acid and its derivatives as amine
formylating reagents,[7] recent developments focused on the
use of sub-stoichiometric amounts of a catalyst and a simpler
formyl donor, such as methanol,[8a–c] carbon monoxide[8d–e] and
CO2.[8f–i] However, these methodologies are significantly restrict-
ed to the use of expensive transition metals and high pressure
of toxic gases. Alternatively, transamidation[9] has been estab-
lished by recent formylating methodologies reported by Wil-
liams,[10a–b] Gamba-Sµnchez[10c] and other groups.[10d–f] Unfortu-
nately, despite of the advances achieved, most of the methods
require excess amounts of activating reagents, high tempera-
tures or extended reaction times to attain reasonable conver-
sions along with a limited scope. In this context, the use of
boron-based catalysts has appeared as an appealing approach
(Scheme 1).[11] Evidences on the acceleration of intramolecular
Formamides are important pharmacophore in various drugs,
such as leucovorin,[1a] formoterol[1b] and orlistat[1c] (Figure 1).
They are also used in the synthesis of valuable heterocycles,
such as quinolone[2a] or imidazole,[2b] and as precursors of iso-
cyanides[3a–c] and formamidines.[3d] Formamides are also used
as Lewis base organocatalysts in allylation and hydrosilylation
Scheme 1. Formylating transamidations promoted by boron-based catalysts.
transamidation of glutamine by sodium borate were reported
as early as 1949.[12] More recently, Sheppard reported a useful
fluorinated boronate, which is able to promote chromatogra-
phy-free synthesis of formamides.[11b] Nevertheless, in both pre-
vious reports, the boron reagent had to be used in large
excess. The first transamidation catalysed by sub-stoichiometric
amount of a boron derivative was reported by Nguyen in 2012
(10 mol% boric acid, solvent-free conditions).[11a] However, in
most of the cases, temperatures as high as 1508C were re-
quired, limiting the scope to stable achiral amine substrates.[13]
In continuation to our recent developments in the field of
boron-based catalysis aiming at developing metal-free alterna-
tives for key chemical transformations,[14] we recently reported
Figure 1. Relevant formamide-containing molecules.
[a] T. M. E. Dine, D. Evans, Prof. J. Rouden, Dr. J. Blanchet
Laboratoire de Chimie MolØculaire et Thio-organique
ENSICAEN et UniversitØ de Caen Basse-Normandie, CNRS
6 bd du MarØchal Juin, 14050 Caen (France)
E-mail: jerome.blanchet@ensicaen.fr
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201600234.
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the borinic acid 1 as an efficient catalyst capable of achieving
the challenging coupling between two non-activated amino
acids.[14b]
its higher Brønsted acidity (entry 14), thus rendering unlikely
the initial hypothesis about the improvement of Brønsted acid-
ity through the coordination of acetic acid with 1.
Herein, we report the remarkable efficiency of borinic acid
1 in catalyzing the transamidation of DMF with amines. As
a first reaction, a combination of 10 mol% of 1 with 20 mol%
of acetic acid was found to promote the unusual transamida-
tion of DMF with benzylamine at room temperature in 73%
isolated yield. Intriguingly, under such conditions, the amide
synthesis involving acetic acid was found to be completely un-
productive[15] (Table 1, entry 1).[16] Notably, the result obtained
In order to gain insight into the mechanism, the reaction
was monitored using borinic acid 4 as a fluorinated probe. Ac-
cordingly, aliquots of the reaction mixture were sequentially
examined by means of 19F NMR spectroscopy after the step-
wise addition of the reagents. After the addition of acetic acid
to borinic acid 4 in DMF, a single signal was observed at d=
À110 ppm, corresponding to pure 4. However, upon the addi-
tion of benzylamine (2a), a new signal immediately appeared
at d=À115.3 ppm, likely corresponding to the amine-borinic
acid complex I (Scheme 2) that remained as the only detecta-
ble signal during 24 h at 658C. A possible proto-deboronation
of 4 during the reaction was ruled out because 2-chloro-4-fluo-
rophenyl boronic acid 6 displayed a different signal at d=
À108.3 ppm under similar conditions.[18]
Table 1. Optimisation of the transamidation of DMF with benzylamine.
Entry
Deviation from standard conditions
Yield [%][a]
1
2
3
none
73
54
6
2-Cl-4-F-(C6H3)2B(OH) 4[b]
Ph2B(OH) 5[b]
4
5
6
7
8
9
10
11
12
13
14
2-Cl-C6H4B(OH)2 6[b]
reaction run for 72 h
reaction run at 458C
reaction run at 658C for 4 h
no 1, no AcOH
only 1
22
82
98
99
0
34
19
15
11
9
only AcOH
only BnCO2H[c]
only HCO2H[c]
only CCl3CO2H[c]
only CF3CO2H[c]
Scheme 2. Second mechanistic hypothesis for the transamidation of DMF.
1
Borinic acid 1 displayed a 11B NMR signal at d=38.7 ppm in
[D7]DMF, representative of a trivalent boron species, indicating
the absence of any detectable interaction between DMF and
the boron centre. Interestingly, upon the addition of two
equivalents of acetic acid, a new signal appeared at d=
5.5 ppm in a 1:1 ratio, consistent with a tetravalent boron
centre.[19] Any attempts to isolate this intermediate yielded
back borinic acid 1, but direct ESI-TOF mass analysis of the
mixture displayed a peak at 309.0 m/z corresponding to
[MÀH]À of 1 complexed with acetic acid (II; Scheme 2). Un-
fortunately, no species involving the activation of DMF by the
borinic acid 1 was detected. However, the observation of com-
plex II suggested a mechanism where the borinic acid 1 was
first converted to a mixed borinic–acetic anhydride possessing
a higher Lewis acidity. Species II would then activate DMF and
promote the addition of benzylamine (Scheme 2).
[a] Isolated yields. [b] Instead of 1. [c] Instead of AcOH.
under very mild conditions compared favorably with the state
of art and prompted us to further investigate this reaction in
details. The efficiency of catalyst 1 was found to be significant-
ly superior to that of borinic acids 4 and 5 (entries 2 and 3) as
well as its analogue 2-chlorophenyl boronic acid 6 (entry 4).
After optimisation, a gentle warming to 458C provided a quan-
titative yield, whereas the duration of the reaction could be
decreased to 4 h upon warming to 658C (entries 6 and 7). In
the course of optimising the different reaction parameters, sev-
eral test experiments were carried out and suggested a syner-
getic mechanism between the borinic acid 1 and acetic acid.
In the absence of both acids, no conversion was observed
(entry 8). However, the presence of borinic acid 1 alone result-
ed in a low yield of 34%, whereas acetic acid alone led to an
even further decrease in the yield. (entries 9–10). These results
suggested a cooperation between the two acids and that
a Lewis acid-assisted Brønsted acid (LBA) catalytic system
could be at play.[17] However, upon examining the reactivity of
different Brønsted acids (entries 11–14), the strongest trifluoro-
acetic acid afforded a barely detectable conversion despite of
With the optimal conditions in hand, the scope of DMF
transamidation was further examined, and it appeared that
borinic acid 1 was efficiently able to formylate a wide range of
amines. The formylation of various primary amines was ach-
ieved at room temperature with good to excellent 77-99%
yields (Table 2, entries 1-5). It is worth noting that only five
equivalents of DMF were found to be sufficient for achieving
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Table 2. Scope of the transamidation of DMF with different amines.[a]
Entry
Formamide
T
[8C]
t [h]
Yield
[%][b]
Entry
Formamide
T [8C]
t
[h]
Yield
[%][b]
1
2
3a
3b
20
20
72
72
86[c] (99)[d]
77[c]
10
11
3j
65
65
12
12
99
3k
96[f]
3
4
3c
20
20
72
72
86[c]
12
13
3l
65
65
12
12
97[f]
99[f]
3d
98 (90)[e]
3m
5
6
3e
3 f
20
65
72
12
99
99
14
15
3n
3o
65
65
12
24
98[f]
99[f]
7
8
3g
3h
65
65
12
24
99
89
16
17
3p
3q
65
65
12
12
99[f]
98[f]
9
3i
65
24
96
18
3r
85
24
26[f,g]
[a] Reaction conditions: Borinic acid 1 (10 mol%), acetic acid (20 mol%), DMF (5 equiv). [b] Isolated yields. [c] Quantitative yield at 658C. [d] Run on 5 mmol
scale at 658C. [e] Run on 5 mmol scale at 208C. [f] DMF was used as solvent. [g] NMR conversion.
room temperature formylation of primary amines functional-
ised with a pyridine, indole, unprotected alcohol or methyl thi-
oether (entries 2–5). More difficult substrates required warming
to 658C to attain acceptable reaction rates. At this tempera-
ture, a-substituted primary and secondary amines gave 89–
99% yields within 12–24 h (entries 6–10). In some occasions,
the starting amine was found to be poorly soluble in the reac-
tion media, resulting in the absence of conversion. However, in
such cases, dilution with DMF led to higher yields. Thus, piper-
azine, cyclohexylamine and other amine substrates afforded
excellent 96–99% yields with DMF being used in excess
amounts (entries 11–17). Among the tested primary amines,
the hindered tert-butylamine was the only to behave sluggish-
ly, thus illustrating that steric hindrance played a significant
role during the reaction (entry 18).
quired a considerable increase in the reaction temperature (up
to 858C) to achieve yields of 78–99% (not reported in this
paper).[21] However, significant racemisation was recorded as
formamides 8b, 8c and 8e were obtained with enantiomeric
excesses of 25, 47 and 80%, respectively. Interestingly, the
methyl glutamate diester provided the macrolactam 9 in 91%
yield instead of the expected formamide 8i, suggesting an un-
expected activation of the side chain involving the ester
moiety (Scheme 3).[22]
Next, our attention was focused on the formylation of the
more challenging a-amino esters. Indeed, this class of func-
tionalised and racemisable substrates is generally less investi-
gated with no precedent for a boron-derived catalysis so far
reported for the synthesis of N-formyl-a-amino ester forma-
mides.[20] As anticipated, glycine methyl ester reacted slowly at
room temperature but afforded a quantitative yield at 658C.
Encouraged by this result, several substituted chiral a-amino
esters were tested. However, less reactive derivatives of ala-
nine, phenylalanine, leucine, valine, methionine and proline re-
Scheme 3. Synthesis of lactam 9 from the aspartic acid derivative.
Consequently, milder conditions were sought and optimised
to decrease racemisation during the transamidation process.
Formamide (HCONH2) was identified[23] as a useful alternative
to DMF allowing transamidation at a lower temperature (458C)
and using a lower catalyst loading (2.5 mol% of 1).[24] Upon
using the new set of conditions, the reactions were noticeably
faster with yields generally superior to 91% (Table 3, entries 1–
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Table 3. Scope of the transamidation of HCONH2 with a-aminoesters.[a]
Scheme 4. One-pot synthesis of isocyanide 10 from benzylamine.
cyanides rapidly decomposed invariably upon different purifi-
cation attempts.
Entry
1
Formamide
Yield [%][b]
99
8a
8b
In conclusion, a new method for the N-formylation of
amines using catalytic amounts of borinic acid 1 and acetic
acid was reported. A short mechanistic study pointed towards
a cooperative involvement of both species and a reactivity
based on an improved Lewis acidity of the boron centre. More
specifically, the N-formylation of amines was examined with
a wide range of primary and secondary amines as well as func-
tionalised a-amino esters. In the case of chiral substrates, the
observed racemisation led to a switch of the formyl donor
from DMF to HCONH2 in order to keep racemisation at a low
level. Additionally, our catalytic system was successfully ex-
tended to one-pot synthesis of isocyanides from primary
amines.
2
3
99
98
8c
4
8d
97
5
6
7
8e
8 f
8g
96 (97)[c]
91
99
Experimental Section
8
9
8h
8i
99
57
General Procedure for the N-formylation of Amines using
DMF
To a round-bottom flask kept under an argon atmosphere were
added 2-chlorophenylborinic acid
1
(12.5 mg, 0.05 mmol,
[a] Reaction conditions: Borinic acid 1 (2.5 mol%), acetic acid (10 mol%),
HCONH2 (5 equiv). [b] Isolated yields. [c] Run on 5 mmol scale (with
656 mg of 7e).
0.1 equiv), acetic acid (6 mL, 6 mg, 0.10 mmol, 0.2 equiv) and dry
DMF (0.19 mL or 7 mL as indicated). The mixture was vigorously
stirred for 15 min at 258C, 658C or 858C, as indicated, and the
amine (0.50 mmol, 1 equiv) was then slowly added using a gastight
syringe. The resulting mixture was stirred for a further 12–72 h, as
reported in Table 2. DMF was removed using Kugelrohr distillation
apparatus (558C) and the crude mixture was further purified by
column chromatography to give the corresponding title com-
pounds. For the amines with low boiling points, the reaction was
conducted in a sealed tube.
8). It is worth mentioning that these milder conditions strongly
limited the racemization, with enantiomeric excesses of 94, 92
and 99% obtained for formamides 8b, 8c and 8e, respectively
(determined by chiral HPLC, see the Supporting Information).
Additionally, racemisation in the case of formamides 8 f and 8i
was notably reduced (ee values >90%). Remarkably, this
method also afforded a quantitative yield of the acid sensitive
Boc-protected lysine 8h (entry 8). Additionally, the methyl glu-
tamate diester led chemoselectively to the desired formamide
8i; however, in a moderate yield of 57% with no traces of the
cyclic dimer 9 detected by TLC or NMR (entry 9).
Finally, our study was completed by taking advantage of the
mildness of our catalytic conditions to develop a one-pot pro-
cedure for the direct preparation of isocyanide 10 from benzyl-
amine.[25] Among the various possible reagents that are able to
dehydrate a formamide, the Burgess reagent[26] was foreseen
as being the most compatible upon using DMF as the solvent.
Indeed, when Burgess reagent and triethylamine were added
after completion of the transamidation step of DMF with ben-
zylamine and further reacted for 48 h, the corresponding
benzyl isocyanide 10 was isolated in 65% yield (Scheme 4).
Furthermore, when other amines were tested (a-methylbenzyl-
amine, decylamine and cyclohexylamine), high conversions
were observed by NMR. Unfortunately, the corresponding iso-
General Procedure for the N-formylation of Amines using
HCONH2
To a sealed tube were added 2-chlorophenylborinic acid 1 (3 mg,
0.0125 mmol, 0.025 equiv), HCONH2 (0.10 mL, 2.5 mmol, 5 equiv)
and acetic acid (3 mL, 3 mg, 0.05 mmol, 0.1 equiv). The mixture was
vigorously stirred for 15 min at 458C and a-amino methyl ester
amine (0.50 mmol, 1 equiv) was then slowly added using a gastight
syringe. The resulting mixture was stirred for a further 24 h. The
crude was purified using column chromatography to yield the cor-
responding title compounds.
Procedure for the One-Pot Synthesis of Isocyanide from
Benzylamine
To a round-bottom flask kept under an argon atmosphere were
added 2-chlorophenylborinic acid
0.1 equiv), acetic acid (6 mL, 6 mg, 0.10 mmol, 0.2 equiv) and dry
DMF (7 mL) at 658C. The mixture was vigorously stirred for 15 min
and benzylamine (0.50 mmol, 1 equiv) was then slowly added
using a gastight 100 mL syringe. The resulting mixture was stirred
1
(12.5 mg, 0.05 mmol,
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J. Zhu, S. H. Gellman, S. S. Stahl, J. Am. Chem. Soc. 2009, 131, 10003–
10008; c) M. Zhang, S. Imm, S. Bahn, L. Neubert, H. Neumann, M. Beller,
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3971–3975. For a recent review: d) R. M. Lanigan, T. D. Sheppard, Eur. J.
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for 12 h at 658C. After 12 h, Burgess reagent (360 mg, 1.5 mmol,
3 equiv), triethylamine (418 mL, 3 mmol, 6 equiv) and 5 Š powdered
molecular sieves (1 g) were added and stirring was maintained for
further 48 h. The reaction was quenched with water and extracted
five times with Et2O, washed with brine, dried over anhydrous
MgSO4 and concentrated under vacuum. Crude isocyanide 10 was
purified by column chromatography.
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Acknowledgements
We gratefully acknowledge ANR “NeoACAT” (ANR-JC-12) for
a fellowship to T.M., Labex SynOrg (ANR-11-LABX-0029) and
rØgion Normandie for financial support.
[11] a) T. B. Nguyen, J. Sorres, M. Q. Tran, L. Ermolenko, A. Al-Mourabit, Org.
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Keywords: amides
methods · transamidation
· amines · boronic acid · synthetic
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[16] For further details on optimisation, refer to the Supporting Information.
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and DMF at 858C for 24 h.
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Received: January 18, 2016
Published online on March 15, 2016
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