Borinic acid catalysed peptide synthesis

Article abstract of DOI:10.1039/c5cc06177f

The catalytic synthesis of peptides is a major challenge in the modern organic chemistry hindered by the well-established use of stoichiometric coupling reagents. Herein, we describe for the first time that borinic acid is able to catalyse this reaction under mild conditions with an improved activity compared to our recently developed thiophene-based boronic acid. This catalyst is particularly efficient for peptide bond synthesis affording dipeptides in good yields without detectable racemization.

Full text of DOI:10.1039/c5cc06177f

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: T. Mohy El Dine,
J. H. Rouden and J. Blanchet, Chem. Commun., 2015, DOI: 10.1039/C5CC06177F.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/chemcomm

Please do not adjust margins
ChemComm
Page 1 of 4
View Article Online
DOI: 10.1039/C5CC06177F
Chemical Communications
COMMUNICATION
Borinic acid Catalysed Peptide Synthesis
Tharwat Mohy El Dine, Jacques Rouden, and Jérôme Blanchet*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The catalytic synthesis of peptides is a major challenge of modern of non-activated carboxylic acids and amines. Regarding peptide
organic chemistry hindered by the well established use of synthesis, 25 mol% of boronic acid 5 was found to promote the
stoichiometric coupling reagents. Herein, we describe the first formation of Boc-Phe-Val-OMe in 50% yield with no detectable
borinic acid able to catalyse this reaction under mild conditions racemization, thus achieving the first synthesis of a dipeptide using
with an improved activity compared to our recently developed a simple chemical catalyst.
thiophene-based boronic acid. This catalyst is particularly efficient Previously, the coupling of protected α-amino acids has been
for peptide bond synthesis affording dipeptides in good yields explored by Whiting8 using stoichiometric quantities of
a
without detectable racemization.
combination of aryl boronic acids 1-2 (Figure 2). However, low to
moderate yields of the corresponding dipeptides were obtained.
Interestingly, a cooperative effect of combined catalyst systems was
suggested to be beneficial when 50 mol% of 1a and 50 mol% of 1b
were used together for the coupling of hindered valine derivatives.9
The synthesis of peptides via the assembly of chiral N-protected α-
amino acids and α-aminoesters is a well-known methodology.1 This
essential transformation could be easily realized with high level of
predictability owing to the progress made with the use of
stoichiometric peptide coupling reagents (Figure 1).2
B(OH)2
F
F
B(OH)2
B(OH)2
R
R
B(OH)2
I
B(OH)2
O
O
S
F
Me2
N
NC
O
NMe2
PF6
OEt
CH3
N
N
1a R = Me
1b R =NO2
2
3a R = H
3b R =OMe
4
5
Cl
N C N
N
Me
N
O
N
N
N
Me2HN
PF6
O
Me
Figure 2. Aryl boronic acids investigated by Whiting, Hall, Tam and Blanchet.
EDCI
HATU
COMU
Although peptides are of great value to pharmaceutical industry,
their catalysed synthesis is still unknown. Therefore, the next
challenge lies in identifying an efficient catalyst that would directly
couple N-protected and C-protected α-amino acids to provide the
desired dipeptides in reasonable yields without racemization.
Since boronic acid 5 has shown moderate activity in peptide
coupling, we decided to shift to a potentially more reactive boron
derivative. We hypothesized that the use of borinic acids, which
possess an amplified Lewis acidity at the boron centre10, can
provide an improved catalytic system. Additionally, they offer an
improved structural flexibility for further tuning of the electronic
and steric properties with two aromatic moieties being attached to
the same boron atom. In this context, we report the use of borinic
acids for catalysed peptide synthesis.
Figure 1. Representative coupling reagents in amide and peptide synthesis.
Despite those great accomplishments, a catalyst that would be able
to achieve similar efficiency is still to be discovered. Albeit the
catalytic synthesis of amides from simple carboxylic acids and
amines has been reviewed several times3, almost no information
concerning the assembly of two α-amino acids derivatives is
available. Compared to simple carboxylic acids and amines,
protected α-amino acids are challenging substrates, due to their
weaker nucleophilic character of the amine function and increased
steric hindrance caused by the side chain.
Inspired by Yamamoto’s pioneering work4, Hall5 (3a-b, Figure 2),
Tam6 (4, Figure 2) and our laboratory (5, Figure 2)7 have recently
reported boronic acids able to promote room temperature coupling
We started our investigations by synthesizing a library of biaryl
borinic acids according to known procedures (See Supporting
Information). The reactivity of these borinic acids was compared to
a set of commercially available boronic acids in the model reaction
carried out between phenylacetic acid 6 and benzylamine 7 (Table
1). Interestingly, higher yields were systematically obtained with
This journal is © The Royal Society of Chemistry 2015
Chemical Communications, 2015, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 2 of 4
View Article Online
DOI: 10.1039/C5CC06177F
COMMUNICATION
Journal Name
the borinic acids. With electron donating group substituted borinic
acids 9c (4-t-Bu), 9d (4-MeO) and 9e (3-MeO) 39-59% isolated yields
were obtained when the corresponding boronic acids were found to
be essentially unreactive (Table 1, Entries 3 and 4).
Entry
Ar2B(OH)
Ar
Isolated Yields (%)
1
2
3
9f
9g
9h
63
81
Table 1. Borinic acids vs boronic acids in the coupling of acid 6 and amine
7.a
99 (99b)d
4
5
6
9i
9j
76
46
Yield with
ArB(OH)2 (%)
Yield with
Ar2B(OH) (%)
Entry
Ar
Ar2B(OH)
9k
98 (99b)d
1
2
9a
9b
13
5
19
15
7
8
9l
80
37
3
4
9c
0
4
59
45
9m
9d
MeO
5
9e
5
39
9
9n
9o
18
a
Reaction conditions: Phenylacetic acid 6 (0.55 mmol), benzylamine 7 (0.50
mmol) and molecular sieves (1g of 5 Å activated powder) were stirred in dry
CH2Cl2 (7 mL) with borinic acids 9a-e or boronic acid (10 mol%) under inert
atmosphere for 48h at room temperature.
10
99 (97c)d
a
Reaction conditions: Phenyl acetic acid 6 (0.55 mmol), benzylamine 7 (0.50
mmol) and molecular sieves (1 g of 5 Å activated powder) were stirred in dry
CH2Cl2 (7 mL) with borinic acids 9f-o (10 mol%) under inert atmosphere for 48h at
room temperature. b 1H NMR conversion after 6 hours. c 1H NMR conversion after
4 hours. d Conversions calculated using 1,3,5-trimethoxybenzene (10 mol%) as an
internal standard.
Encouraged by these results, the screening of various aryl borinic
acids was undertaken. Based on Hall’s previous work,5 we focused
our attention on halogen substituted borinic acids.
The presence of a fluorine atom at the ortho position in 9f
decreased the yield to 63% compared to the 81% yield obtained
with the fluorine at the para position 9g (Table 2, Entries 1 and 2). A
different picture was observed with the introduction of a chlorine
atom. Indeed, the 2-chloro substituted 9h was found to be very
effective delivering a quantitative yield of the amide 8 (Table 2,
Entry 3) and suggesting that increased electronegativity at the ortho
position in 9f lowered the reactivity of the catalyst. The importance
of the ortho-chloro substituent was shown by the lower yields (76%
and 46% respectively) obtained with the meta 9i and para 9j
substituents (Table 2, Entries 4 and 5). 2-Bromo substituted 9k led
to a similar reactivity as 9h (Table 2, Entry 6), however, with a much
less pronounced availability.11 These results suggest the need for a
specific electronic requirement in the ortho position for an optimal
catalytic performance.
However, in our hands, catalyst 9l was less effective affording
amide 8 with 80% yield (Table 2, Entry 7). Substituting the methoxy
group with a chlorine or fluorine atom significantly decreased the
catalyst performance (9m and 9n, 37% and 18% yields respectively,
Table 2, Entries 8 and 9). Interestingly, switching the fluorine
position para to the boron atom in 9o enhanced the catalytic
activity with 97% conversion being attained within 4 hours (99%
yield, Table 2, Entry 10) compared to 99% conversion attained
within 6 h with catalyst 9h (99% yield, Table 2, Entry 3). These
results illustrate the importance of fine-tuning the electronic
properties of the catalyst in terms of electron-density of the ortho-
halogen and boron Lewis acid property.
Having identified 9o as a useful catalyst, we monitored the reaction
using 19F-NMR to collect some insights regarding the mechanism of
the reaction. Indeed, it was important to rule out any boronic acid
Hall and co-workers have reported a dramatic improvement of the
performance of boronic acid 3a (Figure 2) when an electron
donating group was introduced para to the halogen atom. In their
study, the 5-methoxy substituted 3b was identified to provide
faster reactions thus supporting the role of the ortho iodo
substituent as a hydrogen-bond acceptor in the proposed transition
state.5a Based on these results, we prepared the 5-methoxy
substituted borinic acid 9l.
catalysis consecutive to
a side protodeboronation reaction
facilitated by the increased Lewis acidity of borinic acids.12
Table 2. Aryl borinic acid screening.a
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 3 of 4
View Article Online
DOI: 10.1039/C5CC06177F
Journal Name
COMMUNICATION
Ph
O
CO2Me
NH2
10d
catalyst
Ph
CO2
H
Ph
+
Ph
N
H
CO2Me
5 Å mol. sieves
PhF, 65 °C, 36 h
6
11
9h
9h 25 mol%
25 mol%
59% yield
96% yield ee > 99%
94% yield ee 94%
10 mol%
5
Scheme 1. Coupling between phenyl acetic acid 6 and phenylalanine methyl ester 10.
Amide 11 was previously reported by Whiting for its propensity to
provoke racemization, providing an enantiomeric excess of 68%
under similar reaction conditions with catalysts 1a and 1b.8 This
result was previously improved using our catalyst 5 (ee: 94%).7
Interestingly, 9h afforded the desired amide 11 in 96% yield with
the complete retention of enantiopurity (ee > 99%, Scheme 1). First
attempts of coupling two protected α-amino acids at room
temperature resulted in the total absence of conversion. However
using previously optimized conditions (Scheme 1), Boc-protected
phenylalanine (Boc-Phe) was efficiently coupled with various methyl
α-amino esters derived from hindered Valine, Alanine, Glycine and
phenylalanine in average 47-61% yields (Table 3, Entries 1-4).
Similarly, Z-proline (Z-Pro) was coupled with various α-amino esters
(Table 3, Entries 5-11) in 47-73% yields. While the modification of
the nature of the ester group led to marginal change when valine α-
amino ester was used (Table 3, Entries 7-8), a higher yield was
obtained when an ethyl glycine ester (H-Gly-OEt) was used
compared to benzyl or methyl counterparts (Table 3, Entries 9-11).
Highest 80-72% yields were also obtained with the less hindered
Boc-glycine (Boc-Gly-H) as the nucleophilic component of the
reaction (Table 3, Entries 12-13) while Z-methionine (Z-Met) led to a
non-optimized 40% yield (Table 3, Entry 14). Careful examination of
the 1H NMR data of all the synthesized dipeptides incorporating two
chiral centres showed that a single diastereomer was obtained.
Figure 3. Monitoring the catalyst 9o behaviour using 19F probe (375 MHz; CDCl3; 25 ᵒC).
A: isolated 2-Cl-4-F-C6H4-B(OH)2. B: isolated 9o. C: NMR monitoring of the reaction
corresponding to entry 10, table 2, after 6h (97% conversion).
During the whole course of the reaction, a single signal at -114.5
ppm in 19F NMR (Figure 3) was observed, with no intermediates
being detected. This signal matched the isolated 9o, while the
corresponding boronic acid was measured at -106.7 ppm,
suggesting the stability of the catalyst 9o during the reaction.
While 9o was found to be slightly more reactive than 9h, the
combined stability and higher availability of 9h led us to select it as
the optimal catalyst for the further investigations.
Before exploring dipeptide synthesis, a control reaction was carried
out between phenyl acetic acid 6 and phenylalanine methyl ester
10d to ascertain the absence of racemization (Scheme 1). The use
of less reactive α-amino ester 10d required the use of 25 mol% of
catalyst 9h and a switch to a different low polarity solvent and a
higher temperature (fluorobenzene, 65 °C).
Table 3. Scope of the reaction: peptide synthesis.a
O
R2
R2
H
H
9h (25 mol%)
CO2
H
+
N
N
R1
R1
N
H
CO2R2
H2N
CO2R2
5 Å mol. sieves
PhF (0.068 M), 65°C, 36-48 h
R1
R1
12
10
13
Entry
1
2
3
4
5
6
7
8
α-Amino-acid 12
Boc-Phe-H
Boc-Phe-H
Boc-Phe-H
Boc-Phe-H
Z-Pro-H
α-Amino-ester 10
H-Val-OMe
H-Ala-OMe
H-Gly-OMe
H-Phe-OMe
H-Phe-OMe
H-Leu-OMe
H-Val-OMe
H-Val-OBn
Dipeptide 13
Yield (%)
51
47
55
61
60
60
58
59
73
47
51
80
72
40
12a
12a
12a
12a
12b
12b
12b
12b
12b
12b
12b
12c
12c
12d
10a
10b
10c
10d
10d
10e
10a
10f
10g
10h
10c
10h
10d
10d
Boc-Phe-Val-OMe
Boc-Phe-Ala-OMe
Boc-Phe-Gly-OMe
Boc-Phe-Phe-OMe
Z-Pro-Phe-OMe
Z-Pro-Leu-OMe
Z-Pro-Val-OMe
Z-Pro-Val-OBn
Z-Pro-Gly-OEt
Z-Pro-Gly-OBn
Z-Pro-Gly-OMe
Boc-Gly-Gly-OBn
Boc-Gly-Phe-OMe
Z-Met-Phe-OMe
13a
13b
13c
13d
13e
13f
13g
13h
13i
13j
13k
13l
13m
13n
Z-Pro-H
Z-Pro-H
Z-Pro-H
Z-Pro-H
Z-Pro-H
Z-Pro-H
Boc-Gly-H
Boc-Gly-H
Z-Met-H
9
H-Gly-OEt
10
11
12
13
14
H-Gly-OBn
H-Gly-OMe
H-Gly-OBn
H-Phe-OMe
H-Phe-OMe
a Reaction conditions: N-protected α-amino acid 12 (0.46 mmol), α-aminoester 10 (0.46 mmol) and molecular sieves (1 g of 5 Å activated powder) were stirred in dry
PhF (6.8 mL)with the borinic acid catalyst 9h (25 mol%) under inert atmosphere for 32-48h at 65 ᵒC.
Based on Hall’s mechanism with boronic acids, we rationalized that boron anhydride A (Figure 4). This step is greatly facilitated by the
the N-protected α-amino acid is activated in the form of the mixed increased Lewis acidity of the borinic acid 9h. The next step involves
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 4 of 4
View Article Online
DOI: 10.1039/C5CC06177F
COMMUNICATION
Journal Name
Albericio, Chem. Rev., 2011, 111, 6557. (c) T. I. Al-Warhi, H.
M. A. Al-Hazimi and A. El-Faham, J. Saudi. Chem. Soc., 2012,
the additions of the α-amino ester to generate a tetravalent
intermediate B. The collapse of B is then assisted by water and the
Lewis basicity of ortho chlorine substituents to yield eventually the
desired dipeptide.
16, 97. (d) S. C. Stolzew and M. Kaiser, Synthesis 2012, 44
,
1755. (e) V. R Pattabiraman and J. Bode, W. Nature 2011,
480, 471. (f) V. Mäde, S. Els-Heindl and A. G. Beck-Sickinge,
Beilstein J. Org. Chem. 2014, 10, 1197.
2
3
(a) E. Valeur and M. Bradley, Chem. Soc. Rev., 2009, 38, 606.
(b) C. Montalbetti and V. Falque, Tetrahedron, 2005, 61,
10827–10852. (c) K. Ishihara, Tetrahedron 2009, 65, 1085.
For recent reviews about catalytic amide formation, see (a)
H. Lundberg, F. Tinnis, N. Selander and H. Adolfsson, Chem.
Soc. Rev., 2014, 43, 2714. (b) R. M. Lanigan and T. D.
Sheppard, Eur. J. Org. Chem., 2013, 7453. (c) K. Arnold, B.
Davies, R. L. Giles, C. Grosjean, G. E. Smith and A. Whiting,
Adv. Synth. Catal., 2006, 348, 813.
4
(a) K. Ishihara, S. Ohara and H. Yamamoto, J. Org. Chem.,
1996, 61, 4196. See also (b) T. Maki, K. Ishihara and H.
Yamamoto, Tetrahedron, 2007, 63, 8645. For other recent
catalysed amide bond formation see: (c) D. C. Lenstra, F. P. J.
T. Rutjes and J. Mecinović, Chem. Commun., 2014, 50, 5763.
Figure 4. Proposed mechanism for the amide synthesis catalysed by the borinic acid 9h.
(d) H. Lundberg and H. Adolfsson, ACS Catal., 2015, 5, 3271.
To clarify if both chlorine atoms are involved, the unsymmetrical
borinic acid 9p was prepared and tested (Scheme 2). With this
catalyst bearing a single chlorine atom, the yield was reduced from
99% with 9h (Table 2) to 27%.
(e) Y. Terada, N. Ieda, K. Komura and Y. Sugi, Synthesis, 2008,
2318. (f) N. Caldwell, C. Jamieson, I. Simpson and A. J. B.
Watsona, Chem. Commun., 2015, 51, 9495. (g) J. L. Vrijdag, F.
Delgado, N. Alonso, W. M. De Borggraeve, N. Pe´rez-Maciasb
and Jesus Alca´zar, Chem. Commun., 2014, 50, 15094. (h) M.
Tamura, D. Murase and K. Komura, Synthesis 2015, 47, 769.
(i) S. K. Mangawa, S. K. Bagh, K. Sharma and S. K. Awasthi,
Tetrahedron. Lett. 2015, 56, 1960. (j) L. Gu, J. Lim, J. L.
Cheong, S. S. Lee, Chem. Commun. 2014, 50, 7017.
(a) N. Gernigon, R. M. Al-Zoubi and D. G. Hall, J. Org. Chem.,
2012, 77, 8386. (b) R. M. Al-Zoubi, O. Marion and D. G. Hall,
Angew. Chem. Int. Ed., 2008, 47, 2876.
5
Scheme 2. Reactivity of borinic acid 9p in the model reaction between phenylacetic
acid 6 and benzylamine 7.
6
7
8
9
E. K. W. Tam, Rita, L. Y. Liu and A. Chen, Eur. J. Org. Chem.,
2015, 1100.
T. Mohy El Dine, W. Erb, Y. Berhault, J. Rouden and J.
Blanchet, J. Org. Chem., 2015, 80, 4532.
S. Liu, Y. Yang, X. Liu, F. K. Ferdousi, A. S. Batsanov and A.
Whiting, Eur. J. Org. Chem., 2013, 5692.
The cooperative catalytic effect is still unclear but allowed to
obtain Boc-Phe-Val-OMe in 55% yield in the presence of 50
mol% of 1 and 50 mol% of 2 while 100 mol% of 2 delivered
Accordingly, the specific reactivity of 9h and 9k tends to indicate
that both halogen atoms are probably involved in the rate-
determining step and that the intermediate B is formed rather than
B’ (Figure 4).
Indeed, mild hydrogen-halogen bondings might stabilize the
transition state and favour the collapsing of B. Additionally we
suggest that the formation of anhydride A is pre-organized by
similar hydrogen-halogen bonding.
<2% yield.
10 (a) K. Ishihara, H. Kurihara and H. Yamamoto, J. Org. Chem.,
1997, 62, 5664. (b) D. Lee and M. S. Taylor, J. Am. Chem.
Soc., 2011, 133, 3724. (c) T. Soeta, Y. Kojima, Y. Ukaji and K.
Inomata, Tetrahedron Letters, 2011, 52, 2557. (d) M. G.
To summarize, we have uncovered the unique reactivity of borinic
acids for amide and peptide syntheses. Fourteen dipeptides were
prepared in synthetically useful yields without detectable
racemization. Importantly, no stoichiometric peptide coupling
reagents were used and thus minimal waste was generated with
water being the sole side product of the reaction. However, there is
still room for improvements; progress is necessary to match the
requirements of solid phase peptide synthesis such as very high
yields and short reaction times. We believe that those results will
shed light on new strategies aiming at developing the catalysed
peptide synthesis, which exhibits an utmost importance for the
pharmaceutical industry.
Chudzinski, Y. Chi and M. S. Taylor, Aust. J. Chem., 2011, 64
1466. (e) H. Zheng, M. Lejkowski and D. G. Hall, Chem. Sci.,
,
2011,
2013,
2
3
, 1305. (f) E. Dimitrijević and M. S. Taylor, ACS Catal.,
, 945. (g) D. Lee, C. L. Williamson, L. Chan and M. S.
Taylor, J. Am. Chem. Soc., 2012, 134, 8260. (h) Y. Mori, J.
Kobayashi, K. Manabe and S. Kobayashi, Tetrahedron, 2002,
58, 8263.
11 The synthesis of 9k is remarkably low yielding. See
supporting information for details.
12 (a) C.-Y. Lee, S.-J. Ahn and C.-H. Cheon, J. Org. Chem., 2013,
78, 12154. (b) G. Noonan and A. G. Leach, Org. Biomol.
Chem., 2015, 13, 2555.
We gratefully acknowledge ANR "NeoACAT" (JC 2012 program) for a
fellowship to TM and région Basse-Normandie for financial
supports.
Notes and references
1
For recent reviews about peptide synthesis, see (a) S. B. H.
Kent, Chem. Soc. Rev. 2009, 38, 338. (b) A. El-Faham and F.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins