Tris(2-carboxyethyl)phosphine promotes hydrolysis of iminoboronates

Article abstract of DOI:10.1016/j.tetlet.2017.06.069

Iminoboronates are stable and formed fast. Their B[sbnd]N bonds could be reverted by some endogenous biological molecules. The reversible characteristic attracts significant attention in biological and chemical fields. Although synthesis of iminoboronates is well-studied, less efforts have been devoted to disconnecting the units. Here, a series of selected compounds were screened to evaluate their hydrolytic capability of iminoboronates by ¹H NMR or ¹¹B NMR detection. Tris(2-carboxyethyl)phosphine (TCEP), was emerged as an excellent reagent, which decomposed most iminoboronates in short time with high yields. In addition, TCEP is also able to hydrolyze hydrazones and oximes with moderate yields.

Full text of DOI:10.1016/j.tetlet.2017.06.069

Accepted Manuscript
Tris(2-carboxyethyl)phosphine Promotes Hydrolysis of Iminoboronates
Xiaoyu Liu, Zhihong Li, Hongtao Xu, Yuexiong Zhan, Peixiang Ma, Hongli
Chen, Biao Jiang
PII:
S0040-4039(17)30808-0
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.06.069
TETL 49061
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
14 May 2017
21 June 2017
23 June 2017
Please cite this article as: Liu, X., Li, Z., Xu, H., Zhan, Y., Ma, P., Chen, H., Jiang, B., Tris(2-carboxyethyl)phosphine
Promotes Hydrolysis of Iminoboronates, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.
2017.06.069
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Tris(2-carboxyethyl)phosphine promotes
hydrolysis of imonoboronates
Xiaoyu Liu, Zhihong Li, Hongtao Xu, Yuexiong Zhan, Peixiang Ma,* Hongli Chen,* and Biao Jiang*

1
Tetrahedron Letters
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Tris(2-carboxyethyl)phosphine Promotes Hydrolysis of Iminoboronates
Xiaoyu Liu, Zhihong Li, Hongtao Xu, Yuexiong Zhan, Peixiang Ma,* Hongli Chen,* and Biao Jiang*
Shanghai Institute for Advanced Immunochemical Studies, ShanghaiTech University, Shanghai, 201210, P.R.China.
ARTICLE INFO
ABSTRACT
Article history:
Received
Iminoboronates are stable and formed fast. Their B-N bonds could be reverted by some
endogenous biological molecules. The reversible characteristic attracts significant attention in
biological and chemical fields. Although synthesis of iminoboronates is well-studied, less efforts
have been devoted to disconnecting the units. Here, a series of selected compounds were
screened to evaluate their hydrolytic capability of iminoboronates by 1H NMR or 11B NMR
detection. Tris(2-carboxyethyl)phosphine (TCEP), was emerged as an excellent reagent, which
decomposed most iminoboronates in short time with high yields. In addition, TCEP is also able
to hydrolyze hydrazones and oximes with moderate yields.
Received in revised form
Accepted
Available online
ywords:
Iminoboronates
TCEP
Reversible reaction
Efficient disconnection
2009 Elsevier Ltd. All rights reserved.
1. Introduction
HO
OH
HO
OH
R1
R
B
O
B
N
R1
stability
reversibility
Iminoboronate conjugations are stable and could be reverted
by some endogenous biological molecules. Such decent
characteristics have garnered significant attentions in chemical
and biological fields. Iminoboronates are simply formed through
the reaction with 2-formylbenzeneboronic acid or 2-
acetylbenzeneboronic acid to an amine group (Scheme 1).1 Due
to the unique B–N bond properties,2 iminoboronates have been
H2N
+
R
iminoboronates
Scheme 1 The formation of iminoboronates
2. Results and discussion
GSH showed the best performance to hydrolyze the
iminoboronates according to the references3. Its strong
nucleophilicity was presumed to play an important role in
iminoboronate disconnection. Following this hypothesis, a series
of mercaptophenyl nucleophilic compounds were selected to
investigate their potential in the hydrolysis of iminoboronate.
Taking GSH as reference, the nucleophilic compounds included
4-mercaptobenzoic acid (1d), 4-mercaptophenylacetic acid (1e)
and 4-mercaptophenol (1f) as well as TCEP (1g) (Figure 1), a
versatile and useful phosphorus nucleophile with broad
application in reduction of disulfide bonds through nucleophilic
extensively exploited to efficiently modify proteins at the amine
3-7
groups,
amino thiol,
and assist the formation of thiazolidinone on the
which
8-10 11-13
oxime and hydrazone condensations,
are applied to bio-orthogonal reactions.14 Iminoboronate as a
useful tool in the enantiomeric field has also been studied. 15, 16
Many applications do not only require stable linker for
coupling molecular units, but also efficient disconnection to
17, 18
revert the modification on the target molecules.
Although
iminoboronates demonstrate great stability, it could be cleaved by
some endogenous molecules, such as glutathione (GSH) (1a),
dopamine (1b) and fructose (1c) (Figure 1).3 Saccharide and
dopamine facilitate the hydrolysis of boronic acid imines
probably due to their binding to boric acid moiety and formation
of boronate esters. 3, 19-22 Nevertheless, all the methods could not
decompose the iminoboronate completely and 7% of
iminoboronate are still uncleaved after 1 day under the best
performed reagent, GSH, in the previous study.3 In the reversible
reaction, the efficiencies of both the connection and
disconnection processes are vital for the reaction yields. In this
work, we screened a series of compounds and their mixtures with
fructose to evaluate their hydrolytic capabilities compared to
GSH. Tris (2-carboxyethyl) phosphine (TCEP), was emerged as
an excellent reagent, which decomposed many iminoboronates in
short time with remarkable yields.
24
substitution (SN2) reactions.23, Furthermore, to evaluate the
effect of saccharide, mixtures of nucleophilic reagent and
fructose were also employed to cleave the iminoboronates.

Tetrahedron Letters
2
Figure 1 The reported (1a-1c) and screened reagents (1d-1g) to
facilitate the hydrolysis of iminoboronates
The iminoboronate 2 prepared from 2-acetylbenzeneboronic
acid and benzylamine was engaged as a model substrate for
hydrolysis analysis (Scheme 2). The synthesis of iminoboronate
2 was performed in phosphate-buffered saline (PBS, 50 mM, pH
7.4) at room temperature for 5 minutes with quantitative yield. In
1H-NMR spectra, the clean conversion was observed and the
chemical shift of CH2 of benzyl was altered from 3.70 ppm (Hb)
to 4.87 ppm (Ha) obviously (Figure 2). Based on these results, the
efficiency to hydrolyze iminoboronate 2 of compound 1a-1g was
evaluated by 1H NMR.
Scheme 2 The formation and reversible hydrolysis of iminoboronate
2
Figure 3 The hydrolytic efficiency of different reagents to
iminoboronate 2 were evaluated by 1H-NMR
Table 1. The hydrolytic efficiency of different reagents
Entry
1
Reagents
GSH (1a)
Yield %a
79
Figure 21H-NMR data indicating the clean conjugation to form
iminoboronate 2
2
3
4
Dopamine
Fructose
(1b)
(1c)
76
17
49
The reversible hydrolysis of iminoboronate 2 (0.05 mmol) by
1a-1g compounds were carried out in PBS buffer (0.6 mL) with
5% D2O for locking and shimming in NMR. After reaction for 5-
(1d)
(1e)
5
6
7
38
50
1
30 miniutes, H NMR was recorded directly. The ratio of Ha to
Hb signal intensity was used to evaluate the hydrolytic conversion
(Figure 3). The data showed that under the acidic condition (HCl,
pH=3), the iminoboronate was not dissociated at all. GSH (1a)
and dopamine (1b) decomposed the iminoboronate 2 with more
than 75% yield, and a small amount of substrate remained, which
was similar as the results in literature.3 All the mercaptophenyl
nucleophiles 1d-1f hydrolyzed the iminoboronate 2 with a low to
moderate yield (38%-50%). A composition of a nucleophilic
agent (1a, 1d, 1e or 1f) and fructose at a 1:1 ratio improved the
hydrolytic efficiency compared to individual compound itself.
However, the improvement was limited. Surprisingly, TCEP (1g)
was emerged as an excellent reagent, which decomposed
iminoboronate 2 completely in 5 minutes in the initial trail
(Figure 3, Table 1).
(1f)
TCEP
(1g)
100
8
1a + Fructose
1d + Fructose
1e + Fructose
1f + Fructose
HCl pH 3
85
52
43
67
0
9
10
11
12
a. The yield was determined by 1H-NMR: AHb / (AHa + AHb) × 100%; AHa
and AHb represent the relative peak area.
As 11B is sensitive to its chemical environment and rare in
common chemicals, it is especially useful for evaluating the
reaction efficiency in complicated mixtures. We recorded the 11
B
NMR spectra to show the hydrolytic efficiency of the selective
nucleophiles (1e), TCEP (1g), and GSH. Iminoboronate 2
displays a peak around 7.2 ppm. Addition of TCEP (1g) to
iminoboronate 2 resulted that the signal at 7.2 ppm disappeared

3
entirely and new peak at 29.7 ppm appeared which were exactly
consistent with the signals of 2-acetylbenzeneboronic acid, peak
similar to the hydrolysis of imine. Phosphine-containing
compound 1,3,5-triaza-7-phosphaadamantane in acidic
conditions also decomposed the iminoboronate 2 completely
which supported the hypothesis.
9
at 19.3 ppm corresponded to B(OH)3 (Figure 4). In the same
condition, for GSH and compound 1e, signals at 7.2 ppm were
still left. All the results further supported that TCEP was a
potentially effective agent to decompose iminoboronates.
Scheme 3 Proposed mechanism
Table 2 Substrates scope for the hydrolysis of iminoboronates with TCEP
Entry
1
R
R1-NH2
Yield %
100
2
100
CH3
3
4
100
24
5
6
86
Figure
4 The hydrolytic efficiency of different reagents to
100
iminoboronate 2 was detected by 11B NMR. * indicates B(OH)3.
To examine the reaction rate by TCEP, more time points of 11
B
7
100
NMR for hydrolysis were collected (Figure 5). The reaction
reached equilibrium so fast that it was difficult to measure the
reaction rate directly. We optimized the NMR parameters
including tunning, shimming, recycling delay, and scan numbers
in order to reduce the dead time caused by NMR operation. The
amount of H2O was in excess compared with the concentration of
the iminoboronate, thus we fit these data as a pseudo-first order
reaction. The reaction rate was estimated to be in the order of 10-2
S-1.
8
9
100
100
100
H
10
11
12
13
99
98
87
Encouraged by the aforementioned results, we examined the
substrate scope of the TCEP-promoted hydrolytic reaction. The
conversion was determined by 11B NMR. As shown in Table 2,
TCEP decomposed a variety of iminoboronates in satisfactory
results (entry1-3, 6-12). TCEP also displayed a certain ability to
hydrolyze hydrazones and oximes, which possessed greater
intrinsic hydrolytic stability than that of imines (entry 4-5, 13).
3. Conclusion
Iminoboronates have already been used in many biological and
chemical fields. Although iminoboronates are stable, it is
reversible in certain situations. Such reversible reaction is of
great value in drug discovery, drug delivery and self-organizing
system.25 We screened a series of nucleophilic compounds, and
mixtures of the compound and saccharide. Their hydrolytic
efficiency was analyzed by 1H-NMR or 11B-NMR detection.
Saccharide could assist the nucleophilic compounds and improve
the hydrolysis around 10%. Surprisingly, TCEP showed best
Figure 5 Stacked 11B-NMR spectra of iminoboronates as a function
of the hydrolysis time. * indicates B(OH)3.
The proposed mechanism for TCEP-promoted hydrolytic
reaction was shown in scheme 3. Nucleophile attack by
phosphine appeared to be the rate-determining step. After
elimination of the phosphine oxide, the rest procedure was

Tetrahedron Letters
4
hydrolysis capability. It could cleave the iminoboronates
completely in 3 min. When we extended the substrates to
hydrazones and oximes, which possessed greater intrinsic
hydrolytic stability than that of imines, TCEP also displayed a
considerable hydrolytic ability. Our results provide a novel
strategy in the reversible modification with iminoboronates.
4. Acknowledgements
This research was supported by the National Natural Science
Foundation of China (grant number. 31500632 and 21502114),
China Postdoctoral Science Foundation (grant number
2015M581677) and the Open Foundation of the State Key
Laboratory of Drug Research, China (grant number,
SIMM1601KF-05).
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5
Tris(2-carboxyethyl)phosphine (TCEP) performed
more efficient than all the reported reagent;
TCEP decomposed most iminoboronates in short
time with high yields;
TCEP is also able to hydrolyze hydrazones and
oximes with moderate yields.