Reactivity and mechanisms of hydridic hydrogen of B-H in ammonia borane towards acetic acids: The ammonia B-monoacyloxy boranes

Article abstract of DOI:10.1039/d1nj01727f

Non-classical acid-base neutralization reactions of ammonia borane (NH3·BH3, AB) with acetic acids are moderate, affected by strengths of acetic acid and its chloro derivatives, and concentrations and ratios of reactants. The experiment results indicate t

Full text of DOI:10.1039/d1nj01727f

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DOI: 10.1039/D1NJ01727F
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Reactivity and mechanisms of hydridic hydrogen of B-H in
ammonia borane towards acetic acids: the ammonia B-
monoacyloxy boranes
Received 00th January 20xx,
Accepted 00th January 20xx
⊥
⊥
HuizhenLi,*,ab Yunhui Li, a Jiaxin Kang, b Lin Fan,d Qiuyu Yang,*,b Shujun Li,b Abdul Rahman,a Daqi
Chen*,c
DOI: 10.1039/x0xx00000x
The non-classical acid-base neutralization reactions of ammonia borane (NH3·BH3, AB) with acetic acids are moderate,
affected by the strengths of acetic acid and its chloro derivatives, the concentrations and ratios of the reactants. The
experiment results indicate that only one hydridic hydrogen of B–H in AB is substituted and fortunately, four ammonia B-
monoacyloxy boranes (NH3·BH2OOCR,
R = CHnCl3-n, n = 0, 1, 2, 3) are prepared. Two single crystals of
2NH3BH2OOCCH2Cl·C12H24O6 and 2NH3BH2OOCCHCl2·C 12H24O6 were obtained and analyzed firstly. Theoretical calculations
demonstrated that these reactions are two-step elimination-addition process that takes advantage of the formation of
dihydrogen bond (DHB) and hydrogen bond (HB), and aminoborane (NH2BH2, AoB) is identified as the key intermediate.
of AB, in which proton substitutes for BH3 at the ammonia nitrogen,
+
NH4 formed and the released BH3 was rapidly hydrolyzed.[33-34]
Introduction
Interestingly, AB can also serves as an efficient substoichiometric
(10%) precatalyst for the direct amidation of both aromatic and
aliphatic carboxylic acids. [35]
Burgeoning research interest in boron–nitrogen–hydrogen (B–N–H)
compounds resurrected during the past decades, not just because of
their potential use in hydrogen storage[1-7] but also for a new
understanding of their particular structure and properties.[8-16]
Furthermore, the B–N–H compounds have been utilized as reducing
agent[17] for CO2,[18] graphene,[19] aldehyde/ketone,[20] alkenes[21] and
precursors to prepare boron nitride nanotube/ball,[22-23] boron
nitride polymer,[24] white graphene,[25] and metal nanoparticles.[26-27]
Amongst the B–N–H family, ammonia borane (NH3·BH3, AB) is the
most widely studied Lewis acid-base adduct, in which three types of
bonds (N→B dative bond, N−H, and B−H bonds) connected both
protonic (N−Hδ+) and hydridic (B−Hδ-) hydrogens together.
Intermolecular dihydrogen bond (DHB), N–Hδ+δ-H–B and expected
bent B–HH and linear N–HH moieties were present in AB.[10] On
the one hand, the amphoteric reactivity of AB can be explained by
the standard view that electron-deficient BH3 acts as a Lewis acid and
On the other hand, the amphoteric characteristics of AB can be
understood from a new perspective that protic hydrogen (Hδ+) is
−
acidic coupled to basic NH2BH3 and hydridic hydrogen (Hδ-) is basic
+
coupled to acidic NH3BH2 . The hydrogenation of polar unsaturated
compounds (such as olefins and imines) with AB is proposed to be a
double-hydrogen-transfer reaction (both Hδ+ and Hδ- atoms).[36-38]
While AB was treated with Lewis base, such as M, MH (M = Li, Na, K),
the N−H bond was broken, a metal cation replaced only Hδ+, and
metal amidoboranes formed.[39-42] Boronium cations including [(3,5-
(CF3)2C6H3)4B]-[H(OEt2)2]+ and [BH2(amine)(S)]+ (amine = NH3, Me2NH,
MeNH2; S = ethereal solvent) can be obtained from the reactions of
amine borane with some strong Lewis or Brønsted acids, such as
[H(OEt2)2][A] (A− = B[3,5-(CF3)2−C6H2]4, B(C6F5)3 and HOSO2CF3.[43-45]
Similarly, NH3BH2Cl was obtained when AB reacted with dry HCl in
the electron-rich NH3 as a Lewis base. In the presence of a strong
o
Et2O at -40 C.[46] A mixture of NH3·BCl3, NH3·BHCl2 and NH3·BH2Cl
[28-29]
Lewis acid or base, such as BF3/BCl3
or Me3N/Et3N/C5H5N,[30-32]
was found in the products of the reaction of AB with BCl3.[28]
the N→B dative bond was broken, and the displacement of BH3 or
NH3 occurred to form new Lewis acid-base adducts. The breaking of
the N→B dative bond also occurred in the acid-catalyzed hydrolysis
The hydrolysis product of the released BH3 from the acid-
catalyzed hydrolysis of AB was B(OH)3,[33-34] while the product of the
reaction of AB with oxalic acid was a spiral bis(oxalate)borate
([B(C2O4)2]NH4),[47] which is because of the strong interaction
between the electron-deficient B and electron-rich O, the chelation
effect of the oxalic acid may also play an essential role in promoting
the formation of spiroborate compounds. Also, the B–H of AB can
initially displace L from L·BH3 (L = weaker base) to form a new Lewis
acid/base adduct, ammonia diborane [NH3BH2(μ–H)BH3], with a
newly formed B–H–B (3c–2e) bond, which is usually unstable so that
further transformation occurs easily.[48] The analogous B–H–Al bonds
were also observed in aluminum complexes of AB, which resulting
from the coordination of the B–H bonds in AB to Al in AlX3 (X = Br,
a.School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou
510006, P. R. China. E-mail: lihz@gzhu.edu.cn (H. Li)
b.Henan Key Laboratory of Boron Chemistry and Advanced Energy Materials, School
of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang,
Henan 453007, P. R. China. E-mail: yangyu20009@163.com (Q. Yang).
c. School of Mechanical and Electrical Engineering, Guangzhou University,
Guangzhou, 510006, P. R. China. E-mail: daqichen@gzhu.edu.cn (D. Chen)
d.Beijing Normal University Publishing Group, College of Chemistry, Beijing Normal
University, Beijing 100875, P. R. China
† The Supporting Information including Experimental Section and Supporting
Resultsare available free of charge on the ACS Publications website at DOI: *.
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C6F5).[49] Thus it can be seen that the reactivity of Hδ- of B−H bond in stirred for 5 min and filtered. The clear solution was kept in the
View Article Online
DOI: 10.1039/D1NJ01727F
o
AB is far more complicated than it had been expected; an in−depth refrigerator at -10 C, and colorless transparent block crystals (1)
1
understanding of the intrinsic properties of AB is still limited. were obtained. 11B NMR (d3- CD3CN, 128 MHz, δ): -6.8 (t, BH2) H
Therefore, we tested the reactions of AB with about fifty acids, NMR (d3- CD3CN, 400 MHz, δ): 4.6 (brt, 3H, NH3), 4.1 (t, 2H, CH2), 3.6
including alcohol, phenols, and carboxylic acid. However, there are (t, 24H, 12CH2), 3.6 (q, 2H, BH2, overlapped with 24H, CH2). 13C{1H}
no significant reactions of AB with series of alcohols, phenols under NMR (d3- CD3CN, 101 MHz, δ): 169 (s, CO), 118 (s, C of CN), 71 (s, 12C
mild conditions without a catalyst. Only carboxylic acids can react of 12CH2), 43(s, C, CH2), 0.4 (s, C, CH3). Anal. Calcd for C2H7BNClO2
with AB at moderate speed, which is suitable to investigate its (123.35): C, 19.47; H, 5.72; N, 11.36. Found: C, 19.18; H, 5.96; N,
reactivity and reaction mechanism. In the present study, we 11.58.
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systematically studied the reactions of AB with acetic acids. The Preparation and Characterization of 2NH3BH2OOCCHCl2·C 12H24O6
experimental results revealed that only one Hδ- of B–H in AB was (2)
substituted by acyloxy of acetic acid, the ammonia B–monoacyloxy
A similar procedure to that of NH3BH2OOCCH3 was used to prepare
borane (NH3·BH2OOCR, R = CHnCl3-n, n = 0, 1, 2, 3), as the main
NH3BH2OOCCHCl2 (yield: ~88%). A 10 mL THF solution of
product was obtained for the first time, and X-ray single-crystal
NH3BH2OOCCHCl2 (0.16 g, 1 mmol) was prepared and 0.13 g (0.5
structure of two ammonia B–monoacyloxy boranes were also
mmol) of 18-crown-6 was added to the solution. The mixture was
determined. Furthermore, theoretical DFT calculations have
indicated that the reaction of AB with acetic acids is an elimination-
addition process promoted by the DHB and HB, unveiling the
chemical nature of a stepwise dehydrogenation process.
stirred for 5 min and filtered. The clear solution was kept in the
o
refrigerator at -10 C, and colorless transparent block crystals (2)
1
were obtained. 11B NMR (d3- CD3CN, 128 MHz, δ): -5.5 (t, BH2). H
NMR (d3- CD3CN, 400 MHz, δ): 4.6 (brt, 3H, NH3), 4.1 (t, 2H, CH2), 3.6
(t, 24H of 12CH2), 3.6 (q, 2H of BH2, overlapped with 24H, CH2).13C{1H}
NMR (d3- CD3CN, 101 MHz, δ): 168 (s, CO), 118 (s, C, CN), 70 (s, 12C,
12CH2), 67 (s, C, CH2), 0.3 (s, C, CH3). Anal. Calcd for C2H6BNCl2O2
EXPERIMENTAL SECTION
General Methods
(157.79): C, 15.22; H, 3.83; N, 8.88. Found: C, 15.40; H, 4.19; N, 8.36.
Preparation and Characterization of NH3BH2OOCCCl3
Unless stated otherwise, all manipulations were carried out using
standard Schlenk techniques or a nitrogen-filled glove box or glove
bag. Ammonia borane (AB), CH3COOH, CH2ClCOOH, CHCl2COOH, and
CCl3COOH were purchased from Aldrich and used as received
without further purification. Solvents used for the reactions (THF,
Et2O) were dried over sodium/benzophenone and freshly distilled
prior to use. Crystalline 18-crown-6 ether (Aldrich) was ground to a
powder and dried over P2O5 under vacuum.
A similar procedure to that of NH3BH2OOCCH3 was used to prepare
NH3BH2OOCCCl3. Several crystal growing techniques had been tried
to prepare NH3BH2OOCCCl3 crystal, and no crystal had been obtained
yet. After the reaction finished, the solvent was evaporated under a
dynamic vacuum to leave a wax product.11B NMR (d3- CD3CN, 128
MHz, δ): -4.8 (t, BH2).
X-ray Crystallography
A standard reaction procedure was usually followed: a solution
of acids was added dropwise to the solution of AB at room
temperature. The reaction mixture was monitored by 11B NMR at
different intervals. All NMR spectra were recorded with a Bruker
AV400 (11B 128.0 MHz; 1H 400.1 MHz; 13C 100.6 MHz). The 11B NMR
spectra were externally referenced to BF3·OEt2 in C6D6 (δ = 0.00 ppm).
IR spectra were obtained using a Nicolet−Magana 550 FT−IR
spectrophotometer in the range of 4000−400 cm-1 with a resolution
of 4 cm-1. Elemental analysis data were recorded using an Elementar
(Vario EL) instrument.
Single-crystal XRD analyses were conducted on Bruker Kappa CCD
diffractometers. The measurement of 1 and 2 were recorded using
Mo Kα, λ = 0.71073 Å. The linear absorption coefficients, scattering
factors for the atoms, and anomalous dispersion corrections were
taken from the International Tables for X-Ray Crystallography. The
structures were solved using the direct method and refined through
the full-matrix least-squares method on F2 using SHELXS-97.
Anisotropic thermal parameters were used to refine all non-
hydrogen atoms. Further details on the crystal structure
investigation can be obtained free of charge from The Cambridge
Preparation and Characterization of NH3BH2OOCCH3
Crystallographic
Data
Centre
via
At room temperature, a solution of acetic acid was added dropwise
to AB solution with constant stirring. The initial molar ratio of nAB:
nacid and cAB were adjusted according to Table 1 (the same below).
Several crystal growing techniques had been tried to prepare
NH3BH2OOCCH3 crystal, and no crystal had been obtained yet. After
the reaction finished, the solvent was evaporated under a dynamic
vacuum to leave a wax product. 11B NMR (d3- CD3CN, 128 MHz, δ): -
6.5 (t, BH2).
https://www.ccdc.cam.ac.uk/structures/ on quoting the depository
numbers CCDC-1957962 (1), CCDC-1957961 (2).
Computational Details
The Gaussian-09 program suite has been used to investigate the
potential energy landscape associated with ammonia borane and
acetic acid or substituted acetic acid. All the geometrical structures
were optimized at the B3LYP/6-311++G(2d,p) level of theory. The
vibrational frequencies calculations indicate that all the species are
stationary points that the transition states with one imaginary
frequency and other species without imaginary frequency. The
single-point energy calculations for the stationary points are carried
out to get the more reliable and authentic information at the M06/6-
Preparation and Characterization of 2NH3BH2OOCCH2Cl·C12H24O6
(1)
A similar procedure to that of NH3BH2OOCCH3 was used to prepare
NH3BH2OOCCH2Cl (yield: ~86%). A 10 mL THF solution of
NH3BH2OOCCH2Cl (0.12 g, 1 mmol) was prepared and 0.13 g (0.5
mmol) of 18-crown-6 was added to the solution. The mixture was
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311++G(2d,p) level of theory with Solvation Model Based on Density oppositely charged hydrogens combine and a new covalent bond
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DOI: 10.1039/D1NJ01727F
(SMD).
forming has been known for a long time, which can be used to
synthesize new compounds and design new materials.[16,50-51] From
this perspective, the Hδ- of B−H in AB and Hδ+ of O–H in Brønsted acid
can be easily engaged in the intermolecular DHB O–Hδ+···δ-H–B and
subsequent dihydrogen elimination. At room temperature, AB can
react with an excess amount of acetic acid and chloroacetic acids in
THF or Et2O. Fortunately, four ammonia B–monoacyloxy boranes
(NH3·BH2OOCR, R = CHnCl3-n, n = 0, 1, 2, 3) were obtained with H2
release (Eq 1, Table 1, Figures S1–4).
RESULTS AND DISCUSSION
The reactions of AB with acetic acids: the syntheses and structure
of ammonia B–monoacyloxy boranes
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Generally, the reactions of Hδ+ with Hδ- can be regarded as non–
classical acid−base neutralization reactions. Elimination of H2 as
NH3·BH3 + RCOOH → H2 + NH3·BH2OOCR (R = CHnCl3-n, n = 0, 1, 2, 3) (Eq 1).
Table 1. The reactions of AB with acetic acids.
Entry
Reactants (the initial molar ratio of Conditions
Products
Conversion of
nAB: nacid, cAB)
ABa (%)
-
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
AB + CH3COOH (1:1, 0.50 M)
AB + ClCH2COOH (1:1, 0.50 M)
AB + Cl2CHCOOH (1:1, 0.50 M)
AB + Cl3CCOOH (1:1, 0.50 M)
AB + CH3COOH (1:1, 1 M)
AB + CH3COOH (1:1, 2 M)
AB + CH3COOH (1:1, 3 M)
AB + CH3COOH (1:1, 4 M)
AB + CH3COOH (1:1, 5 M)
AB + CH3COOH (1:1, 6 M)
AB + CH3COOH (1:3, 1 M)
AB + CH3COOH (1:6, 1 M)
AB + Cl3CCOOH (1:1.2, 0.13 M)
AB + Cl3CCOOH (1:1.2, 0.38 M)
AB + Cl3CCOOH (1:1.2, 0.50 M)
AB + Cl3CCOOH (1:1.2, 0.63 M)
AB + Cl3CCOOH (1:1.2, 0.75 M)
AB + ClCH2COOH (1:2, 1 M)
AB + ClCH2COOH (1:2, 1 M)
RTb, THF
NRc
RT, THF, 30 d
RT, THF, 56 h
RT, THF, 7 h
NH3BH2OOCCH2Cl, H2
NH3BH2OOCCHCl2, H2
NH3BH2OOCCCl3, H2
NR
20
100
100
-
4
12
35
50
56
100
100
21
73
85
91
97
100
100
RT, THF, 14 h
RT, THF, 14 h
RT, THF, 14 h
RT, THF, 14 h
RT, THF, 14 h
RT, THF, 14 h
RT, THF, 67 h
RT, THF, 24 h
RT, THF, 45 min
RT, THF, 45 min
RT, THF, 45 min
RT, THF, 45 min
RT, THF, 45 min
RT, Et2O, 76 h
RT, Et2O, 10 d
NH3BH2OOCCH3, H2
NH3BH2OOCCH3, H2
NH3BH2OOCCH3, H2
NH3BH2OOCCH3, H2
NH3BH2OOCCH3, H2
NH3BH2OOCCH3, H2
NH3BH2OOCCH3, H2
NH3BH2OOCCCl3, H2
NH3BH2OOCCCl3, H2
NH3BH2OOCCCl3, H2
NH3BH2OOCCCl3, H2
NH3BH2OOCCCl3, H2
NH3BH2OOCCH2Cl, H2
NH3BH2OOCCH2Cl,
NH3BH(OOCCH2Cl)2, H2
complex
20
AB + CH3COOH (1:1, 3 M)
70 oC, THF, 7 h
100
a: calculated by the resonance intensities of boron signals in the 11B NMR spectra of the reaction mixture; b: RT, room temperature; c: NR,
no reaction.
The reactions of AB with acetic acids were monitored by 11B B−monoacyloxy borane has been isolated. We utilized 18−crown−6
NMR, and the products have been characterized by 11B NMR, 1H, 13
C
ether to bind the ammonia B−monoacyloxy boranes by weak HBs and
NMR, FTIR, Elemental analyses, and single−crystal X−ray diffraction two single crystals, 2NH3BH2OOCCH2Cl·C12H24O6 (1),
(SI). Notable features of the 11B NMR, including a triplet of BH2, 2NH3BH2OOCCHCl2·C 12H24O6 (2), were obtained. Single-crystal X-ray
indicate that only one Hδ- of B−H is substituted by acyloxy (Figures diffraction analysis (Fig. 1, Table S2) shows that the two ammonia
S1a, 2a, 3a, 4). Furthermore, the chemical shifts of B in the ammonia B−monoacyloxy boranes and 18−crown−6 adducts are both
B−monoacyloxy boranes move downfield slightly (Table S1), which is crystallized with 0.5 equiv of 18−crown−6, which are monoclinic and
in line with the electron-withdrawing effect of chlorine in belong to the P21/n space group. The structures of the two adducts
chloroacetic acids. The disappearance of characteristic O−H are stabilized by hydrogen-bonding interactions between the
stretching vibration centered at 3400 cm-1 and the new B−O hydrogen atom of an −NH3 of two adjacent ammonia B−monoacyloxy
stretching vibration centered at 1380 cm-1 in FTIR spectra boranes and an oxygen atom of 18−crown−6 (Fig. 1). The HBs in 1
demonstrate unequivocally that the Hδ- of B−H in AB are substituted and
2
are similar to those in 2NH2B2H5·C 12H24O6,
Na[H2N(BH3)2]·C12H24O6·2THF,
by acyloxy, and the B−H stretching vibration centered at 2390 cm-1 NH2BH2NH2BH3·C 12H24O6,
show that not all Hδ- of B−H are substituted (Figures S2d, 3d).
NH2B2H5·THF.[12, 60]
For comparison, the critical bond lengths and bond angles are
It is significant to note that most acyloxyboranes are transient
species, and the intermediates characterization is often summarized in Tables S3, 4. As shown in Table S3, 4, similar to the
challenging.[47, 52-56] Only few acyloxyboranes were synthesized[57] N−B bond length in AB, the N(1)−B(1) bond length is 1.581(3) Å and
[58]
and used as catalyst for asymmetric Diels-Alder reactions,
P. 1.590(7) Å for 1 and 2, which are not affected by one more Cl
Helquist and O. Wiest studied the stereoselectivity in substituent on C(2). The B(1)−O(1), C(1)−C(2) bond lengths for 2
(acyloxy)borane–catalyzed Mukaiyama Aldol reactions by theoritacal (1.513(7) Å, 1.536(9) Å) are slightly longer than those of 1 (1.505(3)
calculations. [59] To the best of our knowledge, there are no ammonia Å, 1.505(4) Å). However, the C(1)−O(2), C(2)−Cl(1) bond lengths for 2
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(1.205(9) Å, 1.750(7) Å) are slightly shorter than those of 1 (1.217(3) 0.13−0.75 M (Table 1, entries 13−17, Figure S6), the reaction of entry
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Å, 1.779(3) Å). The N(1)−B(1)−O(1) angles (109.1(5)o) in 2 is 17 was the most rapid, which can complete within 1 h.
DOI: 10.1039/D1NJ01727F
consistent with sp3 hybridization, while the N(1)−B(1)−O(1) angles
(102.7(2)o) in 1 is slightly smaller.
Quite unexpectedly, only one hydridic (B−Hδ-) hydrogen was
substituted despite different acidity and quantities of acetic acids (t,
BH2, Figure S1a, 2a, 3a, 4), indicating that more substitution of
hydrides of ammonia B−monoacyloxy boranes is difficult. It is only
when the reaction time is long enough, such as 10 days, a small
quantity of ammonia B−diacyloxy boranes can be observed in the 11
B
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NMR spectrum by the reaction of the formed ammonia
B−monoacyloxy boranes with excess ClCH2COOH in Et2O (Table 1,
entries 18−19, Figure S7), which can be probably rationalized with
the appropriate nucleophilicity of the B−H bonding pair electrons and
the suitable acidity of acetic acids. Further investigations in the
substitution process of the second hydridic hydrogen in AB are still in
our group's progress. Besides, the reaction of AB with acetic acids
was strongly dependent on temperature (Table 1, entry 20, Figure
S8). However, the products are too complicated, indicating
increasing the reaction temperature may result in the byproducts
and the dissociation of the main product.
Fig. 1 The molecular structures with 35% of displacement ellipsoids
for A) 2NH3BH2OOCCH2Cl·C12H24O6 (1), B)
NH3BH2OOCCHCl2·C 12H24O6 (2). Colors: N, blue; B, pink; O: red; C,
gray; H, light−blue; Cl, yellow (red dash lines show hydrogen bonds).
The formation mechanism of ammonia B−monoacyloxy boranes by
the reaction of AB with acetic acids
Affecting factors of the reactions of AB with acetic acids
The relatively mild reaction conditions and speed make the reactions
of AB with acetic acids very suitable for mechanistic study when
compared to other highly reactive boranes. To figure out the reaction
mechanism, whether the reaction is carried out kinetically or
thermodynamically, theoretical calculations with density functional
theory (DFT) are performed (SI). The formation mechanism of
ammonia B−monoacyloxy boranes (NH3·BH2OOCR, R = CHnCl3-n, n = 0,
1, 2, 3) by the reaction of AB with acetic acids is shown in Scheme 1.
The energy profile of these reactions at M06/6-
311++G(2d,p)//B3LYP/6-311++G(2d,p) level of theory with SMD
solvent model is shown in Fig. 2 and Fig. S9a-c, 10. The calculated
energies and rate constants are listed in Tables S4-9. The optimized
geometries of all the species are shown in Figures S11a, b.
As shown in Scheme 1, the mechanism includes two steps. In
the first step, the RCOOH catalyzes AB to release hydrogen by the
DHB between the Hδ- of B−H in AB and Hδ+ of O-H in RCOOH and HB
between Hδ+ of N−H in AB and Oδ- of C=O in RCOOH. The transition
states (TS) TS1 has an eight-membered ring via B−Hδ-···δ+H−O and
N−Hδ+···δ-O−C, in which the H···H bond distance is at a range of
0.815~0.862 Å and the H···O distance is at a range of 1.148~1.202 Å
(Fig. 2, Figures S9a-c, Figures S11a, b). The short Hδ-···δ+H and Hδ+···δ-
O distance suggest the elimination of H2 resulting in the formation of
aminoborane (NH2BH2, AoB) and RCOOH. We have tried to trap AoB
by cyclohexene but failed. Synchronously, the Hδ+ of N−H in AoB can
be captured by the terminal O of C=O and the Hδ+ of O−H captured
by the N in AoB, an intermediate complex with a six-membered ring
form (Fig. 2, Figures S9a-c, Figures S11a, b). For the reaction of AB
with CCl3COOH, the intermediate complex (IM4) has a different
structure via an N···Hδ+−O interaction in which the N···H distance is
2.028 Å (Fig. 2, Figure S11b).
The reaction rate of AB with acetic acids is strongly dependent on the
acidity of acetic acids. At the same conditions (the initial ratio of nAB:
nacid = 1:1, cAB = 0.5 M), there was no reaction between AB and
CH3COOH (Table 1, entry 1), the conversion of AB was only 20% in 30
days for the reaction of AB with CH2ClCOOH (Table 1, entry 2), while
AB can react with CHCl2COOH and CCl3COOH at a moderate speed,
which took 76 h and 7 h, respectively, to complete (Table 1, entries
3, 4). Because of the Cl substituents effect, the relative acidity of
acetic acids is known to increase along with the series CH3COOH <
CH2ClCOOH < CHCl2COOH < CCl3COOH and their pKa are 4.76, 2.86,
1.30 and 0.22, respectively. The reaction rates of AB with acetic acids
increased along with the same trend.
Both increasing the concentration of reactants (AB and acetic
acids) and the molar ratio of acetic acids can accelerate the reaction.
Controlling the initial molar ratio of nAB: nacid = 1:1, increasing cAB can
accelerate the reaction significantly. For example, when the
reactions of AB with CH3COOH were monitored by 11B NMR
spectroscopy in 14 h, increasing cAB from 1 M to 6 M resulted in
increased conversion of AB from 0 to 56% (Table 1, entries 5−10,
Figure S5), the fastest reaction could complete in 24 h (Table 1, entry
10). With increasing cAB over 6 M, byproducts were significantly
increased because the reaction was exothermic. Similarly, increasing
the molar ratio of acetic acid can also promote the reaction. By
optimizing the initial ratio of nAB: nacid to 1:3 and 1:6, the reactions of
AB with CH3COOH took 67 h and 24 h to complete, respectively
(Table 1, entries 11, 12). A similar phenomenon was observed for the
reaction of AB with CCl3COOH when increasing the cAB by optimizing
the initial ratio of nAB: nacid to 1:1.2. The conversion of AB was
increasing from 21 to 97%, with the cAB being set at a range of
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Scheme 1 Proposed formation mechanism of ammonia B−monoacyloxy boranes (NH3·BH2OOCR, R = CHnCl3-n, n = 0, 1V,i2ew, 3A)rticle Online
DOI: 10.1039/D1NJ01727F
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Fig. 2 Potential energy surface for the reaction of AB with CCl3COOH.
In the second step, a concerted hydrogen transfer from O−H in
RCOOH to N in AoB and the addition of O in RCOO to the B of AoB
Conclusions
occurred, resulting in the formation of TS2 with a four−membered
ring via N···Hδ+−O and B···O interaction, the N···H and B···O distance
is 1.215~1.307 and 1.734~2.107 Å respectively, and finally the
ammonia B−monoacyloxy borane formed (NH3·BH2OOCCHnCl3-n, n =
0, 1, 2, 3), which does not favor the further reduction of acetic acid.
In a word, the reaction of AB with RCOOH (R = CHnCl3-n, n = 0, 1, 2, 3)
can be regarded as an elimination−addition process promoted by
hydrogen transfer.
All the total free energies (ΔG) are negative (-36.4/-62.3/-62.8/-
66.5 kJ·mol-1), indicating that those reactions are thermodynamically
controlled and can occur spontaneously. The free energy barrier of
TS1 is slightly higher than that of TS2 for the reactions of AB with
acetic acids (Tables S5-8, Figure S9a-c, Figure 2). Simultaneously, all
the free energy barriers of TS decreased with the acidity of acetic
acids increases, which is consistent with the changes in the reaction
rates. Furthermore, the free energy barriers of TS of the reactions of
AB with acetic acids (94.6~124.3 kJ·mol-1, Tables S5-8, Figure S9a-c,
Figure 2) are much smaller than that of the direct decomposition of
AB into AoB and H2 (175.7 kJ·mol-1, Table S9, Figure S10), which
demonstrating that acetic acid is an efficient catalyst and
hydrogen−transfer reagent. Table S10 listed the calculated rate
constants with a temperature range from 258 to 398 K, which shows
that the rate constants are also increasing (k1-k4) with the acidity of
acetic acids.
In summary, using the moderate reactions of AB with acetic acid,
chloroacetic acid, dichloroacetic acid, trichloroacetic acid at
room temperature, the reactivity of AB towards Brønsted acids
has been systematically investigated. The reaction of AB with
acetic acids constitutes an important fundamental reaction
mode of B−N−H compounds, which was not only related to the
activity of the B−H bond and the acidity of Brønsted acid but
also affected by the ratio and concentration of reactants. Only
one hydridic hydrogen in AB can be substituted by acetic acid
and series chloroacetic acid, dichloroacetic acid, trichloroacetic
acid, and four ammonia B−monoacyloxy boranes
(NH3·BH2OOCCHnCl3-n, n = 0, 1, 2, 3) are firstly obtained, in which
two single crystals of 2NH3BH2OOCCH2Cl·C12H24O6 and
2NH3BH2OOCCHCl2·C 12H24O6 were obtained firstly, and their
crystal structures were analyzed. DFT calculations indicate that
the reactions are a two−step elimination−addition process;
both DHB and HB play an essential role in the reaction pathways.
The calculated negative total free energies (ΔG) and lower
energy barriers are consistent because the reaction can
spontaneously occur at room temperature. The reactions of AB
with acetic acids not only offer a facile synthetic procedure for
the synthesis of ammonia B−monoacyloxy boranes with high
yield and purity but also provide a unique opportunity to study
the structure, stability, and behavior of the new
monoacyloxyboranes species. What is more, the ammonia
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13 H. Li, R. Wang, Q. Xia, Q. Yang, P. Wang, C. Wei, N. Ma, X. Chen,
B−monoacyloxy boranes may suggest and allow many
interesting syntheses, incredibly selective and asymmetric
reductions in the future.
The reactivity of B−H bond in R·BH3 towards phenoVl,ieCwhAertmicle. ORnelisn.e,
DOI: 10.1039/D1NJ01727F
2018, 29, 118-124.
14 H. Li, M. Ju, J. Kang, A. Zhou, H. Guan, J. A. Douglas, Y. Yue,
Facile cyclization of sodium aminodiboranate to construct a
boron–nitrogen–hydrogen ring, Dalton Trans., 2020, 49, 16662-
16666.
Author Contributions
15 X. Chen, H. Li, Q. Yang, R. Wang, E. J. M. Hamilton, J. Zhang, X.
Chen, Brønsted and Lewis base behavior of sodium
amidotrihydridoborate (NaNH2BH3), Eur. J. Inorg. Chem., 2017,
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16 X. Chen, J. Zhao, S. G. Shore, The roles of dihydrogen bonds in
amine borane chemistry, Acc. Chem. Res., 2013, 46, 2666-2675.
17. F. Chiara, Boselli. M. Fiorenza, M. Fabrizio, B. Maurizio,
Ammonia borane as a reducing agent in organic synthesis, Org.
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18 G. Menard, D. W. Stephan, Room Temperature reduction of
CO2 to methanol by Al-based frustrated Lewis pairs and ammonia
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19 V. H. Pham, S. H. Hur, E. J. Kim, B. S. Kim, J. S. Chung, Highly
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21 X. Yang, F. Thomas, B. Heinz, Facile metal free regioselective
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⊥
Y. Li and J. Kang contributed equally.
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Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
We are grateful for the financial support from the National
Natural Science Foundation of China (21601051, 62003105).
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