Preparation of cyclic boramides from salicylaldehydes, ammonium acetate and sodium borohydride

Article abstract of DOI:10.1016/j.tet.2014.09.053

Borohydride reduction of salicylaldehyde imines yields surprisingly a cyclic boramide. This is a fairly stable compound, and X-ray analysis shows it has a tetrahedral boron atom. Mechanistic studies show a reaction pathway through an oxazaborinane intermediate. The reaction also works with halogen substituted salicylaldehydes and for the preparation of non-symmetrical boramides.

Full text of DOI:10.1016/j.tet.2014.09.053

Tetrahedron 70 (2014) 8614e8618
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Tetrahedron
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Preparation of cyclic boramides from salicylaldehydes, ammonium
acetate and sodium borohydride
a
a
a
a,
ꢀ
*
ꢀ
ꢀ
ꢀ
Víctor Tena Perez , Angel L. Fuentes de Arriba , Laura M. Monleon , Luis Simon
,
a
b
a,
*
ꢀ
Omayra H. Rubio , Francisca Sanz , Joaquín R. Moran
a Organic Chemistry Department, Plaza de los Caídos 1-5, University of Salamanca, 37008 Salamanca, Spain
b X-ray Diffractometry Service, Plaza de los Caídos 1-5, University of Salamanca, 37008 Salamanca, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2014
Received in revised form 12 September 2014
Accepted 17 September 2014
Available online 28 September 2014
Borohydride reduction of salicylaldehyde imines yields surprisingly a cyclic boramide. This is a fairly
stable compound, and X-ray analysis shows it has a tetrahedral boron atom. Mechanistic studies show
a reaction pathway through an oxazaborinane intermediate. The reaction also works with halogen
substituted salicylaldehydes and for the preparation of non-symmetrical boramides.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Cyclic boramide
Borohydride reduction
Oxazaborinane
1. Introduction
bond formation. The reaction works especially well in the case of
dimethyl malonate,6 as shown in Fig. 1.
Boron compounds have important chemical properties, they are
essential for plants grow, and therefore boramides are used as
fertilizer,1 moreover they have a low toxicity for humans but has
a large activity in the control of certain target species2 as fungi and
insects that degrade susceptible substrates as timber, they are
therefore used as timber preservatives.3
Their mechanical properties are also advantageous and they are
Fig. 1. Imine 1 obtained from salicylaldehyde and dimethyl malonate in the presence
of ammonium acetate.
suitable for antifriction coatings.4
Moreover, cyclic boramides are known in the literature, finding
use in Molecular Recognition. Self assembly structures are easily
obtained making use of boron derivatives.5
In this paper, we describe the preparation of cyclic boramides
starting from salicylaldehyde and related compounds using boron
hydrides as reductors and sources of the boron atom.
Modelling studies show that these imines are more stable that
the corresponding benzoxazines and, therefore, they do not un-
dergo cyclization6 (Fig. 2).
Reduction of imine 1 with hydrides represents an example of
unusual reactivity. Thus, the treatment of the imine with sodium
borohydride, or other boron based reducing agents, as shown in
Table 1, does not yield the expected amine, but a new compound 2.
Similar compounds have been previously reported in the
literature.7
2. Results and discussion
2.1. Preparation of boramide 2
2.2. Characterization of the obtained boramide
Salicyaldehyde reacts with carbon nucleophiles and ammonium
acetate yielding imine compounds strongly stabilized through H-
The new compound 2 shows no signals corresponding to the
malonate group in its NMR spectrum, so this group must have been
lost during the reaction. The most significant signals correspond to
a methoxyl group at 3.30 ppm and an ABX system with the AB part
* Corresponding authors. Tel.: þ34 923294481; fax: þ34 923294574; e-mail
ꢀ
addresses: lsimon@usal.es, luissimonrubio@hotmail.com (L. Simon).
http://dx.doi.org/10.1016/j.tet.2014.09.053
0040-4020/Ó 2014 Elsevier Ltd. All rights reserved.

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V.T. Perez et al. / Tetrahedron 70 (2014) 8614e8618
8615
confirms the proposed structure. The X-ray structure (Fig. 4, right)
also shows a cis fusion between the cycles, which would match
better with the structure obtained in chloroform, where the cou-
pling constants for the methylene protons are consistent with one
of them forming a close to 90ꢀ angle with the NH. Angles around
the nitrogen atom show a tetrahedral structure, while the boron
atom is a slightly distorted tetrahedron since the methoxy oxygen
forms a 116ꢀ angle with the phenol oxygen. This latest fact is
probably due to steric hindrance between the methoxy group ox-
ygen and the axial methylene group hydrogen, which are in close
Fig. 2. Iminine 1 does not cyclize to the corresponding benzoxazine.
Table 1
Reduction of imine 1 with different hydrides
ꢁ
contact at 2.7 A. The endo position of the methoxy methyl group,
which is on top of the heterocyclic ring, also contributes to this
steric collision, since one of the methyl group hydrogen atoms is
ꢁ
only 2.4 A away from the axial methylene group hydrogen.
We find interesting to point out that the methoxy group is easily
lost in the positive electrospray spectrum despite showing a strong
Entry
Reducing agent
Solvent
Time (h)
Yield (%)
1
2
3
4
NaBH4
NaBH4
NaBH3CN
NaBH(OAc)3
MeOH
0.5
0.5
1
91
90
85
82
MeOH/THF
MeOH/THF
MeOH/THF
Fig. 4. Compound obtained from the reduction of imine 1 with sodium borohydride
(left) and its X-ray structure (right).
24
ꢁ
centred around 4.00 ppm and the X part absorbing as a broad signal
at 7.60 ppm. The 13C spectrum shows that the AB protons corre-
spond to a methylene group at 48.0 ppm.
bond with the boron atom, as the bond distance is only of 1.42 A;
meanwhile, the boronenitrogen distance is of 1.61 A.
ꢁ
Since the malonate group is lost during the reaction, it can
logically be expected the generation of the cyclic boramide 2 from
reagents lacking the malonate group. Indeed the reaction of imine 7
with salicyladehyde and sodium borohydride also generates this
cyclic boramide 2 in higher yield (Fig. 5), confirming again the
proposed structure.
The mass spectrum was specially revealing, since the positive
electrospray spectrum shows the presence of a mass fragment at
238. This data together with the isotopic distribution, indicate the
presence of a boron atom in this structure. The negative electro-
spray shows the main signal at 268, confirming the presence of
a methoxyl group, and the molecular formula C14H13BNO2.
These data suggest the cyclic boramide 2 structure shown in
Fig. 3, in which the loss of the methoxyl group easily explains the
positive electrospray mass spectrum obtained for this compound,
and the tetrahedral nitrogen atom justifies the presence of the
diastereotopic protons in the methylene groups. As an interesting
feature, we would like to point out that the coupling constants
between methylene protons and the NH strongly change with the
solvent, moving from 9.0 and 0.0 Hz, respectively in CDCl3 to 4.0
and 6.0 Hz, respectively in DMSO-d6. A modelling study (using the
same level of theory used in the DFT study of the reaction mech-
anism, vide infra) shows that cis and trans boramides have very
similar energies, and therefore the equilibrium can be easily
modified in the presence of different solvents.
Fig. 5. Boramide 2 preparation from salicylaldehyde and imine 7.
Finally, the scope of the reaction has been studied with halogen
substituted salicylaldehydes. Results are very similar as salicy-
laldehyde. Furthermore, the reaction can be used to prepare non-
symmetrical boramides by reacting a preformed salicylaldehyde
imine with the reducing agent and another salicylaldehyde, as
shown in Table 2.
Crystallization of the cyclic boramide 2 in methanol allows the
isolation of high quality crystals, from which an X-ray study
Table 2
Preparation of substituted boramides 3e6 from different salicyl-aldehydes
Entry
R
R0
Imine
Boramide
Yield (%)
1
2
3
4
Cl
Br
H
Cl
Br
Cl
Br
8
9
10
11
3
4
5
6
70
85
50
45
H
Fig. 3. DFT optimized structures of cis (left) and trans (right) boramides.

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V.T. Perez et al. / Tetrahedron 70 (2014) 8614e8618
8616
Since we did not find the formation of boramide 2 obvious, we
a 100 cmꢁ1 cutoff).15 A RTln (24.5)¼1.89 kcal/mol term was added
to account for the change in the default standard state in Gaussian
(P¼1 atm, V¼24.5 dm3, T¼298.15 K) to the more convenient state in
solution ([ ]¼1 mol/dm3, T¼298.15). The energies were referenced
to the starting materials.
investigated the reaction mechanism with DFT calculations using
the program Gaussian09 (Fig. 6).8 Triacetoxy borohydride was se-
lected as the reducing agent since it provides the easiest mecha-
nistic interpretation. Geometry optimization and transition state
searches were performed with the B3LYP9 functional and 6-
31G**10,11 basis set. On the optimized structures, single point en-
ergy was evaluated using the M06-2X12,13 functional and 6-
31þþG** basis set. The solvent (methanol) was included in this
calculation by means of the implicit SMD solvation model.14 Gibbs
free energy calculated with the level of theory employed in the
optimization was added (calculated from the frequencies above
Both the structures considered in this study and the energy
diagram are shown in Fig. 5. After reduction of the imine through
TSa and the formation of intermediate a, the reaction proceeds by
a sequence of addition to boron and elimination of its acetate
groups. We contemplated two possibilities: the borate can react
first with one of the phenol oxygens (TSab), or with the amine
nitrogen (TSab0). The oxygen attack, to generate intermediate b,
shows a smaller energy barrier, probably due to the steric conges-
tion in the nitrogen atom, and the need to break the intramolecular
H-bond with the phenol. Also the BeN16 bond (106 kcal/mol) is
considerably weaker than the BeO bond (125 kcal/mol). The in-
termediate b eliminates then one acetic acid molecule yielding
intermediate c, which can react to make a BeN bond through TScd,
or a new BeO bond through TScd0. The pathway through the
macrocyclic borate shows a slightly smaller energy barrier. Never-
theless, the elimination of acetic acid from the macrocyclic in-
termediate d0 shows a large 29.9 kcal/mol barrier. An alternative
pathway through the oxazaborinane d, which easily exchanges the
acetate for the phenolic oxygen atom, leads to the bicyclic com-
pound requiring only a 17.4 kcal/mol barrier to be formed. The
different behaviour with respect to the elimination of acetate boron
substituent is also present for intermediates b and b0: b0, with
a BeN bond, reacts much faster than intermediate b. This is not
surprising since nitrogen substituted boron compounds are weaker
Lewis acids than its oxygen substituted counterparts, as revealed by
smaller Fluoride affinity.17
3. Conclusion
As a conclusion, the boramide 2 is a fairly stable compound,
which can be obtained under mild conditions from imine 1 or from
imine 8, salicylaldehyde and different boron reducing agents, as
shown in Table 1 and Fig. 5, respectively. DFT calculations revealed
a reaction pathway in which an oxazaborinane is the key in-
termediate to the boramide 2 formation. In addition, we have also
checked that the reaction works too with halogen substituted sal-
icylaldehydes and for the preparation of non-symmetrical
boramides.
4. Experimental section
4.1. General methods
Solvents were purified by standard procedures and distilled
before use. Reagents and starting materials obtained from com-
mercial suppliers were used without further purification. IR spectra
were recorded as neat film and frequencies are given in cmꢁ1
.
Melting points are given in ꢀC. NMR spectra were recorded on
a 200-MHz spectrometer. 1H NMR chemical shifts are reported in
ppm with tetramethylsilane (TMS) as internal standard. Data for 1H
are reported as follows: chemical shift (in ppm), number of hy-
drogen atoms, multiplicity (s¼singlet, d¼doublet, t¼triplet,
q¼quartet, m¼multiplet, br¼broad), coupling constant (in Hz).
Splitting patterns that could not be distinguished clearly are des-
ignated as multiplets (m). Data for 13C NMR are reported in ppm
and hydrogen multiplicity is stated. High-resolution mass spectral
analyses (HRMS) were measured using ESI ionization and a quad-
rupole TOF mass analyzer. Flash chromatography was performed
on 70e200 mesh silica gel.
Fig. 6. Proposed mechanism for boramide 2 formation.

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V.T. Perez et al. / Tetrahedron 70 (2014) 8614e8618
8617
4.2. Procedure I for boramide 2 preparation
(50 MHz DMSO-d6): 48.2 (CH2), 49.2 (CH2), 118.3 (2C), 118.4 (CH),
118.7 (CH), 118.9 (CH), 119.3 (CH), 128.1 (CH), 129.1 (CH), 129.3 (CH),
154.7 (2C), 155.0 (C) ppm; Mþe(HO)2BOMe, found 264.0782.
A solution of the imine 1 (100 mg, 0.3 mmol) in THF:MeOH 5:1
(6 mL) was treated under argon atmosphere with NaBH4 (36 mg,
0.8 mol). After 30 min under agitation, the reaction mixture was
poured over water and the resulting colourless crystals of the
boramide 2 (73.2 mg, 0.27 mmol, 91% yield) were filtered under
suction.
C14H15NO2Cl requires 264.0786.
4.3.5. 12H,14H-[1,3,2]benzoxazaborinino[2,3-b][1,3,2](4-
bromobenzo)oxaza-borinineꢂMeOH (6). Prepared from imine 11
(1.50 mmol), 5-bromosalicylaldehyde (302 mg; 1.50 mmol) and
NaBH4 (65 mg, 1.73 mmol) in 45% yield (235 mg, 0.70 mmol). Mp
177e178 ꢀC (CH2Cl2/MeOH). nmax (Nujol) 1133, 1455, 1499,
4.3. Procedure II for boramides 2e6 preparation
3162 cmꢁ1
; dH (200 MHz MeOD): 3.35 (3H, s), 3.99 (2H, d, J
Imines 7e11 were prepared by treatment of a solution of the
corresponding salicylaldehyde (1.00 mmol) in a 1:1 mixture of
MeOH:THF (1.0 mL) with NH3 gas during 1 min. The reaction
mixture was then used for the preparation of the boramides
without isolating the intermediate imines.
NaBH4 (1.15 mmol) and the corresponding salicylaldehyde
(1.00 mmol) were added in small amounts to the solution of the
corresponding imine (7e11) (1.00 mmol) at room temperature.
After 30 min under agitation, the reaction mixture was poured over
iced water and the resulting crystals (colourless or light yellow) of
the boramide were filtered under suction.
14.6 Hz), 4.24 (2H, d, J 14.6 Hz), 6.65e6.85 (3H, m), 7.02e7.23 (4H,
m) ppm; dC (50 MHz DMSO-d6): 48.2 (CH2), 49.3 (CH2),
115.9 (CH), 118.4 (C), 118.7 (CH), 119.3 (CH), 128.1 (CH), 128.7 (CH),
129.3 (CH), 129.7 (CH), 154.7 (C), 155.0 (C), 157.4 (2C) ppm;
Mþe(HO)2BOMe, found 308.0284. C14H15NO2Br requires
308.0281.
Acknowledgements
ꢀ
Financial support for this work from the Junta de Castilla y Leon
(SA223A11-2) and the University of Salamanca is gratefully ac-
knowledged. We thank to Mass Spectrometry Service and X-ray
Diffraction Service of the NUCLEUS Platform of the University of
Salamanca. We also thank the Ministry of Education of Spain for the
grant to A.L.F.A. (Grant: AP-2009-3338), the University of Sala-
4.3.1. 12H,14H-[1,3,2]benzoxazaborinino[2,3-b][1,3,2]benzoxaza-bor-
inineꢂMeOH (2). Prepared from imine 7 (1.65 mmol), salicylalde-
hyde (200 mg; 1.65 mmol) and NaBH4 (72 mg, 1.90 mmol) in 97%
yield (429 mg, 1.60 mmol). Mp 210e215 ꢀC (CH2Cl2/EtOH). nmax
ꢀ
manca for the grant to O.H.R. (Grant: 70894403P), Dr. Cesar Raposo
(Nujol) 1586, 1612, 3169, 3637 cmꢁ1
; dH (200 MHz DMSO-d6): 3.31
ꢀ
for the mass spectra and useful advice and Dr. Victoria Alcazar for
(3H, s), 3.92 (2H, d, J 14 Hz), 4.21 (2H, dd, J 10, 14 Hz), 6.70 (2H, d, J
8 Hz), 6.75 (2H, t, J 8 Hz), 6.96e7.16 (4H, m), 7.60 (1H, br s) ppm; dC
(50 MHz DMSO-d6): 47.3 (CH2), 48.0 (CH2), 117.7 (C), 118.0 (2CH),
118.2 (C), 127.2 (CH), 127.4 (2CH), 128.1 (CH), 128.4 (2CH), 154.3 (2C)
ppm; HRMS (ESI-QTOF) MþeMeOH, found 238.1031. C14H13BNO2
requires 238.1034.
her kind help.
Supplementary data
1H and 13C NMR, IR and HRMS spectra of compounds 2e6, X-ray
diffraction data of compound 2 and Cartesian coordinates of the
optimized structures are included. Crystallographic data (excluding
structure factors) for the structure in this paper have been de-
posited within the Cambridge Crystallographic Data Centre as
supplementary publication. Copies of the data can be obtained, free
of charge, on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK, (fax: þ44-(0)1223-336033 or e-mail: depos-
it@ccdc.cam.ac.Uk). Supplementary data related to this article can
be found at http://dx.doi.org/10.1016/j.tet.2014.09.053.
4.3.2. 12H,14H-[1,3,2](4-chlorobenzo)oxazaborinino[2,3-b][1,3,2](4-
chlorobenzo)oxazaborinineꢂMeOH (3). Prepared from imine
8
(1.50 mmol), 5-chlorosalicylaldehyde (235 mg; 1.50 mmol) and
NaBH4 (65 mg, 1.73 mmol) in 70% yield (360 mg, 1.05 mmol). Mp
157e161 ꢀC (CH2Cl2/MeOH). nmax (Nujol) 1125, 1488, 3149 cmꢁ1
; dH
(200 MHz MeOD): 3.35 (3H, s), 3.93 (2H, d, J 15.3 Hz), 4.23 (2H, d, J
15.3 Hz), 6.79 (2H, d, J 8.8 Hz), 7.07 (2H, s), 7.13 (2H, d, J 8.8 Hz) ppm;
dC (50 MHz DMSO-d6): 47.6 (2CH2), 120.3 (2CH), 120.4 (2CH), 121.6
(C), 122.4 (2C), 122.9 (C), 127.3 (CH), 128.6 (CH), 153.5 (C), 153.9 (C)
ppm; HRMS (ESI-QTOF) Mþe(HO)2BOMe, found 298.0404.
References and notes
C
14H14NO2Cl2 requires 298.0396.
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9
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; dH
ꢀ
ꢀ
ꢀ
ꢀ
(200 MHz MeOD): 3.31 (3H, s), 3.93 (2H, d, J 15.3 Hz), 4.23 (2H, d, J
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chlorobenzo)oxazaborinineꢂMeOH (5). Prepared from imine 10
(1.50 mmol), 5-chlorosalicylaldehyde (235 mg; 1.50 mmol) and
NaBH4 (65 mg, 1.73 mmol) in 50% yield (230 mg, 0.75 mmol). Mp
176e180 ꢀC (CH2Cl2/MeOH). nmax (Nujol) 1115, 1452, 1488,
€
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3149 cmꢁ1
;
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J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision B.01; Gaussian:
4.23 (2H, d, J 13.2 Hz), 6.70e6.85 (3H, m), 7.03e7.10 (4H, m) ppm; dC
Wallingford, CT, 2009.

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