One-pot synthesis of ammonia-borane and trialkylamine-boranes from trimethyl borate

Article abstract of DOI:10.1021/ol302421t

A one-pot procedure for the preparation of ammonia borane from trimethyl borate in 90% yield and >99% purity has been reported. This methodology has been modified to prepare a series of trialkylamine-boranes in 70-82% yields from trimethyl borate and lithium hydride/aluminum chloride in the presence of the corresponding trialkylamine.

Full text of DOI:10.1021/ol302421t

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
One-Pot Synthesis of AmmoniaꢀBorane
and TrialkylamineꢀBoranes from
Trimethyl Borate
000–000
P. Veeraraghavan Ramachandran,* Bhimapaka C. Raju, and Pravin D. Gagare
Department of Chemistry, 560 Oval Drive, Purdue University, West Lafayette,
Indiana 47907-2084, United States
chandran@purdue.edu
Received August 31, 2012
ABSTRACT
A one-pot procedure for the preparation of ammonia borane from trimethyl borate in 90% yield and >99% purity has been reported. This
methodology has been modified to prepare a series of trialkylamineꢀboranes in 70ꢀ82% yields from trimethyl borate and lithium hydride/
aluminum chloride in the presence of the corresponding trialkylamine.
8
9
Amineꢀboranes, with their wide range of reactivity,
agents, and in electroless plating applications. More
recently, the synthesis of nanoparticles using amineꢀ
boranes has attracted substantial attention from materials
1
,2
have found numerous applications in organic synthesis.
Apart from their traditional use as reagents for the reduc-
3
tion of carbonyls, reductive amination, hydroboration of
4
10
chemists. Their use as borane carriers for exchange
11
reactions has also been demonstrated.
Additionally, ammoniaꢀborane (AB) and other amineꢀ
boranes have received enormous attention as hydrogen
carriers for energy storage applications because of their
high hydrogen content and recycling prospects. A prac-
tical, energy efficient, and cost-effective synthesis remains a
significant challenge for these amineꢀboranes to realisti-
cally become an energy carrier. Various methods have been
5
6
alkenes, and transfer hydrogenation, amineꢀboranes
have also been employed as curing agents or accelerators
7
for the preparation of epoxy resins, as reductive bleaching
12
(
1) Staubitz, A.; Robertson, A. P. M.; Sloan, M. E.; Manners, I.
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13
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(
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(
(
(
3
1
0.1021/ol302421t r XXXX American Chemical Society

mostly using either diborane or metal borohydrides. One of
13a
these describes a large-scale preparation of ammoniaꢀ
3 4
Scheme 1. Preparation of AB from B(OMe) Using LiAlH
borane using sodium borohydride (NaBH ), which in turn,
4
is industrially prepared using Schlesinger’s protocol from
trimethyl borate [B(OMe) ] and sodium hydride (NaH) at
3
15
2
70 °C. We have since reported a one-pot AlCl mediated
3
formation of NaBH and LiBH from B(OMe) in the
presence of the appropriate metal hydride (MH) under
4
4
3
However, the addition of B(OMe) to a solution of LAH
3
1
6
ambient conditions. Herein, we report a novel, one-pot
synthesis of AB and other trialkylamineꢀboranes directly
and NH Cl in THF at rt resulted in the formation of AB in
4
only 46% yield. Changing the solvent to diethyl ether did
not improve the yield. A careful analysis revealed that the
low yield might be due to the reaction of LAH and NH Cl
from B(OMe) .
3
4
toproduce aluminum nitride and LiCl, withthe concurrent
liberation of hydrogen. Consequently, the temperature
and mode of addition were modified.
Table 1. Examination of Ammonium Salts for the Preparation
of AB from B(OMe) and LiH/AlCl
3 3
Table 2. Examination of Ammonium Salts for the Preparation
of AB from B(OMe) and LAH
3
a
entry
4
NH X
yield (%)
a
entry
ammonium salt
yield (%)
1
2
3
4
5
6
NH
NH
4
Cl
68
54
59
63
1
2
3
4
5
6
NH
NH
4
Cl
F
90
67
59
63
47
42
4
F
4
(NH
(NH
4
)
)
2
SO
CO
4
(NH
(NH
4
)
)
2
SO
CO
4
4
2
3
b
4
2
3
NH
NH
4
OAc
NO
NR
b
NH
NH
4
NO
OAc
3
4
3
NR
4
a
b
Isolated yield. NR = no reaction.
a
Isolated yield.
On the basis of our one-pot synthesis of NaBH and
4
1
LiBH from B(OMe) , coupled with our synthesis of AB
6
Thus, LAH in THF was added to a mixture of B(OMe)3
4
3
1
3a
and NH Cl in THF at 0 °C over a period of 1 h and stirred
from NaBH4, we envisaged a oneꢀpot synthesis of AB
4
for 3 h at the same temperature to obtain, after ethereal
1
from B(OMe) . Accordingly, AlCl dissolved in tetrahy-
3
3
1
B
extraction, 90% isolated yield of AB (Scheme 1). A
drofuran (THF) was added to a suspension of B(OMe)3,
lithium hydride (LiH), and ammonium chloride (NH Cl)
in dry THF at room temperature (rt). The reaction,
NMR spectroscopic analysis as well as hydrogen estima-
tionvia hydrolysisrevealed>99%chemicalpurity. Again,
other ammonium salts, such as ammonium fluoride, sul-
fate, nitrate, carbonate, and acetate, provided poorer
yields of AB (Table 2).
4
1
1
monitored by B NMR spectroscopy, was complete with-
in 8 h. Extraction with diethyl ether provided a 68% yield
of analytically pure AB. Other ammonium salts, such as
ammonium fluoride, sulfate, and carbonate, afforded only
moderate yields of AB (Table 1). Ammonium nitrate and
acetate failed to provide any of AB. The corresponding
reaction with NaH and ammonium chloride provided only
poor yields (<25%) of AB.
We surmised that the poor yields obtained earlier during
the AlCl -mediated reaction of trimethyl borate, LiH, and
3
NH Cl (Table 1) could be due to the liberation of gaseous
4
diborane from the presumed intermediate LiBH and/or
4
a lack of reactivity of the ammonium salts under the reac-
tion conditions. This prompted us to capture the liberated
diborane as a trialkylamineꢀborane complex by conduct-
ing the reaction in the presence of trialkylamines to achieve
a novel one-pot synthesis of such amineꢀboranes.
With an aim to improve upon this result, we replaced
LiH/AlCl3 with lithium aluminum hydride (LAH).
(
13) (a) Ramachandran, P. V.; Gagare, P. D. Inorg. Chem. 2007, 46,
810. (b) Mayer, E. Inorg. Chem. 1973, 12, 1954. (c) Hu, M. G.; Van
Paasschen, J. M.; Geanangel, R. A. J. Inorg. Nucl. Chem. 1977, 39, 2147.
d) Heldebrant, D. J.; Karkamkar, A.; Linehan, J. C.; Autrey, T. Energy
7
To our delight, a reaction of trimethyl borate with LiH/
AlCl in the presence of triethylamine provided the corre-
3
(
1
7
Environ. Sci. 2008, 1, 156. (e) Shore, S. G.; Parry, R. W. J. Am. Chem.
Soc. 1955, 77, 6084. (f) Shore, S. G.; Parry, R. W. J. Am. Chem. Soc.
958, 80, 8.
sponding amineꢀborane in 70% isolated yield (Scheme 2).
The increased yield is attributed to the instant capture of the
diborane generated from the borohydride in the presence
of AlCl . In an optimized reaction, B(OMe) was added to
1
(
14) (a) Schaeffer, G. W.; Anderson, E. R. J. Am. Chem. Soc. 1949,
7
2
1, 2143. (b) Nainan, K. C.; Ryschkewitsch, G. E. Inorg. Chem. 1969, 8,
671. (c) Mock, M. T.; Potter, R. G.; O’Hagan, M. J.; Camaioni, D. M.;
Dougherty, W. G.; Kassel, W. S.; DuBois, D. L. Inorg. Chem. 2011, 50,
1914. (d) Baldwin, R. A.; Washburn, R. M. J. Org. Chem. 1961, 26,
3
3
the suspension of LiH (5.5 equiv) in THF followed by the
addition of Et N (0.75 equiv). The reaction mixture was
1
3
3
549.
(
15) Schlesinger, H. I.; Brown, H C; Finholt, A. E. J. Am. Chem. Soc.
953, 75, 205.
16) Ramachandran, P. V.; Hazra, D.; Raju, B. C.; Reddy M. V. R.;
1
(17) A similar reaction of B(OMe)3 and LAH in the presence of
bt r oi er ta hn ye l .a mine provided only 45% isolated yield of triethylamineꢀ
(
Bhattacharyya, A. U.S. Patent US 7601797, 2009.
B
Org. Lett., Vol. XX, No. XX, XXXX

Scheme 2. Preparation of Triethylamine Borane
Table 3. Preparation of AmineꢀBoranes via the Reaction of
3 3
B(OMe)
with LiH and AlCl in the Presence of Amines
11
a
b
entry
amineꢀborane
B NMR (δ) (ppm)
yield (%)
1
2
3
4
5
6
7
BH
i-Pr
DMAP-BH
3
ꢀNEt
3
ꢀ13.3 (q)
ꢀ13.4 (q)
ꢀ13.7 (q)
ꢀ17.9 (q)
ꢀ17.9 (q)
ꢀ18.4 (q)
ꢀ11.0 (q)
70
73
72
74
2
NEt-BH
3
1
1
3
vigorously stirred at rt for 1 h and was monitored by
B
2,6-Lutidine-BH
2,6-Lutidine-BH
3
NMR spectroscopy for the chemical shift change from δ
c
3
77
1
8
(
ppm) 19 to3. Aluminum chloride (1.5 equiv) dissolved in
THF was then added, dropwise, at 0 °C, over a period of
h, and the stirring was continued at rt until the reaction
2,4,6-Collidine-BH3
DABCO-(BH
82
80
3 2
)
1
a
b
Multiplicity q = quartet. Isolated yield. 20 g scale reaction.
c
11
was complete, as determined by the change in B NMR
spectroscopic chemical shift to ꢀ13.3 (q). The solvent was
removed under reduced pressure, and the residue was
stirred in anhydrous pentane for 1 h and filtered through
Celite under inert atmosphere. Removal of pentane in
vacuo provides the amineꢀborane complex. The reaction
was repeated with other metal hydrides such as NaH and
calcium hydride to obtain triethylamineꢀborane in 66%
and 68% yields, respectively. Magnesium hydride did not
provide the amineꢀborane. A briefscreening of Lewis acids
showed that zinc chloride and zirconium tetrachloride were
equally effective under similar conditions.
Accordingly, N,N-diisopropylethylamine, 4-dimethylamino-
pyridine, 2,6-lutidine, 2,4,6-collidine, and 1,4-diazabicyclo-
[
corresponding amine-boranes from B(OMe) in the pre-
2.2.2]octane (DABCO) were utilized to prepare the
3
sence of LiH and AlCl in good to excellent yields (Table 3).
3
While the bisboraneꢀDABCO adduct was formed cleanly
(
entry 7), the monoboraneꢀDABCO adduct was not pure.
1
1
The B NMR spectral data is also summarized in Table 3.
After demonstrating the generality of the reaction, we
examined the feasibility of this protocol for large-scale
applications. Accordingly, the reaction was repeated with
With the optimal procedure in hand, we turned our atten-
tion to the preparation of a series of trialkylamineꢀboranes.
20 g (0.192 mol) of B(OMe) in the presence of 2,6-lutidine
3
and LiH, which provided 2,6-lutidineꢀborane in 77% yield.
In summary, we have reported a new and economical
procedure for the preparation of ammonia borane and
trialkylamineꢀboranes from trimethyl borate. The known
procedures for preparation of trialkylamineꢀborane in-
volve reaction of (i) other complexes of borane, such as
BH ꢀSMe or BH ꢀTHF or in situ generated gaseous
(
18) Vigorous stirring and the purity of LiH is crucial for the
complexation, which is critical for good yields of the amineꢀboranes
Additional time (up to 4 h) and lower yields are observed otherwise.
(
(
(
19) Lang, K.; Schubert, F. U. S. Patent 3,037,985, 1962.
20) Burg, A. B.; Schlesinger, H. I. J. Am. Chem. Soc. 1937, 59, 780.
21) Schaeffer, G. W.; Anderson, E. R J. Am. Chem. Soc. 1949, 71,
2143.
(
22) Brown, H. C.; Schlesinger, H. I.; Cardon, S. Z. J. Am. Chem. Soc.
3
2
3
1
9,20
1942, 64, 325.
diborane with trialkylamine,
21
with ammonium salt, or (iii) transamination of existing
(ii) lithium borohydride
(
23) On a per mole of hydride cost, sodium hydride is ∼6 times more
expensive than lithium aluminum hydride or lithium hydride (Aldrich
online catalog, August 2012).
2
2
amineꢀborane. The advantage ofourprotocol is that the
(
(
24) Ashby E. J. U.S. Patent US 7247286 B2, 2007.
4
25) Preparation of AB from B(OMe) Using LiAlH . Trimethyl
preparation begins with the precursor of the borohydrides
B(OMe) . The reaction has been demonstrated on a molar
3
borate (0.5 g, 0.0048 mol) was added under nitrogen atmosphere to a
suspension of ammonium chloride (0.518 g, 0.0096 mol) in THF (24 mL)
at 0 °C. Under vigorous stirring, lithium aluminum hydride (0.23 g,
3
scale. The optimal yield for AB is achieved using LAH as
the hydride source, whereas the LiH/AlCl combination
3
0.0057 mol) in THF (12 mL) was added, dropwise, over a period of 1 h at
the same temperature. The reaction mixture was warmed to room
provided optimal yields of trialkylamine boranes. Utiliza-
tion of either LAH or LiH/AlCl under mild conditions, in
11
temperature and stirred for 2 h. The B NMR shows the formation of
AB (δ (ppm): ꢀ21.6 (q). The solvent was distilled under reduced pressure
at rt to obtain a powdery solid residue, which was suspended in cold
diethyl ether at 0 °C and stirred for 30 min. The cold ether layer was
filtered and the solvent was removed in vacuo to obtain 0.133 g (90%) of
ammonia-borane as a white crystalline solid in >99% purity. Repre-
sentative Procedure for the Preparation of AmineꢀBoranes. Trimethyl
borate (0.5 g, 0.0048 mol) was added, under nitrogen atmosphere at rt, to
a suspension of lithium hydride (0.210 g, 0.0264 mol) and triethylamine
3
23
contrast to the relatively expensive NaH and energy-
intensive reaction (high temperature) conditions used in
24
the industrial protocol, makes this scalable process more
25
economical. We believe that this synthesis will find applica-
1ꢀ10
tions in organic synthesis
and the efficient synthesis of
(0.5 mL, 0.0036 mol) in THF (10 mL) contained in an oven-dried round-
bottom flask. The reaction mixture was stirred for 1 h and was
monitored by B NMR spectroscopy (change in chemical shift δ
ammoniaꢀborane will aid in hydrogen storage research.
1
1
Acknowledgment. Financial support from General
Atomics, the Department of Defense, and the Department
of Energy is gratefully acknowledged.
(
ppm) from þ19 to þ3). (Note: Depending on the vigor of stirring this
may take up to 4 h.) Aluminum chloride (0.960 g, 0.0072 mol) dissolved
in THF (10 mL) was then added, dropwise, over a period of 1 h, at 0 °C,
and the stirring was continued at rt until the reaction was judged to be
1
1
complete by B NMR spectroscopy (chemical shift δ (ppm): ꢀ13.3 (q)).
The solvent was removed under reduced pressure via cannula distillation
under inert atmosphere, the residue was stirred with anhydrous pentane
for 1 h and filtered through Celite under nitrogen atmosphere, and the
Supporting Information Available. Experimental pro-
cedures and compound characterization data/spectra.
This material is available free of charge via the Internet
at http://pubs.acs.org.
solvent was removed in vacuo to obtain triethylamineꢀborane complex
1
as a colorless liquid, 289 mg, 70%. H NMR (300 MHz, CDCl
3
) δ (ppm):
2
.79 (q, J = 7.3 Hz, 6H), 0.75ꢀ2.00 (broad q, BH
3
), 1.19 (t, J = 7.3 Hz,
3
) δ (ppm): 52.1, 8.3. B NMR (96
1
H). C NMR (101 MHz, CDCl
3
11
9
MHz, CDCl
3
) δ (ppm): ꢀ13.32 (q, J = 95.0 Hz).
The authors declare no competing financial interest.
Org. Lett., Vol. XX, No. XX, XXXX
C