Alkaline-Earth-Catalyzed Dehydrocoupling of Amines and Boranes

Article abstract of DOI:10.1002/anie.201505949

Dehydrocoupling reactions between the boranes HBpin and 9-borabicyclo[3.3.1]nonane and a range of amines and anilines ensue under very mild reaction conditions in the presence of a simple β-diketiminato magnesium n-butyl precatalyst. The facility of the reactions is suggested to be a function of the Lewis acidity of the borane substrate, and is dictated by resultant pre-equilibria between, and the relative stability of, magnesium hydride and borohydride intermediates during the course of the catalysis.

Full text of DOI:10.1002/anie.201505949

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International Edition: DOI: 10.1002/anie.201505949
German Edition: DOI: 10.1002/ange.201505949
À
B N Coupling
Hot Paper
Alkaline-Earth-Catalyzed Dehydrocoupling of Amines and Boranes
David J. Liptrot, Michael S. Hill,* Mary F. Mahon, and Andrew S. S. Wilson
Abstract: Dehydrocoupling reactions between the boranes
HBpin and 9-borabicyclo[3.3.1]nonane and a range of amines
and anilines ensue under very mild reaction conditions in the
presence of a simple b-diketiminato magnesium n-butyl
precatalyst. The facility of the reactions is suggested to be
a function of the Lewis acidity of the borane substrate, and is
dictated by resultant pre-equilibria between, and the relative
stability of, magnesium hydride and borohydride intermediates
during the course of the catalysis.
Scheme 1. Synthesis of a calcium borohydride (Dipp=2,6-iPr2C6H3).
À
À
by the reaction of hydridic B H and protic N H bonds, would
be highly desirable.
À
A
minoboranes, R2N BR’2, find utility in a variety of
interesting transformations. SolØ and Fernandez, for example,
have shown that aminoboranes provide convenient sources of
nucleophilic amide anions for reactions with activated
alkenes, alkynes, and strained lactones.[1] Extensive work by
Suginome et al. has demonstrated that similar reagents enable
ready access to iminium cations through their reactions with
ketones and aldehydes,[2] while the action of aminodi-
(boranes), RN(BR2)2, on ketones has been shown to provide
imines through the formation of thermodynamically favored
boron–oxygen bonds.[3] However attractive, the more wide-
spread uptake of these applications is hindered by the
multistep synthetic routes necessary to yield anything more
than a very narrow scope of aminoboranes.
The dehydrocoupling of amine-boranes, RnNH3Àn·BH3
(n = 0, 1, 2), which typically produces oligo- and polyborazane
products, has elicited intense recent interest for potential
hydrogen storage applications.[10] It is surprising, therefore,
that, beyond an isolated example of rhodium-based dehy-
drocoupling,[11] the sole precedent for the coupling of an
amine and a monohydrido borane arises from our earlier
report of the reaction between [(HC{(CMe)(N{2,6-
iPr2C6H3})}2)Ca(NPh2)(thf)] and 9-BBN (Scheme 1). In this
À
case the stoichiometric reaction took place through Ca N/
À
H B metathesis to yield the aminoborane and a calcium
borohydride.[12] In this contribution we extend this reactivity
to a catalytic regime, thus allowing the facile synthesis of
aminoboranes from readily available amine and borane
precursors.
Although some latent dehydrogenative reactivity exists
between protic amines and the parent borane, B2H6, or highly
Lewis-acidic dialkylboranes such as 9-borabicyclo-
[3.3.1]nonane (9-BBN),[4–6] the synthesis of aminoboranes by
this route is unreliable and often requires forcing conditions,
or fails completely for common borate esters such as pinacol
or catecholborane. More reliably, the action of tin–nitrogen[7]
and silicon–nitrogen[4,8] bonds upon boranes and haloboranes
An initial assessment of the catalytic activity of the b-
diketiminato magnesium alkyl and calcium amide species
[(HC{(CMe)(N{2,6-iPr2C6H3})}2)MgBu] (I) and [(HC{(CMe)-
(N{2,6-iPr2C6H3})}2)Ca{N(SiMe3)2}(thf)] (II) was undertaken
for the reaction of diethylamine and pinacolborane (HBpin).
Although a 10 mol% loading of both precatalysts proved to
be competent for this dehydrocoupling at ambient temper-
ature, the magnesium-mediated process provided superior
reactivity and an effective stoichiometric conversion into
N,N-diethyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-amine.
Encouraged by this observation, we undertook a study into
the scope of the magnesium-catalyzed dehydrocoupling
reactivity. Table 1 summarizes an investigation into the ability
of I to catalyze the dehydrocoupling of a range of primary and
secondary amines and anilines with both HBpin and 9-BBN.
The weak Lewis acid HBpin was observed to couple readily
with aromatic and aliphatic amines of varying bulkiness
(entries 1–8), but failed to react with the very bulky HN-
(SiMe3)2 (entry 9). Most reactions occurred with a pronounced
bubbling and reached completion at room temperature in less
than a day. Reactions of the more Lewis-acidic 9-BBN were
more dependent on the identity of the amine. While less
sterically congested alkyl amines coupled readily (entry 10),
reactions with more bulky substrates (entry 11) were slower
and comparable to the background reactions. Similarly,
smaller anilines coupled at rates significantly in excess of
À
yields aminoboranes and the relevant E X bond. These
methods are disfavored, however, by the formation of
stoichiometric quantities of the group 14 by-products and, in
the case of tin, a toxic waste stream. As a result, most popular
synthetic routes to aminoboranes utilize the reaction of
lithium amides with BCl3.[9] With these limitations in mind,
a general and simple dehydrocoupling route to aminoboranes,
[*] Dr. D. J. Liptrot, Prof. M. S. Hill, Dr. M. F. Mahon, A. S. S. Wilson
Department of Chemistry, University of Bath
Bath BA2 7AY (UK)
E-mail: msh27@bath.ac.uk
Supporting information for this article (details of the synthesis,
characterization data and the crystallographic protocols) is available
on the WWW under http://dx.doi.org/10.1002/anie.201505949.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
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Table 1: Study of the scope of boron–nitrogen dehydrocoupling cata-
lyzed by I.
sium anilido(pinacol)borane and is the first structurally
characterized species of this class. Whilst a variety of
magnesium amidoborane complexes have been structurally
Entry Borane Amine
Product
t [h]
Conv. [%][c]
characterized, such derivatives contain four-coordinate boron
1
2
3
4
5
6
7
8
9
HBpin
nBuNH2
tBuNH2
PhNH2
DippNH2
PhN(H)Me
(CH2)4NH
Et2NH
pinBNHnBu
pinBNHtBu
pinBNHPh
<1[a]
<1[a]
<1[a]
<1[a]
99
99
99
99
99
99
99
99
0
[13]
À
centers and B H···Mg agostic-type interactions. Notably,
a donor interaction with a pinacolate oxygen atom raises the
Mg coordination number of 1 to four. The complexity of the
pinBNHDipp
pinBN(Me)Ph <1[a]
1
pinacol methyl region in the H NMR spectrum can thus be
pinBN(CH2)4
pinBNEt2
pinBNPh2
<1[a]
<1[a]
<1[a]
144[b]
À
rationalized as a result of the persistence of the Mg O bond
in the nondonor deuterobenzene solvent.
Ph2NH
(Me3Si)2NH No reaction
A similar reaction performed with half an equivalent of
the 9-BBN dimer yielded a 11B NMR spectrum comprising
a single resonance at d = 27.5 ppm, which is attributable to
10
11
12
13
14
9-BBN
nBuNH2
tBuNH2
PhNH2
DippNH2
Ph2NH
R2BNHnBu
R2BNHtBu
R2BNHPh
R2BNHDipp
R2BNPh2
<1[a]
99
1
144[b] 83
DippN(H)BR2. The corresponding H NMR spectrum indi-
12[a]
99
cated the clean formation of two species, identified as
DippN(H)BR2 and the previously reported dimeric magne-
sium hydride [(HC{(CMe)(N{2,6-iPr2C6H3})}2)MgH]2.[14]
Notably, a similar reaction performed with a stoichiometric
quantity of 9-BBN provided two new boron-containing
compounds. While DippN(H)BR2 was again observed to
form with complete consumption of the aniline, an additional
species (2) was characterized by a broadened triplet reso-
nance at d = À13.8 ppm in the 11B NMR spectrum. A further
X-ray diffraction analysis confirmed the identity of 2 (Fig-
ure 1b) as a pseudo-square-pyramidal magnesium borohy-
dride reminiscent of the calcium-centered product illustrated
in Scheme 1. Compound 2 is thus closely related to the
more expansive class of magnesium borohydrides containing
[BH4]À anions, including Mountfordꢀs b-diketiminato-
144[b] 80
144[b] 73
Reaction conditions: I (0.01 mmol) with amine (0.1 mmol) and borane
(0.1 mmol) in C6D6 (0.5 mL). [a] At room temperature. [b] At 608C.
[c] Determined by H NMR spectroscopy.
1
Table 2: Catalytic boron–nitrogen dehydrocoupling catalyzed by I.
Entry
Amine
Ratio
Product
t [days]
Conv. [%][c]
HBpin
1
2
3
nBuNH2
tBuNH2
PhNH2
2:1
2:1
2:1
(PinB)2NnBu
(PinB)2NtBu
PinBNHPh
1[b]
99
99
99
<1[a]
2[b]
9-BBN
4
5
nBuNH2
PhNH2
2:1
2:1
R2BHNnBu
R2BNHPh
1[a]
2[b]
99
99
magnesium
tetra(hydrido)borate,
[(HC{(CMe)-(N{2,6-
iPr2C6H3})}2)Mg(BH4)].[15] More pertinently, these observa-
tions suggest that, when in excess, coordination of the Lewis-
acidic 9-BBN to the magnesium hydride and formation of 2 is
competitive with the deprotonation of DippN(H)BR2.
The validity of these deductions was investigated through
a kinetic study of the reactions of N-methylaniline with
HBpin and 9-BBN catalyzed by I. In line with the data
collated in Table 1, the turnover frequency (TOF) of the
HBpin-based reaction [9935(1353) hÀ1] was found to be
significantly higher than for the reaction with 9-BBN [26.4-
(28) hÀ1]. While a background reaction perfomed between
HBpin and N-methylaniline provided no evidence of dihy-
drogen evolution, this latter TOF value also surpassed that of
an uncatalyzed reaction, utilizing 9-BBN, by an order of
magnitude.
The dehydrocoupling of HBpin with N-methylaniline
conformed to global first-order kinetics (see Figure S1 in the
Supporting Information), which varied with a second-order
dependence on catalyst concentration (see Figure S2 in the
Supporting Information). Pseudo-first-order experiments
were indicative of zero- and first-order rate dependences on
[PhNH(Me)] and [HBpin], respectively and the rate law
shown as Equation (1).
Reaction conditions: I (0.01 mmol) with amine (0.1 mmol) and borane
(0.2 mmol) in C6D6 (0.5 mL). [a] At room temperature. [b] At 608C.
[c] Determined by H NMR spectroscopy.
1
the background reactions (entry 12), while bulkier anilines
reverted to the uncatalyzed rate (entries 13 and 14).
Aminodi(boranes) were readily accessible by adjustment
of the reaction stoichiometry between primary amines and
both HBpin and 9-BBN. Reactions performed between two
molar equivalents of both borane substrates and one equiv-
alent of n-butylamine provided quantitative conversions into
the bis(boryl)ated amines (Table 2, entries 1 and 4), albeit
with gentle heating and slightly extended reaction times
compared to those for aminoborane formation. Although tert-
butylamine also coupled twice with pinacolborane (entry 2)
neither borane showed any sign of twofold dehydrocoupling
with aniline (entries 3 and 5).
The nature of these variations was investigated through
a series of stoichiometric reactions. An NMR-scale reaction
performed
(DippNH2) and I, with subsequent addition of HBpin, yielded
11B NMR spectrum (d = 32.1 ppm) which was indicative of
between
2,6-di-isopropylphenyl
aniline
a
a single three-coordinate boron environment. The corre-
1
sponding H NMR spectrum was less informative and com-
2
1
0
ð1Þ
rate ¼ k½IŠ ½pinBHŠ ½PhNHðMeފ
prised a complex set of signals in the regions attributable to
the pinacol methyl and isopropyl resonances. The origin of
these observations was resolved through the isolation of
single crystals of 1, at À388C from toluene, suitable for X-ray
diffraction analysis (Figure 1a). The compound 1 is a magne-
Dehydrocoupling reactions performed with 9-BBN also
provided linear first-order rate plots for more than three half-
lives at a range of loadings of I (see Figure S4 in the
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Figure 1. ORTEP representations of the compounds 1 (a) and 2 (b; 30% probability ellipsoids). Isopropyl methyl groups and hydrogen atoms
omitted for clarity. Selected bond lengths [Š] and angles [8]: (1) Mg1–O2 2.3737(18), Mg1–N3 2.003(2), O2–B1 1.418(3), B1–N3 1.379(4), N3–C30
1.423(3); N1-Mg1-N2 94.72(8), Mg1-O2-B1 80.94(14), O2-B1-N3 116.8(2), B1-N3Mg1 96.96(15), B1-N3-C30 120.6(2); (2) Mg1–B1 2.3092(15),
Mg1–O1 2.0556(9), Mg1–H1a 1.932(14), Mg1–H1b 1.929(15), B1–C35 1.618(2), B1–C39 1.6026(19); N1-Mg1-N2 93.48(4), N1-Mg1-B1 125.62(5),
C35-B1-C39 107.44(11).
Supporting Information). In this case, however, a plot of the
resultant observed rate constants was found to vary linearly
with the square root of [I], thus indicating a one-half-order
dependence on the precatalyst concentration (see Figure S5
in the Supporting Information). The partial reaction order
with respect to the borane substrate was deduced through
a pseudo-first-order methodology in which the consumption
of the limiting reagent with time was found to be indicative of
an inverse dependence on [9-BBN]. Consistent with this
apparent substrate inhibition, a similar approach utilizing an
excess of 9-BBN suppressed the reaction to such an extent
that there was no observable consumption of the aniline
substrate. We interpret this inhibition to indicate the persis-
tence of a borohydride species similar to 2 under reaction
conditions comprising a large excess of the Lewis-acidic
borane. These experimental limitations notwithstanding, the
global first-order kinetics of the reaction infer a second-order
dependence on [PhNH(Me)] and the formulation of a rate
law for this reaction of the form shown in Equation (2).
lyzed by group 2 reagents.[16] Whilst HBpin provided a neg-
ative entropy of activation indicative of a highly ordered rate-
determining transition state, the reaction of 9-BBN and N-
methylaniline was observed to entail a significantly positive
value [20.0(25) calmolÀ1 KÀ1].
These data indicate a considerable mechanistic variance
across the two reactions. We infer that the kinetic profile of
the reaction involving 9-BBN reflects a pre-equilibrium
between a magnesium borohydride, similar to 2, and a dimeric
magnesium hydride (indicated as A and B, respectively, in
Scheme 2). The stability of the former species as an off-loop
resting state of the reaction effectively limits the magnesium
hydride available to engage in onward reactivity. We suggest
that the notable half-order dependence on precatalyst
concentration, [I], the inhibition of the reaction rate by
increasing borane concentration and the implied second-
1
=
2
À1
2
ð2Þ
rate ¼ k½IŠ ½9-BBNŠ ½PhNHðMeފ
The activation parameters for both reactions were
deduced through variable-temperature Arrhenius and
Eyring analyses (Table 3). A now considerable literature
precedent has highlighted the impact of entropic effects on
the rate-determining energetics during many reactions cata-
Table 3: Activation parameters for the catalytic dehydrocoupling of
N-methylaniline with HBpin and 9-BBN mediated by I.
Ea
DH°
DS°
DG°
298
[kcalmolÀ1
]
[kcalmolÀ1
]
[calmolÀ1 KÀ1
]
[kcalmolÀ1
]
HBpin
9-BBN
9.8(7)
29.5(7)
9.2(7)
29.9(7)
À47.9(25)
23.5
23.0
Scheme 2. Proposed mechanism for Mg-catalyzed dehydrocoupling of
amines and boranes.
20.0(25)
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order behavior with respect to [PhNH(Me)] implicates
a monomer–dimer equilibrium between the dinuclear species
B and an adduct of the weakly basic aniline (C) in the rate-
limiting process of the reaction. The onward trajectory of the
catalysis is then predicated on hydrogen elimination from C
and anilide formation (D). We suggest that subsequent and
facile B–N coupling occurs through nucleophilic attack of the
magnesium-bound anilide on the Lewis acidic 9-BBN sub-
strate and rapid b-hydride transfer to the magnesium center
via the borate intermediate E in a sequence reminiscent of
that deduced by both Sadow and co-workers and Sarazin and
co-workers for a closely related silane–amine dehydrocou-
pling and our previous observations of group 2-centered
amine–borane dehydrocoupling catalysis.[17,13]
For the reaction derived from HBpin, we suggest that the
formation of an off-cycle borohydride similar to A is
disfavored by the significantly reduced Lewis acidity of the
borate ester substrate. Although facile aniline protonolysis
will ensue under this regime, direct nucleophilic attack by the
anilide on HBpin is also likely to be disfavored by the
attenuated Lewis acidity of the borane. We suggest, therefore,
that the second-order dependence on [I] implicates the action
of a further borohydrido magnesium species akin to A, and is
necessary to deliver the HBpin substrate to the magnesium
anilide. In this case, it is feasible that the formation of the
[H2Bpin]À anion will facilitate the assembly of a bimetallic
intermediate through the augmentation of the Lewis basicity
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a consequently
enhanced facility for intermolecular Mg-O coordination.
In summary, we have shown that high conversions to
amineboranes may be achieved through the magnesium- and
calcium-catalyzed deydrocoupling of readily available amines
and boranes. Albeit currently restricted to cyclic borane
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Experimental Section
CCDC 1408761 and 1408762 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre.
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Acknowledgements
We thank the University of Bath for a PhD scholarship
(D.J.L.).
À
Keywords: amines · alkaline-earth metals · B N coupling ·
boron · magnesium
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Received: June 29, 2015
Revised: August 13, 2015
Published online: September 11, 2015
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