Mechanism of metal-free hydrogen transfer between amine-boranes and aminoboranes

Article abstract of DOI:10.1021/ja307247g

The kinetics of the metal-free hydrogen transfer from amine-borane Me ₂NH?BH₃ to aminoborane iPr₂N=BH ₂, yielding iPr₂NH?BH₃ and cyclodiborazane [Me₂N-BH₂]₂ via transient Me ₂N=BH₂, have been investigated in detail, with further information derived from isotopic labeling and DFT computations. The approach of the system toward equilibrium was monitored in both directions by ¹¹B{¹H} NMR spectroscopy in a range of solvents and at variable temperatures in THF. Simulation of the resulting temporal-concentration data according to a simple two-stage hydrogen transfer/dimerization process yielded the rate constants and thermodynamic parameters attending both equilibria. At ambient temperature, the bimolecular hydrogen transfer is slightly endergonic in the forward direction (ΔG₁° ₍₂₉₅₎ = 10 ± 7 kJ?mol^-1; ΔG ₁₍₂₉₅₎ = 91 ± 5 kJ?mol^-1), with the overall equilibrium being driven forward by the subsequent exergonic dimerization of the aminoborane Me₂N=BH₂ (ΔG ₂°₍₂₉₅₎ = -28 ± 14 kJ?mol^-1). Systematic deuterium labeling of the NH and BH moieties in Me ₂NH?BH₃ and iPr₂N=BH₂ allowed the kinetic isotope effects (KIEs) attending the hydrogen transfer to be determined. A small inverse KIE at boron (k_H/k_D = 0.9 ± 0.2) and a large normal KIE at nitrogen (k_H/k_D = 6.7 ± 0.9) are consistent with either a pre-equilibrium involving a B-to-B hydrogen transfer or a concerted but asynchronous hydrogen transfer via a cyclic six-membered transition state in which the B-to-B hydrogen transfer is highly advanced. DFT calculations are fully consistent with a concerted but asynchronous process.

Full text of DOI:10.1021/ja307247g

Article
pubs.acs.org/JACS
Mechanism of Metal-Free Hydrogen Transfer between Amine−
Boranes and Aminoboranes
Erin M. Leitao, Naomi E. Stubbs, Alasdair P. M. Robertson, Holger Helten,† Robert J. Cox,
Guy C. Lloyd-Jones,* and Ian Manners*
School of Chemistry, Cantock’s Close, University of Bristol, Bristol BS8 1TS, United Kingdom
S
* Supporting Information
ABSTRACT: The kinetics of the metal-free hydrogen transfer
from amine−borane Me2NH·BH3 to aminoborane iPr2N
BH2, yielding iPr2NH·BH3 and cyclodiborazane [Me2N-BH2]2
via transient Me2NBH2, have been investigated in detail,
with further information derived from isotopic labeling and
DFT computations. The approach of the system toward
equilibrium was monitored in both directions by 11B{1H}
NMR spectroscopy in a range of solvents and at variable
temperatures in THF. Simulation of the resulting temporal−
concentration data according to a simple two-stage hydrogen
transfer/dimerization process yielded the rate constants and
thermodynamic parameters attending both equilibria. At
ambient temperature, the bimolecular hydrogen transfer is
⧧
slightly endergonic in the forward direction (ΔG1°(295) = 10 7 kJ·mol−1; ΔG1
= 91 5 kJ·mol−1), with the overall
(295)
equilibrium being driven forward by the subsequent exergonic dimerization of the aminoborane Me2NBH2 (ΔG2°(295) = −28
14 kJ·mol−1). Systematic deuterium labeling of the NH and BH moieties in Me2NH·BH3 and iPr2NBH2 allowed the kinetic
isotope effects (KIEs) attending the hydrogen transfer to be determined. A small inverse KIE at boron (kH/kD = 0.9 0.2) and a
large normal KIE at nitrogen (kH/kD = 6.7 0.9) are consistent with either a pre-equilibrium involving a B-to-B hydrogen
transfer or a concerted but asynchronous hydrogen transfer via a cyclic six-membered transition state in which the B-to-B
hydrogen transfer is highly advanced. DFT calculations are fully consistent with a concerted but asynchronous process.
Amine−borane dehydrogenation chemistry is also of interest
as a route to polyaminoboranes, [RNH-BH2]n, boron−nitrogen
analogues of polyolefins, and “white graphene”, single-layer
films of hexagonal boron nitride.7,21−25 Furthermore, inspired
by the broad interest in amine−borane dehydrogenation, an
emerging field of amine−borane and aminoborane coordina-
tion chemistry is now attracting much attention.26−32
Elimination of hydrogen from amine−boranes was first
established thermally (>100 °C; extended periods);33−36
more recently, it has been shown to proceed very efficiently
at lower temperatures under catalytic conditions. A wide range
of catalysts has been reported for amine−borane dehydrogen-
ation, based mainly on transition metals2,37−49 but also, in
several cases, main-group elements50−54 (eq 1).
INTRODUCTION
■
Amine−boranes, R3N·BR′3, and aminoboranes, R2NBR′2 (R,
R′ = H, alkyl, aryl), represent two of the simplest and most
studied classes of inorganic molecules. These compounds,
along with related boron−nitrogen species, have played a key
role in the development of inorganic chemistry due to their
isostructural and isoelectronic relationship to well-known
organic compounds.1−8 Amine−boranes are also important as
reagents and are widely utilized as hydroboration agents for
organic substrates and for the reduction of metal salts to
generate metal nanoparticles.9 In this respect, their easy
handling and increased stability has led to a valuable role as
mild and convenient alternatives to commercial borane
reagents L·BH3 (L = THF or SMe2).
As a result of the growing need for the development of
sustainable and environmentally benign alternatives to
petroleum and other fossil fuels, there has been a surge of
interest in amine−boranes over the past decade.10−18 In this
context, ammonia−borane, NH3·BH3, an air-stable solid with a
hydrogen content of 19.6%, has attracted much attention as a
possible alternative to liquid hydrogen as a portable hydrogen
storage material.11 For this to be realistic, the facile release of
hydrogen and rehydrogenation of the resulting spent fuel is
necessary.6,14,18−20
Using Ir, Ru, Fe, Ni, and Rh catalysts, the dehydropolyme-
rization of monoalkylamine−boranes (e.g., MeNH2·BH3,
Received: July 27, 2012
Published: September 27, 2012
© 2012 American Chemical Society
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nBuNH2·BH3) and NH3·BH3 can be achieved by the controlled
elimination of 1 equiv of H2.37,45,55,56 In the case of Ir,
preliminary studies suggest that the initial product of
dehydrogenation may be a transient aminoborane (RNH
BH2, R = H, Me, nBu), which subsequently polymerizes at the
metal center to give poly(alkylaminoboranes)21,37 (Scheme 1).
hydrogenation of substrates such as imines.70−72 A related
example is reversible hydrogen activation by ansa-amino-
boranes, which can be utilized to hydrogenate silyl enol
ethers.73,74 Berke and co-workers have also recently reported
stoichiometric metal-free hydrogenations of organics using
NH3·BH3 as the source of hydrogen, a process that has been
applied to the reduction of polar olefins,75 imines,51 aldehydes,
and ketones.76 Similarly, Musgrave and co-workers have
suggested computationally the metal-free hydrogenation of
carbon dioxide with NH3·BH3.77,78
Scheme 1. Metal-Mediated Amine−Borane
Dehydropolymerization37
A detailed understanding of hydrogen transfer chemistry
between amine−boranes and aminoboranes will be essential for
the effective development of aminoborane hydrogenation. In
addition to direct utility, mechanistic insight will also be
relevant for controlling hydrogen transfer in, e.g., dehydrocou-
pling reactions. Herein we report on a detailed kinetic and
computational investigation of the equilibrium hydrogen
transfer between Me2NH·BH3 (B) and iPr2NBH2 (A) (eq
3).
We recently reported the discovery that aminoboranes
function as hydrogen acceptors to allow the metal-free
dehydrogenation of several amine−boranes at ambient temper-
atures (eq 2).50
RESULTS AND DISCUSSION
■
The inherent polarization of the B−N bond means that net
transfer of hydrogen from an amine−borane to an aminoborane
formally involves N-to-N transfer of a protic hydrogen and
concomitant B-to-B transfer of a hydridic hydrogen. Although
the overall transformation is simple, at least four general
pathways can be envisaged (Scheme 2). The most direct
pathway (1) involves simultaneous cleavage of N−H and B−H
bonds in a concerted, bimolecular process, whereby both the
hydridic and protic hydrogens are transferred in a single and
thus rate-determining step. The second general class of
pathways involves stepwise transfer, in which either N-to-N
hydrogen transfer precedes B-to-B hydrogen transfer, or vice
versa (2 and 3). For both pathways, covalent linear diborazane
intermediates can be envisaged (2a, 3a), or the reactions could
involve stepwise radical (intermediates 2b, 3b) or ionic
(intermediates 2c, 3c) hydrogen transfers or hydrogen transfers
with accompanying B−N bond cleavage (intermediates 2d,
3d).
The final route we considered involves unimolecular
dissociation of the amine−borane adduct B, leaving Me2NH
and BH3 free in solution (4). Even at this stage, because
BH3·THF was not detected at any point in solution by 11B
NMR spectroscopy (δB = −1.1 ppm), formation of
intermediates 2d and 4 was considered unlikely. Moreover,
the addition of BH3 to iPr2NBH2 (A) connects intermediates
4 with 3d, thus resulting in (unobserved) unproductive
reactions, analogous to those occurring with the less substituted
derivatives RNH2·BH3 (R = H, Me).79 In order to more fully
evaluate these mechanistic possibilities (Scheme 2), the kinetic
and thermodynamic parameters of the hydrogen transfer, along
with the influence of solvent and kinetic isotope effects, were
determined by experimental and computational methods.
1. Experimental Data for Metal-Free Hydrogen Trans-
fer from Me2NH·BH3 to iPr2NBH2. 1.1. Equilibrium
between A + B and C + D. We began by conducting an 11B
NMR spectroscopic study of the stoichiometric reaction of
iPr2NBH2 (A, δB = 34.7 ppm, t, JBH = 127 Hz) with
Me2NH·BH3 (B, −13.9 ppm, q, JBH = 97 Hz) at ambient
temperature in THF (eq 3). Analysis by 11B NMR spectros-
copy, 18 h after mixing, revealed the signals arising from A and
B to have decreased in intensity; concomitantly, two new
species formed with signals at δB = −21.5 ppm (q, JBH = 97 Hz)
Interestingly, when R = R′ = Me, the reaction proceeded
much more cleanly (eq 3) than when R′ = Me, R = H or R = R′
= H, where [H2B(μ-H)(μ-NRR′)BH2] is a major side product
(eq 4). In contrast, the undetected aminoborane Me2NBH2,
presumed to be formed after hydrogen transfer, spontaneously
dimerized to generate the well-known cyclodiborazane, [Me2N-
BH2]2.40,57
This result was significant as, to the best of our knowledge,
direct hydrogenation of aminoboranes had not been previously
reported.20 Moreover, the ability to hydrogenate formally
unsaturated boron−nitrogen species is relevant to the problem
of regeneration of spent fuels in hydrogen storage applications.
In addition, the hydrogen transfer reaction between amine−
boranes and aminoboranes offers the potential for the
formation of metal-free oligomers and/or polymers in the
absence of a catalyst, as was observed with hydrogen transfer
from the amine−borane adducts MeNH2·BH3 and NH3·BH3 by
11B NMR spectroscopy (eq 4).50 The hydrogen transfer
reactivity is also highly relevant with respect to the mechanistic
considerations associated with metal-catalyzed group 13−15
dehydrocoupling reactions as, to date, the focus has been
almost entirely on the role of the metal.
This hydrogen transfer chemistry can also be considered
within the context of recent work on metal-free transfer
hydrogenation reactions of organic substrates. For example,
alkenes,58 ketones,59−61 aldehydes,62,63 ketimines, quino-
lines,64−66 and N-heterocycles can be hydrogenated by
employing various protic acids in combination with formal
hydride sources, such as Hantzsch esters.67−69 A further recent
advance is the use of frustrated Lewis pair (FLP) systems such
as iPr2NH and the Lewis acid B(C6F5)3 to enable the catalytic
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Scheme 2. Initial Steps in General Pathways (1→4) for Hydrogen Transfer from Me2NH·BH3 (B) to iPr2NBH2 (A)
and 4.7 ppm (t, JBH = 113 Hz), assigned to the known
compounds iPr2NH·BH3 (C) and [Me2N-BH2]2 (D),
respectively.80,81 At this stage, 54% hydrogenation of A had
occurred, and 58% of B had been dehydrogenated to cleanly
furnish dimer D, as determined by integration of the peaks in
the 11B NMR spectrum. The monomeric precursor to D,
aminoborane Me2NBH2 (M), was not detected by 11B NMR
spectroscopy at this time, consistent with its known propensity
for rapid dimerization in THF solution at 20 °C.82,83 Allowing
the reaction to stir for a further 20 days resulted in only a
marginal increase in the relative intensities of the 11B signals of
C and D, indicating that the system had approached
equilibrium. The hydrogen transfer process was then monitored
by 11B{1H} NMR spectroscopy at 20 °C in THF solution
(Figure 1), demonstrating that, within experimental error,
equilibrium was established after 30 h (Figure 2a).
The same product mixture (A, B, C, D) was generated by a
stoichiometric reaction of C (0.22 M) with D (0.11 M), which
Figure 1. 11B{1H} NMR spectra of the reaction of iPr2NBH2 (A)
required 190 h at ambient temperature (Figure 2b). The
attainment of a dynamic equilibrium was corroborated by
reaction of A with Me2ND·BD3 (d4-B). After 2 h at 20 °C in
THF solution, 11B NMR spectroscopy indicated transfer of D2,
to predominantly generate iPr2ND·BH2D (d2-C), as deter-
mined by the broad triplet characteristic of a BH2D group
(B−D coupling was not resolved, Figure 3b). As the reaction
progressed, scrambling of deuterium into all of the B−H
hydrogen environments was observed, as indicated by the
disappearance of the distinct proton couplings in the 11B NMR
spectra.
and Me2NH·BH3 (B) (20 °C, THF, first 12 h) to generate
iPr2NH·BH3 (C) and [Me2N-BH2]2 (D). Spectra are offset to the
right to show individual peak intensities.
borane M (δB = 36.2 ppm, t, JBH = 125 Hz), generated in low
concentrations in the early stages of the reaction (Figure 4). A
second data set was obtained using an excess of B (0.43 M, see
Figure SI-1), and also the reverse approach to equilibrium (0.22
M C + 0.11 M D) was measured at 22 °C (see Figure SI-2).
Analysis of the initial pseudo-first-order phase of the latter
process (C + 0.5D) allowed direct extraction of the rate
constant for monomerization of D (k−2, Table 1) at 22 °C.
Using this value, the kinetics for the forward reactions were
then simulated (solid lines, Figure 4) according to a simple
1.2. Equilibrium Kinetics. Detailed 11B{1H} NMR spectro-
scopic analysis of the reaction of A (0.22 M) with B (0.22 M)
at 22 °C in THF solution provided temporal−concentration
data for all species, including the elusive monomeric amino-
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Table 1. Rate and Equilibrium Constants Derived from
Simulation of the Kinetics for Two-Stage Reaction (Eq 5) of
iPr2NBH2 (A) with Me2NH·BH3 (B) in THF
temperature /°C
22
30
38
46
54
k1/10−4 M−1·s−1
error /%
5.1
8.3
9.1
11
11
16
18
a
7.3
9.5
8.2
k−1/10−2 M−1·s−1
error /%
3.1
10
4.4
13
5.2
8.6
6.6
11
8.3
10
a
k2/10−2 M−1·s−1
error /%
2.3
4.1
2.7
4.8
4.3
3.5
6.2
4.3
9.0
4.2
a
Figure 2. 11B{1H} NMR spectra (20 °C, THF) of the reactions of (a)
iPr2NBH2 (A) and Me2NH·BH3 (B) after 30 h, and (b)
iPr2NH·BH3 (C) and [Me2N-BH2]2 (D) after 190 h.
b
k−2 /10−7 s−1
2.3
8.1
27
82
240
k−1/k2
1.4
1.7
9.8
1.6
2.0
3.3
1.2
2.1
1.6
1.1
2.4
0.8
0.9
2.2
0.4
K1/10−2
K2/104 M−1
a
b
Overall experimental deviation from the simulated model. Values at
22 and 54 °C determined by analysis of kinetics of C + D; values at
30−46 °C are estimated; see text for discussion.
Figure 3. (a) 11B NMR spectrum of iPr2NBH2 (A) and
Me2ND·BD3 (d4-B) in THF at 20 °C. (b) The same system after 2
h, during which partial equilibration occurred to predominantly
generate iPr2ND·BH2D (d2-C) and [Me2N-BD2]2 (d4-D), plus
isotopologues. On further reaction, complete redistribution of D/H
occurs between all boron centers.
large value of K2, kinetic simulations of the first 50%
equilibration of A + B (ca. 3 × 104 s = 8.3 h) were found to
be insensitive to k−2 (within experimental error) until k−2 was
increased by 2 orders of magnitude.
Simulations were then conducted with experimental data sets
obtained for approach to equilibrium at 30, 38, 46, and 54 °C,
from A (0.22 M) + B (0.22 and 0.43 M; see Figures SI-3−SI-
10) as well as C (0.22 M) + D (0.11 M at 54 °C; see Figure SI-
11). The model was again relatively insensitive to the
magnitude of k−2 during the first 50% of equilibration,84 with
the simulations allowing extraction of k1, k−1, and k2 (Table 1).
The excellent correlations between simulation and experiment
support the conclusion that the equilibrium (K1) between A
and B is driven forward by coupling to the sequential
equilibrium (K2) of M with 0.5D, as outlined in eq 5. The
bimolecular rate constants for hydrogen transfer at 22 °C afford
K1 = (1.7
0.3) × 10−2 (dimensionless). This endergonic
equilibrium (K1; ΔG1° = +10 kJ·mol−1) can be qualitatively
understood to arise from the steric decompression arising on
conversion from tetrahedral to trigonal planar geometry at
nitrogen in the hydrogen donor. Essentially, therefore, the
more hindered iPr2NH·BH3 (C) is a better hydrogen donor
than Me2NH·BH3 (B), and conversely the less hindered
Me2NBH2 (M) is a more effective hydrogen acceptor than
iPr2NBH2 (A).
Figure 4. Reaction profile for the reaction of iPr2NBH2 (A) with
Me2NH·BH3 (B) (both 0.22 M, 22 °C, THF) to yield iPr2NH·BH3
(C) and Me2NBH2 (M), and thus [Me2N-BH2]2 (D) via
dimerization (eq 5). Open circles are experimentally derived (11B
NMR) data; solid lines are simulations using the values for k1, k−1, k2,
and k−2 given in Table 1. Note the orange trace for B is obscured by
that of A.
For the second step in the overall reaction (eq 5), the
bimolecular rate constant k2 = (2.3
0.1) × 10−2 M−1·s−1
(Table 1, 22 °C) compares well with the value of 1.0 × 10−2
M−1·s−1 recently estimated by Weller and Lloyd-Jones for the
dimerization of M generated during Rh-catalyzed dehydrocou-
pling of B in o-difluorobenzene.85 In the current process (eq 5),
the dimerization of M to generate [Me2N-BH2]2 (D) is in
sequential equilibrium model (eq 5), thus yielding the
remaining three rate constants (k1, k−1 and k2) and both
equilibrium constants (Table 1, 22 °C). Consistent with the
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competition with capture by C in the reverse hydrogen transfer
reaction (eq 6).
(k2 and k−2) was investigated. A range of polar, potentially
coordinating solvents such as tetraglyme, acetonitrile, and
dimethoxyethane were considered as well as polar, non-
coordinating solvents such as dichloromethane and difluor-
obenzene and the relatively non-polar, non-coordinating
solvent toluene. Kinetic data were acquired by 11B{1H} NMR
spectroscopy and analyzed by simulation (Figures SI-14−SI-
25) via the same procedures employed for the reaction in THF
at 22 °C (see above), to afford the full complement of rate
constants, and thus equilibrium constants, for each system
(Table SI-3).
A key observation from these experiments is that there was
only a small effect of the solvent on hydrogen transfer rates (k1
ranged from (2.0−19) × 10−4 M−1·s−1, and k−1 from (5.6−23)
× 10−2 M−1·s−1), with both rates being influenced similarly and
thus rather small variations in K1 ((1.1→14) × 10−3). Overall
these features suggest that there are only small changes in
charge distribution between the reactants and the transition
state, this strongly weighing against the formation of ionic
intermediates such as 2c and 3c, and against polarized radical
intermediates 2b and 3b (Scheme 2). In contrast, a more
significant effect was found for the dimerization/monomer-
ization equilibrium, with K2 values ranging from (0.13→24) ×
106 M−1. Acetonitrile was found to substantially accelerate the
dimerization (k2 = 1.3 0.3 M−1·s−1), consistent with recent
reports on Rh-catalyzed dehydrocoupling of B, where M is an
intermediate.82,85
1.5. Kinetic Isotope Effects for Hydrogen Transfer. As
noted above, reversible hydrogen transfer between iPr2NBH2
(A) and Me2ND·BD3 (d4-B) resulted in H/D redistribution at
boron (Figure 3) in an overall process (eq 5) that proceeded
more slowly than with B under the same conditions. To
provide further information on the nature of the hydrogen
transfer, temporal−concentration data were acquired (using
11B{1H} NMR spectroscopy) for four separate reactions at 22
°C, each employing different combinations of deuterium
labeling in substrates A and B. Simulation of the kinetics
(Figures 5 and SI-26−SI-29) according to the standard model
(eq 5), employing k−2 derived from the unlabeled system,
afforded k1, k−1, and k2, and thus kinetic isotope effects (KIEs,
kH/kD) associated with these steps (Table 4).
As expected, the net KIEs for dimerization (k2) of M, where
no B−D cleavage occurs, were small (average kH/kD = 1.1).
However, the KIEs for hydrogen transfer were substantial in
both directions (k1 and k−1), but only for the protic (N-to-N)
transfer (average kH/kD = 6). In contrast, the hydridic (B-to-B)
transfer was generally accompanied by a small inverse KIE
(average kH/kD = 0.9), although it is not evident at this stage
whether this arises from the hydrogen that is transferred or
from the four spectating hydrogens on the two boron centers.
The large, normal KIE associated with deuterium transfer
from nitrogen (Table 4) clearly indicates that N−H cleavage
occurs in the rate-determining step. However, the small inverse
The bimolecular rate constants for these two processes are
similar (k−1/k2 = 1.4 at 22 °C) and about 2 orders of magnitude
greater than for the forward hydrogen transfer (k1); moreover,
until overall equilibrium (eq 5) is approached, the monomer-
ization (k−2) of D is orders of magnitude slower than any of the
other processes. As a result, in the approach to equilibrium
from A + B, the monomeric species M does not accumulate
substantially; instead it reaches a relatively low maximum
concentration ([M]max) early in the reaction. For the example
in Figure 4, where [A,B]o = 0.22 M, the relative rate of reverse
hydrogen transfer over dimerization is approximately constant
(k−1[C]/k2[M] = 1.4) until ca. 1 × 103 s, after which it rises to
the equilibrium value of ca. 232. This results in the maximum
monomer concentration of 0.015 M being reached at ca. 1 ×
103 s and then steadily decaying to reach [Meq] = 0.0008 M.
This propensity to undergo reverse hydrogen transfer (k−1)
results in the efficient redistribution of D and H at boron
observed during the reaction of A and d4-B (Figure 3).
1.3. Equilibrium Thermodynamics. The rate constants
determined at five different temperatures (22, 30, 38, 46, and
54 °C, Table 1) were employed for Eyring analysis of the
forward (k1) and backward (k−1) hydrogen transfers between
iPr2NBH2 (A) and Me2NH·BH3 (B) (Figure SI-12). Linear
regression of the data afforded ΔH⧧ and ΔS⧧ (Table 2) for
both processes. Both hydrogen transfers involve large, negative
entropies of activation (−201 to −210 J·mol−1·K−1), consistent
with bimolecular assembly of a highly ordered activated
complex, but rather low enthalpies (+21 to +29 kJ·mol−1),
suggesting that bond breaking accompanies bond making, i.e.,
that transfer is concerted. The free energies of activation (81−
91 kJ·mol−1 at 295 K) are similar to those found for Diels−
Alder reactions (∼100 kJ·mol−1).86
By allowing samples of A + B (0.22 M in THF) to fully
equilibrate, the apparent equilibrium constant (Keq) for the
overall process was determined by 11B NMR spectroscopy, the
equilibrium concentration of Me2NBH2 (ca. 0.2% of total
boron) being negligible. Using Keq values determined at 25, 35,
45, 55, and 65 °C in THF, a Van’t Hoff analysis was conducted
(Table SI-1 and Figure SI-13). Linear regression afforded ΔH°
and ΔS° (Table 3), with the reaction found to be slightly
exergonic at 22 °C (ΔG°(295) = −7 2 kJ·mol−1, Table 3; see
also Table SI-2). The high entropic cost of dimerization of
1
Me2NBH2 (M) to generate /2[Me2N-BH2]2 (D) results in
the overall reaction becoming endergonic above ca. 65 °C.
1.4. Effect of Solvent. To further probe the transition states
involved in the overall reaction (eq 5), the influence of solvent
on the rates of hydrogen transfer (k1 and k−1) and dimerization
a
Table 2. Thermodynamic Activation Parameters for Hydrogen Transfers between iPr2NBH2 (A) + Me2NH·BH3 (B) and
iPr2NH·BH3 (C) + Me2NBH2 (M) at 22 °C
A + B (k1)
91 5 kJ·mol−1
C + M (k−1)
⧧
⧧
⧧
⧧
ΔG1
ΔS1
ΔH1
ΔG−1
ΔS−1
ΔH−1
81 2 kJ·mol−1
−201 8 J·mol−1·K−1
21 3 kJ·mol−1
(295)
(295)
b
b
−210 11 J·mol−1·K−1
29 5 kJ·mol−1
⧧
⧧
b
b
a
b
From ln(k/T) = ΔS⧧/R + ln(kB/h) − ΔH⧧/RT. Linear regression of Figure SI-12; see Supporting Information.
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Table 3. Thermodynamic Parameters Calculated for the Overall Reaction (Eq 3)
a
b
Van’t Hoff analysis
Eyring analysis
c
c
ΔG°(295)
−7 2 kJ·mol−1
ΔG°(295)
ΔS°
ΔH°
b
−18 18 kJ·mol−1
−187 50 J·mol−1·K−1
−73 23 kJ·mol−1
d
ΔS°
−149 20 J·mol−1·K−1
d
ΔH°
−51 2 kJ·mol−1
a
From ln(Keq) = ΔS°/R − ΔH°/RT, where Keq = K1 K2 = [Ceq]2[Deq]/[Aeq]2[Beq]2. From data in Table 1, ΔXn° = ΔXn⧧ − ΔX−n⧧ and ln(k/T) =
2
c
d
ΔS⧧/R + ln(kB/h) − ΔH⧧/RT. ΔG° = ΔH° − TΔS°. Linear regression of Figure SI-13.
In the latter two pathways, linear but isomeric diborazane
(R2NH-BH2-NR′2-BH3) intermediates 2a and 3a would be
generated. The absence of signals from these species in any of
the 11B NMR spectra would require them to be transient
intermediates. However, preparation of a reference sample of
linear diborazane iPr2NH-BH2-NMe2-BH3 demonstrated that
this material is stable over a period of 24 h under the reaction
conditions (see Supporting Information). With pathways 2, 3,
and 4 largely dismissed via experiment, we turned to theoretical
(DFT) calculations for further insight regarding the mechanism
of hydrogen transfer.
2. DFT Calculations. 2.1. Pathway 1 versus 2, 3, and 4.
All of the pathways for reactions between iPr2NBH2 (A) and
Me2NH·BH3 (B) considered above were explored computa-
tionally by means of DFT calculations (Schemes 2 and 3, Table
Figure 5. Experimental (11B{1H} NMR, circles) and simulated (lines)
temporal−concentration data for iPr2NB(H/D)2 (dn-A) in the
reaction of dn-A (0.22 M) with Me2N(H/D)·B(H/D)3 (dn-B, 0.22 M),
at 22 °C in THF.
Scheme 3. Selected Computed Species, Table 5
Table 4. Rate Constants (Simulation, Figure 5) and KIEs
(kH/kD) for Hydrogen Transfer (k1 and k−1) between
iPr2NB(H/D)2 (dn-A) and Me2N(H/D)·B(H/D)3 (dn-B)
at 22 °C
k1 /10−5
KIE (k1)
kH/kD
k−1 /10−3
KIE (k−1
)
reactants
M−1·s−1
M−1·s−1
kH/kD
NBH2 (A)
NH·BH3 (B)
48
3
−
32
2
−
NBH2 (A)
7.2 0.5
9.2 0.7
6.7 0.9
5.2 0.8
0.9 0.2
0.7 0.1
5.1 1.2
5.9 1.2
6
5
3
2
ND·BH3 (d1-B)
NBD2 (d2-A)
ND·BD3 (d4-B)
NBD2 (d2-A)
NHBD3 (d3-B)
55
66
6
7
23
36
3
4
1.4 0.3
0.9 0.2
5, and Supporting Information). A common factor in the
stepwise pathways (2 and 3, Scheme 2) is that the initial steps
are endergonic. Indeed, the free energies of single hydrogen
atom, proton, or hydride transfer pathways 2 and 3 (via
intermediates 2b,c and 3b,c) are significantly higher than the
free energy of activation for pathway 1, via TS(1). The same is
true for other possible combinations of H•/+/− transfer from
boron or nitrogen of B to boron or nitrogen of A, respectively
(see Supporting Information, Scheme SI-2 and Table SI-4).
Formation of the hydroamination intermediate 2a (Scheme
2) may proceed via a very high barrier (TS(2a), Scheme 3).
The same is true for the formation of the hydroboration
intermediate 3a (Scheme 2) via TS(3a) (Scheme 3). By
comparison, a related version of pathway 3, which is associated
with cleavage of the B−N bond of the amine−borane adduct B
in the transition state TS(3d) (Scheme 3), has significantly
lower free energy of activation.87 However, the barrier is still
higher than that of pathway 1. Moreover, this reaction,
affording the μ-amidodiborane H2B(μ-H)(μ-NiPr2)BH2 and
a
NBH2 (A)
NH·BD3 (d3-B)
a
Reaction proceeded with redistribution of deuterium and hydrogen at
boron.
KIE associated with deuterium substitution on boron is
consistent with B−H cleavage before, during (asynchronous),
or after the rate-liming event, the latter requiring the inverse
KIE to arise from the non-transferred hydrogens on boron.
Thus, while study of the effect of solvent eliminated ionic
intermediates 2b and 3b from consideration, and also weighed
strongly against polarized intermediates 2c and 3c, a number of
pathways, inter alia 1, 2, and 3 (via intermediates 2a and 3a
respectively; Scheme 2) remained feasible, these being at least
consistent with the KIEs.
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hybrid functional PBE0 and the 6-31G(d,p) basis, is shown in
Figure 6.91
Table 5. Computed Thermochemical Parameters for
a
Reactions between iPr2NBH2 (A) and Me2NH·BH3 (B)
ΔG(295)
ΔS(295)
ΔH(295)
reaction or TS
/kJ·mol−1
/J·mol−1·K−1
/kJ·mol−1
b
TS(1)
pathway 1
cd
+86.9
+9.1
−190.8
−8.5
−
−189.1
−202.4
+54.6
+5.1
+38.0
+13.9
−
+154.9
−44.5
+344.1
+308.5
−
b
,
pre-1
−
c
TS(2a)
pathway 2 via 2a
+210.7
+15.3
+328.0
+310.0
−
+115.3
+252.9
+63.6
+326.1
+336.8
+157.0
−
b
b
pathway 2 via 2b
be
,
pathway 2 via 2c
cf
,
TS(2d) via 4
−
+1.3
b
pathway 2 via 2d
+115.7
+194.4
+1.8
c
TS(3a)
−198.2
−209.4
+27.1
+10.1
−161.3
−
b
b
pathway 3 via 3a
pathway 3 via 3b
+334.1
+339.8
+109.3
−
be
,
Figure 6. PBE0/6-31G(d,p)-calculated structure of the transition state
TS(1) for concerted hydrogen transfer from Me2NH·BH3 (B) to
iPr2NBH2 (A). C-bonded hydrogen atoms omitted for clarity;
distances in Å.
pathway 3 via 3c
c
TS(3d)
cf
,
TS(3d) via 4
b
pathway 3 via 3d
+32.5
−22.9
+169.8
+25.7
b
pathway 4
+108.9
+159.0
2.3. Theoretical versus Experimental KIEs. As noted above,
the experimental KIEs for hydrogen transfer were substantial,
but only for the protic (N-to-N) transfer (kH/kD = 6.7 0.9),
with deuteration at boron inducing a small inverse KIE (kH/kD
= 0.9 0.2). In order to reconcile this phenomenon with a
concerted process, hydrogen transfer must be asynchronous,
with the B-to-B transfer of the hydridic hydrogen being
substantially advanced over the N-to-N transfer of the protic
hydrogen at the transition state. The KIEs associated with
deuteration at nitrogen and/or boron were calculated by means
of conventional transition state theory, combined with Wigner’s
tunneling correction92 (Tables 6 and SI-5).
a
See pathways 1−4 in Scheme 2; see also Scheme 3. PBE0/6-
b
31G(d,p), all values relative to separated reactants, A + B. See
c
d
e
Scheme 2. See Scheme 3. Not found. In the cases of reactions that
led to charge separation, the polar medium THF was mimicked by
employing the implicit solvent model PCM (see Experimental
f
Section). Not located, presumably very flat barrier.
Me2NH (intermediates 3d, Scheme 2), is thermodynamically
unfavored.88
The initial formation of solvent-separated Me2NH and BH3
via dissociation of the adduct B (intermediates 4, Scheme 2)
correlates with a significantly larger change in free energy
compared to the formation of TS(1). If dissociation of B
occurred, Me2NH or BH3 could subsequently react with A to
give the linear species iPr2N-BH2-HNMe2 (through TS(2d) via
pathway 4, Scheme 3) or H2B(μ-H)(μ-NiPr2)BH2 (through
TS(3d) via pathway 4, Scheme 3), respectively.89 However, as
both reactions and the preceding dissociation (to produce
intermediates 4, Scheme 2) are thermodynamically unfavorable,
these pathways are presumably not in operation.
Table 6. Theoretical versus Experimental Deuterium KIEs
for Hydrogen Transfer from Me2N(H/D)·B(H/D)3 to
a
iPr2NB(H/D)2
KIE, kH/kD (k1)
b
transfer (B to A)
calculated
experimental
ND·BH3 to NBH2
ND·BD3 to NBD2
NH·BD3 to NBD2
NH·BH2D to NBH2
NH·BD2H to NBD2
6.3
4.9
0.8
1.0
0.8
6.7 0.9
5.2 0.8
0.9 0.2
−
2.2. Concerted Hydrogen Transfer via TS(1). In contrast to
all of the other pathways explored (2−4, see above), the
computed activation parameters (22 °C) for hydrogen transfer
from Me2NH·BH3 (B) to iPr2NBH2 (A) in a concerted
−
a
Theoretical values calculated via TS(1) (PBE0/6-31G(d,p)),
b
fashion (via TS(1)) are in good agreement with the
including Wigner’s tunneling correction. Data from Table 4.
⧧
experimentally determined parameters (ΔG1
calc = 86.9
calc = 38.0
(295)
⧧
kJ·mol−1, exp = 91
kJ·mol−1, exp = 29
5 kJ·mol−1; ΔH1
Deuteration at nitrogen gave a large normal KIE, while
deuteration at boron gave a small and inverse KIE. Altogether,
the calculated values are in good agreement with the
experimental values, supporting the conclusion that hydrogen
transfer proceeds via an asynchronous concerted transition
state TS(1).
In order to better understand the KIEs arising from
deuteration at boron, two further reactions involving selectively
deuterated species were calculated, reactions which were
presently not feasible to investigate experimentally: (i) an
H−D selective hydrogen transfer reaction from Me2NH·BH2D
(d1-B) to iPr2NBH2 (A), and (ii) an H−H selective
hydrogen transfer reaction from Me2NH·BD2H (d2-B) to
iPr2NBD2 (d2-A). For case (i), it was calculated that there
was no KIE present, while for case (ii), a small, inverse KIE was
(295)
5 kJ·mol−1; ΔS1
calc = −190.8
⧧
(295)
J·mol−1·K−1, exp = −210 11 J·mol−1·K−1). The equilibrium
transfer of hydrogen is predicted by the calculations to be
endergonic by 9.1 kJ·mol−1, consistent with the experimentally
derived value (ΔG1°(295) = 10 7 kJ·mol−1). Having found a
pathway consistent with the experimental data, we tested,
computationally, whether an intermediate complex is formed
between A and B prior to concerted double hydrogen transfer.
Using the computational method PBE0/6-31G(d,p), the
complex pre-1 (Scheme 3) was not found.90
Concerted hydrogen transfer was investigated in more detail
using three different density functionals, each in combination
with basis sets of double- and triple-ζ quality. The structure of
the six-membered transition state, TS(1), calculated with the
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obtained. Thus, the net KIEs observed experimentally for
deuteration on boron arise predominantly from inverse
secondary KIEs related to changes in geometry at both boron
centers during the hydrogen transfer process; the primary KIE
for the hydridic transfer is small and normal.
2.4. Dimerization of Me2NBH2 (M). The dimerization of
the aminoborane Me2NBH2 (M) to give [Me2N-BH2]2 (D)
proceeds via a concerted pathway. This [2+2] cycloaddition has
been treated computationally on different occasions before,
whereas the thermochemical character of the reaction has been
calculated with different methods to be either endergonic82,93
or exergonic44,82 at ambient conditions. Using PBE0/6-
31G(d,p), we found the reaction to be exergonic by −20.0
kJ·mol−1 at 22 °C, consistent with the experimentally
determined value (ΔG2°(295) = −28 14 kJ·mol−1, see Table
SI-2). The computed thermochemical reaction and activation
to construct a full energetic landscape for the system (Scheme
4). Transfer occurs via an asynchronous concerted pathway (1,
Scheme 2), with the B-to-B hydridic transfer advanced over the
N-to-N protic transfer at the transition state; these timings are
reflected in small and large primary KIEs, respectively. Eyring
analysis indicates that, at ambient temperature, the hydrogen
transfer reaction is slightly endergonic in the forward direction
(ΔG1°(295) = 10
Me2NBH2 is exergonic (ΔG2°(295) = −28
7 kJ·mol−1), and the dimerization of
14 kJ·mol−1),
predicting the overall reaction to be exergonic (ΔG°(295) = −18
18 kJ·mol−1), in good agreement with the value determined
by Van’t Hoff analysis (ΔG°(295) = −7 2 kJ·mol−1). It is the
coupling of the hydrogen transfer to the slow but exergonic
dimerization of M that, at the concentrations employed (ca.
0.2−0.4 M), drives the process substantially forward (toward C
and D) at ambient temperature.
parameters (Table 7) were also in good agreement with the
It is instructive to compare the hydrogen transfer reaction
(Scheme 4) to the NH3·BH3-mediated reductions of imines
and polar alkenes. Detailed studies by Berke,43,51,75,76,94−97
including isotopic labeling to determine KIEs and theoretical
calculations, have shown these two reactions to be mechanis-
tically distinct, depending on the hydrogen acceptor involved
(Scheme 5).51,75,97
⧧
experimentally determined values (ΔG2
calc = 89.6
calc = 37.2
calc = −177.5
(295)
⧧
kJ·mol−1, exp = 82
kJ·mol−1, exp = 33
8 kJ·mol−1; ΔH2
(295)
9 kJ·mol−1; ΔS2
⧧
(295)
J·mol−1·K−1, exp = −164 30 J·mol−1·K−1; see Table SI-2 and
Figure SI-33 for the calculated structure of the transition state).
Table 7. Computed Thermochemical Parameters for the
Dimerization of Me2NBH2 (M) To Give [Me2N-BH2]2
(D) (PBE0/6-31G(d,p))
Scheme 5. Berke’s Mechanisms for Hydrogen Transfer to (i)
Imines and (ii) Polar Olefins
ΔG(295)
ΔS(295)
ΔH(295)
reaction or TS
/kJ·mol−1
/J·mol−1·K−1
/kJ·mol−1
TS (2M→D)
+89.6
−177.5
−200.0
+37.2
reaction (2M→D)
−20.0
−79.0
3. Overall Energetic Landscape and Comparison with
Other Systems. The above experimental analyses of the
hydrogen transfer and dimerization equilibria, supported by
computational (DFT) studies, provide the kinetic and
thermodynamic information (Tables 2, 3, and SI-2) required
Scheme 4. Energetic Landscape for Overall Hydrogen Transfer Reaction: iPr2NBH2 (A) + Me2NH·BH3 (B)
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The hydrogenation of imines ((i), Scheme 5) is postulated to
occur via a concerted transition state. The experimental KIEs
for this transformation (R = R′ = Ph) are small for both the B-
to-C transfer (kH/kD = 0.87, using NH3·BD3) and the N-to-N
transfer (kH/kD = 1.93, using ND3·BH3), with ND3·BD3 giving
kH/kD = 1.39.51 The imine hydrogenation is strongly exergonic
(DFT calculated ΔG° = −92 kJ·mol−1), thus predicting an early
transition state for concerted transfer and leading to strongly
attenuated primary KIEs. The inverse values for the B-to-C
hydrogen transfer may arise through a secondary KIE from the
BD2 unit, rather than through the hydrogen transfer itself,
analogous to our findings with A + B (Table 6).
The mechanism for hydrogenation of polar olefins is
postulated to involve a fast hydroboration step prior to transfer
of the protic hydrogen on nitrogen in a concerted transition
state ((ii), Scheme 5). This mechanism was proposed on the
basis of the detection of the hydroboration intermediate, as well
as a primary KIE (kH/kD = 1.55) attending the N-to-C transfer
of hydrogen.75
It is thus apparent that the mechanism of hydrogen transfer
to iPr2NBH2 is more similar in character to the reaction of
NH3·BH3 with imines (i) than polar olefins (ii), which is in line
with measured electronegativities.98 However, hydrogen trans-
fer to iPr2NBH2 is much less energetically biased (ΔG1°(295)
= 10 7 kJ·mol−1) than the reaction with imines. As a result,
the much more symmetrical and linear N-to-N transfer of the
protic hydrogen leads to the substantially greater KIE (kH/kD =
6.7) observed with transfer to iPr2NBH2.99,100
EXPERIMENTAL SECTION
■
General Procedures, Reagents, and Equipment. All manipu-
lations were carried out under an atmosphere of nitrogen gas using
standard vacuum line and Schlenk techniques, or under an atmosphere
of argon within an MBraun glovebox. All solvents were dried via a
Grubbs design solvent purification system.101 Deuterated solvents
were purchased from Sigma Aldrich Ltd. and distilled from CaH2 prior
to use. Me2NH·BH3, trimethylsilyl chloride (TMS-Cl), iPr2NH, B(H/
D)3·THF, and B(OiPr)3 were purchased from Sigma Aldrich Ltd. and
purified by sublimation or distillation prior to use. nBuLi (1.6 M in
hexane) was also purchased from Sigma Aldrich Ltd., and was used
without further purification. Me2NH·BD3, Me2ND·BH3, and
Me2ND·BD3 were synthesized according to literature procedures,83
and were all sublimed prior to use. iPr2NBH2, [Me2N-BH2]2 and
iPr2NH·BH3 were synthesized via literature methods40,80 and purified
by distillation or sublimation prior to use.
NMR spectra were recorded using JEOL JNM-ECP300 or JNM-
LA300 spectrometers. Chemical shifts are reported relative to external
standards: BF3·OEt2 (11B). Integration of 11B NMR spectra was
performed using ACD Laboratories Version 9.13 or MestReNova
Version 7.1.1-9649, with an estimated accuracy of 5%. Calibration
tests were performed to determine that the tri- and tetra-coordinate
boron environments have similar relaxation times to ensure that the
integrations were accurate and could be compared throughout. Kinetic
and thermodynamic experiments, where peaks were monitored over
time, were performed in the presence of an internal standard of
B(OiPr)3, neat, in a sealed capillary tube for increased accuracy.
Kinetics were simulated by automated iteration of temporal−
concentration data for all reliable species (generally A, B, C, D, and
Malthough when 11B NMR spectral overlap occurred, selected
species were employed) using Dynochem software (Scale-up Systems
Ltd., Dublin, Eire) until satisfactory fits were obtained within the range
of rate constants and errors reported in Tables 1 and 4, and in the
Supporting Information.
Reaction of iPr2NBH2 (A) with Me2NH·BH3 (B). A 0.83 M
solution of iPr2NBH2 (A, 1.00 mL, 0.83 mmol) in THF was added
to solid Me2NH·BH3 (B, 49 mg, 0.83 mmol) and the mixture was then
stirred over 18 h at 20 °C before analysis by 11B NMR spectroscopy.
11B NMR (96 MHz, THF): 34.7 (t, JBH = 127 Hz, iPr2NBH2), 4.7
(t, JBH = 113 Hz, [Me2N-BH2]2), −13.9 (q, JBH = 97 Hz,
Me2NH·BH3), −21.5 (q, JBH = 97 Hz, iPr2NH·BH3). At this point,
analysis of the peak integrals in the 11B NMR spectrum indicated 54%
conversion of iPr2NBH2 to iPr2NH·BH3 (C) and 58% conversion of
Me2NH·BH3 to [Me2N-BH2]2 (D).
The reaction was then allowed to stir for an additional 30 h at 20 °C
before further analysis by 11B NMR spectroscopy, which indicated a
change in the product distributions, but no change in the products
themselves. After this time period, conversion of iPr2NBH2 (A) to
iPr2NH·BH3 (C), reached 58% and conversion of Me2NH·BH3 (B) to
[Me2N-BH2]2 (D), 62%.
CONCLUSIONS
■
Our in-depth studies have shown that the transfer of hydrogen
from Me2NH·BH3 (B) to iPr2NBH2 (A) occurs directly, in a
bimolecular, concerted, asynchronous, single-step process
involving a six-membered transition state (TS(1), Figure 6,
Scheme 4). This is reminiscent of the mechanism postulated for
the reduction of imines by NH3·BH3 (Scheme 5). It is,
however, endergonic, and the net conversion of A and B into
62% iPr2NH·BH3 (C) and [Me2N-BH2]2 (D) via their
stoichiometric equilibration (0.22 M) at 22 °C is driven by
the exergonic dimerization of the transient Me2NBH2 (M)
produced. At 22 °C, the overall equilibrium is slightly exergonic
(ΔG°(295) = −18 18 kJ·mol−1), with the hydrogen transfer
(ΔG1°(295) = 10
7 kJ·mol−1) only just outweighed by the
dimerization of Me2NBH2 (ΔG2°(295) = −28 14 kJ·mol−1);
the process becomes endergonic above 65 °C. The free energy
necessary to traverse from iPr2NBH2 (A) and Me2NH·BH3
Reaction of [Me2N-BH2]2 (D) with iPr2NH·BH3 (C). A solution of
iPr2NH·BH3 (C, 91 mg, 0.79 mmol) in THF (2 mL) was added to
solid [Me2N-BH2]2 (D, 45 mg, 0.40 mmol). The mixture was then
stirred over 18 h at 20 °C before analysis by 11B NMR spectroscopy.
11B NMR (96 MHz, THF): 34.7 (t, JBH = 127 Hz, iPr2NBH2), 4.7
(B) to the transition state is ΔG1 (295) = 91 5 kJ·mol−1, with
⧧
enthalpic and entropic changes similar to a Diels−Alder
reaction. The kinetic and thermodynamic parameters, including
KIEs, experimentally determined for hydrogen transfer from
Me2NH·BH3 to iPr2NBH2 are in good agreement with DFT
calculations.
Our ongoing research in this area includes the determination
of the steric and electronic effects associated with transfer of
hydrogen, expanding the scope of the reaction, and the
determination of whether the mechanistic model presented
herein can be generalized for all amine−borane to aminoborane
hydrogen transfer reactions. We are also currently exploring the
utility of this interesting and unexpected chemistry in the
formation of polymeric materials and the rehydrogenation of
other multiply bonded acyclic and cyclic boron−nitrogen
species.
(t, JBH = 113 Hz, [Me2N-BH2]2), −13.9 (q, JBH = 97 Hz,
Me2NH·BH3), −21.5 (q, JBH = 97 Hz, iPr2NH·BH3). At this point,
analysis of the peak integrals indicated 6% conversion of iPr2NH·BH3
to iPr2NBH2 and 7% conversion of [Me2N-BH2]2 to Me2NH·BH3.
The reaction was then allowed to stir for an additional 172 h at 20 °C
before further analysis by 11B NMR spectroscopy, which indicated a
change in the product distributions, but no change in the products
themselves. After this time period, 35% conversion of iPr2NH·BH3 to
iPr2NBH2 and 38% conversion of [Me2N-BH2]2 to Me2NH·BH3
was observed.
General Reaction Conditions for Kinetic and Thermody-
namic Measurements of Me2NH·BH3 with iPr2NBH2. A 0.22 M
solution of iPr2NBH2 (A, 2.0 mL, 0.43 mmol) was added to solid
Me2NH·BH3 (B, 25 mg, 0.43 mmol). An aliquot of this mixture (0.7
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mL) was removed to a J-Young NMR tube containing an internal
standard (B(OiPr)3), and the reaction progress was monitored over
time using 11B{1H} NMR spectroscopy at a minimum of 3 min
intervals at a set temperature. The data obtained from these reactions
was simulated to derived rate constants k1, k−1, k2, with k−2 pre-
established or estimated.84 For equilibrium studies, the tube was
heated for 3−4 days at the chosen temperature and then monitored at
the temperature for 12 h without change to determine equilibrium
position.
Synthesis of iPr2NH·BD3. Freshly distilled BD3·THF (25 mL, 25
mmol) was cooled to −78 °C and iPr2NH was added dropwise to the
borane. The solution was stirred at −78 °C for 15 min before warming
to 22 °C for a further hour of reaction time. Volatiles were removed to
yield the product as a clear, colorless oil (0.99 g, 8.4 mmol, 34%). 11B
NMR (96 MHz, CDCl3): −22.6 (s).80
ASSOCIATED CONTENT
* Supporting Information
All experimental reaction profiles of monitored kinetic data
with overlaid simulated data, equilibrium data, and details of the
DFT studies. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
guy.lloyd-jones@bris.ac.uk; ian.manners@bris.ac.uk
■
Present Address
†Institute of Inorganic Chemistry, RWTH Aachen University,
Landoltweg 1, 52056 Aachen, Germany
Synthesis of iPr2NBD2. At −78 °C, 1.6 M nBuLi (5.2 mL, 8.4
mmol) was added dropwise to a solution of iPr2NH·BD3 (0.99 g, 8.4
mmol) in THF (20 mL). The solution was stirred at −78 °C for 15
min before warming to 20 °C for a further hour of reaction time. At
−78 °C, TMS-Cl (1.1 mL, 8.4 mmol) was added via syringe to the
reaction mixture. The solution was stirred at −78 °C for 15 min prior
to warming to 20 °C and stirring for 1 h further. To remove the LiCl
byproduct, the solution was vacuum transferred. The product was left
as a 0.22 M solution in THF for further use. 11B NMR (96 MHz,
THF): 34.1 (br s).80
Qualitative Test for Scrambling with iPr2NBH2 and
Me2ND·BD3. To gain further mechanistic information, the reaction
of iPr2NBH2 (A) with Me2ND·BD3 (d4-B) was monitored by 11B
NMR spectroscopy. Strong evidence of an initial transfer of D2 to
iPr2NBH2 (A) to form iPr2ND·BH2D (d2-C), present as a
broadened triplet at −21.0 ppm in the 11B NMR spectrum, was
observed. After 18 h, 37% conversion of iPr2NBH2 (A) to
iPr2ND·B(H/D)3 (dn-C) was determined, along with 32% conversion
of Me2ND·BD3 (d4-B) to [Me2N-B(D/H)2]2 (dn-D). Scrambling of
hydrogen and deuterium into all components of the reaction mixture
was also apparent by 11B NMR spectroscopy, with the sharp triplet
initially corresponding to iPr2NBH2 rapidly being broadened into a
poorly defined multiplet.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
E.M.L., N.E.S., A.P.M.R., and I.M. acknowledge the EPSRC for
funding. E.M.L. is grateful to the EU for a Marie Curie
postdoctoral fellowship. H.H. thanks the Deutsche Forschungs-
gemeinschaft (DFG) for a postdoctoral research fellowship.
G.C.L.-J. is a Royal Society Wolfson Research Merit Award
Holder. We thank Prof. Dr. Jeremy Harvey for useful
discussions.
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