Spontaneous Reduction of a Hydroborane To Generate a B?B Single Bond by the Use of a Lewis Pair

Article abstract of DOI:10.1002/anie.201605645

The ansa-aminohydroborane 1-NMe₂-2-(BH₂)C₆H₄crystallizes in an unprecedented type of dimer containing a B?H bond activated by one FLP moiety. Upon mild heating and without the use of any catalyst, this molecule liberates one equivalent of hydrogen to generate a diborane molecule. The synthesis and structural characterization of these new compounds, as well as the kinetic monitoring of the reaction and the DFT investigation of its mechanism, are reported.

Full text of DOI:10.1002/anie.201605645

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201605645
German Edition: DOI: 10.1002/ange.201605645
Frustrated Lewis Pairs
À
Spontaneous Reduction of a Hydroborane To Generate a B B Single
Bond by the Use of a Lewis Pair
ꢀtienne Rochette, Nicolas Bouchard, Julien Lꢁgarꢁ Lavergne, Chꢁrif F. Matta, and
Frꢁdꢁric-Georges Fontaine*
Abstract: The ansa-aminohydroborane 1-NMe2-2-(BH2)C6H4
crystallizes in an unprecedented type of dimer containing a
À
B H bond activated by one FLP moiety. Upon mild heating
and without the use of any catalyst, this molecule liberates one
equivalent of hydrogen to generate a diborane molecule. The
synthesis and structural characterization of these new com-
pounds, as well as the kinetic monitoring of the reaction and the
DFT investigation of its mechanism, are reported.
S
ince their discovery by Stephan and co-workers, frustrated
Lewis pairs (FLPs)[1] have been used to activate many
unreactive molecules[2] and to catalyze the hydrogenation,[3]
hydroboration,[4] and hydrosilylation[5] of unsaturated sub-
strates. Conceptually, the absence of a Lewis adduct promotes
cooperativity between a Lewis acid (Z) and a Lewis base (L)
À
to activate R H substrates, thus generating the respective ion
À
+
À
À
pair [Z R] [L H] . Such cooperativity is possible with weak
À
Lewis adducts if L Z bond dissociation is readily accessi-
ble.[2d] However, the functionalization of a R group bound to
a Group XIII Lewis acids, often used as the Z moiety in FLP
chemistry, is more difficult than when bound to transition
metal species, thus making catalyst design more difficult.
Although the chemistry of boron is mostly limited to its
+ 3 oxidation state, boron(II) species, such as diboranes, are
known.[6] These molecules are useful reagents for borylation[7]
and diboration[8] reactions. Diboranes are normally synthe-
sized under highly reducing conditions (Scheme 1A,B).[9]
Other routes to generate these molecules are available,[6b]
such as metal-catalyzed dehydrogenative coupling reactions
of hydroboranes, as notably reported by Braunschweig and
co-workers (Scheme 1C).[10] Himmel and co-workers were
À
Scheme 1. Previously reported approaches to the formation of B B
bonds.
ansa-Aminohydroborane derivatives of the general for-
mula 1-NR2-2-(BR’2)C6H4 have a rich chemistry, notably for
the hydrogenation of alkynes[13] and the C H bond activation
À
of arenes.[14] Although FLP chemistry has been focusing on
molecules possessing very bulky amines, such as the 2,2,6,6-
tetramethylpiperidino group, or highly Lewis acidic boranes,
such as the B(C6F5)2 group, we have been interested in the
chemistry of BH2 derivatives bearing smaller amine groups
that could exhibit FLP-type behavior.[2d] These species were
found to play an important role in the hydrogenation of
CO2.[15] We report herein that the derivative 1-NMe2-2-
(BH2)C6H4 (compound 1) crystallizes in an unprecedented
À
able to use homogeneous catalysts to generate such a B B
bond from a guanidine–BH3 adduct (Scheme 1D).[11] Other
examples of dehydrogenative coupling were observed in the
chemistry of carboranes[12a] and proposed in the dehydrogen-
ation of amine–boranes.[12b,c]
À
type of dimer, in which a B H bond of one molecule is
À
[*] ꢀ. Rochette, N. Bouchard, J. Lꢁgarꢁ Lavergne, Prof. Dr. F.-G. Fontaine
Dꢁpartement de Chimie, Centre de Catalyse et Chimie Verte (C3V)
Universitꢁ Laval
activated by the N B Lewis pair of another molecule.
Surprisingly, upon heating above 808C, this compound
undergoes spontaneous release of H2, in the first completely
characterized example of an uncatalyzed borane dehydro-
coupling reaction, thus opening the way to a whole new range
of reactivities for frustrated Lewis pairs and diboranes.
As based on the work of Wagner and co-workers,[16] the
addition of LiAlH4 to the boronic ester 1-NMe2-2-(B-
(OMe)2)C6H4 led to 1-LiH, the LiH adduct of 1-NMe2-2-
(BH2)C6H4 (1), which was isolated in 60% yield (Scheme 2).
1045 Avenue de la Mꢁdecine, Quꢁbec (Quꢁbec) G1V 0A6 (Canada)
E-mail: frederic.fontaine@chm.ulaval.ca
Prof. Dr. C. F. Matta
Department of Chemistry and Physics
Mount Saint Vincent University
166 Bedford Highway, Halifax (Nova Scotia) B3M 2J6 (Canada)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201605645.
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

Angewandte
Communications
Chemie
respectively. The B1–B2 distance of 2.255 ꢀ is significantly
longer than that reported for B2H6 (1.776 ꢀ)[17] and the 9-
BBN dimer (1.818 ꢀ)[18] and is indicative of the absence of
significant interaction between the two boron atoms. Since
the bridging hydride could be located in the Fourier map, it
was possible to measure the B1–mH and B2–mH distances of
1.27(1) and 1.30(1) ꢀ, respectively.
Whereas these results present the first fully characterized
structure of 1, we previously proposed the dimeric head-to-
tail isomer 1’ (Scheme 3) as the product of the thermal
Scheme 2. Synthesis of compound 1.
The 1H NMR spectrum of 1-LiH exhibits a characteristic
quartet at 1.2 ppm with a B–H coupling constant of 80 Hz.
The sharp 11B{1H} NMR signal observed at À29.3 ppm is
typical of a tetravalent boron atom. The adduct 1-LiH was
also characterized by X-ray crystallography (see Figure S22 in
the Supporting Information). It was possible to remove the
LiH salt by the addition of bromotrimethylsilane (TMSBr),
which led to the isolation of species 1 in 98% yield.
1
A striking feature of the H NMR spectrum of 1 is the
Scheme 3. Equilibrium between 1 and 1’ and synthesis of compound
2.
presence of eight resonances that were assigned to the
hydrogen atoms of the aromatic rings. Furthermore, one
sharp resonance at 2.7 ppm integrating for 6H was attributed
À
À
to one NMe2 moiety, whereas the other NMe2 moiety
included in the six-membered ring appears as two broad
singlets at 3.1 and 2.8 ppm. These two signals coalesced at
408C in [D]chloroform. Two resonances are present in the
11B{1H} NMR spectrum at À10.4 and 3.3 ppm, thus demon-
strating the unsymmetrical nature of 1. It was possible to
obtain X-ray quality crystals of 1 from a cold saturated
solution in toluene. The ORTEP structure (Figure 1) confirms
the unprecedented dimeric arrangement of 1 that derives
degradation of 1-NMe2-2-(BAr2)C6H4 (Ar= 2,4,6- and 2,4,5-
trimethylbenzene) under an atmosphere of molecular hydro-
gen.[15] Although that symmetrical structure was consistent
with the NMR data, no other characterization was possible
for this degradation product. Computational analysis of the
various possible dimeric forms of 1-NMe2-2-(BH2)C6H4 was
also part of this previous study. However, isomer 1 was not
investigated computationally at that time. Additional DFT
calculations were thus performed at the wB97XD/6-31 ++
G** level of theory in the solvent toluene (SMD),[19] and it
was found that isomer 1 is only slightly more stable than 1’
with DH and DG values of 0.1 and À3.3 kcalmolÀ1, respec-
À
from the formation of a six-membered ring by the B H
activation of one monomer by the FLP site of a second. The
N2–B1 bond distance of 1.615 ꢀ is significantly shorter than
the N1–B1 and N1–B2 distances of 3.044 and 3.705 ꢀ,
1
tively. Careful analysis of the H NMR spectrum of 1 con-
firmed the presence of 1’ as a minor component (see the
Supporting Information for details). The thermodynamic
parameters for the equilibrium were determined by variable-
1
temperature H NMR spectroscopy in [D6]benzene (DH8 =
(3.8 Æ 0.1) kcalmolÀ1 and DS8 = (0.02 Æ 0.01) calmolÀ1 KÀ1
)
and are within the expected margin of error of the calcu-
lations (Scheme 3).
Heating a solution of 1 in [D8]toluene led to the formation
of a new compound, 2 (Scheme 3) along with the release of
molecular hydrogen (1H NMR: d = 4.5 ppm). Species 2 was
originally misidentified as compound 1’, since all the NMR
data were consistent with a symmetric dimer.[15] However, X-
ray crystallography unequivocally identified 2 as {1-NMe2-2-
(BH)C6H4}2. Compound 2 is a rare example of a diborane
bearing both a hydrogen atom and an aryl substituent on
boron (Figure 1).[20] The B B bond length (1.740 ꢀ) and the
À
11B NMR signal of 1 (d = 1.79 ppm) are similar to those
reported by Himmel and co-workers for the guanidine-based
derivative (1.772 ꢀ and d = À1.14 ppm; Scheme 1D).[11]
Figure 1. Crystal structure of compounds 1 and 2. Ellipsoids are drawn
at 50% probability. Hydrogen atoms linked to carbon atoms are
omitted for clarity. Selected bond lengths [ꢂ] and angles [8] for 1: N2–
B1 1.615(2), B1–mH 1.27(2), B2–mH 1.30(1), B2–C12 1.584(3), C12–
C11 1.390(2), C11–N2 1.494(2); C11-N2-B1 108.7(1), N2-B1-mH 104.0-
(7), B1-mH-B2 123(1), mH-B2-C12 108.7(7), B2-C12-C11 121.9(2), C12-
C11-N2 115.7(1); for 2: B1–B2 1.740(2), N1–B1 1.671(2), N2–B2
1.677(1); N1-B1-B2 101.42(8), B1-B2-N2 101.20(8).
1
Unfortunately, our attempts to observe JB–H coupling in 2
proved unsuccessful, since only broadening of the resonance
was observed in the proton-coupled 11B NMR spectrum.
Although thermally induced boron–boron dehydrogenative
2
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!

Angewandte
Communications
Chemie
homocoupling was proposed in amine–borane dehydrogen-
studying the transformation of 1D into 2D, we were able to
calculate a kinetic isotopic effect of 2.0 Æ 0.4.
ation,[12c] this rearrangement is to the best of our knowledge
the first structurally characterized example of such a trans-
formation.
We investigated two main pathways computationally
(DFT at the wB97XD/6-31 ++ G** level of theory with
continuum solvation in toluene according to the electron-
density-based universal solvation model SMD)[19] to ration-
alize this transformation. The direct loss of hydrogen to
generate a boron–boron homocoupling product was previ-
ously postulated by Himmel and co-workers with the
guanidine–BH3 adducts.[11a] In that study, the calculated
value of the transition state (DG° = 39.0 kcalmolÀ1) is too
elevated for a significant rate of spontaneous reaction, and
transition-metal catalysts were needed to generate the
diborane product. We calculated a similar transition state to
be significantly more accessible in our system, with respective
DH° and DG° values of 30.6 and 30.4 kcalmolÀ1 (Figure 3,
To get more insight into the reaction mechanism, we
studied the reactivity of 1 and 2 with hydrogen and deuterium
gas. It was found that 1 does not form a stable adduct with
hydrogen, but can nevertheless split the molecule at room
temperature, as evidenced by the appearance of the charac-
1
teristic signal of HD by H NMR spectroscopy when 1 was
stored for 16 h under D2 (3–4 atm) at room temperature.
Moreover, heating at 808C for 16 h led to the complete
1
À
disappearance of the B H signals, as evidenced by H NMR
spectroscopy, with only a trace amount of 2, thus allowing the
ready preparation of 1-NMe2-2-(BD2)C6H4 (1D). In a similar
experiment, 2 was also able to split hydrogen. However,
heating to 808C was needed to clearly observe the HD signal
by 1H NMR spectroscopy.
We monitored the clean and complete transformation of
1 into 2 by 1H NMR spectroscopy at various temperatures to
determine the kinetic parameters of the transformation. The
disappearance of 1 over time was shown to follow a first-order
process with rates varying from 0.0021 to 0.0094 minÀ1 from
85 to 1008C, respectively. An Eyring plot gave a DH° value of
(24.3 Æ 0.7) kcalmolÀ1 and
a
DS° value of (À0.03 Æ
0.02) calmolÀ1 KÀ1 (Figure 2). The first-order rate is consis-
tent with an intramolecular process, and the negligible
entropy contribution at the transition state indicates that no
significant change in geometry occurs at the transition state
À
during B B bond formation. We also measured the rate of the
reaction at 908C under H2 (3–4 atm), and although the kinetic
profile was too complex to analyze, the reaction was found to
be about 50% slower than under a nitrogen atmosphere (see
the Supporting Information for details). The slower trans-
formation under a large concentration of hydrogen indicates
that the rate for the reverse reaction is significantly enhanced,
thus suggesting the reversibility of the process. Finally, by
Figure 3. Computed reaction profile of the thermal dehydrocoupling of
1 into 2 at the wB97XD/6-31+ +G** level of theory (SMD, toluene);
DH (DG) in kcalmolÀ1
.
TS1). Although lower than in the Himmel study, these values
are still too high to account for the experimental kinetic data.
A second pathway was thus investigated in which the free
amine formally deprotonates the bridging hydride with
simultaneous formation of a boron–boron bond (Figure 3,
TS2A). After the corresponding zwitterionic intermediate is
formed (Figure 3, 2-H2), the release of hydrogen could occur
by a typical FLP transition state (Figure 3, TS2B) similar to
that proposed for amine–borane dehydrogenation.[21] There
are two possible pathways, depending on the nature of the
rotamer present (see the Supporting Information). The two
pathways were found to differ slightly in energy, with
respective DH° and DG° values of 24.7 and 26.0 kcalmolÀ1
for TS2A and of 21.9 and 23.5 kcalmolÀ1 for TS2A’. Once the
intermediates have formed (DH (DG) for 2-H2 and 2-H2’ are
19.3 kcalmolÀ1 (20.8 kcalmolÀ1) and 18.9 kcalmolÀ1 (20.1 kcal
Figure 2. Reaction rate of the transformation of compound 1 into
*
*
*
*
compound 2 at different temperatures ( 858C, 908C, 958C,
1008C) and Eyring plot of the transformation. Two duplicates were
measured for each temperature (see the Supporting Information).
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

Angewandte
Communications
Chemie
molÀ1), respectively), the release of H2 occurs with DH° and
DG° values of 25.9 and 29.2 kcalmolÀ1, respectively, for TS2B.
Both transition states in this pathway are very close in energy.
It was not possible to locate TS2B’, but one might expect it to
be lower in energy than TS2B to follow the same trend
observed for TS2A’ and 2H2’.
explore the generality of this method and the reactivity of the
À
À
B B bond, but the possibility for B B bond formation to be
reversible suggests a new scaffold in metal-free catalysis. Such
systems would enable a variety of multiple redox trans-
formations for which transition-metal catalysts are usually
required.
It has been postulated that the activation/elimination of
H2 from a FLP does not present a significant kinetic isotope
effect (KIE),[22] whereas the loss of H2 from species M–
Acknowledgements
NH2BH3, in which the rate-determining step was the cleavage
[23]
À
of a B H bond, exhibits a KIE value close to 1.6. These
This research was supported by the National Sciences and
Engineering Research Council (NSERC) of Canada and the
Centre de Catalyse et Chimie Verte (Quebec). E.R., N.B.,
and J.L.L. acknowledge NSERC and FRQNT for scholar-
ships. We acknowledge Calcul Canada and Calcul Quꢁbec for
computation time, P. Audet for technical help with the NMR
experiments, T. Marris with the crystallographic resolution of
2, and S. A. Westcott, T. B. Marder, T. Autrey, M.-A.
Courtemanche, and M.-A. Lꢁgarꢁ for helpful discussions.
precedents suggest that either TS1 or TS2A is the rate-
limiting step. The complex kinetic profile for the formation of
2 in the presence of H2 supports the hypothesis that TS2A and
TS2B are very close in energy. Although we cannot com-
pletely rule out the possibility of TS1 occurring, the exper-
imental values of DH° and DS° of (24.3 Æ 0.7) kcalmolÀ1 and
(À0.03 Æ 0.02) eu suggest that pathway TS2 is slightly more
favorable, with both steps of comparable energy.
The pathway TS2 is somewhat surprising, since hydrogen
is more electronegative than boron (2.1 for H and 2.0 for B
according to the Pauling scale),[24] thereby making a hydrogen
atom bound to boron hydridic rather than protic. Although
the deprotonation of metal hydrides is not rare,[25] FLP
Keywords: boranes · diboranes · frustrated Lewis pairs ·
reduction · small-molecule activation
À
systems tend to abstract an hydrogen atom from a B H
moiety to generate borenium species.[26] Whereas the depro-
À
tonation of a B H bond in carboranes having boron atoms at
lower-oxidation state is known,[27] to the best of our knowl-
[1] G. C. Welch, R. R. S. Juan, J. D. Masuda, D. W. Stephan, Science
2006, 314, 1124 – 1126.
edge, the deprotonation of a BIII H bond was only reported
once before by Bertrand and co-workers when a carbene
BH(CN)2 adduct was treated with the strong base KHMDS to
form an isolable boryl anion.[28]
À
[2] a) D. W. Stephan, J. Am. Chem. Soc. 2015, 137, 10018 – 10032;
b) D. W. Stephan, G. Erker, Angew. Chem. Int. Ed. 2015, 54,
6400 – 6441; Angew. Chem. 2015, 127, 6498; c) G. Erker, D. W.
Stephan in Frustrated Lewis Pairs I and II, Springer, New-York,
2013; d) F.-G. Fontaine, M.-A. Courtemanche, M.-A. Lꢁgarꢁ, ꢂ.
Rochette, Coord. Chem. Rev. 2016, DOI: 10.1016/
j.ccr.2016.05.005.
Analysis of the electron density according to the Bader
QTAIM theory[29] revealed that for molecule 1, the bridging
hydride has almost the same atomic charge (À0.6438) as the
terminal hydrides (À0.6243 to À0.6435; see the Supporting
Information). In TS2A, it was observed that the nature of the
bridging hydrogen atom switches from hydridic to protic
(+ 0.3240), but that the electron population is transferred
equally to the two boron atoms, whose respective charge
varies from + 1.8992 and + 1.9882 in 1 to + 1.3780 and
+ 1.3865 in TS2A. The atomic electron populations in 2 are
close to those in TS2A: The proton has a charge of + 0.5246
and the boron atoms a charge of + 1.2318 and + 1.3593,
[3] For selected references, see: a) S. Mummadi, D. K. Unruh, J.
Zhao, S. Li, C. Krempner, J. Am. Chem. Soc. 2016, 138, 3286 –
3289; b) M. Lindqvist, K. Borre, K. Axenov, B. Kꢃtai, M. Nieger,
M. Leskelꢄ, I. Pꢅpai, T. Repo, J. Am. Chem. Soc. 2015, 137,
4038 – 4041; c) S. Wei, H. Du, J. Am. Chem. Soc. 2014, 136,
12261 – 12264; d) T. Mahdi, D. W. Stephan, J. Am. Chem. Soc.
2014, 136, 15809 – 15812; e) D. J. Scott, M. J. Fuchter, A. E.
Ashley, J. Am. Chem. Soc. 2014, 136, 15813 – 15816; f) J.
Paradies, Angew. Chem. Int. Ed. 2014, 53, 3552 – 3557; Angew.
Chem. 2014, 126, 3624 – 3629; g) G. Ghattas, D. Chen, F. Pan, J.
Klankermayer, Dalton Trans. 2012, 41, 9026 – 9028; h) A. E.
Ashley, A. L. Thompson, D. OꢆHare, Angew. Chem. Int. Ed.
2009, 48, 9839 – 9843; Angew. Chem. 2009, 121, 10023 – 10027;
i) P. Spies, S. Schwendemann, S. Lange, G. Kehr, R. Frçhlich, G.
Erker, Angew. Chem. Int. Ed. 2008, 47, 7543 – 7546; Angew.
Chem. 2008, 120, 7654 – 7657; j) P. A. Chase, G. C. Welch, T.
Jurca, D. W. Stephan, Angew. Chem. Int. Ed. 2007, 46, 8050 –
8053; Angew. Chem. 2007, 119, 8196 – 8199.
[4] a) R. Declercq, G. Bouhadir, D. Bourissou, M. A. Lꢁgarꢁ, M. A.
Courtemanche, K. S. Nahi, N. Bouchard, F. G. Fontaine, L.
Maron, ACS Catal. 2015, 5, 2513 – 2520; b) M.-A. Courteman-
che, M.-A. Lꢁgarꢁ, F.-G. Fontaine, L. Maron, J. Am. Chem. Soc.
2014, 136, 10708 – 10717; c) M. A. Courtemanche, M. A. Lꢁgarꢁ,
L. Maron, F. G. Fontaine, J. Am. Chem. Soc. 2013, 135, 9326 –
9329; d) M. A. Courtemanche, J. Larouche, M. A. Lꢁgarꢁ, W. Bi,
L. Maron, F. G. Fontaine, Organometallics 2013, 32, 6804 – 6811.
[5] a) A. Y. Houghton, J. Hurmalainen, A. Mansikkamꢄki, W. E.
Piers, H. M. Tuononen, Nat. Chem. 2014, 6, 983 – 988; b) D.
À
respectively. Nevertheless, the deprotonation of a B H bond
by a rather weak Lewis base, such as a dimethylaniline, is not
thermodynamically favorable, since 2-H2 is higher in energy
by 20 kcalmolÀ1 than the starting material; however, the FLP
character can facilitate the H2-elimination process, thus
making the whole transformation thermodynamically possi-
ble.
In conclusion, we discovered that the unusual activation
of the B H bond in hydroborane 1 leads to the first
spontaneous boron–boron dehydrogenative homocoupling
of a hydroborane. The experimental and computational
study of the mechanism shed some light on the unexpected
transformation, which is a rare example of the deprotonation
À
À
of a B H bond. The transformation is made possible by the
FLP molecule, which aids the release of H2. We have yet to
4
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!

Angewandte
Communications
Chemie
Chen, V. Leich, F. Pan, J. Klankermayer, Chem. Eur. J. 2012, 18,
Marder, Science 2015, 349, 473 – 474; c) K. Chernichenko, M.
5184 – 5187.
Lindqvist, B. Kotai, M. Nieger, K. Sorochkina, I. Papai, T. Repo,
J. Am. Chem. Soc. 2016, 138, 4860 – 4868; d) M.-A. Lꢁgarꢁ, ꢂ.
Rochette, J. Lꢁgarꢁ Lavergne, N. Bouchard, F.-G. Fontaine,
Chem. Commun. 2016, 52, 5387 – 5390.
[6] a) R. D. Dewhurst, E. C. Neeve, H. Braunschweig, T. B. Marder,
Chem. Commun. 2015, 51, 9594 – 9607; b) H. Braunschweig,
R. D. Dewhurst, S. Mozo, ChemCatChem 2015, 7, 1630 – 1638.
[7] a) I. A. I. Mkhalid, J. H. Barnard, T. B. Marder, J. M. Murphy,
J. F. Hartwig, Chem. Rev. 2010, 110, 890 – 931; b) T. Ishiyama, J.
Takagi, K. Ishida, N. Miyaura, N. R. Anastasi, J. F. Hartwig, J.
Am. Chem. Soc. 2002, 124, 390 – 391; c) H. Chen, S. Schlecht,
T. C. Semple, J. F. Hartwig, Science 2000, 287, 1995 – 1997; d) T.
Ishiyama, M. Murata, N. Miyaura, J. Org. Chem. 1995, 60, 7508 –
7510; e) A. Bonet, C. Pubill-Ulldemolins, C. Bo, H. Gulyas, E.
Fernandez, Angew. Chem. Int. Ed. 2011, 50, 7158 – 7161; Angew.
Chem. 2011, 123, 7296 – 7299; f) G. A. Chotana, M. A. Rak,
M. R. Smith III, J. Am. Chem. Soc. 2005, 127, 10539 – 10544.
[8] T. B. Marder, N. C. Norman, Top. Catal. 1998, 5, 63 – 73.
[9] a) Y. Shoji, T. Matsuo, D. Hashizume, M. J. Gutmann, H. Fueno,
K. Tanaka, K. Tamao, J. Am. Chem. Soc. 2011, 133, 11058 –
11061; b) Y. Wang, G. H. Robinson, Inorg. Chem. 2011, 50,
12326 – 12337; c) R. J. Brotherton, A. L. McCloskey, L. Petter-
son, H. Steinberg, J. Am. Chem. Soc. 1960, 82, 6242 – 6245; d) T.
Kaese, A. Hꢇbner, M. Bolte, H.-W. Lerner, M. Wagner, J. Am.
Chem. Soc. 2016, 138, 6224 – 6233.
[15] M.-A. Courtemanche, A. P. Pulis, ꢂ. Rochette, M.-A. Lꢁgarꢁ,
D. W. Stephan, F.-G. Fontaine, Chem. Commun. 2015, 51, 9797 –
9800.
[16] D. Franz, M. Bolte, H.-W. Lerner, M. Wagner, Dalton Trans.
2011, 40, 2433 – 2440.
[17] T. H. Jordan, W. E. Streib, W. N. Lipscomb, J. Chem. Phys. 1964,
41, 760.
[18] D. J. Brauer, C. Krꢇger, Acta Crystallogr. B 1973, 29, 1684 – 1690.
[19] A. V. Marenich, C. J. Cramer, D. G. Truhlar, J. Phys. Chem. B
2009, 113, 6378 – 6396.
[20] Although several carboranes exhibit such a bonding mode, the
number of molecular structures is more limited.
[21] H. Li, N. Ma, W. Meng, J. Gallucci, Y. Qiu, S. Li, Q. Zhao, J.
Zhang, J. C. Zhao, X. Chen, J. Am. Chem. Soc. 2015, 137, 12406 –
12414.
[22] F. Schulz, V. Sumerin, S. Heikkinen, B. Pedersen, C. Wang, M.
Atsumi, M. Leskelꢄ, T. Repo, P. Pyykkç, W. Petry, B. Rieger, J.
Am. Chem. Soc. 2011, 133, 20245 – 20257.
[10] a) A. Anaby, B. Butschke, Y. Ben-David, L. J. W. Shimon, G.
Leitus, M. Feller, D. Milstein, Organometallics 2014, 33, 3716 –
3726; b) H. Braunschweig, C. Claes, F. Guethlein, J. Organomet.
Chem. 2012, 706 – 707, 144 – 145; c) H. Braunschweig, F. Gueth-
lein, Angew. Chem. Int. Ed. 2011, 50, 12613 – 12616; Angew.
Chem. 2011, 123, 12821 – 12824.
[11] a) A. Wagner, S. Litters, J. Elias, E. Kaifer, H. J. Himmel, Chem.
Eur. J. 2014, 20, 12514 – 12527; b) N. Schulenberg, O. Ciobanu, E.
Kaifer, H. Wadepohl, H. J. Himmel, Eur. J. Inorg. Chem. 2010,
5201 – 5210.
[23] A. T. Luedtke, T. Autrey, Inorg. Chem. 2010, 49, 3905 – 3910.
[24] L. Pauling, The Nature of the Chemical Bond, 3rd ed., Cornell
University Press, Ithaca, N.Y., 1960.
[25] R. H. Morris, Chem. Rev. 2016, 116, 8588 – 8654.
[26] a) M. A. Dureen, A. Lough, T. M. Gilbert, D. W. Stephan,
Chem. Commun. 2008, 4303 – 4305; b) P. Eisenberger, A. M.
Bailey, C. M. Crudden, J. Am. Chem. Soc. 2012, 134, 17384 –
17387; c) E. E. Clark, A. Del Grosso, M. J. Ingleson, Chem. Eur.
J. 2013, 19, 2462 – 2466.
[27] M. E. Fessler, T. Whelan, J. T. Spencer, R. N. Grimes, J. Am.
Chem. Soc. 1987, 109, 7416 – 7420.
[28] D. A. Ruiz, G. Ung, M. Melaimi, G. Bertrand, Angew. Chem. Int.
Ed. 2013, 52, 7590 – 7592; Angew. Chem. 2013, 125, 7739 – 7742.
[29] R. F. W. Bader, Atoms in Molecules: AQuantum Theory, Oxford
University Press, Oxford, UK, 1990.
[12] a) E. W. Corcoran, L. G. Sneddon, J. Am. Chem. Soc. 1985, 107,
7446 – 7450; b) S.-K. Kim, W.-S. Han, T.-J. Kim, T.-Y. Kim, S. W.
´
Nam, M. Mitoraj, L. Piekos, A. Michalak, S.-J. Hwang, S. O.
Kang, J. Am. Chem. Soc. 2010, 132, 9954 – 9955; c) D. J.
Wolstenholme, K. T. Traboulsee, Y. Hua, L. A. Calhoun, G. S.
McGrady, Chem. Commun. 2012, 48, 2597 – 2599.
[13] K. Chernichenko, A. Madarꢅsz, I. Pꢅpai, M. Nieger, M. Leskelꢄ,
T. Repo, Nat. Chem. 2013, 5, 718 – 723.
[14] a) M.-A. Lꢁgarꢁ, M.-A. Courtemanche, ꢂ. Rochette, F.-G.
Fontaine, Science 2015, 349, 513 – 516; b) S. K. Bose, T. B.
Received: June 10, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Frustrated Lewis Pairs
ꢁ. Rochette, N. Bouchard,
J. Lꢂgarꢂ Lavergne,
C. F. Matta
&&&& — &&&&
Spontaneous Reduction of
À
À
a Hydroborane To Generate a B B Single
Bond by the Use of a Lewis Pair
Your best bet for B B: The ansa-amino-
without the use of a catalyst, this mole-
cule liberates one equivalent of hydrogen
to generate a boron–boron bond (see
scheme).
hydroborane 1-NMe2-2-(BH2)C6H4 crys-
tallizes in an unprecedented type of dimer
containing a hydride bridge between two
boron centers. Upon mild heating and
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!