A stable but highly reactive phosphine-coordinated borenium: Metal-free dihydrogen activation and alkyne 1,2-carboboration

Article abstract of DOI:10.1002/anie.201500959

Borenium cations have been found to be valuable analogues of boranes as a result of their cationic character which imparts high electrophilicity. Herein, we report the synthesis, characterization, and reactivity of a new type of borenium cation employing a naphthyl bridge and a strong intramolecular P→B interaction. The cation reacts with H₂ in the presence of PtBu₃ (frustrated Lewis pair (FLP) approach) but also on its own. The mechanism of the reaction between the borenium cation and H₂ in the absence of PtBu₃ has been investigated using deuterium-labeling experiments and DFT calculations. Both experiments and calculations imply the side-on coordination of H₂ to the B center, followed by heterolytic splitting and B-C bond cleavage. An uncommon syn 1,2-carboboration has also been observed upon reaction of the borenium ion with 3-hexyne. Versatile reactivity: A new type of borenium cation is reported in which a naphthyl bridge supports a strong P→B interaction. Borenium reacts with H₂ through side-on coordination of H₂ to boron, heterolytic splitting, and concomitant cleavage of the B-Mes bond. The molecule also reacts with 3-hexyne through a syn 1,2-carboboration reaction. NTf₂^-=triflimide.

Full text of DOI:10.1002/anie.201500959

Angewandte
Chemie
DOI: 10.1002/anie.201500959
Borenium Cations
A Stable but Highly Reactive Phosphine-Coordinated Borenium:
Metal-free Dihydrogen Activation and Alkyne 1,2-Carboboration**
Marc Devillard, Rꢀmy Brousses, Karinne Miqueu,* Ghenwa Bouhadir,* and Didier Bourissou*
Dedicated to Professor Manfred Scheer on the occasion of his 60th birthday
Abstract: Borenium cations have been found to be valuable
analogues of boranes as a result of their cationic character
which imparts high electrophilicity. Herein, we report the
synthesis, characterization, and reactivity of a new type of
borenium cation employing a naphthyl bridge and a strong
intramolecular P!B interaction. The cation reacts with H2 in
the presence of PtBu3 (frustrated Lewis pair (FLP) approach)
but also on its own. The mechanism of the reaction between the
borenium cation and H2 in the absence of PtBu3 has been
investigated using deuterium-labeling experiments and DFT
calculations. Both experiments and calculations imply the side-
on coordination of H2 to the B center, followed by heterolytic
versatile reactivity on their own or when associated with
Lewis bases in so-called frustrated Lewis pairs (FLP). In
particular, they enable the activation of strong s bonds, such
[1,2]
ꢀ
ꢀ
as H H and H Si, under metal-free conditions.
Such
activity is possible as a result of the high electrophilic
character of boron which typically requires electron-
withdrawing substituents such as perfluorinated aryl groups.
As a result of their cationic character, borenium ions
!
(R2B L)+ are valuable analogues of boranes.[3] Intra- and
intermolecularly stabilized boreniums have been developed
and the nature of the Lewis base has been modified to include
pyridines, amines, N-heterocyclic carbenes (NHC), N-hetero-
cyclic carbene olefins (NHO), carbodiphosphoranes, and
phosphines.[4–6] Accordingly, the variety of stable boreniums
has been considerably extended and our understanding of
their electronic structure has progressed significantly. How-
ever, little is known about their reactivity, although the results
obtained recently in H2 activation with NHC-stabilized
boreniums hold much promise.[7]
ꢀ
splitting and B C bond cleavage. An uncommon syn 1,2-
carboboration has also been observed upon reaction of the
borenium ion with 3-hexyne.
T
he last decade has witnessed an upsurge of interest in Lewis
acidic boron-containing compounds, whose archetypal deriv-
ative is B(C6F5)3. In addition to their role as co-catalysts in
transition-metal-catalyzed reactions, boranes have shown
In this context and given the scarcity of phosphorus-
coordinated boreniums (compared to those involving N and
C donors), we set out to prepare new borenium derivatives of
type A (Figure 1). Related neutral compounds B[8] and C[9]
[*] Dr. M. Devillard, Dr. G. Bouhadir, Dr. D. Bourissou
Laboratoire Hꢀtꢀrochimie Fondamentale et Appliquꢀe
Universitꢀ Paul Sabatier/CNRS UMR 5069
118 Route de Narbonne, 31062 Toulouse Cedex 09 (France)
E-mail: bouhadir@chimie.ups-tlse.fr
dbouriss@chimie.ups-tlse.fr
Homepage: http://lhfa.cnrs.fr/index.php/equipes/lbpb
Dr. R. Brousses
Institut de Chimie de Toulouse (FR 2599)
118 Route de Narbonne, 31062 Toulouse Cedex 09 (France)
Dr. K. Miqueu
Institut des Sciences Analytiques et de Physico-Chimie pour
l’Environnement et les Matꢀriaux/CNRS UMR 5254
Equipe Chimie Physique
Figure 1. Naphthyl-bridged P/B derivatives.
Universitꢀ de Pau et des Pays de l’Adour, Hꢀlioparc
2 Avenue du Prꢀsident Angot, 64053 Pau cedex 09 (France)
E-mail: karinne.miqueu@univ-pau.fr
have been investigated recently. The naphthyl bridge was
shown to impart unique properties and to enforce a strong
P!B interaction despite steric shielding. Herein we report
the synthesis, characterization, and reactivity of a phospho-
rus-stabilized diaryl borenium of type A. The reactivity of this
borenium with H2 and 3-hexyne has been explored. Most
remarkable are the reactions observed in the absence of
[**] The Centre National de la Recherche Scientifique (CNRS), the
Universitꢀ Paul Sabatier (UPS), and the Agence Nationale de la
Recherche (ANR-2011-INTB-1008-01) are acknowledged for finan-
cial support of this work. The theoretical work was facilitated by
access granted to the HPC resources of IDRIS under allocation 2015
(i2015080045) made by GENCI (Grand Equipement National de
Calcul Intensif). UPPA and the MCIA (Mesocentre de Calcul Intensif
Aquitain) are also thanked for providing calculation facilities. We are
very grateful to Prof. F. Jꢁkle (Rutgers University) for insightful
advice on the preparation of MesBBr2.
ꢀ
external base, namely H2 splitting with B C cleavage and syn
1,2-carboboration.
The phosphine-bromoborane 2-Br was prepared in two
steps from 1-diphenylphosphino-8-iodonaphthalene (1;
Scheme 1).[10] In line with previous reports,[9] the naphthyl
bridge enforces a strong intramolecular P!B interaction.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500959.
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electron-rich phosphine and the Lewis acidic boron center is
prevented sterically. However, upon addition of H2 (3 atm)
the 31P NMR spectrum shows the appearance of two new
resonance signals, indicating the occurrence of a slow reaction
(quantitative spectroscopic yield). The signal at d = 56.7 ppm
1
has a large JP-H coupling constant (448 Hz) and corresponds
to the phosphonium tBu3PH+ center.[13] The other 31P NMR
signal appears at d = 9.4 ppm and corresponds to the phos-
phino–hydroborane adduct 2-H. The 11B NMR signal appears
in the high-field region of the spectrum at d = ꢀ13.3 ppm and
1
in the H NMR spectrum a signal for the H atom bound to
boron appears at d = 5.52 ppm.[10] The heterolytic cleavage of
H2 with the 3b/PtBu3 FLP is reminiscent of that described
previously from (tBu3P–Bcat)+ and PtBu3,[14] although it
proceeds under much milder conditions (herein: 3 atm, RT,
39 h, versus 4 atm, 1008C, 24 h).[15]
Scheme 1. Synthesis of the phosphine-stabilized borenium derivatives
3a,b. NTf2 =bis(trifluoromethylsulfonyl)imide. Mes=mesityl.
This can be shown spectroscopically from the high-field
We were next interested in the ability of the borenium 3b
to activate H2 without added base. Indeed, besides FLP, a few
main group compounds have been shown to split H2 at
a unique reactive site.[16] Bertrand et al. pioneered the field
with ambiphilic acyclic and cyclic amino carbenes,[17] Aldridge
and co-workers generalized the approach to aminoboryl and
aminosilyl silylenes,[18] and Piers et al. extended the field to
highly electron-deficient antiaromatic boroles[2a] (see
Refs. [19,20] for additional examples).
resonance signal (d = 0.3 ppm) in the 11B NMR spectrum and
ꢀ
crystallographically from the short P B distance
(2.05(1) ꢀ).[10] Bromine abstraction from 2-Br was readily
achieved with either GaBr3 or AgNTf2 (Scheme 1; NTf2 =
bis(trifluoromethylsulfonyl)imide). The resulting borenium
derivatives 3a,b were isolated in high yields (78–99%). The
molecular structures of 3a,b were unambiguously established
by multinuclear NMR spectroscopy and HRMS (in both
positive and negative modes). The two salts have very similar
NMR spectroscopic characteristics. Most diagnostic is the
resonance signal in the 11B NMR spectrum which is signifi-
cantly shifted to lower field compared to that of 2-Br and
appears at d = 72.1–74.2 ppm, in the typically range for
tricoordinate boron centers.[11] Compounds 3a,b are rare
examples of phosphorus-coordinated boreniums. To our
knowledge, the only related compounds are the intermolec-
ular adducts (tBu2RP–Bcat)+ (Bcat = catecholboryl) pre-
A CH2Cl2 solution of the borenium 3b was pressurized
with dihydrogen (3 atm). The reaction was monitored by
31P NMR spectroscopy, where resonance signals showed the
progressive transformation of 3b into a new compound 4-
HNTf2, with a new resonance signal appearing at d = 4.7 ppm
(Scheme 2). Heating at 808C enabled the clean and complete
conversion of 3b to 4-HNTf2 within 3 h (62% yield of isolated
product). The 11B and 1H{11B} NMR spectra show signals that
can attributed to the tetracoordinate BH moiety in 4-HNTf2
1
pared previously by Stephan et al. upon FLP-induced B H
(d(11B) = ꢀ6.4 ppm, d(1H) = 4.72 ppm). Curiously, in the H
ꢀ
activation.[6] The bonding situation in such compounds is best
described by the superposition of two canonical structures,
a phosphine-stabilized borenium form and a phosphonio–
borane form.[6]
NMR spectrum a unique set of signals for CH3 and CHarom
groups (d = 2.28 and 6.81 ppm) is observed for the Mes group,
ꢀ
ꢀ
suggesting the formation of Mes H through B C bond
cleavage. Accordingly, the borenium 3b would split H2 with
formal transfer of a hydride to boron and concomitant
The reactivity of 3b towards small molecules was then
explored, starting with dihydrogen (Scheme 2).[12] An FLP-
type approach was considered first. No reaction occurs
between PtBu3 and 3b at room temperature, indicating that
the formation of an intermolecular adduct between the
ꢀ
protonolysis of the B Mes bond. This hypothesis was
confirmed by reacting 3b with D2 under similar conditions.
Spectroscopic analyses indicate the formation of 4-DNTf2 and
Mes-D.[10] Crystals of 4-HNTf2 were analyzed by single-crystal
X-ray diffraction (Figure 2).[21] The hydrogen atom at boron
was unambiguously located in the difference Fourier map and
ꢀ
the respective B H bond length was found to be 1.07(3) ꢀ.
ꢀ
The P B distance remains short (1.982(3) ꢀ) and the
ꢀ
triflimide NTf2 is coordinate to boron (B N = 1.602(4) ꢀ),
so that the boron center is tetracoordinate.
The mechanism of H2 activation at main group centers has
stimulated strong interest and various scenarios have been
identified.[1,17,20b] To shed light on how the reaction between
H2 and 3b proceeds, DFT calculations were performed at the
B3PW91/6-31 + G**(CH2Cl2)//B3PW91/6-31G** level of
theory (Figure 3). The borenium derivative 3 (Mes at B and
Ph at P) was used. The NTf2 counterion was not considered
(except in the last stage of the reaction, see below), but
solvent effects (CH2Cl2) were taken into account by single-
Scheme 2. Heterolytic splitting of H2 by the borenium 3b with or
without external base.
2
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Figure 2. X-ray crystal structure of 4-HNTf2. The Ph and NTf2 groups
and the naphthyl spacer are simplified and the hydrogen atoms
(except the one at boron) are omitted for clarity. Selected bond lengths
ꢀ
ꢀ
ꢀ
[ꢂ] and angles [8]: P B 1.982(3), B H 1.07(3), B N 1.602(4); N-B-H
104.4(2), H-B-C3 113.4(2), C3-B-N 118.1(2). Thermal ellipsoids are set
at 50% probability.
Figure 3. Energy profile calculated for the reaction of the borenium 3
with dihydrogen (electronic energy DE in kcalmolꢀ1).
point calculations using the polarizable continuum model
(PCM). The optimized structure of 3 revealed the presence of
a strong P!B interaction (2.006 ꢀ), in line with spectroscopic
data. The reaction of H2 with 3 starts by its side-on
metal-free conditions.[25] Typically, using B(C6F5)3 and other
C6F5-substituted boranes, a variety of 1,1-carboboration
reactions (formal insertion of vinylidene species into B C
ꢀ
coordination to boron (B H 1.460 and 1.511 ꢀ). The
resultant complex 3-s is higher in energy by 10.6 kcalmolꢀ1
and the corresponding activation barrier is 17.2 kcalmolꢀ1. A
few s complexes of H2 to boranes have been computationally
authenticated.[2a,22] Natural bond orbital (NBO) analysis of
the bonding situation in 3-s indicates strong s-H2 to B
donation without back donation. The associated natural
localized molecular orbital (NLMO) is significantly delocal-
ized over B (23%), resulting in noticeable distortion of the
ꢀ
bonds) have been discovered.[25] Clearly, borenium salts are
also promising substrates for carboboration, but to our
knowledge the only precedent for such a reaction is that
reported very recently by Ingleson and Cade.[26] Accordingly,
borenium cations deriving from 8-hydroxyquinoline (and thus
stabilized intramolecularly by N!B donation) were found to
react slowly with 3-hexyne at 20–608C by 1,2-carbobora-
tion.[27] The borenium 3b proved very reactive towards 3-
hexyne. A clean reaction occurs spontaneously upon addition
of the alkyne (1 equiv) to 3b (Scheme 3; conversion is
s-H2 electron density towards the B center.[10] The H H bond
ꢀ
is slightly elongated (0.795 ꢀ versus 0.743 ꢀ in free H2). The
next step is the splitting of H2 leading to the formation of
intermediate 4-HMes, which is lower in energy than 3-s by
ꢀ1
1
ꢀ
18.3 kcalmol . One H atom (H ) is linked to boron (B H
1.198 ꢀ), whereas the other (H2) has been transferred to the
ꢀ
C
ipso atom of the Mes ring (C H 1.093 ꢀ), resulting in the
ꢀ
formation of a Wheland-type structure. The B CMes bond is
noticeably elongated (from 1.581 ꢀ in 3-s to 1.836 ꢀ in 4-
HMes). A transition state TS1 connecting 3-s and 4-HMes
was located 22.3 kcalmolꢀ1 in energy above that of 3 and H2
(starting point). The boron center remains in the plane of the
Mes ring and H2 approaches perpendicularly.[23] Finally,
release of MesH from 4-HMes was found to proceed readily.
The corresponding activation barrier is fairly low (approx-
imately 4 kcalmolꢀ1). The resulting phosphorus-stabilized
hydroborenium 4-H is isoenergetic with 4-HMes and coordi-
nation of the NTf2 counterion to the sterically unhindered
boron center of 4-H drives the reaction forward (taking into
account NTf2, DE = ꢀ25.1 kcalmolꢀ1 was predicted for the
reaction of 3b + H2 !4-HNTf2 + MesH).[10] It is noteworthy
that the phosphine moiety of 3b does not participate directly
in the reaction with H2 but remains engaged in the strong
P!B interaction throughout the process.[24]
Scheme 3. 1,2-Carboboration of 3-hexyne by the borenium salt 3b to
form 5.
complete within 2 h at RT). The resulting product 5 was
characterized by mass spectrometry and multinuclear NMR
spectroscopy. The HRMS (electrospray ionization, positive
mode) shows a signal at m/z = 522.2762, corresponding to the
molar mass of the boron cation plus one 3-hexyne molecule.
The 31P and 11B NMR spectra for 5 (d = ꢀ4.1 and 64.9 ppm,
respectively) show signals which are only slightly shifted
compared with those obtained for of 3b, consistent with the
similar structure of both compounds (phosphorus-stabilized
1
aryl versus a vinyl borenium). The 13C and H NMR spectra
From the work of Berke, Erker, and Piers, highly electro-
philic boranes are known to readily react with alkynes under
are more informative. Two sets of resonance signals attribut-
able to CCH2CH3 groups were observed, in line with the
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1
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ꢀ
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In conclusion, a new type of borenium involving a strong
intramolecular P!B interaction supported by a naphthyl
bridge has been synthesized. Compounds 3a,b have been
readily prepared by bromide abstraction with GaBr3 or
AgNTf2. The steric demand of the mesityl group at boron
prevents the coordination of PtBu3 to boron and the resulting
FLP readily activates dihydrogen. Even in the absence of
a base, the borenium reacts at 808C by heterolytic splitting of
ꢀ
H2 and B Mes bond cleavage. The reactions involve side-on
coordination of H2 to boron followed by concomitant transfer
of a hydride to the boron center and protonation of the Mes
substituent. The phosphorus-stabilized borenium reacts spon-
taneously with 3-hexyne under mild conditions. In this case
ꢀ
also the B Mes bond is cleaved and a vinyl borenium species
is formed through an uncommon syn 1,2-carboboration
reaction.
These results substantiate further the ease with which
boreniums may be structurally modified and highlight their
versatile reactivity. The reactions observed with H2 and 3-
hexyne have some similarity with B(C6F5)3 chemistry but also
show significant differences. Future work in our laboratory
will seek to explore further the chemistry of naphthyl-bridged
P/B compounds.
[12] The NTf2 counterion proved more suitable for reactivity studies
as the corresponding GaBr4 salt 3a was found to readily react
with a coordinating solvent such as Et2O to give back the
phosphino-bromoborane 2Br along with the L·GaBr3 adduct.
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[14] E. R. Clark, A. Del Grosso, M. J. Ingleson, Chem. Eur. J. 2013,
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[15] In the presence of PtBu3, the NHC-stabilized borenium (IiPr2–
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to 3b.[7a]
Received: February 4, 2015
Published online: && &&, &&&&
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4
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[24] As suggested by a reviewer, the reaction of 3 with H2 may lead to
a Ph2PH+ phosphonium/BHMes borane intermediate, but this
would require the cleavage of the strong P!B interaction.
[25] G. Kehr, G. Erker, Chem. Commun. 2012, 48, 1839 – 1850.
[26] I. A. Cade, M. J. Ingleson, Chem. Eur. J. 2014, 20, 12874 – 12880.
[27] For pioneering contributions on 1,2-carboboration of alkynes,
see: a) M. F. Lappert, B. Prokal, J. Organomet. Chem. 1964, 1,
384 – 400; b) B. Wrackmeyer, H. Nçth, J. Organomet. Chem.
1976, 108, C21 – C25.
[23] Another pathway between 3-s and 4-HMes was found that was
higher in energy; see the Supporting Information.
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Borenium Cations
M. Devillard, R. Brousses, K. Miqueu,*
G. Bouhadir,*
D. Bourissou*
&&&& — &&&&
A Stable but Highly Reactive Phosphine-
Coordinated Borenium: Metal-free
Dihydrogen Activation and Alkyne 1,2-
Carboboration
Versatile reactivity: A new type ofbore-
nium cation is reported in which
anaphthyl bridge supports a strong P!B
interaction. Borenium reacts with H2
through side-on coordination of H2 to
boron, heterolytic splitting, and concom-
ꢀ
itant cleavage of the B Mes bond. The
molecule also reacts with 3-hexyne
through a syn 1,2-carboboration reaction.
NTf2ꢀ =triflimide.
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!