Pushing the Lewis Acidity Boundaries of Boron Compounds With Non-Planar Triarylboranes Derived from Triptycenes

Article abstract of DOI:10.1002/anie.201910908

Bending the planar trigonal boron center of triphenylborane by connecting its aryl rings with carbon or phosphorus linkers gave access to a series of 9-boratriptycene derivatives with unprecedented structures and reactivities. NMR spectroscopy and X-ray d

Full text of DOI:10.1002/anie.201910908

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International Edition: DOI: 10.1002/anie.201910908
German Edition: DOI: 10.1002/ange.201910908
Lewis Acids
Pushing the Lewis Acidity Boundaries of Boron Compounds With
Non-Planar Triarylboranes Derived from Triptycenes
Ali Ben Saida+, Aurꢀlien Chardon+, Arnaud Osi, Nikolay Tumanov, Johan Wouters,
Abel I. Adjieufack, Benoꢁt Champagne, and Guillaume Berionni*
Dedicated to Professor Alain Krief on the occasion of his 77th birthday
Abstract: Bending the planar trigonal boron center of
triphenylborane by connecting its aryl rings with carbon or
phosphorus linkers gave access to a series of 9-boratriptycene
derivatives with unprecedented structures and reactivities.
NMR spectroscopy and X-ray diffraction of the Lewis adducts
of these non-planar boron Lewis acids with weak Lewis base
revealed particularly strong covalent bond formation. The first
Lewis adduct of a trivalent boron compounds with the Tf2NÀ
anion illustrates the unrivaled Lewis acidity of these species.
Increasing the pyramidalization of the boron center and using
a cationic phosphonium linker resulted in an exceptional
Scheme 1. Representative triarylboranes and computed NH3 affini-
ties.[5b]
enhancement of Lewis acidity. Introduction of a phosphorus
and a boron atom at each edge of a triptycene framework,
allowed access to new bifunctional Lewis acid-base 9-phospha-
10-boratriptycenes featuring promising reactivity for the acti-
vation of carbon-halogen bonds.
for the activation of H2 and N2O,[7] as well as in materials
sciences for the design of new classes of donor-acceptor
complexes of noble gases.[8] Owing to its strongly pyramidal-
ized boron center integrated at the bridgehead position of
a triptycene scaffold, the NH3 affinity of 6 was computation-
[6]
T
rivalent organoboron compounds are readily available
Lewis acids of importance in academia and industry.[1] In
contrast to triphenylborane (3), which has a propeller-like
structure,[2] enforced planarization of the aryl rings with three
linkers results in air and water stable planar boranes 1–2,
which are unreactive towards most Lewis bases (Scheme 1).[3]
Perfluorinated aryl rings considerably increase the boron
Lewis acidity, as in tris(pentafluorophenyl)borane 4, and
connecting two aryl rings as in borafluorene 5 further
increases the Lewis acidity (Scheme 1).[4] More remarkably,
a CH linker connecting all three phenyl rings of triphenylbor-
ane 3 would provide the 9-boratriptycene 6 featuring
a pyramidal coordination environment around the trivalent
boron atom (Scheme 1).[5]
ally predicted to surpass by far that of B(C6F5)3
4
(Scheme 1).[5]
Since the synthesis of B(C6F5)3 4 by Massey et al. in
1963,[9] the main approaches for increasing the acidity
strength of boron Lewis acids essentially remained to use
electron-withdrawing groups (p-C6F4CF3, OC(CF3)),[2–10a] cat-
+
+
+
+
ionic substituents (NR3 , PR3 , SMe2 , BR2 ),[3a,10b,11] or anti-
aromatic borole rings.[4]
The large number of electron-withdrawing groups how-
ever increases the molecular weight of these triarylboranes,
and perfluorinated scaffolds are often problematic in terms of
synthetic approaches, side-products formation by rearrange-
ments, or fluorine substitutions by strong Lewis bases or
nucleophiles.[12]
Knowing that several free or coordinated non-planar
boron Lewis acids are known, such as borabenzobarrelene 7
(Scheme 2),[13] 1-bora adamantane 8,[14] and 9,[15a,b] we have
been stimulated to generate the boratriptycene Lewis acid 6
and to design Lewis acid/base bifunctional molecules such as
Quantum-chemical calculations revealed that 6 may have
a huge potential in catalysis as a constituent of either
cryptands or frustrated Lewis pairs with potential applications
[*] A. Ben Saida,[+] Dr. A. Chardon,[+] A. Osi, Dr. N. Tumanov,
Prof. J. Wouters, A. I. Adjieufack, Prof. B. Champagne,
Prof. G. Berionni
Department of Chemistry, Namur Institute of Structured Matter
University of Namur
5000 Namur (Belgium)
E-mail: guillaume.berionni@unamur.be
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201910908.
Scheme 2. Cage-shaped boron compounds 6–10.
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10 with a B atom and a P atom at each edge of the triptycene
framework (Scheme 2).
The tris(ortho-halogeno)trityl derivative 11[16] was sub-
jected to a triple halogen/lithium exchange with tBuLi and
was treated with potassium organotrifluoroborates to provide
the 9-boratriptycene ate-complexes 12a–b after cation meta-
thesis and recrystallization (Scheme 3) which have been
Figure 1. a) Molecular structure of the 10-n-butyl-boratriptycene 12a;
plane spanned by the ipso-carbon atoms of the three triptycene
benzene rings shown as a gray plane. b) Molecular structure of the 9-
phosphonium analogue 14a. Here and further structures are shown
with thermal ellipsoids set at the 50% probability level. See the
Supporting Information for further details and CCDC numbers. The
cation in 12a, aromatic and alkyl chain H-atoms omitted for clarity.
À
À
À
Selected bond lengths [ꢀ] in 12a: B C1 1.652(5), B C2 1.641(5), B C3
À
À
1.650(5), B C4 1.612(5), and in 14a (atom numbering as for 12a): B
Scheme 3. Synthesis of 9-boratriptycene ate-complexes 12a,b and
attempted synthesis of 9-boratriptycene 6 via deprotection of 12a or
12b. E+ =Ph3C+BF4À, Me3O+BF4À, Barluenga’s reagent, I2; H+ =HBF4,
HNTf2, HCl. For the synthesis of 11 via a modification from a literature
procedure,[16] see the SI.
À
À
À
C1 1.655(2), B C2 1.661(2), B C3 1.652(2), B C4 1.624(2).
À
bonds (1.524(5) ꢀ) compared to longer C P bonds (1.781-
(2) ꢀ) in 14a (Figure 1).
The phosphonium-borate 14b (and analogue 14e,
Scheme 5) smoothly underwent selective phenyl protode-
boronation with HBF4 to yield the fluoroborates 17–18
(Scheme 5). Their 19F NMR spectrum showed a resonance
unambiguously characterized by X-ray diffraction analysis
(see the Supporting Information and Figure 1). We next
attempted to remove the nBu and Ph groups on the
tetravalent boron atom of 12a–b by using various deboryla-
tion strategies.
Though the nBu chain in triphenylboron ate-complexes
[Ph3(nBu)BÀX+] are readily cleaved via a or b-hydride
abstraction and elimination by carbenium and oxonium
ions,[17] this route proved to be unrewarding for the debor-
ylation of 12a (Scheme 3). Submitting 12b to strong Brønsted
acids or electrophiles only provided complex mixtures
resulting from unselective deborylations (see the Supporting
Information for full details).
at À235 ppm consistent with a F BAr3À bond but low shifted
À
À
[19]
À
À
by 45 ppm from the F B resonance in F B(C6F5)3
.
Their
formation supported a mechanistic scenario involving the
transient formation of the highly Lewis acidic cationic
À
boratriptycenes 15–16 which abstracted a fluoride from BF4
.
We reasoned that replacing the CH bridgehead position in
12 by a P+R phosphonium linker should decrease the electron
density of the triptycene aromatics and favor the selective
Scheme 5. Protodeborylation of 14b and 14e by HBF4·OEt2 and
formation of the zwitterionic fluoroborates 17 and 18. Conditions: i):
À
exocyclic phenyl C B bond protodeboronation. A series of 9-
Na2CO3, Me3O+BF4 (see the Supporting Information for full details).
À
phosphonium-10-boratriptycene aryl and alkyl ate-complexes
14a–d were thus prepared following the method of Sawamura
(Scheme 4).[18]
The boron atom pyramidalization, defined as the distance
between B and the plane spanned by the ipso-carbon atoms of
the three triptycene benzene rings (gray plane in Figure 1),
Treatment of 14b with Br2 or I2 resulted in the in situ
formation of HBr or HI, respectively, due to the strong
Brønsted acidity of the phosphonium moiety, with release of
À
normalized by average B C bond length is greater in
boratriptycenes 12a,b [0.445(3)] than in the phosphorus
analogues 14a,b [0.407(3)]. The higher boron pyramidaliza-
the phosphine which trapped the electrophilic halenium ions
+
À
X . Simultaneous protodeborylation of the phenyl C B bond
by the in situ formed HBr or HI gave 19–20 in high yields
(Scheme 6) and the formation of benzene was detected by
1H NMR spectroscopy (see the Supporting Information and
Figure 2 for the solid-state structures). Translation of those
conditions to the methylated analog 14e furnished the
halogeno-borates 21a–b via halogeno-dephenylation reaction
(Scheme 6), showing that selective deboronation by direct
halogenation with Br2 is also successful.
À
tion in 12a is due to the shorter triptycene back-side C C
Remarkably, the halide attached to the boron atom was
not originating from the Br2 or I2 reagents, but from the
reaction solvent CH2X2. The transient boratriptycene Lewis
acid thus abstracted a halide from the halogenated solvents
Scheme 4. Synthesis of 9-phosphonium-10-boratriptycene ate-com-
plexes 14.
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Scheme 7. Formation of Lewis adducts 23–24 via deallylation of 22.
Scheme 6. a) Proto-dephenylation of 14b by in situ formed HX;
b) Halogeno-dephenylation of 14e by Br2 (solid-state structure of 21b
in Supporting Information).
À
À
Selected bond lengths in [ꢀ]: B N 1.569(9) in 23, B O 1.565(18) in 24,
1.548(3) in 25.
frequency of the coordinated EtOAc from 1607 cmÀ1 in 24 to
1581 cmÀ1 in 25.[24] The nC O stretching frequency in 25 is even
=
lower than in AlCl3·EtOAc (1624 cmÀ1).[24] This strong Lewis
acidity enhancement is the result of the electron-withdrawing
effect of the phosphonium linker and to a transannular
overlap[1c,15a] between the vacant p-orbital of boron and s*
orbital of the phosphonium in the triptycene scaffold.
As both nC stretching frequencies (ca. 1580 cmÀ1) and
=
O
Figure 2. Left to right: Molecular structures of 17 and 20a–c. Aromatic
both C O 13C chemical shifts (ca. 187 ppm) of the coordi-
nated ethyl acetate in 25 and 26 were nearly identical, we
concluded that the phosphorus atom quaternarization by H+
=
À
H-atoms are omitted for clarity. Selected bond lengths [ꢀ]: P H 1.330-
À
À
À
À
(3), B F 1.403(3) in 17; P I 2.364(12), B Cl 1.928(5) in 20a; P I
À
À
À
2.366(11), B Br 2.065(4) in 20b; P I 2.362(13), B I 2.260(5) in 20c.
+
or CH3 had a comparable effect on the boron atom Lewis
acidity (Scheme 8).
assisted by SN2 substitution of the BrÀ or IÀ anions from HBr
or HI.[20]
The 9-phosphonium-10-borates 19–21 were characterized
À
by single-crystal X-ray diffraction, providing the B X bond
lengths for the complete halogen series of boron ate-
À
complexes FÀ, ClÀ, BrÀ and I . The B F (1.403(3) ꢀ) and
À
À
B Cl (1.928(5) ꢀ) distances in 17 and 20a were shorter or
À
À
À
comparable than in the F B(C6F5)3 (1.418(5) ꢀ) and Cl
Scheme 8. Formation of Lewis adducts 26–27 via protodeboronation
B(C6F5)3À (1.919(18) ꢀ) anions, respectively.[21]
À
of 14e. B O length in 26 1.548 ꢀ (solid-state structure of 26 in the
Systematic X-ray diffraction, NMR and IR spectroscopy
investigations of the 9-phospha-10-boratriptycene Lewis
adducts with a series of Lewis bases were performed to
gather quantitative information on the stereoelectronic
properties of their boron atom.
Inspired by the techniques of silylium cations generation
via the allyl leaving group strategy,[22] the MeCN and EtOAc
Lewis adducts 23–24 were synthesized by treating the allyl-
borate 22 with HCl or HBF4 in the desired solvents
Supporting Information). IR spectroscopy, nCN =2379 cmÀ1 for 27.
As a demonstration of the exceptional Lewis acidity of
these 9-phosphonium-10-boratriptycenes, the protodeboryla-
tion of 14b with HNTf2 yielded a unique triflimidate Lewis
adduct 28 (Scheme 9; Figure 3). This unprecedented Lewis
adduct between a trivalent boron Lewis acid and an oxygen
atom of the very non-nucleophilic Tf2NÀ anion[25] showcases
the extreme Lewis acidity of the boron atom in 9-phospho-
nium-10-boratriptycenes.
À
À
(Scheme 7). Both B N and B O bonds in 23 and 24 were
much shorter than in the corresponding B(C6F5)3 adduct[23]
À
with the B N bond length in 23 (1.569(9) ꢀ) being one of the
shortest reported in the Cambridge structural database, even
shorter than in the Lewis adduct of CH3CN with perfluoro-
borafluorene 5 (Scheme 1, 1.581(5) ꢀ)[4] and in the Lewis
adduct 7 (cf. Scheme 2, 1.589(2)).[4b]
Though the phosphorus protonation had no effect on the
boron atom pyramidalization according to the CBC angles
(ca. 108.08 in 24 and 25, see solid-state structures in Support-
ing Information), the boron Lewis acidity dramatically
Scheme 9. Generation of the triflimidate Lewis adduct 28 and subse-
quent Lewis base exchange with PO(Et)3 to give 29 (solid-state
structures in Figure 3).
increased as indicated by the large shift of the nC O stretching
=
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Addition of PO(Et)3 on 28 resulted in the displacement of
the Tf2NÀ anion and produced the Lewis adduct 29
(Scheme 9; solid-state structures in Figure 3) which can be
employed for probing by 31P NMR spectroscopy the boron
Lewis acidity according to the Gutmann-Becket method.[26]
also evaluated (Scheme 10). In the case of fluoride, pseudo-
isodesmic reactions have been used, with the FIA of SiMe3F
as the anchor point evaluated at a reference G3 level.[27a]
The considerably higher Lewis acidity of 9-boratriptycene
6 than of BPh3 3 and of boraadamantane 8 (CBC angles 1168)
is linked with the stronger pyramidalization of its boron atom
environment (CBC angles 1138 in 6). The neutral boratripty-
cenes 6 and 10 are even more Lewis acidic than B(C6F5)3 when
considering their FIAs and is approaching that of SbF5
(536 kJmolÀ1)[27a] and of the “super-B(C6F5)3” 30 in the gas
phase.[2]
The non-isodesmic FIA of 4, 15 and 31 in CH2Cl2
(IEFPCM solvation model)[28b] have also been calculated
since Coulombic attraction and charge effect contribution are
reduced in condensed-phase, and the Lewis acidity of 15
(513 kJmolÀ1) was slightly lower than in B(CF3)3 31
(529 kJmolÀ1). Gas-phase result FIAs clearly showed that 9-
phosphonium-10-boratriptycenes 15 and 16 are stronger
Lewis acids than some electrophilic phosphonium and
silylium cations (Scheme 10b).[27]
Figure 3. Molecular structure of a) the 9-phosphonium-10-boratripty-
cene Tf2NÀ Lewis adduct 28 as a toluene solvate (average values for C-
P-C angles=103.68, C-B-C angles=107.78); b) of the PO(Et)3 adduct
29 (average values for CPC angles=103.28, CBC angles=106.68). H-
À
atoms (except P H groups) and toluene solvate molecule are omitted
for clarity, bond lengths in [ꢀ].
In essence, bending the archetypal planar trigonal boron
coordination environment results in exceptional Lewis acidity
enhancements without requiring perfluorinated scaffolds or
electron-withdrawing groups. Three factors are combined to
reach one of the strongest trivalent boron Lewis acid
(compound 15), which is even able to bind very weakly
nucleophilic anions, such as Tf2NÀ: i) pyramidalized boron
coordination environment; ii) transannular overlap between
the empty p orbitals of boron and the s* orbital of the
phosphonium back-side atom, and iii) cationic charge on the
phosphonium bridge in close proximity to the B atom. The
unprecedented reactivity of 9-phospha-10-boratriptycene
may open innovative avenues in bifunctional Lewis and
Brønsted acid/base catalysis.
The 31P NMR chemical shift of 79.3 ppm for PO(Et)3 in 29
corresponded to an Acceptor Number[26] of 84.7 showing that
the Lewis acidity of tris(perfluorotolyl)borane 30 (AN =
85.0)[2] is reached when considering PO(Et)3 as a reference
Lewis base.
As the Fluoride (FIA) and Hydride Ion Affinity (HIA)
constitute well-established Lewis acidity scales with a reper-
toire of hundreds of Lewis acids,[27a] we computed the FÀ and
HÀ affinities of B(C6F5)3, B(CF3)3, and of boratriptycenes in
the gas phase. Their affinities (DH0) toward ammonia were
Acknowledgements
We acknowledge the University of Namur, the Institute of
Structured Matter (NISM), research college (NARC) and the
Fond National de la Recherche Scientifique FNRS (grant
F.4513.18 for G.B.) for funding. The quantum-chemical
calculations were performed on the computing facilities of
the “Consortium des ꢁquipements de Calcul Intensif”, in
particular at the High-Performance Computing technological
platform (University of Namur), which is supported by the
FNRS-FRFC, the Walloon Region, and the University of
Namur (Conventions No. 2.5020.11, GEQ U.G006.15,
1610468, and RW/GEQ2016). We thank the PC2 technolog-
ical platform for access to X-ray diffraction and NMR
instruments and the Unitꢂ de Chimie Organique team
(Prof. Stꢂphane Vincent and Prof. Steve Lanners) for useful
discussions.
Scheme 10. Gas phase hydride, fluoride, and ammonia affinities, of
selected, a) neutral and, b) cationic Lewis acids (DH0 in kJmolÀ1
)
evaluated using density functional theory with the M06-2X exchange-
correlation functional[28a] and the 6-311G(d) atomic basis set. Values in
parenthesis from the literature with pseudo-isodesmic reactions with
COF2 and BEt3, respectively.[27] Non-isodesmic FIA in CH2Cl2 (IEFPCM
solvation model) equal to 278 kJmolÀ1 for 4.
Conflict of interest
The authors declare no conflict of interest.
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+
À
À
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Manuscript received: August 26, 2019
Accepted manuscript online: September 2, 2019
Version of record online: && &&, &&&&
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Lewis Acids
Pyramid power: Owing to the drastic
pyramidalization around their trivalent
boron atom and to the presence of
a cationic phosphonium back-side
bridge, the Lewis acidity of the cage-
shaped 9-bora-10-phosphonium tripty-
cenes surpasses that of all trivalent boron
Lewis acids generated to date.
A. Ben Saida, A. Chardon, A. Osi,
N. Tumanov, J. Wouters, A. I. Adjieufack,
B. Champagne,
G. Berionni*
&&&& — &&&&
Pushing the Lewis Acidity Boundaries of
Boron Compounds With Non-Planar
Triarylboranes Derived from Triptycenes
6
www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
These are not the final page numbers!