Using Ylide Functionalization to Stabilize Boron Cations

Article abstract of DOI:10.1002/anie.201611677

The metalated ylide YNa [Y=(Ph₃PCSO₂Tol)^?] was employed as X,L-donor ligand for the preparation of a series of boron cations. Treatment of the bis-ylide functionalized borane Y₂BH with different trityl salts or B(C₆F₅)₃ for hydride abstraction readily results in the formation of the bis-ylide functionalized boron cation [Y?B?Y]⁺ (2). The high donor capacity of the ylide ligands allowed the isolation of the cationic species and its characterization in solution as well as in solid state. DFT calculations demonstrate that the cation is efficiently stabilized through electrostatic effects as well as π-donation from the ylide ligands, which results in its high stability. Despite the high stability of 2 [Y?B?Y]⁺ serves as viable source for the preparation of further borenium cations of type Y₂B⁺←LB by addition of Lewis bases such as amines and amides. Primary and secondary amines react to tris(amino)boranes via N?H activation across the B?C bond.

Full text of DOI:10.1002/anie.201611677

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201611677
German Edition: DOI: 10.1002/ange.201611677
Boron Cations
Using Ylide Functionalization to Stabilize Boron Cations
Thorsten Scherpf, Kai-Stephan Feichtner, and Viktoria H. Gessner*
Abstract: The metalated ylide YNa [Y= (Ph3PCSO2Tol)ꢀ]
was employed as X,L-donor ligand for the preparation of
a series of boron cations. Treatment of the bis-ylide function-
alized borane Y2BH with different trityl salts or B(C6F5)3 for
hydride abstraction readily results in the formation of the bis-
+
ꢀ ꢀ
ylide functionalized boron cation [Y B Y] (2). The high
donor capacity of the ylide ligands allowed the isolation of the
cationic species and its characterization in solution as well as in
solid state. DFT calculations demonstrate that the cation is
efficiently stabilized through electrostatic effects as well as p-
donation from the ylide ligands, which results in its high
+
ꢀ ꢀ
stability. Despite the high stability of 2 [Y B Y] serves as
viable source for the preparation of further borenium cations
!
of type Y2B+ LB by addition of Lewis bases such as amines
and amides. Primary and secondary amines react to tris-
ꢀ
ꢀ
(amino)boranes via N H activation across the B C bond.
T
he chemistry of boron compounds is strongly connected
with their exceptional Lewis acidity. Hence, their general
properties, bonding modes and reactivities are dictated by
their tendency to overcome their intrinsic electron deficiency,
such as by adduct formation or oligomerization. Cationic
boron species are naturally even more reactive due to their
higher electron deficiency.[1] Depending on the coordination
number, boron cations are classified into three classes.[2]
While borinium species are highly reactive, two-coordinate
compounds, boronium and borenium cations are four- and
three-coordinate species, that are additionally stabilized by
coordination of neutral donor ligands (L, Figure 1A). Partic-
ularly borenium cations have fostered intense research
interest over the past five years[3] due to the discovery of
a series of applications in stoichiometric as well as catalytic
transformations.[4] For example, the research groups of
Curran, Vedejs, and Ingleson reported on borenium-catalyzed
hydroboration[5] and haloboration[6] reactions, while the
research groups of Stephan and Crudden established hydro-
genation reactions proceeding through FLP-type mechanisms
(FLP = frustrated Lewis pair[7]).[8]
Figure 1. A) Classes of boron cations and B) isolated borenium and
borinium cations.
particularly benefited from the development of sterically
demanding substituents and new ligand systems, such as
strong (and often bulky or multidentate) N-donor ligands,[10]
N-heterocyclic carbenes (NHC)[11] or carbodiphosphoranes
(e.g A–F, Figure 1B).[12] These ligands facilitated their
stabilization and thus their isolation.[13,14] Nevertheless,
almost all isolated boron cations rely on the use of amido or
aryl groups as anionic substituents at boron. They are capable
to efficiently stabilize the cation by minimizing the electron
deficiency at boron through additional p-donation. Our group
has become interested in the coordination chemistry of ylidic
donor ligands. Particularly, metalated ylides with two lone
pairs of electrons at the central carbon atom seemed to be
ideal ligands for the stabilization of electron-deficient species
due to their potential ability to function as carbon-centered s-
and p-donor ligands.[15] Herein, we challenged this donor
capacity of metalated ylides and their applicability as a novel
class of X,L-type ligands for the stabilization of cationic boron
species.
For the preparation of an ylide-stabilized boron cation we
chose a synthetic strategy via hydride abstraction from the
bis(ylide)-functionalized borane 1 (Scheme 1). 1 is conven-
iently accessible from the metalated ylide YNa [Y=
(Ph3PCSO2Tol)ꢀ] and BH3·THF.[15] To access the correspond-
ing boron cation a solution of 1 in CHCl3 was treated with one
equiv. of a trityl salt with a weakly coordinating anion,
[Ph3C][BAr4] (Ar= 3,5-Cl2C6H3, 3,5-(CF3)2C6H3 or C6F5; see
the Supporting Information for experimental details).[16]
Reaction monitoring showed the consumption of the starting
material and formation of one major new product exhibiting
a broad signal at dP = 12.5 ppm in the 31P NMR spectrum (1:
Despite these advances in the application of boron
cations, these species—particularly the two- and three-
coordinated systems—are still relatively underexplored with
only a limited number being isolated.[9] Their chemistry has
[*] T. Scherpf, K.-S. Feichtner, Prof. Dr. V. H. Gessner
Chair of Inorganic Chemistry II
Faculty of Chemistry and Biochemistry
Ruhr University Bochum
Universitꢀtsstraße 150, 44780 Bochum (Germany)
E-mail: viktoria.gessner@rub.de
Homepage: http://www.ruhr-uni-bochum.de/ac2/
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201611677.
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Scheme 1. Preparation of boron cation 2 with different counterions.
dP = 23.8 ppm) and a broad signal at dB = 40.3 ppm in the
11B NMR spectrum. 1H NMR spectroscopy clearly confirmed
the formation of Ph3CH, thus indicating successful formation
of boron cation 2. After 16 h reaction time at 508C the
conversion to 2 was found to be greater than 80%. Most
remarkably, 2 could also be obtained from 1 with the strong
Lewis acid B(C6F5)3.[17] This suggests that the formed boron
cation 2d is—owing to the donor capacity of the ylide
ligands—less electron-deficient than B(C6F5)3 itself.
Despite the high-yielding formation of 2 its isolation
revealed to be complicated due to the highly viscous nature of
the obtained salts. To obtain 2 as solid material smaller
counter-anions were tested to facilitate crystallization. Ion
exchange by addition of [Bu4N]PF6 to the crude reaction
mixture led to 2e as colorless solid and thus allowed its
isolation in an overall yield of 61%. 2 exhibits a high thermal
stability (also in refluxing toluene) and can be stored for
months under inert conditions at room temperature without
showing any decomposition reactions. A thermogravimetric
analysis of 2e showed no decomposition up to 2008C (see the
Supporting Information). In the molecular structure, the
asymmetric unit contains half a molecule which is assembled
to 2e through C2-symmetry.[18] Thereby, the molecule features
Figure 2. a) Molecular structure of 2·PF6 (disorder of the CSO2 moiety
not depicted). Hydrogen atoms and the PF6 anion are omitted for
clarity; 50% displacement parameters. Selected bond lengths [ꢁ] and
angles [8]: C1B-B’ 1.481(7), C1A-B 1.510(9), P-C1A 1.676(8), P-C1B
1.781(8), C1A-S1A 1.669(6), C1B-S1B 1.701(6), S1A-O2A 1.423(4), S1A-
O1A 1.592(5), S1B-O1B 1.428(5), S1B-O2B 1.444(4), O1A-B 1.493(6),
S1A-C1A-P 124.3(4), S1B-C1B-P 119.4(3); C1A-B1-C1B’ 144.0(4), O1A-
B-C1A 96.5(4). b) 31P{1H} NMR spectra of 2 at different temperatures.
c) Different isomers of 2 in solution.
ꢀ ꢀ
a disorder of the B C SO2 moiety, which however could be
de-convoluted and the structure was refined satisfactorily. No
interaction between the PF6 anion and the boron center is
observed in the crystal structure (Figure 2 and the Supporting
Information). However, 2e features no two-coordinate boron
atom. Instead, the sulfonyl groups of the ylide ligands
conform with the p-interaction between the ylide ligand and
the boron center.[15]
ꢀ ꢀ
coordinate to boron, thus leading to a slightly bent C B C
NMR spectroscopic studies of 2 revealed that the
structure observed in solid-state is also preserved in solution.
As such, dichloromethane (DCM) solutions of 2 showed—
independent of the counter-anion—only a single broadened
peak in the 31P{1H} NMR spectrum at dP = 12.5 ppm as well as
moiety with a large angle of 144.0(4)8. Due to the disorder
and the C2 symmetry in the molecular structure the sulfonyl
moiety of both ylide ligands coordinate to the boron center,
resulting in an all-planar L-B-L fragment (L = PCS). Fig-
1
ure 2a shows only one possible arrangement. The two C B1
broadened signals in the H and 13C NMR spectra at room
ꢀ
distances of 1.481(7) and 1.510(9) ꢀ are both shorter than
typical single bonds,[19] thus suggesting a considerable degree
of p-donation from the ylide ligands to the boron center.
temperature. This is consistent with a dynamic behavior and
the alternating coordination of one of the two sulfonyl
moieties to boron. This fluxional behavior can be followed by
VT NMR studies. Already at ꢀ308C the broad 31P NMR
signal splits into two signals (dP = 8.40 and 16.0 ppm) due to
the now prevented oscillation of the boron atom between the
two sulfonyl moieties (anti-2 and anti-2’ in Figure 2). Further
cooling to ꢀ908C results in a further splitting of the two
signals in an approximate 7:3 ratio. This presumably corre-
ꢀ
Thereby, the shorter C B distance corresponds to the exo-
ꢀ
ꢀ
ꢀ
cyclic boron carbon bond (B1 C1B’). The P1 C1 and S1 C1
bond lengths in the ligand backbone are in average longer
ꢀ
(e.g. S1 C1(average): 1.685(6) ꢀ) than in the metalated ylide
[Ph3PCSO2Ph]Na (e.g. S C1: 1.626(2) ꢀ). This is consistent
with a reduced negative charge at C1 and reduced electro-
ꢀ
ꢀ ꢀ
ꢀ
static interactions within the P C S backbone, which is
sponds to the now hindered rotation about the C S bond and
2
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ꢀ
the formation of two isomers with both tolyl groups of the
B C moiety. The HOMO represents the two lone pairs at the
carbon atoms, while the HOMO-1 is the bonding p-inter-
sulfonyl moiety either being located on the same (syn-2) or
opposite side (anti-2/2’) of the YBY plane. In the crystal
structure only the syn-isomer is found due to the interactions
of the aryl groups with the PF6 anion.
ꢀ ꢀ
action over the whole C B C linkage. Hence, the calcula-
tions and experimental data argue for a C B C moiety with
highly polarized C B double bonds.
ꢀ ꢀ
=
DFT calculations were performed to get insights into the
structure and bonding situation in the borenium cation (see
the Supporting Information). The calculated geometrical
parameters of 2Ph (Ph instead of the tolyl groups at sulfur)
match the experimental values very well. The isomer syn-2Ph
observed in the crystal structure is clearly energetically
Next, we addressed the reactivity of the boron cation. At
first, its Lewis acidity was probed by reaction with different
Lewis bases. Dimethylaminopyridine (DMAP) quantitatively
delivered the corresponding borenium ion 3a, as evidenced
by a shift in the 31P{1H} (12.6 ppm for 2 and 21.0 ppm for 3a)
and 11B{1H} NMR spectrum (47.0 ppm for 3a). Adduct
formation was also observed with amides (DMF or N-
methylpyrollidone, NMP), while, no adduct formation was
observed with ketones or phosphines (PMe3, PPh3). The
coordination of NMP and DMF was found to be reversible.
As such, evaporation of a DMF solution of 3c at room
temperature resulted in the re-formation of 2e and the
liberation of DMF. This is confirmed by DFT studies on the
adduct formation with different Lewis bases, which yielded an
exergonic process for amines, but a slightly endergonic
reaction with amides (see the Supporting Information). The
obtained borenium salts 3a–c could be isolated by crystal-
lization at low temperature and their composition was
unambiguously confirmed by NMR spectroscopy as well as
XRD analysis. In all cases, no double coordination of two
molecules of the Lewis base to form a boronium ion was
observed, even when using an excess of Lewis base.[21] The
structures of 3b and 3c are given in Figure 4, 3a in the
Supporting Information. Upon coordination of the Lewis
base the intramolecular sulfonyl coordination is broken to
provide selective access to borenium ions with two equivalent
ꢀ ꢀ
favored over a borinium system with a linear C B C
moiety and no sulfonyl coordination (DG = 49 kJmolꢀ1), but
equal in energy with the anti-isomer anti-2Ph (DG =
ꢀ2.7 kJmolꢀ1).[20] The oscillation of the C B C unit between
ꢀ ꢀ
the two sulfonyl groups showed an activation barrier of only
DG° = 97 kJmolꢀ1, thus being well in line with the NMR
studies. The calculated charges derived from the natural bond
orbital (NBO) analysis of a hydrogen substituted model
system 2H indicate that electrostatic effects play an important
ꢀ ꢀ
role in the stabilization of the C B C linkage (Figure 3). As
ꢀ
ylide ligands. Thus and in contrast to 2e, the B C distances
are almost equal and range between 1.51 and 1.54 ꢀ. These
bond lengths are slightly longer than the average bond length
ꢀ
in 2e, but still in between a typical B C single and double
bond,[19] thus indicating still present p-interaction between
Figure 3. a) Calculated Wiberg bond indices (WBI) and NBO charges
and b) frontier molecular orbitals of 2H (M062X/6-311+G(d,p)).
ꢀ ꢀ
boron and the ylide ligands in 3. Compared to 2e, the C B C
angles in 3a–c are smaller and the ylide ligands are not co-
=
planarly, but slightly twisted arranged to each other. The C O
such, high negative charges are observed at the carbon atoms
bonds in DMF and N-methylpyrrolidone experience a slight
lengthening upon coordination to the boron center, thus
indicating some degree of Lewis acid activation.
In contrast to the coordination of tertiary amines, 2 shows
a totally different behavior towards primary and secondary
(qC = ꢀ1.26 and ꢀ1.30) and a positive charge of qB =+ 0.90 at
ꢀ
boron. The calculated Wiberg bond indices (WBI) of the B C
bonds amount to 0.98 and 1.16, thus suggesting some double
ꢀ ꢀ
bond character within the C B C linkage. The WBI of the
ꢀ
ꢀ
B O interaction of 0.73 underlines its importance for the
amines, which however reflects the high polarity of the C B
stabilization of the boron cation. The NBO analysis of 2H
bond. Treatment of 2b[22] with an excess of p-nitroaniline
ꢀ
ꢀ
confirms the highly unsymmetrical bonding situation within
results in N H activation across the B C bond and cleavage
of both ylide substituents from the boron center.[23] This leads
to the formation of tris(amino)borane 4a together with ylide
ꢀ ꢀ
ꢀ
=
the C B C linkage, showing a C B single and a C B double
bond. However, both bonds [for example, the double bond by
69 (s-bond) and 86% (p-bond)] are strongly polarized
towards the carbon end, thus reflecting the high negative
charges observed at the ylidic carbon atoms. The molecular
orbitals show that the lowest unoccupied orbital (LUMO) is
predominantly localized at the boron center. Hence, the
boron atom remains—independent of the degree of p-
delocalization and coordination of the sulfonyl moiety—the
most electrophilic center in the molecule (c.f. qB =+ 0.90).
The two highest occupied molecular orbitals (HOMOs)
YH and the corresponding phosphonium salt ([YH2][BArF ]).
4
4a is the only product formed in this reaction. Employing
a 1:1 ratio of amine and 2b delivered 4a and unreacted
borenium cation 2b. The same reactivity of 2b was also
observed with aniline or the secondary amines, diethyl amine
or pyrrole, while pyrrole directly leads to the formation of the
corresponding borate [B(pyrrole)4]ꢀ. Thus, despite the high
thermal stability of 2 it still functions as highly reactive boron
cation. Besides the coordination of different Lewis bases and
ꢀ
ꢀ
ꢀ
illustrate the strongly polarized p-delocalization in the C
the N H addition across the B C bond, 2 also acts as fluoride
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Conflict of interest
The authors declare no conflict of interest.
Keywords: boron · boron cations · carbanions ·
Lewis acid base pairs · ylide ligands
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for 3b: S1-C1 1.720(2), P1-C1 1.750(2), C1-B 1.512(3), B-O5 1.478(3),
B1-C27 1.522(3), S2-C27 1.714(2), P2-C27 1.735(2), N-C53 1.283(3),
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ꢀ ꢀ
ane 5, while keeping the Y B Y moiety intact.
In conclusion, we have shown that metalated ylides can be
employed as strong electron-donating X,L-type ligands. Their
donor capacity allowed the isolation of a boron cation of type
+
ꢀ
ꢀ
Y B Y with a remarkable thermal stability due to the
efficient stabilization through the ylide substituents. DFT
calculations indicate that electrostatic interactions as well as
ꢀ ꢀ
p-delocalization within the C B C linkage play the most
important role for the stability of the cationic species. Despite
this stability the boron cation remains its reactivity as cationic
species. As such, it forms Lewis acid base adducts with amines
and amides and reacts with primary and secondary amines by
ꢀ
ꢀ
N H addition across the B C bond. Overall, the facile
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Acknowledgements
This work was supported by the DFG, the Boehringer-
Ingelheim Foundation (Exploration Grant) and the European
Research Council (starting grant number 677749, YlideLi-
gands).
4
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[18] CCDC 1520039– 1520044 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre.
[19] P. Pyykkç, M. Atsumi, Chem. Eur. J. 2009, 15, 12770.
ꢀ ꢀ
[20] The linear C B C structure is only a minimum on the potential
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energy surface in case of a twisted arrangement of the two ylide
moieties. A further conformer with the PPh3 moieties cis to each
other is energetically less stable (DG = 33 kJmolꢀ1).
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Commun. 2005, 2480.
ꢀ
[22] The PF6 anion has to be replaced in these reactions, since
ꢀ
competing P F activation is observed under the reaction
ꢀ
conditions of the N H activation. This side-reaction leads to
the formation of PF5 and fluoroborane 5.
ꢀ
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[23] For a similar reactivity of B C bonds, see Ref. [13].
Manuscript received: November 30, 2016
Revised: January 26, 2017
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Boron Cations
Ylide meets B+: Employing a metalated
ylide as donor ligand makes possible the
isolation of highly stable ylide-function-
alized boron cations. p-Delocalization as
well as electrostatic interactions within
T. Scherpf, K.-S. Feichtner,
V. H. Gessner*
&&&& — &&&&
ꢀ ꢀ
Using Ylide Functionalization to Stabilize
Boron Cations
the C B C linkage play the most impor-
ꢀ ꢀ
tant role for the high stability of the [Y B
Y]+ cation and its Lewis base adducts.
ꢀ
Reaction with amines results in N H
ꢀ
activation by addition across the B C
bond.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!