Reactions of boron-substituted N-heterocyclic carbene boranes with triflic acid. Isolation of a new dihydroxyborenium cation

Article abstract of DOI:10.1021/om201260b

Reactions of NHC-BH₂X (where X = Cl, OTf and NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with 1 equiv of triflic acid provide the stable products of acid/base reactions NHC-BH(OTf)₂ and NHC-BH(OTf)Cl. Further reactions of

Full text of DOI:10.1021/om201260b

Communication
pubs.acs.org/Organometallics
Reactions of Boron-Substituted N-Heterocyclic Carbene Boranes with
Triflic Acid. Isolation of a New Dihydroxyborenium Cation
Andrey Solovyev,† Steven J. Geib,† Emmanuel Lacote,‡ and Dennis P. Curran*,†
̂
†Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
‡ICSN CNRS, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France
S
* Supporting Information
ABSTRACT: Reactions of NHC-BH2X (where X = Cl, OTf and NHC = 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene) with 1 equiv of triflic acid provide the stable
products of acid/base reactions NHC-BH(OTf)2 and NHC-BH(OTf)Cl. Further
reactions of these compounds with additional triflic acid (or direct reactions of NHC-
BH3 with excess triflic acid) produce the isolable dihydroxyborenium triflate [NHC-
B(OH)2]+TfO−. This first-in-class ligated borenium ion has been characterized by NMR
spectroscopy and X-ray crystallography.
ricoordinate boron cations [L-BR2]+ are commonly called
bulky or are capable of conjugation, the dissociation of A to
planar, tricoordinate borenium form B becomes favorable
because of steric destabilization of the tetracoordinate form A,
electronic stabilization of the borenium center in B, or both.
Five NHC-borenium cations with different boron substitu-
ents have been identified (Figure 1). Steric effects are likely
T
borenium ions.1 Formally, they are Lewis adducts of
+
dicoordinate borinium cations (BR2 ) and Lewis bases (L) such
as amines, phosphines, and the so-called carbon(0) ligands.2
Interest in borenium cations is growing because of their novel
structures and their high reactivity as cationic Lewis acids3 and
electrophilic borylation agents.4
̈
responsible for the formation of Gabbaı’s NHC-diarylborenium
18 and Lindsay’s NHC-dialkylborenium 2.9 Both steric and
electronic effects account for the formation of Weber’s NHC-
diaminoborenium 3,10 Robinson’s NHC-aminochloroborenium
4,6 and Aldridge’s dibromoborenium ion 5.11
N-heterocyclic carbenes (NHC) have become popular
ligands for the stabilization of unusual and otherwise unstable
boron species, including borenium ions.5,6 Most NHC-boranes
bearing good leaving groups X on boron prefer structure A,
with a tetracoordinate boron center and a covalent B−X bond
(Figure 1).7 Occasionally, when the groups R on boron are
The synthesis of NHC-boreniums 1−5 was achieved either
by the direct substitution of X in R2B-X by free NHC or by the
treatment of NHC-BR2−H with a strong acid H-X. These
NHC-borenium ions all have amine−borenium analogues:
[DMAP-BMes2]+,12 [Py-9-BBN]+,13 [Py-B(NR2)2]+,14 the β-
diketiminate-supported chloroborenium [L-B(NR2)Cl]+,15 and
[2,6-diMes-Py-BBr2]+.11
Here we describe the isolation and characterization of the
new dihydroxyborenium cation 6, [NHC-B(OH)2]+TfO−. This
cation lacks large groups R on boron, forms in an unusual way,
and has no direct antecedent in other classes of ligated
borenium ions.
The new borenium ion was discovered during a study of
acid/base reactions of substituted NHC-boranes, as summar-
ized in Scheme 1. Treatment of 7 with 1 equiv of triflic acid or
HCl is known to provide monotriflate 8a or monochloride 8b
in situ.7 Reaction of 7 with 2.5 equiv of triflic acid provided the
known but not previously isolated ditriflate 9a.7 This proved to
be rather robust and was isolated in 54% yield after flash
chromatography.
The reaction of NHC-BH2Cl (8b) with TfOH (1.5 equiv)
was quick and complete as well. NHC-BH(OTf)Cl (9b) was
the only boron product observed in the reaction mixture by 11
B
Figure 1. (a) General structures and (b) literature examples of NHC-
borenium cations 1−5 along with the new borenium ion 6 (Mes =
2,4,6-trimethylphenyl; dipp = 2,6-diisopropylphenyl).
Received: December 20, 2011
Published: December 28, 2011
© 2011 American Chemical Society
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Organometallics
Communication
Scheme 1. Reactions of Boron Triflate 8a and Boron
Chloride 8b with Additional Triflic Acid
Figure 2. The X-ray crystal structure of [NHC-B(OH)2]OTf 6.
Selected distances (Å), angles (deg), and torsion angles: B(1)−C(1)
1.591(6), B(1)−O(7) 1.310(6), B(1)−O(8) 1.307(4), O(4)−S(2)
1.433(3), O(5)−S(2) 1.433(2), O(6)−S(2) 1.425(3), O(5)−O(7)
2.681(4), O(4)−O(8) 2.780(4), N(1)−C(1)−N(2) 106.5(2), C(1)−
B(1)−O(7) 114.8(3), C(1)−B(1)−O(8) 117.0(3), O(7)−B(1)−
O(8) 128.2(4); N(1)−C(1)−B(1)−O(7) 32.6(5).
NMR analysis (−3.1 ppm). This was less stable to moisture
than 9a and was isolated in 20% yield after flash
chromatography. However, once isolated, 9b can be handled
under ambient laboratory conditions. The X-ray structure of
compound 9b is shown in the Supporting Information. Both 9a
and 9b exist in the tetracoordinate form A in solution and in
the solid state.
effect of the electron-donating hydroxy groups on the borenium
ion.
We speculate that NHC-BH(OH)2 might be an intermediate
on the way from 9a,b to 6. This could be formed by hydrolysis
with adventitious water. Further acid/base reaction of this
intermediate with the excess triflic acid would then provide 6.
The relative stability of amine−dihydroxyborenium ions
[H3N-B(OH)2]+ compared to that of other amine−borenium
ions [H3N-BR2]+ has been predicted by quantum calcula-
tions,18 but stable ions in this dihydroxy class have yet to be
described. The closest analogues to 6 are perhaps catechol-
borenium cations such as 10 and 11 ([CatB-L]+, Figure 3),
prepared and characterized by Stephan (L = t-Bu3P),19 Ingleson
(L = NEt3),4b and very recently Aldridge.11
In an effort to replace the final “hydride” on the boron atom
of these complexes by a third acid/base reaction, TfOH was
added in excess (5 equiv) to either NHC-BH3 (7) or NHC-
BH2Cl (8b) in CDCl3. Quantitative formation of either NHC-
BH(OTf)2 (9a) or NHC-BH(OTf)Cl (9b) was again observed
by 11B NMR spectroscopy after 10 min. No signals attributable
to NHC-B(OTf)3 or NHC-B(OTf)2Cl were detected. How-
ever, keeping solutions of either 9a or 9b with an excess of
TfOH at room temperature gave an even more interesting
result. In both cases, there was slow formation over 5 days of
the same new compound 6.
The only signal in the 11B NMR spectra of these crude
products was a singlet at +22.5 ppm. This resonance is too far
downfield for the expected tetracoordinate boron products and
suggests instead a tricoordinate boron environment. The
chemical shift of the triflate CF3 signal in the 19F NMR
spectrum at −78.9 ppm is different from the resonances of
triflates bound to the boron atom (−76.0 ppm in 8a and 9a,b)
and is characteristic for the free TfO− anion. Also, the product
did not survive flash chromatography. Thus, the new product
has the general structure [NHC-BR2]+TfO−. But what is R, and
how do the two different precursors give the same product?
Crystallography answered these questions.
Colorless crystals of 6 (33% yield) were grown directly from
the reaction mixture starting from 7 (see the Supporting
Information). These crystals could be handled quickly in air,
but the compound decomposed to the imidazolium triflate
[NHC-H]OTf upon dissolution in Et2O. X-ray crystallographic
analysis established the structure of 6 as an NHC-
dihydroxyborenium triflate. There are two similar molecules
of 6 in the unit cell, and one of these is shown in Figure 2. The
other is shown in Figure S1 of the Supporting Information.
The value of the N(1)−C(1)−N(2) angle (106.5°) lies
between the values of this angle in NHC-BH3 (7; 104.1°)16 and
in imidazolium chloride [NHC-H]Cl (107.7°).17 The hydrogen
atoms on the hydroxy groups of 6 were located, and they form
hydrogen bonds with the triflate counterion to assemble an
eight-membered ring with O−H···O bond distances of 2.681
and 2.780 Å. These H bonds serve to increase the stabilizing
Figure 3. (a) Three of several resonance forms for 6 and (b)
comparable species, including catecholborenium cations 10 and 11,
phenylboronic acid 12, and protonated benzoic acid 13.
The B−O distances (1.310 and 1.307 Å) in 6 are shorter
than B−O bonds in catecholborenium cations 10 (1.350 Å)19
and 11 (1.364 and 1.370 Å)4b or B−O bonds in PhB(OH)2
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Communication
(12; 1.37 Å)20 and are significantly shorter than B−O bonds in
NHC-BH2OTs (1.522 Å)7 and NHC-BH2ONO (1.512 Å).7
Apparently the B−O bonds in 6 have partial double-bond
character, as reflected in resonance form 6c. For comparison,
the length of the BO bond in a coordinated oxoborane (β-
diketiminate)−BO−AlCl3 is 1.304(2) Å.21
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We conclude that the borenium center in 6 is stabilized by
the π donation of hydroxy groups in a manner analogous to the
stabilization of borenium by two amine nitrogens, as in 3.
However, the diazaborole and NHC rings of 3 are orthogonal,
while the torsion angle between the O−B−O plane and the
plane of the NHC ring in 6 is about 30°. Thus, 6 benefits from
additional conjugative stabilization across the two subunits
(NHC and boron with its substituents) that is lacking in 3 and
other NHC-borenium ions.
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Moving farther afield, the NHC rings of NHC-borane
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the neutral phenylboronic acid (PhB(OH)2). Because of the
positive charge, [NHC-B(OH)2]+ must be a much stronger
Lewis and Brønsted acid than PhB(OH)2. The boron atom in 6
can further be replaced by a trivalent carbon atom; thus, 6 is a
distant cousin of [PhC(OH)2]+, which is simply protonated
benzoic acid.
In summary, acid/base reactions of NHC-BH2X (X = Cl,
OTf) provide the stable products NHC-BH(OTf)X. In the
presence of triflic acid, these products slowly convert into
[NHC-B(OH)2]+TfO−. This borenium ion 6 was isolated and
its ionic structure was established by spectroscopic and X-ray
crystallographic methods. It is the first representative of a new
class of borenium cation bearing two hydroxy groups on boron.
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ASSOCIATED CONTENT
■
S
* Supporting Information
(22) Walton, J. C. Angew. Chem., Int. Ed. 2009, 48, 1726−1728.
Text and figures giving full experimental and compound
characterization details and CIF files giving crystallographic
data for all X-ray structures. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: curran@pitt.edu.
ACKNOWLEDGMENTS
■
We thank the U.S. National Science Foundation and the
French Agence Nationale de la Recherche (ANR-11-BS07-008-
01, “NHCX”) for funding this work. A.S. thanks the University
of Pittsburgh for Andrew Mellon and Goldblatt Predoctoral
Fellowships.
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