A dialkylborenium ion via reaction of N-heterocyclic carbene-organoboranes with Bronsted acids - Synthesis and DOSY NMR studies

Article abstract of DOI:10.1039/c1cc10767d

A dialkylborenium ion stabilized by an N-heterocyclic carbene has been prepared for the first time by reaction of IMes·9-BBN-H with triflic acid. The ion-separated nature of the borenium ion was confirmed by ¹H and ¹⁹F diffusion ordered NMR spectroscopy.

Full text of DOI:10.1039/c1cc10767d

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 6650–6652
www.rsc.org/chemcomm
COMMUNICATION
A dialkylborenium ion via reaction of N-heterocyclic
carbene–organoboranes with Brønsted acids—synthesis and
DOSY NMR studieswz
David McArthur,a Craig P. Buttsa and David M. Lindsay*b
Received 9th February 2011, Accepted 31st March 2011
DOI: 10.1039/c1cc10767d
A dialkylborenium ion stabilized by an N-heterocyclic carbene has
been prepared for the first time by reaction of IMesꢀ9-BBN-H
with triflic acid. The ion-separated nature of the borenium ion was
aryl substituents are markedly less stable (though there can be
a stabilizing p-contribution from aryl substituents), and this is
reflected in their relative scarcity in the literature.6
1
confirmed by H and 19F diffusion ordered NMR spectroscopy.
In recent years, N-heterocyclic carbenes (NHCs) have been
widely used as ligands for a variety of transition metal-
mediated processes.7 In contrast, applications of NHC–main
group complexes have begun to appear only recently,8 and the
NHC–main group area in general has received relatively little
attention.9
The intrinsic electron-deficiency of trivalent boron has led to a
variety of organoboron-based compounds finding use as Lewis
acids in organic synthesis.1 Cationic, divalent boron derivatives
have a much higher Lewis acidity than their neutral counterparts,
attracting interest in the two-, three- and four- co-ordinate
boron cations termed borinium- 1, borenium- 2 and boronium
ions 3, respectively (Fig. 1).2
We have an interest in developing main group–NHC complexes
for use in a variety of organic processes, and in particular
the application of NHC–boron complexes in organic synthesis,
including the use of chiral NHC–boranes and –diorganoboranes
in the asymmetric reduction of ketones.10 The electron-rich nature
and steric bulk of some of the common NHCs should make them
ideal for stabilization of the highly electrophilic dialkyl boro-
cations and we therefore initiated a programme of research based
on NHC-stabilized borocations. Although some early reports
hinted at the potential of NHCs in stabilizing diaminoborenium
ions,11 this general class of species has been largely overlooked
until Matsumoto and Gabbaı recent report on an NHC-stabilized
diaryl borenium ion.12 We hereby disclose our preliminary
findings on the synthesis of an NHC-stabilized dialkyl borenium
ion, generated from an NHC–diorganoborane complex by
reaction with Brønsted acids.
The extremely high reactivity of borinium ions 1 towards
even solvents and counter-ions often precludes their study in
all but the gas phase and their utility in organic synthesis is
clearly limited. However, there is scope for further studies, and
applications, of the more stable borenium- and boronium ions
2 and 3. The use of borocations in organic synthesis is
currently limited to the oxazaborolodinium cations reported
by Corey et al.,3 and a catalyst for propylene oxide poly-
merization reported by Wei and Atwood.4 The Corey ‘‘Lewis
superacids’’ are exceptionally potent, rendering unreactive
Diels–Alder substrates reactive at very low temperatures.
Borocations are not only of general interest, but also significant
as a source of new, powerful Lewis acids for organic synthesis.
Many of the borenium ions reported to date possess
heteroatom substituents; the lone pairs on these atoms
conferring some extra stability on the highly electron deficient
boron.5 However, borenium ions bearing hydrido-, alkyl- or
Strategies reported to date for the synthesis of borenium
ions include: (a) halide abstraction from tetra-co-ordinate ate
complexes;2,12 (b) displacement of anionic leaving groups from
boron by a neutral ligand;2 (c) conversion of a formally
neutral N-substituent to a positively charged species by
protonation, or co-ordination of a Lewis acid;2 and (d) hydride
abstraction using the trityl cation with a non-co-ordinating
counter-ion.2 Distinct from these four approaches is the
borenium ion formation reported by Fox et al., employing
protic cleavage of a carborane B–B bond.13
Fig. 1 Borocations: borinium-, borenium- and boronium ions.
The hydride abstraction method described above is attrac-
tive, since it allows the synthesis of borenium ions from ate
complexes of a wide-range of diorganoboranes, which are, in
turn, either commercially available or readily prepared using
hydroboration chemistry. We envisioned the possibility of
forming NHC-stabilized borenium ions 5 from the NHC–
organoboron ate complexes 4, via hydride abstraction by
acid-induced cleavage of the B–H bond (Scheme 1).
a School of Chemistry, University of Bristol, Cantock’s Close, Bristol,
BS8 1TS, UK
b Department of Chemistry, University of Reading, Whiteknights,
Reading, RG6 6AD, UK. E-mail: d.lindsay@reading.ac.uk;
Tel: +44 (0)118 378 4707
w This paper is dedicated to the memory of Mr David Hinton.
z Electronic supplementary information (ESI) available: Experimental
procedures, spectral and analytical data for all new compounds. See
DOI: 10.1039/c1cc10767d
c
6650 Chem. Commun., 2011, 47, 6650–6652
This journal is The Royal Society of Chemistry 2011

View Article Online
Scheme 1 Proposed Brønsted acid-mediated borenium ion synthesis.
Scheme 3 Formation of an NHC-stabilized dialkylborenium ion.
and triethylamine16 complexes of the 9-BBN borenium ion,
are reported as having an 11B chemical shift of +59.2 and
+85.1 ppm, respectively. The large, positive 11B NMR chemical
shift of 11 is good evidence for a dissociated triflate and a
genuine, NHC-stabilized dialkylborenium ion. Furthermore,
the magnitude of the chemical shift suggests that the NHC
offers little in the way of p-stabilisation, given how similar the
value is to that reported by Vedejs et al. for the corresponding
triethylamine complex.16
We investigated the feasibility of this B–H protonolysis
route by first studying the reaction of NHC–boranes with
Brønsted acids.
IMesꢀBH3 69e and IMesꢀ9-BBN-H 710 were prepared in
excellent yields by the reaction of IMes with BH3ꢀSMe2 and
9-BBN-H, respectively. The tetra-co-ordinate ate nature of
these air-stable complexes 6 and 7 was confirmed by their 11B
NMR chemical shifts, displaying a sharp quartet at ꢁ38.3 ppm,
and a doublet at ꢁ16.6 ppm, respectively.
Definitive evidence of the borenium ion nature of 11
can be derived from diffusion ordered NMR spectroscopy
(DOSY NMR)—measuring the diffusion co-efficients of the
NHC–boron moiety and the triflate, respectively. Significantly
different diffusion co-efficients for the cationic and anionic
components would confirm the ion-paired, borenium character
of 11. Such studies are well-known in transition metal chemistry,
thanks to the pioneering work of Pregosin,17 but to the best
of our knowledge, this technique has not yet been employed in
determining ion-pairing in borocation chemistry. Instead, in
the cases where there is an absence of X-ray crystallographic
data, 11B NMR chemical shifts have been relied on to determine
the nature of borocation species. The diffusion co-efficient of
In agreement with the observations of Lacote and Curran
et al. in NHC–boranes employing the DIPP ligand system,8f
reaction of IMesꢀBH3 6 with both HCl and TsOH proceeded
smoothly, with the evolution of hydrogen, to yield new
NHC–borane complexes 8 and 9 in high yield (Scheme 2).
Treatment of IMesꢀBH3 6 with triflic acid provided the
monotriflate complex IMesꢀBH2OTf 10. Chloro- and tosyl
complexes 8 and 9 are air-stable solids, whereas triflate complex
10 was found to be air-sensitive. This complex was not
isolated, but was formed quantitatively as demonstrated by
11B NMR analysis.
The 11B chemical shift and multiplicity of 8–10 suggested
that all three complexes exist as tetra-co-ordinate boronates
and not borenium ions; no ligand system is known which
stabilizes the elusive dihydridoborenium ion.5a,6c We reasoned
that instead, a dialkylborane complex might be induced to
expel triflate and form a borenium ion, due to both the
increased steric congestion and inductive electronic stabilization,
compared to the corresponding dihydrido complex.14 Thus,
treatment of IMesꢀ9-BBN-H 7 with triflic acid cleanly produced
the corresponding IMesꢀ9-BBN OTf 11 (Scheme 3). In contrast
to 10, the 11B NMR chemical shift of 11 (+81.4 ppm in
CD2Cl2) and width at half-height (715.8 Hz) suggested a three
co-ordinate boron and a dissociated triflate counter-ion, i.e. a
borenium ion.
1
the NHC–boron moiety was measured by a H NMR DOSY
experiment and 19F DOSY was used to obtain the triflate
diffusion co-efficient, in CD2Cl2 solution. Fluorobenzene
was used as an inert internal standard in both experiments
to ensure consistency between the 1H and 19F experiments.
The NHC–9-BBN moiety of 11 was found to have a diffusion
co-efficient of 5.8 ꢂ 1010 m2 sꢁ1 whilst the triflate’s diffusion
co-efficient was 6.8 ꢂ 1010 m2 sꢁ1 15
.
The ratio of the cation
and anion diffusion co-efficients is reflective of the amount of
ion-pairing in the system, with a ratio of 1 being the theoretical
value for the covalently bound anion and cation (i.e. 100%
ion-pairing),18 values of 0.98–1.00 suggest complexes that are
close to complete ion pairing, and ratios of o0.96 are indicative
of a separated ion pair.18 Using this approach, the ratio of
diffusion co-efficients for the cation and anion in borenium
salt 11 is 0.85, unquestionably in the realm of a separated
ion pair.19 Hence these DOSY NMR measurements provide
compelling evidence for a separated ion pair, indicating the
first N-heterocyclic carbene-stabilized dialkylborenium ion.
The composition of complex 11 was also confirmed by
an independent synthetic method. Addition of IMes to a
commercially available solution of 9-BBN-OTf in hexanes
produced a single species by 11B NMR with an identical
chemical shift to 11. Given the thermal instability of even
N- or O-stabilized borenium ions, we were surprised at
the remarkable stability of NHC–dialkylborenium complex
11: a sample of 11 (CD2Cl2 solution under N2) showed no
signs of decomposition after storage for two weeks at room
temperature, providing clear evidence for the thermodynamic
For reference, 9-BBN-OTf exhibits an 11B NMR chemical
shift of +66 ppm (in hexanes),15 and the two previously-
reported 9-BBN-based borenium ions, the 2,6-lutidine-6a
Scheme 2 Reaction conditions: (a) 2 M HCl in Et2O, CH2Cl2, rt, 1 h,
100%; (b) TsOH, CHCl3, rt, 1 h, 85%; (c) TfOH, CH2Cl2, ꢁ40 1C, 0.5 h.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6650–6652 6651

View Article Online
stability conferred by the NHC ligand, as well as the kinetic
shielding provided by the bulky mesityl groups.
7 (a) N-Heterocyclic Carbenes in Synthesis, ed. S. P. Nolan, Wiley-VCH,
Weinheim, 2006; (b) Topics in Organometallic Chemistry, ed.
F. Glorius, Springer, Berlin, Heidelberg, 2007, vol. 21.
Thus, we have prepared the first example of a dialkylborenium
ion stabilized by an NHC, via a novel route employing triflic
acid to effect protolytic cleavage of the B–H bond of an
NHC–diorganoborane complex. The remarkable stability of this
system augers well for their application as Lewis acids in a range
of organic transformations; such studies are underway in our
laboratory and will be reported on in due course.
8 (a) S. H. Ueng, M. M. Brahmi, E. Derat, L. Fensterbank, E. Lacote,
M. Malacria and D. P. Curran, J. Am. Chem. Soc., 2008, 130, 10082;
(b) K. S. Lee, A. R. Zhugralin and A. H. Hoveyda, J. Am. Chem.
Soc., 2009, 131, 7253; (c) Q. Chu, M. M. Brahmi, A. Solovyev,
S. Ueng, D. P. Curran, M. Malacria, L. Fensterbank and E. Lacote,
Chem.–Eur. J., 2009, 15, 12937; (d) S. Ueng, A. Solovyev, X. Yuan,
S. J. Geib, L. Fensterbank, E. Lacote, M. Malacria, M. Newcomb,
J. C. Walton and D. P. Curran, J. Am. Chem. Soc., 2009, 131, 11256;
(e) J. Monot, A. Solovyev, H. Bonin-Dubarle, E. Derat,
D. P. Curran, M. Robert, L. Fensterbank, E. Lacote and
M. Malacria, Angew. Chem., Int. Ed., 2010, 49, 9166;
(f) A. Solovyev, Q. Chu, S. J. Geib, L. Fensterbank, M. Malacria,
E. Lacote and D. P. Curran, J. Am. Chem. Soc., 2010, 132, 15072.
9 (a) N. Kuhn, G. Henkel, T. Kratz, J. Kreutzberg, J. Boese and
A. H. Maulitz, Chem. Ber., 1993, 126, 2041; (b) D. Enders,
K. Breuer, J. Runsink and J. H. Teles, Liebigs Ann., 1996, 2019;
(c) A. J. Arduengo, F. Davidson, R. Krafczyk, W. J. Marshall and
R. Schmutzler, Monatsh. Chem., 2000, 131, 251; (d) X. Zheng and
G. E. Herberich, Organometallics, 2000, 19, 3751; (e) T. Ramnial,
H. Jong, I. D. McKenzie, M. Jennings and J. A. C. Clyburne, Chem.
Commun., 2003, 1722; (f) Y. Wang, B. Quillian, P. Wei,
C. S. Wannere, Y. Xe, R. B. King, H. F. Schaeffer III, P. v.
R. Schleyer and G. H. Robinson, J. Am. Chem. Soc., 2007, 129,
12414; (g) D. Holschumacher, T. Bannenberg, C. G. Hrib, P. G. Jones
and M. Tamm, Angew. Chem., Int. Ed., 2008, 47, 7428; (h) P. A. Chase
and D. W. Stephan, Angew. Chem., Int. Ed., 2008, 47, 7433.
10 D. M. Lindsay and D. McArthur, Chem. Commun., 2010, 46, 2474.
11 (a) L. Weber, E. Dobbert, H.-G. Stammler, B. Neumann, R. Boese
and D. Blaser, Chem. Ber., 1997, 130, 705; (b) A. Wacker,
H. Pritzkow and W. Siebert, Eur. J. Inorg. Chem., 1998, 789.
12 T. Matsumoto and F. Gabbaı, Organometallics, 2009, 28, 4252.
13 M. G. Davidson, M. A. Fox, T. G. Hibbert, J. A. K. Howard,
A. Mackinnon, I. S. Neretin and K. Wade, Chem. Commun., 1999,
1649.
We thank the EPSRC for a studentship to D.M. (EP/
C531922/1) and an Advanced Research Fellowship to D.M.L.
(GR/S52100/02). We thank Mr York Schramm for technical
assistance in running the DOSY NMR experiments.
Notes and references
1 Science of Synthesis Organometallics: Boron Compounds, in vol. 6,
ed. D. E. Kaufmann and D. S. Matteson, Georg ThiemeVerlag,
Stuttgart, New York, 2004.
2 For a review of borocations, see: W. E. Piers, S. A. Bourke and
K. D. Conroy, Angew. Chem., Int. Ed., 2005, 44, 5016.
3 (a) Y. Hayashi, J. J. Rohde and E. J. Corey, J. Am. Chem. Soc.,
1996, 118, 5502; (b) E. Canales and E. J. Corey, J. Am. Chem. Soc.,
2007, 129, 12686 and references therein.
4 P. Wei and D. A. Atwood, Inorg. Chem., 1998, 37, 4934.
5 For recent examples, see: (a) T. S. De Vries and E. Vedejs,
Organometallics, 2007, 26, 3079; (b) M. A. Dureen, A. Lough,
T. M. Gilbert and D. W. Stephan, Chem. Commun., 2008, 4303;
(c) C. Bonnier, W. E. Piers, M. Parvez and T. S. Sorensen, Chem.
Commun., 2008, 4593; (d) A. Del Grosso, R. G. Pritchard,
C. A. Muryn and M. J. Ingleson, Organometallics, 2010, 29, 241;
(e) A. J. V. Marwitz, J. T. Jenkins, L. N. Zakharov and S.-H. Liu,
Angew. Chem., Int. Ed., 2010, 49, 7444; (f) A. Del Grosso,
P. J. Singleton, C. A. Muryn and M. J. Ingleson, Angew. Chem.,
Int. Ed., 2011, 50, 2102.
6 (a) C. K. Narula and H. Noth, Inorg. Chem., 1985, 24, 2532;
(b) P. Jutzi, B. Krato, M. Hursthouse and A. J. Howes, Chem.
Ber., 1987, 120, 1091; (c) E. Vedejs, T. Nguyen, D. R. Powell
and M. R. Schrimpf, Chem. Commun., 1996, 2721;
(d) I. Ghesner, W. E. Piers, M. Parvez and R. McDonald,
Chem. Commun., 2005, 2480; (e) M. K. Uddin, Y. Nagano,
R. Fujiyama, S.-I. Kiyooka, M. Fujio and Y. Tsuno,
Tetrahedron Lett., 2005, 46, 627; (f) C.-W. Chiu and
F. Gabbaı, Organometallics, 2008, 27, 1657; (g) T. S. De Vries,
A. Prokofjevs, J. N. Harvey and E. Vedejs, J. Am. Chem. Soc.,
2009, 131, 14679.
14 Triflate is covalently bound in IMesꢀInMe2OTf; see:
J. H. Cotgreave, D. Colclough, G. Kociok-Kohn, G. Ruggiero,
C. G. Frost and A. S. Weller, Dalton Trans., 2004, 1519.
15 See ESIw.
16 A. Prokofjevs, J. W. Kampf and E. Vedejs, Angew. Chem., Int. Ed.,
2011, 50, 2098.
17 For reviews, see: (a) M. Valentini, H. Ruegger and P. S. Pregosin,
Helv. Chim. Acta, 2001, 84, 2833; (b) P. S. Pregosin, Prog. Nucl.
Magn. Reson. Spectrosc., 2006, 49, 261.
18 See, for example, A. Moreno, P. S. Pregosin, L. F. Veiros,
A. Albinati and S. Rizzato, Chem.–Eur. J., 2008, 14, 5617.
19 For reference, the corresponding ratio for 1H and 19F in fluoro-
benzene was 0.99. See ESIw.
c
6652 Chem. Commun., 2011, 47, 6650–6652
This journal is The Royal Society of Chemistry 2011