Mesoionic carbene-boranes

Article abstract of DOI:10.1021/om400743c

Mesoionic carbenes (MICs), derived from alkylation and deprotonation of triazoles, are shown to form stable complexes with BH₃. The synthesis of triazoles via Huigsen cycloadditions provides considerable structural diversity. Several routes to MIC-boranes are described, and their structures have been characterized by X-ray crystallography. As predicted on the basis of the increased σ-donor capacity of MICs by comparison to NHCs, the MIC-borane adducts are more reactive reducing agents.

Full text of DOI:10.1021/om400743c

Communication
pubs.acs.org/Organometallics
Mesoionic Carbene−Boranes
Luiza Baptista de Oliveira Freitas,†,∥ Patrick Eisenberger,‡,∥ and Cathleen M. Crudden*,‡,§
†
́
Departamento de Quımica, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais 31270-901, Brazil
‡Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario K7L 3N6, Canada
§Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Chikusa, Nagoya, Japan
S
* Supporting Information
ABSTRACT: Mesoionic carbenes (MICs), derived from alkylation and
deprotonation of triazoles, are shown to form stable complexes with BH3.
The synthesis of triazoles via Huigsen cycloadditions provides considerable
structural diversity. Several routes to MIC−boranes are described, and their
structures have been characterized by X-ray crystallography. As predicted on
the basis of the increased σ-donor capacity of MICs by comparison to NHCs,
the MIC−borane adducts are more reactive reducing agents.
diminished B−H bond stability. Together, this should result in a
more robust and reactive neutral hydride equivalent.32−34
Moreover, the remarkable versatility of synthetic routes to the
1,2,3-triazole core21 provides considerable opportunities for the
rapid generation of diverse libraries of MIC−boranes 1.
N-heterocyclic carbenes have become staple ligands in organo-
metallic chemistry, transition-metal catalysis, and organo-
catalysis1−7 and are making inroads in bioinorganic8 and
materials chemistry.9−15 The stabilization of the singlet state
by flanking heteroatoms leads to exceptionally stable carbenes
that can be crystallized, distilled, and stored for extended periods
of time without decomposition.16−18 Recently, N-heterocyclic
carbenes (NHCs) flanked by only one stabilizing heteroatom
have gained attention due to their increased σ-donor proper-
ties.19−25 In particular, pioneering work by Albrecht and co-
workers illustrated the facility with which 1,3,4-trisubstituted
carbenes can be prepared from the 1,2,3-triazole core, which is
itself prepared by a simple and modular Huisgen cyclo-
addition.21,26,27 The utility of these ligands in a wide range of
metal-catalyzed reactions has been reported, and their unique
properties relative to typical NHCs are particularly note-
worthy.19,21,28,29
In the context of employing MICs and NHCs outside the
realm of transition-metal chemistry, we became interested in
synthesizing previously unknown MIC−borane adducts 1
(Figure 1). Related NHC−boranes 2 have been employed as
precursors to stabilized borenium ions.30,31 We anticipate that
the increased σ-donor ability of the MIC as an ancillary ligand
should result in increased C−B bond strength and in turn
Prior to the pioneering work of Curran and co-workers
describing the use of NHC·BH3 complexes 2 as reagents in
organic synthesis,35−40 such complexes were sporadically
described in the literature out of inherent interest in their
structure,41−45 as tools to characterize the structure and/or
reactivity of carbenes and carbene complexes,46−49 or as
precursors to carbenes themselves.50−52 The Stephan group
has also recently reported the use of NHC-stabilized boranes as
precursors to catalytically active borenium salts.30,31 Despite the
rich chemistry of Lewis base borane adducts, it is somewhat
remarkable that no examples of MIC−borane complexes have
been reported in the literature thus far.35 Difficulties in the
isolation of free 1,2,3-triazol-5-ylidines may complicate the use of
these species in the generation of borane adducts, in comparison
with the case for NHC derivatives.19,53 In particular, less bulky N-
alkyl-substituted 1,2,3-triazol-5-ylidenes are known to undergo a
variety of unwanted decomposition reactions in the free
state.19,53 We thus reasoned that a one-pot protocol avoiding
isolated MICs would be desirable for the efficient synthesis of
MIC−boranes 1. Herein we describe the facile and high-yielding
synthesis of alkyltriazolium- and aryltriazolium-based carbene−
borane complexes 1a−f. Structural data for compounds 1a−c,f
are presented, along with preliminary reactivity data.
Special Issue: Applications of Electrophilic Main Group Organo-
metallic Molecules
Figure 1. Mesoionic triazolylidene boranes (1) and imidazolylidene N-
heterocyclic carbene boranes (2).
Received: September 19, 2013
Published: October 29, 2013
© 2013 American Chemical Society
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Organometallics
Communication
Synthesis and Characterization of MIC−Boranes. Alkyl-
and aryl-substituted 1,2,3-triazoles 3a−e were prepared by the
well established CuI-catalyzed Huisgen cyclization54 of terminal
alkynes with alkyl or aryl azides generated in situ from alkyl
halides55 or synthesized from amines.56,57 Alternatively, the base-
catalyzed reaction of terminal alkynes with organic azides can be
employed for the preparation of 1,5-disubstituted 1,2,3-triazoles
such as 3f (Scheme 1).58
Scheme 2. Synthesis of MIC−Boranes 1a−f
Scheme 1. Synthesis of 1,2,3-Triazolium Salts 4a−f
reaction had to be carried out at low temperature under rigorous
exclusion of moisture to avoid rapid decomposition of the free
MIC or other competing side reactions. The presence of
adventitious water results in diminished product yields due to
benzylic C−N bond scission in 3 via generated hydroxide ion.
Moreover, the migration of backbone N-alkyl groups to the
carbene carbon center in an SN2 fashion, resulting in self-
cannibalization of the carbene, has been reported for similar,
even more sterically hindered triazolylidenes, albeit at slightly
elevated temperatures (50 °C).19
In the event, the sequential addition of base followed by
borane (after 20 min) at −78 °C and then gradual warming of the
reaction mixture gave the desired MIC·BH3 complexes 1 in good
yield with a minimal degree of side product formation. Pure
borane adducts 1a−f can be isolated after evaporating the solvent
and subjecting the crude mixture to column chromatography.
For those complexes that show decomposition on silica gel (for
instance, 1a), filtration through a plug of Celite followed by
recrystallization was effective. The slightly decreased yield of less
sterically protected 1e may be rationalized by decomposition of
the MIC in solution prior to coordination to BH3.
+
−
Alkylation using an excess of Meerwein’s salt (MeO3 BF4 ;
Scheme 1, conditions A) at ambient temperature resulted in the
exclusive preparation of the 3-methylated 1,2,3-triazolium
tetrafluoroborate salts in 80−90% yield after flash column
chromatography.59 This method proved to be general and
applicable to a variety of 1,2,3-triazoles (4a-BF4, 4b-BF4, 4d-
BF4). However, the limited commercial availability of Meerwein-
type reagents restricts this method to the introduction of methyl
(or potentially ethyl) substituents. Less expensive and more
general alkylation protocols could also be employed that relied
on readily available, bench-stable reagents such as MeOTs and
alkyl halides (MeI or 2-iodopropane; Scheme 1, conditions B). In
our hands, the reaction of 3a with 2.8 equiv of MeI in MeCN
required slightly forcing conditions to provide high isolated
yields, namely heating the reaction mixture to 80 °C in a closed
ACE pressure tube, which gave the desired product in good
isolated yield (84%).60 With the exception of (S)-4c-I, which
gave only moderate yields, these conditions worked well for all
substrates examined.
The11B NMR chemical shifts of 1a−f fall in the typical range
reported for four-coordinate carbene−BH3 complexes. For
instance, the boron center in MIC−borane 1a resonates as a
quartet at δB −36.1 with a coupling constant of 1JB−H = 85.9 Hz,
comparing well to δB −35.13 (q, 1JB−H = 86.2 Hz) for the related
2-borane-l,3-diethyl-4,5-dimethylimidazole.41 The MIC·BH3
proton resonance in 1a appears as a broad quartet shifted
upfield to δH 1.22 (br q, 1JB,H = 86.5 Hz) in comparison to BH3·
Me2S (δH 1.50 (br q, JB,H = 104 Hz)).64 This observation is
1
rationalized by increased shielding due to coordination of the
electron-rich MIC to boron. A similar upfield shift is observed for
the resonance of the nitrogen-bound methyl group in 1a (δH
3.97) in comparison to 4a-BF4 (δH 4.19). Notably, in all cases,
the 13C resonances of the carbene carbon atom of MIC−boranes
1a−f could not be observed in the 13C NMR spectra, due to
quadrupolar broadening resulting from coordination to the
boron atom. All of these NMR data are consistent with the
formulation of the structure as a carbene-stabilized BH3 unit (1).
To unambiguously elucidate the connectivity, single crystals of
1a−c,f were obtained by slow diffusion of hexanes into a solution
of the MIC−borane in CH2Cl2. Their structures, as determined
by X-ray crystallography, are shown in Figure 2.
Since aryl substituents cannot be installed via SN2 chemistry,
the triphenyltriazolium salt 4f was prepared in 90% isolated yield
following the method of You, Gao, and co-workers (Scheme 1,
conditions C).61 This strategy is complementary to existing
methods for the synthesis of 1,3,4-triaryl-1,2,3-triazol-5-ylidenes,
reported by the laboratories of Grubbs and Bertrand,53 based on
1,3-diaryltriazenes as synthetic intermediates,62 which results in a
symmetrical 1,3-substitution pattern.
With a variety of triazolium salts 4 in hand, we set out to reveal
the corresponding triazolylidenes by deprotonation at the C(5)
position with KHMDS (potassium hexamethyldisilazide),
followed by subsequent addition of a BH3 source, as shown in
Scheme 2. The successful use of sterically demanding
hexamethyldisilamide bases was previously demonstrated in
the one-pot syntheses of NHC−boranes.63 Importantly, the
The observed C(5)−B bond distances (see Figure 2) are
comparable to those in reported NHC·BH3 compounds (for
instance: 2-borane-l,3-dimethylimidazole, 1.600(4) Å;65 2-
borane-l,3-diethyl-4,5-dimethylimidazole, 1.603(3) Å;41 (2-
phenyl-2,5,6,7-tetrahydropyrrolo[2,1-c][1,2,4]triazol-4-ium-3-
yl)trihydroborate, 1.611(3) Å).17 The N(1)−C(5)−C(4) bond
angle of 102.03(12)° in 1a is decreased in comparison to
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Organometallics
Communication
by 1a in the absence of additives. However, clean reduction
occurred after the addition of 10 mol % of the Lewis acid
Sc(OTf)3, with high conversion of the substrates obtained in 1−3
h at ambient temperature. The reactions were conducted without
any special precautions in vials open to air and only required a
catalytic amount of Sc(OTf)3 as Lewis acid to achieve high
conversion.
Similarly, the use of silica as an acidic promoter allowed the
isolation of the corresponding benzylic alcohols in high yields
after 1−3 h reaction time. When directly compared with 1,3-
dimethylimidazol-2-ylidene−borane, 1a gave the desired prod-
uct in slightly higher yield (71% vs 62%) despite the significantly
greater steric hindrance in the case of MIC 1a (See Supporting
Information). Furthermore, unreacted 1a could be conveniently
recovered from the reaction mixture under either of the reaction
conditions by eluting the silica gel with dichloromethane after
product isolation. Since the MIC−borane 1a itself was recovered
intact and not as a complex of the reduced carbonyl (entries 1−3
and 5−7), this implies that the initially formed MIC−
benzyloxyborane, resulting from the reduction of the first C−
O double bond, is a more reactive reducing agent than the parent
MIC−borane 1a itself. As shown in entries 4 and 8, quantitative
reduction was observed when a 1:3 ratio of borane to aldehyde
was employed; therefore, all three hydrides are available for
reduction.
Figure 2. ORTEP-3 representations of the X-ray single-crystal
structures of 1a−c,f. The structures of 1b,c were refined without
addressing absolute stereochemistry, and thus the representation as the
S isomer is arbitrary. Ellipsoids are plotted at 50% occupancy, and non-
heteroatom-bound hydrogen atoms are omitted for clarity. Nitrogen
atoms are represented in blue, boron in pink, and carbon in gray. Bond
distances (Å) and bond angles (deg) are given in the Supporting
Information.
To test the versatility of this species as a reducing agent in the
presence of trityl salts, the hydroboration of allylbenzene was also
attempted36 and proceeded smoothly at ambient temperature to
give exclusively the linear product after oxidation to the
corresponding alcohol (see Supporting Information).
Conclusions. In conclusion, we have developed a one-pot
synthesis of bench-stable crystalline MIC−boranes 1 and
demonstrated their use in the stoichiometric reduction of
C−O double bonds in ketones and aldehydes and as catalysts for
the hydroboration of allylbenzene via the intermediacy of
borenium ions. Importantly, as predicted, MIC−borane 1a has
increased hydricity in comparison to NHC−boranes in the
reduction of ketones and aldehydes, which translates to greater
reducing power despite greater steric hindrance. The synthesis of
more complex MIC−boranes and their use in reduction
chemistry and catalysis will be reported in due course.
66
106.02(18)° in 4a-PF6 with concomitant elongated bond
distances for N(1)−C(5) and C(4)−C(5) in comparison to the
salt 4a-PF6 (N(1)−C(5) = 1.350(2) Å, C(4)−C(5) = 1.367(2)
Å). Similar values for these bond angles and distances are
observed for the other three solid-state structures. These trends
for the geometry of the carbene carbon are typical for NHC and
related MIC coordination compounds of transition metals and
related NHC−boranes.16,21,35,66 The diminished bond angles
about the carbene carbon reflect an increase in s character for the
lone pair orbital typically observed for other singlet carbenes.20
We next examined the activity of the new MIC−borane
complex 1a in the reduction of electrophilic C−O double bonds
in 4-bromoacetophenone and 4-bromobenzaldehyde as depicted
in Table 1, employing a methodology inspired by Lindsay and co-
workers.63 Consistent with reports on the reducing capacity of
NHC−boranes,37 neither ketones nor aldehydes were reduced
ASSOCIATED CONTENT
* Supporting Information
■
S
Table 1. Sc(OTf)3- and Silica-Catalyzed Reduction of 4-
Bromoacetophenone and 4-Bromobenzaldehyde using MIC−
Borane 1a
Text, figures, a table, and CIF files giving full experimental details
and spectroscopic and characterization data for new compounds
and crystallographic data for 1a−c,f. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*cruddenc@chem.queensu.ca.
■
recovered
1a/%
a
entry
R
1a:substrate
catalyst
time/h yield /%
1
2
3
4
5
6
7
8
Me
H
1:1
1:1
1:2
1:3
1:1
1:1
1:2
1:3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
silica
3
1
3
3
3
3
3
1
93 (89)
99 (89)
83 (80)
98 (94)
98 (85)
100 (91)
100 (91)
90 (88)
(53)
(49)
(33)
(0)
Author Contributions
∥These authors contributed equally to this work.
H
Notes
H
The authors declare no competing financial interest.
Me
H
(47)
(45)
(26)
(0)
silica
ACKNOWLEDGMENTS
■
H
silica
Financial support from the Natural Sciences and Engineering
Research Council of Canada (NSERC) is gratefully acknowl-
edged in terms of operating, equipment, accelerator, and strategic
grants to C.M.C. The Canada Foundation for Innovation (CFI)
H
silica
a
Yield calculated by NMR using hexamethylbenzene as internal
standard; isolated yields given in parentheses.
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Communication
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(LOF) Grant to C.M.C. L.B.d.O.F. thanks the Conselho
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Nacional de Desenvolvimento Cientifico e Tecnologico
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ABBREVIATIONS
MIC, mesoionic carbene; NHC, N-heterocyclic carbene
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