Carboranes as Aryl Mimetics in Catalysis: A Highly Active Zwitterionic NHC-Precatalyst

Article abstract of DOI:10.1002/chem.201700209

Modern catalysis takes advantage of aryl-based interactions to tune and control reactions. In the design of N-heterocyclic-carbene catalysts, both the electronic and steric nature of the nitrogen substituents play a crucial role. Although hydrocarbon-based systems and especially aryl residues have contributed considerably to overcome multifaceted catalytic challenges, the unique properties of carborane moieties, including delocalized charge, potential planar chirality, and well-known thermodynamic stability, offer unprecedented opportunities to develop new catalysts while being employed as aryl mimetics. We report a straightforward synthetic route to a novel zwitterionic triazolium-based N-heterocyclic carbene (NHC) precatalyst bearing a 7,8-dicarba-nido-undecaboranyl substituent. The catalyst's excellent activity and its broad applicability are demonstrated in a wide range of organocatalytic transformations. Comparison of the performance with known N-aryl NHC catalysts offers preliminary insights into the stereoelectronic nature of this nido-carboranyl substituent.

Full text of DOI:10.1002/chem.201700209

A Journal of
Accepted Article
Title: Carboranes as Aryl Mimetics in Catalysis: a Highly Active
Zwitterionic NHC Precatalyst
Authors: Christoph Selg, Wilma Neumann, Peter Lönnecke, Evamarie
Hey-Hawkins, and Kirsten Zeitler
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Carboranes as Aryl Mimetics in Catalysis: a Highly Active
Zwitterionic NHC Precatalyst
Christoph Selg,[a] Wilma Neumann,[b]‡ Peter Lönnecke,[b]§ Evamarie Hey-Hawkins[b] and Kirsten Zeitler*[a]
Abstract: Modern catalysis takes advantage of aryl-based
interactions to tune and control reactions. In the design of
N-heterocyclic carbene catalysts both electronic and steric nature of
the nitrogen substituents play a crucial role. While hydrocarbon-based
systems and especially aryl residues have considerably contributed
to surmount multifaceted catalytic challenges, the unique properties
of carborane moieties such as delocalized charge, potential planar
chirality and their well-known thermodynamic stability offer
unprecedented opportunities to develop new catalysts while
employing these as aryl mimetics. We herein report a straightforward
synthetic route to a novel zwitterionic triazolium based N-heterocyclic
carbene (NHC) precatalyst bearing a 7,8-dicarba-nido-undecaboranyl
substituent. The catalyst´s excellent activity and its broad applicability
are demonstrated in a wide range of organocatalytic transformations.
Comparison of the performance with known N-aryl NHC catalysts
offers preliminary insights into the stereoelectronic nature of this nido-
carboranyl substituent.
prevalent precatalyst system.[8] An astonishing array of
transformations has since been established, not least by virtue
of the possibility to assemble tailor-made catalysts for every
application.[6,9]
A typical triazolium NHC-precatalyst consists of the essential
heterocyclic core bearing the procarbenic carbon atom, three
substitution sites, and a counterion. To provide a rigid scaffold
for chiral substituents and to circumvent the formation of meso-
ionic (abnormal) carbenes,[10] two of the substitution sites are
mostly incorporated in a bicyclic structure (Figure 1, top right).
The spare N-centered site is crucial for optimizing both
selectivity and reactivity of the catalyst.[11] Hence, the fine-tuning
of the steric and electronic properties of this substituent is key to
modern NHC-catalyst design.
Previous Work:
Y
N
N
N
Design Concept
Ar
X
X
= Cl, BF4
Ar = C6H5, C6F5, Mes
= CH2, OCH2, (CH2)2
carborane clusters as aryl
Introduction
mimetics in organocatalysts
Y
The influence of non-covalent interactions is ubiquitous
in chemistry. Especially aromatic rings and their contribution
in π-stacking, cation/π-, anion/π- or CH/π-interactions are
well-known to often play a key role for the outcome of
(organo)catalytic reactions.[1] Despite this significant
importance, the development of catalysts which employ aryl
This Work:
Carboranyl-substituted triazolium system
- no counterion
N
- 3D aromaticity
H
N
N
- manifold possibilities for B- and C-substitution
- delocalized negative charge anionic NHC
- unique steric properties
= C, CH
= BH
- optional planar chiral modifications
mimetics as
elusive.[2]
a structural element has remained largely
Examples of boron cluster-tethered imidazolium NHC precursors: ligand use only - not tested in catalysis
Since the demonstration of the catalytic activity of vitamin B1
in the benzoin reaction by Ukai and co-workers in 1943[3] and
Breslow's subsequent mechanistic attribution to a stable
carbene species, formed in situ by deprotonation of the
thiazolium moiety,[4] NHC-catalysts have emerged as "super-
heroes of organic chemistry"[5] with remarkable structural
diversity.[6] The majority of catalytic systems initially was
based on thiazolium-, imidazolium- and imidazolinium-
heterocycles, while only the introduction of a facile synthetic
route by Enders and Teles in 1995[7] has sparked the
advancement of the more acidic triazolium salts to become the
N
N
N
N
N
N
N
N
H
Br
HN
Lavallo, 2014
Lavallo, 2015
Willans, 2016
Figure 1. Carboranes as aryl mimetics in catalysis. Novel carboranyl
triazolium NHC-precatalysts for organocatalysis (Mes
trimethylphenyl: mesityl).
=
2,4,6-
The stability of a free NHC is partly attributed to the shielding
by sterically demanding substituents, but primarily to the
interaction of the sp²-hybridized carbenic carbon with the lone
pairs of the adjacent heteroatoms. Electronic properties of
their substituents will thus significantly influence the Lewis
basicity of the NHC. Despite elaborate investigation on the
N-substituents´ influence on the catalyst´s reactivity[11]
reported residues are currently limited to carbon-based,
preferentially aryl-type, systems.[12]
[a]
[b]
M. Sc. C. Selg, Prof. Dr. K. Zeitler
Institut für Organische Chemie
Universität Leipzig
Johannisallee 29, 04103 Leipzig (Germany)
E-mail: kzeitler@uni-leipzig.de
Dr. W. Neumann‡, Dr. P. Lönnecke§, Prof. Dr. Dr. h.c. E. Hey-Hawkins
Institut für Anorganische Chemie
Universität Leipzig
Johannisallee 29, 04103 Leipzig (Germany)
Current address: Department of Chemistry, Massachusetts Institute
of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139
(USA)
[‡]
[§]
X-Ray structure analysis
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/chem.201700209.
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Design Plan
generated from the commercially available closo analog
(ortho-C2B10H12) by alkoxide-promoted deboronation.[22,23] Higher
thermal stability and lesser susceptibility to NHC-mediated
(auto-)decomposition[24] render the nido-carborate as a well suited
substituent of our choice (as compared to its closo species). Upon
the removal of one vertex, the nido cluster exhibits a flat open
face[25] with a negative charge delocalized throughout the cluster.
Combination with the positively charged triazolium moiety would
hence result in a single, zwitterionic compound without a counterion
as precursor for an anionic NHC.
To address these limitations and to extend our ability to
control catalyst reactivity, we questioned whether replacement of
the N-substituent by aryl-mimetic scaffolds[13] would be possible
and we hereby could expand catalyst diversity.
Isosteric and bioisosteric substitution of phenyl rings by boron
clusters is a well-established and powerful tool in medicinal
chemistry.[14] Despite numerous successful applications[13,15] and
the widely accepted versatility of these aryl surrogates together
with the position dependent, extreme electronic influences
available by carboranes in ligands,[2b,c] there is a dearth for their
introduction in (organo-)catalytic scaffolds. A limited number of
imidazolium-based carboranyl NHCs (Figure 1, bottom) have
been investigated recently.[16] However, all of these have only
been examined for their coordination chemistry as ligands in
metal complexes with in situ preparation. To the best of our
knowledge, (organo)catalytic activity has not yet been reported.
Herein, we present the successful marriage of the thriving
triazolium-based catalyst system and an aryl-mimetic nido-carborate
moiety.
Results and Discussion
Our straightforward, optimized synthesis builds on the three
step sequence of the Rovis group starting from lactim ether 3 and
a hydrazine (Scheme 1).[26,27] The corresponding carboranyl
hydrazinium salt 2 can be obtained quantitatively from the
Boc-protected hydrazine 1.[28] It is important to note, that, while
deprotection of precursor 1 with trifluoroacetic acid was shown to
result in decomposition,[28] the usage of dry gaseous HCl yields
the deprotected hydrochloride 2 quantitatively as a bench-stable
white powder.
The final step includes both the introduction of the procarbenic
carbon and concurrent deboronation of the carborane cluster.
Trialkyl orthoformates were employed as C1-building blocks and
additionally – with respect to their alkoxide leaving groups – as
suitable Lewis base source for carborate decapping.
a
Scheme 1. Reaction conditions: dry HCl gas, Et2O, 0 °C, 3 h, quant.;
c
d
b (MeO)2SO2, neat, 60 °C, 18 h, 50%; DCM, r.t., 18 h, quant.; HC(OEt)3,
PhCl, 130 °C, 18 h, 74%.
Carboranes are polyhedral clusters of BH and CH vertices that
are stabilized by σ-electron-delocalized covalent bonding in the
skeletal framework and show extraordinary reactivity profiles.[17] Due
to their electronic structure they are considered as three-
dimensional benzene analogs or "superaromatic". Aside from a
negative inductive effect,[18] the clusters exhibit unique steric
properties. Unlike the ortho,ortho'-substitution pattern of typical aryl-
based N-substituents in NHCs (vide supra), these clusters bear
additional B- and C-centered substitution sites providing access to
steric fine-tuning in three dimensions. Conjugation between the
σ-aromatic system of the cluster and the π-aromatic system of the
triazole ylidene moiety may result in additional stabilization of the
free NHC.[1]
As a key element of design we considered a nido-carboranyl-
substituent to be best suited. The versatile, eleven-vertex 7,8-
dicarba-nido-undecaborate anion ([C2B9H12]1–) is among the most
prominent and well examined carborane species and can be easily
Figure 2. Monitoring of the deboronation of closo-carborane 4 to nido-
carborane
5
during the final cyclization by 11B{1H} NMR spectroscopy.
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Figure 4. Comparison of the 13C NMR chemical shifts of the procarbenic carbon atoms of a range of triazolium precatalysts (in DMSO-d6).
Triethyl orthoformate provided best results with 74% yield,
while triisopropyl orthoformate with its more Lewis basic, yet
sterically more demanding isopropionate leaving group, only
furnished 20–30% of the desired triazolium species 5.
decreased while the signals of the dicarba-nido-
undecaboranyl residue of 5 (δB = −20.1, −22.9, −24.5, −21.9,
−32.5, −36.7) concurrently evolved within 18 hours. The
new broad singlet at δB
=
19.6 was attributed to
triethylborate (6), stemming from the Lewis base-mediated
deboronation.[29]
The racemate 5[25] crystallized in the monoclinic space
group P21/n (Figure 3).[30] The volume buried by the
carborate moiety of zwitterion 5 is slightly bigger than that
of a rotating phenyl ring[14c] (Figure 3, middle).
Developing iminium-π-interaction
Developing iminium-π-interaction
N
N
N
Rn
N
O
N
O
N
- -
Anion π
interaction
H
π-Stacking
H
O
H
O
In the postulated transition state of an umpolung reaction
(e.g., enaminol in a benzoin reaction) aryl-based catalysts´
N-substituents turn out-of-plane (Figure 3, top).[20,21]
However, due to the 3D nature of carboranyl-based
systems, the torsion angle between the N-substituent and
the plane of the triazole moiety has considerably less
influence on the space occupied (Figure 3, middle left).
Besides steric aspects non-bonding interactions are known
to play a key role for the product formation and selec-
tivity.[31,32] While for typical NHC catalysts π-stacking of the
N-aryl substituent and a covalently bound substrate is pro-
posed to be crucial for selectivity (Figure 3, top right), the
negatively charged carboranyl-residue may add anion-π
interactions[33,34] as an alternative selection parameter.
H
H
H-Bonding
Aryl-basedcatalystsystem
H-Bonding
Carboranyl-basedcatalyst system
For an estimation of the electronic parameters of compound
5, we compared the 13C NMR chemical shifts of the procar-
benic carbon atoms of selected common triazolium
precatalysts (Figure 4). With a value of 161 ppm, zwitterion
5 (Figure 4, V) is found in the range of the most prominent
organocatalyst systems and thus, the electron density and
acidity should be suitable for covalent organocatalysis.
Remarkably, the influence of modifications of the
N-substituents (I to V) on the chemical shift is rather small
compared to changes in the backbone (II vs. VI).
Figure 3. Comparison of selected bond lengths and angles obtained from
crystal structures of triazolium species 5 (middle, left) and a phenyl-substituted
derivative (middle, right).[26] Postulated transition states with relevant
non-bonding interactions for a carboranyl- (top, left) and an aryl-based (top,
right) NHC-catalyst system.[31,32]
With an optimal synthetic route for the carboranyl-based
triazolium species 5 at hand, we further set out to examine
its catalytic activity.
The deboronation, co-occuring with the heterocyclic ring closure,
was monitored by 11B{1H} NMR spectroscopy (Figure 2). The
characteristic signal pattern of the dicarba-closo-dodecaboranyl
residue of amidrazone 4 (δB = 4.7, 9.9, −12.4, −15.3)
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Reactivity Profile
and
VI), representing electron withdrawing
N-aryl
residues,[38,39]
are well established and are known for
increased rates for the initial adduct formation. Using
triazolium precatalyst 5 we examined a broad set of homo
and cross benzoin reactions employing aromatic (8a,d–e)
and aliphatic (8b,c) aldehydes as well as trifluoromethyl-
ketones[31] (8f) and α-keto esters[40] (8g) (Scheme 2, bottom
left). Interestingly, our catalyst was highly selective for the
transformation of aromatic substrates, while other electro-
philic partners could not be converted.
For a first proof–of–principle, we chose the classic NHC-
catalyzed benzoin reaction (Scheme 2) as an unambiguous
test reaction as this transformation clearly evidences catalytic
activity as no background may occur without the presence of a
catalyst to mediate this umpolung reaction.[6,8] An initial attempt
with a catalyst loading of 10 mol% of carboranyl precatalyst 5
using benzaldehyde (7) under basic conditions[35] was found to
be highly successful; benzoin (8a) was obtained as colourless
crystalline needles in 96% yield. Control experiments with non-
functionalized, anionic dicarba-nido-undecaborate cluster
irrevocably prove the catalytic activity to be attributed to the
NHC part of 5.[30,36,37]
Owing to the assumingly low acidity of precatalyst 5, weaker
bases such as carbonates and amines mostly proved to be
ineffective. An intramolecular Stetter reaction could only be
realized with DBU as base, increasing the yield of
chromanone 10 to 41% (Scheme 2, bottom right).
Low acidity is also typical for more electron-rich, sterically
demanding NHCs, such as N-mesityl type NHCs, which are
known
to
promote
so-called
extended
umpolung
reactions,[9a,41] including homoenolate-type and redox
reactions, oxidations[42] as well as annulations.[43,44] We found
that our new carboranyl NHC precatalyst 5 is highly effective
in an annulation of cyclic ketimine 11 with unsaturated
aldehyde 12 (Scheme 2, top left),[45] providing the desired
lactam 13 in 95% yield and 74% de (lit.: 95% yield, 60% de)
with a catalyst loading as low as 0.5 mol %.
To further illustrate the stability of triazolium
precatalyst 5 towards air and moisture, we performed a
lactonization of aldehyde 12 and acetylacetone by in situ
oxidation in a vessel open to air (Scheme 2, top center).[46]
Lactone 14 was obtained in 62% yield further underlining the
extraordinary versatility of 5.[30]
Finally, we turned our attention to reactions typically
mediated by more electron-rich NHCs, where N-substituents
such as o,o'-dimethoxyphenyl as electron donating N-aryl
residues, have been shown to improve intramolecular
hydroacylation reactions of unactivated double bonds
(Scheme 2, top right).[47] However, here our zwitterionic
precatalyst 5 (10 mol%) proved to be ineffective for this
transformation and this hence denotes the scope limit of its
reactivity.[30,46]
The reactivity profile highlights the broad applicability of the
novel aryl mimetic nido-carboranyl NHC, which already
largely exceeds the versatility of the analogous N-phenyl-
derivative. In addition, this reactivity map also allows for a
preliminary classification of the aryl mimetic nido-carborate´s
combined electronic and steric properties as compared to
typical arene residues.
Scheme 2. Reactivity map: Scope of reactions employing carboranyl
precatalyst 5. Isolated yields are reported. Detailed experimental procedures
are provided in the supporting information.[30] a Yields determined by 1H NMR
with CH2Br2 as internal standard.
Conclusions
Having successfully demonstrated catalytic activity of our
novel carborane-tethered triazolium ylide, we then started to
further explore the catalyst´s reactivity profile. With the
objective to compile a useful kind of reactivity map we
selected a broad array of exemplary reactions mediated by
commonly used N-aryl substituted NHC precatalysts of
different steric and electronic properties.
In conclusion, we have developed
straightforward synthetic route to
a
high-yielding,
a
novel bench-stable
zwitterionic scaffold as NHC precursor incorporating an aryl-
mimetic N-tethered dicarba-nido-undecaborate moiety. Its
unique steric properties have been revealed by crystal structure
analysis. Moreover, for the first time, a carboranyl triazolium
species was shown to be a highly active and versatile catalytic
precursor for a broad range of C-C-connecting reactions.
We started out with classical umpolung transformations,
where N-pentafluorophenyl substituted precatalysts (Fig. 4, II
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Forschungsgemeinschaft (DFG, FOR1296); funding by the
Graduate School Building with Molecules and Nano-objects
(BuildMoNa) (W.N., E.H.H.) is gratefully acknowledged.
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The first N-carboranyl-substituted
Christoph Selg, Wilma Neumann, Peter
Lönnecke, Evamarie Hey-Hawkins and
Kirsten Zeitler
INTERCHANGING Aryl by nido-Carboranyl
zwitterionic
triazolium
NHC-pre-
catalyst for covalent organocatalysis is
presented. Its broad applicability and
reactivity profile are examined in a
wide range of test reactions.
Presentation and discussion of the
unique spectroscopic and crystallo-
graphic properties pave the way for
versatile applications of this novel
class of catalysts, which are bench-
stable and easily accessible.
H
Page No. – Page No.
Carboranes as Aryl Mimetics in
Catalysis: a Highly Active Zwitterionic
NHC Precatalyst
N
H
N
N
New Tools for Catalysis
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