Selective Activation of Fluoroalkenes with N-Heterocyclic Carbenes: Synthesis of N-Heterocyclic Fluoroalkenes and Polyfluoroalkenyl Imidazolium Salts

Article abstract of DOI:10.1002/chem.201601029

Selective reactions between nucleophilic N,N′-diaryl-heterocyclic carbenes (NHCs) and electrophilic fluorinated alkenes afford NHC fluoroalkenes in high yields. These stable compounds undergo efficient and selective fluoride abstraction with Lewis acids to give polyfluoroalkenyl imidazolium salts. These salts react at Cβ with pyrrolidine to give ammonium fluoride-substituted salts, which give rise to conjugated imidazolium-enamine salts through loss of HF. Alternatively, reaction with 4-(dimethylamino)-pyridine provides a Cα-pyridinium-substituted NHC fluoroalkene. These compounds were studied using multinuclear NMR spectroscopy, mass spectrometry, and X-ray crystallography. Insight into their electronic structure and reactivity was gained through the use of DFT calculations. The unusual suspects: N,N′-Diaryl-heterocyclic carbenes react with fluorinated alkenes to afford N-heterocyclic fluoroalkenes in high yields. Efficient and selective fluoride abstraction with Lewis acids affords the first examples of polyfluoralkenyl imidazolium salts. These salts react with pyrrolidine and DMAP through different pathways to provide unique compounds.

Full text of DOI:10.1002/chem.201601029

DOI: 10.1002/chem.201601029
Communication
&
Carbenes
Selective Activation of Fluoroalkenes with N-Heterocyclic
Carbenes: Synthesis of N-Heterocyclic Fluoroalkenes and
Polyfluoroalkenyl Imidazolium Salts
Matthew C. Leclerc, Serge I. Gorelsky, Bulat M. Gabidullin, Ilia Korobkov, and R. Tom Baker*[a]
[7]
reagents such as HF, CrVI, and SbF5 or environmentally persis-
tent additives such as perfluorooctanoic acid.[8] The manipula-
tion of CÀF bonds is challenging owing to their large thermo-
dynamic and kinetic stabilities.[9] As such, desirable methods
for achieving controlled transformations of fluorinated organics
are in great demand, particularly if these can be achieved with-
out the use of synthetically complex reagents. Fluoroalkenes
are an important class of electrophilic substrates, owing to the
electron-withdrawing effects of various fluorinated substitu-
ents. Thus, the unique nucleophilic nature of NHCs makes
them attractive candidates for reactions with electron deficient
fluoroalkenes. Herein, we demonstrate the utility of NHCs for
the selective and efficient manipulation of fluoroalkene CÀF
bonds.
Abstract: Selective reactions between nucleophilic N,N’-
diaryl-heterocyclic carbenes (NHCs) and electrophilic fluo-
rinated alkenes afford NHC fluoroalkenes in high yields.
These stable compounds undergo efficient and selective
fluoride abstraction with Lewis acids to give polyfluoroal-
kenyl imidazolium salts. These salts react at Cb with pyrro-
lidine to give ammonium fluoride-substituted salts, which
give rise to conjugated imidazolium-enamine salts
through loss of HF. Alternatively, reaction with 4-
(dimethylamino)-pyridine provides a Ca-pyridinium-substi-
tuted NHC fluoroalkene. These compounds were studied
using multinuclear NMR spectroscopy, mass spectrometry,
and X-ray crystallography. Insight into their electronic
structure and reactivity was gained through the use of
DFT calculations.
We report on the formation and facile isolation of a variety
of NHC fluoroalkene compounds. When common NHCs such
as SIPr [1,3-bis-(2,6-diisopropylphenyl)imidazolidin-2-ylidene] or
SIMes [1,3-bis-(2,4,6-trimethylphenyl)imidazolidin-2-ylidene] are
treated with a variety of fluorinated alkenes, the corresponding
NHC fluoroalkenes are easily obtained and isolated (Scheme 1).
As quintessential examples of singlet carbenes, N-heterocyclic
carbenes (NHCs) have been featured as ancillary ligands on
early-, late-, and post-transition metals[1] and serve as key stabi-
lizing elements for novel low-valent p-block element com-
pounds.[2] The nucleophilicity of the carbon center, increased
in part by the ability of nitrogen to p-donate into the empty
p-orbital on carbon, makes this class of compounds especially
useful for a variety of applications. Whereas metal NHC com-
plexes serve as effective catalysts for alkene metathesis and CÀ
C bond formation,[3] NHCs are themselves effective organocata-
lysts for a number of different processes including selective
polymerization.[4] Since the isolation of the first stable NHC in
1991,[5] a variety of derivatives have been successfully synthe-
sized and their nucleophilic character has made them attrac-
tive reaction partners with electrophiles.
Scheme 1. Synthetic scheme for N-heterocyclic fluoroalkenes.
The first report of this reaction was mentioned in the Support-
ing Information of work performed by Ogoshi et al.,[10] wherein
the product derived from the reaction of IPr [1,3-bis-(2,6-diiso-
propylphenyl)imidazol-2-ylidene] and tetrafluoroethylene (TFE)
was a by-product of their organometallic reactions. During the
preparation of this manuscript, work by Arduengo et al. dem-
onstrating the reactivity between imidazol(in)-2-ylidenes and
fluoroalkenes was published, including the study of compound
2a.[11a] The reactions introduced here proceed at room temper-
ature in THF or toluene and afford a single product, except
when using vinylidene fluoride (VDF), which gives rise to two
products, one of which has not yet been identified. The other
product has been characterized as the salt formed between
Fluorine-containing compounds and materials have found
numerous useful applications, ranging from non-stick polymers
and refrigerants to solvents and lubricants.[6] Unfortunately,
many of the methods still used to produce them employ toxic
[a] M. C. Leclerc, Dr. S. I. Gorelsky, B. M. Gabidullin, I. Korobkov, Prof. R. T. Baker
Department of Chemistry and Biomolecular Sciences and
Centre for Catalysis Research and Innovation
University of Ottawa
30 Marie Curie, Ottawa, Ontario K1N 6N5 (Canada)
E-mail: rbaker@uottawa.ca
the fluoroalkenyl cation and
a
fluoride counteranion
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201601029.
1
(Scheme 2, Int 1). The direct observation of Int 1 through H
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Scheme 2. Proposed reaction pathway for NHC fluoroalkene formation from
HFP (red) and VDF (blue). See the Supporting Information for more informa-
1
tion on the observation of Int 1 through H and 19F NMR analysis.
Figure 2. Comparison between NHC fluoroalkenes presented in this work
and the Breslow intermediate.
and 19F NMR spectroscopy supports the previously proposed
mechanistic pathway.[11] TFE, hexafluoropropene (HFP), and tri-
fluoroethylene (HTFE) derivatives have been isolated in high
yields and characterized by 1H, 13C{1H}, and 19F NMR spectrosco-
py, as well as mass spectrometry. Additionally, compound 1b
has been studied through X-ray crystallography (Figure 1).
expected, even with heating. Instead, the fluorine atoms on
the carbon directly bound to the alkene are activated. Indeed,
the average CÀF bond length of the difluoromethylene group
in 1b is 1.37 Š, compared to the average CÀF bond length of
the trifluoromethyl group of 1.33 Š. Additionally, the C=C
bond in 1b is the same length (1.35 Š) as the analogous bond
in the crystallographically studied IMes=CH(OMe) and SIMes=
CH(OMe), supporting the lack of a major contribution from
a zwitterionic/azolium form.[14] Finally, the lack of symmetry ob-
1
served through H NMR for all derivatives of 1 and 2 supports
the alkene form being favoured. Variable temperature NMR ex-
periments using [D8]toluene with heating up to 1008C, cou-
pled with NOESY experiments, demonstrated that there was no
appreciable rotation around the C=C bond, and the appear-
ance of symmetry was not observed.
There are only a few examples of analogous hydrocarbon N-
heterocyclic alkenes (NHCAs) and their reactivity in the litera-
ture.[15] Since the first report by Kaska et al. in 1979,[16] further
work has established the ability of NHCAs to perform organic
transformations,[17] including organocatalysis[18] and transition
metal-free polymerization,[19] as well as their reactivity towards
main-group elements,[20] transition metal complexes,[21] and
lanthanides.[22] Additionally, it has been shown that NHCs react
with dimethyl fumarate to form stable NHCAs, proposed to
occur through a 1,2-H shift.[23]
Figure 1. Crystallographic representation of 1b with 30% thermal ellipsoids.
H atoms and a second molecule within the unit cell are omitted for clarity.
Selected bond lengths and angles are presented in Table S1 in the Support-
ing Information.
It is interesting to note that these addition reactions do not
proceed with all fluoroalkenes; no reaction was observed with
cis-1,2-difluoroethylene, even upon heating for extended peri-
ods of time. It is probable that the latter lacks a sufficiently
electrophilic carbon center; the reactive gases all have at least
one electrophilic CF2 fragment, which is presumably where the
initial attack occurs.
These products can be thought of as fluorinated variations
on the Breslow intermediate (Figure 2), an important product
arising from the reaction between an NHC and an aldehyde.[12]
First proposed in 1958,[13] and successfully isolated in 2012,[14]
this enamine-like compound, along with its many variations,
represents an important example of umpolung in organic
chemistry[14] and has demonstrated rich reactivity towards elec-
trophilic substrates, such as aldehydes, esters, and Michael ac-
ceptors.[3] These results have provided systems capable of
transesterification and transition metal-free polymerization.[4a]
Contrary to the classic Breslow intermediate, the compounds
presented here do not react with benzaldehyde as might be
On addition of the gas to a solution of the free NHC, an im-
mediate and vivid color change from the initial pale yellow so-
lution occurs and persists for approximately 15 seconds before
changing to the final color of the respective products (see Sup-
porting Information for more details on the color changes for
the different reactions). This phenomenon is proposed to arise
from a short-lived zwitterionic intermediate, leading to a delo-
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calized cation within the N-C-N fragment of the heterocycle,
and terminal carbanion on the remaining fragment
a
(Scheme 2). Subsequent expulsion of a fluoride to form the
corresponding alkenyl salt, followed by attack of the fluoride
on the electrophilic terminal fragment affords the final prod-
uct. Presumably, the vinyl fragment formed in the reaction
with VDF is not sufficiently electrophilic for the fluoride to
attack, bearing two hydrogen substituents and lacking the flu-
orine or trifluoromethyl substituents present in the other sys-
tems. Attempts to observe the proposed zwitterion intermedi-
ates by low-temperature NMR spectroscopy have not yet been
successful.[24]
Interestingly, of the gases used in this study, only TFE reacts
cleanly with the unsaturated IPr, as demonstrated previously.[10]
Reactions with HFP and HTFE led to a multitude of different
products. The importance of the electronic and steric proper-
ties of the NHC in these reactions is also apparent. Reactions
with smaller and electronically different alkyl NHCs (IMe4 and
IiPr) led to NHC decomposition and complex mixtures that
were not studied further.[25] When a bulkier alkyl NHC was used
(ItBu), no reaction whatsoever was observed. Heating to 658C
led to decomposition of the NHC. The TFE derivatives (1a and
2a) are stable in the presence of water (20 equiv in CD3CN),
but the HFP derivatives (1b and 2b) and the HTFE derivatives
(1c and 2c) react quickly under the same conditions to form
several different products that have not yet been fully charac-
terized.
We next examined the CÀF bond reactivity of these NHC flu-
oroalkenes. Treatment with a Lewis acid, boron trifluoride (di-
ethyl etherate), in diethyl ether at room temperature leads to
the formation of novel polyfluoroalkenyl imidazolium salts in
high yields within 2 h (Scheme 3). These compounds can be
Figure 3. Crystallographic representation of 3b (top) and 6 (bottom) with
30% thermal ellipsoids. H atoms and tetrafluoroborate anions are omitted
for clarity. There is disorder within the tetrafluoroborate and trifluoromethyl
fragments due to rotation in 3b. One molecule each of toluene, dichlorome-
thane and pentane was removed from 6. Selected bond lengths and angles
are presented in Tables S2 and S3 in the Supporting Information.
isomer is observed through NMR analysis, and the orientation
is further corroborated by convincing coupling constant
values. To the best of our knowledge, there is only one analo-
gous compound in the literature, formed by the presence of
adventitious water.[11b] Additionally, Lehmann et al.[21b] have
characterized a boron adduct that shares certain structural
characteristics to these polyfluoroalkenyl systems. This fluoride
abstraction leads to several decomposition products when the
previously reported IPr derivative of the TFE adduct undergoes
reaction.[10] Importantly, there does not appear to be any for-
mation of the corresponding polyfluoroalkenyl imidazolium
salt.
Scheme 3. Synthetic scheme of polyfluoroalkenyl imidazolium salts.
readily isolated as off-white powders due to their insolubility in
diethyl ether. Selective abstraction of a fluoride occurs in all
cases, leading to a single product. These reactions also occur
cleanly and in high yields using Me3SiOTf (OTf=SO3CF3) as
a Lewis acid in DCM. The selective activation of alkyl CÀF
bonds by Lewis acids has previously been demonstrated
through the use of silylium-carborane-,[26] boron-,[27] and alumi-
nium-based[28] catalysts.
The polyfluoroalkenyl imidazolium salts are likely to exhibit
rich reactivity with nucleophiles. In fact, reaction of 3b with
pyrrolidine in DCM or MeCN leads to the selective substitution
of one fluorine on Cb with an ammonium fluoride fragment
(Scheme 4, compound 5). Further treatment with DBU affords
the pyrrolidine-containing species 6. The alkenyl fragment is
The molecular structure of 3b (Figure 3, top) clearly demon-
strates the importance of aryl ring rotation away from the al-
kenyl fragment, as well as the bending of the iso-propyl
groups to relieve further steric strain. Furthermore, the CF3
substituent is located trans with respect to the NHC fragment.
This phenomenon is also present in solution, where a single
1
retained, as supported by the symmetry observed through H
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The HOMO of 1b is localized on the NHC C=C bond coupled
with the lone pairs of the nitrogen atoms (Figure S1a in the
Supporting Information). The LUMO of 1b is localized on the
aryl group (Figure S1b in the Supporting Information). The
Mayer bond order[34] for the formally double C=C bond is 1.66.
The ESP of 1b (Figure S1c in the Supporting Information) indi-
cates a polarization of the molecule, where the positive area is
located near the backbone of the NHC and the negative area
is located near the F and CF2CF3 groups.
The HOMO of 3b is localized on the aryl group (Figure S2a
in the Supporting Information). The LUMO of 3b, which is re-
sponsible for reactivity of 3b with nucleophiles, is localized on
the FC=CF bond, coupled with the p orbitals of the N-C-N
atoms of the NHC (Figure 4). The alkenyl carbon bonded to
Scheme 4. Reaction of 3b with pyrrolidine to form ammonium fluoride de-
rivative 5 and further reaction with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
to form enamine product 6 (iminium resonance form is also shown). Calcu-
lated Mayer bond orders of 1.16 for C11ÀC18, 1.43 for C18ÀC20 and 1.31
for C20ÀN25 indicate that, although 6 is indeed a mixture of the two reso-
nance forms, the enamine form is major. The reaction of 4b[OTf] with
DMAP is also shown.
and 19F NMR analysis, which persists from the initial salts. Fur-
thermore, NMR coupling constants and crystal structure stud-
ies (Figure 3, bottom) convincingly demonstrate that the pyrro-
lidine substituent is oriented trans with respect to the N-het-
erocyclic fragment, presumably for steric relief, and only one
isomer is observed using NMR spectroscopy.
Figure 4. LUMO of 3b with % atomic contributions (alkenyl atom contribu-
tions are shown in red). H atoms are omitted for clarity.
Alternatively, reaction between the triflate salt of 4b,
(4b[OTf])[29] and 4-(dimethylamino)-pyridine (DMAP) in di-
chloromethane affords a novel Ca-pyridinium-substituted N-
heterocyclic fluoroalkene (Scheme 4, compound 7) in high
yield. The perfluoroethyl fragment obtained with the original
HFP adduct is reformed and net substitution at Ca occurs.
Without the option of HF loss, it is possible that, following an
initial nucleophilic attack of the DMAP on Cb, a [1,2] fluoride
shift occurs from Ca to Cb and the DMAP is finally able to
bind to Ca. Although reactivity with HFP derivatives is demon-
strated as an example, the different derivatives presented in
this work also give rise to analogous products under the same
conditions.
the NHC, Ca, has a contribution of 13% to the LUMO, whereas
the other alkenyl carbon, Cb, has a larger contribution of 18%.
This makes Cb a preferential site for a nucleophilic attack. As
such, DFT supports the selectivity observed experimentally in
this work with pyrrolidine. The nature of the DMAP reactivity is
currently under study in our laboratory. The Mayer bond order
for the FC=CF bond is 1.70, which indicates a fairly weak delo-
calization of the double-bond character in 3b. The ESP of 3b
(Figure S2c in the Supporting Information) indicates that the
positive potential (attractive to a negative charge) is located
around the outside surface of the cation. The HOMO and the
LUMO of 6 are localized on the FC=C(CF3) bond (Figure S3 in
the Supporting Information), with the LUMO showing stronger
coupling with the p orbitals of the N-C-N atoms of the NHC
(Figure S3a in the Supporting Information). Relative to 3b, the
p orbitals of the C(4)=C(5) bond in 6 are more strongly cou-
pled with the adjacent bonds and, as a result, the Mayer bond
order for the formal double C(4)=C(5) bond is only 1.43. Similar
to the ESP of 3, that of cationic 6 (Figure S3c in the Supporting
Information) indicates that the positive potential (attractive to
a negative charge) is located around the outside surface of the
cation.
It is interesting to note that, although DMAP is very reactive
towards 4b[OTf], closely related substrates do not react under
the same conditions, or with added heating. Specifically, tri-
ethylamine, pyridine, and N,N-dimethylaniline are not reactive,
presumably due to the fact that their respective pKb values are
all larger than those of DMAP, and thus less basic.[30] It appears
unlikely that steric hindrance plays a role with these structural-
ly related compounds, and so relative basicity is seemingly the
governing factor.
DFT[31] calculations at the B3LYP[32]/TZVP[33] level were used
to study the electronic structure of neutral 1b, and cationic 3b
and 6, as well as to better understand the selectivity observed
in the reactivity of 3b with pyrrolidine. The single-point calcu-
lations of the geometries corresponding to the X-ray structures
with CÀH bond lengths adjusted to 1.08 Š were conducted to
evaluate the electron density distribution, the bond orders,
and the electrostatic potential (ESP).
In conclusion, we have synthesized and fully characterized
six N-heterocyclic fluoroalkenes, and demonstrated that their
exposure to Lewis acids leads to the clean, selective, and high-
yielding formation of six polyfluoroalkenyl imidazolium salts.
Initial studies demonstrate the importance of an electrophilic
carbon center on the fluoroalkenes for the initial attack of the
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[13] R. Breslow, J. Am. Chem. Soc. 1958, 80, 3719–3726.
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cleanly with pyrrolidine at Cb to afford ammonium fluoride de-
rivatives, which can be further deprotonated to their enamine
forms. Additionally, reaction with DMAP generates a pyridini-
um-substituted N-heterocyclic fluoroalkene through formal Ca
substitution. Insight into the electronic structure and the reac-
tivity of these new compounds was gained through DFT calcu-
lations. Studies on the reactivity of these compounds are cur-
rently underway in our laboratory.
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Acknowledgements
We thank the NSERC and the Canada Research Chairs program
for generous financial support and the University of Ottawa,
Canada Foundation for Innovation, and Ontario Ministry of
Economic Development and Innovation for essential infrastruc-
ture. M.C.L. gratefully acknowledges support from NSERC. Dr.
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Keywords: carbenes · fluoroalkenes · imidazolium · NMR
spectroscopy · pyrrolidine
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mation of 2a to take three days, but we found the reaction to be com-
plete within minutes; small crystals of this hydrolysis product were
studied crystallographically, but the compound was not characterized
further.
Received: March 8, 2016
[12] X. Bugaut, F. Glorius, Chem. Soc. Rev. 2012, 41, 3511–3522.
Published online on May 3, 2016
Chem. Eur. J. 2016, 22, 8063 – 8067
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8067
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