A closer look at the reactivity between N-heterocyclic carbenes and fluoroalkenes

Article abstract of DOI:10.1016/j.jfluchem.2017.05.012

The fundamental reactivity leading to N-heterocyclic fluoroalkene adducts is explored in detail, featuring a total of 15 N-heterocyclic carbenes (NHCs) with various electronic and steric environments. The activity of these carbenes towards tetrafluoroethylene (TFE), hexafluoropropene (HFP), trifluoroethylene (HTFE) and vinylidene fluoride (VDF) is assessed in THF and toluene. Attempts were made to correlate the observed reactivity with electronic (Tolman Electronic Parameters) and steric (% buried volume) parameters unique to each NHC, but a trend has yet to be fully determined. However, the unique steric constraints of a cyclic (alkyl)(amino)carbene (CAAC) were shown to modify the initial point of nucleophilic attack on HTFE, providing selective transformation to a different adduct than has been observed to date with all reactions involving this fluoroalkene.

Full text of DOI:10.1016/j.jfluchem.2017.05.012

Journal of Fluorine Chemistry xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
A closer look at the reactivity between N-heterocyclic carbenes and
fluoroalkenes
⁎
Matthew C. Leclerc, Jason G. Da Gama, Bulat M. Gabidullin, R. Tom 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
A B S T R A C T
The fundamental reactivity leading to N-heterocyclic fluoroalkene adducts is explored in detail, featuring a total
of 15 N-heterocyclic carbenes (NHCs) with various electronic and steric environments. The activity of these
carbenes towards tetrafluoroethylene (TFE), hexafluoropropene (HFP), trifluoroethylene (HTFE) and vinylidene
fluoride (VDF) is assessed in THF and toluene. Attempts were made to correlate the observed reactivity with
electronic (Tolman Electronic Parameters) and steric (% buried volume) parameters unique to each NHC, but a
trend has yet to be fully determined. However, the unique steric constraints of a cyclic (alkyl)(amino)carbene
(CAAC) were shown to modify the initial point of nucleophilic attack on HTFE, providing selective transfor-
mation to a different adduct than has been observed to date with all reactions involving this fluoroalkene.
1. Introduction
very twisted ring, which can lead to desirable orientation of the N-
substituents upon coordination [9,10]. A unique class of carbenes
The incredible diversity of isolable carbenes available to researchers
termed cyclic (alkyl)(amino)carbenes (CAACs), introduced by Bertrand
et al., has been shown to possess unique steric environments and
electronic parameters [11–13].
has contributed to these molecules becoming mainstays of transition
metal catalysis as ancillary ligands and in main-group chemistry as
potent stabilizers for low-valent species [1,2]. More recently, N-het-
erocyclic carbenes (NHCs) and thiazol-2-ylidenes have also been fea-
tured prominently as organocatalysts, with most examples of such re-
activity proceeding via the Breslow intermediate and involving a formal
umpolung of the carbon in the initially electrophilic substrate [3–5].
Since the pioneering work of Bertrand et al. [6] and the seminal
isolation of the first persistent carbene by Arduengo et al. in 1991 [7],
there has been a significant push towards the synthesis of an ever-in-
creasing amount of derivatives and analogues, many of which exhibit
marked differences in reactivity from one another. The design of such
molecules was greatly aided when it became apparent that the stability
of free carbenes was not necessarily governed by sterics, but rather by
important σ and π electronic effects, as evidenced by the stability of
As powerful nucleophiles, it is somewhat surprising that the re-
activity between NHCs and electrophilic alkenes remains relatively
unexplored. Arduengo and coworkers reported on the reactivity of cy-
anocarbons with imidazole-2-ylidene carbenes, which includes the very
electrophilic tetracyanoethylene (TCNE) [14]. Following up on this
study, they chose to focus on the reactivity between imidazolin-2-yli-
denes and fluoroalkenes [15]. Closely following the publication of this
manuscript, we published our own report examining the formation of
NHC fluoroalkene adducts [16]. More recently, we have demonstrated
that NHCs can promote facile CeF bond activation from NHC fluor-
oalkene adducts and polyfluoroalkenyl imidazolium salts to form a
variety of CeE (E = C, N, O, S) and C-M bonds (M = Mn, Mo) [17].
Recently, several reports have emerged demonstrating the ability of
NHCs and CAACs to affect CeF bond activation in various aryl fluor-
ides, further demonstrating the affinity of these systems towards these
types of activations [18–24]. Due to the difficulties associated with the
formation and manipulation of CeF bonds, the selectivity and ease of
activation observed in these systems is somewhat surprising, and herein
we aim to gain a better understanding of the fundamental reactivity
between various carbenes and fluoroalkenes.
4
IMe (1,3,4,5-tetramethylimidazol-2-ylidene) [8]. The field is largely
dominated by N-heterocyclic carbenes, with the possibility of a satu-
rated or unsaturated two-carbon backbone for systems based on imi-
dazoline or imidazole fragments, respectively. The nitrogen atoms in
these types of carbenes can possess both aryl or alkyl substituents, and
the backbone can also be functionalized. Furthermore, six- and seven-
membered expanded ring NHCs are known to be more basic than their
five-membered analogues, with the seven-membered species featuring a
⁎
Corresponding author.
E-mail address: rbaker@uottawa.ca (R.T. Baker).
http://dx.doi.org/10.1016/j.jfluchem.2017.05.012
Received 27 April 2017; Received in revised form 24 May 2017; Accepted 26 May 2017
0022-1139/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Leclerc, M.C., Journal of Fluorine Chemistry (2017), http://dx.doi.org/10.1016/j.jfluchem.2017.05.012

M.C. Leclerc et al.
Journal of Fluorine Chemistry xxx (xxxx) xxx–xxx
Fig. 1. Various NHCs (top) and fluoroalkenes (bottom) studied in this work. Terminal ]CF
2
fragments on fluoroalkenes are highlighted in red, indicating principal point of nucleophilic
attack by the carbene. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2. Results and discussion
2.1. Reactivity with N,N’-diaryl NHCs
To probe the limitations and requirements of the reactivity between
As we have previously reported [16], both SIMes and SIPr react
cleanly with TFE, HFP and HTFE in THF at room temperature within
seconds to afford NHC fluoroalkene adducts, including the SIMes ad-
duct with TFE also reported by Arduengo et al. [15]. The reaction of IPr
NHCs and fluoroalkenes, a total of 15 relatively common NHCs (Fig. 1)
were explored. Their reactivity with tetrafluoroethylene (TFE), hexa-
fluoropropene (HFP), trifluoroethylene (HTFE) and vinylidene fluoride
(
VDF) was examined. Owing to the element’s large electronegativity,
6 6
with TFE in C D was reported by Ogoshi et al. in the Supporting in-
the presence of more fluorine substituents on an alkene might appear to
render it more electrophilic, but the π-donor abilities of fluorine must
also be considered as a factor that mitigates electrophilicity in these
systems. Consequently, the reactivity observed in this work must take
both effects into account.
formation of their work as a by-product to their reactions [25]. In our
hands, IPr was shown to react unfavourably with HFP and HTFE,
providing a multitude of unidentified products, even at −78 °C. Ad-
ditionally, less sterically demanding IMes was found to require low
temperatures to afford a clean adduct with TFE, and did not provide
identifiable products with HFP. When a solution of IMes in THF was
exposed to HTFE at room temperature, several unidentified products
were observed and separation proved difficult. However, when the re-
We have previously demonstrated the importance of a sufficiently
2
electron deficient sp carbon center for reactivity with an NHC to occur
[
16]. Specifically, it was found that cis-1,2-difluoroethylene offered no
reactivity whatsoever, while seemingly every other fluoroalkene with a
terminal ]CF fragment reacts in some way. The proposed mechanistic
pathway for this reaction involves initial attack of the carbene at the
CF position of the fluoroalkene, leading to a transient zwitterionic
action was performed in toluene or C
slowly diffuse into the solution a colour change to pale yellow was
observed, along with clean formation of 1 (IMes = CF(CF H)). Mon-
6 6
D and HTFE was allowed to
2
2
]
2
itoring the reaction by 19F NMR revealed that the reaction is over
within less than 2 min, but the product is not stable for more than
10 min in solution, decomposing to a mixture of dark brown, almost
black products. Attempts to isolate 1 promptly led to decomposition.
Substitution of the backbone hydrogen atoms with electron with-
drawing chlorine atoms lowers the basicity of IMes by an appreciable
intermediate [15,16]. Expulsion of a fluoride then leads to a poly-
fluoroalkenyl imidazolium fluoride species, which will rearrange via
formal 1,2-F shift to form the observed NHC fluoroalkene adduct. Our
previous isolation and characterization of the proposed imidazolium
fluoride salt lends support for this pathway.
The general reaction studied in this work is presented in Scheme 1,
and the products shown are those expected from the different fluor-
oalkenes based on previous work. Due to the extreme reactivity ob-
served between most carbenes and fluoroalkenes in this work, every
reaction was performed a minimum of two times, in both polar THF and
non-polar toluene. When many different products were observed, the
reactions were repeated at −78 °C.
2 2 3
degree [26]. Indeed, Cl IMes forms 2a (Cl IMes = CF(CF )) cleanly
with TFE without the need for low temperature reaction conditions.
Product 2a can be isolated in very good yield (83%) as a bright yellow
crystalline solid. Unfortunately, HFP still proved too reactive at room
1
9
temperature; however, the expected adduct could be observed via
F
NMR if the reaction was performed at −78 °C. Unfortunately, this
product was one of several and was thus not fully characterized. The
2 2 2
reactivity between Cl IMes and HTFE to give 2b (Cl IMes = CF(CF H))
is analogous with IMes. The formation of 2b is noticeably slower,
however the stability of the product is only marginally increased. In this
Scheme 1. General reaction scheme for the transformation studied in
this work.
2

M.C. Leclerc et al.
Journal of Fluorine Chemistry xxx (xxxx) xxx–xxx
Scheme 2. Synthetic scheme for 5a and 5b.
case, the reaction is shown to be complete within 5 min, and 2b is
stable in solution for approximately 15 min, upon which decomposition
to dark orange products is observed.
Due to the positive effect of backbone substitution with chloride, we
2
chose to examine (CO) SIMes. This carbene requires deprotonation at
zwitterion and is especially noticeable with TFE and HFP. However, we
have been unable to trap this proposed intermediate despite exploring a
large variety of substrates and reaction conditions. When 6-Mes is ex-
posed to TFE or HFP, an immediate change from colourless to a very
bright yellow is observed. Curiously, this colour change persists for
approximately 15 s before settling on a paler yellow for TFE and a deep
red mixture of decomposition products for HFP. When these reactions
are performed at −78 °C, the brightly-coloured intermediate could be
observed and maintained for approximately 2 h before eventually
giving rise to the light yellow or deep red colors of the final reaction
mixtures. Unfortunately, limitations pertaining to the introduction of a
fluorinated gas into a cooled NMR tube in an NMR probe has prohibited
low temperatures and immediate in situ reactivity due to its thermally
accessible triplet state, which leads it to readily dimerize [27]. Un-
fortunately, generation of the free carbene and injection of fluoroalk-
enes at low temperature did not provide any reactivity. When the so-
lution was slowly warmed up to room temperature, immediate
dimerization was observed and only unreacted fluoroalkene was ob-
1
9
served by F NMR.
1
9
us from visualizing this proposed zwitterionic intermediate by
F
2
.2. Reactivity with N,N’-dialkyl NHCs
NMR. Attempts to inject the gas and immediately lower the NMR tube
into a pre-cooled probe (−50 °C) also proved unsuccessful. Finally, 6-
Mes did not provide clean reactivity with HTFE. The reactivity ob-
served with 7-Mes was analogous to that of 6-Mes, giving rise to 4 (7-
i
4
We have previously noted that smaller NHCs, IMe and I Pr, do not
react cleanly with the fluoroalkenes in our work [16]. Here, we have
tried to isolate clean products by performing reactions at −78 °C but no
change in reactivity was observed, prompting us to believe that the
adducts formed in these reactions are inherently unstable. Attempts to
limit further reactivity between the theoretically formed adducts and
excess gas by instead utilizing an excess of NHC led to no change in the
product distribution. Conversely, I Bu was reported to not react with
our studied fluoroalkenes, even upon heating. Unfortunately, we failed
to specify that this only applies to TFE and HTFE. Indeed, I Bu reacts
with HFP to form a multitude of products, the F NMR spectra of which
Mes = CF(CF
3
)) in very similar yield (92%).
A thiazol-2-ylidene, which we’ve termed Me
2
ThiaDipp, failed to
provide any adducts with the fluoroalkenes studied in this work. With
TFE and HFP, an immediate colour change to deep red was observed,
which is indicative of dimerization for this carbene [28]. This was
confirmed by 1H NMR and the values matched those reported in the
literature. This carbene is known to undergo immediate dimerization in
the presence of trace amounts of protic acids, and it is currently unclear
what catalyzes this dimerization in the presence of TFE and HFP. No
t
t
1
9
are difficult to interpret. When reactions with IAd were attempted,
2
reactivity was observed when HTFE was introduced to Me ThiaDipp.
19
analogous reactivity was obtained. A detailed inspection of the
F
t
NMR spectra obtained using I Bu and IAd with HFP reveals that al-
though both reactions are messy, there is very little overlap between the
products that are formed with the two NHCs. Analogously, ICy failed to
provide any clean or identifiable products with the fluoroalkenes stu-
died herein.
2.4. Reactivity with CAACs
The unique steric constraints imposed by Me Dipp(CAAC), wherein
2
the methyl groups on carbon are relatively near the carbene centre and
are also in and out of the plane of the heterocyclic fragment, implies
these CAACs might react in unique ways with fluoroalkenes. Reactivity
2.3. Reactivity with ring-expanded NHCs and a thiazol-2-ylidene
with TFE and HFP afforded the expected adducts 5a ({Me
2
Dipp
(
CAAC)} = CF(CF )) and 5b ({Me Dipp(CAAC)} = CF(CF CF )), re-
3
2
2
3
Most NHCs feature five-membered core structures, but examples of
spectively (Scheme 2). The products are formed in high yields and can
be isolated as a pale yellow free-flowing solid (5a) or a pale pink-red
crystalline solid (5b). Although 5b is stable indefinitely at room tem-
perature in the solid state, it is not stable in solution for more than a few
hours.
ring-expanded structures have been reported [9,10]. Due to our lack of
success with alkyl-substituted NHCs, we chose to utilize aryl-substituted
carbenes 6-Mes and 7-Mes. When 6-Mes is allowed to react with TFE,
3
clean transformation to 3 (6-Mes = CF(CF )) is observed within sec-
onds at room temperature, and can be isolated in excellent yield (94%)
as a pale beige crystalline solid (Scheme 4). Conversely, the reactivity
with HFP was messy and no distinguishable products were observed.
We have previously noted that most of these reactions undergo a
vivid, and very short-lived (ca. 1–2 s), colour change when exposed to
fluoroalkenes [16]. This has been proposed to arise from the transient
2
When HTFE was slowly diffused into a solution of Me Dipp(CAAC)
in toluene, a color change from nearly colorless to dark red and then a
very dark green occurred. Surprisingly, a new isomer was identified
(5c, (Me
2
Dipp(CAAC)) = CH(CF
3
)) and characterized for the first time
H) isomer that has
(Scheme 3). As opposed to the expected ]CF(CF
2
3
been observed thus far in successful reactions, the ]CH(CF ) isomer is
Scheme 3. Synthetic scheme for 5c.
3

M.C. Leclerc et al.
Journal of Fluorine Chemistry xxx (xxxx) xxx–xxx
electronic parameters. The Tolman cone angle [29] is still the most
common way of evaluating the steric impact of a wide variety of
phosphines and phosphites, but this model was shown to be an in-
efficient metric to evaluate NHCs and related carbenes. Instead, the
percent buried volume (%Vbur) is commonly employed as a more ac-
curate representation of this effect [30–34]. A recent review by Nolan
et al. explores this subject in depth, and offers an extensive collection of
values reported to date [35]. Defining the electronic parameters of an
NHC is somewhat less straightforward than its steric effects. However,
the Tolman electronic parameter (TEP) [29] remains the most widely
utilized method to accomplish this, as elaborated upon in another
useful review by Nolan et al. [36].
Although the values between different systems do vary slightly for
both%Vbur and the TEP, there is sufficient work supporting the general
trends observed for these series of values to feel confident in comparing
these relative values [35,36]. Finally, it is important to note that TEP
values fail to provide any accurate information about the π-acidity of
NHCs. For many years, these species were viewed solely as σ-donors,
but it is now known that a more accurate depiction of NHC bonding
must involve at least some amount of π-backbonding [37–41]. The two
primary methods of establishing or quantifying the degree of π-acidity
of NHCs involve the formation of carbene-phosphinidene [37,39] and
carbene-selenium [38,40] complexes.
As evidenced in Table 1, SIPr and SIMes remain the best candidates
for reactivity with electrophilic fluoroalkenes. They are the only NHCs
thus far that have proven capable of affording clean adducts, at room
temperature and in various solvents, with TFE, HFP and HTFE. Gen-
erally, HFP has proven to be too reactive, while HTFE appears to often
give rise to unstable products that are prone to decomposition. The
Fig. 2. Crystallographic representation of 5c with 30% probability thermal ellipsoids. H
atoms (except H5) are omitted for clarity. Selected bond lengths and angles are presented
in Table S1 in the Supporting information.
obtained in this case. In fact, this is the only isomer present and there is
2
no evidence for the formation of the ]CF(CF H) product. The products
1
19
can be distinguished with ease from their H and F NMR spectra, due
to the different coupling patterns. Thus far, this represents the only
isolated example of a fluoroalkene affording a regioisomeric adduct in a
clean fashion. The isolation of 5c proceeds in decent yield (67%) and
affords bright green-yellow needles. It is probable that this change in
reactivity is encouraged by the unique steric environment present in
cleaner reactivity obtained with TFE when utilizing Cl
to IMes, and the complete lack of reactivity observed with (CO)
2
IMes as opposed
SIMes,
2
Me
sterically demanding ]CH(F) carbon instead of the bulkier, more
electrophilic ]CF carbon. Following this initial attack, the same re-
2
Dipp(CAAC), wherein the nucleophilic attack proceeds at the less
does appear to be indicative of a system that is sensitive to the elec-
tronic parameters of the NHC. However, the isolation of clean adducts
with carbenes that are more nucleophilic than IMes, i.e., Me Dipp(C-
2
AAC), 6-Mes and 7-Mes, suggests that there are several other factors at
play. To correlate the reactivity presented in this work with key elec-
tronic and steric parameters of NHCs, the relative TEP and%Vbur values
of these carbenes were plotted and are presented in Fig. S22.
2
arrangement that forms the other NHC fluoroalkene adducts would lead
to 5c. As TFE is symmetrical it cannot offer a less hindered site of at-
tack, while the reactivity with HFP already proceeds via the less en-
cumbered site.
Single crystals of 5c were grown by cooling (−35 °C) a concentrated
solution in pure hexanes, and the proposed structure was confirmed
crystallographically (Fig. 2). The data confirm the orientation of the H
Although it would be appealing to draw sweeping conclusions from
these data, it is still unclear if a correlation can be made between the
observed reactivity and the NHC parameters presented in this work. In
3
and CF substituents with respect to the carbene, wherein H is located
t
particular, I Bu and IAd stand out as exceptions. It is thus somewhat
closer to the Dipp substituent (Dipp = 2,6-diisopropylphenyl) and CF
3
curious that no clean reactivity with these carbenes has been observed
yet, with various solvents and reaction conditions. Finally, attempts to
incorporate the relative π-acidity of these carbenes when looking at the
TEP values to perhaps obtain a more complete picture of the electronic
effects of these carbenes have been inconclusive. In fact, IAd has been
is oriented towards the side of the two methyl substituents. Analogous
to crystallographic data we have previously reported, the Dipp frag-
ment is rotated perpendicular to the plane of the heterocycle. At this
time, we do not have any evidence for the formation of the isomer
3
where H and CF would be switched.
t
shown to be a significant π-acceptor, along with I Bu. As with the other
factors, it appears that the desired NHCs are spread out over the π-
acidity scale and it is difficult to draw any significant conclusions from
these values. It does appear, however, that a moderate amount of steric
bulk is required for the successful formation and isolation of NHC
fluoroalkene adducts. Thus far, no adducts have been isolated with
carbenes having a%Vbur below ca. 32%. However, it is important to
note that although a generous cross-section of carbenes have been
studied herein, the incredible number and variety of these species
having been reported in the literature means that there may be other
classes that could provide clean adducts. Indeed, the isolation of 5c
should encourage studies involving the use of carbenes with unique or
atypical steric arrangements, perhaps aiming towards the formation of
other novel isomers or of more labile systems, perhaps capable of ef-
fecting organocatalysis or controlled fluoroalkene polymerization.
2
.5. Reactivity summary
A summary of the reactivity observed between NHCs and fluor-
oalkenes studied in this work is presented in Table 1. Although not
discussed until now, VDF has not yet provided any clean or identifiable
products. In fact, it leads to decomposition and unstable product mix-
tures in most cases, except with I Bu, IAd and Me
t
2
ThiaDipp, where no
1
9
reactivity was observed. It bears mentioning that the F NMR shift of
the fluorine atom bound directly to the alkene in TFE adducts is very
sensitive to the electronic nature of the NHC. Considering data from the
IPr adduct by Ogoshi et al. [25] and our own previous reports with SIPr
and SIMes [16], as well as the examples with Cl
and Me Dipp(CAAC) reported in this work, this shift was shown to
vary between ca. δ( F) = −170 to −218 ppm.
To attempt and correlate the reactivity observed between the var-
ious NHCs and fluoroalkenes with certain structural or electronic
characteristics, we have chosen to focus on a few key steric and
2
IMes, 6-Mes, 7-Mes
2
1
9
3. Conclusions
In summary, a combination of 15 NHCs with various electronic and
4

M.C. Leclerc et al.
Journal of Fluorine Chemistry xxx (xxxx) xxx–xxx
Table 1
Summary of the reactivity observed between NHCs and fluoroalkenes, as well as TEP (cm−1) and%Vbur values.
TEP (cm 1)a
−
%Vburb
NHC
TFE
HFP
HTFE
VDF
✓
✓
✓
decomp.
2050.8 [36]
36.9 [35]
✓
✓
✓
decomp.
2051.1 [36]
47.0 [35]
✓
✓
(−78 °C)
decomp.
decomp.
✓ (unstable)
decomp.
decomp.
2049.6 [36]
2050.2 [36]
36.5 [35]
45.4 [35]
decomp.
✓
decomp.
✓ (unstable)
decomp.
2054.2 [42]
32.7d [43]
decomp.
decomp.
decomp.
decomp.
decomp.
decomp.
decomp.
decomp.
2050.3 [36]
2051.7c [44]
27.5 [35]
26.2 [35]
n/r
decomp.
decomp.
n/r
n/r
2048.9 [36]
2049.5 [36]
39.6 [35]
27.5 [35]
decomp.
decomp.
decomp.
n/r
decomp.
n/r
n/r
2048.3 [36]
2069.0 [36]
39.8 [35]
34.7 [45]
dimerization
dimerization
dimerization
dimerization
✓
✓
decomp.
decomp.
✓ (unstable)
✓ (unstable)
decomp.
decomp.
2042.6d [36]
2041.9d [36]
42.2 [10]
42.9 [10]
f
dimerization
dimerization
n/r
n/r
2053.6 [36]
2046.0e [46]
32.4 [4]
✓
✓
✓ (alternate isomer)
decomp.
38.0e [46]
a
2
Obtained from Ir(Cl)(CO) (NHC) complexes.
Obtained (NHC)AuCl complexes.
Obtained from DFT calculations.
b
c
d
e
f
Obtained from Rh(Cl)(CO)
Obtained from Ni(CO) (NHC) complexes.
Obtained from the corresponding HClO salt.
2
(NHC) complexes.
3
4
5

M.C. Leclerc et al.
Journal of Fluorine Chemistry xxx (xxxx) xxx–xxx
steric environments and 4 fluoroalkenes were studied to obtain a better
understanding of their reactivity to form NHC fluoroalkene adducts. To
date, a stable adduct with VDF has not been isolated. Reactivity be-
tween a cyclic (alkyl)(amino)carbene and HTFE provided an alternate
isomer than is typically observed with this fluoroalkene, the first time
this switch in reactivity has been demonstrated. Unfortunately, we were
unable to discern any clear correlations between electronic (TEP) or
steric (%Vbur) factors, and carbenes providing positive reactivity.
650 °C, 15 g scale, product stabilized with (R)-(+)-limonene (Aldrich,
97%), giving TFE of ≥97% purity] [47]. The synthesis of [SIMes][HCl]
[48], [SIPr][HCl] [49], [IMes][HBF
[26], IMe [49], [ICy][HBF ] [50], [IAd][HBF
(Cl) [27], [6-Mes][HBF ] [9], [7-Mes][HBF
[HClO ] [28] and [Me Dipp(CAAC)][HOTf] [11] have been previously
described. Free SIMes, SIPr and Me ThiaDipp carbenes were synthe-
4
] [50], [IPr][HBF
] [51], (CO
] [9], [Me
4
] [50], Cl
)SIMes(H)
ThiaDipp]
2
IMes
4
4
4
2
4
4
2
4
2
2
sized from the appropriate imidazolium chloride salts by reaction with
sodium hydride (2 equiv.) and catalytic potassium tert-butoxide (5 mol
%) in THF overnight with vigorous stirring. The resulting solution was
filtered through Celite with THF washings and the solvent removed in
vacuo to afford flaky white solid of pure free N-heterocyclic carbene.
Following an analogous procedure, but using KHMDS as the base, free
2
However, the unexpected result obtained by using Me Dipp(CAAC) is
encouraging, and should prompt further studies into carbenes featuring
atypical steric demands. By employing flexible steric bulk and varying
the initial point of attack of the carbene on a fluoroalkene, it is inter-
esting to envision forming novel adducts that could potentially be in-
volved in exciting umpolung chemistry or polymerization reactions
with other fluoroalkenes.
2
IMes, IPr, (CO) SIMes (at −78 °C), 6-Mes and 7-Mes carbenes were
prepared from their respective salts. Free ICy and IAd carbenes were
t
2
similarly prepared by using KO Bu as the base. Finally, free Me Dipp
3
3
.1. Experimental section
(CAAC) carbene was prepared by deprotonation with LDA at −78 °C.
All the carbenes utilized in this work were recrystallized according to
their respective literature procedures prior to being screened for re-
.1.1. General considerations
All manipulations were carried out using standard Schlenk techni-
activity and stored in the freezer at −35 °C. 1H, F and F{ H} NMR
spectra were recorded on either a Bruker Avance 300 or Bruker Avance
II 300 spectrometer at room temperature. 13C{ H} NMR spectra were
recorded on a Bruker Avance 400 spectrometer at room temperature.
19
19
1
ques or in an MBraun glove box. All glassware was oven-dried
at > 150 °C for a minimum of 2 h prior to use, or flame-dried using a
torch. Toluene, tetrahydrofuran (THF), diethyl ether (Et
anes were dried on columns of activated alumina using a J. C. Meyer
1
2
O) and hex-
1
H NMR spectra were referenced to the residual proton peaks associated
®
(formerly Glass Contour ) solvent purification system. Dichloromethane
(DCM), chloroform (CHCl ), chloroform-d (CDCl ) and acetonitrile
(MeCN) were dried by refluxing over calcium hydride under a nitrogen
with the deuterated solvents (C
6
D
6
= 7.16 ppm, CDCl
3
= 7.26 ppm).
13
C NMR spectra were referenced to the signal associated with CDCl
3
3
3
(77.16 ppm). It is important to note that 13C NMR signals coupled to
19
F
flow, followed by distillation and filtration through a column of acti-
vated alumina (ca. 10 wt.%). Methanol (MeOH) and ethanol (EtOH)
nuclei are broadened out significantly, and although coupling constant
values and multiplicity can sometimes be extracted it is often im-
possible to do so. As such, the data is presented to the best of our ability
and all efforts are made to avoid any ambiguity in the presentation of
were dried by refluxing over Mg/I
2
under nitrogen, followed by dis-
tillation. Benzene-d (C ) was dried by standing over activated alu-
6
6 6
D
the data. 19F and F{ H} NMR spectra were referenced to internal 1,3-
19
1
mina (ca. 10 wt.%) overnight, followed by filtration. All solvents were
stored over activated (heated at 250 °C for > 6 h under vacuum) 4 Å
molecular sieves, except EtOH (stored over activated 3 Å molecular
sieves). Acetone (Sigma Aldrich, 99.5 +%) and dimethylsulfoxide
bis(trifluoromethyl)benzene (BTB) (Aldrich, 99%, deoxygenated by
purging with nitrogen and stored over 4 Å molecular sieves), set to −
1
63.5 ppm. H NMR data for BTB: (300 MHz, C
6
D
6
) δ 6.60 (m, 1H, Ar-5-
(
DMSO) (Sigma Aldrich, 99.9 +%) were used as purchased, without
H), 7.12 (m, 2H, Ar-4,6-H), 7.76 (m, 1H, Ar-2-H). A Micromass Q-ToF 1
(positive mode) was used for electrospray ionization (ESI), with sam-
ples diluted to ca. 5 μg/mL in acetonitrile. A Mel-Temp II was used for
the determination of melting points.
further drying. The following chemicals were used as purchased,
without further purification: 2,6-diisopropylaniline (Alfa Aesar, 90 +
%), 2,4,6-trimethylaniline (Alfa Aesar, 98%), triethyl orthoformate
(
Alfa Aesar, 98%), glacial acetic acid (Alfa Aesar, 99 +%), N,N-diiso-
propylethylamine (Alfa Aesar, 99%), formic acid (Alfa Aesar, 97%),
ethyl acetate (Alfa Aesar, 99 +%), trimethylsilyl chloride (Sigma
Aldrich, 99 +%), sodium tetrafluoroborate (Strem Chemicals, 98%),
potassium bis(trimethylsilyl)amide (Sigma Aldrich, 95%), carbon tet-
rachloride (Sigma Aldrich, anhydrous, 99,5 +%), 1,3-diisopropylimi-
3.1.2. Synthesis of IMes = CF(CF
In a glove box, IMes (20 mg, 0.07 mmol) was placed in a vial with a
stir bar and dissolved in C (∼ 0.6 mL). The solution was transferred
2
H) (1)
6 6
D
to a screw-cap septum NMR tube. Outside of the glove box, a 3 mL
plastic syringe was filled with HTFE and purged, before an additional
3 mL were added and slowly injected into the NMR tube. The gas was
allowed to slowly diffuse through the solution to avoid the formation of
unwanted, unidentified products. A color change from nearly colorless
to pale yellow could be observed within a few minutes of gas addition.
The product is formed quickly and decomposes to a mixture of dark
brown, almost black products within ca. 10 min if left in solution or if
attempts are made to isolate it (see main text for more details). The
i
dazolium chloride (I Pr) (Strem Chemicals, 97%), 1,3-di-tert-butylimi-
t
dazol-2-ylidene (I Bu) (Strem Chemicals, 98%), 1-hexanol (Sigma
Aldrich, 98%), 3-hydroxy-2-butanone (Sigma Aldrich, 98 +%), N,N’-
dimethylthiourea (Sigma Aldrich, 99%), potassium (Sigma Aldrich,
98%), cyclohexylamine (Alfa Aesar, 98 +%), tetrafluoroboric acid
(
Strem Chemicals, 48% aqueous solution), oxalyl chloride (Sigma
Aldrich, 98%), sodium bis(trimethylsilyl)amide (Sigma Aldrich, 95%),
potassium carbonate (Sigma Aldrich, 99 +%), 1,3-dibromopropane
product was thus not isolated, and characterized to the best of our
ability using 19F NMR spectroscopy. F NMR (282 MHz, C
19
D ) δ
(
Alfa Aesar, 98%), 1,4-diiodobutane (Alfa Aesar, 99%), sodium hy-
6 6
2
3
droxide (Sigma Aldrich, 97 +%), carbon disulfide (Sigma Aldrich, an-
hydrous, 99 +%), 3-chlorobutan-2-one (Sigma Aldrich, 97%), sodium
perchlorate (Sigma Aldrich, 98 +%), isobutyraldehyde (Oakwood
Chemicals, 99%), lithium diisopropylamide (Sigma Aldrich, 1.0 M in
THF/hexanes), isobutylene oxide (Oakwood Chemicals, 97%), tri-
fluoromethanesulfonic anhydride (Oakwood Chemicals, 98%), sodium
hydride (Strem Chemicals, 60% in oil), hexafluoropropene (HFP)
−107.6 (dd,3
J
FH ≈ 52 Hz3,
J
FF ≈ 15 Hz, 1F, IMes = CF(CF
2
H)),
19
−221.1 (dt,
J
FH ≈ 22 Hz,
J
FF ≈ 16 Hz, 1F, IMes = CF(CF
2
H)).
F
1
3
{ H} NMR (282 MHz, C
6
D
6
) δ −107.6 (d, JFF ≈ 15 Hz, 1F, IMes = CF
(CF
2
H)), −221.1 (t, 3
J
FF ≈ 16 Hz, 1F, IMes = CF(CF H)).
2
3.1.3. Synthesis of Cl
2
IMes = CF(CF
3
) (2a)
In a glove box, Cl
2
IMes (100 mg, 0.27 mmol) was placed in a 50 mL
(
9
SynQuest Labs, 98.5%), trifluoroethylene (HTFE) (SynQuest Labs,
8%) and 1,1-difluoroethylene (VDF) (SynQuest Labs, 99%).
round bottom Schlenk flask with a stir bar and dissolved in THF
(∼ 4 mL). The flask was sealed with a septum and hooked up to a
Schlenk line outside of the glove box. A 10 mL plastic syringe was filled
with nitrogen and purged, before being filled with 10 mL of TFE. The
gas was quickly injected into the flask, and an additional 5 mL were
Tetrafluoroethylene (TFE) was made by pyrolysis of polytetra-
fluoroethylene (PTFE) (Scientific Polymer Products, powdered) under
vacuum, using a slightly modified literature procedure [10–20 mTorr,
6

M.C. Leclerc et al.
Journal of Fluorine Chemistry xxx (xxxx) xxx–xxx
added immediately after. Immediately upon addition of TFE, a color
change from nearly colorless to bright yellow could be observed. After
stirring for 30 min, the septum was replaced with a glass stopper, and
all volatiles were removed in vacuo, affording an off-white residue. The
flask was returned to the glove box, and the product was extracted with
CDCl
3
) δ 18.66, 19.26, 20.86, 21.03, 25.28, 47.67, 48.42, 119.55 (dq,
1
2
J
J
FC ≈ 42 Hz,
FC ≈ 263 Hz,
3
JFC ≈ 202 Hz, 6-Mes = CF(CF )), 123.52 (dq,
1
2
3
JFC ≈ 34 Hz, 6-Mes = CF(CF )), 129.82, 130.02,
133.84, 134.77, 134.80, 134.96, 136.12, 141.78, 141.82, 142.64,
2
3
19
143.62 (dq, JCF ≈ 15 Hz, JCF ≈ 2 Hz, 6-Mes(C) = CF(CF
3
)). F NMR
FF ≈ 16 Hz, 3F, 6-Mes = CF(CF )),
FF ≈ 16 Hz, 1F, 6-Mes = CF(CF )). MS [ESI (positive
) +
]: 421.23 (100), 422.23 (26), 423.23 (3). Found m/z
(% intensity): 421.2201 (100), 422.2305 (26), 423.2442 (3).
®
3
hexanes (∼ 4 mL) and filtered through Celite with hexanes washings
(282 MHz, C
6
D
J
6
) δ −63.4 (d,
J
3
3
(
2 × 1 mL). The solution was concentrated under reduced pressure
−198.0 (q,
3
until the formation of solid could be observed, and then recrystallized
from this cloudy solution at −35 °C. The product was isolated by fil-
tration on a frit and dried under vacuum, affording bright yellow
mode), solvent: MeCN] Calc. m/z (% intensity) for [6-Mes = CF(CF
3
+
+
H
29 4 2
, C24H F N
crystalline solid. Yield: 105 mg, 83% based on Cl
2
IMes. mp:
) δ 2.22 (br s, 12H, Ar-CH ), 2.32
), 6.94 (br, 4H, Ar-H). C{ H} NMR (101 MHz, CDCl ) δ
CF ≈ 196 Hz,
IMes = CF(CF )),
1
1
62–163 °C. H NMR (300 MHz, CDCl
3
3
3
3.1.6. Synthesis of 7-Mes = CF(CF ) (4)
1
3
1
(
s, 6H, Ar-CH
3
3
In a glove box, 7-Mes (100 mg, 0.30 mmol) was placed in a 50 mL
round bottom Schlenk flask with a stir bar and dissolved in THF
(∼ 4 mL). The flask was sealed with a septum and hooked up to a
Schlenk line outside of the glove box. A 10 mL plastic syringe was filled
with nitrogen and purged, before being filled with 10 mL of TFE. The
gas was quickly injected into the flask, and an additional 5 mL were
added immediately after. Immediately upon addition of TFE, a color
change from nearly colorless to yellow could be observed. After stirring
for 30 min, the septum was replaced with a glass stopper, and all vo-
latiles were removed in vacuo, affording an off-white residue. The flask
1
2
1
Cl
7.86, 21.31, 112.94 (ov s), 118.79 (dq,
J
CF ≈ 48 Hz,
J
1
2
3
IMes = CF(CF )), 123.90 (dq,
J
CF ≈ 264 Hz, Cl
2
3
1
1
29.06, 129.39, 131.38, 132.33, 137.20, 137.83, 139.20, 139.60,
2
3
19
39.86 (dq,
J
CF ≈ 20 Hz,
J
CF ≈ 2 Hz, Cl
2
IMes(C) = CF(CF
3
)).
IMes = CF
)). MS [ESI
positive mode), solvent: MeCN] Calc. m/z (% intensity) for
F
3
NMR (282 MHz, CDCl
3
) δ −60.1 (d,
JFF ≈ 16 Hz, 3F, Cl
2
3
(
CF
3
)), −213.1 (q,
) + H+, C23
H23Cl F N ]: 473.12 (100), 475.11
2 4 2
2 3
JFF ≈ 16 Hz, 1F, Cl IMes = CF(CF
(
+
[
Cl
2
IMes = CF(CF
3
(
(
64), 476.12 (16). Found m/z (% intensity): 473.1174 (100), 475.1247
60), 476.1349 (14).
was returned to the glove box, and the product was extracted with THF
®
(
∼ 4 mL) and filtered through Celite with THF washings (2 × 1 mL).
3
.1.4. Synthesis of Cl
In a glove box, Cl
with a stir bar and dissolved in C
2
IMes = CF(CF
IMes (20 mg, 0.05 mmol) was placed in a vial
(∼ 0.6 mL). The solution was
2
H) (2b)
The solution was concentrated under reduced pressure until the for-
mation of solid could be observed, and then recrystallized from this
cloudy solution at −35 °C. The product was isolated by filtration on a
frit and dried under vacuum, affording a pale beige crystalline solid.
2
6 6
D
transferred to a screw-cap septum NMR tube. Outside of the glove box,
a 3 mL plastic syringe was filled with HTFE and purged, before an ad-
ditional 3 mL were added and slowly injected into the NMR tube. The
gas was allowed to slowly diffuse through the solution to avoid the
formation of unwanted, unidentified products. A color change from
nearly colorless to pale yellow could be observed within a few minutes
of gas addition. The product is formed quickly and decomposes to a
mixture of dark orange products within ca. 15 min if left in solution or if
attempts are made to isolate it (see main text for more details). The
product was thus not isolated, and characterized to the best of our
1
Yield: 120 mg, 92% based on 7-Mes. mp: 150–153 °C. H NMR
(300 MHz, C
CH ), 2.12 (s, 6H, Ar-CH
N-CH CH CH -N), 6.80 (ov m, 4H, Ar-H). C{ H} NMR (101 MHz,
CDCl
6
D
6
) δ 1.03 (ov m, 4H, N-CH
2
CH
2
CH
2
-N), 2.11 (s, 6H, Ar-
3
3
), 2.30 (s, 6H, Ar-CH
3
), 3.12-3.66 (ov m, 4H,
1
3
1
2
2
2
3
) δ 18.42, 19.92, 20.67, 20.75, 20.89, 21.16, 28.58, 28.68 (br),
30.51, 54.03, 55.07, 101.03, 118.83, 121.32 (m), 123.69 (m), 126.44,
129.71 (ov s), 130.38 (ov m), 134.72, 135.20, 142.30, 143.45 (ov s),
143.53, 144.00. 19F NMR (282 MHz, C
) δ −63.7 (d,
3
JFF ≈ 14 Hz,
6
D
3
6
3F, 7-Mes = CF(CF
(CF )). MS [ESI (positive mode), solvent: MeCN] Calc. m/z (% intensity)
for [7-Mes = CF(CF
3
)), −184.4 (q,
JFF ≈ 14 Hz, 1F, 7-Mes = CF
1
9
19
ability using F NMR spectroscopy. F NMR (282 MHz, C
−
6
D
6
) δ
3
111.1 (dd, 2JFH ≈ 52 Hz,
3
J
FF ≈ 17 Hz, 1F, Cl
IMes = CF(CF
H)),
) + H+, C25
H F N ]: 435.24 (100), 436.25
+
2
2
3 31 4 2
3
3
19
−
215.8 (dt, JFH ≈ 21 Hz, JFF ≈ 17 Hz, 1F, Cl
2
IMes = CF(CF
2
H)).
F
(27), 437.25 (3). Found m/z (% intensity): 435.2572 (100), 436.2694
(11), 437.2495 (6).
1
3
{
H} NMR (282 MHz, C
IMes = CF(CF H)), −215.8 (t,
H)).
6
D
6
)
δ
−111.1 (d,
J
FF ≈ 17 Hz, 1F,
IMes = CF
3
Cl
2
2
JFF ≈ 17 Hz, 1F, Cl
2
(
CF
2
3.1.7. Synthesis of (Me
In a glove box, Me
2
Dipp(CAAC)) = CF(CF
Dipp(CAAC) (100 mg, 0.35 mmol) was placed in
3
) (5a)
2
3
.1.5. Synthesis of 6-Mes = CF(CF
In a glove box, 6-Mes (100 mg, 0.31 mmol) was placed in a 50 mL
round bottom Schlenk flask with a stir bar and dissolved in THF
∼4 mL). The flask was sealed with a septum and hooked up to a
3
) (3)
a 50 mL round bottom Schlenk flask without a stir bar and dissolved in
toluene (∼4 mL). The flask was sealed with a septum and hooked up to
a Schlenk line outside of the glove box. A 10 mL plastic syringe was
filled with nitrogen and purged, before being half-filled with 5 mL of
TFE. The gas was slowly injected into the flask, and allowed to slowly
diffuse through the solution to avoid the formation of unwanted, uni-
dentified products. This process was repeated twice with 5 mL portions
of TFE. Soon after the addition of TFE, a color change from nearly
colorless to yellow-green could be observed. Following the final gas
addition, and after allowing the gas to diffuse through the solution for
1 h, the septum was replaced with a glass stopper, and all volatiles were
removed in vacuo, affording a pale yellow residue. The flask was re-
turned to the glove box, and the product was extracted with hexanes
(
Schlenk line outside of the glove box. A 10 mL plastic syringe was filled
with nitrogen and purged, before being filled with 10 mL of TFE. The
gas was quickly injected into the flask, and an additional 5 mL were
added immediately after. Immediately upon addition of TFE, a color
change from nearly colorless to very bright yellow could be observed,
which persisted for a few seconds before settling on yellow-orange.
After stirring for 30 min, the septum was replaced with a glass stopper,
and all volatiles were removed in vacuo, affording an off-white residue.
The flask was returned to the glove box, and the product was extracted
®
®
with THF (∼4 mL) and filtered through Celite with THF washings
(∼4 mL) and filtered through Celite with hexanes washings
(
2 × 1 mL). The solution was concentrated under reduced pressure
(2 × 1 mL). The solution was concentrated under reduced pressure
until the formation of solid could be observed, and then recrystallized
from this cloudy solution at −35 °C. The product was isolated by fil-
tration on a frit and dried under vacuum, affording a pale yellow, free-
until the formation of solid could be observed, and then recrystallized
from this cloudy solution at −35 °C. The product was isolated by fil-
tration on a frit and dried under vacuum, affording a pale beige crys-
talline solid. Yield: 123 mg, 94% based on 6-Mes. mp: 175–177 °C. 1
H
flowing solid. Yield: 99 mg, 74% based on Me
Dipp(CAAC). mp:
87–90 °C (decomposition). H NMR (300 MHz, CDCl ) δ 1.11 (d,
), 1.28 (d,
), 2.12 (s, 2H, CH ),
2
1
NMR (300 MHz, C
H, Ar-CH ), 2.12 (s, 6H, Ar-CH
NeCH CH CH eN), 6.78 (ov m, 4H, Ar-H). C{ H} NMR (101 MHz,
6
D
6
) δ 1.41 (ov m, 2H, NeCH
2
CH
2
CH
2
eN), 2.11 (s,
3
3
6
3
3
), 2.29 (s, 6H, Ar-CH
3
), 2.99 (ov m, 4H,
J
J
HH ≈ 6.6 Hz, 6H, CH(CH
3
)
2
), 1.17 (s, 6H, C(CH
3 2
)
1
3
1
3
2
2
2
HH ≈ 6.6 Hz, 6H, CH(CH
3
)
2
), 1.51 (s, 6H, C(CH
3
)
2
2
7

M.C. Leclerc et al.
Journal of Fluorine Chemistry xxx (xxxx) xxx–xxx
could be observed, and then recrystallized from this cloudy solution at
−35 °C. The product was isolated by filtration on a frit and dried under
vacuum, affording bright green-yellow needles. Yield: 86 mg, 67%
3
3
.13 (sept, JHH ≈ 6.6 Hz, 2H, CH(CH
3
)
2
), 7.09-7.29 (ov m, 3H, Ar-H).
1
3
1
C{ H} NMR (101 MHz, CDCl
9.80 (br m), 57.80, 65.37, 123.86, 127.83, 148.51 (d,
3
) δ 23.93, 25.19, 25.22, 28.94, 29.22,
2
2
J
CF ≈ 4 Hz,
) δ −60.9
)), −170.98 (q,
)). MS [ESI (positive
Dipp(CAAC)
]: 386.25 (100), 387.25 (24), 388.25
1
9
1
Me
2
Dipp(CAAC)(C) = CF(CF
3
)). F NMR (282 MHz, CDCl
Dipp(CAAC) = CF(CF
Dipp(CAAC) = CF(CF
3
based on Me
2
Dipp(CAAC). mp: 92–93 °C. H NMR (300 MHz, CDCl
3
) δ
), 1.28 (d,
3 2
) ), 2.10 (s, 2H,
3
3
(
d,
J
JFF ≈ 13 Hz, 3F, Me
2
3
1.15 (d, JHH ≈ 6.7 Hz, 6H, CH(CH
3
)
2
), 1.22 (s, 6H, C(CH
), 1.55 (m, 6H, C(CH
), 2.99 (sept, JHH ≈ 6.7 Hz, 2H, CH(CH ), 3.44 (q, JHF ≈ 10 Hz,
1H, Me Dipp(CAAC) = CH(CF )), 7.22-7.40 (ov m, 3H, Ar-H). C{ H}
NMR (101 MHz, CDCl ) δ 23.74, 26.33, 28.67, 29.47 (br m), 42.39,
56.39, 64.36, 79.36 (q,
3 2
)
3
3
FF ≈ 13 Hz, 1F, Me
2
3
J
HH ≈ 6.7 Hz, 6H, CH(CH
3
)
2
3
3
mode), solvent: MeCN] Calc. m/z (% intensity) for [Me
2
CH
2
3 2
)
+
+
13
1
=
CF(CF
3
) + H , C22
H
32
F
4
N
2
3
(
3). Found m/z (% intensity): 386.2356 (100), 387.2316 (35), 388.2249
3
3
(
5).
J
CF ≈ 38 Hz, Me
2
Dipp(CAAC) = CH(CF
3
)),
Dipp
6 6
D ) δ −46.9 (d,
19 1
3
1
24.99, 128.76, 132.58, 149.48, 165.53 (q,
J
CF ≈ 5 Hz, Me
NMR (282 MHz, C
Dipp(CAAC) = CH(CF )).
) δ −46.9 (s, 3F, Me Dipp(CAAC) = CH(CF
2
19
3.1.8. Synthesis of (Me
2
Dipp(CAAC)) = CF(CF
Dipp(CAAC) (100 mg, 0.35 mmol) was placed in
2
CF
3
) (5b)
(CAAC)(C) = CH(CF
3
)).
F
3
In a glove box, Me
2
J
FH ≈ 10 Hz, 3F, Me
2
3
F{ H} NMR
)).MSCH
a 50 mL round bottom Schlenk flask without a stir bar and dissolved in
toluene (∼4 mL). The flask was sealed with a septum and hooked up to
a Schlenk line outside of the glove box. A 10 mL plastic syringe was
filled with nitrogen and purged, before being half-filled with 5 mL of
HFP. The gas was slowly injected into the flask, and allowed to slowly
diffuse through the solution to avoid the formation of unwanted, uni-
dentified products. This process was repeated twice with 5 mL portions
of HFP. Soon after the addition of HFP, a color change from nearly
colorless to pale pink could be observed. Following the final gas addi-
tion, and after allowing the gas to diffuse through the solution for 1 h,
the septum was replaced with a glass stopper, and all volatiles were
removed in vacuo, affording a pale pink, almost white residue. The flask
was returned to the glove box, and the product was extracted with
(282 MHz, C
6
D
6
2
3
(CF
3
))·MS [ESI (positive mode), solvent: MeCN] Calc. m/z (% intensity)
+
for [Me
2
3 33 3
Dipp(CAAC) = CH(CF H F N ]: 368.26 (100),
) + H+, C22
369.26 (24), 370.26 (3). Found m/z (% intensity): 368.2135 (100),
369.2284 (27), 370.2165 (5).
Acknowledgements
We thank the NSERC for generous financial support and the
University of Ottawa, Canada Foundation for Innovation and Ontario
Ministry of Economic Development and Innovation for essential infra-
structure. M.C.L. gratefully acknowledges support from the NSERC.
®
hexanes (∼ 4 mL) and filtered through Celite with hexanes washings
References
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2 × 1 mL). The solution was concentrated under reduced pressure
until the formation of solid could be observed, and then recrystallized
from this cloudy solution at −35 °C. The product was isolated by fil-
tration on a frit and dried under vacuum, affording a pale pink-red
crystalline solid. The product is stable in the solid state, but decomposes
in solution over several hours, and we were thus unable to characterize
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(
1
3
1
it cleanly by C{ H} NMR. Yield: 135 mg, 89% based on Me
2
Dipp
1.10 (d,
), 1.29 (d,
), 2.12 (s, 2H, CH ),
), 7.07-7.30 (ov m, 3H, Ar-H).
2
1
(
CAAC). mp: 82–83 °C.
H
NMR (300 MHz, CDCl
), 1.17 (s, 6H, C(CH
), 1.52 (s, 6H, C(CH
3
) δ
3
3
6
J
J
HH ≈ 6.7 Hz, 6H, CH(CH
HH ≈ 6.7 Hz, 6H, CH(CH
3
)
2
3 2
)
3
)
2
3
)
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1
9
3
4
F NMR (282 MHz, CDCl
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ESI (positive mode), solvent: MeCN] Calc. m/z (% intensity) for
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+
+
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F
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N
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3.1.9. Synthesis of (Me
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9