Eliminative Deoxofluorination Using XtalFluor-E: A One-Step Synthesis of Monofluoroalkenes from Cyclohexanone Derivatives

Article abstract of DOI:10.1021/acs.orglett.7b01581

The eliminative deoxofluorination of cyclohexanone derivatives using XtalFluor-E is described. The corresponding monofluoroalkenes are obtained in up to 79% yield. Notably, this one-step procedure occurs at room temperature using readily accessible and cost-effective reagents.

Full text of DOI:10.1021/acs.orglett.7b01581

Letter
pubs.acs.org/OrgLett
Eliminative Deoxofluorination Using XtalFluor-E: A One-Step
Synthesis of Monofluoroalkenes from Cyclohexanone Derivatives
Mathilde Vandamme and Jean-Franco̧ is Paquin*
CGCC, PROTEO, Dep
́
artement de chimie, Universite
́ ́ ́ ́
Laval, 1045 Avenue de la Medecine, Quebec, Quebec G1V 0A6, Canada
S
* Supporting Information
ABSTRACT: The eliminative deoxofluorination of cyclo-
hexanone derivatives using XtalFluor-E is described. The
corresponding monofluoroalkenes are obtained in up to 79%
yield. Notably, this one-step procedure occurs at room
temperature using readily accessible and cost-effective reagents.
lkenes bearing one or more fluorine atoms represent a
triisopropylbenzenesulfonyl hydrazide (TrisNHNH2). Reaction
with n-BuLi followed by fluorination of the resulting vinyl
lithium species using NFSI afforded the fluoroalkenes. More
recently, the palladium-catalyzed fluorination of cyclic vinyl
triflates was reported (Scheme 1, eq 3).9 In this case, ketones
were first converted to their corresponding vinyl triflates, which
were then fluorinated using a newly developed phosphine
ligand, a palladium source, TESCF3 as an additive, and KF as
the fluoride source. While all reactions represent key
contributions, they suffer from issues limiting their applications.
Indeed, in the first case, the source of alumina was found to be
critical. In the last two reactions, the starting ketone must be
transformed into an appropriate precursor which results in a
lower overall yield. Additionally, in the case of the Shapiro
reaction, both the hydrazine and fluorinating agent are
expensive10 and the basic conditions employed limit the
functional groups tolerated. In the case of the Pd-catalyzed
fluorination, not all the ligands employed are commercially
available, and the reaction requires 30 mol % of TESCF3, an
expensive additive,9 but most importantly, the fluorination step
needs to be performed in a glovebox under strictly anhydrous
conditions. Given those limitations, the development of
additional complementary methods is necessary.
A
valuable subclass of fluorine-containing molecules. Indeed,
monofluoroalkenes are utilized in medicinal chemistry as,
among other things, amide isosteres1,2 and enol mimics.3
Fluoroalkenes also have potential applications in material
sciences,4 and they can be used in synthetic organic chemistry
as fluorinated building blocks for further functionalization.5 The
synthesis of monofluorinated alkenes, despite their potential
uses in various fields, still remains a synthetic challenge.6 An
attractive strategy would be the direct conversion of a ketone to
the corresponding fluoroalkene. Toward this end, three
synthetic approaches have been reported. In the first case, an
alumina-promoted elimination of difluorocycloalcanes was
described (Scheme 1, eq 1).7 In the second approach, the
Shapiro reaction was used as the key step (Scheme 1, eq 2).8
Ketones were first transformed into hydrazones using 2,4,6-
Scheme 1. Previous and Current Work
Our inspiration for the present work was the report that
fluoroalkenes were sometimes observed as side products for the
deoxofluorination of ketones using XtalFluor-E ([Et2NSF2]-
BF4)11,12 or with other related reagents.13−15 We imagine that if
conditions favoring the formation of the fluoroalkene could be
found, it would represent a practical one-step alternative to the
above methods. Herein, we report the direct conversion of
cyclohexanone derivatives to monofluoroalkene using Xtal-
Fluor-E (Scheme 1, eq 4).16 Notably, this one-step procedure
occurs at room temperature using readily accessible and cost-
effective reagents10 without the need for a glovebox.
An extensive optimization of the reaction conditions was
undertaken, and selected key results are shown in Table 1.17
Under conditions reported for the deoxofluorination of ketone
Received: May 25, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01581
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Key Optimization Results for the Eliminative Deoxofluorination of 4a Using XtalFluor-E
b
c
c
d
entry XtalFluor-E (equiv) fluoride source (equiv)
solvent
additive (equiv)
temp (°C) time (h) conversion (%) ratio 5a/6a yield (%)
1
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2
Et3N·2HF (3)
Et3N·2HF (3)
Et3N·2HF (3)
Et3N·2HF (3)
Et3N·2HF (3)
Et3N·2HF (3)
Et3N·2HF (3)
Et3N·2HF (3)
Et3N·2HF (3)
Et3N·2HF (3)
Et3N·2HF (3)
Et3N·2HF (1.5)
Et3N·3HF (3)
Et3N·3HF (2)
Et3N·3HF (2)
Et3N·3HF (2)
Et3N·3HF (2)
Et3N·3HF (2)
CH2Cl2
toluene
EtOAc
Et2O
−
−
−
−
−
−
−
−
−
−
−
−
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
60
rt
rt
rt
rt
rt
rt
rt
16
16
16
16
16
16
16
16
16
16
2
100
100
90
1:6.7
1:2.4
1:4.7
1:5.5
1:1.8
1:2.9
2.5:1
5:1
13
2
22
3
14
4
100
86
13
5
THF
24
6
CH3CN
DMF
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
100
100
100
87
12
7
33
8
15
9
5.7:1
6.8:1
4.6:1
9.1:1
5.4:1
5.5:1
5.8:1
5.5:1
3.2:1
2.9:1
34
e
10
11
12
13
14
15
16
3
100
100
98
75 (79)
23
3
3
120
16
16
16
16
16
16
73
3
−
100
94
70
3
DBU (1)
60
3
i-Pr2EtN (1)
Me-imidazole (1)
−
97
70
3
97
44
f
17
18
3
97
19
g
3
−
89
29
a
b
See the Supporting Information for the detailed experimental procedures. Et3N·2HF is generated in situ by adding Et3N (1 equiv) to Et3N·3HF (2
c
d
equiv). Determined by 19F NMR analysis of the crude mixture after workup. Yield of 5a determined by 19F NMR analysis of the crude mixture
e
f
g
after workup. Isolated yield of 5a contaminated with inseparable 6a (5a/6a = 5.9:1). Me-DAST was used instead of XtalFluor-E. Deoxofluor was
used instead of XtalFluor-E.
using XtalFluor-E (Table 1, entry 1),11 a 13% NMR yield of the
monofluoroalkene 5a was observed, yet the major product was
the difluoromethylene compound 6a (ratio 5a/6a = 1:6.7).
Next, different solvents were tested including toluene, EtOAc,
Et2O, THF, or CH3CN (Table 1, entries 2−6).17 For all of
them, low NMR yields of 5a were detected (13−24%), and in
all cases, product 6a was the major compound with the best 5a/
6a observed with THF (1:1.8). Interestingly, when DMF was
ratio employed as the solvent, a reversal of the selectivity was
observed, as 5a was now the major product (5a/6a = 2.5:1)
with an NMR yield of 33% (Table 1, entry 7). Using a related
solvent, dimethylacetamide (DMA), improved the 5a/6a ratio
to 5:1, but at the expense of the yield (15% by NMR) (Table 1,
entry 8). Increasing the amount of XtalFluor-E to 3 equiv
(Table 1, entries 9−10) resulted in a 75% NMR yield of the
desired monofluoroalkene 5a with a 5a/6a ratio of 6.8:1.
Diluting the reaction in CH2Cl2 using various ratios (9:1, 1:1,
or 1:9) of a CH2Cl2/DMA mixture did not furnish better
results (not shown). Similarly, conducting the reaction at 60 °C
resulted in a lower NMR yield of 5a (23%) (Table 1, entry 11).
Reducing the amount Et3N·2HF to 1.5 equiv resulted in a
much slower reaction. Nonetheless, after 5 days, a 73% NMR
yield of 5a was observed with a higher 5a/6a ratio of 9.1:1
(Table 1, entry 12). When Et3N·3HF was used as the fluoride
source, a slightly inferior NMR yield was observed (70%)
(Table 1, entry 13) most likely due to the lower nucleophilicity
of Et3N·3HF compared to Et3N·2HF.18 Replacing the added
Et3N by other amine bases (DBU, i-Pr2EtN, or Me-imidazole)
had limited effect on conversion and on the 5a/6a ratio (Table
1, entries 14−16). Other fluoride sources such as TBAF (1 M
soln in THF), TBAF·3H2O, or DMPU·HF19 (with or without
added Et3N) were ineffective (not shown). Also, various
additives17 were tested, but none offered significantly better
results (not shown). Finally, for comparison, Me-DAST and
Deoxofluor, two classical deoxofluorinating agents, were tested
(Table 1, entries 17−18). In both cases, a low yield (19−29%)
and lower selectivity (up to 3.2:1) were obtained, demonstrat-
ing the unique reactivity of XtalFluor-E. Overall, the conditions
shown in Table 1, entry 10 were chosen to be optimimal.
We next evaluated the reactivity of various cyclohexanone
derivatives under the optimized conditions (Scheme 2). 4-
Piperidone bearing various amine protecting groups performed
well. In this case, monofluoroalkenes bearing a carbamate or
sulfonyl-based protecting groups (5a−c) were isolated in better
yields than the one having a benzyl group (5d). Cbz-, Boc-, or
Ts-protected 4-aminocyclohexanone also provided the corre-
sponding monofluoroalkenes (5e−g) in moderate yields. In the
case of protected 4-hydroxycyclohexanone, the use of a benzoyl
group furnished the monofluoroalkene 5h in a better yield than
when using a benzyl group (5i). An ethyl ester substituent is
well tolerated at both the 4- and 3-position. In the former,
monofluoalkene 5j is isolated in 76% yield. In the latter, the
product 5k is isolated in moderate yield as a mixture of
inseparable monofluoroalkenes in a ratio of 1.2:1 favoring the
alkene distal to the ester moiety. Monofluoroalkenes derived
from 1,4-cyclohexanedione monoacetal (5l and 5m) were
obtained in good yield. Finally, cyclohexanone bearing a phenyl
group or a n-pentyl chain at the 4-position provided the desired
products 5n and 5o in 38% and 14% respectively.
A number of other ketones were also tested, but did not
provide the desired monofluoroalkenes. For instance, for 1-
tetralone, no conversion was observed whereas, for 1-indanone,
B
DOI: 10.1021/acs.orglett.7b01581
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Eliminative Deoxofluorination of Various
Cyclohexanone Derivatives Using XtalFluor-E
Scheme 3. Mechanistic Hypothesis
a b
,
a
−
The BF4 counterion has been omitted for clarity.
reaction on intermediate 9 with HF would produce 10 directly.
Alternatively, ionization would lead to the fluorine-stabilized
carbocation 11,23 which could then react with HF to produce
10. For monofluoroalkene 12, the first possibility would involve
an E2 mechanism triggered by Et3N conducting directly to 12.
Or else, ionization to carbocation 11 followed by elimination
(E1 pathway) would also lead to monofluoroalkene 12. While
the current conditions do not allow us to discriminate between
the two pathways, the observation that substrates bearing an
electron-withdrawing substituent performed better suggest that
the mechanism involving the carbocation is likely not the main
pathway. Concerning the role of DMA (or DMF) for the
control of the selectivity between monofluoroalkene (12) and
difluoromethylene (10), our current hypothesis24 points toward
a possible role for the hydrogen bond acceptor ability of the
solvent. Indeed, both DMA and DMF are strong hydrogen
bond acceptors (pKBHX = 2.44 and 2.10 respectively) whereas
all the other solvents are a significantly weaker hydrogen bond
acceptor (pKBHX = −0.36 (toluene) to 1.28 (EtOAc)).25
Hence, with HF being an excellent hydrogen bond donor, a
significant interaction between HF with DMA would slow
down the SN2 attack of HF onto intermediate 9 (to produce
10) and would thus favor the elimination pathway instead
(leading to 12).
a
See the Supporting Information for the detailed experimental
b
c
procedures. Isolated yield. Monofluoroalkene/difluoromethylene
ratio determined by 19F NMR in the purified product when complete
d
separation by flash chromatography was not possible. On a 1 mmol
scale, 75% (ratio 5a/6a = 6.1:1) of 5a was obtained.
In summary, we have described the eliminative deoxofluori-
nation of cyclohexanone derivatives using XtalFluor-E. Notably,
this one-step procedure for the synthesis of monofluoroalkenes
occurs at room temperature using readily accessible and cost-
effective reagents without the need for a glovebox. Overall, this
new approach complements the previous reported methods.
Mechanistic studies and extension of the reaction to other
ketones, including acyclic ones, are currently underway and will
be reported in due course.
<20% conversion was observed by NMR, but no fluorinated
products were formed. When using α-substituted cyclo-
hexanones, some conversion (23% and 72% for 7b and 7c
respectively) was observed, but no fluorinated products could
be detected. A moderate conversion (69%) was obtained for a
five-membered ring derivative (7d), but no fluorinated
products were formed. Finally, when using an acyclic ketone
such as 7e, only degradation was observed and no fluorinated
products were observed.
ASSOCIATED CONTENT
* Supporting Information
■
S
Our current mechanistic hypothesis is shown in Scheme 3.
First, the ketone (8) would get converted to the fluoroalkoxy-
N,N-diethylaminodifluorosulfane (9).20 This could occur either
through HF addition to the carbonyl and reaction of the
resulting α-fluoro alcohol with XtalFluor-E as claimed for
DAST21 or, alternatively, via reaction of the ketone with
XtalFluor-E followed by addition of HF as proposed for SF4.22
In any case, from intermediate 9, two related pathways would
be possible for the formation of difluoromethylene 10 and
monofluoroalkene 12. For difluoromethylene 10, an SN2
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01581.
Detailed experimental procedures and full spectroscopic
data for all new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jean-francois.paquin@chm.ulaval.ca.
C
DOI: 10.1021/acs.orglett.7b01581
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
ORCID
Letter
(12) For other contributions from our group using XtalFluor-E, see:
(a) Pouliot, M.-F.; Angers, L.; Hamel, J.-D.; Paquin, J.-F. Org. Biomol.
Chem. 2012, 10, 988−993. (b) Pouliot, M.-F.; Angers, L.; Hamel, J.-D.;
Jean-Franco̧ is Paquin: 0000-0003-2412-3083
Notes
Paquin, J.-F. Tetrahedron Lett. 2012, 53, 4121−4123. (c) Mahe,
Desroches, J.; Paquin, J.-F. Eur. J. Org. Chem. 2013, 2013, 4325−4331.
(d) Pouliot, M.- F.; Mahe, O.; Hamel, J.-D.; Desroches, J.; Paquin, J.-F.
Org. Lett. 2012, 14, 5428−5431. (e) Keita, M.; Vandamme, M.; Mahe,
́
O.;
The authors declare no competing financial interest.
́
́
ACKNOWLEDGMENTS
O.; Paquin, J.-F. Tetrahedron Lett. 2015, 56, 461−464. (f) Desroches,
J.; Champagne, P. A.; Benhassine, Y.; Paquin, J.-F. Org. Biomol. Chem.
2015, 13, 2243−2246. (g) Keita, M.; Vandamme, M.; Paquin, J.-F.
Synthesis 2015, 47, 3758−3766. (h) Vandamme, M.; Bouchard, L.;
Gilbert, A.; Keita, M.; Paquin, J.-F. Org. Lett. 2016, 18, 6468−6471.
(i) Lebleu, T.; Paquin, J.-F. Tetrahedron Lett. 2017, 58, 442−444.
(13) Monofluoroalkenes were observed when the deoxofluorination
of cyclohexanone (72% GC yield) and 4-methoxyacetophenone (15%
GC yield) were performed using 2,2-difluoro-1,3-dimethyl-
imidazolidine (DFI); see Hayashi, H.; Sonoda, H.; Fukumura, K.;
Nagata, T. Chem. Commun. 2002, 1618−1619.
(14) A patent describing the eliminative deoxofluorination using
DAST under strong acidic conditions was reported; see: Boswell, G.
A., Jr. Preparation of vinylene fluorides. U.S. Patent 4,212,815, 1980.
(15) The deoxofluorination of β-diketones with N,N-diethyl-α,α-
difluoro-m-methylbenzylamine provides β-fluoro-α,β-unsaturated ke-
tones, see: (a) Sano, K.; Fukuhara, T.; Hara, S. J. Fluorine Chem. 2009,
130, 708−713. Reaction of cyclohexanone with the same reagent
provides fluorocyclohexane in 58% yield along with 32% of
difluorocyclohexane, see: (b) Hara, S.; Fukuhara, T. Method of
fluorination. U.S. Patent 20060014972, 2006.
(16) For selected recent alternative approaches to monofluoroalkene-
containing cyclohexane derivatives, see: (a) Furuya, T.; Ritter, T. Org.
́
Lett. 2009, 11, 2860−2863. (b) Alonso, P.; Pardo, P.; Fananas, F. J.;
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2014, 136, 14381−14384. (d) Nihei, T.; Kubo, Y.; Ishihara, T.;
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■
This work was supported by the Natural Sciences and
Engineering Research Council of Canada, the Fonds de
recherche du Quebec−Nature et technologies, OmegaChem,
́ ́
and the Universite Laval. Eliane Soligo (Universite Laval) is
thanked for preliminary experiments.
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D
DOI: 10.1021/acs.orglett.7b01581
Org. Lett. XXXX, XXX, XXX−XXX