Base promotedgem-difluoroolefination of alkyl triflones

Article abstract of DOI:10.1039/d1cc01132d

A new synthesis ofgem-difluoroalkenes from readily available alkyl triflones and difluorocarbene precursors such as TMSCF₂Br has been reported. The reaction, regardless of electronic effect, givesgem-difluoroalkenes in good to excellent yields. The mechanism may involve deprotonation of triflones, nucleophilic addition, and the elimination of SO₂CF₃

Full text of DOI:10.1039/d1cc01132d

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Base promoted gem-difluoroolefination of alkyl
triflones†
Cite this: Chem. Commun., 2021,
57, 4831
Ren-Yin Yang, Hui Wang and Bo Xu
*
Received 1st March 2021,
Accepted 8th April 2021
DOI: 10.1039/d1cc01132d
rsc.li/chemcomm
A new synthesis of gem-difluoroalkenes from readily available alkyl elimination of –SO2CF3 may lead to gem-difluoroalkenes 2
triflones and difluorocarbene precursors such as TMSCF2Br has (Scheme 1d). Difluorocarbene is the most stable dihalocarbene15
been reported. The reaction, regardless of electronic effect, gives and is a highly reactive electrophilic species that could be easily
gem-difluoroalkenes in good to excellent yields. The mechanism in situ generated from precursors such as TMSCF2Br and
may involve deprotonation of triflones, nucleophilic addition, and CF3TMS.16 Herein, we are glad to report a gem-difluoroolefin
the elimination of SO2CF3.
synthesis via the reaction of difluorocarbenes with alkyl triflones
(Scheme 1d).
gem-Difluoroalkenes not only display remarkable biological
We selected the reaction of triflone 1a and TMSCF2Br as the
activity1 but also serve as versatile building blocks in organic model system for the gem-difluoroolefination of triflones
synthesis.2 In the past decades, many efficient syntheses of gem- (Table 1). When 2.0 equiv. of KOtBu was employed, the desired
difluoroalkenes have been developed.2r,3 For example, the product, difluoroalkene 2a was obtained in 61% yield (Table 1,
Wittig-type reaction,4 Julia–Kocienski-type reaction,5 and Hor- entry 1). Next, we investigated the effects of the base (Table 1,
ner–Wadsworth–Emmons-type reaction6 have been used to entries 2–8). The bases such as LiOtBu, NaOtBu, and KOtBu were
prepare difluoroalkenes from aldehydes or ketones. The also effective and gave moderate yields of 2a. However, the
defluorination of trifluoromethylated alkenes are also flexible conversions were not complete. Other bases, including strong
routes for synthesizing difluoroalkenes.2d,2p,7 Recently, groups bases such as LDA, LiHMDS, NaHMDS, and KHMDS, weak bases
of Hu,8 Wang,9 and Xiao10 reported elegant approaches to gem- such as K2CO3 and TEA, were not effective (Table 1, entries 4–8).
difluoroalkenes starting from diazo compounds and difluoro-
carbene precursors (Scheme 1a). Crudden and Nambo have
revealed that the alkyltriflones could transform to gem-
¨
difluoroalkenes via Ramberg–Backlund reaction promoted by
Grignard reagents (Scheme 1b).11 However, there are still
limitations, such as unstable starting materials or harsh reac-
tion conditions in these syntheses.
Sulfones are versatile synthons in C–C, C–X, C–N bond
formations owing to their nucleophilic-electrophilic amphi-
bious properties.12 Wang and coworkers described an efficient
desulfonylmethylation of N-sulfonylhydrazone with sulfone
catalyzed by Cu(I) (Scheme 1c).13 To the best of our knowledge,
the methodologies for the construction of terminal difluori-
nated alkenes via metal-free desulfonylative coupling are rare.14
We envisioned that electrophilic carbenes could react with alkyl
triflone nucleophiles in the presence of the base, and the
College of Chemistry, Chemical Engineering and Biotechnology, Donghua
University, Shanghai 201620, P. R. China. E-mail: bo.xu@dhu.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cc01132d
Scheme 1 Literature background.
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Table 1 Optimization of the gem-difluoroolefination of triflone 1aa
Entry
Promoter (10 mol%)
Base
Solvent
Yieldb (%)
1
2
3
4
5
6
7
—
—
—
—
—
—
—
—
KOtBu
THF
THF
THF
THF
THF
THF
DMF
THF
THF
61
64
49
11
12
6
NaOtBu
LiOtBu
LiHMDS
NaHMDS
KHMDS
K2CO3
0
0
8
TEA
9c
—
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaH
70
81
75
57
52
49
5
10c
11c
12c
13c
14c
15c
16c
17d
LiI
TBAB
LiI
LiI
LiI
LiI
LiI
NaI
THF
THF
CH3CN
DCM
Dioxane
DMF
Toluene
THF
14
42
a
Reaction conditions: 1a (0.1 mmol), TMSCF2Br (0.2 mmol), base
b
(0.2 mmol), solvent (1.0 mL), 0 1C. Yields were determined by LC–MS.
c
d
3.0 equiv. of NaOtBu was used. TMSCF3 was used instead of
TMSCF2Br, NaH (60% in mineral oil), 60 1C, 5 h.
A larger amount of NaOtBu (3.0 equiv.) led to an improved
chemical yield (70%, Table 1, entry 9). The use of a catalytic
amount of Lewis base promoter (LiI) further improved the
chemical yield to 81% and reduced the formation of byproducts
(Table 1, entries 10 and 11). Subsequently, various solvents such
as DCM, CH3CN, DMF, dioxane, and toluene were investigated
(Table 1, entries 12–16). Among them, THF was the optimal
solvent. Additionally, a relatively inexpensive difluorocarbene
precursor–TMSCF3, also gave the desired product, but the yield
was only moderate (Table 1, entry 17). Thus, the optimized
reaction conditions were established (10% LiI, 3.0 equiv. of
NaOtBu, 2.0 equiv. of TMSCF2Br, 0 1C, 0.5 h) (Table 1, entry 10).
We subsequently investigated the substrate scope of the
reaction under the optimized conditions (Scheme 2). In gen-
eral, a-benzyl benzylic triflones, a-allyl benzylic, a-aryl benzylic
triflones, and a-benzyl alkyl triflone reacted with difluorocar-
Scheme 2 Substrate scope for the synthesis of gem-difluoroalkenes
from triflonesa,b. aReaction conditions: 1 (0.1 mmol), TMSCF2Br (0.2 mmol),
NaOtBu (0.3 mmol), LiI (10 mol%), 0 1C in 1 mL of THF. bAll yields are
isolated yields. cTBAB (10 mol%) was used instead of LiI (10 mol%); the
reaction was heated at 60 1C rather than room temperature. dThe reaction
were performed on 0.5 mmol, and TBAB (10 mol%) was used rather than LiI
bene afforded gem-difluoroalkenes 2 in moderate to excellent (10 mol%). eReaction condition: 1ah (0.1 mmol), THF (1.0 mL), NaH
(0.25 mmol), NaI (0.2 mmol), TMSCF3 (0.2 mmol), 60 1C, 5 h.
yields. Triflone 1 containing electron-neutral groups such as
phenyl, tert-butyl, methyl groups on the benzyl ring were
suitable substrates (Scheme 2, 2a–e). Triflone 1, containing
weak electron-withdrawing groups such as halogens (Br and I) in 95% yield (Scheme 2, 2m). The triflones 1, containing
and strong electron-withdrawing groups such as cyano, ester, heteroaromatic rings such as thiophene and indole, reacted
trifluoromethyl on the benzyl, resulted in excellent yields with TMSCF2Br, forming corresponding gem-difluoroalkenes in
(Scheme 2, 2f–k). It should be noted that the highly electron- 72% and 91%, respectively (Scheme 2, 2n and 2o). Moreover,
withdrawing group–nitro group, was well tolerated (Scheme 2, the a-benzyl meta-substituted benzylic triflones also worked
¨
2l). On the contrary, the Ramberg–Backlund reaction of the well under the optimized conditions, giving the desired pro-
benzyl triflone bearing EWG on the para-position of the benzyl ducts with satisfactory yields (Scheme 2, 2p, 2q). However, the
yields the fluorinated alkenes in low efficiency.11 The substrate yields of reaction significantly decreased when substituents on
1 containing a strong electron-donating group methoxy at the the ortho-position of the benzyl ring. For instance, a-(p-tolyl)-1-
para-position of the benzyl group gave difluoroalkene product naphthyleneylic triflone and a-(p-tolyl)-2-bromobenzyl triflone
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only gave 22% and 25% yields, respectively (Scheme 2, 2r
and 2s). Furthermore, a-benzyl alkylic triflone was a suitable
substrate (Scheme 2, 2t). The electronic properties of aromatic
substituents (electron-deficient or electron-rich) on the
a-benzyl of the triflones (R1) played a small role; good yields
were obtained regardless (Scheme 2, 2aa and 2ab).
Bulky a-phenyl (4-phenyl)benzylic triflone gave a relatively
poor chemical yield (40%) under the standard conditions. To
our delight, the reaction yield increased to 65% using slightly
modified conditions (10% TBAB, 60 1C) (Scheme 2, 2ac). The
vinyl and alkynyl groups, which could react with carbenes via
[2+1] cyclopropanation, were untouched under our conditions
(Scheme 2, 2ad–2af); only the yields were slightly lower.
a-Methyl (4-phenyl)benzylic triflone only afforded the target
product in trace amount. Triflone containing an aliphatic chain
was also a suitable substrate, albeit a lower yield was obtained
(50%, Scheme 2, 2ag). It should be noted that mono-substituted
difluoroalkene could also be obtained, but the yields were
relatively low despite many attempts (Scheme 2, 2ah).
Scheme 3 Plausible mechanism.
trifluoromethyl benzene 6a in 51% yield (eqn (4)).
(4)
Our plausible mechanism was illustrated in Scheme 3. First,
the base–NaOtBu abstracts a proton from the a-position of
sulfone 1, forming an anion intermediate A at a low tempera-
ture. Anion A then captures the difluorocarbene in situ gener-
ated from TMSCF2Br promoted by Lewis base, giving another
anion intermediate B. Finally, a b-elimination of –SO2CF3 from
anion intermediate B gives the final difluoroalkene 2.
In conclusion, we have developed a facile methodology to
synthesize gem-difluoroalkenes from easily accessible alkyltri-
flones under metal-free conditions. Moreover, the reaction not
only has a broad substrates scope but also achieved high
efficiency. The novel strategy for constructing fluorinated C–C
double bonds has potential application in more complicated
synthetic chemistry and medicinal chemistry.
To demonstrate the synthetic usefulness of our method, we
conducted a gram-scale transformation (2.5 mmol) under the
standard conditions. The larger scale had little impact on the
yield of the protocol; product 2a was obtained in 84% yield as a
white solid (eqn (1)).
(1)
We also attempted to synthesize gem-difluorinated
a,b-unsaturated ketones from a-acyl benzylic triflones and
difluorocarbene under the standard conditions. However,
gem-difluorinated a,b-unsaturated ketone was not obtained.
Instead, a good yield of O-difluoromethylation17 compound
We are grateful to the National Science Foundation of China
for financial support (NSFC 21871046). RY is grateful for the
Fundamental Research Funds for the Central Universities and
Graduate Student Innovation Fund of Donghua University
(CUSF-DH-D-2021027).
1
5a was isolated (eqn (2)). The H NMR and 19F NMR spectrum
of compound 5a indicated a E : Z ratio of 4 : 1, and the presence
of CF2H group and SO2CF3 group has remained.18 Our protocol
could be used in the dichloroolefination of alkyltriflone using
the readily available chloroform as the dichlorocarbene pre-
cursor, albeit the chemical yield was only moderate (eqn (3)).
Conflicts of interest
The authors declare no competing financial interest.
Notes and references
(2)
(3)
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