Synthesis of gem-Difluoroallylboronates via FeCl2-Catalyzed Boration/β-Fluorine Elimination of Trifluoromethyl Alkenes

Article abstract of DOI:10.1021/acs.orglett.7b00168

The first ferrous chloride catalyzed boration/β-fluorine elimination of trifluoromethyl alkenes is described. Thus, a full range of gem-difluoroallylboronates were obtained in high yield under mild conditions. As an important fluorinated building block, gem-difluoroallylboronate can be readily converted into diverse difluoro-substituted species.

Full text of DOI:10.1021/acs.orglett.7b00168

Letter
pubs.acs.org/OrgLett
Synthesis of gem-Difluoroallylboronates via FeCl2‑Catalyzed
Boration/β-Fluorine Elimination of Trifluoromethyl Alkenes
Yang Liu, Yuhan Zhou,* Yilong Zhao, and Jingping Qu
State Key Laboratory of Fine Chemicals, School of Pharmaceutical Science and Technology, Dalian University of Technology, Dalian
116024, China
S
* Supporting Information
ABSTRACT: The first ferrous chloride catalyzed boration/β-
fluorine elimination of trifluoromethyl alkenes is described.
Thus, a full range of gem-difluoroallylboronates were obtained
in high yield under mild conditions. As an important
fluorinated building block, gem-difluoroallylboronate can be
readily converted into diverse difluoro-substituted species.
Surprisingly, the preparation of gem-difluoroallylboronates
ntroduction of fluorine atom(s) or fluorinated moieties into
Iorganic molecules can dramatically alter the reactivity, has scarcely been described. Their synthesis based on the
stoichiometric reaction of iodomethylboronate with difluor-
ovinyllithium was first reported by Ramachandran and co-
workers.12 More recently, Zhang and co-workers described the
preparation of a shelf-stable β-tosyloxy-γ,γ-difluoroallylboronic
acid pinacol ester through an analogous way.13 However, the
reaction conditions required for the preparation of difluor-
ovinyllithium species and their homologation to the corre-
sponding allylboronate limit the scope of this synthetic route.
To the best of our knowledge, the sole example of the catalytic
synthesis of gem-difluoroallylboronate was presented by
Hoveyda and co-workers, showing that (3,3,3-trifluoroprop-1-
en-2-yl)benzene could lead to the corresponding gem-
difluoroallylboronate in a moderate yield by a copper-catalyzed
boration/β-fluorine elimination using a N-heterocyclic carbene
Cu catalyst.14 Otherwise, the rhodium, iridium, or copper salt
catalyzed reactions of HBpin or B2pin2 with trifluoroalkenes
have been reported to deliver the expected 3,3,3-trifluoropro-
pylboronates.15
Owing to its earth-abundance, low price and low toxicity,
iron is an attractive metal for the development of new catalytic
processes, and this field has received much attention.16 In this
context, we report the FeCl2-catalyzed addition/β-fluorine
elimination of trifluoromethyl alkenes leading to gem-
difluoroallylboronates.
The FeCl2-catalyzed reaction between 4-chloro-2-(3,3,3-
trifluoroprop-1-en-2-yl)benzene (1a) and B2pin2 2 was first
selected as the model reaction for optimization of the reaction
conditions. At the outset, several bases were examined (Table 1,
entries 1−10). In contrast to our previous work on iron-
catalyzed hydroboration of aryl alkenes,17 low selectivity and
yield were achieved in the presence of t-BuOK (Table 1, entry
1). Replacing t-BuOK with t-BuONa (Table 1, entry 2) or
MeOLi (Table 1, entry 6) increased the selectivity, but only
physical, chemical, and biological properties of the target
molecule. As a consequence, fluorine-containing compounds
are widely used in various fields such as material science and
agricultural and pharmaceutical chemistry.1 Among numerous
fluorine-containing compounds, gem-difluorovinyl (1,1-di-
fluoro-1-alkene) functionality is an important structural motif
of many biologically active compounds. It appears in
mechanism-based enzyme inhibitors2 and is also considered
as a bioisostere of the corresponding aldehyde and ketone in
drug design.3 Moreover, the gem-difluorovinyl is a useful
monomer for the preparation of fluorinated polymers4 and can
be further converted into different fluorinated functionalities in
organic synthesis.5
Over the past 50 years, owing to their important applications,
a number of examples of the preparation of gem-difluorovinyls
have been reported. First, the Wittig, Horner−Wadsworth−
Emmons, and Julia−Kocienski reaction for the difluoroolefina-
tion of aldehydes or ketones are classical methods.6 Recently,
gem-difluoroolefination of diazo compounds as an efficient
approach toward gem-difluorovinyl derivatives has also been
reported.7 Alternatively, gem-difluorovinyl derivatives have been
prepared via a direct introduction of gem-difluorovinylidene
unit with gem-difluorovinyl lithium, copper, zinc, tin, silicon,
halogen, tosylate, or borane.8 Finally, nucleophilic addition of
nitrogen, sulfur, and carbon nucleophiles on trifluoromethy-
lalkenes followed by β-fluorine elimination is also an efficient
approach to gem-difluorovinyl derivatives.9
With the widely use in Suzuki−Miyaura cross-coupling
reactions and conversion into the corresponding alcohols,
aldehydes, amines, etc., alkylboronates have become one of the
most versatile building blocks in organic synthesis.10
Allylboronates are also known to be reagents of choice for
allylation reactions.11 Introducing an electron-donating boro-
nate group into gem-difluorovinylidenes should therefore afford
gem-difluoroallylboronates as versatile carbon nucleophilic units
embedding a gem-difluorovinyl framework.
Received: January 18, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00168
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a,b
Table 1. Optimization of the Reaction Conditions
Scheme 1. FeCl2-Catalyzed Synthesis of gem-Difluorovinyl
Derivatives
a,b
entry
base
conv (%)
yield (%)
1
t-BuOK
t-BuONa
t-BuOLi
MeOK
MeONa
MeOLi
KOH
69
59
100
88
23
43
39
21
22
25
33
30
99
93
36
74
62
99
99
98
8
55
99
45
4
2
3
4
5
6
41
10
5
7
8
Cs2CO3
K2CO3
K3PO4
9
5
10
11
3
0
c
12
t-BuOLi
t-BuOLi
t-BuOLi
t-BuOLi
t-BuOLi
t-BuOLi
t-BuOLi
t-BuOLi
t-BuOLi
trace
99
90
34
59
60
99
99
98
d
13
e
14
f
15
g
16
h
17
i
18
a
Reaction conditions: 1 (1.0 mmol), 2 (1.1 mmol, 1.1 equiv), FeCl2 (5
mol %), t-BuOLi (1.1 mmol, 1.1 equiv), THF (8 mL), 65 °C, 12 h.
j
19
k
20
b
c
Isolated yields. Reaction conditions: 1 (1.0 mmol), 2 (1.5 mmol, 1.5
a
Reaction conditions: 1a (0.4 mmol), 2 (0.6 mmol, 1.5 equiv), catalyst
(10 mol %), base (0.48 mmol, 1.2 equiv), THF (6 mL), 65 °C, 12 h.
equiv), FeCl2 (10 mol %), t-BuOLi (1.2 mmol, 1.2 equiv), THF (8
d
mL), 65 °C, 18 h. 1H NMR yield.
b
1
Conversion and yield were determined by H NMR with 1,1,2,2-
c
d
tetrachloroethaneas an internal standard. No FeCl2. High-purity
FeCl2 (99.99%). FeCl3 as catalyst. Reaction was performed at 25 °C
for 24 h. 2 (0.6 equiv). t-BuOLi (0.6 equiv). 2 (1.1 equiv), t-BuOLi
e
f
yields (3a−j). In addition, the electron-withdrawing group
chloro substituent and electron-donating group methoxyl
substituent were tolerated at all of the positions on the
benzene ring (3a,b,k−n). The substrates with polysubstituted
phenyl also proceeded smoothly with 2 to afford good to
excellent yields (3o−q). In addition, heterocyclic and naphthyl
substrates provided the desired products in good yields (3r−
t,u). To our delight, alkyl substrates including bulky adamantyl
group can also undergo the reaction to afford the desired
products in high yields (3v−x). Finally, internal alkene
provided the desired product in good yield too (3y).
Unfortunately, cyano and nitro substituents were not tolerated
in this reaction.
On the basis of the previous reports on the mechanism of
boration19 and addition/β-fluorine elimination of trifluoro-
methyl alkenes,9h−j a possible reaction pathway using 1a as a
representative substrate is proposed in Scheme 2. First,
complex A was generated via addition of t-BuOLi to B2pin2.
Then, upon the cleavage of B−B bond in complex A, the
addition of −Bpin anion to the iron center afforded
intermediate B. Species C was formed by coordination of
substrate to the intermediate B followed by the insertion of
double bond to the Fe−B bond. Finally, intermediate C
underwent β-fluorine elimination to generate the product and
the iron catalyst.
g
h
i
j
(1.1 equiv). FeCl2 (5 mol %), 2 (1.1 equiv), t-BuOLi (1.1 equiv).
k
FeCl2(99.99%, 5 mol %), 2 (1.1 equiv), t-BuOLi (1.1 equiv).
moderate conversion was obtained. To our delight, excellent
conversion and yield were achieved when t-BuOLi was used
(Table 1, entry 3). Other bases such as MeOK, MeONa, KOH,
Cs2CO3, K2CO3, and K3PO4 were found to be unfavorable
(Table 1, entries 4, 5, and 7−10), while no product was
obtained in the absence of a base (Table 1, entry 11). Use of a
blank control experiment and high-purity (99.99%) FeCl2 as
catalyst ensured that this reaction was catalyzed by iron salt
(Table 1, entries 12 and 13). Moreover, experiments involving
trace metals present in FeCl2 further support the involvement
of iron (Tables S1 and S2).18 A slightly lower yield was
observed when FeCl3 was used as a catalyst (Table 1, entry 14).
The reaction was significantly slower at lower temperature.
Only 34% yield was achieved at room temperature even for 24
h (Table 1, entry 15). In addition, several other solvents were
explored (Table S3). THF is the optimized solvent, and ether
solvents were more suitable for this reaction generally. Further
study showed that a slight excess of t-BuOLi and B2pin2 was
beneficial for the conversion (Table 1, entries 16−18). Slightly
lower loading of FeCl2 to 5 mol % still led to a high yield
(Table 1, entry 19) and using high-purity (99.99%) FeCl2 as
catalyst gave similar results (Table 1, entry 20).
To demonstrate the practical utility of the ferrous chloride
catalyzed boration/β-fluorine elimination process, a gram-scale
reaction was conducted, and gem-difluoroallylboronate 3a was
obtained with maintained yield (Scheme 3). Bearing both gem-
difluorovinyl and boronate groups, gem-difluoroallylboronate is
an important fluorinated building block. It can be readily
converted into the corresponding gem-difluoroallyl alcohol 4
(Scheme 4). Furthermore, compound 3c can readily be
With the standard reaction conditions (Table 1, entry 19) in
hand, we next explored the substrate scope of the reaction
(Scheme 1). Substrates bearing electron-donating and electron-
withdrawing groups in the para positions on the phenyl ring
provided the corresponding products in moderate to excellent
B
DOI: 10.1021/acs.orglett.7b00168
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
gave good yields. Functional groups such as halogen, ester,
trifluoromethyl, alkoxy, alkyl, etc. were tolerated well in the
reaction. Furthermore, the gem-difluoroallylboronates can be
readily converted into corresponding alcohols and difluor-
omethylboronates and used as intermediates for the synthesis
of a GABAB receptor agonist.
Scheme 2. Proposed Mechanism of the Reaction
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b00168.
Tables of reaction condition development, full exper-
imental protocols, and characterization data for all new
compounds (PDF)
Scheme 3. Gram-Scale Reaction
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zhouyh@dl.cn.
ORCID
Yuhan Zhou: 0000-0002-1860-8669
Notes
Scheme 4. Conversion of gem-Difluoroallylboronates
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (Nos. 21576041,
21376040, and 21231003) and the program for Changjiang
Scholars and Innovative Research Team in University (No.
IRT13008). We also thank Prof. Ce Hao (Dalian University of
Technology, China) for O3 and Prof. Laurent Micouin
(Universite
́
Paris Descartes, France) for helpful suggestions.
converted into the corresponding difluoromethylboronate 5
(Scheme 4) and reacts with benzaldehyde in the presence of
acid catalyst to afford the homoallylic alcohol adduct 6
(Scheme 4) in 84% yield.
Finally, gem-difluoroallylboronates are extremely useful
precursors and can be used to synthesize bioactive molecules.
Recently, compound 8 was reported as a new agonist of the γ-
aminobutyric acid type B (GABAB) receptor by Colby and co-
workers.20 Herein, an alternative route for the synthesis of 8
based on the gem-difluoroallylboronate is presented. Com-
pound 3x reacted with 4-acetylbenzaldehyde to afford the
homoallylic alcohol adduct 7 in 92% yield. Oxidation under
ozone and reduction by dimethyl sulfide led to 8 in 95% yield
(Scheme 5).
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