Visible-Light-Induced Radical Defluoroborylation of Trifluoromethyl Alkenes: An Access to gem-Difluoroallylboranes

Article abstract of DOI:10.1002/adsc.202000257

A photoredox-catalyzed defluoroborylation of trifluoromethyl alkenes with N-heterocyclic carbene boranes is described for the synthesis of gem-difluoroallylboranes. This protocol exhibits a broad substrate scope and good functional group compatibility, which enables the late-stage functionalization of structurally complex compounds. Further transformations of the defluoroborylation products to valuable CF₂-containing molecules are also demonstrated. (Figure presented.).

Full text of DOI:10.1002/adsc.202000257

Title: Visible-Light-Induced Radical Defluoroborylation of
Trifluoromethyl Alkenes: An Access to gem-Difluoroallylboranes
Authors: Guojun Chen, Liling Wang, Xiaozu Liu, and Peijun Liu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202000257
Link to VoR: https://doi.org/10.1002/adsc.202000257

Advanced Synthesis & Catalysis
10.1002/adsc.202000257
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Visible-Light-Induced Radical Defluoroborylation of
Trifluoromethyl Alkenes: An Access to gem-
Difluoroallylboranes
a
a
a,b,
a,b,
Guojun Chen, Liling Wang, Xiaozu Liu, * and Peijun Liu *
a
Key Laboratory of Biocatalysis & Chiral Drug Synthesis of Guizhou Province, Generic Drug Research Center of
Guizhou Province, School of Pharmacy, Zunyi Medical University, Zunyi 563000, P. R. of China.
E-mail: pjliu@zmu.edu.cn, xiaozliu@126.com
Tel/Fax: (+86)-0851-28609726
Key Laboratory of Basic Pharmacology of Ministry of Education and Joint International Research Laboratory of
b
Ethnomedicine of Ministry of Education, Zunyi Medical University, Zunyi 563000, P. R. of China.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. A photoredox-catalyzed defluoroborylation of
trifluoromethyl alkenes with N-heterocyclic carbene
boranes is described for the synthesis of gem-
difluoroallylboranes. This protocol exhibits
a broad
substrate scope and good functional group compatibility,
which enables the late-stage functionalization of
structurally complex compounds. Further transformations
of the defluoroborylation products to valuable CF
2
-
containing molecules are also demonstrated.
Keywords: photocatalysis; iridium; defluoroborylation; N-
heterocyclic carbene boranes; trifluoromethyl alkenes
Organofluorine compounds play a vital role in
modern medicine. Approximately 20% of all
pharmaceuticals in the market nowadays contain
[1]
fluorine. Among all the organic functional groups
related to fluorine, the gem-difluoroalkene is an
intriguing structural motif that is commonly found in
molecules of pharmaceutical interests.[2] In terms of
the electrosteric properties, gem-difluoroalkene
resembles carbonyl groups but has less liability to the
in vivo metabolism, serving as a potentially valuable
carbonyl bioisostere with improved pharmaceutical
properties.[ Moreover, gem-difluoroalkenes are also
versatile building blocks for the construction of more
Scheme 1. Methods for the synthesis of fluorinated
organoboron compounds.
3]
[4]
complicated organofluorine molecules.
These
On the other hand, organoboron compounds are
one of the most popular intermediates in organic
synthesis due to the high versatility of the C−B bonds
in the formation of other C−X (X = C, N, O, etc.)
features have triggered substantial research efforts
directed towards the development of more efficient
synthetic strategies for their syntheses, including
direct difluoroolefination of carbonyl or diazo
groups,[ the incorporation of gem-difluorovinyl-
[9]
bonds. Substantial efforts have been devoted to the
5]
development of efficient strategies to access these
[6]
containing precursors into the target molecules, and
synthetically
valuable
fluorine-substituted
defluorinative functionalization of trifluoromethyl
[10]
organoboron compounds in this aspect. Among the
protocols reported, the catalytic defluoroborylation of
fluorine-containing compounds has emerged as one
alkenes.[
7,8]
However, for the sake of modular
synthesis, the construction of gem-difluoroalkenes
bearing easily transformable functional groups is
highly desirable but attracts far less attention.
of the most straightforward pathways to achieve this
goal (Scheme 1a).
[8e-h,11]
The current transformations
1
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Advanced Synthesis & Catalysis
10.1002/adsc.202000257
largely depend on the assistance of transition metals
Table 1. Optimization of the reaction conditions.[a]
(
Ni, Cu, Fe, etc.) in the C−B bond formation.
Therefore,
a
new pathway to achieve the
defluorinative borylation is highly desired.
From the pioneering works by MacMillan and
Yoon,[ visible-light-induced catalysis, highlighted
by the environment-friendliness, the mild conditions,
and the low-energy irradiation, has attracted
significant interests in organic synthesis. This has led
to a growing demand for developing environmentally
12]
[13]
Solvent Yield[b]
DMSO 33
benign and sustainable synthetic protocols. On the
other hand, the chemistry of boron-centred radicals,
particularly the N-heterocyclic carbene (NHC)‒boryl
Entry Photocatalyst RSH Base
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
PC1
PC1
PC1
PC1
PC1
PC1
PC1
PC1
PC2
PC3
PC4
PC5
PC1
PC1
PC1
PC1
PC1
PC1
PC1
PC1
‒
S1
S1
S1
S1
S1
S1
S1
S1
S1
S1
S1
S1
S2
S3
S4
S1
S1
S1
‒
K
K
K
K
K
LiOH
Li CO
Na CO
K
K
K
K
K
K
K
K
K
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
3
MeCN
DMF
42
10
[14]
radicals, has gained increasing popularity. Progress
has been made recently towards the inverse
hydroboration of imines with NHC‒boranes by
acetone 61
DCE 29
acetone 32
acetone 55
acetone 36
acetone trace
[
15]
synergistic organocatalysis and photocatalysis.
Inspired by this result as well as our more recent
2
3
work on radical hydroboration and hydrosilylation of
2
3
gem-difluoroalkenes (Scheme 1b),[
10m]
we envisioned
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
that the NHC‒boryl radical could be generated
through a photoinduced hydrogen atom abstraction
process (HAAP) under mild conditions, which would
circumvent the use of potentially toxic radical
initiators and high temperature. Herein, we would
like to present a concise and efficient assembly of
0
1
2
3
4
5
6
7
8
9
0
1
2
acetone
acetone
acetone
0
0
0
acetone 23
acetone 35
acetone 50
acetone 54
acetone 62
acetone 67
acetone 37
acetone 43
gem-difluoroallylboranes
from
trifluoromethyl
[c]
alkenes and NHC‒boranes under photoredox
conditions (Scheme 1c).
[d]
[d,e]
[e]
K
K
‒
K
K
2
CO
3
We started with the treatment of 4-(3,3,3-
trifluoroprop-1-en-2-yl)-1,1'-biphenyl 1a (1.0 equiv.)
and NHC‒borane 2a (1.2 equiv.) with photocatalyst
CO
2 3
[d,e]
[d]
S1
S1
S1
2
2
CO
CO
3
acetone
acetone
0
0
Ir(ppy)
2
(dtbbpy)PF
6
(PC1, 1 mol %), cocatalyst tert-
CO (1.0 equiv.)
[d,e,f]
PC1
3
dodecanethiol (S1, 20 mol %), and K
2
3
in DMSO under irradiation with a 12 W blue LEDs at
room temperature for 18 h, which provided the
desired defluoroborylation product 3a in 33%
isolated yield (Table 1, entry 1). The structure of 3a
was unambiguously confirmed by spectroscopic data
[16]
and X-ray diffraction analysis of the single crystal.
Encouraged by the promising result, further screening
of the reaction conditions was performed. Among the
solvents examined, acetone gave the best result
(
entries 2‒5). Replacing K
or Na CO proved to be detrimental to the yield
entries 6–8). Other photocatalysts including
2
CO
3
with LiOH, Li
2
CO
3
,
2
3
(
Ir[dF(CF
Ru(bpz)
3
)ppy]
][PF
2
(dtbbpy)PF
6
(PC2), Ir(ppy) (PC3),
3
+
[
3
]
6 2
(PC4), and Mes-Acr (PC5) did not
catalyse the anticipated reaction (entries 9–12).
Further experiments revealed that other thiols,
including
2-methylpropane-2-thiol
(S2),
triisopropylsilanethiol (S3), and 2,4,6-triisopropyl-
thiophenol (TRIP thiol, S4), were inferior to S1
[
a]
Reaction conditions: 1a (0.3 mmol, 1.0 equiv.), 2a (0.36
mmol, 1.2 equiv.), photocatalyst (1 mol %), thiol (20
mol %), base (0.3 mmol, 1.0 equiv.), solvent (3 mL), 12
W blue LEDs, argon atmosphere, room temperature, 18
h.
Isolated yield.
S1 (30 mol %) was used.
S1 (10 mol %) was used.
PC1 (2 mol %) was used.
(
entries 13–15). Further adjustment of the thiol
loading to 10 mol % and photocatalyst loading to 2
mol %, the yield was improved to 67% (entry 18).
The yield decreased dramatically in the absence of
thiol or base, and no reaction occurred in the dark or
in the absence of photocatalyst, suggesting that light,
photocatalyst, thiol, and base were all essential for
efficient defluoroborylation (entries 19‒22).
[
[
[
[
[
b]
c]
d]
e]
f]
Without LEDs.
2
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Advanced Synthesis & Catalysis
10.1002/adsc.202000257
Table 2. Substrate scope with respect to trifluoromethyl
After the establishment of the optimal reaction
conditions, we then examined the generality of this
defluoroborylation reaction by evaluating the
substrate scope of trifluoromethyl alkenes (Table 2).
A series of aryl substituted trifluoromethyl alkenes
bearing electron-donating or electron-withdrawing
substituents on the aromatic ring were subjected to
alkenes.[
a]
the reaction with NHC‒BH 2a. The reactions all
3
proceeded smoothly to deliver the desired
corresponding products in moderate to excellent
yields (3a-3t) regardless of electronic nature of the
substituents. Interestingly, substrate 1h underwent
this reaction selectively without any side reaction at
the terminal alkene group. Substrate 1g with an active
thioether moiety was also compatible to the reaction
conditions, delivering the desired product 3g in 70%
yield. α-CF styrenes bearing chloro- and methoxy-
3
substitution all proved to be applicable substrates
regardless of their substitution position, leading to the
desired products in moderate to good yields (3d, 3k,
3
u-3x). It is worth noting that this protocol was also
efficient for heteroaryl-, naphthyl-, and alkynyl-
substituted trifluoromethyl alkenes, and the
corresponding products were obtained in moderate to
good yields. Furthermore, the reaction of internal
alkene 1ae with 2a furnished the desired product 3ae
in 83% yield, further demonstrating good
applicability of the method. Unfortunately, in the case
of aliphatic trifluoromethyl alkenes as substrates, the
reactions failed to afford the desired products (see the
Supporting Information for details).
Based on the results above, we decided to pursue
the application of our method in the late-stage
modification of drug molecules. Two trifluoromethyl
alkenes derived from indomethacin and naproxen
were then examined under the present reaction
conditions. Gratifyingly, the defluoroborylation
products (3ai, 3aj) were successfully obtained from
this transformation in 46% and 83% yields,
respectively.
Next, the scope of the reaction was explored with
respect to different NHC‒boranes (Table 3). Several
NHC–boranes that were derived from 1,3-
dialkylimidazolium iodides were reactive towards
this reaction, affording the corresponding products in
6
4% to 74% yields (4ab-4ae). Notably, CN-
substituted borane 2f could smoothly participate in
the process to provide the desired product 4af in 67%
yield.
To demonstrate the practicality and synthetic value
of the protocol, the reaction between 1f and 2a was
scaled up under the standard reaction conditions and
the product 3f was obtained in 66% yield (Scheme
2
a). Subsequent treatment of 3f with pinacol under
acidic conditions provided gem-difluoroallylboronate
[
a]
Reaction conditions: 1 (0.5 mmol, 1.0 equiv.), 2a (0.6
[17]
5
in 71% yield (Scheme 2b) , which is a useful
mmol, 1.2 equiv.), Ir(ppy)
dodecanethiol (10 mol %), K
2
(dtbbpy)PF
CO
6
(2 mol %), tert-
(0.5 mmol, 1.0
synthon for further transformations. The addition
2
3
reaction of 5 and 4-chlorobenzaldehyde 6 furnished
the difluorinated homoallylic alcohol 7 in 73%
equiv.), acetone (5 mL), 12 W blue LEDs, argon
atmosphere, rt, 12‒36 h. Yields of isolated products are
given.
[8e]
yield.
A three-component Petasis-type gem-
difluoroallylation reaction of 5, 6, and 2-aminophenol
led to gem-difluorohomoallylamine 9 in 89% yield.
[
[
b]
c]
MeCN (5 mL) was employed instead of acetone (5 mL).
8
2
a (0.75 mmol, 1.5 equiv.).
3
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Advanced Synthesis & Catalysis
10.1002/adsc.202000257
Table 3. Substrate scope with respect to NHC‒boranes.[a]
[
a]
Reaction conditions: 1a (0.5 mmol, 1.0 equiv.), 2 (0.6
mmol, 1.2 equiv.), Ir(ppy)
dodecanethiol (10 mol %), K
2
(dtbbpy)PF
CO
6
(2 mol %), tert-
(0.5 mmol, 1.0
2
3
equiv.), acetone (5 mL), 12 W blue LEDs, argon
atmosphere, rt, 17‒36 h. Yields of isolated products are
given.
Scheme 3. Control experiments and proposed mechanism.
observed when stoichiometric amount of 2,6-di-tert-
butyl-4-methylphenol (BHT) was added into the
catalytic system. With the addition of 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO), the reaction
was completely suppressed under the standard
reaction conditions. Notably, the radical adduct 11
was detected by HRMS analysis (Scheme 3a). Based
[15,19]
on these observations and previous reports,
a
plausible mechanism for the reaction is outlined in
Scheme 3b. Initially, a photoexcitation of ground-
state Ir(III) complex A by blue light generates
excited-state Ir*(III) species B, which undergoes a
single electron transfer (SET) with the thiol anion D
to deliver an Ir(II) species C and thiol radical E. At
this stage, the NHC−boryl radical F can be produced
through a hydrogen atom transfer (HAT) process
between the thiol radical E and 2a, along with the
regeneration of thiol S1. A regioselective radical
addition of the radical F to trifluoromethyl alkene 1
followed by a SET reduction by Ir(II) C takes place
to provide a carbanion H and reproduce the ground-
state Ir(III) A. Finally, an E1cB-type β-fluoride
elimination of H leads to the desired product 3.
Scheme 2. Scaled-up reaction and further transformations
of gem-difluoroallylborane 3f.
In summary, we have developed a visible-light-
induced defluoroborylation of trifluoromethyl alkenes
using NHC‒boranes as the radical precursors under
[
18]
Finally, 3f was oxidized to give gem-difluoroallyl
alcohol 10 in 71% yield.[
To verify the radical reaction mechanism of this
defluoroborylation, radical trapping experiments were
performed. A significantly reduced yield of 3a was
17]
[20]
mild conditions,
which enabled a convenient
access to a wide range of gem-difluoroallylboranes
that exhibited remarkable potential for further
4
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Advanced Synthesis & Catalysis
10.1002/adsc.202000257
derivatization to valuable fluorinated molecules.
Moreover, this method can also be applied to the late-
stage functionalization of complicated bioactive
substrates.
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Chem. Soc. 2003, 125, 6348-6349; b) G. Magueur, B.
Crousse, M. Ourévitch, D. Bonnet-Delpon, J.-P. Bégué,
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Experimental Section
General Procedure for the Synthesis of gem-
[
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390-402.
Difluoroallylboranes
To an oven-dried reaction tube (25 mL) equipped with a
magnetic stir bar, trifluoromethyl alkene 1 (0.5 mmol, 1.0
equiv.), NHC‒borane
2
(0.6 mmol, 1.2 equiv.),
2 6 2 3
Ir(ppy) (dtbbpy)PF (9.2 mg, 0.01 mmol, 2 mol %), K CO
(
70.0 mg, 0.5 mmol, 1.0 equiv.), and acetone (5 mL, pre-
degased) were added. The tube was sealed with a rubber
septum and cooled to -10 °C. Then the tube was evacuated
and backfilled with argon for 3 times. tert-Dodecanethiol
[
5] For selective examples of direct difluoroolefination of
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Robins, Org. Lett. 2005, 7, 721-724; b) Y. Zhao, W.
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J.-H. Lin, Y. Guo, J.-C. Xiao, Chem. Commun. 2013,
(
12.0 μL, 0.05 mmol, 10 mol %) was added via a syringe.
The reaction mixture was stirred at room temperature
under irradiation of 12 W blue LEDs (distance app. 3 cm)
for the specified length of time (TLC tracking detection).
After the reaction was finished, the reaction mixture was
diluted with EtOAc (5 mL). The resulting mixture was
filtered, and the filtrate was concentrated under the reduced
pressure. The residue was purified by column
chromatography on silica gel (petroleum ether/ethyl
acetate/triethylamine) to give the desired product 3 or 4.
4
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Acknowledgements
Financial support from the National Natural Science Foundation
of China (No. 21562052 and 21462055) and National First-Rate
Construction Discipline of Guizhou Province (Pharmacy)
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