Carbotrifluoromethylation of Allylic Alcohols via 1,2-Aryl Migration Promoted by Visible-Light-Induced Photoredox Catalysis

Article abstract of DOI:10.1002/adsc.201800726

Visible-light-enabled photocatalytic carbotrifluoro-methylations of allylic alcohols and sodium triflinate were explored through 1,2-migration of an aryl group, affording an efficient method for synthesis of β-trifluoromethyl-α-substituted carbonyl compou

Full text of DOI:10.1002/adsc.201800726

Title: Carbotrifluoromethylation of Allylic Alcohols via 1,2-Aryl Migration
Promoted by Visible-Light-Induced Photoredox Catalysis
Authors: Shunyou Cai, Yu Tian, Jinwang Zhang, Zhiji Liu, Maojian Lu,
Wen Weng, and Mingqiang Huang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800726
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10.1002/adsc.201800726
Advanced Synthesis & Catalysis
Carbotrifluoromethylation of Allylic Alcohols via 1,2-Aryl Migration
Promoted by Visible-Light-Induced Photoredox Catalysis**
Shunyou Cai,*a,b Yu Tian,a Jinwang Zhang,a Zhiji Liu,a Maojian Lu,a Wen Weng,a Mingqiang Huanga
aKey Laboratory of Modern Analytical Science and Separation Technology of Fujian Province, School of Chemistry, Chemical Engineering
and Environment, Minnan Normal University, Zhangzhou, 363000, China.
bKey Laboratory of Chemical Genomics, School of Chemical Biology and Biotechnology, Shenzhen Graduate School, Peking University,
Shenzhen, 518055, China.
E-mail: caishy05@mnnu.edu.cn
Supporting information for this article is available on the WWW under https://doi.org/xxxxx/adsc.
co-workers for the preparation of β-trifluoromethyl-α-aryl ketones
(Scheme 1b).[8b] Alternatively, a metal-free protocol for the direct
Abstract: Visible-light-enabled photocatalytic carbotrifluoro-
aryltrifluoromethylation of allylic alcohols with CF3SO2Na was
disclosed by Yang and Xia for β-trifluoromethyl-α-aryl ketones
synthesis by using (NH4)2S2O8 as the oxidant (Scheme 1c). However,
the established protocols[8] generally suffer from drawbacks such as
the employment of transition metal catalysts, as well as unstable and
expensive trifluoromethyl sources or/and strong oxidants. Therefore,
a strong need remains for uncovering new and straightforward
methods for the facile generation of β-trifluoromethyl-α-aryl ketones
via visible-light-enabled photocatalytic radical 1,2-migration, with
simpler substrate structures under transition-metal-free conditions.
methylations of allylic alcohols and sodium triflinate were explored
through 1,2-migration of an aryl group, affording an efficient
method for synthesis of β-trifluoromethyl-α-substituted carbonyl
compounds under mild reaction conditions. This catalyst system is
well tolerant of various synthetically useful functional groups.
Keywords: Photocatalysis; visible light; carbotrifluoromethylation;
trifluoromethyl radical; Langlois reagent; 1,2-aryl migration
Incorporation of the trifluoromethyl (CF3) group into organic
molecules has become an increasingly crucial subject in modern
organic synthetic chemistry because it is capable of improving the
biological properties of small molecules through increasing their
metabolic stability, elevated electronegativity, and lipophilicity.[1]
Consequently, an explosion of research efforts has been triggered to
develop efficient methods for the introduction of the trifluoromethyl
group into small molecules.[2-3] Many effective strategies have been
+
-
developed, leveraging various “CF3 ” or “CF3 ” reagents such as
Umemoto’s reagent, and Togni’s reagent, etc.[4] However, these
established methods generally restrict their practical usage due to
the high cost and/or scarce availability of these trifluoromethylation
reagents. As a response to this problem, it is more appealing to take
advantage of CF3SO2Na (Langlois reagent), which is a stable,
inexpensive, and easy-to-handle trifluoromethyl source, to develop
new and efficient catalytic variants for trifluoromethylation of
alkenes under mild reaction conditions.
In recent years, photoredox catalysis[5,6] enabled by visible light
irradiation has been demonstrated to be a powerful synthetic
protocol to produce trifluoromethyl radical species. In particular,
utilizing this strategy, a number of trifluoromethylations of C=C
bonds have been successfully established with the concomitant
formation of C–N, C–O, C–X and C–S bonds employing various
Scheme 1. Synthetic routes to β-trifluoromethyl-α-aryl ketones
Owing to our sustaining research interest in the design and
discovery of robust synthetic reactions relying on visible-light-
enabled photoredox catalysis,[9] we disclose herein our recent
achievement in radical carbotrifluoromethylation of allylic alcohols,
which illustrates an approach for the convenient preparation of β-
trifluoromethyl-α-aryl ketones using CF3SO2Na as the
trifluoromethyl source (Scheme 1c). The event of radical abstraction
CF3 reagents, affording
a facile approach to a variety of
synthetically useful and biologically significant CF3-containing
frameworks.[7]
In 2013, Wu and co-workers reported an inspirational β-
trifluoromethyl-α-aryl ketones synthesis via copper-catalyzed
trifluoromethylation-triggered radical 1,2-migration of an aryl group
in α,α-diaryl allylic alcohols (Scheme 1a).[8a] Subsequently, other
transition-metal catalysts were also demonstrated to be effective in
similar scenarios. Fe(OAc)2-catalyzed carbotrifluoromethylation of
allylic alcohols and Togni’s reagent was discovered by Sodeoka and
might be triggered on CF3SO2Na [Eox = +1.05 V][6i] under the
1/2
synergistic actions of an organic fluorophores 1,2,3,5-
tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) [Ered1/2(*P/P-)
= +1.35 V][10] and visible light irradiation, thereby leading to the
formation of trifluoromethyl radical species. Subsequent of radical
1
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10.1002/adsc.201800726
Advanced Synthesis & Catalysis
addition and 1,2-aryl migration would take place to yield β-
trifluoromethyl-α-aryl ketones.
practically simple source of oxidant. Fortunately, a satisfactory 86%
yield of desired product was obtained by increasing the loading of
2a (1.8 equiv) (entry 10). Further increasing the amount of 2a was
found to be detrimental for the product yield (entry 11). The choice
of a photocatalyst was proven to be extremely significant, since the
use of eosin Y, rose bengal, Ru(bpy)3Cl2, Ir(ppy)2(dtbpy)PF6 (Ir-cat.
1), or even Ir[dF(CF3)ppy]2(dtbpy)PF6 (Ir-cat. 2) only resulted in
critical reductions or complete inhibitions in reactivities (entries 13-
16). Finally, reactions carried out under the absence of either visible
light irradiation or a photocatalyst confirmed definitely that the
reproducible synthesis of 3a requires both (entries 17 and 18).
Table 1. Screenings of reaction conditions
b
c
aIrradiation with white LED. Irradiation with blue LED. Reaction
d
e
mixture was degassed. [Ir-cat.1] = Ir(ppy)2(dtbpy)PF6. [Ir-cat. 2]
f
g
= [Ir[dF(CF3)ppy]2(dtbbpy)]PF6. No light was employed. Yield of
isolated product. h4CzIPN = 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-
dicyanobenzene.
Allylic alcohol 1a and sodium triflinate 2 were employed as the
model substrates for screening the optimal reaction conditions
(Table 1). Eventual success demonstrated that the capacity to deliver
the β-trifluoromethyl-α-aryl ketones in synthetically meaningful
efficiency relies heavily on the synergistic effects of photocatalyst,
light source, and solvent. Using a catalytic amount of eosin Y as
photocatalyst under white light emitting diode (LED) irradiation, no
desired product was detected when the reaction was performed in
MeCN under a balloon-nitrogen atmosphere at room temperature
(entry 1, Table 1). Fortunately, the desired β-trifluoromethyl-α-aryl
ketone 3a was obtained in 10% isolated yield by employing 1,2,3,5-
tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) [Ered1/2(*P/P-)
= +1.35 V, Eox1/2(P/P-) = -1.21 V vs. SCE],[10] in place of eosin Y
(entry 2). Under otherwise identical reaction conditions, a simple
switch of the light source white LED to blue LED exhibited great
improvement of the reactivity (41% isolated yield, entry 3). A
Scheme 2. Reactivity screenings on symmetrical allylic alcohols
A range of symmetrical α,α-diaryl allylic alcohols 1 were next
exposed to the above-identified optimal reaction conditions; the
results were summarized in Scheme 2. The substitution patterns on
the aryl rings appeared to have posed little effect on the reaction
efficiency. Substrates 1 bearing a para- and a meta-methyl
substituent on the aromatic ring proceeded smoothly to furnish the
corresponding products 3b and 3c in 84 and 71% isolated yields,
respectively. Both electron-donating (3d) and electron-withdrawing
(3e-3h) substituents on the aromatic ring were well tolerated, and
the desired products were generated in good-to-excellent yield. It
should be noted that β-trifluoromethyl ketones (3i and 3j) containing
a quaternary carbon center could also be obtained by this method.
However, pentafluorophenyl 1k was found to be almost inert in our
conditions. The conformational effects were demonstrated to have a
vital influence on the reactivity, in which complicated mixtures
were detectable with the substrate having a tricyclic moiety (1l).
To expand the substrate scope of this method further, a variety of
number
of
solvents,
including
methanol
(MeOH),
dimethylformamide (DMF), N-methylpyrrolidone (NMP), 1,2-
dichloroethane (DCE), or dichloromethane (DCM) were next
screened, from which the DCE stood out for its remarkable capacity
to afford the desired product in moderate isolated yield (65%, entry
7). These results indicated that the trace amout of oxygen in the
reaction system might serve as the oxidant in this migration. It
merits attention that, in some of the above cases, accompanying 3a
was the formation of benzophenone as the major byproduct, arising
possibly from the excess oxygen in the reaction system promoted
cleavage of the C=C double bond of 1a.[11] Indeed, when the
reaction mixture was purged carefully with a stream of nitrogen
(with 0.5 mol% of oxygen) for half an hour, we were pleased to find
that 3a could be obtained in an improved yield (79%, entry 9), thus
suggesting that 0.5 mol% of oxygen in nitrogen was chosen as a
structurally variable, unsymmetrical allylic alcohols
4 next
attempted to react with Langlois reagent 2. As exemplified in
Scheme 3, the reactivity seems to be general, smoothly producing
the corresponding β-trifluoromethyl-α-aryl ketones 5 in good-to-
excellent isolated yields (66-90%) with high chemoselectivity in
each case. Substrates possessing electron-donating groups, e.g.,
methyl (4a and 4b), methoxy (4c), and phenyl (4d) groups on the
aromatic ring, afforded the corresponding products with the
2
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10.1002/adsc.201800726
Advanced Synthesis & Catalysis
migration of the phenyl group in a preferential manner. When
unsymmetrical allylic alcohols
4 having electron-withdrawing
substituents on the aromatic ring were employed as the substrates,
the more electron-deficient aryl group was found to migrate
preferentially to generate the β-trifluoromethyl-α-aryl ketones 5e-5i
in each case. The results, in conjunction with the previous reports,[8]
strongly suggested that radical intermediates should be involved in
this migration. It is interesting that substitution patterns on the aryl
rings in unsymmetrical allylic
Scheme 4. Control experiments
To elucidate the potential reaction pathways, several control
experiments were performed (see the Supporting Information), and
some results are shown in Scheme 4. When α,α-dicyclohexyl allylic
alcohol 1m was exposed to the standard reaction conditions to react
with Langlois reagent 2, no desired β-trifluoromethyl-α-cyclohexyl
ketone 3m was yielded at all. Next, 2,2,6,6-tetramethyl-1-
piperidinyloxy (TEMPO) was consciously added into the reaction
mixture as a radical scavenger, and complete inhibition in reactivity
was observed. These collected results also supported that the
reaction should be a radical pathway.12 Interestingly, when the
reaction was performed in the presence of a ballooned O2
atmosphere under otherwise identical reaction conditions,
benzophenone 5 was furnished in excellent isolated yield, but no
migratory product 3a was observed.[13] Finally, to identify the effect
of photostimulation, we explored on-off switching of the light
source in the reaction of 1f with CF3SO2Na under standard reaction
conditions, in which a full consumption of 1f required continuous
irradiation of visible light, thus suggesting that a radical chain
process might be ruled out in this migration.[14]
Scheme 3. Reactivity screenings on unsymmetrical allylic alcohols 4
alcohols 4 were demonstrated to be significant. When the substrate
4j, which carries 2-fluorophenyl and 4-fluorophenyl groups, was
subjected to the standard reaction conditions, two inseparable
isomers were obtained in a 1:1.6 ratio and 86% yield. Notably, a
switch of a 4-fluorophenyl group to a 4-chlorophenyl group in the
case of 4k was found to improve the chemoselectivity dramatically,
from which migration of 4-chlorophenyl ring was faster than that of
the 2-fluorophenyl group. Furthermore, preferential migration of a
para-substituted aryl ring was also recorded in the cases of 5l-5q,
regardless of whether the ortho- or meta-substituent was electron-
withdrawing or electron-donating. It is worth noting that the
reactions were also successful with α-heterocyclic or alkyl-
substituted allylic alcohols; the corresponding β-trifluoromethyl-α-
aryl ketones (5r-5u) were thus yielded in 66-74% yield.
Scheme 5. Proposed mechanistic network
A plausible mechanistic network enabled by these observations is
illustrated in Scheme 5. Upon exposure to visible light irradiation,
sulfinate anion 2 would be reductively quenched by the excited-state
species 4CzIPN to generate SO2 and
a relatively stable
trifluoromethyl radical, which should react with alkene 1a to give
rise to the key radical A.[8] Subsequent 1,2-aryl migration via
spiro[2,5]octadienyl[8a] radical B yields the radical intermediate C.
Finally, intermediate C would undergo a sequence of oxidation and
deprotonation to deliver the final β-trifluoromethyl-α-aryl ketone
product, 3a.
In conclusion, by subjecting a series of α,α-diaryl allylic alcohols
and Langlois reagent to visible-light photoredox catalysis under
mild reaction conditions, we disclosed a novel technology for β-
3
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10.1002/adsc.201800726
Advanced Synthesis & Catalysis
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trifluoromethyl-α-aryl ketones synthesis through photocatalytic
radical 1,2-aryl migration. This catalyst system is tolerant of various
synthetically useful functional groups, including bromo, chloro,
fluoro, trifluoromethoxy, methoxy, and phenyl. Compared with the
previous established methods, this method, not requiring any strong
oxidants and transition-metal catalysts and using easily accessible
and relatively low cost starting materials, adds further to its
applications in organic synthesis.
Experimental Section
A flame-dried round bottom flask (25 mL) was equipped with
magnetic stir bar and charged with α,α-diaryl allylic alcohols 1
(0.145 mmol, 1.0 equiv), CF3SO2Na 2 (0.261 mmol, 1.8 equiv),
1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene
(4CzIPN)
(0.0029 mmol, 0.02 equiv), and DCE (7.5 mL). The reaction
mixture was degassed carefully by purging thoroughly with nitrogen
(with 0.5 mol % of oxygen) for half an hour, then irradiated by blue
LED (18 W) under a balloon nitrogen (with 0.5 mol % of oxygen)
atmosphere at room temperature until the starting material
disappeared from the TLC. After that, the reaction mixture was
directly concentrated under reduced pressure and the crude product
was purified by silica gel column chromatography to afford the
desired product.
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Acknowledgments
We thank the National Natural Science Foundation of China (No.
21502086, and No. 41575118), Natural Science Foundation of
Fujian Province (No. 2015J05028), Outstanding Youth Science
Foundation of Fujian Province (No. 2015J06009), and Program for
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10.1002/adsc.201800726
Advanced Synthesis & Catalysis
Graphical Abstract
5
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