UV light-mediated difunctionalization of alkenes with CF3SO2Na: Synthesis of trifluoromethyl phenanthrene and anthrone derivatives

Article abstract of DOI:10.1039/c6ob00912c

A metal-free and cost-effective protocol for UV light-mediated difunctionalization of alkenes with CF₃SO₂Na was developed. This strategy realized the direct formation of Csp³-CF₃ and C-C bonds through a proposed

Full text of DOI:10.1039/c6ob00912c

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UV Light-Mediated Difunctionalization of Alkenes with CF3SO2Na:
Synthesis of Trifluoromethyl Phenanthrene and Antrone
Derivatives
Received 00th January 20xx,
Accepted 00th January 20xx
Bing Li, Dan Fan, Chao Yang, Wujiong Xia*
A metal-free and cost-effective protocol of UV light-mediated difunctionalization of alkenes with CF3SO2Na was developed.
This strategy realized the direct formation of Csp3-CF3 and C-C bonds through a proposed tandem radical cyclization
DOI: 10.1039/x0xx00000x
www.rsc.org/
process, which produced
a
variety of phenanthrene and anthrone derivatives in moderate yields.
Introduction
The trilfluoromethyl group plays
a
significant role in
R2
R2
CF3
pharmaceuticals and agrochemicals because its introduction into
drug candidates could strengthen chemical and metabolic stability,
electronegativity, ameliorate lipophilicity, bioavailability, and
enhance binding selectivity.1,2 In recent years, a number of synthetic
methods for CꢀCF3 bond formation have been constantly developed.3
R1
R1
O
OH
hv CF3
F3C
5
These strategies can be classified into CspꢀCF3,4 Csp2ꢀCF3 and
6
Csp3ꢀCF3 bond construction according to trifluoromethylation
R
R
Ar
Ar
reaction of hydrocarbons. As the formation of Csp3ꢀCF3 bonds was
relatively limited, the difunctionalization of alkenes has arose as a
highly attractive strategy.7 However, most of these reactions go
through by transitionꢀmetalꢀmediates, such as palladiumꢀ and
copperꢀcatalyzed trifluoromethylation, which tend to require
expensive catalysts and trifluoromethyl reagents, as well as high
O
O
Scheme 1 Trifluoromethylation with CF3SO2Na via
UV light irradication
temperatures.8ꢀ10 Therefore, development of an environmental (Table 1) as the model substrate, separately irradiated with the CF3
friendly process is appealing for the trifluoromethylation area.
reagents of CF3SO2Na, CF3SO2Cl, CF3I and Tognis’ reagent in dried
In the realm of organic synthesis, synthetic methodologies, and MeCN by a 500W medium pressure lamp through a Pyrex filter, but
radical chemistry,11 organic photochemical research is one of most no reaction occurred. Then we added benzophenone (BP) as a
important branch. Based on our previous work on the photochemical sensitizer, to our delight the product 3a was obtained in 54% yield
behaviour of radicals12 and the studies on radical by detection on GCꢀMS (Table 1, entry 1) in the system of
trifluoromethylation,13 we tried to realize the trifluoromethylation of CF3SO2Na. Such a result promoted us to further study of this
alkenes under photochemical condition. Successfully, we found that reaction. Then we tried other sensitizers, such as anthraceneꢀ9,10ꢀ
the inexpensive and stable solid CF3SO2Na (Langlois’ reagent) could dione (AQ), benzoquinone (BQ), terephalonitrile (DCB), benzeneꢀ
.
generate CF3 via UVꢀlight in the presence of sensitizer and smoothly 1,2,4,5ꢀteracarbonitrile (TCB), naphthaleneꢀ1,4ꢀdicabonitrile (DCN),
carry out difunctionalization of alkenes followed by C−C bond and andanthraceneꢀ9,10ꢀdicarbonitrle (DCA), which led to lower
formation.14 Herein we report our recent advances on yields (Table 1, entries 2ꢀ7). A solvent screening revealed that other
trifluoromethylation of aromatic unsaturated ketones (Scheme 1).
solvents did not or only sluggishly promote this reaction (Table 1,
entries 8−12). Other wavelength, such as 300 nm or 350 nm gave the
desired product in 21% and 25% yields, respectively (Table 1,
entries 13 and 14). When the reaction proceeded in dried solvent in
air, the complicated products were obtained. In addition, the reaction
conducted in undried MeCN under N2 atmosphere led to 21% yield
(Table 1, entry 15). Finally, the reaction condition was optimized to
use a 500 W medium mercury lamp as the light source, 2 equiv of
CF3SO2Na, 0.2 equiv of BP as the sensitizer, and dried MeCN as the
solvent.
Results and discussion
Originally, we chose the aromatic α,βꢀunsaturated ketone 1a
State Key Lab of Urban Water Resource and Environment, School of Chemistry
and Chemical Engineering, Harbin Institute of Technology, Harbin, 150080, China
E-mail: xiawj@hit.edu.cn
† Electronic Supplementary Informaꢀon (ESI) available: Experimental procedures
and 1H and 13C NMR spectra for all new compounds. See DOI: 10.1039/x0xx00000x
We therefore prepared a series of aromatic α,βꢀunsaturated
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Table 1 Screening for optimal reaction conditiona
Journal Name
Table 2 scope of α,β-unsaturated ketones 1a
entry
1
Sens.
BP
hv
Solvent
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
Yield [%]b
54
280 nm
280 nm
280 nm
280 nm
280 nm
280 nm
280 nm
280 nm
280 nm
280 nm
280 nm
280 nm
350 nm
300 nm
280 nm
2
AQ
BQ
DCB
TCB
DCN
DCA
BP
13
3
32
4
trace
37
5
6
26
7
10
8
trace
trace
trace
trace
trace
25
9
BP
DMSO
toluene
DCM
10
11
12
13
14
15 c
BP
BP
BP
THF
BP
MeCN
MeCN
MeCN
BP
21
BP
21
a Reaction conditions: 1a (0.2 mmol), 2 (0.4mmol), Sens. (0.04 mmol), hv,
solvent (anhydrous, 10 mL) under N2 atmosphere. b Yields were detected by
GCꢀMS. c Undried MeCN
aReaction condition: 1 (0.2 mmol), 2 (0.4 mmol), BP (0.04 mmol), hv (280
ketones to investigate the scope of the reaction. As shown in Table 2,
the reactions were completed within 4ꢀ10 hrs to form the desired
products in 30ꢀ70% yield. The substrates bearing electronꢀ
withdrawing or ꢀdonating groups at the metaꢀ and paraꢀpositions of
R1 were tolerant of the reaction conditions to provide the desired
products (3aꢀ3d, 3jꢀ3o). The electronꢀwithdrawing groups of R2 led
to better yield within shorter reaction time, e.g. 70% yield for R2 = F
(3h). Otherwise, when the metaꢀposition of R2 was methoxyl, the
product of 3q afforded the isomers with a ratio of 2:1. In addition, a
thiophene group was also tolerated under the optimal conditions to
furnish the corresponding product in acceptable yield (3p). However,
no desired product was observed when the hydrogen at terminal
position of CꢀC double bond of the alkene was replaced by a phenyl
group, only the cisꢀisomer was isolated.
To further investigate the applicability of this protocol, aromatic
γ,δꢀunsaturated ketone 4a (Table 3) was therefore prepared and
subjected to the reaction conditions. We envisioned that, the addition
of generated CF3 radical to the double bond of 4a followed by a
subsequential intramolecular radical cyclization might lead to the
formation of anthrone derivatives. It was nice to see that the desired
product 5a was obtained in 23% yield after 70 minutes. When the
sensitizer was replaced by anthraceneꢀ9,10ꢀdione (AQ), the yield
was improved to 55%. Then a variety of substituted aromatic γ,δꢀ
unsaturated ketones were prepared and submitted to the standard
reaction conditions using AQ as the sensitizer, and the results were
summarized in Table 3. The reactions were completed at room
temperature within 70 minutes to form the desired products in 35ꢀ
55% yield. The electronic effect on the pendent aryl group has no
nm), MeCN (anhydrous 10 mL) under N2 atmosphere. bIsolated yield.
significant influence on the results of the reactions, such as the aryl
group substituted with electronꢀwithdrawing or ꢀdonating groups,
including methyl, F, Cl, Br, tꢀBu, and methoxy were smoothly
converted to the corresponding products in moderate yields.
Moreover, although two orthoꢀpositions of the Ar group were
available for reaction, only one isomer was obtained possibly due to
the influence of election cloud density (5gꢀ5k). When a hydrogen
atom at the end of double bond was replaced by a methyl group, the
desired product 5m could be isolated in 51% yield. However, when
the methyl group connected to the CꢀC double bond of the
trisubstituted alkene was replaced by a hydrogen atom, no main
product was obtained. Importantly, the desired product could also be
prepared when Ar was naphthalene (5l), pyridine (5n) or thiophene
(5o). It should be noted that it was indispensable for the substrates
furnished with two aryl or conjugated groups, for example, when Ar
was replaced by an isoꢀbutyl group, no trifluoromethylation
occurred. In addition, low conversion and yield were obtained when
the substrate was furnished without a carbonyl group even if longer
time was involved (5r).
Before a reaction mechanism was proposed, a control experiment
of 1a in the presence of 2.2 equiv of TEMPO15 was carried out under
the standard conditions for 4 h, TEMPOCF3 (M = 225 g/mol) and
only trace amount of product 3b was detected based upon GCꢀMS
analysis. Such a result suggested that the reaction involves a radical
pathway. Accordingly, a plausible mechanism of this reaction is
depicted in scheme 2.16,9c The CF3 radical, which was generated
from CF3SO2Na was added to the CꢀC double bond of 1 to form Cꢀ
2 | J. Name., 2012, 00, 1-3
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Table 3 Scope of aromatic γ,δ-unsaturated ketones 4a
F3C
R
S
cheme 2 Proposed mechanism
O
various CF3ꢀcontaining phenanthrene and anthrone derivatives via
cascade trifluoromethyl radical addition. The reaction was
highlighted by operational simplicity, excellent functional group
tolerance, and a wide range of compatible substrates.
5a, R = H, 55%
5b, R = F, 50%
5c, R = Me, 48%
5d, R = Br, 40%
5e, R = t-Bu, 47%
5f, R = OMe, 45%
Experimental
General Procedure
General procedure for the photoꢀinduced trifluotomethylation of 1:
In a Pyrex filter, the magnetic stir bar was added. Then it was
charged with substrate 1 (0.2 mmol), CF3SO2Na (0.4 mmol), dried
MeCN (10 mL) and sensitizer BP (0.04 mmol). The mixture was
charged with N2 for 30 minutes under room temperature and then
irradiated by UV lamp (280 nm). After the substrate was consumed
(monitored by TLC), the mixture was filtered and concentrated in
vacuo. The residue was purified by column chromatography on silica
gel (petroleum ether/ ethyl acetate) to afford the desired product 3.
10ꢀ(2,2,2ꢀtrifluoroethyl) phenanthrenꢀ9ꢀol (3a): White solid. 1H
NMR (400 MHz, CDCl3): δ 8.71 (d, J = 8.4 Hz, 1H), 8.68 (d, J = 8.4
Hz, 1H), 8.17 (d, J = 7.6 Hz, 1H), 7.96 (d, J = 8.4 Hz, 1H), 7.74ꢀ7.63
(m, 3H), 7.57 (t, J = 7.6 Hz, 1H), 5.47 (s, 1H), 4.02 (q, J = 10.8 Hz,
2H); 13C NMR (151 MHz, CDCl3): δ 148.8, 131.8, 131.4, 127.7,
127.5, 127.1, 126.9, 126.7 (q. J = 278.7 Hz), 125.2, 124.7, 123.6,
123.1, 123.0, 121.4, 106.7, 30.4 (q, J = 30.2 Hz); 19F NMR (376
MHz, DMSO): δ ꢀ62.2 (s, decoupled, 3F); GCꢀMS (EI): 276, 256,
236, 207, 179, 152, 118, 89, HRMS (ESI): [M+H]+ calcd. for
C16H12F3O+: 277.0835, found: 277.0835.
aReaction condition: 3 (0.1 mmol), 2 (0.2 mmol), AQ (0.02mmol), hv (280
nm), MeCN (anhydrous, 5 mL) under N2 atmosphere, 70 mins. bIsolated
yield.
centered radical A (blue). Subsequently, the radical attacked the
orthoꢀposition of benzene to produce the intermediate B. Then
enolization of ketone to afford the intermediate C. The final product
3 was formed after loss of a hydrogen radical. Similarly, the CF3
radical was added to the CꢀC double bond of 4 to form the
intermediate D (black). Subsequently, a radical addition the aromatic
ring occurred to perform the intermediate E, which donated a
hydrogen radical rapidly to give the final product 5.
Acknowledgements
We are grateful for the financial supports from China NSFC (Nos.
21272047, 21372055 and 21472030), SKLUWRE (No. 2015DX01),
the Fundamental Research Funds for the Central Universities (Grant
No. HIT.BRETIV.201310) and HLJNSF (B201406).
Notes and references
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Smart, J. C. Tatlow, Eds. Organofluorine chemistry:
Conclusions
In summary, we have developed an efficient, costꢀeffective, and
metalꢀfree protocol for the direct trifluoromethylation of alkenes
through UV light synthetic strategy using CF3SO2Na as the CF3
source. This approach provided a highly atomꢀeconomic access to
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