The Journal of Organic Chemistry
Article
Scheme 7. Proposed Photocyclization Reaction Mechanism of 1d in ACN
dimethoxybenzoate was obtained.22 The methyl 3,5-dimethoxyben-
zoate was used for the next reaction. Then, to a dry flask containing a
solution of acetophenone (0.6 g, 5.1 mmol) and methyl 3,5-
dimethoxybenzoate (1.0 g, 5.1 mmol) in THF (30 mL) was added
quickly 60% sodium hydride (0.2 g, 5.0 mmol). The reaction mixture
was heated under argon at 60 °C for 72 h. The solution was then
acidified with dilute HCl and extracted with CH2Cl2. After removal of
solvent, the pure product 1-(3,5-dimethoxyphenyl)-3-phenylpropan-
1,3-dione was obtained as a yellow solid over a silica gel column.22
Finally, after the 1-(3,5-dimethoxyphenyl)-3-phenylpropan-1,3-dione
(1.136 g, 4.0 mmol) was dissolved in ACN (50 mL), LiClO4 (0.260 g,
2.44 mmol) was added dropwise over a period of 10 min. Then, NCS
(0.59 g, 4.4 mmol) was added and stirring for an additional 3 h. The
solvent was evaporated in a vacuum. The rest was dissolved in CH2Cl2
(50 mL) and washed twice with H2O (25 mL). The organic phase
was separated and dried with anhydrous Na2SO4. After evaporation of
solvent, the crude product was obtained. Crystallization was done
from the mixture of CH2Cl2 and hexane, and pure 1b was obtained as
a yellow crystalline solid (920 mg, 71.9% yield).12 1H NMR (CDCl3,
400 MHz): δ −8.01−7.96 (m, 2H), 7.63−7.56 (m, 1H), 7.50−7.43
(m, 2H), 7.11 (d, 2H, J = 2.3 Hz), 6.67 (t, 1H, J = 2.3 Hz), 6.39 (s,
1H), 3.79 (s, 6H). 13C{1H} NMR (CDCl3, 100 Hz): δ −189.3, 189.1,
161.2, 135.7, 134.5, 133.9, 129.4, 129.1, 107.0, 106.9, 62.9, 55.8.
Synthesis of 2-Chloro-2-fluoro-1,3-diphenylpropan-1,3-
dione (1c). To a stirred solution of dibenzoylmethane (1.13 g, 5
mmol) in ACN (100 mL) at room temperature was added 1-
chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis-
(tetrafluoroborate) (F-TEDA-BF4) (3.40 g, 10 mmol). The mixture
was stirred for 3.5 h. The solvent was evaporated in a vacuum, and to
the rest, CH2Cl2 (50 mL) was added and washed with saturated
NaHCO3 solution and H2O. The organic phase was separated and
dried with anhydrous Na2SO4, the solvent was evaporated in a
vacuum, and the crude product was obtained. Crystallization was
done from the mixture of CH2Cl2 and hexane, and pure 2-fluoro-1,3-
diphenylpropan-1,3-dione was isolated.23 The pure 2-fluoro-1,3-
diphenylpropan-1,3-dione was used for the next reaction. After the
2-fluoro-1,3-diphenylpropan-1,3-dione (968 mg, 4.0 mmol) was
dissolved in ethyl acetate (80 mL), LiClO4 (254.4 mg, 2.4 mmol)
was added at room temperature. After 5 min of stirring at room
temperature, NCS (587.4 mg, 4.4 mmol) was added. The stirring was
continued for an additional 1.5 h. The reaction mixture was washed
with water. The organic phase was separated and dried with
anhydrous Na2SO4. After evaporation of the solvent, the crude
product was obtained. Recrystallization from hexane furnished 1c as a
pale yellow solid (894 mg, 80.5% yield).12 1H NMR (CDCl3, 400
MHz): δ −8.06−7.97 (m, 4H), 7.67−7.58 (m, 2H), 7.52−7.42 (m,
4H). 13C{1H} NMR (CDCl3, 100 Hz): δ −186.7, 186.5, 134.8, 131.5,
131.4, 130.6, 130.5, 129.0.
methoxyphenyl)propan-1,3-dione (533 mg, 1.76 mmol) was dissolved
in ACN (35 mL), LiClO4 (113 mg, 1.06 mmol) was added at room
temperature. After 5 min of stirring at room temperature, NCS (258
mg, 1.93 mmol) was added. The stirring was continued for an
additional 3 h. The solvent was evaporated in a vacuum; the rest was
dissolved in CH2Cl2 (35 mL) and washed with water. The organic
phase was separated and dried with anhydrous Na2SO4. Purification
was done by column chromatography using SiO2 for the stationary
phase and CH2Cl2 for elution, and pure 1d was obtained as a white
crystalline solid (458 mg, 77% yield).12 1H NMR (CDCl3, 400 MHz):
δ −8.02 (d, 4H, J = 8.0 Hz), 6.91 (d, 4H, J = 8.0 Hz), 3.86 (s, 6H).
13C{1H} NMR (CDCl3, 100 Hz): δ −185.2, 185.0, 164.8, 133.2,
133.1, 124.4, 114.3, 55.7.
For characterization of 1a, 1b, 1c, and 1d, please see the 1H NMR
2,2,6,6-Tetramethylpiperidine-N-oxyl (TEMPO) used as the
radical inhibitor was added to the sample solutions to prepare
photolysis solutions. The concentrations of TEMPO were 5, 10, 15,
20, and 25 equiv of samples. Sample solutions (1c (0.1 mM), 1b (0.05
mM), and 1d (0.02 mM)) and sample solutions added with TEMPO
in quartz vessels during irradiation in a Rayonet (RPR 100)
photochemical reactor using 254 nm lamps. The photolysis process
was monitored by measuring the UV−vis absorbance.
The fs-TA experiments use a commercial regenerative amplified
Ti:sapphire laser system and an automated data acquisition system.
The instrument response function was determined to be about 150 fs.
For the present measurements, sample solutions were excited by a 266
nm pump beam (the third harmonic is 800 nm, and the regenerative
amplifier is the fundamental wave) and probed by a white-light
continuum probe beam (330−800 nm) produced from a CaF2 crystal.
A 40 mL sample solution with an optical density of unity at 266 nm
was flowed through a 2 mm path-length cuvette throughout the data
acquisition. The ns-TA experiments were recorded on a LP980 laser
flash photolysis spectrometer (Edinburgh Instruments, UK). The
solutions were purged with Ar for 30 min before measurements. More
detailed descriptions on the fs-TA and ns-TA experiments can be
found in ref 24.
The DFT computations employed the (U)M06-2X/6-311G**
level to find the optimized geometries and electronic absorption
spectra for the transients considered to be possible intermediates for
the reactions. No imaginary frequency mode was observed at the
stationary states of the optimized structures. The effect of solvent
polarity was estimated by utilizing the polarizable continuum model
(PCM) in ACN.25 The DFT26 and TD-DFT27 calculations presented
in this study were carried out using the Gaussian 09 program suite.28
Synthesis of 2-Chloro-2-fluoro-1,3-di(4-methoxyphenyl)-
propan-1,3-dione (1d). To a 50 mL flask were added 1,3-bis(4-
dimethoxyphenyil)propan-1,3-dione (0.71 g, 2.5 mmol), F-TEDA-
BF4 (0.85 g, 2.5 mmol), and ACN (25 mL). The reaction mixture was
stirred at room temperature for 20 h. The solvent was evaporated in a
vacuum, and the CH2Cl2 (50 mL) was added and washed with
saturated NaHCO3 solution and H2O. The organic phase was
separated and dried with anhydrous Na2SO4, the solvent was
evaporated in a vacuum, and the crude product was obtained.
Crystallization was done from the mixture of CH2Cl2 and hexane, and
pure 2-fluoro-1,3-bis(4-methoxyphenyl)propan-1,3-dione was iso-
lated.23 The pure 2-fluoro-1,3-bis(4-methoxyphenyl)propan-1,3-
dione was used for the next reaction. After the 2-fluoro-1,3-bis(4-
ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
Synthetic route of compounds with the 1H NMR spectra
discussed in the text, fs-TA and ns-TA spectra for some
of the intermediate species, DFT calculated electronic
absorption spectra, and the Cartesian coordinates and
energies for all calculated structures discussed in the text
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J. Org. Chem. 2021, 86, 7594−7602