The Journal of Organic Chemistry
Article
known compounds and were prepared using previously reported
procedures.18a,35
Preparation of Substrates. Sulfonylketones 2 were prepared by
arylsulfination of the corresponding bromides. Preparations of 2d, 2e,
and 2g are described below. 2a,36 2b,37 2c,36 and 2f38 are known
compounds.
ratios are governed by the contact versus solvent separated
nature of radical ion intermediates. We found that photo-
excited BIH-NapOH promotes reactions of sulfonylphenones
under aerobic conditions that occur by competitive reduction
(forming alkylphenones) and oxidation (producing α-hydrox-
yphenones) through pathways involving the intermediacies of
ketone radical anions and α-ketoalkyl radicals. An assessment
was made to determine how the ratios of reduction and
oxidation products are influenced by several factors including
molecular oxygen concentration, solvent polarity, nature of
substituent on sulfonylketones, and structures of benzimidazo-
line donors. The results suggest that oxidation products are
produced by dissociation of α-ketoalkyl radicals from initially
formed, solvent-caged radical ion pairs followed by reaction
with molecular oxygen. In addition, the observations indicate
that reduction products are generated by PT in solvent-caged
benzimidazoline and sulfonylphenone-derived radical ion pairs.
Moreover, we proposed that arylsulfinate anions arising by
carbon-sulfur bond cleavage of sulfonylketone radical anions
act as reductants for oxidative conversion of initially formed α-
hydroperoxyketones to α-hydroxyketones. Finally, DFT
calculations were performed to explore the structures and
properties of radical ions and other intermediates generated
from BIH-NapOH as well as α-sulfonylketones. Although
numerous examples of PET reactions of carbonyl substrates
and amines have been reported, to the best of our knowledge,
the current investigation focusing on competitive reduction
and oxidation reactions taking place via radical intermediates
derived from PET processes under the aerobic conditions is
unique.21 In addition, because of the rising interest taking place
in applications of BIH-Ar and BIH-ArOH,10−16 the observa-
tion of a solvent-dependent nature of their PET reactivities is
important. Our continuing investigations of PET reactions
utilizing BIH-Ar and BIH-ArOH as photoreductants17−19 are
aimed at gaining a deeper understanding of reaction
mechanisms and at developing mechanism-based protocols
to control their PET reaction.
Synthesis of 2-Phenylsulfonyl-2-methyl-1-(4-methylphenyl)-1-
propanone (2d). A DMF (1.0 mL) solution containing 2-bromo-2-
methyl-1-(4-methylphenyl)-1-propanone (121 mg, 0.50 mmol) and
PhSO2Na·2H2O (201 mg, 1.0 mmol, 2.0 equiv) was purged with N2
and then stirred at room temperature for 17 h. The mixture was
diluted with water (30 mL) and extracted with EtOAc (20 mL × 3).
The combined extracts were washed with water (30 mL × 2), sat. aq
Na2S2O3 (30 mL), and brine (30 mL), dried over anhydrous MgSO4,
and concentrated in vacuo to give a white solid. The solid was washed
with water to give 2d (114 mg, 0.38 mmol, 76%); mp 82.0−84.0 °C;
1H NMR (400 MHz, DMSO-d6): δ 7.82−7.74 (m, 5H), 7.66 (t, J =
7.8 Hz, 2H), 7.30 (d, J = 8.0 Hz, 2H), 2.37 (s, 3H), 1.60 (s, 6H).
13C{1H} NMR (100 MHz, CDCl3) 198.5, 143.2, 135.9, 135.4, 134.9,
134.3, 130.6, 129.5, 129.0, 73.6, 22.8, 21.7; HRMS (ESI) m/z: calcd
for C17H18O3S [M + H]+, 303.1049; found, 303.1044.
Synthesis of 2-Phenylsulfonyl-2-methyl-1-(4-chlorophenyl)-1-
propanone (2e). A DMF (1.0 mL) solution containing 2-bromo-2-
methyl-1-(4-chlorophenyl)-1-propanone (131 mg, 0.50 mmol) and
PhSO2Na·2H2O (201 mg, 1.0 mmol, 2.0 equiv) was purged with N2
and then stirred at room temperature for 15 h. The mixture was
diluted with water (30 mL) and extracted with EtOAc (20 mL × 3).
The combined extracts were washed with water (30 mL × 2), sat. aq
Na2S2O3 (30 mL), and brine (30 mL), dried over anhydrous MgSO4,
and concentrated in vacuo to give a white solid. The solid was washed
with water to give 2e (114 mg, 0.35 mmol, 70%); mp 124.0−126.0
°C; 1H NMR (400 MHz, CDCl3): δ 7.95 (d, J = 8.4 Hz, 2H), 7.77 (t,
J = 7.6 Hz, 2H), 7.68 (t, J = 7.6 Hz, 1H), 7.54 (t, J = 7.2 Hz, 2H),
7.43 (d, J = 8.8 Hz, 2H), 1.69 (s, 6H). 13C{1H} NMR (100 MHz,
CDCl3) 198.0, 138.9, 135.9, 135.1, 134.4, 130.8, 130.5, 128.9, 128.7,
73.5, 22.8; HRMS (ESI) m/z: calcd for C16H15O3SCl [M + H]+,
323.0503; found, 323.0502.
Synthesis of 2-[(4-Methoxyphenyl)sulfonyl]isobutyrophenone
(2g). A mixture of α-bromo isobutyrophenone (114 mg, 0.50
mmol) and sodium 4-methoxybenzenesulfinate (194 mg, 1.0 mmol,
2.0 equiv) in DMF (1.5 mL) was stirred at room temperature for 19
h. The mixture was diluted with water (30 mL) and extracted with
EtOAc/hexane = 1/4 (20 mL × 3). The combined extracts were
washed with sat. Na2S2O3 aq (30 mL × 2) and brine (30 mL), dried
over anhydrous MgSO4, and concentrated in vacuo to give a colorless
oil. The oil was distilled (0.1 mmHg, 170 °C) to yield 2g (56 mg, 0.17
EXPERIMENTAL SECTION
■
1
General Methods. H and 13C{1H} NMR spectra were recorded
1
on CDCl3, DMSO-d6, and CD3CN with tetramethylsilane (Me4Si) as
mmol, 34%). H NMR (400 MHz, CDCl3): δ 7.95 (d, J = 7.2 Hz,
1
an internal standard at 400 MHz for H NMR and 100 MHz for 13C
2H), 7.71 (t, J = 8.8 Hz, 2H), 7.53 (t, J = 7.4 Hz, 1H), 7.44 (t, J = 7.6
Hz, 1H), 6.98 (d, J = 8.8 Hz, 2H), 3.38 (s, 3H), 1.69 (s, 6H).
13C{1H} NMR (100 MHz, CDCl3) 199.8, 164.3, 137.9, 132.7, 132.1,
129.0, 128.32, 126.5, 114.1, 73.4, 55.8, 22.8; HRMS (ESI) m/z: calcd
for C17H18O4S [M + H]+, 319.0999; found, 319.0995.
NMR. High-resolution mass spectra (HRMS) were recorded on an
electrospray ionization (ESI) Orbitrap spectrometer. Uncorrected
melting points were reported. Reduction potentials in MeCN were
measured using cyclic voltammetry and a previously described
procedure.34 Conversion of the measured potentials (V vs Ag/
AgNO3) to the reported potentials (V vs SCE) was performed by
using the formal potential of ferrocene/ferrocenium couple (0.439 V
vs SCE) and the difference (0.373 V) of the potentials between Ag/
AgNO3 and SCE. Half-wave reduction potentials (Er1e/d2) reported in
the manuscript were obtained from peak potentials by adding 0.029 V.
The light source for photoreactions was a 7.3 W white LED (Toshiba
bulb-type LED light: LDA7N-G-K). Column chromatography was
performed with silica gel. Anhydrous solvents for reactions were
obtained as follows. THF was distilled over sodium benzophenone
under N2. CH2Cl2 and PhCH3 were purified in the same manner by
the treatment with H2SO4, water, 5% NaOH, water, and CaCl2 and
then distilled over CaH2. MeCN was distilled over P2O5 and
subsequently distilled with K2CO3. Anhydrous DMF, DMSO, MeOH,
EtOH, and iPrOH were purchased and used without distillation.
Other reagents and solvents were used without further purification.
Preparation of Benzimidazolines. 1,3-Dimethylbenzimidazo-
line derivatives 1a,18a 1b,35 1c,35 1d,19b 1e,19b 1f,18a and 1g9a are
Photoreaction Procedure. A general procedure for reactions
under air is described below (for Tables 1, 3, 4, and 7). A solution of
1 (0.12 mmol) and 2 (0.10 mmol) in a solvent (1.0 mL) in a Pyrex
test tube (1.4 cm diameter) fitted with a drying tube containing CaCl2
is irradiated with a 7.3 W white LED at room temperature (the
distance between the test tube and the LED lamp is approximately 2.0
cm). In the photoreaction of 2 with 1 under O2 (see entries 1, 4 and 7
in Table 1) or N2 (see entries 3, 6 and 9 in Table 1), the solution is
purged with O2 or N2 for 10 min and then irradiated. The photolysate
is diluted with water (30 mL) and extracted with CH2Cl2 or EtOAc
(20 mL × 3). The combined extract is washed with water (30 mL ×
2) and brine (30 mL), dried over anhydrous MgSO4, and
concentrated in vacuo to give a residue. The conversion of 2 and
1
the yield of products are determined using H NMR analysis of the
residue with triphenylmethane as an internal reference in CDCl3.
Ketones 3b,39 3d,40 and 3e40 and hydroxyketones 4b,41 4d,42 and
4e42 are known compounds, while 3a, 3c, and 4a are commercial
materials. Ketone 3a and hydroxyketone 4a are produced from the
2564
J. Org. Chem. 2021, 86, 2556−2569