Anodic methoxylation of 2-acyloxy-3,3,3-trifluoropropyl sulfides accompanying with [1,2]-rearrangement of the acyloxy group and anodic cyclization of 2-(t-butoxycarbonyl)oxy-3,3,3-trifluoropropyl sulfide

Article abstract of DOI:10.1149/2.1601713jes

Anodic methoxylation of 2-acyloxy-3,3,3-trifluoropropyl sulfides was carried out in methanol containing Et₃N-3HF as a supporting electrolyte and mediator using an undivided cell to provide the corresponding α-methoxylated products and unexpecte

Full text of DOI:10.1149/2.1601713jes

Journal of The Electrochemical Society, 164 (13) G121-G127 (2017)
G121
0013-4651/2017/164(13)/G121/7/$37.00 © The Electrochemical Society
Anodic Methoxylation of 2-Acyloxy-3,3,3-trifluoropropyl
Sulfides Accompanying with [1,2]-Rearrangement
of the Acyloxy Group and Anodic Cyclization of
2-(t-Butoxycarbonyl)oxy-3,3,3-trifluoropropyl Sulfide
Motoaki Isokawa,a Masaru Sano,a Kiyotaka Kubota,a Katsutoshi Suzuki,a Shinsuke Inagi,b,∗
and Toshio Fuchigamia,∗∗,z
aDepartment of Electronic Chemistry, Tokyo Institute of Technology, Yokohama 226-8502, Japan
bDepartment of Chemical Science and Engineering, Tokyo Institute of Technology, Yokohama 226-8502, Japan
Anodic methoxylation of 2-acyloxy-3,3,3-trifluoropropyl sulfides was carried out in methanol containing Et3N-3HF as a sup-
porting electrolyte and mediator using an undivided cell to provide the corresponding α-methoxylated products and unexpected
β-methoxylated products owing to [1,2]-rearrangement of the acyloxy group in the latter case. It was also found that the regioselec-
tivity and diastereoselectivity of the anodic methoxylation were greatly affected by the concentration of methanol in an electrolytic
solvent, electrolytic temperature, and bulkiness of acyloxy group. This is the first example of anodic methoxylation of sulfides
accompanying with rearrangement of an acyloxy group. Furthermore, anodic intramolecular cyclization of 2-(t-butoxycarbonyl)oxy-
3,3,3-trifluoropropyl sulfide was achieved in 0.01 M Et3N-3HF/MeCN eliminating a t-butyl cation to provide the corresponding
trifluoromethylated ethylene carbonate derivative in good yield.
© 2017 The Electrochemical Society. [DOI: 10.1149/2.1601713jes] All rights reserved.
Manuscript submitted August 8, 2017; revised manuscript received October 3, 2017. Published October 31, 2017.
Anodic substitution is known to be one of characteristic organic
electrochemical reactions. For example, anodic α-methoxylation of
organonitrogen compounds has been well-established.1–3 However,
the analogous α-methoxylation of organosulfur compounds is rather
limited.3 Namely, the methoxylation proceeds only when organosul-
fur compounds have a strongly electron-withdrawing group4–6 or
a good leaving group like silyl7–13 and boryl groups14–19 at the α-
position to the sulfur atom. On the other hand, we have found that
a fluoride ion markedly promotes anodic α-methoxylation of sulfides
even when the sulfides have relatively weak electron-withdrawing
and electron-donating groups as shown in Scheme 1.20–22 It was
also shown that Et3N-3HF was more effective than tetrabutylam-
monium fluoride. Since a fluoride ion mediator is highly effective
for the anodic generation of α-carbocation to a sulfur atom, a flu-
oride ion mediator was successfully applied to anodic intramolec-
ular cyclization of α-(phenylthio)acetamides to provide indole and
3-oxotetrahydroisoquinoline derivatives.23
On the other hand, organofluorine compounds are highly useful for
development of novel functional materials, pharmaceuticals and agro-
chemicals. However, methods for their synthesis are strictly limited in
many cases because of specific properties of fluorine atoms. For ex-
ample, nucleophilic substitution at α- and β-positions to a CF3 group
is generally difficult because of its strongly electron-withdrawing
nature.24–27
With these facts in mind, we applied the fluoride ion-mediator
to anodic methoxylation of 2-acyloxy-3,3,3-trifluoropropyl sulfides
and intramolecular cyclization of 2-(t-butoxycarbonyl)oxy-3,3,3-
trifluoropropyl sulfide to form cyclic carbonate derivative. Since all
of the starting sulfides have a stereogenic center, the stereoselectivity
of the products would be interesting.
Scheme 1. Fluoride ion-mediated anodic α-methoxylation of sulfides.
GCMS-QP-2000A or JEOL JMS-700 mass spectrometer. 19F NMR
yields and diastereomeric excess (d.e.) were estimated using α,α,α-
trifluorotoluene as an internal standard.
Materials.—2-Acyloyl-3,3,3-trifluoropropyl phenyl sulfides 1a-
1e.—Acid anhydride (60 mmol) was added dropwise to a stirred so-
lution of 2-hydroxy-3,3,3-trifluoropropyl phenyl sulfide.34 (50 mmol)
derived from thiophenol and 3,3,3-trifluoropropene oxide in 50 ml
of CH2Cl2 containing Et3N (60 mmol) at 0◦C and the solution was
stirred for 1 h at 0◦C. After the solution was stirred at room tempera-
ture overnight, water was added to the solution, followed by addition
of sat. aq. NaHCO3 solution to neutralize, and then the product (oily
layer) was extracted repeatedly with CH2Cl2. The extracts were dried
over anhydrous MgSO4 and the solvent was removed by evaporation
under reduced pressure. The products 1a-1e were isolated by silica gel
column chromatography (eluent: hexane/AcOEt = 12:1). The yields
of 1a-1e were 96%, 87%, 79%, 94%, and 93% yield, respectively.
2-Acetoxy-3,3,3-trifluoropropyl phenyl sulfide (1a) colorless
oil; 1H NMR δ 7.25-7.44 (m, 5H), 5.37-5.50 (m, 1H), 3.15-3.30 (m,
2H), 2.04 (s, 3H); 13C NMR δ 168.88, 131.20, 129.25, 129.22, 127.51,
123.03, 68.47, 32.92, 20.02; 19F NMR δ −0.12 (d, J = 6.21 Hz); MS:
m/z = 264 (M+); analysis calculated for C11H11F3O2S: C, 49.99; H,
4.20; F, 21.57; S, 12.13. Found: C, 49.71; H, 4.09; F, 21.47; S, 12.01.
2-(n-Propylcarbonyloxy)-3,3,3-trifluoropropyl phenyl sulfide
(1b) colorless oil; 1H NMR δ 7.28-7.43 (m, 5H), 5.43-5.48 (m, 1H),
3.08-3.29 (m, 2H), 2.28 (t, J = 3.3 Hz, 2H), 1.59-1.72 (m, 2H),
0.95 (t, J = 7.26 Hz, 3H); 13C NMR δ 171.12, 133.42, 130.73, 127.03,
122.05, 67.70, 34.97, 32.63, 17.70, 12.92; 19F NMR δ 0.08 (d, J = 6.82
Hz); MS: m/z = 292 (M+); analysis calculated for C13H15F3O2S: C,
53.41; H, 5.17; F, 19.50; S, 10.97. Found: C, 53.44; H, 5.17; F, 19.99;
S, 10.73.
Experimental
General.—1H, 13C and 19F NMR spectra were recorded on JEOL
JNM EX-270 (1H: 270 MHz, 13C: 67.8 MHz, 19F: 254.05 MHz) spec-
trometer in CDCl3. The chemical shifts for 1H and 13C were given in
δ (ppm) downfield from internal TMS (0.00) and 19F NMR chemical
shifts are given in δ (ppm) upfield from external trifluoroacetic acid,
respectively. Preparative electrolysis experiments were carried out
with Hokuto Denko HABF 501 potentiostat/galvanstat. Mass spec-
tra and high-resolution mass spectra were obtained with a Shimazu
∗
Electrochemical Society Member.
Electrochemical Society Fellow.
∗∗
zE-mail: fuchi@echem.titech.ac.jp
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Journal of The Electrochemical Society, 164 (13) G121-G127 (2017)
phenyl sulfide40 (50 mmol) in 50 ml of THF cooled in an ice bath, and
then the mixture was stirred for 1 h. After that, the mixture was stirred
at room temperature over night, and then water was added and neu-
tralized with 2 M HCl. The resulting solution was extracted with ethyl
acetate repeatedly, and the extracts were dried over anhydrous MgSO4.
The solvent was removed by evapolation under reduced pressure, and
the remaining oil was purified by silica gel column chromatography
(eluent: hexane/AcOEt = 9:1) to provide 4 as a light yellow oil in
54% yield.
2-(t-Butylcarbonyloxy)-3,3,3-trifluoropropyl phenyl sulfide
(1c) colorless oil; 1H NMR δ 7.29- 7.42 (m, 5H), 5.36-5.43 (m, 1H),
3.08-3.32 (m, 2H), 1.27 (s, 9H); 13C NMR, δ 171.12, 133.42, 130.73,
128.73, 127.03, 122.05, 67.70, 34.97, 32.63, 17.70, 12.92;19F NMR δ
−0.22 (d, J = 6.20 Hz); MS: m/z = 306 (M+); analysis calculated for
C14H17F3O2S: C, 54.89; H, 5.9; F, 18.60; S, 10.47. Found: C, 54.87;
H, 5.78; F, 18.91; S, 10.27.
2-Benzoyloxy-3,3,3-trifluoropropyl phenyl sulfide (1d) color-
less oil; 1H NMR δ 7.02- 8.00 (m, 10H), 5.66-5.71 (m, 1H), 3.25-3.42
(m, 2H), 13C NMR δ 134.27, 134.20, 131.91, 131.73, 130.55, 129.70,
128.93, 128.05, 123.69, 69.52, 33.95; 19F NMR δ 0.13 (d, J = 6.82
Hz); MS: m/z = 326 (M+); analysis calculated for C16H13F3O2S: C,
58.89; H, 4.02; F, 17.47; S, 9.83. Found: C, 59.16; H, 4.16; F, 17.48;
S, 9.83.
1H NMR δ 7.45-7.25 (m, 5H), 5.10-5.25 (m, 1H), 3.4-3.26 (m,
2H), 1.51 (s, 9H); 13C NMR δ 151.5, 135.6, 131.23, 129.1, 127.9,
122.9 (q, J = 270 Hz), 83.9, 71.3 (q, J = 32 Hz), 33.5, 27.6; 19F NMR
δ −0.26 (d, J = 7.4 Hz); MS: m/z = 322 (M+); HRMS: m/z calcd for
C14H17F3O3S 322.0846, found 322.0809.
2-(p-Methoxybenzoyloxy)-3,3,3-trifluoropropyl phenyl sulfide
(1e) colorless oil; 1H NMR δ 7.93 (d, J = 2.31 Hz, 2H), 7.20-7.92 (m,
5H), 6.92 (d, J = 2.30 Hz, 2H), 5.66-6.93 (m, 1H), 3.86 (s, 3H), 3.23-
3.39 (m, 2H); 13C NMR δ 164.29, 164.00, 133.94, 132.22, 131.38,
129.21, 127.49, 123.32, 120.70, 113.73, 68.72, 55.47, 33.8; 19F NMR
δ 0.14 (d, J = 6.82 Hz); MS: m/z = 356 (M+); analysis calculated for
C17H15F3O3S: C, 57.30; H, 4.24; F, 15.99; S, 9.0. Found: C, 57.24; H,
4.24; F, 15.99; S, 8.63.
A typical procedure for anodic methoxylation.—Constant current
(6.25 mA/cm2) anodic oxidation of 1 (1.0 mmol) was carried out with
a platinum anode and cathode (2 cm × 2 cm) in an undivided cell
containing 25 ml of MeOH or MeOH/MeCN containing 0.13 M Et3N-
3HF (10 eq. of F– to 1) under a dry nitrogen atmosphere. After 4 F/mol
of electricity was passed, the electrolytic solution was neutralized with
a saturated aqueous solution of NaHCO3, and organic solvent was
removed on a rotary evaporator. The residue was extracted repeatedly
with AcOEt and the extracts were dried over anhydrous MgSO4.
After the solvent was removed under reduced pressure, the NMR
yield and diastereomeric excess (d.e.) of product 2 were estimated
from 19F NMR spectra using CF3C6H5 as an internal standard. After
the estimation, the crude mixture was passed through a silica gel
column (eluent: hexane/AcOEt = 13:1), and then α-methoxylated and
rearrangement products 2 and 3 were isolated by medium-pressure
liquid chromatography (silica gel Merk Lichroprep Si 60; 37 φ × 440
mm) on elution with hexane/2-propanol (99.9:0.1).
2-Acetoxy-3-fluoropropyl phenyl sulfide (1f). A solution of sec-
BuLi (1.04 M in cyclohexane/hexane, 50 ml) in dry THF was slowly
added dropwise to a solution of thioanisole (6.5 ml, 55 mmol) in
a minimun amount of dry THF at –78◦C. After 1 h ethyl fluoroac-
etate (4.8 ml, 50 mmol) was added, and the mixture was stirred for
1 h. Then water was added, it was rosen slowly to room temperature.
After the reaction mixture was neutralized by 2 M HCl aqueous so-
lution, the product was extracted with ethyl acetate and washed with
water, and brine. The extract was dried over MgSO4 and concen-
trated. The residue was purified by column chromatography on silica
gel using hexane to hexane/ethyl acetate (10/1) as an eluent to give
1.0 g (11%) of 3-fluoro-1- phenylthiopropan-2-one as a colorless oil.
Then, a NaBH4 (0.4 g, 10 mmol) was added dropwise to 3-fluoro-1-
phenylthiopropan-2-one (0.9 g, 5 mmol) in MeOH (10 ml) at 0◦C for
1 h and stirred at room temperature for 24 h. After the reaction mixture
was neutralized by 2 M HCl aqueous solution, it was concentrated,
the product was extracted with ethyl acetate and washed with water,
and brine. The extract was dried over MgSO4 and concentrated. The
residue was purified by column chromatography on silica gel using
hexane to hexane/ethyl acetate (4/1) as an eluent to give 0.8 g (86%)
of 2-hydroxy-3-fluoropropyl phenyl sulfide as a colorless oil. Next,
2-hydroxy-3-fluoropropyl phenyl sulfide (0.7 g, 4 mmol) and triethy-
lamine (0.7 ml, 5 mmol) in CH2Cl2 was slowly added acetyl chloride
(1.4 ml, 20 mmol) at 0◦C for 1 h and stirred at room temperature for
24 h. After the reaction mixture was concentrated, water was added,
and then the product was extracted with ethyl acetate and washed with
water, and brine. The extract was dried over MgSO4 and concentrated.
The residue was purified by column chromatography on silica gel us-
ing hexane to hexane/ethyl acetate (7/1) as an eluent to give 0.8 g
(85%) of 1f as a colorless oil. 1H NMR δ 2.03 (s, 3H), 3.10-3.26 (m,
2H), 4.56 (ddd, J = 48.7 Hz, 10.3 Hz, 3.0 Hz, 1H), 4.64 (ddd, J =
45.8 Hz, J = 12.4 Hz, J = 4.1 Hz, 1H), 5.01-5.15 (m, 1H), 7.19-7.43
(m, 5H); 19F NMR δ -156.10 (td, J = 47.3 Hz, J = 23.0 Hz); 13C
NMR δ 20.9, 32.9 (d, J = 6.2 Hz), 71.3 (d, J = 19.5 Hz), 82.1 (d, J =
173.5 Hz), 126.7, 129.3, 129.8, 134.8, 170.1; MS m/z 228 (M+), 123
(M+ -CH(OAc)CH2F); HRMS: m/z calcd for C11H13FO2S 228.0620,
found 228.0609.
2-Acetoxy-1-methoxy-3,3,3-trifluoropropyl phenyl sulfide (2a)
(less polar isomer) Colorless oil; 1H NMR δ 7.32-7.48 (m, 5H), 5.29
(dq, J = 8.25 Hz, J = 6.20 Hz, 1H), 4.76 (d, J = 8.25 Hz, 1H), 3.56 (s,
3H), 2.16 (s, 3H); 19F NMR δ 3.47 (d, J = 6.20 Hz); MS: m/z = 294
(M+); HRMS calcd. for C12H13F3O3S: 294.0537. Found; 294.0528.
2-Acetoxy-1-methoxy-3,3,3-trifluoropropyl phenyl sulfide (2a)
1
(highly polar isomer) Colorless oil; H NMR δ 7.33-7.54 (m, 5H),
5.63 (dq, J = 6.60 Hz, J = 3.63 Hz, 1H), 4.84 (d, J = 3.30 Hz, 1H),
3.61 (s, 3H), 2.06 (s, 3H); 19F NMR δ 3.32 (d, J = 6.82 Hz); MS: m/z
= 294 (M+); MS: m/z = 294 (M+); HRMS calcd. for C12H13F3O3S:
294.0537. Found; 294.0529.
1-Acetoxy-2-methoxy-3,3,3-trifluoropropyl phenyl sulfide (3a)
(less polar isomer) Colorless oil; 1H NMR δ 7.36-7.76 (m, 5H), 5.65
(d, J = 6.27 Hz, 1H), 4.28 (dq, J = 6.27, 5.58 Hz, 1H), 3.33 (s, 3H),
1.65 (s, 3H); 19F NMR δ 0.33 (d, J = 5.58 Hz); MS: m/z = 294 (M+);
HRMS calcd. for C12H13F3O3S: 294.0537. Found; 294.0533.
1-Acetoxy-2-methoxy-3,3,3-trifluoropropyl phenyl sulfide (3a)
1
(highly polar isomer) Colorless oil; H NMR δ 7.34-7.64 (m, 5H),
5.59 (d, J = 5.61 Hz, 1H), 4.53 (dq, J = 5.94, 5.61 Hz, 1H), 3.38 (s,
3H), 1.65 (s, 3H); 19F NMR δ = −1.13 (d, J = 6.20 Hz); MS: m/z
= 294 (M+); MS: m/z = 294 (M+); HRMS calcd. for C12H13F3O3S:
294.0537. Found; 294.0530.
2-(n-Propylcarbonyloxy)-1-methoxy-3,3,3-trifluoropropyl
phenyl sulfide (2b) (less polar isomer) Colorless oil; 1H NMR δ
7.31-7.47 (m, 5H), 5.32 (dq, J = 1.45, 6.60 Hz, 1H), 4.77 (d, J = 7.92
Hz, 1H), 3.59 (s, 3H), 2.39 (t, J = 7.26 Hz, 2H), 1.71 (qt, J = 7.26,
14.56 Hz, 2H), 0.99 (t, J = 7.26 Hz, 3H); 19F NMR δ 3.45 (d, J =
6.82 Hz); MS: m/z = 322 (M+); calcd for C14H17F3O3S: 322.0851.
Found; 322.0858
2-Acetoxypropyl phenyl sulfide (1g) Sulfide 1g was prepared
from 2-hydroxypropyl phenyl sulfide derived from propyrene oxide
and thiophenol in a similar manner to the preparation of 1a–1e. Col-
orless oil; 1H NMR δ 7.41-7.19 (m, 5H), 5.08-5.01 (m, 1H), 3.20-2.94
(m, 2H), 1.96 (s, 3H), 1.33 (d, J = 5.70 Hz, 3H); MS: m/z = 210
(M+); HRMS calcd. for C11H14O2S: 210.0715. Found; 210.0697.
2-(n-Propylcarbonyloxy)-1-methoxy-3,3,3-trifluoropropyl
1
phenyl sulfide (2b) (highly polar isomer) Colorless oil; H NMR δ
7.32-7.54 (m, 5H), 5.32 (dq, J = 2.97, 6.60 Hz, 1H), 4.85 (d, J = 3.63
Hz, 1H), 3.60 (s, 3H), 2.31 (t, J = 6.93 Hz, 2H), 1.68 (qt, J = 7.26,
14.52 Hz, 2H), 0.96 (t, J = 7.26 Hz, 3H); 19F NMR δ 4.05 (d, J =
7.34 Hz); MS: m/z = 322 (M+); calcd for C14H17F3O3S: 322.0851.
Found; 322.0855.
2-(t-Butoxycarbonyl)oxy-3,3,3-trifluoropropyl phenyl sulfide
(4).—K2CO3 was added slowly to a stirred solution of pyrocarbonic
acid di-tert-butyl ester (50 mmol) and 2-hydroxy-3,3,3-trifluoropropyl
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Journal of The Electrochemical Society, 164 (13) G121-G127 (2017)
G123
1-(n-Propylcarbonyloxy)-2-methoxy-3,3,3-trifluoropropyl
phenyl sulfide (3b) (less polar isomer) Colorless oil; 1H NMR δ
7.20-7.65 (m, 5H), 5.64 (d, J = 6.40 Hz, 1H), 4.27 (dq, J = 5.94,
12.21 Hz, 1H), 3.32 (s, 3H), 1.85 (t, J = 7.26 Hz, 2H), 1.22 (qt, J =
6.93, 13.86 Hz, 2H), 0.93 (t, J = 7.26 Hz, 3H); 19F NMR δ −0.77
(d, J = 5.58 Hz); MS: m/z = 322 (M+); calcd for C14H17F3O3S:
322.0851. Found; 322.0823.
1-(n-Propylcarbonyloxy)-2-methoxy-3,3,3-trifluoropropyl
phenyl sulfide (3b) (highly polar isomer) Colorless oil; 19F NMR δ
0.38 (d, J = 5.58 Hz); MS: m/z = 322 (M+); calcd for C14H17F3O3S:
322.0851. Found; 322.0833.
2-(t-Butylcarbonyloxy)-1-methoxy-3,3,3-trifluoropropyl
phenyl sulfide (2c) (less polar isomer) Colorless oil; 1H NMR δ
7.31-7.34 (m, 5H), 5.28 (dq, J = 6.60, 8.25 Hz, 1H), 4.81 (d, J =
8.57 Hz, 1H), 3.54 (s, 3H), 1.27 (s, 9H); 19F NMR δ 3.36 (d, J =
6,82 Hz); MS: m/z = 336 (M+); calcd for C15H19F3O3S: 336.1002.
Found; 336.1012.
0.76 (d, J = 5.58 Hz); MS: m/z = 386 (M+); calcd for C18H17F3O4S:
386.0800. Found; 386.0796.
1-(p-Methoxybenzoyloxy)-2-methoxy-3,3,3-trifluoropropyl
1
phenyl sulfide (3e) (highly polar isomer) Colorless oil; H NMR δ
7.50-7.59 (m, 5H), 7.36 (d, J = 5.61 Hz, 2H), 6.91 (d, J = 8.90 Hz,
2H), 5.71 (d, J = 5.60 Hz, 1H), 4.69 (dq, J = 5.41, 5.94 Hz, 1H),
3.82 (s, 3H), 3.33 (s, 3H); 19F NMR δ 0.57 (d, J = 6.20 Hz); MS: m/z
= 386 (M+); calcd for C18H17F3O4S: 386.0800. Found; 386.0789.
2-Acetoxy-1-methoxy-3-fluoropropyl phenyl sulfide (2f) (less
polar isomer). 1H NMR δ 7.57-7.27 (m, 5H), 5.32-5.23 (m, 1H),
4.79 (d, J = 6.3 Hz, 1H), 4.86-4.48 (m, 2H), 3.53 (s, 3H), 2.05 (s,
3H); 19F NMR δ -156.66 (td, J = 20.2 Hz, 46.9 Hz); MS m/z 258
(M+), 153 (M+ -CH(OAc)CH2F); calcd for C12H15FO3S: 258.09447.
Found; 258.09551.
2-Acetoxy-1-methoxy-3-fluoropropyl phenyl sulfide (2f)
1
(highly polar isomer). H NMR δ7.49-7.22 (m, 5H), 5.13-5.00 (m,
1H), 4.77 (d, J = 7.3 Hz, 1H), 4. 72–4.51 (m, 2H), 3.52 (s, 3H), 2.13
(s, 3H); 19F NMR δ -159.08 (td, J = 23.9 Hz, 47.1 Hz); MS m/z 258
(M+), 153 (M+-CH(OAc)CH2F); calcd for C12H15FO3S: 258.09447.
Found; 258.09632.
2-(t-Butylcarbonyloxy)-1-methoxy-3,3,3-trifluoropropyl
1
phenyl sulfide (2c) (highly polar isomer) Colorless oil; H NMR δ
7.44-7.53 (m, 5H), 5.57 (dq, J = 4.61, 6.92 Hz, 1H), 4.89 (d, J =
4.29 Hz, 1H), 3.55 (s, 3H), 1.26 (s, 9H); 19F NMR δ 3.70 (d, J =
6.82 Hz); MS: m/z = 336 (M+); calcd for C15H19F3O3S: 336.1002.
Found; 336.1010.
1-(t-Butylcarbonyloxy)-2-methoxy-3,3,3-trifluoropropyl
phenyl sulfide (3c) (less polar isomer) Colorless oil; 1H NMR δ
7.34-7.39 (m, 5H), 5.51 (d, J = 8.25 Hz, 1H), 4.46 (dq, J = 5.61, 8.25
Hz, 1H), 3.36 (s, 3H), 0.88 (s, 9H); 19F NMR δ 1.24 (d, J = 5.58);
MS: m/z = 336 (M+); calcd for C15H19F3O3S: 336.1002. Found;
336.9985.
2-Acetoxy-1-methoxypropyl phenyl sulfide (2g) (less polar iso-
mer) 1H NMR δ 7.52-7.26 (m, 5H), 5.15-5.08 (m, 1H), 4.67-4.60
(m, 1H), 3.49 (s, 3H), 1.99 (s, 3H), 1.34 (d, J = 2.60 Hz, 3H); MS:
m/z = 222 (M+).
2-Acetoxy-1-methoxypropyl phenyl sulfide (2g) (highly polar
isomer) 1H NMR δ 7.52-7.26 (m, 5H), 5.15-5.08 (m, 1H), 4.67-4.60
(m, 1H), 3.52 (s, 3H), 2.06 (s, 3H), 1.36 (d, J = 2.60 Hz, 3H); MS:
m/z = 222 (M+).
Anodic cyclization of sulfide 4.—Constant current (10 mA/cm2)
anodic oxidation of 4 (1.0 mmol) was carried out with platinum elec-
trodes (2 cm × 2 cm) in an undivided cell containing 20 ml of MeCN
containing various supporting electrlytes (Table III) at various tem-
peratures. When 3 F/mol of electricity was passed, the electrolysis
was stopped, and then work up and estimation of product yields were
performed similarly to the anodic methoxylation of 1. Isolation of
cyclized product 5 and fluoro by-product 6 was carried out by using
silica gel columun chromatography (eluent:hexane/AcOEt = 8:1).
trans-4-Phenylthio-5-trifluoromethyl-1,3-dioxolan-2-one (5)
1H NMR δ 7.59-7.43 (m, 5H), 5.86 (d, 1H, J = 4.1 Hz), 4.41 (dq, J =
5.9, 5.6 Hz, 1H); 19F NMR δ -2.39 (d, 3F, J = 5.6 Hz); MS m/z 264
(M+); 13C NMR δ 150.8, 134.8, 130.5, 129.9, 121.6 (q, J = 281 Hz),
82.1, 76.5 (q, J = 35 Hz), 29.7; calcd for C10H7F3O3S: 264.0068.
Found; 264.0060.
1-(t-Butylcarbonyloxy)-2-methoxy-3,3,3-trifluoropropyl
phenyl sulfide (3c) (highly polar isomer) Colorless oil; H NMR δ
1
7.53-7.58 (m, 5H), 5.66 (d, J = 8.25 Hz, 1H), 4.29 (dq, J = 5.28,
8.25 Hz, 1H), 3.29 (s, 3H), 1.03 (s, 9H); 19F NMR δ 1.49 (d, J =
5.58 Hz); MS: m/z = 336 (M+); calcd for C15H19F3O3S: 336.1002.
Found; 336.1000.
2-Benzoyloxy-1-methoxy-3,3,3-trifluoropropyl phenyl sulfide
(2d) (less polar isomer) Colorless oil; 1H NMR δ 7.28-8.07 (m, 10H),
5.53 (dq, J = 3.60, 6.20 Hz, 1H), 4.94 (d, J = 3.30 Hz, 1H), 3.61 (s,
3H); 19F NMR δ 3.66 (d, J = 6.20 Hz); MS: m/z = 356 (M+); HRMS
calcd. for C17H15F3O3S: 356.0694. Found; 356.0690.
2-Benzoyloxy-1-methoxy-3,3,3-trifluoropropyl phenyl sulfide
1
(2d) (highly polar isomer) Colorless oil; H NMR δ 7.23-8.07 (m,
10H), 5.86 (dq, J = 3.96, 6.60 Hz, 1H), 4.98 (d, J = 3.96 Hz, 1H),
3.62 (s, 3H); 19F NMR δ 3.77 (d, J = 6.73 Hz); MS: m/z = 356 (M+);
HRMS calcd. for C17H15F3O3S: 356.0694. Found; 356.0692.
1-Benzoyloxy-2-methoxy-3,3,3-trifluoropropyl phenyl sulfide
(3d) (less polar isomer) Colorless oil; 1H NMR δ 7.31-7.59 (m, 10H),
5.79 (d, J = 6.83 Hz, 1H), 4.33 (dq, J = 6.83, 5.94 Hz, 1H), 3.36 (s,
3H); 19F NMR δ 0.71 (d, J = 6.20 Hz); MS: m/z = 356 (M+); calcd.
for C17H15F3O3S: 356.0694. Found; 356.0686.
2-(t-Butoxycarbonyl)oxy-1-fluoro-3,3,3-trifluoropropyl
1
phenyl sulfide (6) (diastereomeric mixture) H NMR δ 7.62-7.31
(m, 5H), 5.86 (dd, 1H, J = 6.8, 5.1 Hz), 5.26 (m, 1H), 1.54 (s, 9H);
19F NMR δ -85.9 (m, 1F), -81.7 (m, 1F), 3.04 (dd, 3F, J = 7.4, 5.6
Hz), 2.88 (dd, 3F, J = 7.4, 5.6 Hz); MS m/z 340 (M+), 321 (M+-F);
calcd for C14H14F4O3S: 340.0756. Found; 340.0750.
1-Benzoyloxy-2-methoxy-3,3,3-trifluoropropyl phenyl sulfide
1
(3d) (highly polar isomer) Colorless oil; H NMR δ 7.20-8.01 (m,
Results and Discussion
10H), 5.71 (d, J = 5.94 Hz, 1H), 4.71 (dq, J = 3.30, 5.94 Hz, 1H),
3.33 (s, 3H); 19F NMR δ −0.52 (d, J = 6.21 Hz); MS: m/z = 356
(M+); calcd. for C17H15F3O3S: 356.0694. Found; 356.0690.
2-(p-Methoxybenzoyloxy)-1-methoxy-3,3,3-trifluoropropyl
phenyl sulfide (2e) (less polar isomer) Colorless oil; 1H NMR δ
7.99-8.03 (m, 2H), 7.27-7.44 (m, 5H), 6.93-6.97 (m, 2H,), 5.52 (dq,
J = 6.60, 7.27 Hz, 1H), 4.93 (d, J = 7.91 Hz, 1H), 3.89 (s, 3H), 3.60
(s, 3H); 19F NMR δ 3.64 (d, J = 6.20 Hz); MS: m/z = 386 (M+);
calcd for C18H17F3O4S: 386.0800. Found; 386.0794.
Anodic methoxylation of 2-acetoxy-3,3,3-trifluoropropyl phenyl
sulfides using a fluoride ion-mediator.—Anodic methoxylation of 2-
acetoxyl sulfide 1a was carried out at various temperatures in MeOH
and MeOH/MeCN containing Et3N-3HF. The results are summarized
in Table I.
Anodic oxidation of 1a at 25◦C in MeOH containing Et3N-3HF
as a supporting salt provided the corresponding α-methoxylated prod-
uct 2a mainly and unexpected β-methoxylated product 3a was also
formed (Entry 1). The total yield was 68% and the both products were
diastereisomeric mixture. The product 3a seems to be formed owing
to [1,2]-rearrangement of the acetoxy group of 1a. In comparison,
anodic oxidation of 1a was carried out similarly using Et4NOTs in-
stead of Et3N-3HF as a supporting salt, 2a was formed solely in much
lower yield (23%) with similar diastereoselectivity (30% d.e.). When
the electrolytic solvent was changed to MeOH/MeCN (1:1), 2a and 3a
were formed in almost same yield (Entry 2). Furthermore, when the
2-(p-Methoxybenzoyloxy)-1-methoxy-3,3,3-trifluoropropyl
1
phenyl sulfide (2e) (highly polar isomer) Colorless oil; H NMR δ
7.98-8.03 (m, 2H), 7.50-7.53 (m, 5H), 6.89-6.95 (m, 2H,), 5.83 (dq,
J = 3.53, 6.96 Hz, 1H), 4.97 (d, J = 3.96 Hz, 1H), 3.87 (s, 3H), 3.61
(s, 3H); 19F NMR δ 3.76 (d, J = 6.82 Hz); MS: m/z = 386 (M+);
calcd for C18H17F3O4S: 386.0800. Found; 386.0796.
1-(p-Methoxybenzoyloxy)-2-methoxy-3,3,3-trifluoropropyl
phenyl sulfide (3e) (less polar isomer) Colorless oil; 19F NMR δ
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G124
Journal of The Electrochemical Society, 164 (13) G121-G127 (2017)
Table I. Fluoride ion-mediated anodic methoxylation of sulfide 1a.
Yield(%)a,b
2a 3a
42 (27) 26 (19) 0.62
33 (46) 30 (23) 0.91
33 (59) 57 (22) 1.7
34 (56) trace
21 (59) 43 (25) 2.1
6 (73) 19 (28) 3.2
Solvent
Entry MeOH/MeCN
Temperature Conversion
(◦C)
25
(%)a
91
3a / 2a
Scheme 2. Anodic methoxylation of sulfides 1f and 1g.
1
2
3
4
5
6
100/0
50/50
10/90
10/90
10/90
10/90
25
69
25
90
fide 1b having an n-PrCOO group proceeded to give α-methoxylated
product 2b with the highest diastereoselectivity as 72% d.e. On the
other hand, the diastereoselectivity of the β-methoxylated products
3a-3d is much lower than that of α-methoxylated products 2a-2d.
The results thus obtained indicate that the diastereoselectivity of α-
methoxylated products 2 and the product selectivity of β-methoxylated
products 3 are affected by the concentration of MeOH, electrolytic
temperature, and bulkiness of acyloxy group.
15
80
-
35
94
45
68
aDetermined by 19F NMR.
bDiastereomeric excess (d.e, %) is shown in parentheses.
ratio of MeOH to MeCN was reduced to 1:9, β-methoxylated product
3a was formed mainly and the total yield was also increased to 90%
(Entry 3). Notably, the diastereoselectivity of 2a also increased with
decrease of the ratio of MeOH to MeCN, while the diastereselectivity
of 3a was not appreciably affected by the solvent ratio (Entries 1–3).
Next, temperature effect on the rearrangement of acetoxy group
was also examined. The rearrangement did not take place at a low
temperature as 15◦C (Entry 4). In this case, the diastereselectivity of
2a was slightly decreased. With increase of temperature, the prod-
uct selectivity ratio of β-methoxylated product 3a to α-methoxylated
product 2a increased (Entries 3, 5, 6). However, the total yield was
decreased with increase of temperature.
Effects of β-fluoroalkyl group of sulfides on the anodic [1,2]re-
arrangement of acyloxy group.—In order to disclose the effects of
fluoroalkyl groups at the β-position of sulfides on the anodic [1,2]re-
arrangement of acyloxy group, we comparatively investigated anodic
methoxylation of 2-acetoxy-3-fluoropropyl phenyl sulfide (1f) and
nonfluorinated analogue 1g in Et3N-3HF/MeOH-MeCN (10:90) sim-
ilarly. In sharp contrast to the case of sulfides 1a-1e, these sulfides
did not undergo rearrangement at all, and α-methoxylated products 2f
and 2g were formed solely although the yield was quite low (23% and
8%) (Scheme 2). These results clearly indicate that a strongly electron-
withdrawing CF3 group significantly contributes to the rearrangement
to result in β-methoxylation.
Effects of acyloxy groups on the anodic [1,2]-rearrangement of 2-
acyloxy-3,3,3-trifluoropropyl phenyl sulfides.—In order to clarify the
effects of acyloxy groups on the anodic [1,2]-rearrangement, anodic
methoxylation of other sulfides 1b-1e having various acyloxy groups
at the β-position was carried out similarly in MeOH/MeCN (10/90).
The results are summarized in Table II.
As shown in Table II, acyloxy groups affected the anodic rearrange-
ment considerably. In a series of RCOO groups, the rearrangement of
an acyloxy group occurs more easily in the following order: MeCOO
>p-MeOC6H4 > PhCOO = n-PrCOO >>t-BuCOO. Namely, in the
case of sulfide 1a having a sterically smallest acetoxy group, acetoxy
rearrangement takes place more preferentially than α-methoxylation
(Entry 1). On the other hand, sulfide 1c having a bulky t-BuCOO
group undergoes anodic α-methoxylation more favorably compared
to acyloxy rearrangement (Entry 3). Anodic α-methoxylation of sul-
Reaction mechanism for anodic methoxylation and [1,2]-
rearrangement of acyloxy group.—More than 75 years ago, Win-
stein and his co-workers reported a number of papers deal-
ing with roles of neighboring groups in nucleophilic substi-
tution reactions.28–32 For example, the solvolysis of trans-2-
acetoxycyclohexyl p-toluenesulfonate in acetic acid proceeds via cis-
cyclohexaneacetoxonium ion to provide 1,2-diacetoxycyclohexane as
shown in Scheme 3.33
Paulsen and his co-workers also reported simple synthesis of acy-
loxonium salts by the reaction of 1,2-diol esters with SbCl5 in CH2Cl2
as shown in Scheme 4.33,34 This indicates that cyclic acyloxonium
cations are generally stable enough.
Moreover, it is well-known that neighboring group participation
of an acetoxy group plays an important role to control stereochemical
pathway in nucleophilic substitutions of carbohydrates.35–37
In consideration to these reports and the mechanism of anodic
methoxylation of sulfides via a fluorosulfonium ion intermediate
(Scheme 1),20–23 a plausible mechanism can be illustrated as shown
in Scheme 5.
Table II. Effects of acyl groups on the anodic rearrangement of
acyloxy groups of sulfides 1.
One electron oxidation of sulfide 1 generates radical cation inter-
mediate A followed by reaction with a fluoride ion to form fluorosul-
fonium ion B. Elimination of HF from B generates cation intermediate
C. Since C is destabilized by the strongly electron-withdrawing CF3
group, the acyloxy group would stabilize the cathionic intermediate
Yield (%)a,b
Entry
R
Me (1a)
n-Pr (1b)
t-Bu (1c)
Ph (1d)
Conversion (%)a
2
3
3 / 2
1
2
3
4
5
90
99
86
97
61
33 (59) 57 (22) 1.7
28 (72) 38 (30) 1.4
24 (63) 17 (46) 0.71
32 (57) 40 (37) 1.3
24 (52) 34 (48) 1.4
p-MeOC6H4 (1e)
aDetermined by 19F NMR.
bDiastereomeric excess (d.e, %) is shown in parentheses.
Scheme 3. Solvolysis of trans-2-acetoxycyclohexyl p-toluenesulfonate in
acetic acid.
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Journal of The Electrochemical Society, 164 (13) G121-G127 (2017)
G125
Scheme 4. Synthesis of acyloxonium Salts.
Scheme 6. Temperature dependency on the structure of anodically generated
cation intermediate.
C to form cyclic cationic intermediate D (i.e. neighboring group par-
ticipation). When a methanol electrolytic solution is used without
co-solvent, methanol should attack mainly C immediately after the
generation of intermediate C (Path 1), namely before such neighbor-
ing group participation. This should result in favorable formation of 2
and lower diastereoselectivity. On the other hand, when the methanol
concentration in the electrolytic solution is reduced by using acetoni-
trile co-solvent, methanol seems to attack D preferentially after the
neighboring group participation. Acyloxonium cation intermediate D
has two reactive sites, which react with MeOH to afford α- and β-
methoxylated products 2 and 3 (Path 2 and 3, respectively). Since
the β-position to the sulfur atom of intermediate D is also adjacent
to strongly electron-withdrawing CF3 group, the β-position is more
positive compared to the α-position. Therefore, the methoxylation
at the β-position should occur more preferentially than that at the
α-position. To support this, sulfides devoid of a strongly electron-
withdrawing group 1f and 1g provided α-methoxylated products
solely and [1,2]-rearrangement did not take place at all (Scheme 2).
This result clearly indicates that a strongly electron-withdrawing CF3
group is neccessary for the [1,2]-rearrangement.
Intermediate C might also undergo [1,2]-rearrangement via in-
termediate D to generate another open-chain cation intermediate E,
which may react with MeOH to give a β-methoxylated product (Path
4). However, it is well known that the generation of α-cation to a CF3
group is particularly difficult and SN1 reaction via a trifluoroethyl car-
benium ion has been unsuccessful26 unless a sulfur or nitrogen atom
exists at α-position to the CF3 group.5,38 Therefore, Path 4 is not likely.
In order to verify further more β-methoxylation via cation inter-
mediate D, optically active 1a was subjected to anodic methoxylation
in Et3N-3HF/MeOH and obtained β-methoxy product 3a was found
to be optically active.39 This clearly suggests that β-methoxylation
proceeds mainly via D in a similar way to SN2 type reaction. Thus,
major β-methoxylation via acyloxonium cation intermediate D can be
reasonably explained.
In the case of sulfide 1c, the formation of D seems to be less
favorable compared to other sulfide derivatives owing to a bulky t-
BuCOO group. This is probably one of main reasons for less preferable
rearrangement of the t-BuCOO group.
β-Methoxylation derived from rearrangement of an acyloxy group
is also enhanced by increase of electrolytic temperature. This can be
explained in terms of preferential formation of cyclic cationic inter-
mediate D at higher temperature as shown in Scheme 6. However,
at high temperature like 45◦C, free Et3N is released from Et3N-
3HF and generated Et3N is anodically oxidized prior to oxidation
of sulfide 1a. Therefore, conversion of 1a decreased significantly
(Table I, Entry 6).
It is notable that the diastereoseletivity of α-methoxylation is much
higher than that of β-methxylation, however the reason is not clear at
present time.
Anodic
cyclization
of
2-(t-butoxycarbonyl)oxy-3,3,3-
trifluoropropyl phenyl sulfide using a fluoride ion-mediator.—In
consideration to the mechanism for [1,2]-rearrangement of an acyloxy
group via an acyloxonium ion, we expected that anodic intramolecular
cyclization of 2-(t-butoxycarbonyl)oxy-3,3,3-trifluoropropyl phenyl
sulfide (4) would be achieved by using a fluoride ion-mediator to
Scheme 5. Plausible mechanism for anodic methoxylation and
[1,2]-rearrangement of an acyloxy group of sulfides.
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G126
Journal of The Electrochemical Society, 164 (13) G121-G127 (2017)
high temperature (45◦C), the yield was almost same as that at room
Table III. Fluoride ion-mediated anodic cyclization of sulfide 4.
temperature (Entries 2 and 4). Therefore, the electrolysis at a room
temperature was found to be suitable for the formation of 5. In sharp
contrast to these results, other supporting electrolytes like Et4NClO4
and Bu4N-HSO4 were not effective at all or much less effective
(Entries 6 and 7). The product 5 seems to be a useful fluorobuilding
block.
Based on the results and the proposed mechanism for the [1,2]-
rearrangement of acyloxy group, a plausible reaction mechanism can
be illustrated as shown in Scheme 7.
Yield (%)a
Entry Supporting electrolyte Temperature (◦C)
5
6b
1
0.1 M Et3N-3HF
0.05 M Et3N-3HF
0.05 M Et3N-3HF
0.05 M Et3N-3HF
0.01 M Et3N-3HF
0.05 M Et4NClO4
0.05 M Bu4NHSO4
r.t.
r.t.
0
45
r.t.
r.t.
r.t.
40 (25)c
36 (10)c
Fluoride ion effectively promotes anodic generation of α-cation
intermediate F of sulfide 4. Then, neighboring participation takes
place to form cyclic cation intermediate G and H. Since t-butyl cation
is a good leaving group, elimination of t-butyl cation from G and H
takes place to give 5. On the other hand, nucleophilic attack of fluoride
ion to F, H, and G affords 6. Regarding the stereoselectivity of 5, a
CF3 group is much more bulky than a CH3 group, therefore, exclusive
formation of trans form of 5 is reasonable.
2
3
4
5
6
7
65
45
66
73
0
18
15
15
7
0
0
18
aDetermined by 19F NMR.
bDiastereomeric excess (d.e. %) is 57–61%.
cIsolated yields are shown in parentheses.
Conclusions
We found the first example of anodic methoxylation of sulfides ac-
companying with [1,2]-rearrangement of acyloxy group. Furthermore,
electrochemical synthesis of trifluoromethylated ethylene carbonate
derivative was achieved by anodic cyclization of sulfide having a car-
bonate moiety using a fluoride ion mediator. Thus, we have shown that
a fluoride ion-mediator is highly useful for electrochemical molecular
conversion of organosulfur compounds.
provide the corresponding ethylene carbonate derivative. The anodic
oxidation of 4 was carried out in MeCN under various conditions.
When of Et3N-3HF was used as a supporting electrolyte, expected
intramolecular cyclization took place eliminating a t-butyl group to
provide 1-phenylthio-2-trifluoromethylethylene carbonate derivative
5 as well as the corresponding α-fluorosulfide 6 (Table III, Entry
1–5). Regardless of electrolytic conditions, trans form of 5 was
always formed exclusively. At a high concentration (1.0 M) of
Et3N-3HF, a large amount of α-fluorosulfide 6 was formed as a
by-product considerably (Entry 1). In order to avoid the formation of
6, the concentration of Et3N-3HF was reduced and eventually desired
product 5 was obtained in good yield (73%) at a low concentration
(0.01 M) of Et3N-3HF (Entry 5). In this case, the formation of
by-product 6 was suppressed considerably. At a low temperature
(0◦C), the yield of 5 was decreased significantly (Entry 3) while at a
Acknowledgments
We thank Dr. Seiichiro Higashiya for his measurement of high
resolution mass spectra.
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Scheme 7. Reaction mechanism for anodic cyclization of sulfide 4.
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Journal of The Electrochemical Society, 164 (13) G121-G127 (2017)
G127
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26
Section. 1a (75% ee): [α]D = 28.60 (c 0.92, MeOH). β-Methoxylated product 3a
26
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