Oxidation of sulfides to sulfoxides with 3-aryl-2-tert-butyloxaziridines under high pressure

Article abstract of DOI:10.1039/a705022d

3-Phenyl-2-tert-butyloxaziridine has been shown to behave as an oxidant under 800 MPa at 100°C to oxidize sulfides to sulfoxides, in spite of having been reported to be inactive as an oxidant and to undergo thermal rearrangement to W-tert-butyl-α-phenylnitrone at atmospheric pressure. It has been shown that under thermal and high-pressure conditions the ability of the oxaziridine to react changes dramatically. The mechanism for the high-pressure reaction is also discussed.

Full text of DOI:10.1039/a705022d

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Oxidation of sulfides to sulfoxides with 3-aryl-2-tert-
butyloxaziridines under high pressure
Masao Shimizu,* Isao Shibuya, Yoichi Taguchi, Satoshi Hamakawa,
Kunio Suzuki and Takashi Hayakawa
National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305,
Japan
3-Phenyl-2-tert-butyloxaziridine has been shown to behave as an oxidant under 800 MPa at 100 ЊC to
oxidize sulfides to sulfoxides, in spite of having been reported to be inactive as an oxidant and to
undergo thermal rearrangement to N-tert-butyl-á-phenylnitrone at atmospheric pressure. It has been
shown that under thermal and high-pressure conditions the ability of the oxaziridine to react changes
dramatically. The mechanism for the high-pressure reaction is also discussed.
Table 1 Oxidation of the sulfides 2 with oxaziridines 1 under high
High pressure sometimes accelerates organic reactions which
are difficult or slow at atmospheric pressure to give high yields
of the desired products; this phenomenon arises from the close
packing of the molecules under such conditions. High-pressure
effects were particularly noticeable in the reactions of sterically
crowded molecules such as 2,6-disubstituted pyridines where
N-alkylation was accelerated under such conditions.1
pressurea
Yieldb (%)
Entry
Oxaziridine 1
Sulfide 2
3
4
5
6
1c
2d
3e
4
5
6
7
8
9
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1f
2a
2a
2a
2a
2b
2c
2a
2a
2a
2a
2a
2a
2
3
7
64
25
3
42
6
8
8
6
22
62
53
5
62
6
7
54
81
6
18
36
38
18
58
63
8
18
12
22
6
Although some oxaziridines, which are unique three-
membered heterocycles having a carbon-, nitrogen- and oxygen-
containing ring,2 are oxidants,3 2-tert-butyl-3-phenyloxaziridine
1a has little such ability4 and acts in this capacity only with
trisubstituted phosphines5 and halides6 to give phosphine
oxides and halogens, respectively. The lower reactivity of 1a
is the result of steric hindrance around active sites which
obstructs attack by substrates. Since we expected that high
pressure would effectively counter such steric hindrance, we
attempted to oxidize sulfides to sulfoxides with 3-aryl sub-
stituted 2-tert-butyloxaziridines. The oxaziridine 1a was packed
and sealed in a Teflon tube with an excess of methyl phenyl
sulfide 2a, and then subjected to 800 MPa at 100 ЊC. After 20
h, the reaction products were analyzed by GLC and the yields
calculated on the basis of the amount of starting oxaziridine
consumed. The sulfide 2a was successfully oxidized to methyl
phenyl sulfoxide 3a (64%) and N-tert-butylbenzaldimine 4a,
N-tert-butyl-α-phenylnitrone 5a and benzaldehyde 6a were also
identified in the reaction mixture. The imine 4a was produced
as the result of oxygen transfer from the oxaziridine ring to the
sulfide while the nitrone 5a was formed by thermal
rearrangement of the oxaziridine ring. This oxidation was
repeated under a variety of conditions, and the results are
summarized in Table 1. The product distribution was dramat-
ically changed between 400 (entry 2) and 800 MPa (entry 4), the
highest yield of 2a being obtained for a reaction at 800 MPa
and 100 ЊC.
8
20
—
—
15
—
8
10
11
12
37
18
39
66
36
51
23
13
1g
—
a Reaction conditions, 1 (1 mmol) and 2 (5 mmol), 800 MPa, 100 ЊC, 20
h. b Calculated based on 1. c Under atmospheric pressure. d 400 MPa.
e 40 ЊC.
O
H
C
N
But
R2
S
R3
+
R1
1
2
O
O
N
R2
S
R3
R1CH
N
But
R1CH
But
R1CHO
+
+
+
3
6
4
5
R2
Ph
Me
Ph
R3
R1
Ph
2, 3
1, 4, 5, 6
a
b
c
Me
Me
Ph
a
b
c
d
e
f
p-O2NC6H4
p-MeC6H4
p-MeOC6H4
2-Pyridyl
The sulfide 2a was oxidized under high pressure with other
oxaziridines, higher product yields being obtained for com-
pounds having electron-withdrawing substituents (Entries 7, 10
and 12) compared with those having electron-donating sub-
stituents (Entries 8, 9 and 11). This result agrees with those
reported by Tamagaki et al. for the oxidation of triphosphine
with oxaziridines.5
3-Pyridyl
g
4-Pyridyl
Scheme 1 Reaction conditions: 800 MPa, 100 ЊC, 20 h
The effect of the high pressure in this oxidation is explained
as follows. Because their ring size is small, oxaziridines are
always subject to structural strain. When a mixture of 1a and 2a
was heated under atmospheric pressure, the bulky substituent
on the nitrogen atom of 1a prevented approach of 2a to the
adjacent oxygen atom of the oxaziridine ring; in other words,
the oxidation centre and substrate were insufficiently close to
react. Addition of thermal energy to the oxaziridine, however,
reduced the ring strain, the result being rearrangement of the
oxaziridine ring to the thermally stable nitrone compound then
becoming the main reaction (Entry 1). Having lost its oxidation
activity,6 the rearrangement nitrone 5a was unable to oxidize
the sulfide 2a even at 800 MPa and 100 ЊC, and was recovered
unchanged after 20 h. In contrast, under high pressure condi-
J. Chem. Soc., Perkin Trans. 1, 1997
3491

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tions, the reaction system brought 1a and 2a into close contact
with the result that the oxaziridine-ring strain was reduced by
oxygen atom transfer to the substrate, and the imine compound
4a and the sulfoxide 3a were formed. The reaction was, in effect,
a competitive one between oxaziridine-ring rearrangement
and sulfide oxidation. Therefore, in the oxidation of diphenyl
sulfide 2b, which has bulkier substituents than methyl phenyl
sulfide, the yield of diphenyl sulfoxide 3c was very low and
the ring rearrangement became the main reaction (Entry 6).
In conclusion, 3-aryl-2-tert-butyloxaziridines have been
shown to behave as oxidants under high pressure, even with a
compound such as thioanisole which has been reported to be
resistant to oxidation. These results can be explained in steric
terms as high pressure overcomes the hindrance engendered by
the bulkiness of the oxaziridine substituents and brings the
oxaziridine and sulfides sufficiently close to reaction.
pyridyl)nitrone 5e, mp 62–63 ЊC (Found: C, 67.6; H, 7.9; N,
15.7. C10H14N2O requires C, 67.4; H, 7.9; N, 15.7%); νmax(neat)/
cmϪ1 2980, 1560, 1437, 1400, 1197, 1123, 911, 816 and 733;
δH(300 MHz; CDCl3) 1.64 (9H, s, But), 7.26–7.30 (1H, m), 7.79
(1H, td, J 8, 2), 7.90 (1H, s, CH᎐), 8.64 (1H, dd, J 2, 5) and 9.22
᎐
(1H, d, J 8). N-tert-Butyl-α-(3-pyridyl)nitrone 5f, mp 81–82 ЊC
(hexane) (Found: C, 67.6; H, 8.0; N, 15.6. C10H14N2O requires
C, 67.4; H, 7.9; N, 15.7%); νmax(neat)/cmϪ1 2990, 1560, 1417,
1361, 1199, 1131, 909, 832 and 710; δH(300 MHz; CDCl3) 1.64
(9H, s, But), 7.36 (1H, dd, J 5, 8), 7.61 (1H, s, CH᎐), 8.59 (1H,
᎐
br, d, J 4), 8.98 (1H, br, s) and 9.11 (1H, dt, J 2, 8). N-tert-Butyl-
α-(4-pyridyl)nitrone 5g, mp 84–86 ЊC (hexane) (lit.,11 99–
101 ЊC) (Found: C, 67.3; H, 8.05; N, 15.7. C10H14N2O requires
C, 67.4; H, 7.9; N, 15.7%); νmax(neat)/cmϪ1 2980, 1644, 1568,
1417, 1365, 1137, 990, 843, 669 and 530; δH(300 MHz; CDCl3)
1.63 (9H, s, But), 7.58 (1H, s, CH᎐), 8.07 (2H, d, J 6) and 8.69
᎐
(2H, br, s).
Experimental
General oxidation procedure
Mps determined on a Yanagimoto micro-melting point appar-
atus are uncorrected. H NMR spectra were obtained with a
A homogeneous mixture of the oxaziridine (100 mg) and the
sulfide (300 mg) in a sealed Teflon tube was compressed to
800 MPa and heated at 100 ЊC for 20 h in high-pressure
equipment.12 After the reaction, the reaction mixture was
cooled to room temperature and the products were analyzed
with GLC with naphthalene as a standard.
1
Hitachi R-40 High-Resolution Spectrometer (90 MHz) and
Varian Gemini 300 BB (300 MHz) with tetramethylsilane as
an internal standard; J values given in Hz. IR spectra were
recorded on a JASCO FTIR-7000 Fourier transfer infrared
spectrophotometer. Gas chromatographic analyses were per-
formed on a Shimadzu GC-14A chromatograph fitted with a
Neutrabond-1 column (OV-1 equivalent).
References
Oxaziridines were prepared by reported methods.7,8 Com-
pounds 1a,7 1b6 and 1c–e,g9 were found to have spectral char-
acteristics and physical properties identical with those reported.
2-tert-Butyl-3-(3-pyridyl)oxaziridine 1f, mp 32–33 ЊC (pentane)
(Found: C, 67.7; H, 7.8; N, 15.5. C10H14N2O requires C, 67.4; H,
7.9; N, 15.7%); νmax(neat)/cmϪ1 2976, 1599, 1580, 1479, 1435,
1390, 1365, 1328, 1267, 1205, 1027, 863, 806 and 710; δH(300
MHz; CDCl3) 1.19 (9H, s, But), 4.73 (1H, s, C-3), 7.32 (1H, dd,
J 5, 8). 7.73 (1H, dt, J 8, 2), 8.64 (1H, br, d, J 4) and 8.71 (1H, br,
s).
1 W. J. le Noble and Y. Ogo, Tetrahedron, 1970, 26, 4119.
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63; J. P. Freeman, in The Chemistry of Heterocyclic Compounds, eds.,
A. Weissberger and E. C. Taylor, Part 3, Wiley, New York, 1985,
p. 283; F. A. Davis and A. C. Sheppard, Tetrahedron, 1989, 45, 5703.
3 Y. Hata and M. Watanabe, J. Org. Chem, 1981, 46, 610; L. C.
Vishwakarma, O. D. Stringer and F. A. Davis, Org. Synth., 1987,
66, 203; V. A. Petrov and G. Resnati, Chem. Rev., 1996, 96, 1809.
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5 S. Tamagaki, K. Sakaki and S. Oae, Bull. Chem. Soc. Jpn., 1972, 45,
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Sulfoxides were identified by comparison with com-
mercially available authentic samples (concordant retention
times on GLC).
6 W. D. Emmons, J. Am. Chem. Soc., 1957, 79, 5739.
7 W. D. Emmons and A. S. Pagano, Org. Synth., Coll. vol. 5, p. 191.
8 F. A. Davis, S. Chattopadhyay, J. C. Towson, S. Lal and T. Reddy,
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Nitrones obtained by high pressure oxidation were isolated
fromthereactionmixtureswithcolumnchromatographyonsilica
gel with dichloromethane–acetone–methanol (100:10:2 or
100:40:8) as eluent. N-tert-Butyl-α-phenylnitrone 5a, mp 71–
72 ЊC (hexane) (lit.,6 75–76 ЊC). N-tert-Butyl-α-(p-nitrophenyl)-
nitrone 5b, mp 130–131 ЊC (lit.,6 134–135 ЊC). N-tert-Butyl-α-
(p-methylphenyl)nitrone 5c, mp 69–70 ЊC (hexane) (lit.,10
72–74 ЊC). N-tert-Butyl-α-(p-methoxyphenyl)nitrone 5d, bp
202 ЊC/270 Pa. (Found: C, 68.7; H, 8.2; N, 6.65. C12H17-
NO2ؒ0.1H2O requires C, 68.9; H, 8.2; N, 6.7%); νmax(neat)/cmϪ1
2978, 1605, 1363, 1261, 1172, 1125, 1033 and 843; δH(90 MHz;
CDCl3) 1.57 (9H, s, But), 3.81 (3H, s, CH3O), 6.92 (2H, d, J 10),
Paper 7/05022D
Received 11th July 1997
7.47 (1H, s, CH᎐) and 8.29 (2H, d, J 10). N-tert-Butyl-α-(2-
Accepted 22nd August 1997
᎐
3492
J. Chem. Soc., Perkin Trans. 1, 1997