Oxidatively sonochemical dealkylation of various N-alkylsulfonamides to free sulfonamides and aldehydes

Article abstract of DOI:10.1016/S0040-4020(01)00725-6

Various N-alkylsulfonamides were easily dealkylated to give the corresponding free sulfonamides in moderate to good yields in the presence of (diacetoxyiodo)benzene and iodine under ultrasonic irradiation. Application of this methodology to various N-prot

Full text of DOI:10.1016/S0040-4020(01)00725-6

TETRAHEDRON
Pergamon
Tetrahedron 57 '2001) 7481±7486
Oxidatively sonochemical dealkylation of various
N-alkylsulfonamides to free sulfonamides and aldehydes
Masashi Katohgia and Hideo Togoa,b,p
aGraduate School of Science and Technology, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan
bDepartment of Chemistry, Faculty of Science, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan
Received 19 June 2001; accepted 13 July 2001
AbstractÐVarious N-alkylsulfonamides were easily dealkylated to give the corresponding free sulfonamides in moderate to good yields in
the presence of 'diacetoxyiodo)benzene and iodine under ultrasonic irradiation. Application of this methodology to various N-protected
alkylamines with sulfonyl, phosphonyl, and acyl groups was carried out, and the oxidative conversion occurred only in N-sulfonyl-protected
phenylalkylamines to give the corresponding aldehydes together with free sulfonamides in moderate to good yields. q 2001 Elsevier Science
Ltd. All rights reserved.
1. Introduction
direct oxidation of amines to carbonyl groups is known,9
and N-dealkylation of trialkylamines to dialkylamines was
studied.10
Recently, synthetic application of ultrasonic irradiation has
been a focus of interest in organic synthesis.1 Sometimes
this technique leads not only to the simple rate enhance-
ment of chemical reactions, especially in organometallic
chemistry, but also to the induction of radical pathways,
giving products which are normally not accessible via
ionic reactions.2 Thus, ultrasonic irradiation for organic
reactions has become popular to improve yields, to reduce
reaction time, and to develop new reactions. On the other
hand, extensive study on hypervalent iodine compounds has
been carried out and their utilization for organic synthesis
has been popularized.3 Especially, 'diacetoxyiodo)benzene
has been widely used in organic synthesis in view of its
potential utility for oxidation and radical reactions.
In spite of these advantages, sonochemical reactions of
a 'diacetoxyiodo)benzene/iodine system are extremely
limited.4 A part of our study on the synthetic use of sul-
fonamidyl radicals with 'diacetoxyiodo)benzene as a radical
precursor,5 we would like to report on the oxidative
dealkylation of various N-alkylsulfonamides6 to give the
corresponding aldehydes together with free sulfonamides
in moderate to good yields by ultrasonic irradiation in the
presence of 'diacetoxyiodo)benzene and iodine. Nowadays,
the removal of the sulfonyl group 'deprotection) in sulfona-
mides has been well investigated.7 However, most of these
approaches were focused on cleavage of the S±N bond in
sulfonamides; meanwhile, study on the C±N bond cleavage
of N-alkylsulfonamides is scarcely known,8 though the
2. Results and discussion
2.1. Sonochemical dealkylation of various N-alkyl-
sulfonamides
At ®rst, sonication of N-ethyl'o-methyl)benzenesulfona-
mide '1a-II) was studied. The reaction was carried out
under various reaction conditions as shown in Table 1.
Deethylation of N-ethyl'o-methyl)benzenesulfonamide
proceeds smoothly in 1,2-dichloroethane to give
o-methylbenzenesulfonamide '2a) in 69% yield under
ultrasonic irradiation '200 W; 28 kHz) in the presence of
Table 1. Sonochemical deethylation of N-ethyl'o-methyl)benzenesulfon-
amide 1a±II
Entry
Equiv.
PhI'OAc)2
Yield '%)
Recovery '%)
I2
1
2
3
4
5
3.0
3.0
±
3.0
3.0
1.0
±
69
0
0
98
96
94
83
1.0
1.0a
1.0b
0
0
0
Keywords: 'diacetoxyiodo)arene; N-alkylsulfonamide; oxidative dealkyl-
ation; ultrasonic irradiation.
a
p
THF was used as a solvent.
Br2 was used instead of I2.
Corresponding author. Fax: 181-43-290-2874;
e-mail: togo@scichem.s.chiba-u.ac.jp
b
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020'01)00725-6

7482
M. Katohgi, H. Togo / Tetrahedron 57 02001) 7481±7486
Table 2. Substituent effect of 'diacetoxyiodo)arenes
Entry
1
Ar
Yield '%)
42
2
3
4
68
69
60
Scheme 1. Selective removal of the N-ethyl group.
'diacetoxyiodo)benzene '3.0 equiv.) and iodine '1.0 equiv.).
N-Iodo-N-ethyl'o-methyl)-benzenesulfonamide was quanti-
tatively obtained by the reaction of N-ethyl'o-methyl)-
benzenesulfonamide, 'diacetoxyiodo)benzene and iodine
without ultrasonic irradiation. The reaction does not proceed
at all with 'diacetoxyiodo)benzene or iodine alone, and with
'diacetoxyiodo)benzene and bromine instead of iodine,
under ultrasonic irradiation. Moreover, a tetrahydrofuran
or ethanol solvent inhibits the present deethylation com-
pletely, due to the hydrogen-atom abstraction from the
solvent by the formed sulfonamidyl radical. Use of [bis'tri-
¯uoroacetoxy)iodo]benzene instead of 'diacetoxyiodo)-
benzene under the same conditions does not induce the
deethylation, and
a wet 1,2-dichloroethane solvent
'including 1% v/v H2O) instead of dry 1,2-dichloroethane
also does not give rise to deethylation. In these systems, the
starting amide 1a-II was recovered.
Table 3. Sonochemical dealkylation of various N-alkysulfonamide 1
The substituent effect of 'diacetoxyiodo)arenes for the
present deethylation was examined as shown in Table 2.
'Diacetoxyiodo)benzene showed the best reactivity and
gave the compound 2a in 69% yield, though there is no
big difference among 'diacetoxyiodo)benzene, 'diacetoxy-
iodo)-p-toluene, and 'diacetoxyiodo)-p-chlorobenzene, and
'diacetoxyiodo)-p-methoxybenzene was less effective.
Probably, this difference comes from the fact that iodine
is consumed through the iodination of the formed p-iodo-
anisole by an acetylhypoiodite species derived from the
'diacetoxyiodo)-p-methoxybenzene/iodine system.11
Entry
R-
R0
1
2/Yield '%)
1
2
±CH3
±CH2CH3
1a-I
1a-II
0
69
2a
2b
3
4
5
±CH2CH3 1b-II
±CH2CH2CH3 1b-III
±CH2'CH2)2CH3 1b-IV
63
54
43
Various N-alkylarenesulfonamides and N-alkylalkane-
sulfonamides were used as substrates as shown in Table 3.
Here, N-ethyl, N-propyl and N-butylsulfonamides were
dealkylated in moderate to good yields 'entries 1±7);
especially, the N-ethyl group shows the best reactivity.
Demethylation of N-methylsulfonamide does not proceed
at all, and the starting material was recovered in 93%
yield 'entry 1). In addition, dealkylation of N,N-dimethyl-,
N,N-diethyl-, N-isopropyl-, N-tert-butylsulfonamides and
N-ethylbenzamide did not occur. Moreover, deethylation
of N-ethylarenesulfonamides bearing an electron-with-
drawing group such as a methanesulfonyl 'entry 11), nitro,
and N,N-diethylaminocarbonyl groups on the aromatic ring,
did not proceed. In these reactions, the starting amides were
recovered. So the present dealkylation reaction is signi®-
cantly affected by both steric effects and electronic effects.
6
7
±CH3
±CH2CH3
1c-I
1c-II
0
2c
2d
2e
2f
50a
8
±CH2CH3
±CH2CH3
±CH2CH3
±CH2CH3
1d-II
1e-II
1f-II
1g-II
66
40
51a
0
9
10
11
12
13
±CH2CH3
±CH2CH3
±CH2CH3
1h-II
1i-II
1j-II
71
69
81
2h
2i
The selective removal of the N-ethyl group was studied as
shown in Scheme 1. When a mixture of N,N-diethyl-2-
phenylethanesulfonamide 3 and N-ethyl-2-phenylethane-
sulfonamide 1j-II was treated with 'diacetoxyiodo)benzene
and iodine under irradiation with an ultrasonic cleaning
bath, only sulfonamide 1j-II was dealkylated to give the
corresponding dealkylated sulfonamide 2j in 79% yield,
14
2j
a
Irradiated for 6 h.

M. Katohgi, H. Togo / Tetrahedron 57 02001) 7481±7486
7483
from 'diacetoxyiodo)benzene and iodine.11 Thus, aldehydes
are the side product in the present sonochemical
dealkylation of sulfonamides. When the amides were
protected by a diethyl phosphate or p-methylbenzoyl
group, the corresponding aldehydes were not formed and
both starting amides were recovered in 94% yields, respec-
tively 'entries 6 and 7). The addition of potassium carbonate
'1.0 equiv.), did not accelerate the reactions.
Table 4. Oxidative deamination of various N-protected 3-phenyl-1-propyl-
amines 1 'Z=protecting group)
Next, we worked on changing the number of methylene
units of the starting alkylamines. Various N-tosylamines
bearing an aromatic ring at the a-, b-, g-, and d-positions,
respectively, were treated under similar sonochemical
conditions. The results are summarized in Table 5. Here,
in entries 3 and 4, oxidative deamination occurred
ef®ciently to give p-toluenesulfonamide 2c and aldehydes
4 in good yields. In addition, in entry 4, the Hofmann±
Entry
±Z
1
Yields '%)
4-V 'A/B)
2
1
2
3
1k-V
1c-V
1h-V
76
70
74
64 '50/14)
72 '61/11)
91 '76/15)
È
Lof¯er±Freytag type reaction partly occurred to form
N-tosyl-2-phenylpyrrolidine as a by-product in 19% yield,
through cyclization of the formed sulfonamidyl radical.
N-Tosybenzylamine did not react ef®ciently 'entry 1), and
the starting sulfonamide was recovered in 18% yield in spite
of increasing the irradiation time. In entry 2, the reaction
proceeded ef®ciently; however, the phenethyl group may
react further to give a complicated reaction mixture via
the formation of phenylacetaldehyde, together with toluene-
sulfonamide in 72% yield 'Table 5).
4
5
6
7
a
1l-V
1m-V
5
74
66
0a
90 '65/25)
63 '24/39)
0
0
6
0a
Starting material was recovered in 94% yield.
To optimize the reaction conditions, the effects of output
and frequency in the ultrasonic device were also studied
'Table 6). Here, as the frequency was elevated, the yield
of sulfonamide 2c decreased and the recovery yield of the
starting material was increased. Thus, as general reaction
conditions, high output and low frequency of the ultrasonic
device were effective for this reaction 'entry 4).
together with the complete recovery of sulfonamide 3
'96%).
2.2. Oxidative deamination of various N-protected
phenylalkylamines
Now, what is the side product in the sonochemical
dealkylation of N-alkylsulfonamides? As an extension of
the present N-deethylation methodology, reactivity of
various N-protected 3-phenyl-1-propylamines 1 was studied
as shown in Table 4. Here, the reaction proceeds effectively
to give 3-phenylpropionaldehyde 4-V-A and its p-iodinated
aldehyde 4-V-B together with dealkylated sulfonamide 2.
p-Iodinated aldehyde was formed by the iodination of the
starting sulfonamide 1 or the aldehyde 4-V-A through the
ionic pathway with an acetylhypoiodite species generated
Based on these results, a reasonable reaction pathway of the
present reaction is shown in Scheme 2. Here, the formation
of the N-iodinated compound under dark conditions and the
formation of an unstable imino compound after ultrasonic
irradiation were observed by 1H NMR measurements.
Moreover, the addition of a galvinoxyl free radical
'1.0 equiv.) inhibited this reaction completely. Thus, the
present reaction proceeds through the oxidative b-hydrogen
elimination of the formed sulfonamidyl radical to give an
Table 5. Oxidative deamination of various N-tosylphenylalkylamines
Entry
n
1
Yields '%)
Recovery '%)
2c
4 'A/B)
Trace
Trace
1
2
3
4
0
1
2
3
1c-VI
1c-VII
1c-V
45a
72
18
0
0
70
71b
73 '61/11)
56 '33/23)
1c-VIII
0
a
Irradiated for 6 h.
N-Tosyl-2-phenylpyrrolidine was obtained in 19%.
b

7484
M. Katohgi, H. Togo / Tetrahedron 57 02001) 7481±7486
Table 6. Effects of output and frequency of ultrasonic device
Entry
Output 'W)
Frequency 'kHz)
Yields '%)
4-V 'A/B)
Recovery '%)
2c
1
2
3
4
100
100
100
200
28
45
100
28
64
53
33
70
63 '52/11)
42 '38/4)
24 '21/3)
72 '61/11)
0
32
65
0
Scheme 2. Plausible reaction mechanism.
imino derivative. The dealkylated sulfonamide and alde-
hyde come from the hydrolysis of the corresponding
imino compound. Hence, N-demethylation probably did
not occur at all due to the slightly stronger C±H bond in
the N-methyl group than that in the N-ethyl group.
3.2. Materials
'Diacetoxyiodo)benzene and [bis'tri¯uoroacetoxy)iodo]-
benzene are commercially available. The other 'diacetoxy-
iodo)arenes were prepared by the oxidation of the
corresponding iodoarenes based on the literature method.12
Most N-alkylsulfonamides were easily prepared by the
reaction of sulfonyl chlorides and alkylamines.13 o-Toluene-
sulfonamide, p-toluenesulfonamide, benzenesulfonamide,
methanesulfonamide and 3-phenylpropionaldehyde were
identi®ed with commercially available compounds.
In conclusion, novel and ef®cient sonochemical dealkyl-
ation of various N-alkylsulfonamides and oxidative deami-
nation of various N-protected alkylamines were studied.
Ultrasound promises to be a new and powerful technique
in hypervalent iodine chemistry. To the best of our
knowledge, this type of dealkylation of sulfonamides to
form free sulfonamides and aldehyde has not hitherto
been reported. A similar conversion from an amino group
to aldehyde is known in vitamin B6 using pyridoxal. Thus,
the establishment and application of this methodology in the
bioorganic ®eld is also interesting.
3.3. General procedureÐoxidative deamination of
various N-alkylsulfonamides
'Diacetoxyiodo)benzene '1.5 mmol) and iodine '0.5 mmol)
were added to a solution of N-alkylsulfonamide '0.5 mmol)
in 1,2-dichloroethane '7 ml). The mixture was stirred pre-
liminarily for 10 min. under dark conditions, and then
irradiated with an ultrasonic cleaning bath '200 W;
28 kHz or 100 W; 28, 45, and 100 kHz) for an appropriate
time under an argon atmosphere in the range of 30±408C.
After the reaction, the mixture was poured into ethyl acetate
and washed with aq. sodium sul®te 'Na2SO3) solution and
subsequently with water. The organic layer was dried over
sodium sulfate 'Na2SO4). After removal of the solvent under
reduced pressure, the residue was chromatographed on
silica gel by preparative thin-layer chromatography using
a mixture of hexane and ethyl acetate '3:1±5:1) as an eluent.
3. Experimental
3.1. General methods
1H NMR Spectra were recorded on 400 MHz spectrometer.
Chemical shifts are reported as ppm down®eld from tetra-
methylsilane 'TMS) in d units. J values are given in Hz.
Mass spectra were measured with QMS 'EI), and high-
resolution mass spectra 'HRMS) were measured with a
110A mass spectrometer. Silica gel C-200 was used for
column chromatography, Kieselgel 60 F 254 'Merck) was
used for TLC, and Wakogel B-5F was used for preparative
thin-layer chromatography 'p-TLC). Sonication was
performed with Tokyo Rika Kikai AC-70 CO '200 W;
28 kHz) or HONDA W-113 '100 W; 28, 45 and 100 kHz)
ultrasonic cleaning device.
3.3.1. 2,5-Dimethylbenzenesulfonamide /2b). Mp 142.5±
143.58C 'lit.14 145.5±146.58C); IR 'KBr) 3350, 3255, 1290,
1
1140 cm21; H NMR '400 MHz, CDCl3) d 2.37 's, 3H),
2.64 's, 3H), 4.75 'bs, 2H), 7.20 'd, Jˆ7.7 Hz, 2H), 7.27
'd, Jˆ8.6 Hz, 1H), 7.84 's, 1H); MS 'EI) found M1ˆ185.

M. Katohgi, H. Togo / Tetrahedron 57 02001) 7481±7486
7485
3.3.2. 2,4-Dimethylbenzenesulfonamide /2d). Mp 135.5±
Acknowledgements
136.58C 'lit.14 136.5±137.08C); IR 'KBr) 3365, 3260, 1320,
1
1175 cm21; H NMR '400 MHz, CDCl3) d 2.38 's, 3H),
We are grateful for the ®nancial support from a Grant-in-
Aid for Scienti®c Research '13554028) from the Ministry of
Education, Science, Sports and Culture of Japan. We also
thank Ms Ritsuko Hara for measurement of HRMS in
Chemical Analysis Center of Chiba University.
2.64 's, 3H), 4.89 'bs, 2H), 7.10 'd, Jˆ8.0 Hz, 2H), 7.13
's, 1H), 7.87 'd, Jˆ8.0 Hz, 1H); MS 'EI) found M1ˆ185.
3.3.3. 2,3,4,5-Tetramethylbenzenesulfonamide /2e). Mp
180.0±181.58C 'lit.14 183.5±184.08C); IR 'KBr) 3300,
1
3240, 1295, 1155 cm21; H NMR '400 MHz, CDCl3) d
2.25 's, 3H), 2.26 's, 3H), 2.31 's, 3H), 2.59 's, 3H), 4.77
References
'bs, 2H), 7.72 's, 1H); MS 'EI) found M1ˆ213.
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3.3.4. 5-Bromo-2-methylbenzenesulfonamide /2f). Mp
163.0±164.58C 'lit.15 164.0±165.08C); IR 'KBr) 3400,
1
3295, 1295, 1160 cm21; H NMR '400 MHz, CDCl3) d
2.63 's, 3H), 4.92 'bs, 2H), 7.21 'd, Jˆ8.0 Hz, 1H), 7.58
'dd, Jˆ8.2, 2.2 Hz, 1H), 8.15 'd, Jˆ2.2 Hz, 1H); MS 'EI)
found M1ˆ249, 251.
3.3.5. Benzylsulfonamide /2i). Mp 98.5±100.08C 'lit.16
101.0±102.08C); IR 'KBr) 3380, 3320, 1325, 1125 cm21
;
1H NMR '400 MHz, CDCl3) d 4.30 's, 2H), 4.72 'bs, 2H),
7.37±7.44 'm, 5H); MS 'EI) found M1ˆ171.
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3.3.6. Phenylethanesulfonamide /2j). Mp 114.0±115.58C;
IR 'KBr) 3360, 3250, 1310, 1160 cm21
;
1H NMR
'400 MHz, CDCl3) d 3.14±3.18 'm, 2H), 3.39±3.43 'm,
2H), 4.85 'bs, 2H), 7.21±7.27 'm, 3H), 7.30±7.35 'm,
2H); MS 'EI) found M1ˆ185.
3.3.7. 4-Methoxybenzenesulfonamide /2k). Mp 107.5±
109.08C 'lit.17 109.08C); IR 'KBr) 3350, 3260, 1330,
1
1160 cm21; H NMR '400 MHz, CDCl3) d 3.87 's, 3H),
4.96 'bs, 2H), 6.98 'd, Jˆ9.2 Hz, 2H), 7.86 'd, Jˆ8.9 Hz,
2H); MS 'EI) found M1ˆ187.
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3.3.8. 4-Bromobenzenesulfonamide /2l). Mp 158.5±
159.58C 'lit.17 162.08C); IR 'KBr) 3330, 3240, 1330,
1
1150 cm21; H NMR '400 MHz, CDCl3) d 4.93 'bs, 2H),
7.67 'd, Jˆ8.7 Hz, 2H), 7.80 'd, Jˆ8.7 Hz, 2H); MS 'EI)
found M1ˆ235, 237.
3.3.9. 3-/4-Iodophenyl)propionaldehyde /4-V-B). Oil; IR
'neat) 2925, 2855, 1720, 1590, 1480 cm21 1H NMR
;
'400 MHz, CDCl3) d 2.75 'td, J1ˆ7.3, J2ˆ1.2 Hz, 3H),
2.89 't, Jˆ7.3 Hz, 2H), 6.94 'd, Jˆ8.5 Hz, 2H), 7.60 'd,
Jˆ8.5 Hz, 2H), 9.79 't, Jˆ1.2 Hz, 1H); HRMS 'EI) found
M1ˆ259.9687, calcd for C9H9OI Mˆ259.9698.
3.3.10. 4-Phenylbutyraldehyde /4-VIII-A). Oil; IR 'neat)
2930, 2860, 1720, 1605, 1495 cm21; 1H NMR18 '400 MHz,
CDCl3) d 1.97 'quint., Jˆ7.3 Hz, 2H), 2.46 'td, J1ˆ7.3,
J2ˆ1.5 Hz, 2H), 2.66 't, Jˆ7.6 Hz, 2H), 7.17±7.31 'm,
5H), 9.76 't, Jˆ1.6 Hz, 1H); HRMS 'EI) found M1ˆ
148.0891, calcd for C10H12O Mˆ148.0888.
Â
4. Costa, S. C. P.; Moreno, M. J. S. M.; Sa e Melo, M. L.;
Campos Neves, A. S. Tetrahedron Lett. 1999, 40, 8711.
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Tetrahedron 1999, 55, 14885. 'b) Togo, H.; Harada, Y.;
Yokoyama, M. J. Org. Chem. 2000, 65, 926.
3.3.11. 4-/4-Iodophenyl)butyraldehyde /4-VIII-B). Oil;
1
IR 'neat) 2925, 2855, 1720, 1595, 1485 cm21; H NMR
6. As a preliminary report: Katohgi, M.; Yokoyama, M.; Togo,
H. Synlett 2000, 1055.
'400 MHz, CDCl3) d 1.93 'quint., Jˆ7.5 Hz, 2H), 2.45
'td, J1ˆ7.3 Hz, J2ˆ1.5 Hz, 2H), 2.60 'td, J1ˆ8.0, J2ˆ
1.5 Hz, 2H), 6.93 'd, Jˆ8.5 Hz, 2H), 7.61 'd, Jˆ8.2 Hz,
2H), 9.76 't, Jˆ1.6 Hz, 1H); HRMS 'EI) found M1ˆ
273.9866, calcd for C10H11OI Mˆ273.9855.
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