Ultrasound-promoted deoxygenation of sulphoxides by samarium - NH4Cl

Article abstract of DOI:10.1055/s-2001-14911

Several aromatic and aliphatic sulphoxides are selectively deoxygenated to the corresponding thioethers in high to quantitative yields by samarium metal in methanolic ammonium chloride under sonication. Other functional groups such as halides, esters, ethers, nitriles, olefins and ketones are unaffected under the present reaction conditions.

Full text of DOI:10.1055/s-2001-14911

854
LETTER
Ultrasound-Promoted Deoxygenation of Sulphoxides by Samarium NH4Cl
Jhillu. S. Yadav,* Basi V. Subba Reddy, C. Srinivas, P. Srihari
Organic Chemistry Division 1, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Fax +91 40 7170512; E-mail: yadav@iict.ap.nic.in
Received 9 April 2001
Several sulphoxides were selectively deoxygenated to the
Abstract: Several aromatic and aliphatic sulphoxides are selective-
corresponding thioethers by samarium metal and ammo-
nium chloride in methanol under sonic waves. The sul-
phoxides are prepared by oxidation of the corresponding
ly deoxygenated to the corresponding thioethers in high to quantita-
tive yields by samarium metal in methanolic ammonium chloride
under sonication. Other functional groups such as halides, esters,
ethers, nitriles, olefins and ketones are unaffected under the present sulphides with sodium metaperiodate in aqueous metha-
nol.10 The reactions were clean, high-yielding and com-
reaction conditions.
pleted within 3-7 hours.11 The reaction rates and yields
Key words: samarium, ultrasound, sulphoxides, sulphides
were dramatically enhanced by ultrasound. In the absence
of sonic waves, Sm-NH4Cl mediated reactions gave mod-
erate yields of products after a long reaction time (10-15
Sulphoxides and sulphones find wide applications in or-
hours) at room temperature. This is because the surface of
ganic synthesis, particularly in carbon-carbon bond form-
the samarium metal is unreactive, which can be activated
ing processes.1 Especially, sulphoxides deserved much
by ultrasound. The rate enhancement under ultrasound
attention as important chiral auxiliary in asymmetric syn-
may be attributed through the cavitation and the cleaning
of the metal surface by sonic waves. The reaction condi-
tions were compatible with other functional groups such
as halides, esters, ethers, nitriles, olefins and ketones. Sul-
phones were unreactive by Samarium metal in methanolic
ammonium chloride. Both aromatic and aliphatic sul-
phoxides were reduced to the corresponding thioethers
and the results are summarized in Table. A wide range of
structurally varied sulphoxides underwent smoothly
deoxygenation by samarium metal under neutral condi-
tions to provide thioethers in high yields. Methanol was
found to be the best solvent for this reaction. Several met-
als such as zinc, tin, indium and samarium were employed
for this transformation. Among them samarium was found
to be effective in terms of reaction time and yields.
thesis.2 Generally, sulphoxides are removed by a two step
process3 involving deoxygenation to sulphides which are
further reduced by Raney nickel or lithium in liquid NH3.
Many reagents4-6 such as TiCl4-Zn, TiCl4-LiAlH4, TiCl4-
NaI, CrCl2, VCl2, SnCl2, WCl6-Zn and 2,6 dihydroxypyri-
dine have been reported for the deoxygenation of sulphox-
ides to sulphides. However, many of these procedures3,7
employ strong acids,6a strong Lewis acids, strong nucleo-
philes or highly reactive carbenes,7a expensive, hazardous
and moisture sensitive reagents.5a Further, some of these
methods are associated with limitations regarding
selectivity4c and incompatibility7b,c with other functional
groups. The difficulties associated with these procedures
can be overcome by the use of samarium metal under son-
ication. In recent years there has been increasing interest
in samarium mediated transformations because of certain
unique properties possessed by samarium. Even though,
Samarium has been used as a potential reducing agent in
various transformations,8 it has not been used for the
deoxygenation of sulphoxides. In continuation of our
work on the applications of metals in various transforma-
tions,9 herein we report a novel and highly efficient proce-
dure for the deoxygenation of sulphoxides using
samarium metal in methanolic ammonium chloride under
sonication.
In conclusion, the procedure describes a facile deoxygen-
ation of sulphoxides using samarium metal under neutral
reaction conditions. The method offers several advantages
including mild reaction conditions, greater selectivity,
high yields of product compatibility with other functional
groups, cleaner reactions, simple experimental and prod-
uct isolation procedures, which makes it a useful and at-
tractive strategy for the reduction of sulphoxides.
Acknowledgement
Three of us (BVS, ChS and PSH) thank CSIR, New Delhi for the
award of fellowships.
References and Notes
IICT Commun. No.4644
(1) Magnus, P. D. Tetrahedron 1977, 33, 2019.
(2) a) Solladie, G. Synthesis 1981, 185. b) Carreno, M. C. Chem.
Rev. 1995, 95, 1717. c) Davies, S. G.; Loveridge, T.; Clough,
J. M. Synlett 1997, 66.
Synlett 2001, No. 6, 854–856 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Ultrasound-Promoted Deoxygenation of Sulphoxides by Samarium NH4Cl
Table Reduction of sulfoxides to sulfides using samarium metal
855
a) All products were characterized by 1H NMR, IR and MS.
b) Isolated yields after purification.
(3) a) Madesclaire, M. Tetrahedron, 1988, 44, 6537.
b) Drabowicz, J.; Numata, T.; Oae, S. Org. Prep. Proced. Int.
1977, 9, 63.
(4) a) Drabowicz, J.; Mikolajczyk, M. Synthesis 1978, 138.
b) Balicki, R. Synthesis 1991, 155. c) Bird, C. W. J. Chem.
Soc. 1968, 1230.
(5) a) Akita, Y.; Inaba, M.; Uchida, H.; Ohta, A. Synthesis 1977,
792. b) Ohah, G. A.; Surya Prakash, G. K.; Ho, T. L. Synthesis
1976, 810.
(6) a) Ho, T. L.; Hall, T. W.; Wong, C. M. Synthesis 1973, 206.
b) Firouzabadi, H.; Karimi, B. Synthesis 1999, 3, 500.
c) Miller, S. J.; Collier, T. R.; Wu, W. Tetrahedron Lett. 2000,
41, 3781.
(7) Drabowicz, J.; Togo, H.; Mikolajczyk, M.; Oae, S. Org. Prep.
Proc. Intl. 1984, 16, 171.b) Alper, H.; Keung, E. C. H.
Tetrahedron Lett. 1970, 53. c) Chasar, D. W. J. Org. Chem.
1971, 36, 613.
(8) a)Wang, L.; Zhou, L.; Zhang, Y. Synlett 1999, 7, 1065.
b) Banik, B. K.; Zegrocka, O.; Banik, I.; Hackfeld, L.; Becker,
F. F. Tetrahedron Lett. 1999, 40, 6731. c) Huang, Y.; Zhang,
Y.; Wang, Y. Tetrahedron Lett. 1997, 38, 1065. d) Ghatak, A.;
Becker, F. F.; Banik, B. K. Tetrahedron Lett. 2000, 41, 3793.
(9) a) Yadav, J. S.; Subba Reddy, B. V.; Muralidhar Reddy, M.
Tetrahedron Lett. 2000, 41, 2663. b) Sampath Kumar, H. M.;
Subba Reddy, B. V.; Tirupathi Reddy, P.; Yadav, J. S.
Tetrahedron Lett. 1999, 40, 5387. c) Sampath Kumar, H. M.;
Synlett 2001, No. 6, 854–856 ISSN 0936-5214 © Thieme Stuttgart · New York

856
J. S. Yadav et al.
LETTER
Subba Reddy, B. V.; Anjaneyulu, S.; Yadav, J. S. Tetrahedron
Lett. 1999, 40, 8305.
(10) Vogel’s Text book of Practical Orgnic Chemistry, fifth
edition. PP 792.
Representative spectroscopic data for compound 1a: 1H NMR
(CDCl3): 3.60 (m, 2H), 5.25 (dd, 2H, J = 5.6 Hz and 11.2
Hz), 5.60 (m, 1H), 7.60 (m, 3H), 7.95 (m, 3H), 8.15 (s, 1H).
EIMS m/z: 216 (M+), 200 (M+-16), 185, 175, 160, 147, 127,
115. IR (KBr): : 3050, 2951, 1634, 1586, 1343, 1091, 1041,
822 cm-1.
(11) Experimental Procedure: A mixture of sulfoxide (5 mmol)
samarium metal (7.5mmol) and solid NH4Cl (15 mmol) in
methanol (20 mL) was sonicated (Bransonic model 2210 R-
DTH) at room temperature for an appropriate time (Table).
After complete conversion, as indicated by TLC, the reaction
mixture was diluted with water and extracted twice with
dichloromethane (2 15 mL). The combined organic extracts
were dried over anhydrous Na2SO4, concentrated in vacuo and
purified by column chromatography on silica gel (Merck 100-
200 mesh, ethyl acetate hexane, 1:9) to afford thioether in
pure form.
Compound 2a: 1H NMR (CDCl3): 3.65 (d, 2H, J = 5.7 Hz),
5.25 (m, 2H), 5.95 (m, 1H), 7.45 (m, 3H), 7.80 (m, 4H). EIMS
m/z: 200 (M+), 185, 167, 159, 128, 115. IR (KBr): 3052,
1624, 1588, 1500, 1338, 1132, 810 cm-1.
Article Identifier:
1437-2096,E;2001,0,06,0854,0856,ftx,en;S08400ST.pdf
Synlett 2001, No. 6, 854–856 ISSN 0936-5214 © Thieme Stuttgart · New York