...someone pointed this reference from March 5th edition that may shed some light in some misconception........March Textbook 5th edition page 531
"10-88 Desulfurization
HYDRODE-THIOSUBSTITUTION, AND SO ON,
RSH RH
RSR' RH + R'H
RS(O)nR' RH + R'H
Thiols and thioethers,1397 both alkyl and aryl, can be desulfurized by hydrogenolysis
with Raney nickel.1398 The hydrogen is usually not applied externally, since Raney
nickel already contains enough hydrogen for the reaction. Other sulfur compounds
can be similarly desulfurized, including disulfides, thiono esters,1399 thioamides,
sulfoxides, and thioacetals. Reduction of thioacetals is an indirect way of
accomplishing reduction of a carbonyl to a methylene group (see 19-33), and it
can also give the alkene if a hydrogen is present.1400 In most of the examples given,
R can also be aryl. Other reagents1401 have also been used.1402
Lithium aluminum hydride reduces most sulfur compounds with cleavage of the
C—S bond, including thiols.1403 Thioesters can be reduced with Ni2B (from NiBr2/
NaBH4).1404 P-Ketosulfones are reduced with TiCl4-Zn,1405 TiCl4-Sm,1406 or
Bu3SnCl-NaCNBH3/AIBN.1407
An important special case of RSR reduction is desulfurization of thiophene
derivatives. This proceeds with concomitant reduction of the double bonds. Many
compounds have been made by alkylation of thiophene, followed by reduction:
" B--N. 3--R' T-h* R-(CH2)4-R'
j . R ^-g^ R Raney Ni
115
Thiophenes can also be desulfurized to alkenes (RCH2CH=CHCH2R' from 115)
with a nickel boride catalyst prepared from nickel(II) chloride and NaBH4 in
methanol.l408 It is possible to reduce just one SR group of a dithioacetal by treatmentwith borane-pyridine in trifluoroacetic acid or in CH2CI2 in
the presence of AICI3.1409 Phenyl selenides RSePh can be reduced to RH with Ph3SnH1410 and with nickle boride.1411 Cleavage of the C—Se bond can
also be achieved with Sml2.1412
The exact mechanisms of the Raney nickel reactions are still in doubt, though
they are probably of the free radical type.1413 It has been shown that reduction of thiophene proceeds through butadiene and butene, not through
1-butanethiol or other sulfur compounds, that is, the sulfur is removed before the double bonds are reduced.
This was demonstrated by isolation of the alkenes and the failure to isolate any potential sulfur-containing intermediates.1414
OS IV, 638; V, 419; VI, 109, 581, 601. See also OS VII, 124, 476.
REf.
1397 For a review of the reduction of thioethers, see Block, E. in Patai. The Chemistry of
Functional Groups, Supplement E, pt. 1; Wiley: NY, 1980, p. 585.
1398 For reviews, see Belen'kii, L.I. in Belen'kii Chemistry of Organosulfur Compounds; Ellis
Horwood: Chichester, 1990, p. 193; Pettit, G.R.; van Tamelen, E.E. Org. React, 1962,12,
356; Hauptmann, H.; Walter, W.F. Chem. Rev., 1962, 62, 347. See Node, M.; Nishide, K.;
Shigeta, Y; Obata, K.; Shiraki, H.; Kunishige, H. Tetrahedron, 1997, 53, 12883.
1 3 9 9See Baxter, S.L.; Bradshaw, J.S. J. Org. Chem., 1981, 46, 831.
1400Fishman, J.; Torigoe, M.; Guzik, H. J. Org. Chem., 1963, 28, 1443.
1401 For lists of reagents, with references, see Ref. 568, p. 31. For a review with respect to
transition metal reagents, see Luh, T.; Ni, Z. Synthesis, 1990, 89. For some very efficient
nickel-containing reagents, see Becker, S.; Fort, Y; Vanderesse, R.; Caubere, P. J. Org.
Chem., 1989, 54, 4848.
1402For example, diphosphorus tetraiodide by Suzuki, H.; Tani, H.; Takeuchi, S. Bull. Chem.
Soc. Jpn., 1985, 58, 2421; Shigemasa, Y; Ogawa, M.; Sashiwa, H.; Saimoto, H.
Tetrahedron Lett., 1989, 30, nil; NiBR2-Ph3P-LiAlH4 by Ho, K.M.; Lam, C.H.;
Luh, T. J. Org. Chem., 1989, 54, 4414.
1403 Smith, M.B.; Wolinsky, J. J. Chem. Soc, Perkin Trans. 2, 1998, 1431.
1404 Back, T.G.; Baron, D.L.; Yang, K. J. Org. Chem., 1993, 58, 2407.
1405 Guo, H.; Ye, S.; Wang, J.; Zhang, Y. J. Chem. Res. (S), 1997, 114.
1406Wang, J.; Zhang, Y Synth. Commun., 1996, 26, 1931.
1407Giovannini, R.; Petrini, M. Synlett, 1995, 973.
1408Schut, J.; Engberts, J.B.F.N.; Wynberg, H. Synth. Commun., 1972, 2, 415.
1409Kikugawa, Y. J. Chem. Soc, Perkin Trans. 1, 1984, 609.
1410Clive, D.L.J.; Chittattu, G.; Wong, C.K. J. Chem. Soc, Chem. Commun., 1978, 41.
1411 Back, T.G. J. Chem. Soc, Chem. Commun., 1984, 1417.
1412Ogawa, A.; Ohya, S.; Doi, M.; Sumino, Y; Sonoda, N.; Hirao, T. Tetrahedron Lett., 1998,
39,6341.
1413 For a review, see Bonner, W.A.; Grimm, R.A. in Kharasch; Meyers. The Chemistry of
Organic Sulfur Compounds, vol. 2; Pergamon: NY, 1966, pp. 35, 410. For a review of the
mechanism of desulfurization on molybdenum surfaces, see Friend, CM.; Roberts, J.T.
Ace Chem. Res., 1988, 21, 394.
1414Owens, P.J.; Ahmberg, C.H. Can. J. Chem., 1962, 40, 941.
[Edited on 3-10-2009 by solo]
[Edited on 3-10-2009 by solo]
It's better to die on your feet, than live on your knees....Emiliano Zapata.
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