Weygand/Hilgetag Preparative Organic Chemistry

650 Formation of carbon-sulfur bonds
Pure monomeric thioketones have only recently become accessible 406 but
can now be prepared in any desired amount. The following methods have
proved suitable: catalytic removal of hydrogen sulfide from geminal dithiols
at 200°; 407 thermolysis of 2#,6#-l,3-dithiins 408 (obtained from geminal
dithiols and malonodinitrile or its alkylidene or benzylidene derivatives); 409
and treatment of ketals with hydrogen sulfide in the presence of acid catalysts.
410 The last method in particular is widely variable and very effective;
it is only necessary to see that overdosage with hydrogen sulfide does not
convert the thione into the geminal dithiol by addition.
R'R"C(OR)2 + H2S > R'R"CS + 2ROH
Enol ethers react similarly, giving a mixture of enethiols and thioketones:
—CH=C(OR)— **^> __CH=C(SH)— ' —CH2—CS—
—ivOrl
It is best to carry out the operation under an inert gas and in the presence
of a little hydroquinone.
General preparation of monomeric thioketones: 410 One mole of the desired ketal or enol
ether is treated with twice the amount by weight of glacial acetic acid and about 1 gram of
hydroquinone, then 3 drops of concentrated sulfuric acid are added and a rapid stream of
hydrogen sulfide is passed in until the theroretical amount has been absorbed. The reaction
is usually complete in 15-20 min. The final temperature is about 35°. The mixture is poured
into ice-water, and the organic phase is taken up rapidly in light petroleum, purified, and
distilled through a column. The thioketones are unstable deep red oils with an extremely
unpleasant and penetrating odor.
This method has given: 2-butanethione, 43^45% yield, b.p. 120-124°; 3-pentanethione,
80-85%, b.p. 55-56°/57mm; 4-heptanethione, 50-55%, b.p. 52-62°/12mm; cyclopentanethione,
30-35%, b.p. 60-67°/18 mm; and cyclohexanethione, 50%, b.p. 81-84°/17 mm.
Aromatic and heterocyclic thioketones show no tendency to polymerize
and with few exceptions they crystallize well. For their preparation the oxygen
analogs are treated (i) with hydrogen sulfide in anhydrous hydrogen fluoride
[e.g., thiobenzophenone, 4,4'-dimethylthiobenzophenone, 1-naphthyl phenyl
thioketone, or 4-(dimethylamino)thiobenzophenone 411 ] or in the presence
of hydrogen chloride (e.g., thiobenzophenone 412 or thiofluorenone 413 ) or
(ii) with phosphorus pentasulfide (e.g., dimethylthiochromone 361 , 4-thioflavone,
414 or thiophthalimide 416 ).
406 Review: R. Mayer, J. Morgenstern, and J. Fabian, Angew. Chem. Int. Ed., Engl., 3,
277 (1964).
407 S. Bleisch and R. Mayer, Chem. Ber., 99, 1771 (1966).
408 J. Morgenstern and R. Mayer, /. Prakt. Chem., |iv], 34, 116 (1966).
409 J. Jentzsch and R. Mayer, /. Prakt. Chem., [iv], 18, 211 (1962).
410 R. Mayer and H. Berthold, Chem. Ber., 96, 3096 (1963).
411 R. M. Elofson, L. A. Baker, F. F. Gadallah, and R. A. Sikstrom, /. Org. Chem., 29,
1355 (1964)f
412 H. Staudinger and H. Freudenberger, Ber. Deut. Chem. Ges., 61, 1576 (1928).
413 E. Campaigne and W. B. Reid Jr., /. Amer. Chem. Soc, 68, 769 (1946).
414 H. Simonis and S. Rosenberg, Ber. Deut. Chem. Ges., 47, 1232 (1914).
415 W. Baker, J. B. Harborne, and W. D. Ollis, /. Chem. Soc, 1952, 1303.
416 R. J. W. Cremlyn, /. Chem. Soc, 1961, 5055.