Weygand/Hilgetag Preparative Organic Chemistry

Conjugated ethylenic bonds 831
cases. Much success has been achieved in dehydrogenation of partially unsaturated
compounds by sulfur or selenium. Dehydrogenation by sulfur is
usually effected by methods worked out by Ruzicka: 125 the substance to be
dehydrogenated is mixed with the calculated amount of sulfur and heated in
a metal bath; evolution of hydrogen sulfide begins at about 180°, is allowed to
proceed at 200-220°, and is completed, when necessary, at 260°; the reaction
requires a few hours; working up is by vacuum-distillation, when possible.
The preparation of 1-phenylnaphthalene from its 3,4-dihydro compound will
serve as example: 126
Powdered sulfur (6g, 0.18 mole) and 3,4-dihydro-1-phenylnaphthalene (35 g, 0.17 mole)
are placed in a Claisen flask (200 ml) and heated in a metal-bath at 250-270°; evolution of
hydrogen sulfide is complete in 0.5 h. The viscous residue distils at 134-135°/2 mm or 189 to
190°/12 mm. The yield is 91-94% (32-33 g).
Selenium was first used as dehydrogenation catalyst by Diels, 127 when by
this method he established cyclopenta[#]phenanthrene as the parent skeleton
of steroids, bile acids, steroid hormones, and other natural products. Dehydrogenation
by selenium requires the use of higher temperatures and is thus often
accompanied by side reactions.
The substance to be dehydrogenated is mixed with the calculated amount of selenium
and heated at 250-280° in a flask fitted with an air-condenser. The temperature is slowly
raised further, but generally not to above 350°. The reaction requires 20-100 h. Working
up is by extraction with ether or benzene and fractionation of the residues from the extracts.
Caution: The experiment must be conducted in a very efficient fume-cupboard, as hydrogen
selenide is exceedingly poisonous, even more so than hydrogen sulfide.
Chloranil is a mild dehydrogenating agent. 128 Xylene (b.p. 140°) is used
as solvent, so that dehydrogenation occurs at a relatively low temperature.
2-Phenylnaphthalene, for example, is obtained in 90% yield when 3,4-dihydro-2-phenylnaphthalene
is boiled for 15 h with chloranil in xylene. 129
Pentaphenylbenzene is formed in 99% yield when a mixture of l,2,3,4,5-pentaphenyl-l,3hexadiene
and chloranil in benzene is irradiated under nitrogen for 2.5 h by a mercury
vapor lamp. 130
Chloranil is particularly suitable for dehydrogenation of tetrahydrocarbazole
derivatives. By its use Barclay and Campbell 131 obtained twenty different
carbazoles, nitro and carboxyl groups being unaffected in the process.
Other quinones, besides chloranil, can be used for such dehydrogenations,
e.g., 2,3-dichloro-5,6-dicyano-l,4-benzoquinone and tetrachloro-l,2-benzoquinone.
132 In especially favorable cases atmospheric oxygen, 133 selenium
dioxide, 134 and sulfuric acid act as dehydrogenating agents. A reaction of
125 L. Ruzicka, Fortschr. Chem. Phys., Phys. Chem., 19,1 (1928); L. Ruzicka and J. Meyer,
Helv. Chim. Ada, 4, 505 (1921).
126 R. Weiss, Org. Syn., 24, 84 (1944).
127 O. Diels, Ber. Deut. Chem. Ges., 69, A, 195 (1936).
128 R. T. Arnold and C. J. Collins, /. Amer. Chem. Soc, 61, 1407 (1939).
129 N. Campbell and D. Kidd, /. Chem. Soc, 1954, 2154.
130 G. R. Evanega, W. Bergmann, and J. English, /. Org. Chem., 27, 13 (1962).
131 B. M. Barclay and N. Campbell, /. Chem. Soc, 1945, 530.
132 E. A. Braude, A. G. Brook, and R. P. Linstead, /. Chem. Soc, 1954, 3569.
133 H. Lewis, G. Ramage, and R. Robinson, /. Chem. Soc, 1935, 1412.
134 E. Borgwardt and E. Schwenk, /. Amer. Chem. Soc, 56, 1185 (1934); G. Stein,
Angew. Chem., 54, 146 (1941).