Weygand/Hilgetag Preparative Organic Chemistry

Addition of hydrogen to C=C bonds and aromatic systems 9
homologs have been recommended as solvents. Subsequent addition of ammonium
salts or alcohols removes the excess of metal and simultaneously
protonizes the carbanions that are formed as primary product. Free carboxylic
acids react only with difficulty under the above conditions and are advantageously
neutralized beforehand with sodium amide. The following directions have
been given for preparation of 9,10-dihydro-9-phenanthroic add: 34
9-Phenanthroic acid (5.69 g), sodium amide (1 g), and liquid ammonia (90 ml) are placed
in a reaction vessel fitted with a reflux condenser that can be cooled to —70° by alcohol-
Dry Ice and with a magnetic stirrer. The mixture is stirred until the sodium amide is wholly
dissolved (ca. 4 h) and thereafter freshly cut, small pieces of sodium (1.2 g) are added during
1 h. The dark red mixture is stirred for a further 1.5 h and then slowly decolorized by ethanol
(10 ml). The ammonia is then allowed to evaporate, the residue dissolved in water and
acidified with hydrochloric acid, and the thick brown oil is extracted in ether (two 25-ml
portions). The united extracts are washed with water, dried over anhydrous sodium sulfate,
and treated with Norite. Evaporation of the ether yields the dihydro acid of m.p. 122-124°.
Solutions of the more active metals lithium and potassium have been valuable
for hydrogenation of C=C bonds of #,/?-unsaturated ketones 2a ' 35 * 36 and aryl
ethers. 37
A striking improvement in technique was found in the addition of alcohols.
This procedure, discovered by Wooster 7a and developed chiefly by Birch, 38
reduces isolated aromatic rings. Benzene and its homologs are reduced to
1,4-dihydro derivatives, 39 which can be rearranged by NaNH2 in liquid
ammonia 40 to the corresponding conjugated dienes and then further transformed
into cyclohexene derivatives. Condensed aromatic systems are, of course,
also attacked: for instance, 1,4,5,8-tetrahydronaphthalene is obtained from
naphthalene 6 ' 39 3,8-dihydropyrene from pyrene, 41 and hexahydrophenanthrene
in 94% yield from tetrahydrophenanthrene; 42 1-naphthol is converted
into 5,8-dihydro-l-naphthol in 97% yield. 43 The method has found particularly
wide application for partial hydrogenation of aryl ethers 44 which, without the
O
(ref. 46)
34 H. de Koning et aL, Rec Trav. Chim., 83, 364 (1964).
35 G.L. Chetty et aL, Tetrahedron, 22, 2311 (1966); R. H. Jager, Tetrahedron, 2, 326
(1958).
36 F. H. Howell and D. A. H. Taylor, /. Chem. Soc, 1958, 1248; D. H. R. Barton et aL,
J. Chem. Soc, 1954,903; R. E. Schaub and M. J. Weiss, Chem. & Ind. (London), 1961, 2003;
R. Vilotti and A. Bowers, Gazz. Chim. Ital, 93, 1695 (1963); P. Westerhof, Rec. Trav. Chim.,
83, 1069 (1964).
37 A. Sandoval et aL, J. Amer. Chem. Soc, 11, 148 (1955); J. A. Barltrop and N. A. J.
Rogers, /. Chem. Soc, 1958, 2566.
38 A. J. Birch et aL, J. Chem. Soc, 1944, 430; 1946, 593; 1951, 1945.
39 W. Hiickel et aL, Chem. Ber., 88, 388, 346 (1955); C. A. Grob and P. W. Schiess, Helv.
Chim. Acta, 43, 1546 (1960).
40 A. J. Birch, /. Chem. Soc, 1950, 1551.
41 O. Neunhoeffer, H. Woggon, and S. Dahne, Ann. Chem., 612, 98 (1958).
42 S. Mejer, Bull. Acad. Polon. ScL, Sir. Sci. Chim., 10, 463 (1962).
43 C. D. Gutsche and H. H. Peter, Org. Syn., 37, 80 (1957).
44 R. Grewe, E. Nolte, and R.-H. Rotzoll, Chem. Ber., 89, 600 (1956).