Weygand/Hilgetag Preparative Organic Chemistry

1044 Cleavage of carbon-carbon bonds
pounds include 1,2-diols, ketols, a-hydroxy acids, a>hydroxy amines, 1,2-diamines,
#-amino acids, and oxalic acid. They are all cleaved smoothly between
the carbon atoms carrying the functional groups either by lead tetraacetate
in Criegee's method 140 or by periodic acid in Malaprade's method. 141 The
fission products are carbonyl compounds or imines, though the latter are usually
further dehydrogenated to nitriles:
(OH) > Nco + \co
CHR(OH)—COOH > RCHO + CO2
RR'C(OH)—COOH > RR'CO-f CO2
CHR(NH2)—COOH • CO2 + CHR=NH > RCN
Lead tetraacetate is the reagent most used in such reactions because it is
soluble in organic solvents. Baer, Grossheintz, and Fischer 142 have, however,
shown that it can be applied as an emulsion in water or in mixtures of water
and other solvents. Periodic acid is used in an aqueous medium, often with
addition of another solvent such as dioxan or acetic acid or with an emulsifier.
Osmium tetraoxide has also proved a valuable oxidant in special cases. 143
Although, in general, the two oxidants lead tetraacetate and periodic acid
give the same results 144 and the former has usually been preferred because
it is cheaper, it should be noted that periodic acid has the virtue of
being more selective. For instance, lead tetraacetate but not periodic acid
attacks ditertiary glycols and a-hydroxy acids; thus only lead tetraacetate,
as shown by Oeda, 145 converts 2-hydroxy-3-phenylpropionic acid
into phenylacetaldehyde:
C6H5CH2CH(OH)COOH > C6H5CH2CHO
2-Hydroxy-3-phenylpropionic acid (10 g; prepared from phenylalanine) is boiled in benzene
(150 ml) until most of it has dissolved and then lead tetraacetate (26 g) is added gradually
with stirring. The lead(n) acetate that separates is filtered off and washed with benzene.
Distillation of the benzene solutions in a vacuum affords phenylacetaldehyde (4.2 g, 58%),
b.p. 84-86°/17 mm.
Tartaric acid, however, can be converted into glyoxylic acid only by periodic
acid since lead tetraacetate attacks all four carbon atoms of tartaric acid. 146
Weygand nevertheless worked out a method of obtaining glyoxylic esters from
tartaric esters by means of lead tetraacetate because a-hydroxy esters are as
resistant to cleavage as are esterified or etherified glycols.
ROOCCH(OH)CH(OH)COOR > 2OHCCOOR
Ethyl glyoxylate: Diethyl tartrate (232 g) is dissolved in benzene (1200 ml; dried over
sodium) and cooled in ice, then lead tetraacetate (500 g) is added with stirring during 1 h,
the temperature being kept at not more than 10°. Reaction is completed by a further 12 hours'
140 R. Criegee, Angew. Chem., 53, 324 (1940).
141 L. Malaprade, C. R. Hebd. Seances Acad. Set, 186, 382 (1928).
142 E. Baer, J. M. Grossheintz, and H. O. Fischer, /. Amer. Chem. Soc, 61, 2607, 3379
(1939).
143 R. Criegee, Ann. Chem., 522, 75 (1936).
144 P. Caro and R. Hirohata, Helv. Chim. Ada, 16, 959 (1933).
145 H. Oeda, Bull. Chem. Soc. Japan, 9, 13 (1934).
146 Freury and Bon-Bernatets, /. Pharm. Chim., [viii], 23, 85 (1936).