Weygand/Hilgetag Preparative Organic Chemistry

1024 Cleavage of carbon-carbon bonds
sulfonate, a reaction that is comparable with the degradation of acylamino
acid chlorides and probably occurs by way of an unusual mixed anhydride. 63
The following observations should also be included among the heterolytic
decarbonylations:
The enol lactone of levulic acid is cleaved to carbon monoxide and an unsaturated
ketone when heated with pieces of silica at 570° : 64
HC-CH2
ruJi r>/ CO —* CO + CH3COCH=CH2
v. ri 3C —KJ
When o- or /?-hydroxy (or amino)benzaldehyde is titrated with bromine in
aqueous solution at 40-45° 2,4,6-tribromophenol or -aniline is obtained
almost quantitatively; the aldehyde group is not oxidized to carboxyl but
replaced by bromine; it is eliminated as carbon monoxide which originates
from decomposition of formyl bromide that is formed as intermediate. 65 At
10° and 15°, respectively, these hydroxy and amino aldehydes are, however,
merely converted into the dibromo aldehydes with retention of the CHO
group, e.g.:
Br Br
2. Radical decarbonylation
Although not of much real preparative interest, radical decarbonylation
has been observed with a large number of aldehydes and ketones at high
temperatures and/or under ultraviolet irradiation or in the presence of peroxides.
For example, carbon monoxide is eliminated in 90% yield when
3-methyl-3-phenylbutanal is heated for five hours at 130° in the presence of
di-tert-butyl peroxide; fractionation of the residue then affords 70% of a
1: 1 mixture of terf-butyl- and isobutyl-benzene, so that the /?,/?-dimethylphenethyl
radical formed on elimination of carbon monoxide from the
C6H5C(CH3)2CH2CO— radical must be assumed to have partially
rearranged. 66
When this experiment is carried out at 80° under otherwise identical conditions no carbon
monoxide or butylbenzene is formed; the acyl radical is stable at this temperature and reacts
with carbon tetrachloride to give the acid chloride or with an olefin to give a ketone.
Aromatic aldehydes can be decarbonylated even at the boiling point in the
presence of palladium-charcoal; although the products are almost always
accessible more easily by other routes, the high yields (61-94%) of the decarbonylation
are remarkable. 67
63 J. C. Sheehan and J. W. Frankenfeld, /. Org. Chem., 27, 628 (1962).
64 Brit. Pat. 601,922; Chem. Abstr., 42, 7319 (1948).
65 A. W. Francis and A. J. Hill, /. Amer. Chem. Soc, 46, 2498 (1924).
66 S. Winstein and F. H. Seubold, Jr., /. Amer. Chem. Soc, 69, 2917 (1949).
67 J. O. Hawthorne and M. H. Wilt, /. Org. Chem., 25, 2215 (1960).