Weygand/Hilgetag Preparative Organic Chemistry

1012 Cleavage of carbon-carbon bonds
It has been found that the decarboxylating acylation probably begins with
the equilibrations:
2C6H5CH2COOH + (CH3CO)2O
\\
C6H5CH2COOCOCH3 + C6H5CH2COOH + CH3COOH
\\
(C6H5CH2CO)2O + 2CH3COOH
The formation of phenylacetone can then be formulated as:
C6H5-CH2-CO C6H5—CH2 + CO2
\o • CH3—CO + O—CO
CH3-CO-O CO CH3-COCH3
CH3—CO CH3
c. 0-Oxo acids
Like malonic acids, acetoacetic acid and analogous /?-oxo acids are more
readily decarboxylated as undissociated acids than as their anions. The rate
equation for the two types of acid is the same:
v — k [oxo acid] + h' [oxo acid anion] k>k'
Thence it can be concluded that such decarboxylations can occur in either of
two ways; neither of these two routes involves the enol form of the acid or
its salt since dimethylacetoacetic acid behaves in the same way. The two terms
in the rate equation thus correspond simply to the monomolecular decomposition
of the oxo acid or its anion, respectively. Here too decomposition of
the oxo acid 27 must be assumed to proceed by way of a cyclic intermediate
containing an intramolecular hydrogen bridge, as formulated in the introduction
(page 1004) for a type 2 reaction.
Alongside that reaction there is decomposition of the anion by way of an
anionic intermediate that becomes stabilized as the ketone:
CH3COCH2COO" -^+ [CH3COCH2-
+H20
-££* (CH3)2CO + HO"
The intermediate carbanion or ketone enol can be trapped by carrying out
the decarboxylation in the presence of bromine or iodine, 28 which leads to
the formation of the bromo or iodo ketone; the rate of this reaction is unaffected
by the concentration of oxo acid, whence it follows that the halogenation does
not precede the decarboxylation; also halogenation is not subsequent to
formation of the ketone, since the ketone is not halogenated under the conditions
used for the decarboxylation. If then halogenation follows decarboxylation
and precedes ketone formation, the only possibility is reaction of
the short-lived intermediate carbanion or enol.
27 F. H. Westheimer and W. A. Jones, /. Amer. Chem. Soc, 63, 3283 (1941).
28 K. J. Pederson, /. Amer. Chem. Soc.9 51, 2098 (1929); 58, 240 (1936).