Organic Reactions Volume 4 - Sciencemadness Dot Org

THE ACYLOINS 259
A mechanism similar to that outlined above has been proposed for
the reduction of aromatic acids to benzoins by the binary system
magnesium-magnesium iodide. 8 This reagent, however, has not been
applied to aliphatic esters.
Another proposal involves the sodium enolate 9 of the ester as an
O
R2CHC-OR/ + Na
/ONa
2R2C=C< + 4[H]
X)R'
ONa
/
R2C=C-OR' + [H]
R2CHC-ONa
R2CHC-ONa
+ 2R'0H
intermediate in the formation of acyloins. The most convincing objection
to this mechanism is the inability to explain the ready formation of
diketone and acyloin from ethyl trimethylacetate, which has no enolizable
a-hydrogen.
A study of the action of sodium in liquid ammonia on a number of
esters of aliphatic as well as aromatic acids has resulted in tiie suggestion
that a series of equilibria and an intermediate free radical are involved
in the formation of diketones and acyloins. 10 Even if this is a correct
O ONa
2RC—0R' -^» 2RC—OR'
i
ONa
2RC—OR'
I
Na
O
ONa
RC-OR'
RC-OR'
ONa
I
2Na
2RC-Na + 2NaOR'
RC=O
RC=O
RC-ONa
RC-ONa
+ 2NaOR'
+ 2NaOR'
representation of the reduction of esters by sodium in liquid ammonia,
it does not follow that the mechanism is similar in an inert solvent in
8 Gomberg and Bachmann, J. Am. Chem. Soc, 50, 2762 (1928).
9 Scheibler and Voss, Ber., 53, 388 (1920); Scheibler and Emden, Ann., 434, 265 (1923).
10 Kharasch, Sternfeld, and Mayo, J. Org. Chem., 5, 362 (1940); see also Blicke, J. Am.
Chem. Soc, 47, 229 (1925).