Organic Reactions Volume 4 - Sciencemadness Dot Org

THE SYNTHESIS OF BENZOINS 291
The ether is removed, and the aqueous layer is extracted with two 100ml.
portions of ether. The extracts are discarded. The acid solution is
neutralized with aqueous ammonia, and the yellow precipitate which
forms is separated, dissolved in 50 ml. of ethanol, and filtered while hot.
On cooling, yellow needles separate, which weigh 1.5 g. (45% yield).
After three recrystallizations from ethanol, a pure product results, m.p.
159-160°.
THE REDUCTION OF BENZILS
Benzils have been reduced by a variety of reagents, usually with the
formation of the benzoins. Although the transformation is simple and
direct, it has the serious limitation that most benzils are accessible only
ArCOCOAr' -> ArC=CAr' -> ArCOCHOHAr'
OH OH
through oxidation of the corresponding benzoins. The probable mechanism
of reduction in the absence of certain catalysts is 1,4-addition of the
hydrogen with subsequent rearrangement of the enediol intermediate.
The reagent used and the conditions employed determine whether any
of the following reduction products are obtained in place of or in addition
to the benzoin: ArCH2COAr', ArCHOHCHOHAr', ArCH2CHOHAr',
ArCH2CH2Ar'. In unsymmetrical benzils the relative reactivity of the
two carbonyl groups is so markedly different that usually only one of
them is reduced; the stable isomeric form of the corresponding benzoin
is formed. 83,89 Catalytic hydrogenation and reduction with magnesium
and magnesium iodide or bromide are the preferred procedures of the
variety of systems that have been studied.
The reduction of benzil with zinc dust and sulfuric acid in the presence
of acetic anhydride 109 ' 110 yields two stereoisomeric forms of the enediol
diacetate; each is converted by hydrolysis with ethanolic potassium
hydroxide into benzoin. Only one of the two stereoisomeric enediol
C6H5COCOC6H5 -> C6H5C =CC6H5 -> C6H5C=CC6H5 ->
OCOCH3 OCOCH3 OH OH
cis and trans
C6H5COCHOHC6H5
diacetates is obtained if benzil is reduced in ether solution by sodium
and then treated with acetic anhydride. 110 Hydrolysis of some of the
diacetates leads to enediols which can be isolated.
109 TMoIe, Ann., 306, 142 (1899).
110 Nof, Ann., 308, 283 (1899).