Organic Reactions Volume 4 - Sciencemadness Dot Org

THE SYNTHESIS OF BENZOINS 293
effective. 4,118 The cis and trans enediols are formed from such benzils;
they can be separated, and on treatment with piperidine acetate or with
ethanolic hydrochloric acid 1>9 they isomerize to the benzoins.
Experimental Procedures
Catalytic Reduction (General Procedure). The catalytic reduction of
benzil 94 - 120 is carried out conveniently in a Burgess-Parr apparatus (with
the modification described on p, 285). A solution of 0.01 mole of the
substance in about 50 ml. of the appropriate solvent (ethanol, ethyl
acetate, pyridine, or acetic acid), to which 0.5 g. of platinum oxide is
added, is warmed to 70-75° and allowed to absorb 0.01 mole of hydrogen.
The reduction is then stopped to prevent the formation of hydrobenzoin.
The catalyst is removed, the solvent partially evaporated, and the
product recrystallized from a suitable solvent. Anisil, benzil, furil, and
piperil are reduced to the corresponding benzoins in 86-93% yield.
Reduction of 4,4'-ToIiI with Magnesium-Magnesium Bromide. 52 To
an ether solution containing 10 g. of anhydrous magnesium bromide in
a 70-ml. test tube is added 4.76 g. of tolil and 20 ml. of benzene. A
deep-yellow solution results. A weighed magnesium rod is inserted, and
the tube is corked and placed on a shaking machine. A deep-brown
coloration soon forms around the ends of the magnesium rod; after
several hours the entire solution has become very deep brown. The
reduction is apparently complete after twenty hours' shaking. The
magnesium rod is removed and washed with benzene, and the solution
is quickly decomposed with water. The ether-benzene solution is dried
over anhydrous sodium sulfate. The product crystallizes after partial
evaporation of the solvent. The yield of pure toluoin is 3.9 g. (82%),
m.p. 88-89°.
THE REDUCTION OF AROMATIC ACIDS AND THEIR DERIVATIVES
The reduction of aromatic acids, their chlorides, esters, and peroxides
can be accomplished by treatment with magnesium and magnesium
iodide in a mixture of ether and benzene as solvents. With benzoic acid
or benzoyl peroxide two steps are involved: the formation of the magnesium
iodide salt and the reduction. With an acid chloride or ester
the magnesium iodide derivative of the enediol forms directly and is
hydrolyzed to the benzoin. Intermediate enediols have been isolated
only from reduction of hindered acid chlorides. The method has been
used in reducing benzoic, 4-toluic, a- and ^-naphthoic, and phenylbenzoic
acid, benzyl and methyl benzoate, benzoyl peroxide, and 2,4,6-