Of benzoin

Oxidation of benzoin with concentrated nitric acid or by catalytic amounts of cupric salts in acetic acid solution, which are regenerated continuously by ammonium nitrate, yields the diketone benzil ... 

The latter procedure gives a purer product it is difficult to remove the last traces of benzoin from the benzil obtained by the nitric acid method. 

Method 2. Place 0-2 g. of cupric acetate, 10 g. of ammonium nitrate, 21 2 g. of benzoin and 70 ml. of an 80 per cent, by volume acetic acid -water solution in a 250 ml. flask fitted with a reflux condenser. Heat the mixture with occasional shaking (1). When solution occurs, a vigorous evolution of nitrogen is observed. Reflux for 90 minutes, cool the solution, seed the solution with a crystal of benzil (2), and allow to stand for 1 hour. Filter at the pump and keep the mother liquor (3) wash well with water and dry (preferably in an oven at 60°). The resulting benzil has m.p. 94-95° and the m.p. is unaffected by recrystallisation from alcohol or from carbon tetrachloride (2 ml. per gram). Dilution of the mother liquor with the aqueous washings gives a further 1 Og. of benzil (4). 

In a 250 ml. bolt-head flask, fitted with a reflux condenser, place a mixture of 10 g. of benzoin (Section IV,125) and 20 g. (25 ml.) of rectified spirit together with an aqueous solution of 8 0 g. of hydroxylamine hydrochloride which has previously been neutralised with 4-4 g. of sodium hydroxide. Reflux for 60 minutes. Add water to precipitate the benzoinoxime, and cool in an ice bath. Filter the solid with suction at the pump, wash it with water, and recrystaUise from dilute alcohol. Alternatively, the dry sohd may be recrystalhsed from ether. The yield of pure a-benzoinoxime, m.p. 151°, is 5 g. 

The cyclic carbonate of benzoin (4,5-diphenyl-l,3-dioxol-2-one, prepared from benzoin and phosgene) blocks both hydrogen atoms of primary amines after dehydration acid stable, easily crystallizable Sheehan oxazolinones are formed, which are also called Ox derivatives. The amine is quantitatively deblocked by catalytic hydrogenation in the presence of 1 equiv. of aqueous acid (J.C Sheehan, 1972, 1973 M.J. Miller, 1983). An intelligent application to syntheses of acid labile -lactams is given in the previous section (p. 161). 

The alkyl ethers of benzoin undergo dkect photofragmentation upon absorption of uv energy at ca 360 nm to produce two free-radical intermediates. 

The copper sulfate-pyridine mixture is readily reoxIdized by passing a current of air through it for thirty-six hours (Note 4). To this resulting solution is now added 200 g. of pyridine and it is then used for oxidizing another batch of 1696 g. of benzoin. 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

The synthesis of deoxybenzoin from phenacetyl chloride and benzene by the Friedel-Crafts reaction has been described. For symmetrically substituted deoxybenzoins, direct reduction of the readily accessible benzoin is a more convenient method. Reduction of benzoin by zinc dust and acetic acid, and by hydrochloric acid and granulated tin or amalgamated powdered tin has been reported. The present method is based on a publication of the authors. ... 

The lDcii70in and nltiic acid are heated on the water-bath with an. >ir condenser, the flask being occasionally shaken. Nitrous fumes arc evolved, and the crystals of benzoin are converted into a yellow oil, which, after two hours heating, is free from un-(hanged benzoin. The contents of the flask are now poured into water, and the yellow crystalline deposit separated by filtration, washed with water, and recrystallised fiom alcohol. Yield, 10—12 grams. 

Although esters and ethers (327, R = alkyl or acyl) of the enediol form (327, R = H) of benzoin are known, the parent compound exists... 

By the oxidative catalytic dehydration of benzoin. Zetzsche and Zala, Helvetica Chim. Acta 9, 288 (1926). 

An illustrative example of the Michael reaction is that of the thiirene dioxide 19b with either hydroxylamine or hydrazine to give desoxybenzoin oxime (87) and desoxybenzoin azine (88), respectively, in good yields6 (see equation 29). The results were interpreted in terms of an initial nucleophilic addition to the a, j8-unsaturated sulfone system, followed by loss of sulfur dioxide and tautomerization. Interestingly, the treatment of the corresponding thiirene oxide (18a) with hydroxylamine also afforded 86 (as well as the dioxime of benzoin), albeit in a lower yield, but apparently via the same mechanistic pathway6. 

Oxidation of benzoin, PhCH(OH)COPh (above) yields benzil, PhCOCOPh (133), and this, in common with non-enolisable 1,2-diketones in general, undergoes base-catalysed rearrangement to yield the anion of an a-hydroxy acid, benzilate anion, Ph2C(0H)C02e (134). This is, almost certainly, the first molecular rearrangement to be recognised as such. The rate equation is found to be,... 

A jug containing calcium hypochlorite (probably as moist solid) was used as a disposal receptacle for cyanide wastes from student preparations of benzoin. When a little acetic acid residue was inadvertently added, an explosion occurred, attributed to a cyanide-chlorine redox reaction. 

Hoyos, P., Buthe, A., Ansorge-Schumacher, M.B. et al. (2008) Highly efficient one pot dynamic kinetic resolution of benzoins with entrapped Pseudomonas stutzeri lipase. Journal of Molecular Catalysis B, Enzymatic, 52-53,133-139. 

Figure 1. Absorbance spectra of benzoin ethyl ether before and after 30 minutes of illumination illustrating the absorbance decrease at wavelengths from 300 to 400 nm.

Figure 2. Absorbance decay of benzoin ethyl ether at 328 nm as a function of illumination time. The experiments were performed on 3.0 ml solutions of 0.1 wt. % BEE in 1-propanol.

---