Benzil from benzoin

Hydrogen sulfide is introduced into an ice-cooled solution of 0.2 mol of a 1,2-diketone and 0.02 mol of piperidine in 30 ml of dimethylformamide for 1 -4 hours. Elemental sulfur is precipitated during the introduction of hydrogen sulfide. The mixture is acidified with dilute hydrochloric acid the sediment is filtered with suction and dissolved in warm methanol the undissolved sulfur is separated, and the product is isolated by evaporation of the methanol and purified by crystallization. If the product after the acidification is liquid it is isolated by ether extraction and distillation after drying of the ether extract with sodium sulfate. Yield of benzoin from benzil after 1 hour of treatment with hydrogen sulfide is quantitative. 

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. 

Oxidation of chalcones with TTN has been studied in detail (95, 96), and it has been shown that the products obtained depend on the amount of reagent and the solvent employed. Oxidation with 1 equivalent of TTN in methanol, methanol-chloroform, or methanol-boron trifluoride leads to acetals of the type (XXXIV) (see also Scheme 21) in yields of 20-80%. When 3 equivalents of TTN are employed, however, and aqueous glyme containing a little perchloric acid used as solvent, the products are benzils. This remarkable transformation, which proceeds in yields varying from moderate to good (40-80%), involves three distinct oxidations by TTN, and these are outlined in Scheme 22. Each individual step in this reaction sequence has been investigated in detail, with the result that useful procedures have been developed for the oxidation of both deoxybenzoins and benzoins to benzils with TTN (96). 

Aliphatic aldehydes and ketones and also aliphatic-aromatic ketones can be converted into the corresponding hydrocarbons alkyl-phenols can be obtained from phenolic-aldehydes and -ketones p-hydroxy-benzophenone yields p-benzylphenol benzoin and benzil yield dibenzyl anthraquinone yields anthracene dihydride. 

Diols may be prepared by reduction of a-diketones or a-hydroxy ketones such as biacetyl, benzoin, and benzil. Substituted benzoins containing methoxyl and p-dimethylamino groups have been reduced catalytically over platinum oxide and by sodium amalgam and alcohol. Levorotatory propylene glycol is made from acetol, CHjCOCHjOH, by an enzymatic reduction with yeast. ... 

A similar rearrangement occurs when a-epoxyketones are refluxed with 30% aqueous sodium hydroxide. Best directions ate those for benzilic acid (90%) from benzoin, sodium bromate, and sodium hydroxide. Oxidation of the benzoin to benzil and rearrangement of benzil to benzilic acid are accomplished in one step. a-Ketoaldehydes and potential a-ketoalde-hydes undergo a similar internal oxidation-reduction reaction in excellent yields, viz.,... 

The equation representing the oxidation of benzoin to benzil has been given under the preceding preparation. The analogues of benzoin also give, on oxidation, compounds of the benzil series. Thus from anisoin and cuminoin, anisil and cuminil respectively are obtained ... 

Photoinitiation System. In most cases the curing is initiated by free radicals generated by UV light and photoinitiators. The free radicals can be formed from photocleavage or hydrogen abstraction mechanisms. Benzoin ethers, benzil ketals, and acetophenone derivatives are typical photoinitia,tors. 

Benzilic acids from benzoins via benzils a. 2, 622 (Ar)sC(OH)COOH... 

Oxidation of benzoins to benzils. Benzoins can be converted to benzils by oxidation with cupric sulfate in HMPT with air (or O2) ebullition yields are 60-95%. This method was used for preparation of 4,4 -divinylbenzil from 4,4 -divinylbenzoin (73% yield). 

Oxidation of alcohols. The reagent oxidizes benzoin to benzil at -78° in methylene chloride in 97% yield. However, yields of ketones from most secondary alcohols are in the range of 50-60%. Considerable amounts of methylthio-methyl ethers are usually formed. 

Oxidation of benzoins to benzils Benzoins (1) are oxidized to benzils(2) in high yield by catalytic amounts of this lanthanide nitrate and 1 eq. of HCl in aqueous glyme. Benzil can also be obtained from desoxybenzoin and diphenyl-acetylene under these conditions. This oxidation has recently been reported with... 

Triaryl-substituted imidazoles were synthesized from benzil or benzoin, aldehyde, and ammonium acetate. A literature survey reveals several reports for the synthesis of triarylimidazoles using SSA as a catalyst. Shaabani et al. (2007) reported the synthesis of triarylimidazoles using SSA under conventional heating as well as microwave irradiation (Scheme 5.15). Microwave irradiation afforded the corresponding products in shorter reaction times (5-10 min). Catalytic reusability... 

Enantiomerically pure hydrobenzoin could be prepared from benzil by employing a diamino-type ruthenium(ii) complex as depicted in Scheme 2.21. The success of this reaction was made possible due to the stepwise reduction of benzil to hydrobenzoin via the intermediate benzoin. This intermediate was itself configurationally labile and stereomutated rapidly. 

The experiment demonstrates multistep synthesis of benzilic acid starting from benzaldehyde. In Experiment 32A, benzaldehyde is converted to benzoin using a thiamine-catalyzed reaction. This part of the experiment demonstrates how a "green" reagent can be utilized in organic chemistry. In Experiment 32B, nitric acid oxidizes benzoin to benzil. Finally, in Experiment 32C, benzil is rearranged to benzilic acid. The scheme on the next page shows the reactions. 

SCHEME 22 Synthesis of 2,4,5-triaryl-lH-imidazoles from benzoin or benzil, ammonium acetate, and aromatic aldehydes using MCM-41 as catalyst. 

Recrystallise 5 0 g. from about 40 ml. of hot rectified (or methylated) spirit upon cooling, 4-5 g. of pure benzoin (a white, crystaUine solid, m.p, 137°) separate. Reserve the remainder of the preparation for benzil and benziUc acid (Sections IV.126 and IV.127 respectively). 

Method 1. Place 20 g. of crude benzoin (preceding Section) and 100 ml. of concentrated nitric acid in a 250 ml. round-bottomed flask. Heat on a boiling water bath (in the fume cupboard) with occasional shaking until the evolution of oxides of nitrogen has ceased (about 1 -5 hours). Pour the reaction mixture into 300-400 ml. of cold water contained in a beaker, stir well until the oil crystallises completely as a yellow solid. Filter the crude benzil at the pump, and wash it thoroughly with water to remove the nitric acid. RecrystaUise from alcohol or methylated spirit (about 2-5 ml. per gram). The yield of pure benzil, m.p. 94-96°, is 19 g. 

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). 

Symmetrical and unsymmetrical benzoins have been rapidly oxidized to benzils in high yields using solid reagent systems, copper(II) sulfate-alumina [105] or Oxone-wet alumina [105, 106] under the influence of microwaves (Scheme 6.32). Conventionally, the oxidative transformation of a-hydroxy ketones to 1,2-diketones is accomplished by reagents such as nitric acid, Fehling s solution, thallium(III) nitrate (TTN), ytterbium(III) nitrate, ammonium chlorochromate-alumina and dayfen. In addition to the extended reaction time, most of these processes suffer from drawbacks such as the use of corrosive acids and toxic metals that generate undesirable waste products. 

Scheme 6.35 Benzoins obtained from the reduction of benzils in the presence of thiourea derivative rac-36 and sodium dithionate.

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