Hydrobenzoin, Oxidation with

Electron-withdrawing groups close to the alcohol functionality may likewise destabilize intermediate carbocations and result in very slow oxidations. For instance, sterol 88 is oxidized with DDQ at the allylic alcohol two hundred times slower than the corresponding compound lacking the fluorine atom,94 and the treatment of hydrobenzoin (89) with DDQ results in the oxidation of a single alcohol because a second oxidation would involve a carbocation highly destabilized by the presence of a carbonyl group.95f... 

Benzoin can be oxidized to the a-diketone, benzii, very efficiently by nitric acid or by copper(II) sulfate in pyridine. On oxidation with sodium dichromate in acetic acid the yield is lower because some material is converted into benzaldehyde by cleavage of the bond between two oxidized carbon atoms and activated by both phenyl groups (a). Similarly, hydrobenzoin on oxidation with dichromate or permanganate yields chiefly benzaldehyde and only a trace of benzii (b). 

Enantiomer selective coloration of optically active amines, our important project, was realized by chiral azophenol crown 4 incorporated with two units of optically active hydrobenzoin. The synthetic route is shown in Scheme 2. Reaction of 2,6-bis(bromomethyl)-l,4-dimethoxybenzene 22, which is derived from hy-droquinone monomethylether 19 by a three-step procedure, with the dibutyltin derivative 26 of optically active dihydrobenzoin gives optically active podand 23 in 63% yield. Cyclization of 23 with the ditosylate of polyethylene glycol, followed by oxidation with ceric ammonium nitrate (CAN) and treatment with dinitrophenylhy-drazine, affords the desired chiral azophenol crowns 4n. 

Phenylethanol, Benzhydrol, Hydrobenzoin, 4,4 -Dimethoxyhydrobenzoin, 4,4 -Dichlorohy-drobenzoin, and 4-Chloro-benzhydrol These alcohols can be oxidized with conversions higher than 90 % and high yields into the desired products [3]. 

Acyloxylation of aryl olefins probably involves radical cations as intermediates. Acetoxylation of frans-stilbene in anhydrous acetic acid/sodium acetate yields mainly meso-diacetate, while in moist acetic acid mainly threo-2-acetoxy-l,2-di-phenylethanol is formed 100 Anodic oxidation of trans- and ds-stilbene in ace-tonitrile/benzoic acid produces with both olefins the same mixtures of meso-hydrobenzoin diacetate (62) and f/ireo-2-benzoyloxy-l,2-dip]ienylethanol (63) l01 Product formation is best rationalized by a ECiqE-sequence leading to theienerge-tically most favorable acyloxonium ion (64) (Eq. (125) ) ... 

Immobilization of 133 on a polymeric resin shows several advantages over the corresponding monomeric species.199 Oxidation of 133 with sodium periodate affords the corresponding bis-(p ra-methoxyphenyl) tellurone (An2Te02) 134, which presents a peculiar reactivity.198 For example, the tellurone 134 oxidizes benzyl alcohols to benzaldehydes in good yields (Scheme 72), and also converts hydrobenzoin in benzaldehyde in good yield. 

In the second approach, a chiral nitrogen-containing compound has most often been used as the ligand to achieve enantioselectivity. Thus, oxidation of ( )-stilbene (22 equation 9) with a stoichiometric quantity of osmium tetroxide in toluene at room temperature, in the presence of dihydroquinine acetate (23), yielded r/ireo-hydrobenzoins (24) after reductive hydrolysis, with an enantiomeric excess of 83.2% in favor of the (15,25)-(-)-isomer performing the reaction at -78 C increased the eiuuitiomeric excess to 89.7%. 

This procedure has been modified to become an effective catalytic procedure in which iV-methyl-moipholine A -oxide is used as the secondary oxidant. In this manner, ( )-stilbene has been converted into (+)-r/irco-hydrobenzoin (55% yield after two reciystallizations, >99% ee) on a one molar scale, by treatment with osmium tetroxide (0.002 mol equiv.) and iV-methylmoipholine 1 -oxide (1.2 mol equiv.) in aqueous acetone in the presence of dihydroquinidine p-chlorobenzoate (0.134 mol equiv.). The latter compound can be recovered in 91% yield. 

The reaction of cyclohexene with AT-(phenylseleno)phthalimide in the presence of (S,S)-hydrobenzoin in methylene chloride afforded two diastereomeric oxyselenides (38 and the (li , 2R) diastereomer) in a 1 1 ratio. Compound 38 was separated and converted into the olefin 39 via selenoxide elimination. The second PhSeOTf promoted oxyselenenylation reaction gave only the cis fused bicyclic dioxane 40. Oxidation and subsequent elimination provided the olefin 41. This is the key intermediate for the syntheses of the cyclitols 42 and 43, that were obtained from a series of classical reactions as indicated in the Scheme. Oxyselenenylation reactions have also been employed to promote glycosylation reactions [43]. 

Optically active hydrobenzoins. Optically active hydrobenzoins 4a and 4b can be obtained by reaction of the racemic oxide of rra j-stilbene (2) with ( + )-cumphanic acid to form two threo-esters, 3a and 3b. These are hydrolyzed to the two enantiomeric forms of hydrobenzoin, 4a and 4b. ... 

Furthermore, it is sometimes possible to measure polarographic-ally simultaneously the concentration change for more than one component of the reaction mixture. When two or more limiting currents at applied potentials are measured, two or more galvanometers are used.( l The recording of whole curves is often necessary Thus in the oxidation of hydrobenzoin with periodic acid (Fig. 45) the decrease in the concentration of periodic acid, the increase in the concentration of the benzaldehyde formed and the constant concentration of iodate can be followed. Such measurements enable us to verify the kinetics studied by the use of different equations. 

Further, itwas shown [89] that electrolysis of benzoic acid in acetonitrile with triethylamine and cis- and trans-stilbene gives only meso-hydrobenzoin dibenzoate irrespective of the stilbene configuration. The authors attempted an explanation of this unexpected result on the assumption that stepwise oxidation and inversion of an intermediate unstable product occur. An important part is played here by adsorption and oxidation of stilbene itself at the electrode, although no evidence was obtained for its possible isomerization under the experimental conditions. 

It was observed that hydrobenzoin was selectively oxidized to benzoin (99% yield), when exposed to microwaves for 20 sec or to benzyl in quantitative yields, when exposed for 2 min using NBS and neutral Al Oj. It is a quite simple and efficient solvent-free protocol for the microwave accelerated oxidation of hydrobenzoins, benzoins and alcohols with N-bromosuccinimide-neutral alumina. The reaction conditions were optimized by carrying out reactions of hydrobenzoin as a model substrate and changing the molar ratios of NBS and AI2O3 besides variable micro-wave irradiation. 

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