Hydrobenzoins

Usually prepared by the action of NaCN on benzaldehyde in dilute alcohol. It is oxidized by nitric acid to benzil, and reduced by sodium amalgam to hydrobenzoin PhCHOHCHOHPh by tin amalgam and hydrochloric acid to des-oxybenzoin, PhCH2COPh and by zinc amalgam to stilbene PhCH = CHPh. It gives an oxime, phenylhydrazone and ethanoyl derivative. The a-oxime is used under the name cupron for the estimation of copper and molybdenum. 

Benzoin yields hydrobenzoin on reduction with sodium amalgam, and clesoxybenzoin, C H,CO.CHj.C,Hj, when reduced with zinc and hydrochloric acid. 

The stereoselective synthesis of hexacoordinated phosphate anions was also reported by the same group. A general one-pot process was developed for the preparation of C2-symmetric anions 15,16 and 17 containing enantiopure BINOL, hydrobenzoin, and tartrate-derived ligands respectively [38-40] Cpsymmetric anion 18 being prepared similarly in two steps from methyl-a-... 

Cr(II) reduction of benzaldehyde in aqueous acidic ethanol also yields largely hydrobenzoin although other products were characterised, e.g. hydrobenzoin monoethyl ether, and a 1 1 stoichiometry is preserved. The rate law in ethanol acidified both with HCIO4 and HCl is complex for example, in HCl-ethanor°°,... 

Rovis and co-workers have extended the application of redox transformations to generate chiral acylazolium species from a-haloaldehydes 245 and the NHC derived from pre-catalyst 247, allowing the desymmetrisation of mei o-hydrobenzoin 246 to give ester 248 in good yield and enantioselectivity (Scheme 12.54) [30]. 

The enantioselective synthesis of an allenic ester using chiral proton sources was performed by dynamic kinetic protonation of racemic allenylsamarium(III) species 237 and 238, which were derived from propargylic phosphate 236 by the metalation (Scheme 4.61) [97]. Protonation with (R,R)-(+)-hydrobcnzoin and R-(-)-pantolactone provided an allenic ester 239 with high enantiomeric purity. The selective protonation with (R,R)-(+)-hydrobenzoin giving R-(-)-allcnic ester 239 is in agreement with the... 

It oxidizes thiophenol to diphenyl disulphide, hydroquinone to p-benzoquinone, benzoin to benzyl, benzylic alcohols to the corresponding carbonyl compounds and cleaves hydrobenzoin to benzaldehyde. 

In the context of our work in the area of chiral nucleophilic carbenes and their utility in organic synthesis, we have developed a conceptually distinct approach to catalyzed acylation using a-haloaldehydes as acylation precursors. The use of a chiral triazolium salt in the presence of base allows an enantioselective desymme-trization of meio-hydrobenzoin to proceed in 83% ee and good yield ... 

A Raney nickel surface is also suitable for electrocatalytic hydrogenation [205]. This surface is prepared by electrodepositing nickel from a solution containing suspended Raney nickel alloy (Ni 50% A1 50%). Some alloy particles stick to the surface, which is then activated by leaching the aluminium using hot aqueous sodium hydroxide. Cyclohexanone, acetophenone and benzil have been converted to the corresponding alcohol and there is no stereoselectivity for the formation of hydrobenzoin from benzil. 

The use of tartrate esters was an obvious place to start, especially since both enantiomers are readily available commercially and had already found widespread application in asymmetric synthesis (Figure 11) (e.g.. Sharpless asymmetric epoxidation).23.24 Reagents 36-38 are easily prepared and are reasonably enantioselective in reactions with achiral, unhindered aliphatic aldehydes (82-86% ee) typical results are given in Figure 12.3c,h Aromatic and a,p-unsaturated aldehydes, unfortunately, give lower levels of enantioselection (55-70% e.e.). It is also interesting to note that all other C2 symmetric diols that we have examined (2,3-butanediol, 2,4-pentanediol, 1,2-diisopropylethanediol, hydrobenzoin, and mannitol diacetonide, among others) are relatively ineffective in comparison to the tartrate esters (see Table ll).25... 

Figure 14. Chiral 1,2-diols that have been incorporated into monocyclic crown compounds. Listed under (o) are piecursors with C2 symmetry obtained from L-taitaric acid and L-thieitol, as well as from (5S)-hydrobenzoin as its p-methoxy analog, under (b) are precursors with Cj symmetiy obtained from D-mannitol and L-iditol, and under (c) are asymmetric precursors obtained from (5)-lactic acid, (5)-mandelic acid, and L-glyceraldehyde dithioethylacetal.

Both the (RR)- and (5S)-enantiomers of hydrobenzoin (Figure I4a) have been made to react (110, 117, 118) successfully in a (2 + 2) cyclization with ethylene glycol ditosylate in the presence of bases (e.g., NaOH in dioxane or NaH in MA -dimethylformamide) to give the enantiomeric tetiaphenyl-18-crown-6 derivatives (RRRR)-73 and (SSSS)-73. The corresponding optically pure tetra-anisyl, tetra-a-naphthyl, and tetra-/3-naphthyl-18-crown-6 derivatives 74, 75, and 76, respectively, have also been prepared in similar fashion (119). It should be noted that (RRRR)- and (,SSSS)-73 have also been obtained as a result of a base-promoted (1 + 1) cyclization (120) between the chiral extended diol and... 

The 18-crown-6 derivative (RRRR)-73 [obtained from (/Uf)-hydrobenzoin] with BH3NH3 (118). 

The first method is successful on a large scale with methadone (3) and l,2-diphenyl-l,2-ethanediol (hydrobenzoin, 4). 

Table 7 shows a series of alcohols analyzed with the Alexakis reagent. In the 31P-NMR spectra, alcohol derivatives appear as two singlets (P—H decoupled) corresponding to each diastereomer. Shift differences are about 10 times larger in the primary, nonthionated product. No racemization could be observed during the derivatization step as demonstrated with ethyl lactate and hydrobenzoin. 

The only good method for the preparation of benzoin1 appearing in the literature is by the action of potassium cyanide upon an alcoholic solution of benzaldehyde, a method described first by Wohler and later modified slightly by Zinin and Zincke. The other methods are of theoretical importance only and have, therefore, not been studied. Thus, by the reduction of benzil,2 and by the oxidation of hydrobenzoin,3 benzoin can be formed it happens, however, that both benzil and hydrobenzoin are commonly made from benzoin. 

Figure 5. Mass chromatogram of photoinitiators in a polymer system. Peak 1 benzoin, molecular weight 212.24 (theory), 212.6 (MC-2). Peak 2 hydrobenzoin, molecular weight 214.26 (theory),...

Both benzil and benzoin are reduced by TiCl3 in an acidic medium to hydrobenzoin (95% yield) but in this case the meso-isomer is strongly favored over the d/-isomer (—80 20). 

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