Benzils, formation

When benzii is heated with potassium hydroxide solution, it undergoes a molecular rearrangement with the formation of the potassium salt of benzilic acid, or diphenyl lycollic acid ... 

This rapid formation of the crystalline quinoxaline derivative can therefore be used to identify 1,2-diketones conversely, a nuclear-substituted o-phenyl-enediamine can be identified by the quinoxaline derivative which it forms with a known 1,2-diketone such as benzil. 

Finally a proton transfer leads to formation of carboxylate anion 3. Of particular interest is the benzilic acid rearrangement of cyclic diketones such as 4, since it... 

The first step in the nonreversible degradation reactions is the formation of a reactive a-dicarbonyl species through the p-elimination of a hydroxide ion. The subsequent reaction pathways to all degradation products can be described by just five reaction types, namely, p-elimination, benzilic acid rearrangement, a-dicarbonyl cleavage, aldol condensation, and retro-aldol condensation (see Fig. 7).31 Retro-aldol condensation and a-dicarbonyl cleavage involve C-C bond... 

Only a few other cobalt complexes of the type covered in this review (and therefore excluding, for example, the cobalt carbonyls) have been reported to act as catalysts for homogeneous hydrogenation. The complex Co(DMG)2 will catalyze the hydrogenation of benzil (PhCOCOPh) to benzoin (PhCHOHCOPh). When this reaction is carried out in the presence of quinine, the product shows optical activity. The degree of optical purity varies with the nature of the solvent and reaches a maximum of 61.5% in benzene. It was concluded that asymmetric synthesis occurred via the formation of an organocobalt complex in which quinine was coordinated in the trans position (133). Both Co(DMG)2 and cobalamin-cobalt(II) in methanol will catalyze the following reductive methylations ... 

The diazaphosphetidinones (9 R = OMe or NMej) gave stable phos-phoranes (10) with benzil at - 70 °C, but the adduct of (9 R = NMcz) with diacetyl could not be isolated although there was n.m.r. evidence for its formation. ... 

Reaction of perthiophosphonic anhydrides (64) with amines leads first to (105) and then, by further attack, to (106). With ammonia itself the second addition proceeds at the same phosphorus atom as the initial attack, giving (107) and (108). The anhydride (64) is also reported to react with 1,3-dioIs to give cyclic phosphonyl disulphides (109). Thermal decomposition of phenylphosphinic anhydride (110) may lead to the formation of PhP since in the presence of benzil the formation of the phosphorane (111) was observed. ... 

The symmetric diarylthiirene oxides (18) are much more thermally stable than the corresponding saturated thiiranes and unsaturated thiirene dioxides. Thus, the thiirene oxide 18a shows only slight decomposition after 24 hours of reflux in benzene, whereas the analogous sulfone 19b fragments completely to SO2 and diphenylacetylene after less than six hours under the same conditions Irradiation of the oxide 18a, however, does result in the elimination of sulfur monoxide and formation of diphenylacetylene. Its thermolysis at 130 °C afforded benzil as the only isolable product, implying that SO is not being eliminated in this thermolytic process. 

The synthesis of methylimidazole-thiophene compounds was reported by Santos et al. [46] and has been included for completeness, although no biological activity has been reported for these heterocycles. The formation of these imidizole-thiophenes (24a-d), occurs via the condensation of 2-formylthiophene (25) with benzil derivatives (26a-d) in the presence of ammonium acetate to yield the imidazole-thiophene compounds (27a-d). These compounds can then be N-methylated by treatment with iodomethane in... 

An identical compound is formed from benzil by the action of phenylhydrazine, and from benzaldehyde phenylhydrazone by autoxida-tion (Busch). The formation of osazones from a-hydroxyketones (and a-hydroxyaldehydes) will be discussed later (p. 298). 

Attempts to trap the digermene 74 in solution with benzil afforded only the cycloadduct 109 because of the [1 + 4] cycloaddition with germylene 10863 [Eq. (26)]. Another proof of the formation of germylene 108 in solution from 74 was given by cryoscopic studies.63... 

Saccharinic acid formation has been studied for several years. The four-step reaction proceeds rapidly in alkaline solution because of basic catalysis, particularly in the last two steps. Initially formed is an enediol that can undergo j8-elimination of a functional group, usually a hydroxyl group. The final two steps involve tautomerization to an a,j8-dicarbonyl intermediate followed by a benzilic acid rearrangement. This sequence is shown in Scheme 6 for the formation of the a- and j8 -xylometasac-charinic acids (30) by way of 3-deoxy-D-g/ycero-pentos-2-ulose (29). 

Because a benzilic acid rearrangement is the last step in the formation of saccharinic acids the difference between these results could be due to complexation of hydroxyl groups and alkoxide anions by the divalent calcium ions. Such a complexation apparently promotes isomerization and, hence, a fragmentation-recombination to the xylosaccharinic acid. This route has been questioned, and yet it provides a rational explanation of the results. 

Rate constants and Arrhenius parameters for the reaction of Et3Si radicals with various carbonyl compounds are available. Some data are collected in Table 5.2 [49]. The ease of addition of EtsSi radicals was found to decrease in the order 1,4-benzoquinone > cyclic diaryl ketones, benzaldehyde, benzil, perfluoro propionic anhydride > benzophenone alkyl aryl ketone, alkyl aldehyde > oxalate > benzoate, trifluoroacetate, anhydride > cyclic dialkyl ketone > acyclic dialkyl ketone > formate > acetate [49,50]. This order of reactivity was rationalized in terms of bond energy differences, stabilization of the radical formed, polar effects, and steric factors. Thus, a phenyl or acyl group adjacent to the carbonyl will stabilize the radical adduct whereas a perfluoroalkyl or acyloxy group next to the carbonyl moiety will enhance the contribution given by the canonical structure with a charge separation to the transition state (Equation 5.24). 

ReCl3(PPh3)(benzil)] reacts with bipy and related ligands or terpy to form a number of rhe-nium(III) and rhenium(II) compounds which are useful precursors for the synthesis of lower-valent rhenium complexes. " Thus, reduction of [Re(bipy)3][PF6]2 with zinc amalgam results in the rhenium(I) compound [Re(bipy)3][PF6] in excellent yields. The corresponding terpyridyl bis-chelate [Re(terpy)2][PF6] has been prepared in a similar manner. " The electrochemistry of the products provides a convenient measure of the chemical reactivity associated with the redox processes. Thus, the one-electron oxidation of [Re(bipy)3]" is reversible at -0.33 V, whereas the Re"/Re" redox couple is irreversible and occurs at relatively low potentials (-1-0.61 V) which is consistent with the instability of [Re(bipy)3] + in solution. However, in the presence of a small coordinating molecule such as CNBu, oxidation to the rhenium(III) state is readily available by the formation of seven-coordinate complexes of the composition [Re(bipy)3(L)]. " ... 

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. 

A quick perusal of Table 3 shows inverted stability for a and /3 isomers of phenyl-glyoxime and p-tolylglyoxime. Given uncertainties as to stereochemistry it is plausible that the structural assignments of Z and E, syn and anti, even a and /3 were reversed. This is understandable. However, it is inexplicable that the tolyl species have enthalpies of formation at least 150 kJ moH more positive than the phenyl compounds. This is an altogether implausible result. The solid phase reaction 35 is calculated to be ca 75 kJmol exothermic if we use the averaged value for the various stereoisomers of phenylglyoxime and benzil dioxime. This, too, is implausible. 

The bismuth ylides, Ph3Bi=CHCOR, do not react with simple ketones and electron-rich olefins probably because of their relatively low electrophilic character. However, Ph3Bi=CHCOR reacts with a-keto esters [46, 67, 68], benzils [46, 67-69], orf/to-quinones [46, 67, 68], and acenaphthenequinone [70] to give epoxides, (9-arovl enolates, 3-hydroxytropones, and 3-hydroxyphenalenones, respectively, accompanied by the formation of Ph3Bi (Scheme 11). In particular, transposition and ring expansion reactions are of interest from a mechanistic point of view, since these reaction modes are unprecedented in ylide chemistry. 

The first two reactions of the sequence are similar to reactions that occur in acidic medium. The 1,2- and 2,3-enediols, and the unsaturated elimination-products derived from them, are present both in acidic and basic solutions. In general, however, reactions in basic solution are much faster than in acidic solution, because of the greater catalytic effect of the hydroxyl ion on the transformation reactions Mechanistic differences between the media become operative in steps c and d. In acid, further dehydration, if it is possible, occurs rapidly, before equilibrium of the deoxy-enediol with the dicarbonyl compound has been established,17 and the products are furans. In alkaline solution, the rapid formation of the tautomeric dicarbonyl compound permits the benzilic acid rearrangement42 to proceed. 

The 2-O-methylaldoses undergo //-elimination to form the 2-methyl ethers of the enol form of the 3-deoxyhexosuloses.163 These ethers are relatively stable benzilic acid rearrangement is not possible, because a carbonyl group cannot form at C-2, and further dehydration by elimination of the 4-hydroxyl group, which occurs readily in acid, does not proceed in base.69 Although formation of these ethers has been demonstrated in many studies,164-171 their reactions after... 

---