Benzil, base-catalysed

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,... 

Base-catalysed ring fission of 3,4-diphenylcyclobut-3-ene-l,2-diones (103) in 50% (v/v) aqueous DMSO proceeds by rapid reversible addition of hydroxide ion followed by rate-determining benzilic acid-type rearrangement to form an intermediate 1-hydroxycyclopropane-1-carboxylic acid which ring opens to the corresponding (Z)-2-oxo-3,4-diphenylbut-3-enoic acid (Scheme 8).173 This is supported by the value of Hammett p = 1.3 (for variation of substituents on one or both rings), the kinetic solvent effects, and the three-oxygen enrichment of (107) from reaction of (103) in 50% H2 180-DMSO. 

A similar investigation of the base-catalysed ring opening of 3,4-diphenylcyclobut-3-ene-l,2-diones (77) to give (Z)-2-oxo-3,4-diphcnylbut-3-cnoatcs (78) has been carried out in aqueous DMSO.108 The evidence points towards a rapid, reversible addition of hydroxide to one carbonyl, followed by a benzilic acid-type rearrangement to give a cyclopropene intermediate (79), which ring opens. 

It has been shown that benzils (323) react with Michael addition acceptors (324) in the presence of a catalytic amount of cyanide ion to yield 1,4-diketones (325). The authors proposed that (325) are produced through the formation of the O-aroylmandelonitrile anion, followed by Michael addition and rearrangement of the aroyl group with decyanation (see Scheme 77). The mechanism of the base-catalysed ring fission of 2,2-dihydroxyindane-l,3-diones has been investigated and the pathway set out in Scheme 78 has been proposed for the transformation. The base-catalysed ring... 

The scheme was verified usii chronopotentiometric and potentio-static methods (Alberts and Shain, 1963). A similar scheme was considered for the reduction of o- and p-nitrophenols and nitroanilines but the theoretical treatment is more involved owing to the irreversibility of the first four-electron reduction step. Similarly also a chemical reaction is interposed between the first and second electrode process in the reductions of a-substituted ketones of the type RCO.CHg—X (for X=NR2, NR, SR, SRf, OR, PRJ and halogen) and in the reductions of a,)9-unsaturated ketones. In both these systems enolate is a primary electro-inactive reduction product that must be first transformed into the electro-active keto-form. The rate of this transformation, which is acid-base catalysed, limits the wave-height of the more negative wave of the saturated ketone. Similarly it was explained why the more negative wave observed on curves of benzil is smaller than expected for the given concentration of benzoin. 

Interest in the structure-activity relationships of the gibberellins has led to the partial synthesis of the four stereoisomers about positions 5 and 6 of the hexa-hydrofluorene (57). The unsaturated acid anhydride (56) was prepared by the benzilic acid ring-contraction of the 6,7-diketones obtained from oxidation of desisopropyl dehydroabietic acid, and converted into the saturated acids by reduction and base-catalysed epimerisation. 

The remainder of this section will focus specifically upon several recent mechanistic studies that have identified acid catalysis in HTW and SCW. It should not be assumed that the increase in the value for water in HTW and SCW will result in an increase in the observed rate of acid catalysis. Many organic reactions can be both acid- and base-catalysed and, as the heterolytic fission of water produces equivalent amounts of H and OH, an observed increase in the rate of the reaction may arise from either the acid-or base-catalysed pathway or indeed both routes simultaneously. Furthermore, the dominant reaction pathway under ambient conditions can change in HTW and SCW. For example, Savage has shown that the rearrangement of benzil to benzilic acid, which is exclusively base catalysed under normal conditions, can also exhibit solvent and acid catalysis in HTW (Scheme 3.2). Thus, the... 

The rearrangement of 1,2-diketones to a-hydroxy carboxylic acids is referred to as Benzilic acid rearrangement. It is a base catalysed reaction whereby a-diketones are converted into a-hydroxy acids. 

Schaltegger et al reported the base-catalysed benzilic acid rearrangement of cyclic diketone 29, which, resulted in ring contraction affording the acid 30. 

A rapid and efficient microwave-assisted synthesis of hydantoins and thiolydan-toins was reported by Muccioli et al. [74], The reaction is known as Biltz synthesis and is the base-catalysed condensation of benzil and urea. It was observed that microwave irradiation reduced the reaction time as well as increased the yield of the product as compared to the conventional method. The reason behind this is the involvement of polar transition state, i.e. nucleophilic addition of a neutral NH2 group to a carbonyl site (Scheme 11.21). The yield via the conventional method was 36 % when heated for 2 h whereas the yield via microwave irradiation was 80 % when heated only for 30 min. 

Scheme 3.2 The mechanisms for the base-, solvent- and acid-catalysed rearrangement of benzil to benzilic acid.

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