Benzilic acid, base-catalysed rearrangement

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. 

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... 

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

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. 

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