Ketones, bromination

The optical rotatory dispersion curves of steroidal ketones permit a distinction to be made between the conformations, and assignment of configuration is possible without resorting to chemical methods (see, e.g. ref. 36) which are often tedious. The axial halo ketone rule and, in the more general form, the octant rule summarize this principle and have revealed examples inconsistent with the theory of invariable axial attack in ketone bromination. 2-Methyl-3-ketones have been subjected to a particularly detailed analysis. There are a considerable number of examples where the products isolated from kinetically controlled brominations have the equatorial orientation. These results have been interpreted in terms of direct equatorial attack rather than initial formation of the axial boat form. 

DlBENZ[c,e][l,2]AZABOKINE, 5,6-DIHY-DRO-6-METHYL-, 46, 6S Dibenzyl ketone, bromination of, 47, 62... 

Thus the approach to equilibrium always follows a first-order rate law, Equation (14), with the pH-dependent rate constant kobs = kE + kK. Figure 1 shows the concentration changes in time starting from a 1m solution of pure enol (full line) and of pure ketone (dashed line). The individual, unidirectional rate constants kE and kK can be determined as follows For most ketones the equilibrium enol concentration is quite small, i.e., tE = cE(oo)/ ck(oo)<<1. Hence kE kK [Equation (1)], so that enol ketonization is practically irreversible and kE may be neglected, kobskK. The rate constant of enolization kE, on the other hand, is equal to the observed rate constant of reactions for which enolization is rate-determining, such as ketone bromination (Scheme 2). 

The anethole ring has two functional groups - an ether and a hydrocarbon side chain with a double bond. The ether is synthesized first - by a Williamson ether synthesis from phenol and CH3I. The hydrocarbon side chain results from a Friedel-Crafts acylation of the ether. Reduction of the ketone, bromination and dehydrohalogenation are used to introduce the double bond. 

The overall result of the HeU-Volhard-Zeiinskii reaction is the c formation of an acid into an u-bromo acid. Note, thou, that the key step involves a substitution of an add bromide enol rather than a rarboxdic acid enol. The reaction is analogous in all respects to what occurs durjg ketone bromination. 

Interactive mechanism for base-catalysed ketone bromination... 

Photo-catalytic bromination of epoxides derived from terminal alkenes results in the formation of ketones brominated exclusively on the less substituted a-carbon atom [equation (47)]. 5,5-Dibromo-2,2-dimethyl-4,6-dioxo-l,3-dioxan (32) has been shown to monobrominate saturated aldehydes and ketones... 

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