Migratory aptitude, alkyl groups

The carbocations generated by non-Kolbe electrolysis can rearrange by alkyl, phenyl or oxygen migration. The migratory aptitudes of different alkyl groups have been studied in the rearrangement of a-hydroxy carboxylic acids (Eq. 34) [323]. 

It is considered that [1,5] alkyl shifts usually require temperatures above 330 °C and proceed with free energies of activation greater than 180 kJmol-1. The 1,5-migratory aptitude of the formyl group is comparable with that of a trimethylsilyl group. However, 5-acetyl- and 5-ethoxycarbonyl-PMCPDs under the same conditions show a temperature-invariant XH NMR spectra, i.e. the migratory aptitude decreases in order CHO COMe COOMe114. 

Electrolysis of /J-hydroxy carboxylic acid (90) gave ketone (91) as the main product, which was converted to dl-muscone (Scheme 34) [113]. The migratory aptitude of alkyl substituents at the fi-position in the intermediate -hydroxy carbenium ion was scrutinized to show that vinyl, cyclopropyl, and benzyl groups rearrange predominantly. 

Answer. Three aspects of the scheme shown in Figure 13.2 may be examined by orbital interaction theory (a) the binding of CO compared to ethylene (b) the reactivity of coordinated carbonyl compared to ethylene with respect to reactivity toward intramolecular nucleophilic attack and (c) the migratory aptitude of alkyl versus carbonyl groups. 

The relative ease with which alkyl and aryl groups migrate is called their migratory aptitude. Unfortunately, migratory aptitudes are not absolute quantities values determined in one reaction under one set of conditions may differ enor-... 

An experimental and theoretical study of the Baeyer-Villiger (BV) oxidation of ketones has examined its uncatalysed and acid-catalysed forms, and fluoro- and chloro-substituted substrates.344 In assessing migratory aptitudes, fluoroalkyl groups are only slightly less favourable than alkyl (by 0.3/0.5 kcal mol-1, calculated/observed), whereas migration of a chlorinated substituent is significantly more difficult (ca 2.6 kcal mol-1). 

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