Anisyl migration

The most reasonable interpretation of Curtin s work is outlined in Fig. 28.2. This assumes a situation exactly analogous to that indicated by Collins work, a reasonable assumption since anisyl and phenyl are of the same bulk. Whether phenyl or anisyl migrates predominantly depends on which group is in the proper location in the first-formed carbonium ion, and this again depends on the most stable conformation of the parent diazonium ion. The minor product in each case is due to front-side migration of the aryl group brought into position by the easiest rotation of the carbonium ion. 

The rearrangement of IV-triarylmethylhydroxylamines gives the migration aptitudes -anisyl > phenyl > -chlorophenyl > -nitrophenyl.87... 

The migration aptitudes in the reaction of ketones with peracetic acid in acetic acid as a solvent are in the order -anisyl > phenyl > >-nitro-phenyl.82 Cyclohexyl and phenyl are about equal in migration aptitude... 

The migratory aptitudes obtained in this way were /i-anisyl, 880 /i-tolyl, 40 phenyl, 1 /i-chlorophenyl, 0.47 Depovere points out that the larger values for the migratory aptitudes of />-anisyI and -tolyl here as compared to Bachmann s data are due to the twofold role of the electron-donating groups in 37 They migrate better from the j3 carbon and they facilitate ionization at the a carbon. 

Conformational factors can determine more than the stereochemistry of rearrangement. In light of Collins findings, let us examine work done earlier by D. Y. Curtin (of the University of Illinois) with 2-amino-1-anisy 1-1-phenyl-1-propanol. This resembles Collins labeled compound (p. 899), except that an anisyl group (p-methoxyphenyl group) takes the place of one of the phenyls. Here, the competition in migration is between a phenyl and an anisyl, instead of between labeled and unlabeled phenyl groups. 

Figure 28.2. Pinacolic deamination of diastereomeric 2-amino-l-anisyl-1-phenyl-1-propanols. In each case the most abundant conformer. Via or Vila, of the diazonium ion yields a cation in which an aryl group is in position for back-side migration via a trans transition state anisyl in lb, phenyl in VII6. Such rearrangement predominates. Some of each first-formed cation is converted through rotation into another cation, in which the other aryl group is in position for front-side migration via a trans transition state phenyl in Vic, anisyl in VIIc. Such rearrangement gives the minor product.

The pinacolic deamination of 2-amino-1,1 -diaryl-1 -propanols provides a convincing case for conformational control. Optically active 2-amino-l,l-diphenyl-l-propanoI 209) affords a-phenylpropiophenone (210) with 88% inversion plus 12% retention (76% inversion plus 24% racemization) at the terminus of phenyl migration 196K The diastereomer (211) of 2-amino-l-anisyl-l-phenyl-l-propanol reacts with preferential... 

By oxidation of the labeled products [28 and 29 or 30ab] to labeled benzoic and substituted benzoic acids followed by assay of these purified acids for carbon-14, the relative migratory abilities of the Ar groups with respect to phenyl were determined. Thus in the dehydration, with phosphorus pentoxide in xylene, of a series of carbinols (27, X = OH), the following migratory aptitudes (Ar Ph) were determined p-xenyl, 1-3 m-tolyl, 1-6 p-i-propylphenyl, l-8 3,4-dimethyl-phenyl, 1-9 p-tolyl, 2-0 p-ethylphenyl, 2-2 p-t-butylphenyl, 3-2 and p-anisyl, 21-2. Burr (1953) demonstrated the p-tolyl phenyl migration ratio to be 2-5 during the solvolysis of 2-phenyl-2- p-tolylethyl-l-i C tosylate (27, Ar = p-tolyl, X = tosyl). In a series of deaminations of the primary amines (27, X = NH ) and 31 (Ar = p-tolyl and o-tolyl) the p-tolyl phenyl (1 2), p-biphenyl phenyl (1 0) p-anisyl phenyl... 

The minor products (91) of the deamination of amino-alcohol 89 must not have been formed through the cis-transition state, which would lead to structure 91a, but must instead have been produced by migration of the p-anisyl group to the same side of the migration terminus which was originally bonded to carbon. Structure 89 should then yield the mirror image of 91a and not structure 91a itself as the minor component of the deamination reaction. 

If insufficiently solvated (as in DEE), the lithium atom exerts a pull effect on the halogen and thus triggers its departure. The migrating aryl assists this process nucleophilically. Therefore, 2-p-anisyl substituted a-chlorovinyllithiums rearrange more rapidly and 2-p-chloro substituted ones more slowly than the parent compounds. ... 

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