Rearrangement compared with substitution

Ironically, the most recent calculational treatments of the rearrangement from 56 to 57 indicate that even at room temperature tunneling does, in fact, dominate the H-shift process. Phenyl substitution in 56 should lower the energy barrier for rearrangement compared with 54, not only increasing the rate of classical rearrangement... 

Substituted dibenzo[6,/]thiepins can be generated from thioxanthene derivatives by the rearrangement of carbocation 1. Compared with other possible cations, the tropylium ion type 1C is favored because of its resonance energy. Depending on the reaction conditions, the thiepin cation can react to give thiepins by loss of a proton, or by trapping a nucleophile, followed by elimination. 

Now for the relative proportions of products. Only 3% of the unrearranged product shows just how unfavourable the secondary carbocation is compared with the rearranged tertiary carbocation. The relative proportions of the other two alkenes are explained by the increased thermodynamic stability of the more-substituted alkene, though this is not sufficient to produce just the single product. 

Nucleophilic substitution reactions are general. Reactions in side-chains are comparable with the same type of reaction in azines. Dimroth-type rearrangements in appropriately substituted azine rings may occur. 

Substituent effects in the allyl ester rearrangements are very similar to those observed in the ester reverse ene-type eliminations. This is apparent from the relative rate comparisons of Table 8. At the a- and y-carbons, reaction rates are observed to increase in the order CF3 < H < CH3. The rate accelerations by methyl substitution for hydrogen at the a-carbons are factors of 40 and 23, and at the y-carbon are factors of 55 and 23. These effects should be compared with the rate accelerations by methyl for hydrogen substitution at the a-carbon in the ester ene reactions, i.e., from Table 2, i-PrOAc/EtOAc = 18.7 and t-BuOAc/i-PrOAc = 53. One may conclude that the positive formal charge densities at the a- and... 

Rearrangements of a-substituted cyclopropylmethyl radicals afford mixtures of the (E)-and (Z)-butenyl radicals usually with a preponderance of the Z-isomer. This stereoselectivity is probably a consequence of the higher proportion of the a /-conformer compared to the j n-conformer, the former being lower in energy for steric reasons i.e. nonbonded interactions between the substituent and ring hydrogens are less important. The -alkene is also thermodynamically more stable than the Z-alkene. ... 

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