Driving force for rearrangement

The driving force for rearrangement is relief of steric strain between the isopropyl group and one of its adjacent methyl groups. Isomerization is acid-catalyzed. Protonation of the ring generates the necessary carbocation intermediate and rearomatization occurs by loss of a proton. 

The driving force for rearrangement of water molecules before instantaneous FC proton-electron charge transfer presents a problem. A five-molecule water superdipole assembly might acquire +25.9kT of thermal energy, but there will be a low probability that its subsequent rearrangement will direct it towards that for an intermediate state for FC electron transfer and proton discharge. This is not necessarily true for... 

The driving force for rearrangements of this type is conversion of a less stable car-bocation to a more stable one. Proton transfer to H2O then gives the alkenes. As in other cases of acid-catalyzed dehydration of alkenes, the Zaitsev rule applies, and the more substituted alkene predominates. 

Let us note here some of the basic principles underlying models, theories, and calculations of hydrophobic effects. The quantity of first interest is the free energy associated with the interaction of the solute with the aqueous environment. This is the chemical potential or partial molar Gibbs free energy of the solute. This quantity provides a driving force for rearranging molecules in thermodynamic systems and many quantities of interest are fundamentally connected to this free energy. Consider first an atomic solute of type A. For example, perhaps A = Ar. The chemical potential at extreme dilution may be expressed as... 

The driving force for ring expansion of 17-hydroxy-20-keto steroids probably comes from relief of strain accompanying conversion of trans-fused hydrindanes to tra/75-fused decalins. The greater susceptibility to rearrangement of 17/ -hydroxy-20-ketones as compared to the 17a-hydroxy-20-... 

In the case of an appropriate substrate structure, the carbenium ion species can undergo a 1,2-alkyl shift, thus generating a different carbenium ion—e.g. 4. The driving force for such an alkyl migration is the formation of a more stable carbenium ion, which in turn may undergo further rearrangement or react to a final product by one of the pathways mentioned above—e.g. by loss of a proton to yield an alkene 3 ... 

In this synthesis, we have witnessed the dramatic productivity of the intramolecular enone-olefin [2+2] photocycloaddition reaction. This single reaction creates three contiguous and fully substituted stereocenters and a strained four-membered ring that eventually provides the driving force for a skeletal rearrangement to give isocomene. 

The transformation from the radical cation to arenium ion is proposed to proceed in the manner shown in Figure 12. Here, the release of strain, which is calculated to be 42.9 kcal/mol at the B3LYP/6-31G(d) level, accompanying the contraction of the central six-membered ring to the five-membered ring, is considered to be an important driving force for this rearrangement. 

Even 2,3-disubstituted indoles can be achieved if internal olefins are used. Regioselective hydroformylation of a styrene-type olefin and subsequent hy-drazone formation and Fischer indolization gives an intermediate indole with a quaternary center in 3-position. The regained aromaticity is the driving force for the rearrangement of one substituent into the 2-position of the indole core (Scheme 39). 

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