Sigmatropic migration of carbon

FIGURE 18.5 Hypothetical orbital movement for a thermal [1,5] sigmatropic migration of carbon. To move from one — lobe to the other — lobe, the migrating carbon use only its own — lobe, retaining its configuration. 

L8.6 Hypothetical orbital movement for a thermal [1,3] sigmatropic migration of carbon. The migrating carbon moves from a — to a + lobe, requiring it to switch its own bonding lobe from — to -f, inverting its configuration. 

Unlike [l,3]-sigmatropic migrations of H-atom, which cannot occur under thermal conditions, the [ 1,3] -sigmatropic migrations of carbon can occur under thermal conditions. This is because carbon has two-lobed p-orbital. Therefore, carbon can simultaneously interact with the migrating source and the migrating terminus using either one of its lobes or both of its lobes, as shown in Fig. 8.58. 

The thermal expansion of a vinylcyclopropane to a cyclopentene ring is a special case of a [l,3]-sigmatropic migration of carbon, although it can also be considered an internal [ 2 + 2]-cycloaddition reaction (see 15-63). It is known as a vinylcyclopropane rearrangementThe reaction has been carried out on many vinylcyclopropanes bearing various substituents in the ring or... 

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