Bent bonds and the antiperiplanar hypothesis – a simple model to rationalize [1,3]-sigmatropic alkyl shifts†
The bent bond/antiperiplanar (BBA) hypothesis has been applied to the analysis of [1,3]-sigmatropic alkyl shifts. These thermal rearrangements, for which there is evidence that they proceed through diradical intermediates, can be interpreted by considering their transient allyl radical structures. For the thermolysis of cyclic molecules, the preferred generation of pyramidal allyl radicals in staggered conformations is postulated on the basis of the BBA hypothesis. This accounts for the preference of suprafacial rearrangement pathways as well as the extent of inversion or retention of configuration at the migrating carbons.