Isopropenyl aryl groups

T. Ishida, T. Matsumoto, J. Caldwell, A. Drake, Enantioselective Biotransformation of Aryl-isopropyl, -isopropenyl, and -propenyl Groups by the Rabbit or by the Rat , Enantiomer 1998, 3, 133 - 147. 

Alkenyl (vinyl), aryl, and alkynyl carbocations are particularly unstable with respect to alkyl carbocations. Let s compare the isopropyl cation with the iso-propenyl cation. In the latter, the central C has two cr bonds, one it bond, and one empty orbital, so it is sp-hybridized (linear). Both ions are stabilized by the C(sp3)-H cr bonds of the CH3 group on the right. In the isopropyl cation there is an additional interaction with C(sp3)-H cr bonds on the left, whereas in the isopropenyl cation there is an additional interaction with C(sp2)-H cr bonds on the left. Because C(sp2) orbitals are lower in energy than C(sp3) orbitals, the... 

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