A Bispericyclic Transition Structure Allows for Efficient Relief of Antiaromaticity Enhancing Reactivity and Endo Stereoselectivity in the Dimerization of the Fleeting Cyclopentadienone

B3LYP/6-31G calculations account for the enhanced reactivity and endo stereoselectivity in the dimerization of the fleeting antiaromatic cyclopentadienone. Secondary orbital interactions promote endo stereoselectivity and a full merging of 4+2 and 2+4 allowed paths in an endo bispericyclic transition structure. Electrostatic effects increase reactivity and selectivity but the driving force to enhanced reactivity is the loss of antiaromaticity in the dimerization TSs while enhanced selectivity derives from the more efficient relief of antiaromaticity in the bispericyclic array.