Cycloreversion reactions for five to seven membered rings

The cycloreversion of the following ring systems was studied: (1) Cyclopentyl anion → allyl anion + ethylene, (2) Cyclohexene → butadiene + ethylene, (3) Cyclohept-4-enyl cation → butadiene + allyl cation. The potential energy surfaces of these reactions were investigated by the semiempirical MO method SINDO1 under inclusion of a 4 × 4 CI. All internal coordinates were varied. We found major differences in the nature and geometry of transition states and intermediates of these reactions. Diradical character is involved only in the fragmentation of the six membered ring.