Sigmatropic rearrangements Cope rearrangement degeneration

Fivefold degenerate reversible [3,3]-sigmatropic shifts were first reported in 1988116,117 in the CPD-amidine system 257, where AG g = 117 to 120 kJmol-1 (equation 88) (for aza-Cope rearrangements see Section IV.E.2). In addition, a slow accumulation of a colored by-product was observed at elevated temperatures. This was identified as a product of a novel intramolecular carbon to nitrogen 1,4-shift of the methoxycarbonyl... 

The way in which all 10 hydrogens (or carbons) in bullva-lene become equivalent involves a series of degenerate (i.e., leading to an equivalent structure) isomerizations known as Cope rearrangements (or [3.3] sigmatropic shifts). One such rearrangement is shown below (the dot is there to keep track of Cd) ... 

The Cope rearrangement is a 3,3-sigmatropic rearrangement of 1,5-hexadienes. Although the rearrangement is degenerate for 1,5-hexadiene itself, substituents will alter the position of the equilibrium in accord with the relative stabilities of the two hexadienes. 

---