The three mechanisms for cope rearrangement of 1,5-hexadiene

✍ Scribed by Donald C. Wigfield; Kevser Taymaz

• Book ID: 104224608
• Publisher: Elsevier Science
• Year: 1975
• Tongue: French
• Weight: 205 KB
• Volume: 16
• Category: Article
• ISSN: 0040-4039
• DOI: 10.1016/s0040-4039(00)91473-4

✦ Synopsis

There is evidence that Cope Rearrangements may occur by as many as four distinct mechanisms, of which only one is the usual concerted mechanism (1). In 1972, Goldstein and Benzon reported the discovery of a second degenerate rearrangement of 1,5-hexadiene, occurring only at high temperature (250-3000), experimentally distinct from the normal concerted mechanism by virtue of loss of stereointegrity from 1,5-hexadiene-1,3,4,6-d4 (2). If, as the authors suggest, this high temperature rearrangement is concerted, it represents yet another mechanistic variety in the Cope Rearrangement -a second concerted mechanism. The evidence presented, however, seemed to us not to completely exclude the possibility that the new high temperature rearrangement might be a further example of the radical dissociation-recombination mechanism which clearly would also be consistent with the observed racemization and with the observed near zero value of AS+ (-3.0 f 3.6 e.u.) (3). In particular, the activation parameters reported (AH+ = 44.7 kcals/mole; AS+ = -3.0 e.u.), while quite different from enthalpy and entropy differences between 1,5-hexadiene and two ally1 radicals (AH = 62 kcals/mole; AS = +35 e.u.), combine to give a value of free energy (AG' = 46.3 kcals/mole) similar to that calculated from the equilibrium data (AG = 44 kcals/mole) (4). Thus, in the absence of a barrier to radical recombination it would appear that at 250' both the new rearrangement and dissociation-recombination would take place at comparable rates. Even taking into account the value of the free energy barrier to radical recombination, which can be estimated from the data of Benson and co-workers (5) as approximately 5 kcals/mole, the value of AG# for dissociation (49 kcal/mole) is still within experimental error with the AGf