Allylic radicals with captodative substitution : Easy homolysis of 1,5-hexadienes.

Abstract

Dissociation of the new 1,3,4,6-captodative substituted 1,5-hexadienes 1 to 1,3-bis-captodative allylic radicals 4 has been observed around room temperature. Dissociation enthalpies ΔH vary from 8 to 25 Kcal/mol. In contrast the dimer 3a of the 1,3-bis-dicapto allylic tetraester radical dissociates to 6a only around 140°C (ΔH = 38.1 Kcal/mol). The replacement of one of the olefinic ester groups on carbons 1 and 6 in 3a by a donor substituent (without influencing the crowding around the central C-C bond), such as in 2a and b lowers ΔH from 38.1 to 24.0 and 28.2 Kcal/mol respectively. These results demonstrate the synergy for radical stabilisation by captodative substitution.