Captodative stabilization in geminally substituted allylic radicals assessed by means of the EPR-spectral D parameter of 1,3-cyclopentanediyl triplet diradicals

The EPR-spectral zero-field parameter D of low-temperature persistent triplet diradicals T was used to assess the electronic substituent effects in EA/ED and ED/ED geminally substituted allylic radicals. From these EPR data, the interaction parameter (DeltaS) was calculated for a set of doubly functionalized radicals, which expresses the ability of the unpaired electron to interact either individually with the functional groups or simultaneously in either a synergistic or antagonistic way. We show that the ED/ED combinations interact antagonistically with the odd electron, whereas the R2N/EA and the MeO/EA pairs display a pronounced captodative delocalization. Compared to MeO/EA, the MeS/EA functionalities manifest just an additive effect of the individual substituents. These results are rationalized in terms of appropriate electron-transfer-type mesomeric structures. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.