EPR Studies on Carboxylic Esters. Part 17.¹ EPR Spectra and Spin Densities in the Radical Anions of Thio and Dithiocoumarins

Abstract

The radical anions of 4-methyl-1-thiocoumarins and 4-methyl-1-thio-2-thionocoumarins are generated by internal electroreduction and studied by EPR spectrosopy. The spin density distribution is evaluated from the proton hfs coupling constants and by MO calculations. The radical anions of the corresponding coumarins and 2-thionocoumarins are not persistent enough for EPR spectroscopic measurements.

EPR Studies on Carboxylic Esters. Part 20. EPR Spectra and Spin Densities in Radical Anions of Isocoumarin, Benzocoumarin and their Sulfur Analogues

“…This shift can be explained by the enhanced polarisability of sulfur as compared with oxygen and, in particular, of the thiocarbonyl group versus the carbonyl group, which facilitates the uptake of an electron into the molecule. We have observed this effect also in the coumarin series 2 and in open-chain esters and their sulfur analogues. [5][6][7][8][9] Furthermore, a significant shift in the same direction occurs between the isocoumarins 1-4 and the corresponding benzocoumarins 5-8.…”

“…This effect should be due to the more extended p-electron system of the latter with a smaller HOMO-LUMO difference. Remarkably, the reduction potentials E ½ of the benzocoumarins are similar to E ½ of the corresponding coumarin derivatives 2 whereas the E ½ -values of the isocoumarins deviate significantly. Thus, in terms of the SET step, the benzocoumarin system is obviously more closely related to the coumarin than to the isocoumarin system, each of which formally represents a constituent of the benzocoumarin system.…”

“…The compounds to study, 1-8, were prepared according to literature procedures (see Experimental). Their polarographic half-wave reduction potentials E ½ corresponding to the formation of radical anions by single electron transfer (SET) of 1-8, which we have determined as described earlier, [2][3][4] are compiled in Table 1.…”

“…Recently we have reported on the radical anions of coumarins and their sulfur analogues, 1,2 which may be thought of as cyclic cinnamate esters and thioesters. Their isomeric counterparts, the isocoumarins, on the other hand, represent cyclo-conjugated benzoate esters and thioesters, whereas the corresponding benzoanellated derivatives exhibit a combination of both these features.…”

Enhanced photocatalytic dehalogenation of aryl halides by combined poly-p-phenylene (PPP) and TiO2 photocatalysts

“…For the chosen aryl halides, all reported reduction potential values were determined using the same Ag/AgBr system, in which the potential of the Ag/AgBr system was estimated at -520 mV vs. SCE or -276 mV vs. NHE. [31,32] In addition to 1, the aryl halides (3-10) used in this study are shown in Scheme 2.…”