Reactivity of unsaturated substrates under reductive electron transfer conditions. Part 1. Reduction of 1,2,3-triphenyl-1H-indene with sodium metal in various solvents

Abstract

The reduction of 1,2,3-triphenyl-1H-indene 1 with sodium metal in various solvents was carried out in order to compare the results with those previously obtained in the electrochemical reduction of 1(Hg cathode, dimethylformamide, Bu₄N⁺ ClO₄^– as supporting electrolyte). The most general reaction observed was reduction of the double bond of the pentatomic cycle to afford the corresponding indanes of different configurations. In two cases (1,2-dimethoxyethane, dioxane) this reaction was accompanied by reduction of the C(1 )–H bond of 1 with formation of the corresponding anion and hydrogen. No rearrangement or cleavage of groups was observed. In a protic solvent (pentan-1-ol) the results are consistent with a reduction process occurring via protonation of the radical anion of 1, affording quantitatively the isomeric indanes with a slight preference for those of cis-configuration around the C(2)–C(3) bond. In liquid ammonia, in contrast, the results are indicative of a reduction process occurring via protonation of dianionic species, leading to the almost exclusive formation of indanes of trans-configuration around the C(2)–C(3) bond. The self-protonation reaction, which is the characteristic process of reduction of 1 under electrochemical conditions in the absence of specific proton donors, was found to be followed only in the case of tetrahydrofuran, where a pseudo-unimolecular decay of a paramagnetic species, likely the radical anion of 1, could be followed in an EPR cavity.