Further investigations into the use of ionic liquids as green solvents in photoinduced electron transfer reactions

Abstract

Two photoinduced electron transfer (PET) reactions were re-investigated in an ionic liquid as a solvent as opposed to the conventional solvent acetonitrile. The first reaction studied was the previously-reported photosensitised electron transfer reaction between 1,4-dicyanobenzene and 2,3-dimethylbut-2-ene m the presence of phenanthrene. Two isomeric substitution products were obtained, 1-( 4-cyanophenyl) 2,3-dimethyl-2-butene and 3-( 4-cyanophenyl)-2,3-dimethyl-1-butene which were identified by NMR. The reaction was carried out on a large scale in the ionic liquid, 1 butyl-3-methylimidazolium tetrafluoroborate, where the products were obtained in a 1:1 ratio, thus showing a change from the 1:1.5 ratio obtained in acetonitrile. These results obtained were interpreted as being due to the effect of the ionic nature of the solvent, its increased basicity over acetonitrile, its polarity and its high viscosity. The ionic solvent used for the large-scale photolysis was recovered and reused successfully for another photosubstitution reaction. However, an accurate determination of differences in yields and rates of reaction, between two simultaneous reactions, carried out in fresh and recovered ionic liquid respectively was not possible. This was due to the unexpected partial degradation of the fresh ionic liquid, resulting in both decreased yields and rates of reaction. Nevertheless, this investigation shows that the more environmentally-friendly ionic liquid solvents may be used for this photoinduced electron transfer reaction on a large-scale, thus avoiding the use of toxic acetonitrile. The second PET reaction investigated was the photocyclodimerisation of 1,1 diphenylethene. Two products were obtained, namely, 1,1,4-triphenyl-1,2,3,4 tetrahydronaphthalene and 1,1,4-triphenyl-1,2-dihydronaphthalene, with the former being the major product when 1,4-dicyanobenzene was used as the photosensitiser and the latter being the major product when 2,6-diphenylpyrylium perchlorate was used as the photosensitiser. The reaction proceeded at a fast rate in acetonitrile, however, it was very slow in ionic liquid, and therefore, this reaction was carried out on a large scale in acetonitrile, whereby 1,1,4-triphenyl-1,2-dihydronaphthalene was isolated and acetonitrile, whereby 1,1,4-triphenyl-1,2-dihydronaphthalene was isolated and characterised spectroscopically. The inversion in product ratio observed was attributed to the participation of the pyrylium system in a redox reaction with one of the key intermediates in the reaction.