Sterically hindered free radicals. Part 22. Dimerization and EPR spectroscopy of indanedionyl and 9-acylfluorenyl radicals

Abstract

2-Phenylindanedionyl radicals 5a, their p- and o-monosubstituted derivatives and related compounds 5b–h recombine by C–C bond formation giving 6a–h. In contrast o-substituted derivatives 5i–k and the 2-tert-butylindanedionyl radicals 5l react to give the enol ethers 10a–d by C–O coupling. The EPR data indicate that the substituents are more bulky in 5i–l than in 5a–h. From the proton splitting constants it is concluded that 5a is planar and 5h nearly planar (twist angle of the phenyl group ca. 30°). The phenyl group in 5i is twisted considerably with an angle of about 50°. The g values of 5i–k are larger than those of 5a–h indicating a higher spin density in the carbonyl groups.

9-Acylfluorenyl radicals 12b–c, representing the planar conformations of diarylmethyl radicals 1c, d, give no enol ethers like 4, but recombine by C–C bond formation yielding 13b–c. Their g values indicate that the acyl groups are twisted out of the radical plane.