A Guide to Tris(4-Substituted)-triphenylmethyl Radicals

Triphenylmethyl (trityl, Ph₃C•) radicals have been considered the prototypical carbon-centered radical since their discovery in 1900. Tris(4-substituted)-trityls [(4-R-Ph)₃C•] have since been used in many ways due to their stability, persistence, and spectroscopic activity. Despite their widespread use, existing synthetic routes toward tris(4-substituted)-trityl radicals are not reproducible and often lead to impure materials. We report here robust syntheses of six electronically varied (4-RPh)₃C•, where R = NMe₂, OCH₃, ^tBu, Ph, Cl, and CF₃. The characterization reported for the radicals and related compounds includes five X-ray crystal structures, electrochemical potentials, and optical spectra. Each radical is best accessed using a stepwise approach from the trityl halide, (RPh)₃CCl or (RPh)₃CBr, by controllably removing the halide with subsequent 1e^- reduction of the trityl cation, (RPh)₃C⁺. These syntheses afford consistently crystalline trityl radicals of high purity for further studies.