The synthesis and ESR analysis of pentaarylethyl free radicals

Abstract

A practically unstudied class of stable free radicals was investigated. Seven pentaarylethyl free radicals were synthesized and characterized by their ESR spectra. These radicals included: 9-triphenylmethylfluorenyl, 9-tris(pentadeuteriophenyl)-methylfluorenyl, 9-tris-(4-tert-butylphenyl)methylfluorenyl, 9-(9'-phenylfluorenyl)-fluorenyl, 1,1-bis-(4-tert-butylphenyl)-2,2,2-triphenylethyl, penta-(4-tert-butylphenyl)ethyl and pentaphenylethyl. The work of W. Schlenk and H. Mark on this subject was repeated, but the authors' conclusions could not be substantiated. The radicals were in most cases prepared by the reaction of the corresponding anion with 2,3-dibromo-2,3-dimethylbutane. This procedure had been described by K. Ziegler and was modified for our purposes. Also, triphenylmethyl radical and the carbene fluorenylidene were coupled to yield 9-triphenylmethylfluorenyl radical. The ESR spectra generated by the four "fluorenyl-like" pentaarylethyl radicals were identical to one another. These spectra contained unresolved hyperfine interactions which led to line broadening. These hyperfine interactions were resolved in one experiment. The ESR spectra generated by the two tert-butyl substituted pentaphenylethyl radicals were identical to each other, and the ESR data suggested that the aryl groups on the radical centers were greatly twisted out of planarity. The ESR data obtained from the pentaphenylethyl radical indicated that a rapid, reversible 1,2-phenyl shift occurs. Computer simulations were performed to substantiate the reported ESR values.