Sterically hindered free radicals. 18. Stabilization of free radicals by substituents as studied by using triphenylmethyls

Abstract

The relative stabilization of 10 4-monoand 24 4,4'-disubstituted triphenylmethyl radicals 1 has been measured by recording the degree of dissociation of the corresponding quinonoid dimers 2 by means of ESR. The following substituents or combinations of two of them have been used: H, CF3, r-Bu, OMe, OPh, CN, COPh, COMe, Ph, SMe, and N02. Both donors and acceptors enhance the stability in the ground state of the radicals, which is evaluated in terms of ' values and a Hammett-like equation. Two donors act additively, as do two acceptors. No specific synergism of a donor with an acceptor (capto-dative stabilization) has been found. Most efficient for the relative stabilization are the electroneutral substituents Ph and SMe. Most of the ESR spectra of these trityls are new. An exact assignment became possible via the corresponding ENDOR spectra, which are listed in detail. Many of the substituted trityls including dimers and precursors have been prepared

Synthesis and Properties of a Highly Congested Tri(9‐anthryl)methyl Radical

“…The C−C σ‐bond was formed at the 10‐position of the anthracene unit, in contrast to the head‐to‐tail trityl dimer (Figure a). The trityl radical and its dimer are in equilibrium in solution; the equilibrium constant for dimerization, log K D , is −3.48, indicating that the monomeric radical form is significantly preferred under dilute conditions. In contrast, 1 ′ mostly exists in the dimeric form, that is, 1 ′ 2 , in dilute solution (10 −5 –10 −4 m ).…”

Chemical Bond-Making, Bond-Breaking, and Electron Transfer in Solution

“…Despite the fundamental importance of homolysis and heterolysis for the quantitative treatment of organic reactivity, few data are available for the wide variety of bonding situations encountered in the broad range of organic compounds (6,7). An inherent problem for the quantitative study of both homolysis and heterolysis is the high reactivity of the free radicals and ions that are produced by the two processes, so that it is difficult to separate the energetics of the initial bond cleavage from secondary processes.…”

Synthesis and Properties of a Highly Congested Tri(9‐anthryl)methyl Radical

“…The C À C s-bond was formed at the 10-position of the anthracene unit, in contrast to the head-to-tail trityl dimer (Figure 1a). Thetrityl radical and its dimer are in equilibrium in solution; the equilibrium constant for dimerization, log K D ,isÀ3.48, [18] indicating that the monomeric radical form is significantly preferred under dilute conditions.Incontrast, 1' mostly exists in the dimeric form, that is, 1' 2 ,indilute solution (10 À5 -10 À4 m). Thesolution showed avery weak absorption band at 600 nm, which became weaker upon increasing the concentration from 10 À5 m to 10 À4 m.Inaddition, adilute toluene solution of 1' 2 showed avery weak ESR signal at room temperature.Thus av ariable-temperature ESR measurements were conducted to determine the thermodynamic parameters of the dimerization of 1' to 1' 2 ,and the DH and DS values were estimated by using the vantH off equation to be À19.7 kcal mol À1 and À35.4 cal mol À1 K À1 ,respectively [19] (Supporting Information).…”

“…However, 1' could not be isolated;instead, its tail-to-tail dimer, 1' 2 ,was obtained in good yield (65 %). Thetrityl radical and its dimer are in equilibrium in solution; the equilibrium constant for dimerization, log K D ,isÀ3.48, [18] indicating that the monomeric radical form is significantly preferred under dilute conditions.Incontrast, 1' mostly exists in the dimeric form, that is, 1' 2 ,indilute solution (10 À5 -10 À4 m). The C À C s-bond was formed at the 10-position of the anthracene unit, in contrast to the head-to-tail trityl dimer (Figure 1a).…”