Neutral Radicals Containing a Thiophene Ring

The simple 2- and 3-thienylmethyl radicals 98 and 99 were observed by ESR spectroscopy by Hudson and co-workers who generated them by steady-state photolysis of dibenzoyl peroxide in the appropriate methyl-thiophene. Their hyperfine splittings are indicated in gauss in the formulas  

These same radicals have been observed following y-radiolysis of methyl-thiophenes in an adamantane matrix at room temperature. In solid solution the differences in the methylene splittings were not resolved, but otherwise hyperfine sphttings essentially similar to those found for fluid solution were observed. 

The 2-hydroxy-5-(2-thienyl)-2-liydrotliienyl radical (100) has been observed during radiolysis of thiophene 100 (which may be regarded formally to be a derivative of 98) arises by addition of hydroxyl radical to 2,2 -bithienyl, a product of the radiolysis. 

The isotropic ESR spectra of 101 at temperatures accessible to the radical show signs neither of the existence of separate rotamers nor of line-broadening effects suggestive of their rapid interconversion. An INDO MO study has shown that the radical probably exists in two preferred propeller-like conformations (that of C3 symmetry is shown in 101 that of Cj symmetry is obtained by a rotation of any single thiophene ring in 101 through 180°). The calculations indicate the C3 conformer to be more stable by 0.75 kcal mol and the barrier to interconversion of the two forms, by a preferred two-ring-flip mechanism, to be about 6 kcal mol . This low barrier to interconversion is consistent with the failure to observe effects of the isomerism in the ESR spectra at temperatures above those at which the radical dimerizes. 

A number of thienyl nitroxyl radicals have been reported. Sleight and Sutcliffe irradiated 2-nitrothiophene in THE and obtained a radical 102 

The relative homolytic reactivities of 2- and 3-methylthiophenes toward hydrogen abstraction from the side chain by tert-butoxyl radicals to form 98 and 99 have been investigated by the spin-trapping method. The 

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