William B. Gara

An ESR study of the reactions of trialkylsiloxyl and trialkylsilyl radicals with dialkyl sulphides

Abstract Trimethylsiloxyl radicals add to dialkyl sulphides to produce sulphuranyl radicals, whilst triethylsilyl radicals bring about homolyric dealkylation of dialkyl sulphides.

Mechanism of cyclization of aryl radicals containing unsaturated ortho-substituents

Aryl radicals generated by interaction of tributylstannane with aryl iodides containing unsaturated ortho-substituents cyclize regiospecifically to afford products containing the newly formed-radical centre exocyclic to the newly-formed ring. The relative rates of cyclization of a number of related radicals have been determined. The results are consistent with the hypothesis that the transition state for homolytic addition to an olefin bond is formed by primary interaction of the semi-occupied orbital with one lobe of the π* orbital.

An electron spin resonance study of the reactions of oxidising radicals with dialkyl sulphides. Radical cations derived from anodic oxidation of R2S, (R2N)2S, and (R2N)2SO

Constant current electrochemical oxidation of R2S, (R2N)2S, and (R2N)2SO gives rise to radical cations which have been detected by e.s.r. spectroscopy. Dialkyl sulphides, except di-t-butyl sulphide which afforded the spectrum of But2S+˙, gave the dimer cations (R2SSR2)+˙. Bis(dialkylamino) sulphides or sulphoxides gave monomer cations in which a large fraction of the spin density is on nitrogen. The g factors of (R2SSR2)+˙ increase with the bulk of R, and the g factors of both (R2SSR2)+˙ and But2S+˙ are dependent on the temperature, the solvent, and the counteranion. The sulphide cation radicals have also been generated by photolysis of solutions containing R2S, ButOOBut, and CF3CO2H, and formation probably involves oxidation of the sulphide by ButȮH. The neutral radical ButO· and Me3SiO· react with dialkyl sulphides by abstraction of hydrogen from an α-C–H group or by competing addition to sulphur to form a sulphuranyl radical. The e.s.r. spectra of the adducts R2ṠOSiMe3 have been detected and these radicals appear to be non-planar at sulphur with the unpaired electron probably confined to an S–O σ* orbital.

Some intramolecular reactions of ortho-substituted aryl radicals

ortho-Substituted aryl radicals have been generated by reduction of arenediazonium salts in aqueous solution by use of a flow cell. The e.s.r. spectra indicate that radicals containing olefinic bonds in the 5,6- or 6,7-positions with respect to the radical centre undergo rapid cyclization by addition to the 5- and 6-positions respectively to give the thermodynamically less stable products. No evidence was obtained for addition to the remote termini of the double bonds. The direction of cyclization is not determined primarily by the case of approach of the radical centre to the olefinic carbon atoms. Aryl radicals containing saturated ortho-substituents undergo rearrangement by 1,5-intramolecular hydrogen atom transfer. The conclusions based on e.s.r. spectroscopic studies have been supported by the isolation of rearranged products from the reduction of suitable arenediazonium salts.

An electron spin resonance study of the reactions of alkoxyl and trimethylsiloxyl radicals with dialkyl sulphoxides

Qualitative and quantitative studies have been made of the dealkylation of dialkyl sulphoxides by t-butoxyl and trimethylsiloxyl radicals. Dimethyl sulphoxide undergoes SH2 reaction at sulphur much less readily with ButO˙ than with HO· or Me3SiO·. The rate of t-butoxydealkylation of symmetrical and of mixed dialkyl sulphoxides increases with the stability of the displaced alkyl radical, providing steric effects are not dominant. Siloxydealkylation of sulphoxides appears to be more rapid and less selective than t-butoxydealkylation. and it is suggested that the electrophilicity of the attacking oxyl radical is an important factor in determining rate and selectivity. The cyclic sulphoxides [graphic omitted]O(n= 2–4) undergo increasingly rapid ring opening as n decreases, a trend attributed to relief of angle strain.No e.s.r. spectra of suiphuranyloxyl radicals. R2S(O)OX or R(RO)S(O)OX, were detected during the reactions of XO· with sulphoxides or with alkyl alkanesulphinates, respectively.

An electron spin resonance study of the electrochemical oxidation of phosphorus(III) compounds

Radical cations generated during electrochemical oxidation of a series of trivalent phosphorus compounds, alone or in the presence of 3,3-dimethyl-2-t-butylbut-1-ene, have been studied using e.s.r. spectroscopy. The oxidations were carried out in nitrile or dichloromethane solvents and the cell containing the working electrode was situated in the cavity of the spectrometer. Oxidation of X3P alone gives rise to the spectrum of the phosphine dimer cation radical, (X3PPX3)+, produced by rapid reaction of the initially formed phosphinium radical cation, X3P+, with a further molecule of phosphine. Trialkyl phosphites did not give detectable concentrations of dimer cation radicals in nitrile solvents, probably because of preferential reaction of (RO)3P+ with the solvent. In the presence of 3,3-dimethyl-2-t-butylbut-1-ene the spectra of the dimer cation radicals were completely or partially quenched and replaced by those of the adducts But2ĊCH2PX3. The phosphorus hyperfine splittings of both (X3PPX3)+ and But2ĊCH2PX3 increased with the electronegativity of the ligands X, and these trends are interpreted in terms of changes in hybridisation at phosphorus.

Electrochemical generation of the radical cations [X3PPX3]+

The radical cations [X3PPX3]+ have been generated by electrochemical oxidation of phosphines in solution at low temperatures and their e.s.r. spectra have been studied.