Absolute Rate Expressions for Intramolecular Displacement Reactions of Primary Alkyl Radicals at Sulfur

Article abstract of DOI:10.1021/jo00387a028

Arrhenius parameters are presented for the intramolecular displacement reactions of 4-(alkylthio)butyl radicals, where alkyl = benzyl, propyl, and tert-butyl, giving tetrahydrothiophene and the corresponding alkyl radical (R^.).Arrhenius expressions were determined by competition of the rearrangement (displacement) with abstraction of hydrogen from tributylstannane.The Arrhenius parameters for the intermolecular displacement reactions were as follows ., log (A/s^-1), E_a (kcal/mol), at the 95percent confidence interval>: benzyl, 9.92 +/- 0.22, 8.63 +/- 0.40; tert-butyl, 10.77 +/- 0.24, 11.36 +/- 0.40; n-propyl, 10.17 +/- 0.56, 12.15 +/- 1.16.The 4-(benzylthio)butyl and 4-(propylthio)butyl radicals undergo a competing intramolecular 1,6-hydrogen migration to form the corresponding α-(butylthio)benzyl and (butylthio)propyl radicals.The Arrhenius parameters -1), E_a (kcal/mol), 95percent confidence interval>, per hydrogen atom abstracted, for these latter two reactions were as follows: 4-(benzylthio)butyl,9.51 +/- 0.28, 8.87 +/- 0.44; 4-(propylthio)butyl, 9.10 +/- 1.9, 9.98 +/- 3.9.The Arrhenius parameters of the intramolecular displacement reaction are consistent with a product-like rate-determining step, as opposed to the formation of an intermediate sulfuranyl radical followed by nonselective ligands loss.INDO calculations predict the SOMO of the trialkylsulfuranyl radicals to be a ?^* orbital with electron density distributed about sulfur and two pseupoapical carbons.The pseudoapical C-S-C angle defined by this SOMO suggests a predominance of exocyclic (axial-axial) displacement over endocyclic (equatorial-axial) displacement for the intramolecular displacement reactions forming a tetrahydrothiophene ring.MM2 calculations of internal rotational barriers of tert-butyl, hex-5-en-1-yl, and the 4-(alkylthio)butyl radicals and of the exocyclic ligands of models of the displacement transition structures are presented to aid in the interpretation of displacement A factors.