TEMPS, the Chalcogen Cousin of the Infamous Stable Radical TEMPO, Traps Radicals via Substitution on Its Dimer (TEMPS)2

Abstract

Nitroxides such as 2,2,6,6-tetramethylpiperidin-N-oxyl (TEMPO) are the quintessential organic radicals. They are commonly used to trap alkyl radicals in controlled radical polymerizations, synthetic transformations and mechanistic experiments, and peroxyl radicals when applied as radical-trapping antioxidants (RTAs). Relatively little is known about their sulfur analogs (N-thiyl radicals, R₂NS• and dithiobisamines, R₂NS-SNR₂). We found that dithiobisamines of moderate steric bulk (including (TEMPS)₂, the dimer of the sulfur analog of TEMPO), trapped peroxyl radicals with rate constants up to 1.2 × 10⁶ m⁻¹s⁻¹ (at 100 °C) – similar to TEMPO. Unlike TEMPO, their radical-trapping activity was not catalytic, independent of the rate of radical initiation and still operative in saturated hydrocarbons – implying a distinct mechanism. Additional experimental data and computations point to a homolytic substitution mechanism, analogous to that recently elucidated for tetrasulfides. In the absence of O₂, alkyl radicals and dithiobisamines react to form sulfenamides, which may be exploited for synthetic purposes.