Free‐radical reactions of allene the interaction of allene and thiyl radicals

Abstract

The free-radical addition of 1-propanethiol, benzenethiol (thiophenol) and a-toluenethiol (benzyl mercaptan) to allene in benzene solution has been investigated.At high allene/thiol ratios the two sulfides resulting from the addition of one mole of RSH to allene (isopropenyl-SR and allyl-SR) are formed in yields of the same order of magnitude.At low allene/thiol ratios the more reactive isopropenyl sulfide is not present among the products, since it is quantitatively converted into 1 ,2-di-SR-propane. Thiol addition to the ally1 sulfide to yield 1,3-di-SR-propane is a much slower process.

Thiyl Radicals in Organic Synthesis

“…Given that radical addition to olefin usually affords the most stable radical, it might be expected that attack at the central sp -carbon atom of allenes would be favored as this would form a resonance-stabilized allylic radical, whereas attack at the terminal carbon would furnish a reactive vinylic radical. However, radical 155 resulting from attack of the central carbon of allene 151 resembles a primary radical in the transition state rather than an allylic one as the unpaired electron does not overlap with the π-orbital of the remaining double bond (Scheme ). , As a result, the free-radical addition of thiols onto the simple allene 151 generally leads to a mixture of allyl sulfide 152 and isopropenyl sulfide 154 . A 90° rotation allows nonallylic radical 155 to interconvert rapidly into the more stable allylic radical 156 , which cannot undergo β-fragmentation, thus rendering the addition of thiyl radical at the central carbon atom practically nonreversible, , although this aspect has been debated.…”

“…Addition of thiyl radicals onto allenes was studied by a number of research groups in the early 1960s. ,− Reaction of ethanethiol with allene led preferentialy to allyl sulfide 158 arising through attack at the terminal carbon atoms, while vinyl sulfide 157 resulting from the addition at the internal position was rapidly converted into 1,3-diethylthiopropane 159 upon addition of a second molecule of ethanethiol (Scheme , eq a). , The regioselectivity of the initial attack proved to depend upon both the nature of the thiol and the temperature . Upon irradiation with UV-light, 1,3-diaddition products were obtained selectively from allene with various thiols, including aliphatic thiols, dialkyldithiophosphoric acids, thiolacetic acid, and mercaptoacetic acid .…”