Free-Radical Addition of Heteroarenethiols and Heteroarylmethanethiols to Hexyne and Phenylacetylene. Chemical Behavior of the Transient β-Sulfanylvinyl Radicals

Article abstract of DOI:10.1021/jo00129a039

The free-radical reaction of a number of heteroarenethiols (including 2-thiophene-, 2-benzofuran-, and 2-benzothiophenethiol) and heteroarylmethanethiols (including 2-furyl-, 2-thienyl-, and 3-thienylmethanethiol) with hex-1-yne and phenylacetylene has been investigated in benzene solution both at 100 deg C in the presence of AIBN and at room temperature in the presence of BEt3/O2.Under both of these conditions the above thiols generally furnished transient 2-sulfanylvinyl radicals through regioselective addition of corresponding sulfanyl radicals to the terminal alkyne carbon, but 2-benzofuranthiol failed to react with either alkyne in the presence of BEt3/O2 and unexpectedely gave 2-(ethylsulfanyl)benzofuran to a significant extent.The produced 2-(2-heteroarylsulfanyl)vinyl radicals largely preffered to undergo intramolecular H-abstraction reaction rather than intramolecular 5-endo cyclization onto the heteroaryl moiety of the 2-sulfanyl substituent.The 2-<(2-thienylmethyl)sulfanyl>- and, especially, 2-<(2-furylmethyl)sulfanyl>vinyl radicals, besides H-abstraction, promptly underwent intramolecular 5-exo cyclization to give spiro radicals that interestingly underwent β-scission of their respective C-S and C-O bond resulting in ring cleavage of the original heteroaryl group.The 2-<(3-thienylmethyl)sulfanyl>vinyl radicals did not exhibit any similar 5-exo cyclization, but did undergo a 6-endo cyclization to a very slight extent.