FREE-RADICAL ADDITION OF HETEROARENETHIOLS AND HETEROARYLMETHANETHIOLS TO HEXYNE AND PHENYLACETYLENE - CHEMICAL BEHAVIOR OF THE TRANSIENT BETA-B-SULFANYLVINYL RADICALS

The free-radical reaction of a number of heteroarenethiols (including 2-thiophene-, 2-benzo[b]furan-and 2-benzo[b]thiophenethiol) 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 degrees C in the presence of AIBN and at room temperature in the presence of BEt(3)/O-2. 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-benzo[b]furanthiol failed to react with either alkyne in the presence of BEt(3)/O-2 and unexpectedely gave 2-(ethylsulfanyl)benzo[b]furan to a significant extent. The produced 2-(2-heteroarylsulfanyl) vinyl radicals largely preferred to undergo intermolecular 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 B-exo cyclization to give spiro radicals that interestingly underwent beta-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 B-exo cyclization, but did undergo a 6-endo cyclization to a very slight extent.