Electrolytically Induced Allene-Alkyne Isomerization

Article abstract of DOI:10.1021/jo00283a027

Preparative-scale electrolyses at mercury cathodes in dimethylformamide containing tetra-n-butylammonium perchlorate have revealed that 1-phenyl-1,2-hexadiene exhibits no evidence of isomerization to form 1-phenyl-1-hexyne via an electrolytically induced, base-catalyzed rearrangement; similarly, 1-phenyl-1,2-butadiene exhibits only a slight tendency (1-2percent) to form 1-phenyl-1-butyne.In contrast, voltammetric and electrolytic experiments demonstrate that isomerization of 1-phenyl-1-butyne, 1-phenyl-1-pentyne, and 1-phenyl-1-hexyne to the corresponding allenes occurs muchmore readily.Observations from several investigations show that the extent of allene-to-alkyne rearrangement increases from 1-phenyl-1,2-hexadiene to 1-phenyl-1,2-butadiene to phenylpropadiene, whereas the reverse alkyne-to-allene isomerization decreases in significance from 1-phenyl-1-hexyne to 1-phenyl-1-pentyne to 1-phenyl-1-butyne to 1-phenyl-1-propyne.Steric and electronic factors, which affect protonation of the conjugate bases of the various starting materials at acrbons 1 and 3, can be invoked to explain these trends.