John H. MacMillan Alfred Viola.
The Acetylenic-Oxy-Cope Rearrangement of 1,5-Hexadiyne-3-ol and Methyl Substituted Derivatives, Ccl.net ArchiveVapor phase thermolysis of 1,5-hexadiyne-3-ol at 350oC afforded phenol and 4-methylene-2-cyclobuten-1-carboxaldehyde as products. The 4-methyl derivative gave only ortho cresol and 4-ethylidene-2-cyclobutene-1-carboxaldehyde. The 3-methyl and 4,4-dimethyl derivatives yielded no aromatic products. 3-Acetyl-4-methylene cyclobutene was the sole product from thermolysis of the 3-methyl derivative, while 4-isopropylidene-2-cyclobutene-1-carboxaldehyde was the sole product from thermolysis of the 4,4-dimethyl derivative. The phenol and ortho-cresol aromatic products are postulated to result from cyclization of intermediate 1-hydroxy-1,2,4,5-tetraenes (bis allenes), produced by acetylenic oxy-Cope rearrangements. Subsequent supra facial 1,5-hydrogen shifts restore the aromaticity. The 3-methyl and 4,4-dimethyl derivatives cannot aromatize by this pathway. Exclusive formation of ortho cresol eliminates prismane or benzvalene intermediates, as such structures should also produce meta and para cresols. The mechanism of cyclobutene derivative formation for all compounds is proposed to involve cyclization of the identical bis allenes, via diradical pathways. The 3-hydroxy functionality greatly facilitated the rate of the aromatizations, consistent with rate accelerations seen in other oxy-Cope and acetylenic oxy-Cope systems