In the previous papers in this series we have demonstrated that dimerization of acyclic allenes exhibits a stereoselectivity which favors the formation of those isomers in which the exocyclic methylene substituents take the inward, more crowded orientation. This stereoselectivity was attributed to favorable steric interactions in the orthogonal 2,2'-bisallyl biradicals leading to these crowded isomers.
Yolecular models indicate that two adamantyl substituents cannot occupy the inward positions of these dimers so that an adamantyl-substituted allene should test to the fullest this inward-orieKting stereoselectivity.
Treatment of 1-adamantyl-Z-propyn-l-01 with thionyl chloride in dibutyl ether afforded a mixture of 1-adamantyl-3-chloroallene and 1-adamantyl-1-chloro-2-propyne, which was separated by column chromatography on silica gel in 38% and 24% yields, respectively. Dimerization of the allene was about 90% complete after twenty hours at 125-130" in solution in an equal weight of hexane; very little higher molecular weight material was present. The dimers were best separated by column chromatography on acidic alumina, and are shown in Figure I.
Structures are based on the n.m.r. spectra of the dimers, given in Table I.