Organometallic Chemistry of Propargylallenes: Syntheses, Reactivity, Molecular Rearrangements and Future Prospects

Alkynylallenes offer the varied reactivity patterns of two different multiple bond linkages either separately or in concert. Initially, a short overview of their syntheses, structures, rearrangement mechanisms and synthetic utility, especially when treated with transition metal reagents such as gold(I), silver(I), platinum metals or metal carbonyls, is presented. Subsequently, we focus on the particular case of 1,2-dien-5-ynes (propargylallenes), whereby the shortness of the single atom bridge, and the consequent proximity of the allenyl and alkynyl moieties, facilitates metal-mediated interactions between them. It is shown how these metals can coordinate to either the alkyne or the allene fragment, thus leading to different cyclisation or rearrangement products, dependent also on whether it is the proximal or the distal double bond of the allene that participates in the reaction. Dimerisation of bromo-substituted fluorenylideneallenes bearing silyl or ferrocenyl substituents can occur in either head-to-head or head-to-tail fashion, thereby yielding propargylallene derivatives that undergo unexpected and novel rearrangements, including the formation of molecules possessing unusually long carbon-carbon single bonds. Fluorenyl-bearing propargylallenes react with silver nitrate or iron carbonyl to form novel organic polycyclic systems. Finally, suggestions are offered for future advances in the area.