Allenylidenes (2) are linear carbenes in which the divalent carbon occupies the terminus of an allenic linkage. The parent member of this series, propadienylidene (2a; R=H), is a C3H2 species of much relevance to interstellar and combustion chemistry. Although a few allenylidenes have been isolated and characterized in cryogenic gas matrices, their solution chemistry remains unknown. In continuation of our ongoing investigation into phenanthrene-based methylenecyclopropanes as photochemical precursors to unsaturated carbenes, we synthesized and characterized 1a (R = H). Photolysis of 1a in solution results in the liberation of 2a, which may be intercepted by an alkene to form adduct 3a. A similar approach was also taken to prepare 1b (R = Ph) to generate the phenyl-substituted allenylidene 2b, and trap it as the adduct 3b. Our experimental results, as well computational studies using density functional theory and coupled-cluster methods, are presented.

Cosmic carbenes: Approaches to allenylidenes

ACS Spring 2022

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