Generation of Cyclocarbons with 4n Carbon Atoms (C_(12), C_(16), and C_(20)) by [2 + 2] Cycloreversion of Propellane-Annelated Dehydroannulenes

摘要

Recent studies on the spectroscopic, chemical, and physical behavior of carbon clusters revealed that small clusters of C_(10)-C_(20) possess monocyclic form. All-carbon molecules of this type, called cyclo[n]carbons, are of particular interest because they play an important role in the formation of fullerenes and because for some of them more than one structure has been predicted by calculations. We and Diederich et al. have synthesized a number of precursors of cyclo[n]carbons with well-defined cyclic structures and readily removable groups, which eventually generated cycol-C_(18) (and cycol-C_(24) and cyclo-C_(30) as well) by laser desorption induced [4 + 2] cycloreversion (retro-Diels- Alder reaction), decarbonylation, or [2 + 2] cycloreversion.The advantage of the [4.3.2]propellane-type precursors lies in their thermal stability and facility to undergo cycloreversion on both laser desorption and ultraviolet irradiation. However, these methods have been only applied so far to generate cyclocarbons with 6n carbon atoms (n = 3-5), and no precursors to cyclocarbons of other ring sizes were known despite the importance of these carbon clusters. Moreover, cyclocarbons having 4n carbon atoms should give dianions by two-electron reduction'9' and form transition metal complexes (in the case of cycol-C_(12)), in view of the chemistry of the dehydroannulenes with 4n π-electron systems. In this connection, we report herein the synthesis of hexadehy-dro[12jannulene 1. octadehydro[16]annulene 2, and decadehy-dro[20]annulene 3 with annelated [4.3.2]propellatricne units and their laser desorption induced fragmentation generating cyclo-C_(12), cyclo-C_(16), and cyclo-C_(20), respectively, by extrusion of indan.