Crystal structure of a cyclotetramer from a strained cyclic allene.
The low-temperature treatment of 1,1-dibromo-1a,9b-cyclopropa[l]phenanthrene (1) with butyllithium and copper(II) chloride in THF affords a dibenzoannellated 1,2,4,6-cycloheptatetraene which undergoes a rare cyclotetramerization. The crystal structure of this "formal" 2 + 2 + 2 + 2 cyclotetramer (2) reveals a central eight-membered ring folded in a zigzag fashion with hydrogen atoms and exocyclic double bonds occupying axial positions. B3LYP/6-31+G** calculations indicate that the strained cyclic allene is significantly distorted and could be formed by ring expansion of a putative cyclopropylidene intermediate.
Department of Chemistry, Colby College, Waterville, Maine 04901, USA.
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