Methyleneketenes and Methylenecarbenes. XIV. Evidence for the Rearrangement of Cyclopent-2 enylidenecarbene to Benzene and of Cyclopentadienylidenecarbene to Benzyne

Abstract

Flash vacuum pyrolysis of 2,2-dimethyl-5-(endo-tricyclo[5,2,1,0^2,6]dec-8'-ylidene)-1,3-dioxan-4,6-dione (2) at 550-800º yielded cyclopentadiene, acetone and benzene. The production of benzeneis rationalized in terms of the formation and rearrangement of cyclopent-2-enylidenecarbene (5). 2,2-Dimethyl-5-(octahydro-1',2',4'-metheno-1'H-cyclobuta[cd]pentalen-3'-ylidene)-1,3-dioxan-4,6-dione(7) yielded cyclopentadiene, acetone, biphenylene (27 %), triphenylene (12 %) and fluorenone (14%) as major products on flash vacuum pyrolysis at 550-700º. The last three compounds are considered to arise from o-benzyne, formed by rearrangement of the intermediate cyclopentadienylidenecarbene (9).