[2 + 2] Cycloreversion of [4.3.2]Propella-1,3,11-trienes:  An Approach to Cyclo[<i>n</i>]carbons from Propellane-Annelated Dehydro[<i>n</i>]annulenes

Abstract

As a method to generate all-carbon molecules having highly reactive polyyne units from stable precursors, the [2 + 2] cycloreversion of [4.3.2]propella-1,3,11-triene derivatives was developed. To test the efficiency of this method, the reaction was first applied to simple diethynyl- and dibutadiynyl-substituted propellatrienes, which produced upon UV-irradiation linear hexatriyne and decapentayne derivatives, respectively. Next, dehydro[12]-, [16]-, [18]-, [20]-, and [24]annulene derivatives annelated by the [4.3.2]propellatriene units were prepared as precursors to the corresponding cyclo[ n ]carbons, a monocyclic form of carbon clusters. Laser-desorption mass spectra of the dehydroannulenes exhibited, in the negative mode, peaks due to the corresponding cyclo[ n ]carbon anions ( n = 12, 16, 18, 20, and 24) formed by successive losses of aromatic indane fragments. Solution photolysis of the dehydro[16]annulene and dehydro[18]annulene derivatives formed reactive polyyne intermediates by [2 + 2] cycloreversion which were intercepted by furan to give the corresponding Diels−Alder adducts. The structures and spectroscopic properties of the dehydroannulenes annelated by the [4.3.2]propellatriene units, the precursors to cyclo[ n ]carbons, and those annelated by the oxanorbornadiene units, the products of the photolysis in furan, are discussed.