[2+2] cycloreversion of [4.3.2]propella-1,3,11-trienes: An approach to cyclo[n]carbons from propellane-annelated dehydro[n]annulenes

As a method to generate all-carbon molecules having highly reactive polyyne units from stable precursors, the [2 + 2] cycloreversion of [4.3.2]propell a-1,3,11-triene derivatives was developed, To test the efficiency of this m ethod, the reaction was first applied to simple diethynyl- and dibutadiynyl -substituted propellatrienes, which produced upon UV-irradiation linear hex atriyne and decapentayne derivatives, respectively. Next, dehydro[12]-, [16 ]-, [18]-, [20]-, and [24]annulene derivatives annelated by the [4.3.2]prop ellatriene units were prepared as precursors to the corresponding cyclo[n]c arbons, 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 pol yyne intermediates by [2 + 2] cycloreversion which were intercepted by fura n to give the corresponding Diels-Alder adducts. The structures and spectro scopic properties of the dehydroannulenes annelated by the [4.3.2]propellat riene units, the precursors to cyclo[n]carbons, and those annelated by the oxanorbornadiene units, the products of the photolysis in furan, are discus sed.