Action of potassium fluoride on 5-benzensulfonyl-5,10-dihydro-10-trimethylsilylindeno[1,2-b]indole. Attempted synthesis of dibenzo[b,f-1]azapentalene

Abstract

Mild and neutral experimental conditions were employed in order to maximize the chance of success in the isolation of dibenzo[b,f-1]azapentalene (3), a compound expected to exhibit anti-aromatic character, from a precursor. Thus, 5-benzensulfonyl-5,10-dihydro-10-trimethylsilylindeno[1,2-b]indole (7) was allowed to react with potassium fluoride at room temperature. The outcome of the reaction was complex, but the major product was identified as 5,5′,10,10′-tetrahydro-10,10′-biindeno[1,2-b]indolylidene (15). Apparently, the anion8 that was generated by the action of potassium fluoride on7, did not suffer direct elimination of the benzensulfenate anion. Instead, presumably due to the antiaromacitity of3, the benzensulfenate anion was eliminated only after a series of electron transfer reactions, resulting in15. Also, unexpectedly, dibenzo[b,f-1]azapentalene dianion (5) was silylated exclusively at the 10-carbon in preference to silylation at nitrogen. Experimental data indicated that the 10-carbon of5 was silylated directly with chlorotrimethylsilane.