Reactions of 4-fluoren-9-ylidene-2-methylpent-2-ene and 2-fluoren-9-yl-2- methylpentan-4-one

Abstract

The photochromic title hydrocarbon undergoes a photochemical 1,5-H shift to 4-fluoren-9-yl-2-methylpenta-1,3-diene and photocyclisation to red 3,3a-dihydro-1,3,3-trimethylfluoranthene; the latter undergoes a thermal 1,9-H shift in toluene and a photochemical 1,7-H shift in ethanol. The title ketone (17) cyclises in the presence of hydrogen bromide in acetic acid to yield 1,10b-dihydro-1,1,3-trimethylfluoranthene which, on catalytic hydrogenation, yields the same tetrahydrofluoranthene derivative as is obtained when 2-fluoren-9-yl-2-methylpentan-4-ol (20) is cyclised with sulphuric acid. On treatment with PCl₅, the ketone (17) yields 4-chloro-2-fluoren-9-yl-2-methylpent-4-ene, which reacts with sodium in xylene to give trans-2-fluoren-9-yl-2-methylpent-3-ene, obtained also when the alcohol (20) is heated at 180 °C with KHSO₄.