o-Quinonoid compounds. Part 18. Stabilised 2,3-naphthoquinodimethanes via transient 1,3-diphenylbenz[f]inden-2-one

1,3-Diphenylbenz[f]inden-2-one (8) can be reversibly generated by thermolysis of its formal (π4s+π4s)-dimer (11) which is itself available in 5 steps from benz[f]indan-1,3-dione; (8) adds stereospecifically to cis- and trans-but-2-ene. Photodecarbonylation of the adducts (23), (21), and (22) derived by trapping (8) with norbornadiene, trimethylmaleimide, and the dienophile (24) gives the sterically stabilised 2,3-naphthoquinodimethanes (29), (25), and (26) respectively. These are long-lived in fluid solution at 20 °C and were characterised by u.v.–visible spectroscopy and as their adducts with 4-phenyltriazoline-3,5-dione (PTD). 1,3,4,9-Tetraphenylbenz[f]inden-2-one (37) generated by reaction of its dibromide (10; X = O, Y = Br) with sodium iodide forms the formal (π6s)+π6s)-dimer (35). Dissociation of (35) in the presence of N-phenylmaleimide gives the adduct (15).