ortho Diquaternary Aromatic Compounds. IV. Acid-catalyzed Rearrangements of Polyalkyltetralones and Related Ketones

The ketones 1,1,4,4-tetramethyl-2-tetralone (1a), 1,1,4,4,7-pentamethyl-2-tetralone (1b), and 1,1,4,4,6-pentamethyl-2-tetralone (1c) undergo rearrangement on heating with aluminum chloride or ferric chloride in tetrachloroethane or nitromethane into mixtures of their respective isomeric ketones, 1-acetyl-1,3,3-trimethylindane (2a) and 2,2,4,4-tetramethyl-1-tetralone (3a) from 1a, 1-acetyl-1,3,3,6-tetramethylindane (2b) and 2,2,4,4,7-pentamethyl-1-tetralone (3b) from 1b, and 1-acetyl-1,3,3,5-tetramethylindane (2c) and 2,2,4,4,6-pentamethyl-1-tetralone (3c) from 1c. A quantitative study using g.l.c. of the distribution of the ketones with time shows that the sequence of the rearrangement is: [Formula: see text]. A methyl group homologously para to the carbonyl (1c) accelerates the rearrangement. The same kind of rearrangement takes place during Friedel–Crafts cyclialkylation with 2,2,5,5-tetramethyltetrahydrofuranone. A unifying reaction mechanism is postulated to account for the rearrangements. This mechanism also accounts for the rearrangement and fragmentation products from treatment of 1,1,4,4-tetramethyl-2,3-dioxotetralin (9) with aluminum chloride or sulphuric–acetic acids. The products from 9 include 1-acetyl-1,3,3-trimethyl-2-indanone, 1,1,3-trimethyl-2-indanone, and, from the aluminum chloride – catalyzed rearrangement, 5(or 6)-acetyl-1,1,3-trimethyl-2-indanone.