An Effective and Selective Route to 1,5-Dihydropolyalkylated <i>s</i>-Indacenes: Characterization of Their Mono- and Dianions by Silylation. Structure of <i>trans</i>-1,5-Bis(trimethylsilyl)-2,6-diethyl-4,8-dimethyl-<i>s</i>-indacene

2,5-Dimethyl-1,4-bis(carboxyalkyl)benzenes were prepared in almost quantitative yield by alkylation of malonic esters from 2,5-dimethyl-1,4-dibromomethylbenzene obtained by bromomethylation of 1,4-dimethylbenzene. The methyl groups on the central benzene ring induce intramolecular, regiospecific cycloeliminations leading selectively to the diketones, the precursors of the corresponding 1,5-dihydro-s-indacenes, which also were obtained in very high yield. These new ligand precursors were easily converted to the mono- or dianions, which were characterized by means of the mono- or disilylated compounds. Disilylation occurs in cis and trans position. The structure of the trans-1,5-bis(trimethylsilyl)-2,6-diethyl-4,8-dimethyl-s-indacene was obtained by X-ray diffraction. Hexaalkyl-1,5-dihydro-s-indacenes underwent regioselective monosilylation and regiospecific and stereoselective disilylation. Disilylation of the less hindered tetraalkyl-1,5-dihydro-s-indacenes was regioselective and stereoselective.