LIMITING THE RELATIVE ORIENTATION OF BRIDGED CYCLOPENTADIENYL ANIONS - MONONIONS AND DIANIONS DERIVED FROM 4,4,8,8-TETRAMETHYLTETRAHYDRO-4,8-DISILA-S-INDACENES

Two isomers of 4,4,8,8-tetramethyltetrahydro-4,8-disila-s-indacene have been deprotonated under various conditions to give novel anions in which a cyclopentadienyl (Cp) and a cyclopentadiene or two Cp's are doubly bridged by Me2Si fragments to give the monoanion 3 or the dianion 4. Use of NaH or 1 equiv of n-BuLi selectively yielded 3 whereas KH or 2 equiv of n-BuLi gave 4. Temperature-dependent NMR spectroscopy showed the two enantiomers 3a/b of the monoanion to undergo both a degenerate and a nondegenerate silatropic rearrangement with a barrier of E(a) = 48.5 kJ mol-1. The intermediate in this process contains a 1,3-disilacyclopent-4-ene fragment which is energetically unfavorable so that its abundance is only 2.4% at 29-degrees-C. The dilithium derivative 4Li2 of the dianion forms predominantly solvent-separated or contact ion pairs in solution depending on the solvent. The solid-state structure of 4Li2(TMEDA)2 was determined by X-ray structure analysis: triclinic, P1BAR (No. 2), a = 8.216 (1) angstrom, b = 9.014 (1) angstrom, c = 11.527 (1) angstrom, alpha = 111.10 (1)degrees, beta = 100.66 (1)degrees, gamma = 88.85 (1)degrees, R(w) = 0.075 for 154 refined parameters and 2193 observables. Anti orientation was found for the Li(TMEDA) moieties, and a slight double folding, for the dianionic skeleton.