Probing the Effect of Organic and Organometallic Functionalization on [1,5]-Silicon Shifts in Indenylsilanes

Indenylsilanes bearing organic and organometallic substituents have been prepared in order to probe the effect of substitution on the rate of [1,5]-silicon shifts in this class of compounds. In an attempt to prepare 1,1,3-tris(trimethylsilyl)indene (7), the hitherto unknown silicon-functionalized bis(trimethylsilyl)dibenzo[a,d]fulvalene (9) was unexpectedly generated; this species was characterized by use of both NMR spectroscopy and X-ray crystallography and was rationally prepared in 68% yield from 3,3‘-bis(1-(trimethylsilyl))indene (16). The molecular dynamics of 1,3-dimethyl-1-(trimethylsilyl)indene (18) and the crystallographically characterized chromium complex (η6-1,3-dimethyl-1-exo-(trimethylsilyl)indene)tricarbonylchromium (22) were examined by use of 1D-selective inversion and 2D-EXSY NMR techniques; surprisingly, the presence of chromium and methyl substituents has a negligible effect on the rate of [1,5]-silicon shifts (Δ G ⧧ = 23−24 kcal mol-1) versus the parent compound 1-(trimethylsilyl)indene (3) (Δ G ⧧ ≈ 24 kcal mol-1). In the case of 18, the intermediate isoindene 18- iso was intercepted with tetracyanoethylene as the crystallographically characterized [4 + 2] cycloadduct 5,6-benzo-2,2,3,3-tetracyano-1,4-dimethyl-7-(trimethylsilyl)bicyclo(2.2.1)hept-5-ene (19).