Multidimensional NMR Study of Tris(indenyl)methylsilane: Molecular Dynamics Mapped onto a Hypercube

(Trimethylsilyl)indene exists as a pair of enantiomers which interconvert by means of Me3Si migrations. The mechanism involves successive [1,5]-suprafacial sigmatropic shifts via the corresponding isoindene, which can be trapped with tetracyanoethylene as the Diels−Alder adduct, 7. The X-ray crystal structure of 7 reveals that the TCNE approaches from the opposite face to the Me3Si substituent. Tris(indenyl)methylsilane, 5, also undergoes a series of [1,5]-silatropic shifts which interconvert the RRR, RRS, RSS, and SSS isomers, where the R and S labels refer to the absolute configuration of C(1) in each indenyl ring. By use of 1H−1H COSY, 1H−13C and 1H−29Si shift-correlated NMR spectra, the proton and carbon-13 nuclei in all four different indenyl ring environments can be unequivocally assigned. The molecular dynamics of 5 have been elucidated by means of 1D-selective inversion experiments together with 2D-EXSY data, and the exchange pathways between indenyl sites can be conveniently mapped onto a hypercube. Moreover, 5 yields a triple Diels−Alder adduct, 12, with TCNE; the Si-CH3 1H NMR resonance in 12 is found at −1.8 ppm suggesting that this methyl lies inside a cavity formed by the three phenyls.