As far as I know, negative hyperconjugation means, that we have electrons that are transferred from a p- or $\pi$- to a $\sigma^*$-orbital. In the lecture, we analyzed a TMS-substituted tricyclopentane derivate (tricyclo[$^{2,5}$]-pentane-3-one). Apparently, the first one on the left can't be deprotected even by TBAF or boiling MeLi, but the second one easily. If I could deprotect the first one, we'd obtain a 3° carbanion that would do negative hyperconjugation with the TMS group. Would't this be favourable and should be readily feasible?

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  • $\begingroup$ The sigma bonding C-Si electrons would feed up the p orbital in carbon, but as it is occupied, a net destabilization will result, as the antibonding orbital produced by the hyperconjugation would be more destabilized than the bonding orbital would be stabilized. There is big energy difference between the p orbital on carbon and the sigma antibonding orbital C-Si for hyperconjugation to take place to a significant extent, $\endgroup$ – Marko Apr 10 '16 at 15:54

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