Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: loves2Spl00ge on October 10, 2016, 10:49:37 PM
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Do (Z) trimethylsilane substituent alkenes undergo faster reaction rates (reaction with sp2 hybridized carbons) than (E) trimethylsilane alkenes? I've always wondered if it was due to the positioning of the SiMe3 group relative to neighboring atoms. (A(1,3) strains and A(1,2) strains) and where the carbocation intermediate ends up.
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Looks like this has something to do with beta silicon effect (how the C-Si bond is very electron donating and statbilizes carbocation intermediates. I would guess it's more specific to certain ring conformations and not general to E or Z.
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I've run PM3 on E and Z Me3Si-propenyl, and the Z doesn't look very strained. Its computed heat of formation is also only 1.6kJ higher.
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I'm going to make a guess even though I really don't know (as if that's different?). I think this is a problem aided by models (which I'm too lazy to dig out). I don't know if the ratio is a difference in cyclization or selectivity to the cis-fused ring. I suspect it may indicate a greater preference for the latter. If simply drawn in a plane, the Z-isomer places the TMS group over the phenyl group. This doesn't happen with the E-isomer.
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A model did make it 10x easier. The sigma C-Si orbital from the intermediate could easily reach the empty p orbitals of the carbocation forming that pi bond. thanks !