[Heory]

 

[Heory]

And I don't understand the " for E/syn case". Should the C-Si σ bond overlap with the C-C π* orbital when the the 6-memebered cycle is being formed? And is the boat-like TS necessary? The cycle can form through chair-like TS and a carbon cation is generated on the alpha-position of TMPS group, then TMPS group is eliminated. (I means that the cycle formation and the elimination of the TMPS group doesn't happen simutaneously)

[azmanam]

I disagree with your statement that an aldehyde oxygen atom is a better nucleophile than ROTMS.  Redraw with H+ instead of TMS+.  In that scenario, oxocarbenium formation might be easier to see. 

And yes, the orbitals need to overlap in the transition state leading to the tetrahydropyran ring.  Any time bonds break, it is because electrons have been pumped into a corresponding antibonding orbtial.  For the reaction we described, the orbital alignment requirement is met in the chair-like transition state.  In the E/syn case, the chair-like transition state has the C-Si σ bond perpendicular to the C-C π* orbital.  As a result, there is no favorable overlap, and the THP forming step does not occur.  Only through the boat-like transition state is the orbital overlap requirement met and the reaction can occur.

[Heory]