[Kaladiscope]

I am looking for the mechanism of the dealkylation of esters using TMSBr.

Anyone has any idea where can I find a good source?

[Dan]

Well, TMSBr is a strong Lewis acid. What do you think will happen first in the mechanism?

[nox]

Hint: Si is strongly oxophilic

[Babcock_Hall]

Hint:  The dealkylation is really a transesterification.

[Kaladiscope]

I guess that Si-Me3 will bind to one of the lone pairs of the oxygen in the carbonyl group. Afterwards the Br- will attack the methyl group (nucleophilic attack) generating then Me-Br. Addition of MeOH or water will just react with SiOMe3 generating HOSiMe3 and dealkylating the corresponding ester.

Is that right?

[camptzak]

Heres my attempt

The carbonyl oxygen attacks the TMSBr by Sn₂

The pi bond of the carbonyl comes up to become a lone pair on the oxygen

The oxygen attached to the carbon chain forms a pi bond as the other pi bond leaves (octet resonance)

The bromine attacks the backside of the carbon chain by Sn₂ and the transesterification is complete with trimethyl silicon in place of the carbon chain. and the carbon chain has been brominated.


close? right? completley wrong?

why is do silicon and oxygen form such a stable bond? I dont understand that. is it because it is in the 3rd energy level with lots of electron density to donate to the electronegative oxygen?

[discodermolide]

Heres my attempt

The carbonyl oxygen attacks the TMSBr by Sn₂

The pi bond of the carbonyl comes up to become a lone pair on the oxygen

The oxygen attached to the carbon chain forms a pi bond as the other pi bond leaves (octet resonance)

The bromine attacks the backside of the carbon chain by Sn₂ and the transesterification is complete with trimethyl silicon in place of the carbon chain. and the carbon chain has been brominated.


close? right? completley wrong?

why is do silicon and oxygen form such a stable bond? I dont understand that. is it because it is in the 3rd energy level with lots of electron density to donate to the electronegative oxygen?

In the deprotection of an ester with TMSBr, the oxygen of the ester group (COOR) is attacked by the Silicon to give a COOSiR+ species. Then the bromine, Br- attacks the CH group in the ester to give the alkyl bromide and the silylated ester.

[camptzak]

Heres my attempt

The carbonyl oxygen attacks the TMSBr by Sn₂

The pi bond of the carbonyl comes up to become a lone pair on the oxygen

The oxygen attached to the carbon chain forms a pi bond as the other pi bond leaves (octet resonance)

Thanks!

The bromine attacks the backside of the carbon chain by Sn₂ and the transesterification is complete with trimethyl silicon in place of the carbon chain. and the carbon chain has been brominated.


close? right? completley wrong?

why is do silicon and oxygen form such a stable bond? I dont understand that. is it because it is in the 3rd energy level with lots of electron density to donate to the electronegative oxygen?

In the deprotection of an ester with TMSBr, the oxygen of the ester group (COOR) is attacked by the Silicon to give a COOSiR+ species. Then the bromine, Br- attacks the CH group in the ester to give the alkyl bromide and the silylated ester.

[discodermolide]

The Si-O bond is very strong, so there is your thermodynamic driving force.
have a look at this link.
http://www.minsocam.org/ammin/am65/am65_321.pdf

[orgopete]

The Si-O bond is very strong, so there is your thermodynamic driving force.
have a look at this link.
http://www.minsocam.org/ammin/am65/am65_321.pdf

This is a useful paper to read and at least in part to understand why Si-O bonds should be strong. I say in part, because I do not agree with Pauling on some of his explanation. None the less, there is data which shows the Si-O bonds are 20 pm shorter and as calculated by Pauling, contain about 50% double bond character.

For me, this is an interesting topic and one needing explanation. For me, protons are positive and electrons negative so the notion that silicon or oxygen contain partial charges does not explain how or why an Si-O bond should be stronger. The inverse square law and the bond angles are consistent with strong bonds. Since several third row elements have expanded octets, it appears consistent with this trend.

Let me further point out that H2S is more acidic than H2O. If this is a result of a greater nuclear charge effect, then how might silicon differ from carbon? (That is a rhetorical question for those willing to think outside of the textbook.)