[g_orbital]

In the following reaction (taken from a procedure in a paper), the O-Acetyl groups were replaced by O-TMS (tetramethylsilyl) groups.
I wonder:
1) Whether the following mechanism (attached herewith as an image file) makes sense?
2) Why the experimental procedure uses Acetyl Chloride in Methanol instead of a simple HCl (in order to protonate the oxygen)?
3)  Why isn't the "left"-terminal methyl-ester group affected by the protonation?

Thank you very much in advance.

[Dan]

In the following reaction (taken from a procedure in a paper), the O-Acetyl groups were replaced by O-TMS (tetramethylsilyl) groups.
I wonder:
1) Whether the following mechanism (attached herewith as an image file) makes sense?

Well, my tutor would string me up if I suggested this (exact) mechanism. This is acid hydrolysis, the possibility of ^-OMe hanging around are slim to none. The nucleophile should be methanol. Same applies to the magical deprotonation the second step.

2) Why the experimental procedure uses Acetyl Chloride in Methanol instead of a simple HCl (in order to protonate the oxygen)?

I can't think of a good reason, since you will rapidly generate HCl in situ anyway.

3)  Why isn't the "left"-terminal methyl-ester group affected by the protonation?

Draw out that group in full... RCOOMe, where R is the chain with all the other stuff on it, it will be protonated, it will react (you could use ¹⁸O labelled methanol to prove this), but hydrolysis of a methyl ester with methanol regenerates the same methy ester.

[movies]

You have the right idea for the mechanism, but you protonated the wrong oxygen.  In an ester, the carbonyl oxygen is much more basic because the resulting cation can be stabilized by resonance, whereas the structure you drew cannot be stabilized by resonance.  Once you protonate the carbonyl oxygen, MeOH attacks (as Dan said, it's not MeO-) to form a tetrahedral intermediate.  After that, you would transfer the hydrogen that came along with the MeOH to the other ether oxygen (the oxygen you showed as protonated) and then collapse the electrons from the OH group to make a carbonyl and release the alcohol.

As for the combination of reagents, MeOH/AcCl, this is a common way to generate dry HCl in solution.  Otherwise you would have to use HCl gas.  Aqueous HCl can't be used because it would hydrolyze your methyl ester to the carboxylic acid with water in the pot.

[Dan]

As for the combination of reagents, MeOH/AcCl, this is a common way to generate dry HCl in solution.  Otherwise you would have to use HCl gas.  Aqueous HCl can't be used because it would hydrolyze your methyl ester to the carboxylic acid with water in the pot.

Ahhh, of course! I'll remember that for my finals (next week!).