[Heory]

The following decarboxylation reaction looks so incredible, doesn't it? Could anyone explain it?

[tmartin]

Does the steroechemistry at the beta position of the carbonyl that leaves stay intact or is it destroyed (as you're indicating?)?

[Heory]

I coppied it from my textbook (Hydrolization of penicillin). I don't know whether the steroechemistry stays intact or destroyed, for it's not mentioned in the book. But I coppied the picture, so I thought it's destroyed. However, I'm concerned about the decarboxylation, not the sterochemistry.

[stewie griffin]

I bet that the stereocenter is destroyed. I proposed a mechanism below. Basically I think you've got to have something to similar to a beta-keto acid for decarboxylations to occur... thus the proposed ring opening to form a thionium ion (if you can call it that). That destroys the stereochemistry.

[tmartin]

Well, stewie beat me to this, but since I drew it all out, I'll post it too.

[Heory]

Solved so fast!
stewie griffin and tmartin, how smart you are! I must learn hard.

[Heory]

The last step breaks Baldwin's rule (5-trig-endo is forbidden). I think B's rule doesn't fit for this situation. Or maybe the mechanism is wrong?

[stewie griffin]

I would ignore Baldwin's rules here for the last step. The last step is completely analagous to acid induced acetal formation with a 1,2-diol (ie formation of an acetonide from a diol). 

[azmanam]

agreed.  Because of the resonance in the, er, thionium(?) ion, there is significant positive charge character on the carbon atom.  Perhaps it looks more like a carbocation with an empty p-orbital than a 'regular' double bond with full sp² character?  If it were a typical double bond, the orbital alignment of the nitrogen lone pair into the C(sp²) pi* orbital might not be favorable.  I suspect the resonance structure giving positive charge character to the carbon atom is in play here.

[movies]

thionium(?) ion

Thiocarbenium ion.  Thionium would be S sigma-bonded to three different things, like in trimethylsulfonium iodide.

[jinclean]

i'm confusing about the Thiocarbenium because of its huge polarity and the bonding orbital,the S should ues the d orbit to form the S-C  bond,but the d orbital have the higher energy than the carbon's p orbital.what is the possible configuration in the carbon anion and the thiocarbenium?what is the C(-)-S(+) bond in actually?

[stewie griffin]

re: movies
Thanks for the word. For some reason I couldn't remember thiocarbenium when I was typing it all up 
re: jinclean
It's interesting to see how you are thinking about this C=S double bond. I don't see a need to bring in d orbitals on sulfur to explain this. Really what we have here is just a fancy carbocation. So the carbon was initially sp3, then some reaction occurs, and it's now an sp2 hybrid with an empty p orbital (in other words it's a carbocation). The sulfur atom is sp3 hybridized with two sets of lone pairs (just like an oxygen atom in a water molecule or in an ether). To stabilize the carbocation's empty p orbital, we can imagine that sulfur uses one set of it's lone pairs to donate into the empty p orbital. We can represent that stabilization as a second bond between the carbon atom and sulfur atom.
Note that you don't have to draw it as a C=S double bond. That's just one of two resonance structures. If you understand it better to just leave it as a full blown carbocation, you may do so. But you should know that in terms of the resonance rules, it's best to give everyone a full octet. Thus the C=S structure is the better resonance structure of the two. Hopefully the figure will help out a bit.

[jinclean]

re:stewie griffin
 thanks for your explaination,but what i am confusing like SMe3(+) I(-) is the  quadrivalence sulfur with the three sigma bonds.that is very strange to me.and in your opinion.the S in the C=S bond uses the SP3 hybird orbit not the P obital? can't the sulfur changes its configurtion to get the better bonding with the carbon?