Semi-Pinacol Reaction

Hi guys

I wonder if anyone could show me the mechanism for the attached reaction. Specifically the alcoholic alkene forming the carbocation using acid. I cant seem to find it or work it out.
Dont worry about the other 2 on the image, just the one which is circled and I only need it up until the carbocation is formed as the rest is fine.
Thanks in advance - its really appreciated as this has been driving me crazy!
Even had the molymod out :biggrin:

Reply 1

EierVonSatan

Theres no image there i'm afraid :p:

The semipinacol is essentially just the pinacol rearrangement with 'no choice'. I've attached an example using tosyl chloride and compared it to the pinacol rearrangement of the corresponding diol. Theres a special case of the semipinacol called the 'Tiffeneau-Demjanov reaction' where beta-amino-alcohols are treated with HNo2/HCl to form the diazonium salt, but I'm not sure you want that :smile:

Dstbgre

Even had the molymod out :biggrin:

ooo i say

semipinacol.pdf48.7KB

Reply 2

Dstbgre

EierVonSatan

Theres no image there i'm afraid :p:

Whoops lol

Here is the image.

Thanks for the reply but I am already familiar with the pinacol reaction which you have specified above. Its the specific one in the image - I just cant work out how the H+ eliminates the CH2!

Semi-pinacol_reactions.JPG19.9KB

Reply 3

EierVonSatan

Dstbgre

Thanks for the reply but I am already familiar with the pinacol reaction which you have specified above. Its the specific one in the image - I just cant work out how the H+ eliminates the CH2!