ISSUE:

Good afternoon,
in the aforementioned reaction of a hydrochloric acid with H2, Pd /BaSO4 in xylene to give an aldehyde with hydrochloric acid, there is a rearrangement after attacking the Pd to the carbonyl, after which enCl is in the beta position and the Pd in alpha position, the question is how/why does this transposition occur and what is the role of BaSO4 in the reduction?
All the best

Hi Meric,
The Rosemund reduction does not use hydrochloric acid as a raw material, it uses an acid chloride and Pd catalyst, poisoned with Barium Sulfate, this is precisely its function to poison the catalyst so that it loses power, and in this way it does not also reduce the aldehyde resulting.
Although the mechanism of the reaction is not well defined, steps have been established that explain the transformation:
1..- Poisoning of palladium by sulfate (formation of the complex)
2.- Oxidative addition of palladium to the C-Cl bond
3.- Ligand/Hydrogen Exchange
4.- Reductive elimination