Hello! Me and my partner are trying to figure out the initial mechanism of Mitsunobu azidation with PPh3/DIAD/DPPA. In any mechanism, as well as the original paper where azidation with DPPA is reported (Lal, Pramanik, Manhas & Bose, 1977, DIPHENYLPHOSPHORYL AZIDE A NOVEL REAGENT FOR THE STEREOSPECIFIC SYNTHESIS OF AZIDES FROM ALCOHOLS) , after phosphonium ion installs on one of the nitrogens of DEAD's N=N, double bond breaks and the remaining nitrogen obtains formal negative charge which is shown to be eliminated by protonation. However, there is nothing that evidently is capable of donating the proton to stabilize that negative charge on the N.
We've been trying to solve this enigma for a few days now and have grown quite desperate, as our paper deadline is in 8 hours from now :)
Thank you!
Artur
Your pro-nucleophile (HN3) or alcohol intermediate (after bonding to phosphorus) are your proton sources. Most general mechanisms for Mitsunobu reactions show this even if they use benzoic acid not HN3 as the pro-nucleophile the mechanism is the same http://www.organic-chemistry.org/namedreactions/mitsunobu-reaction.shtm