Effect of pKa value of acid component in Mitsunobu Reaction?
I figured out the answe

The desired Mitsunobu reaction proceeds through path (a). On the contrary, in case of the reaction with less acidic HA (pKa>15) , the hydrazo anion (4) attacks the alkoxyphosphonium (5) directly to afford alkylated the hydrazine derivative (6) as a by-product path(b).

Since the anion (4) is not efficient in deprotonating the weakly acidic HA. In order to overcome these drawbacks, the acidic component should have pKa&lt;15 to get desire product.