Yes, I dont know the acidity of the phosphonate, maybe its more acidic then the Fmoc-proton. Its a good suggestion to mix the base and phosphonate first.
Some time ago I found a pKa
of 18.6 for (EtO)2
Et, which I believe is in DMSO. I have not yet found pKa
values for common β-sulfonylphosphonates, but I think that they are likely to be in this vicinity. The presence of the lithium ion in the reaction will lower the pKa
of the β-sulfonyl phosphonate, but I don't know by how much. The 1984 Tet. Lett. paper by Blanchette M et al
., suggests that the effect is roughly 7 pH units relative to the potassium ion with respect to the carboxylate derivative.