Ansa versus Spiro Substitution of Cyclophosphazenes: Is Fluorination Essential for Ansa to Spiro Transformation of Cyclophosphazenes?

Fluorinated ansa substituted cyclophosphazenes endo-FcCH2P(S)(CH2O)2[P(F)N]2(F2PN) [Fc = ferrocenyl] (1) and
exo-FcCH2P(S)(CH2O)2[P(F)N]2(F2PN) (2) readily transform to the spirocyclic compound [FcCH2P(S)(CH2O)2PN](F2PN)2 (3) not only in the presence of CsF but also with non-fluorinated bases such as Cs2CO3, K2CO3, KOBut,
Et3N, DABCO, DBN, and DBU. The analogous tetrachloro ansa compound exo-FcCH2P(S)(CH2O)2[P(Cl)N]2(Cl2PN) (5), however, did not transform to the chlorinated spiro compound (6) in the presence of these bases. With
excess of CsF, P−Cl bonds of 5 were found to undergo fluorination leading to the formation of 2, which transformed
to spirocyclic compound 3. Time dependent 31P NMR spectroscopy was used to monitor this transformation. Crystal
structure studies on the ansa substituted compounds 4 and 5 have shown weak bonding interactions involving
C−H···Cl, C−H···O, and C−H···S interactions.