Ansa versus spiro substitution of cyclophosphazenes: is fluorination essential for ansa to spiro transformation of cyclophosphazenes?

Fluorinated ansa substituted cyclophosphazenes endo-FcCH(2)P(S)(CH(2)O)(2)[P(F)N](2)(F(2)PN) [Fc = ferrocenyl] (1) and exo-FcCH(2)P(S)(CH(2)O)(2)[P(F)N](2)(F(2)PN) (2) readily transform to the spirocyclic compound [FcCH(2)P(S)(CH(2)O)(2)PN](F(2)PN)(2) (3) not only in the presence of CsF but also with non-fluorinated bases such as Cs(2)CO(3), K(2)CO(3), KOBu(t), Et(3)N, DABCO, DBN, and DBU. The analogous tetrachloro ansa compound exo-FcCH(2)P(S)(CH(2)O)(2)[P(Cl)N](2)(Cl(2)PN) (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 (31)P 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.