Syntheses of novel exo and endo isomers of ansa-substituted fluorophosphazenes and their facile transformations into spiro isomers in the presence offluoride ions

Reactions of the dilithiated diols RCH2P(S)(CH2OLi)(2) [R = Fe (1), Ph (2) (Fc = ferrocenyl)] with N3P3F6 in equimolar ratios at -80 OC result exclusi vely in the formation of two structural isomers :of ansa-substituted compou nds, endo-RCH2P(S)(CH2O)(2)[P(F)N](2)(F2PN) [R = Fe (3a), Ph (4a)] and exo- RCH2P(S)(CH2O)(2)[P(F)N](2)(F2PN) [R = Fe (3b), Ph (4b)], which are separat ed by column chromatography. Increasing the reaction temperature to -40 deg rees C results in more of the exo isomers 3b and 4b at the expense of the e ndo isomers. The formation of the ansa-substituted compounds is found to de pend on the dilithiation of the diols, as a reaction of the silylated phosp hine sulfide FcCH(2)P(S)(CH2OSiMe3)(2) (5) With N3P3F6 in the presence of C sF does not yield either 3a or 3b but instead fives the spiro isomer [FcCH( 2)P(S)(CH2O)(2)PN](F2PN)(2) (6) as the disubstitution product of N3P3F6. Th e ansa isomers 3a and 3b an transformed into the spiro compound 6 in the pr esence of catalytic amounts of CsF at room temperature in THF, while 4a and 4b are transformed into the spiro compound [PhCH2P(S)(CH2O)(2)PN](F2PN)(2) (7) under similar conditions. The novel conversions of ansa-substituted ph osphazenes into spirocyclic phosphazenes Were monitored by time-dependent P -31 NMR spectroscopy. The effect of temperature on a transformation was stu died by carrying out reactions at various temperatures in the range from -6 0 to +33 degrees C for 3b. In addition, compounds 3a, 3b, 4a, and 6 were st ructurally characterized. In the case of the ansa compounds, the nitrogen a tom flanked by the bridging phosphorus sites was found to deviate significa ntly from the plane defined by the five remaining atoms of the phosphazene ring.