Basicity solution, phosphazenes

The exceptional basicity of phosphazenes (iminophosphoranes) has been discovered by Schwesinger [83]. The phosphazene derivatives have been proved to be chemically very stable, kinetically active and highly versatile, and the large number of these bases has been synthesized. The gas phase and solution equilibrium basicity measurements for a large number of phosphazenes are conducted by Kaljurand et al. [84], and their PA and pAia values are published in various papers. These measurements show that phosphazenes surpass in their basicity the derivatives of acychc or bicyclic guanidines, amidines and vinamidines (Table 2.11). Extraordinary basicity of phosphazenes was theoretically rationalized by Maksic et al. [85], in terms of effective resonance stabilization of protonated molecules [86]. 

Kaljurand, I., Rodima, T., Pihl, A. et al. (2003) Acid-hase equilibria in nonpolar media 4. Extension of the self-consistent basicity scale in THF medium. Gas-phase basicities of phosphazene. The Journal of Organic Chemistry, 68, 9988-9993 Soovali, L., Rodima, T., Kaljurand, I. et al. (2006) Basicity of some PI phosphazenes in water and in aqueous surfactant solution. Organic and Biomolecular Chemistry, 4, 2100-2105. 

Table 2.11 Solution and gas phase basicity data for phosphazenes [90,91]...

Soovali, L., Rodima, T., Kaljurand, I. et al. (2006) Basicity of some PI phosphazenes in water and in aqueous surfactant solution. Organic and Biomolecular Chemistry, 4, 2100-2105. 

Pihl, A. et al. (2002) Acid-base equilibria in non-polar media 2. Self-consistent basicity scale in THF solution ranging from 2-methoxypyridine to EtPl(pyrr) phosphazene. The Journal of Organic Chemistry, 67, 1873-1881. 

The basicities in water and in aqueous surfactant solution of another 15 aryl-Pl(pyrrolidino) [(NC4Hs)3P=N-Ar] and aryl-Pl(dimethylamino) [(NMe2)3P=N Ar] phosphazenes (Ar = mono or di substituted phenyl rings) have been measured and correlated with previous data using acetonitrile and THE. A comparison with other bases such as guanidines, amines and pyridines showed that, in all cases, the phosphazenes were the strongest bases. The results confirmed earlier conclusions about the partly ylidic character of the NP double bond. Density functional theory calculations have shown that the phosphazenes (41) and (42) are very powerful neutral organic superbases at the site marked by an asterisc (the calculated pA a values in acetonitrile were 44.8 and 37.8, respectively). ... 

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