Other Phosphorus Containing Superbases Verkades Proazaphosphatranes

7 Other Phosphorus Containing Superbases Verkade s Proazaphosphatranes 

Other phosphorus containing superbases are phosphines and phosphorus yhdes. The most detailed studied class is Verkade s proazaphosphatranes (cyclic azaphosphines). Because of the extraordinary basicity and relatively weak nucleophilicity of nonionic proazaphosphatranes, they have been found to be efficient catalysts and promoters for many reactions 

The originally reported pA a value of 41.2 for 139, which was in the vicinity of that for P3 and P4 phosphazene bases, was recently re-evaluated at 32.82 in acetonitrile by UV-Vis spectrophotometric titration, as shown in Table 2.13 [100]. This value is similar to the phosphazene base P2-Et (pAa = 32.74). The pATa values for 139-141 determined in acetonitrile by P NMR spectroscopy fall in the same range (32.84—33.63). X-ray-determined P-Nax bond distances of 141 and 142 are 2.037 and 1.958 A. An even larger pXa value for 143 was determined by P NMR spectroscopy in acetonitrile (34.49) [101], while the parent compound 138 has a pATa value of 29.6 in DMSO [102]. The greater stability and therefore weaker acidity of cation 138H+ compared with 139 (R = CH3) and 144 

Theoretical predictions have been made that yhdes of phosphorus, nitrogen and sulfur are potentially superstrong neutral organic bases. Limited experimental results showed that the Ph3P=CH2 ylide in DMSO has pXa = 22.5 [103], while the pAia of (Ph(CH3)2N)3P = C(CH3)2 ylide in THF was estimated to be in the range between 26 and 28 units [104]. 

Calculations have revealed that the proazaphosphatrane bases are approximately equal in thermodynamic basicity to the Schwesinger P2 phosphazene bases (Table 2.14). The RHF/ 6-3IG calculations of proton affinities indicate that phosphazene (Z = NH) and phosphorus ylide (phosphorane, Z = CH2) counterparts (146 and 145) are stronger than the parent Verkade superbase 139. Higher basicity is associated with the higher degree of delocalization of the positive charge in the protonated iminophosphoranes and phosphorus ylides as a result of the more electropositive character of phosphorus atom [67]. On the other hand, phosphoms oxides 147 and 150 are weaker than 139, but still above the superbasicity borderline. The strained polycycles 138, 149 and 148, as representatives of superbases 

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Other Phosphorus Containing Superbases Verkade s Proazaphosphatranes 39 Table 2.13 Basicity values for proazaphosphatranes... 

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