Basicity 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... 

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

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... 

In summary, guanidinophosphazenes belong to the most basic, experimentally determined class of superbases, followed by phosphazenes, proazaphosphatranes and guanidines. Amidines and classical proton sponges generally show less pronounced basicity. 

The proazaphosphatrane bases serve as an effective ligand for the palladium catalysed amination of a wide array of bromides and iodides. Other bicyclic or acyclic triaminopho-sphines, even those of similar basicity and/or bulk, were inferior. The palladium catalysed... 

The variable basicity of (187 R = Me) due to the flexible transannular bond is believed to play an important role in the superior catalytic activity of this compound for the cyclotrimerization of phenylisocyanate <93AG(E)896>. The relative stability of (188) has been attributed to its partially annulated structure (P—N 3.250 A) <92JA3129>. The reaction of the proazaphosphatrane (189) with methyl iodide gave both the annulated —S—Me and open N+—Me regioisomers <93JA1660>. 

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