Synthesis of electron-rich sterically hindered P1 phosphazene bases by the staudinger reaction

Article abstract of DOI:10.1002/ejoc.201201400

The synthesis of electron-rich P₁ phosphazene bases with a sterically protected basic center by a Staudinger reaction is reported. The initial products of the reaction between peralkylated triaminophosphanes and bulky alkyl azides, phosphazides 1a-f, were isolated in all cases in good to quantitative yield. The structures of 1d and 1e were confirmed by single-crystal X-ray diffraction. Acidic hydrolysis of pyrrolidino-substituted phosphazide (pyrr)₃PN₃tBu 1d led to the quantitative formation of aminophosphonium salt (pyrr)₃PNH₂⁺· BF₄^- 8, a direct precursor to a Schwesinger's "building block" synthetic unit. Thermally induced denitrogenation of the phosphazides, which is the second step performed in most cases under solvent-free conditions gave P₁ phosphazene bases 2a-f in moderate to excellent yields. A "one-pot" two-step synthesis of phosphazene bases from commercially available triaminophosphanes was discovered. Most of the syntheses were performed on a large laboratory scale. The basicities of the newly synthesized bases 2e and 2f were determined. X-ray crystal structures were obtained for base 2e and for protonated species 2d·HBF₄, 2e·HBF₄, and 2f·HOTs, which provided the crucial geometrical parameters around the basic center. A rationale for the higher basicity of the pyrrolidino- (pyrr)₃P=NR than the piperidino- (pip)₃P=NR phosphazenes is presented. A Staudinger reaction was successfully used for the synthesis of a number of sterically congested electron-rich P₁ phosphazene bases in moderate to quantitative yield. The intermediate phosphazides were isolated and characterized in all cases; their application in the preparation of higher-order phosphazenes is described. Copyright