Abstract
The synthesis of electron-rich P-1 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)(3)PN(3)tBu 1d led to the quantitative formation of aminophosphonium salt (pyrr)(3)PNH2+center dot 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 P-1 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 center dot HBF4, 2e center dot HBF4, and 2f center dot HOTs, which provided the crucial geometrical parameters around the basic center. A rationale for the higher basicity of the pyrrolidino- (pyrr)(3)P=NR than the piperidino- (pip)(3)P=NR phosphazenes is presented.