Hydroxyphosphoranes tautomers of ? hydroxylated phosphoric esters - structure and acidity

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Abstract
The crystal structure of the triethylammonium salts of hydroxyphosphoranes 1a and 2 was resolved by X Ray diffraction. The first one has a TBP geometry slightly deformed with the phosphorus atom at the center, and the second one is a polycylic dimer containing two TBP which present the same deformations. In both cases, the P-O− bond lengths are short and dioxaphospholane rings planar. These two particular properties can be related to the strong Bronsted acidity of compounds 1a and 2. Effectively, the pKa of hydroxyphosphoranes 1a, 1b and 2, determined by potentiometrical titration in DMF or DMSO solutions are characteristic of strong acids.