Bridgehead phosphorus chemistry:: in-out inversion, intrabridgehead P•••P bonding, and reactivity

Propellane diphosphonium dications undergo addition reactions with a range of nucleophiles, and the products often have in,out-geometry with some P-P interaction, as shown by (1)J(PP) values ranging from 46-253 Hz. X-Ray structures of 1-benzyl-6-phospha-1-phosphoniabicyclo[4.4.4]tetradecane trifluoromethanesulfonate, 1-benzyl-6-phospha-1-phosphoniabicyclo[4.4.3]tridecane hexafluorophosphate, and 1-hydroxy-6-phospha-1-phosphoniabicyclo[4.4.3]tridecane hexafluorophosphate are reported; these show that there is no simple relationship between P-P distance and (1)J(PP) values, although the latter do correlate with the apicophilicity of the attached group. Hydride adducts, although formally protonated phosphines, can react as hydride sources, and salts of the 6-phospha-1-phosphoniabicyclo[4.4.3]tridecane ion, while stable in solution, undergo ring opening to the 1-propyl-1-phosphonia-6-phosphabicyclo[4.4.0]decane ion in the solid state. Addition of hydroxide to the 1,6-diphosphoniatricyclo[4.4.4.0(1,6)]tetradecane ion leads, via a non-isolable hydroxide adduct, to out,in-1 lambda (5),6-diphosphabicyclo[4.4.4]tetradecane 1-oxide. Addition to the 1,6-diphosphoniatricyclo[4.4.3.0(1,6)]tetradecane dication leads to isolable 1-hydroxy-6-phospha-1-phosphoniabicyclo[4.4.3]tridecane hexafluorophosphate, converted by excess hydroxide to the diphosphine monoxide which exists as a dynamic mixture of out,in- and out,out-isomers, the equilibrium being dependent on solvent polarity, the water content of the solvent, and the presence of metal ions. Addition of hydroxide to the 1,6-diphosphoniatricyclo[4.3.3.0(1,6)]tetradecane dication leads to an out,in-hydroxide adduct which is rapidly converted to the out,out-monoprotonated phosphine oxide.