Bridgehead phosphorus chemistry: in–out inversion, intrabridgehead P ⋯ P bonding, and reactivity

Abstract

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 ¹J_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 ¹J_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-1λ⁵,6-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.