Bridgehead Diphosphines in the Bicyclo[3.3.3]undecane and Bicyclo[4.4.4]tetradecane Series: Synthesis, Structure and Properties.

Several propellane diphosphonium salts 8 are prepared by the reaction of 1,k + 2-diphosphabicyclo[k.l.0]alkanes 6 with α,ω-alkanediol bis-triflates [bis(trifluoromethanesulfonates)]; their properties are strongly dependent on ring size. 1,6-Diphosphoniatricyclo[4.4.4.0]tetradecane bis-triflate 8d is stable in acidic aqueous solution, and reacts with nucleophiles [X = F–, MeO–, H– (from BH4–) and R– (from Grignard and alkyllithium reagents)] to give products 13–18 with partial X–P–P+ bonding, hydroxide ion gives the diphosphine monooxide 12b but even this may retain some P–P bonding. However 1,5-diphosphoniatricyclo[3.3.3.0]undecane bis-triflate 8a is hydrolysed irreversibly and much more rapidly than its [4.4.4.0] counterpart. 1,6-Diphosphoniatricyclo[4.4.3.0]tridecane bis-triflate 8c reacts with NaBH4 to give a hydride adduct 15a which is deprotonated by BunLi to give 1,6-diphosphabicyclo[4.4.3]tridecane 9c. However several attempts to prepare 1,6-diphosphabicyclo[4.4.4]tetradecane 9d led to a deep-seated rearrangement to give 1,4-bis(1-phospholan-1-yl)butane. These included reaction of hydride adduct 15b with BunLi, and debenzylations of 1-benzyl-1-phosphonia-6-phosphabicyclo[4.4.4]tetradecane trifluoromethanesulfonate 16b and 1,6-dibenzyl-1,6-diphosphoniabicyclo[4.4.4]tetradecane bromide triflate 21 with LiAlH4. Reaction of cis-1,5-dibenzyl-1,5-diphosphacyclooctane with CH2(CH2OTf)2 gives 1,5-dibenzyl-1,5-diphosphoniabicyclo[3.3.3]undecane bis-triflate which is debenzylated with LiAlH4 to 1,5-diphosphabicyclo[3.3.3]undecane 9a. Attempts to prepare 1,6-diphosphabicyclo[4.4.4]tetradecane by related methods lead to oligomerisation reactions. The structure of 1,5-diphosphabicyclo[3.3.3]undecane is reported; its He(I) photoelectron spectrum shows two well separated bands at 7.58 and 8.14 eV, and RHF/6-31G* ab initio calculations indicating that this is due to through-bond interactions.