Bridgehead diphosphines in the bicyclo[3.3.3]undecane and bicyclo[4.4.4]tetradecane series: synthesis, structure and properties

Abstract

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.