Medium-ring diphosphines from diphosphabicyclo[k.l.0]alkanes: stereoselective syntheses, structure and properties

Abstract

A series of 1,k + 2-diphosphabicyclo[k.l.0]alkanes 2b–e are prepared by BuⁿLi-promoted cyclisation of 1,ω-diphosphinoalkanes, followed by alkylation and cycloalkylation with 1,ω-dihaloalkanes. These compounds appear to be exclusively cis-isomers except 1,6-diphosphabicyclo[5.4.0]undecane 2e which is a 3:1 cis/trans mixture. Mono-quaternisation of cis-1,k + 2-diphosphabicyclo[k.l.0]alkanes, followed by treatment of the mono-quaternary salts with alkyllithium or Grignard reagents produces cis-1,n-disubstituted-1,n-diphosphacycloalkanes 4 exclusively; examples containing 8-, 9- and 10-membered rings and a range of substituents on phosphorus are described. Di-quaternisation of 1,k + 2-diphosphabicyclo[k.l.0]alkanes, followed by hydrolysis, yields the trans-isomers of 1,n-disubstituted-1,n-diphosphacycloalkane monooxides 6 exclusively; reduction of these with LiAlH₄ in benzene gives largely trans-1,n-disubstituted-1,n-diphosphacycloalkanes 7, but is not completely stereoselective. The structure and properties of these diphosphacycloalkanes are discussed. He(I) photoelectron spectra of 1,5-diphosphabicyclo[3.3.0]octane, 1,6-diphosphabicyclo[4.3.0]nonane and 1,6-diphosphabicyclo[4.4.0]decane show little evidence of interaction between phosphorus lone pairs, unlike the corresponding hydrazines. The medium-ring diphosphacycloalkanes, 1,5-dimethyl-1,5-diphosphacyclooctane and 1,6-dimethyl-1,6-diphosphacyclodecane also show little evidence of interaction between phosphorus lone pairs.