Organophosphazenes. 26. Factors controlling the pathways observed in the reactions of ethynyl lithium reagents with hexafluorocyclotriphosphazene

摘要

In contrast to previously studied reactions of ethynyl lithium reagents, the reactions of propynyl and hexynyl lithium with N _3P _3F _6 lead to predominantly nongeminal isomers. A modest cis stereo selectivity was observed. The sequential addition of a lithio acetylene reagent which follows a predominately geminal pathway (lithiophenylacetylene) and an aryl lithium reagent (p-propenylphenyl lithium) which follows a predominantly nongeminal pathway were examined. The relative order of addition of the two reagents was interchanged, resulting in a change of reaction pathway demonstrating ring substituent control of the regio and stereo chemical pathways. Hydrogenation of the ethynyl unit in the phosphazene derivatives provides a facile pathway to the difficult to prepare alkylphosphazenes. The chemical shift of the organosubstituted phosphorus center undergoes a remarkably large change (46-47 ppm) on going from the ethynyl to the alkyl derivatives, which reduces the complex ~(31)P and ~(19)F NMR spectra of the ethynyl derivatives to easily interpretable first-order spectra, thus allowing for structure assignment. The ~(13)C NMR data shows that nongeminal regio selectivity increases with the amount of s character on the ethynyl carbon atom attached to the phosphorus center. These observations allow for an understanding of the factors controlling regio and stereo chemical control in the reactions of carbanionic nucleophiles with N _3P _3F _6.