Crown ethers with converging neutral binding sites: Synthesis and complexation with t-butylammonium hexafluorophosphate

The influence of substituents in close proximity to crown ether cavities, on the stability of complexes of the crown ethers with t-butylammonium salts, has been investigated. Crown ethers with intra-annular donor substituents (2–4) were prepared by the reaction of 2-acetylresorcinol (1) with polyethylene glycol ditosylates and subsequent modification of the acetyl group. Crown ethers with substituents above and below the plane of the crown ether 0 atoms were synthesized by the reaction of 2,2'-dihydroxy-1,1'-biphenyls with polyethylene glycol ditosylates. Chloromethylation of 5,5'-dimethyl-1,1'-biphenyl crown ethers (6) yielded 4,4'-bis(chloromethyl)-1,1'-biphenyl crown ethers (10). 3,3'-Disubstituted-1,1'-biphenyl crown ethers (13–24) were synthesized by the reaction of 3,3'-diallyl-2,2'-dihydroxy-1,1'-biphenyl (12) with polyethylene glycol ditosylates. The allyl groups of 13 were isomerized with sodium hydride to propen- 1-yl groups. Ozonolysis of 13 and 14 gave the corresponding dialdehydes (15 and 18) which were converted into other 3,3'-disubstituted biphenyl-20-crown-6 derivatives (RCH₂COOMe, CH₂COOH, CH₂OH, CH₂Cl, CH₂OMe, OH and Me) by standard operations. The thermodynamic stability of the complexes of these functionalized crown ethers with t-butylammonium hexafluorophosphate has been studied in deuterochloroform in competition experiments with m-xyleno-18-crown-5 and benzo-15-crown-5 as the reference compounds. The nature of the 2-substituents in the crown ethers 2 and 3 has little effect on the stability of the complexes. The stability of the complexes of 3,3'-disubstituted biphenyl crown ethers depends of ringsize and the size and nature of the substituents. The most stable complexes are those of 24 (R = Me) and 14 (R=CH=CHMe).The Me groups in 24 represent the optimum between relief of O-O repulsion in the polyether ring and steric hindrance of complexation. The propen-1-yl substituents of 14 stabilize the complex because they provide extended π-electron donor stabilization. Substitution at the 4- and 4'-positions of the aryl groups has little effect on the stability of the complexes.