Synthesis and preliminary studies on novel enantiopure crown ethers containing an alkyl diarylphosphinate or a proton-ionizable diarylphosphinic acid unit

Abstract

This paper reports the synthesis, characterization and electronic circular dichroism (ECD) spectroscopic studies of a new type of crown ethers and their achiral analogues containing a tetrahedral phosphorous centre. The synthetic routes to the two chiral phosphinate derivatives [( R, R)- 10 and ( R, R)- 11] were similar, starting from the earlier reported ethyl bis(2-hydroxyphenyl)phosphinate and the unreported methyl bis(2-hydroxyphenyl)phosphinate, respectively. The enantiopure crown ether containing phosphinic acid unit ( R, R)- 14 was obtained by hydrolysis of the phosphinates ( R, R)- 10 and ( R, R)- 11, respectively. ECD spectroscopy was used for investigation of the chiroptical properties as well as complex formation ability of the novel enantiopure ligands. Owing to the presence of the aryl substituents the ECD spectra are rich in bands in the 1B b, 1L a and 1L b regions (190–250 nm and 260–330 nm, respectively). In the case of ( R, R)- 14, a solvent dependent conformational behaviour was observed due to the strong dimer or aggregate forming ability of the POOH groups. This finding was supported by theoretical calculation of the monomer and the dimer forms. Phosphinates ( R, R)- 10 and ( R, R)- 11 form complexes with α-phenylethylammonium perchlorate (PEA) and α-(1-naphthyl)ethyl ammonium perchlorate (NEA) but do not discriminate between their enantiomers. All three chiral crown ethers bind strongly cations of ionic radii <∼1 Å.