Crystal structures of crown ethers containing an alkyl diarylphosphinate or a diarylphosphinic acid unit

This paper describes the structures of pseudo-18-crown-6 compounds (2, R,R-4 and 5) in the crystals together with theoretical calculations of the electronic circular dichroism (ECD) spectra. The achiral macrocyclic phosphinic acid 5 forms hydrogen-bonded dimers in the crystal. The O1-O2 distance (2.489 Ǻ) indicates strong H-bondings. The conformations of the macrorings of the achiral phosphinate 2 and the monomers of the achiral phosphinic acid 5 are chiral. A comparison of the torsion angles of the achiral methyl phosphinate 2 and the monomeric units of achiral 5 indicates a similar geometry. The torsion angles of the chiral methyl phosphinate (R,R)-4 differ more significantly from those in achiral methyl phosphinate 2. A negative ¹B_b exciton couplet was observed in the ECD spectrum of monomeric (R,R)-6 in MeOH and H₂O as in the spectra of (R,R)-4 in all solvents. To support the idea that (R,R)-4 has basically the same conformation in the crystal and in solution, the ECD spectrum of (R,R)-4 was calculated using the geometry of the molecule in the crystal. The calculated ECD spectrum shows a reasonable agreement with the ECD spectra obtained in solution. This shows that the steric structure observed in the crystal is predominant in solution as well.