Studies on the mechanism of the Carr–Price blue colour reaction

Abstract

The reaction of retinoids (retinol, retinyl acetate and anhydroretinol) with Brønsted acids was studied as a model system for the Carr–Price reaction. The anhydroretinylic cation was characterised by VIS and 2D NMR spectroscopy, including an estimate of the charge distribution and region of bond inversion, observed in a mixture of identified E/Z isomers. Products obtained by quenching with NaOMe–MeOH were identified by HPLC and MS. The classical Carr–Price reaction between retinol (vitamin A) and the Lewis acid SbCl₃ in saturated chloroform solution was reinvestigated by VIS, NMR, EPR, dynamic light scattering and chemical quenching. Whereas product instability and failure to provide informative NMR spectra indicated a radical cation, EPR results excluded free-radical species. Dynamic light scattering experiments, in comparison with model systems, revealed strong aggregation for the Carr–Price complex, rationalising the low stability, NMR problems and dimerisation observed by chemical quenching. The VIS data support structural similarity of the blue Carr–Price product with the delocalised anhydroretinylic cation, and a detailed structure of the antimony complex is evaluated.