Molecular modelling and synthetic studies in novel approaches to hair coloration

Abstract

Molecular modelling techniques such as PPP-MO, ZINDO and CNDO/S have been applied to derive colour-structure relationships in a series of donor-acceptor nitroaromatic chromogens based on diazines. Selected 2,4-disubstituted amino-5 nitropyrimidine derivatives have been synthesised and their colouristic properties correlated with molecular modelling results. Attempts were made to synthesise the more bathochromic predicted compounds in the pyrimidine and pyrazine series. During the course of this investigation, IH-15N heteronuc1ear shift correlation spectra using gHMQC and gHMBC with x-ray crystallography were used for the characterisation of certain compounds exhibiting tautomerism. Molecular modelling techniques have also been used to understand the chemical and physical behaviour of colorants towards uptake by the hair fibre to assist development of the concept of pro-pigmentation. This new technology is based on a water soluble pigment precursor, which is converted chemically back in situ to the parent pigment when applied to the hair fibre. A molecular modelling method has been devised and implemented to take into account the shape and size of the molecules as a measure of their diffusivity into the hair. Some pro-pigments which met the requirements were selected for synthesis. Although the chemistry proved feasible with indigo, with other pigments it was less straightforward. A significant breakthrough was the discovery of pro-disperse dyeing. This technology is based on the same concept as the pro-pigmentation, but using disperse dyes. The first pro-disperse red dye was obtained. It was proved that in vitro and "in vivo" the disperse dye was regenerated and can impart a bright red colour to the hair which withstood more than ten shampoos.