Paraphenylenediamine (PPD) is commonly used in permanent hair dye formulations. PPD penetrates the hair fibers and undergoes oxidation reactions to form darkly colored compounds. However, PPD also is a well-known contact allergen. This is particularly problematic when it is used in higher concentrations to achieve darker shades of hair. Good, nontoxic alternatives for customers that are allergic to PPD would be useful. The alternative compounds should not readily bind to or penetrate the skin  and have good water solubility.

Gopalakrishnan Venkatesan, National University of Singapore, and colleagues have synthesized seven different aromatic 1,4-diamines and evaluated their potential as PPD alternatives. The team first synthesized the corresponding nitroaniline intermediates, which were then reduced using H₂ and a Pd/C catalyst. In addition to the two amine groups, the products contain different hydrophilic functional groups at the ortho position to improve their solubility in water. The team tested the hair dyeing efficacy, solubility, toxicity, skin permeation ability, and skin sensitization properties of the products.

The researchers found that five of the compounds created dark shades of hair and the other two imparted lighter colors. The compounds are less cytotoxic, show lower skin permeation, and are less sensitizing than PPD. According to the team, the products could be useful nontoxic substitutes for PPD that are less prone to cause an allergic response.

• Synthesis and Assessment of Non-allergenic Aromatic Amine Hair Dyes as Efficient Alternatives to Paraphenylenediamine,
  Gopalakrishnan Venkatesan, Yuri Dancik, Yub Raj Neupane, Aneesh V. Karkhanis, Paul Bigliardi, Giorgia Pastorin,
  ACS Sustainable Chem. Eng. 2022.
  https://doi.org/10.1021/acssuschemeng.1c06313