Indigo Beyond Tradition: Scalable Derivatization, Extraction from Waste Denim Textiles, and Boc-Protected Intermediates

This study demonstrates the feasibility of industrial-scale, column-free synthesis of <i>tert</i>-butoxycarbonyl (Boc)-protected indigo derivatives and highlights their application potential in the textile industry for the dyeing of previously undyeable synthetic textiles with indigo. The direct synthesis of <i>N</i>-mono-<i>tert</i>-butoxycarbonyl (Boc) indigo, an important intermediate for indigo substitution studies, was achieved under very mild conditions and in a moderate yield. For the first time, the molecular structures of all synthesized compounds were comprehensively characterized through detailed NMR and HRMS analyses as well as the single-crystal X-ray diffraction method. The influence of the Boc groups on the photophysical and thermal properties, oxidation-reduction behavior, and dyeing performance was thoroughly investigated and defined. Importantly, the oxidation mechanism of dihydro-di-Boc indigo (i.e., reduced di-Boc indigo) was elucidated by combined experimental and theoretical approaches. Furthermore, this study shows that indigo-dyed textile waste can serve as valuable starting materials for the synthesis of Boc-protected indigo derivatives. This innovative methodology offers significant promise not only for textile-to-textile recycling technologies but also as a sustainable strategy to address global waste management and environmental pollution challenges.