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 tert-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 N-mono-tert-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.