# A new synthesis of Tyrian purple (6,6’-dibromoindigo) and its corresponding sulfonate salts

## TL;DR

This paper presents a new, cost-effective method to synthesize Tyrian purple, a historic dye, and creates water-soluble versions of it.

## Contribution

A new four-step synthesis of Tyrian purple using inexpensive reagents and avoiding harsh oxidation methods.

## Key findings

- The new synthesis achieves 14.5% overall yield from p-bromotoluene.
- The method uses a Kornblum oxidation to avoid harsh chromium-based reagents.
- Water-soluble sulfonate salts of 6,6’-dibromoindigo were successfully produced.

## Abstract

6,6’-Dibromoindigo is the major component of a historic pigment, famous since ancient times, known as Tyrian purple. In this work, we report a new strategy for the synthesis of 6,6’-dibromoindigo in four steps from p-bromotoluene in 14.5% overall yield. A key improvement in the reported synthesis is the oxidation of the benzylic methyl group of 4-bromo-2-nitrotoluene to 4-bromo-2-nitrobenzaldehyde, which is accomplished by benzylic bromination followed by a Kornblum oxidation. This gentle oxidation avoids the need for chromium trioxide-mediated or nitrone-based methods. While other published syntheses of 6,6’-dibromoindigo have resulted in higher overall yields, our approach offers the advantages of inexpensive starting reagents, operationally simple reactions, and minimal purification of intermediates. Moreover, this work reports the successful sulfonation of 6,6’-dibromoindigo, producing water-soluble derivatives of this historically relevant dye.