Baeyer-Drewson indigo synthesis | SpringerLink
The Bayer-Drewson reaction to make PNI (this is thesame reaction used in the original synthesis of the famous pigment indigo,using ortho nitrobenzaldehyde in place of our nitro naphthaldehyde) clearly hasbecome a serious bottleneck to further progress. We decided to invest inexploration of a superior route to these compounds.
(2003) Baeyer-Drewson indigo synthesis.
The primary objectives and achievements to date havefocused on discovering and optimizing efficient synthetic routes toperi-naphthindigo (PNI) and its derivatives. We discovered thatBaeyer-Drewson type reaction of 8-nitro-1-naphthaldehyde with acetone underbasic conditions produced the parent PNI 1 in low yield (Scheme 1). Asanticipated, this molecule is an intensely absorbing green chromophore(absorption maximum 647 nm, extinction coefficient ~25,000). However, and nottotally unexpectedly, this flat species is quite insoluble in all commonsolvents.
Given its once economic significance, indigo has been prepared by many methods. The Baeyer-Drewson indigo synthesis dates back to 1882 but was impractical. The first practical route is credited to Pfleger in 1901. In this process, -phenylglycine is treated with a molten mixture of sodium hydroxide, potassium hydroxide, and sodamide. This highly sensitive melt produces indoxyl, which is subsequently oxidised in air to form indigo. Variations of this method are still in use today. An alternative and also viable route to indigo is credited to Heumann in 1897. It involves heating -(2-carboxyphenyl)glycine to 200 °C in an inert atmosphere with sodium hydroxide. The process is easier than the the Pfleger method but the precursors are more expensive. Indoxyl-2-carboxylic acid is generated. This material readily decarboxylates to give indoxyl, which oxidises in air to form indigo.