One-step construction of aminosquaraine backbone

• Amidation reaction unexpectedly gives reduced aminosquaraine as the major product.
• Reaction mechanism and substrate requirement are proposed and studied.
• The reduced aminosquaraine can be facilely oxidized to conjugated aminosquaraine.

One-step construction of the amino-substituted squaraine dye backbone was discovered unexpectedly in an amidation reaction. Under the catalysis of Mukaiyama's reagent, (4-dimethylamino)phenylacetic acid and bulky secondary amine 2,2,6,6-tetramethylpiperidine give the reduced form of aminosquaraine instead of amide as the major product. The reduced form of aminosquaraine can be oxidized to a dark-colored conjugated aminosquaraine within seconds. The method provides a novel and facile way to functionalize the four-membered core of squaraine dye.

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