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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