Efficient and Green Approaches for the Synthesis of 4H-Benzo[g]chromenes in Water, Under Neat Conditions, and Using Task-Specific Ionic Liquid

Abstract

Facile and convenient procedures have been described for the synthesis of 4H-benzo[g]chromenes by one-pot condensation of aromatic aldehydes, malononitrile/ethyl cyanoacetate, and 2-hydroxy-1,4-naphthoquinone. The reaction has been accomplished using catalytic cetyltrimethylammonium bromide (CTAB) in water under reflux or under neat conditions at 110 °C. A task-specific ionic liquid, 1-butyl-3-methyl imidazolium hydroxide ([bmim]OH), has also been found to be an effective catalyst for this transformation. The protocols prove to be environmentally benign and offer advantages of good yields, short reaction times, and simple workup procedures. The ionic liquid could be recycled and reused for up to four runs without appreciable loss in the activity.

GRAPHICAL ABSTRACT

Keywords:

• [bmim]OH
• CTAB
• 4H-benzo[g]chromene
• 2-hydroxy-1,4-naphthoquinone
• three-component condensation

ACKNOWLEDGMENT

D. M. and A. C. thank the Council of Scientific and Industrial Research, New Delhi, India, for the senior and junior research fellowships.

Notes

^a Reactions were performed in water as reaction media.

^b Reactions were performed under solvent-free conditions.

^a Method A: Reaction in water under reflux, CTAB (0.2 mmol).

^b Method B: Reaction under neat conditions at 110 °C, CTAB (0.2 mmol).

^c Method C: Reaction in neat at 50–60 °C, bmim[OH] (0.2 mmol).