Sustainable Diels–Alder syntheses in imidazolium ionic liquids

ABSTRACT

An effective and simple protocol for the Diels–Alder reactions catalyzed and mediated by ionic liquid in conjunction with Lewis acid was studied. Diels–Alder adduct of the reaction of 1,4-benzoquinone with isoprene and 2,3-dimethyl-1,3-butadiene was obtained in good to excellent yields within the time span of 5.00–4.00 min under microwave irradiation. Due to the excellent activity of 2,3-dimethyl-1,3-butadiene as a diene, its efficiency was further analyzed by performing against four different dienophiles under various reaction conditions. The ionic liquid, 3-methyl-1-octyl-imidazolium tetrachloroaluminate mixed with Lewis acids, could be recycled and reused for consecutive cycles. The recovered ionic liquid in conjunction with Lewis acid displayed almost similar activity without any significant loss. This system is an essential upgrade and related to the reactions consuming common organic solvents at high temperatures and longer duration. Diels–Alder reactions under solventless conditions have also been investigated for the aforesaid reactions.

GRAPHICAL ABSTRACT

KEYWORDS:

• 1,4-Benzoquinone
• Diels–Alder reaction
• ionic liquids
• Lewis acid
• microwave

Acknowledgments

We are indebted to Bharathiar University, Coimbatore and Sir Theagaraya College, Chennai, India for the continuous support and inspiration rendered. Also, we would like to thank Madurai Kamaraj University, Madurai, for providing the microwave reactor facility to perform all the microwave-assisted experiments.