Recent Advances in Organocatalyst‐Mediated Benzannulation Reactions

Abstract

Aromatic compounds are omnipresent in organic chemistry, which are the broadest skeletons of bioactive molecules, medicines, and materials. Due to their importance, conventional methods to assemble aromatic rings have attracted considerable attention. The direct formation of benzene cores from readily available acyclic precursors offers a highly versatile and superior approach to prepare both small organic molecules and sophisticated natural products. Recently, organocatalyst-mediated annulation reaction, including phosphine-, tertiary amine-, secondary amine-, primary amine-, and NHC-mediated benzannulation reactions, is considered as an efficient method to access multi-substituted arenes. In this review, we will emphasize the selected examples of organocatalyst-mediated benzannulation reactions of available acyclic precursors for the construction of diverse aromatics.

Polymeric Networks Containing Amine Derivatives as Organocatalysts for Knoevenagel Reaction within Continuously Driven Microfluidic Reactors

“…The poor conversion (2 to 4%) of the reaction between 4-nitro benzaldehyde and malononitrile without a catalyst in the MFR was taken as a reference. [ 9 ] The gel dots containing primary amines showed a better conversion of about 83–90% with aliphatic aldehyde (isobutyraldehyde) and 86–100% with aromatic aldehydes (4-methoxy benzaldehyde and benzaldehyde), when compared to the tertiary amines (52–59% with aliphatic aldehyde and 77–93% with aromatic aldehydes), because the nucleophilicity and basicity of the primary amines are higher than the tertiary amines in the polar solvents [ 29 , 30 ]. Furthermore, the catalytic efficacy of the Knoevenagel reaction was changed with solvent mixtures DMSO: 2-propanol ( v : v = 1:1) and DMSO: 2-propanol: water ( v : v : v =2:2:1).…”

(3 + 3) Annulation of acetoxy allenoates with enolisable carbonyl substrates leading to fused pyrans

“…The discovery of Lu's (3 + 2) cycloaddition brought out an emerging area of research for the construction of various carbo- and heterocycles. 4–6 In the presence of nitrogen or phosphorus-containing bases, acetoxy allenoate forms a diene ammonium/phosphonium intermediate that behaves as an electrophile and facilitates attack by appropriate nucleophiles, leading to several carbo- and heterocycles. 7,8…”

Organocatalytic (5+1) benzannulation of Morita–Baylis–Hillman carbonates: synthesis of multisubstituted 4-benzylidene pyrazolones

“…In recent years, from the standpoint of green and sustainable chemistry, ongoing research on the construction of the multifunctional benzene derivatives through organocatalytic benzannulation has emerged as a hot topic. 7 For example, Chi's group reported the construction of benzene through N-heterocyclic carbene-catalyzed (3+3) annulation. In 2019, our group developed the (4+2) annulation catalyzed by a secondary amine to synthesize multifunctionalized CF 3 -benzene.…”