Synthesis of Naphtho‐Fused Imidazo[1,2‐a]pyridines through Copper‐Catalyzed Cascade Reactions

Abstract

Ah ighly efficient copper-catalyzed one-pot tandem protocol has been developed for the synthesis of naphtho-fusedi midazo[1,2-a]pyridines. The transformation involves aK noevenagel condensation followed by ac hemoselective cross-coupling reaction along with ac arbon-carbon bond cleavage. This protocol can tolerate av ariety of functional groups and provided naphtho[1',2':4,5]imidazo[1,2-a]pyridines in moderate to excellent (35-91 %) yields. The photophysical studies of the naphtho-fusedi midazo[1,2-a]pyridines show ab athochromic shift between the band maximao ft he absorption ande mission spectra.T hese compounds emit with high fluorescencequantum yields.

2‐Aryl‐3‐Vinyl Substituted Imidazo[1,2‐a]pyridines and Fluorescent Electrocyclization Derivatives therefrom

“…Polyheterocycles 8 , when irradiated under a handheld UV lamp, showed evident fluorescence (Figure ), and we considered that such property surely deserved a detailed study, in order to define the absorption and emission maxima, the Stokes shifts and the relative fluorescence quantum yields. Similar systems have been recently reported and their photophysical properties studied, but generally speaking, only a limited number of naphtho‐imidazopyridines has ever been prepared and analyzed . Fluorescence data obtained for compounds 8 are collected in Table .…”

“…The value reported in Table for λ emiss (λ max , ems ) is that relative to the highest peak; the Stokes shift was calculated as Δ=1/λ max , abs – 1/λ max , ems ; the quantum yields were evaluated using a solution of 4‐amino‐4‐methylcoumarine in ethanol as standard. Only for compound 8ba , fluorescence data were found in the literature, where quantum yields were calculated with quinine sulphate in 0.1 N sulfuric acid as standard.…”

Recent Developments in Transition‐Metal‐Catalyzed Regioselective Functionalization of Imidazo[1, 2‐a]pyridine

“…2‐(2‐bromophenyl)imidazo[1, 2‐ a ]pyridine‐3‐carbaldehyde on reaction with malanonitrile form naphtho‐fused product having photophysical properties and this was reported by Saini and his co‐workers in 2015 (Scheme 31). [54] This Cu‐catalyzed reaction was carried out in presence of ligand and resulted in low to good yields of the desired product. The mechanism (Scheme 32) of this reaction initially undergoes Knoevenagel condensation which then binds with catalyst and finally affords the product via reductive elimination.…”

The synthesis ofN‐containing heterocyclic compounds catalyzed by copper/L‐proline

“…[ 45 ] In 2015, Kumar’s group made a progress with CuCl 2 / L ‐proline‐catalyzed tandem condensation/C‐C cleavage reaction of 2‐(2‐bromophenyl)imidazo[1,2‐ a ]pyridine‐3‐carbaldehydes 63 and malononitrile 64 to synthesize naphtho‐fused imidazo[1,2‐ a ]pyridines 65 (Scheme 13). [ 46 ] The absorption and emission maxima of the products were bathochromically moved, and these products emitted good fluorescence quantum yields. Despite this reaction had broad substrate scope, o ‐chlorochalcone could not participate in this transformation.…”