Imidazo[1,2-a]pyridines susceptible to excited state intramolecular proton transfer: one-pot synthesis via an Ortoleva-King reaction.
A short and efficient route to a broad range of imidazo[1,2-a]pyridines from 2-aminopyridines and acetophenones is achieved by a tandem, one-pot process starting with an Ortoleva-King reaction. Optimal conditions for the first step were established after examining various reaction parameters (solvent, reagent ratios, and temperature). The conditions identified (1st step, neat, 2.3 equiv of 2-aminopyridine, 1.20 equiv of I(2), 4 h, 110 °C; 2nd step, NaOH(aq), 1 h, 100 °C) resulted in the formation of imidazo[1,2-a]pyridines in 40-60% yields. The synthesis is compatible with various functionalities (OH, NMe(2), Br, OMe). Products containing a 2-(2'-hydroxyphenyl) substituent undergo excited state intramolecular proton transfer (ESIPT) in nonpolar and polar-aprotic solvents. Although ESIPT-type emission in nonpolar solvents is weak, the Stokes shifts are very high (11000 cm(-1)). The comparison of the properties of six ESIPT-capable imidazo[1,2-a]pyridines shows the influence of various substituents on emission characteristics. All of them also display strong, solid-state emission in blue-green-yellow region. 2-Aryl-imidazo[1,2-a]pyridines not capable of ESIPT emit in the blue region, displaying high fluorescence quantum yield.
Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland.
Anton J
AJ
Stasyuk
Marzena
M
Banasiewicz
Michał K
MK
Cyrański
Daniel T
DT
Gryko
D015394:Molecular Structure; D011522:Protons; D011725:Pyridines / Q000138:chemical synthesis* / Q000737:chemistry; D013237:Stereoisomerism
D011522:Protons; D011725:Pyridines; C001439:imidazo(1,2-a)pyridine