Synthesis of crown ether appended 25-Oxasmaragdyrins and their BF2-Complexes

Abstract

Three examples of crown ether appended 25-oxasmaragdyrins were synthesized in 15–20% yields by [3 + 2] oxidative condensation of 16-oxatripyrrane and crown ether appended meso-aryl dipyrromethane in CH2Cl2 in the presence of catalytic amount TFA under inert atmosphere followed by oxidation with DDQ in open air. The crown ether appended 25-oxasmaragdyrins were treated with BF3.OEt2 in CH2Cl2/TEA on hot water bath with 50 °C for 30 min to afford crown ether appended BF2-oxasmaragdyrins in 60–70% yields. The identities of crown ether appended 25-oxasmaragdyrins and their BF2 complexes were confirmed by molecular ion peak in mass spectra and detailed 1D & 2D NMR, absorption, fluorescence and DFT/TD-DFT techniques were used to study the macrocycles. DFT studies revealed that the crown ether appended 25-oxasmaragdyrin and its BF2 complex were almost planar with the meso-phenyl ring containing the crown ether moiety deviated by an angle of 54.86◦ with respect to mean plane defined by three meso carbon atoms. The absorption studies indicated that crown ether appended smaragdyrins and their BF2 complexes showed one or two strong Soret bands in the region of 440–480 nm and four weak Q-bands in 500–750 nm region. The crown ether appended 25-oxasmaragdyrins and their BF2 complexes were weakly fluorescent.