Tetra-Coordinated Boron-Functionalized Phenanthroimidazole-Based Zinc Salen as a Photocatalyst for the Cycloaddition of CO₂ and Epoxides

Abstract

A unique B–N coordinated phenanthroimidazole-based zinc salen was synthesized. The zinc salen thus synthesized acts as a photocatalyst for the cycloaddition of carbon dioxide with terminal epoxides under ambient conditions. DFT study of the cycloaddition of carbon dioxide with terminal epoxide indicates the preference of the reaction pathway when photocatalyzed by zinc salen. We anticipate that this strategy will help to design new photocatalysts for CO2 fixation.

B−N Coordinated Phenanthroimidazole‐Based Zinc‐Salen as a Photocatalyst for the Synthesis of Oxazolidinones using Carbon Dioxide as a C1 Source under Mild Reaction Conditions

“…The catalytic cycle can be thought of as being initiated with the binding of aziridine N‐atom with the Lewis acidic zinc center in its excited state and thereby activating the aziridine ring to form the species A. Intramolecular nucleophilic attack by the bromine anion on the less‐hindered side of the activated aziridine leads to the ring opened aziridine, affording an intermediate of metal‐bound alkylamide species B. CO 2 insertion allowed the formation of species C which subsequently undergoes intramolecular nucleophilic (S N 2) reaction to give the corresponding oxazolidinone and regenerates the excited state catalyst 1 . Theoretical studies performed using propylene oxide (PO) as a reactant reveal that B−N coordinated phenanthroimidazole based Zn‐salen‐PO complex shows a low energy difference between the excited and the ground state compared to simple Zn‐salen‐PO complex [16] . We believe that the boron coordination helped to achieve a high degree of planarity and effective extension of conjugation in 1 , thus helps to accelerate the reaction under blue light.…”

“…Theoretical studies performed using propylene oxide (PO) as a reactant reveal that BÀ N coordinated phenanthroimidazole based Zn-salen-PO complex shows a low energy difference between the excited and the ground state compared to simple Zn-salen-PO complex. [16] We believe that the boron coordination helped to achieve a high degree of planarity and effective extension of conjugation in 1, Table 2. Substrate scope of the coupling of CO 2 with different aziridines catalyzed by photocatalyst 1.…”

“…A possible catalytic cycle for the cycloaddition of CO 2 and aziridines is proposed based on the literature precedents [16,23] (Scheme 1). The catalytic cycle can be thought of as being initiated with the binding of aziridine N-atom with the Lewis acidic zinc center in its excited state and thereby activating the aziridine ring to form the species A. Intramolecular nucleophilic attack by the bromine anion on the less-hindered side of the activated aziridine leads to the ring opened aziridine, affording an intermediate of metal-bound alkylamide species B. CO 2 insertion allowed the formation of species C which subsequently undergoes intramolecular nucleophilic (S N 2) reaction to give the corresponding oxazolidinone and regenerates the excited state catalyst 1.…”

“…Motivated by our recent work in which we showed that 1 (Figure 1) exhibited interesting photocatalytic activity for the cycloaddition of CO 2 and epoxides, [16] we investigated its catalytic property for the synthesis of oxazolidinone from 1‐butyl‐2‐phenylaziridine under the irradiation of blue LED (6 W, 435–445 nm) in solvent free conditions with 1 bar carbon dioxide pressure. Reaction of 1‐butyl‐2‐phenylaziridine with 0.5 mol% of 1 , 2.5 mol% of tetrabutylammonium iodide (TBAI) as a co‐catalyst at 1 bar CO 2 pressure under blue light for 24 h resulted 20% of the 5‐substituted oxazolidinone product with 100% regio‐selectivity (Table 1, entry 1).…”

“…[15] We also reported the use of BÀ N coordinated PAH based zinc-salen for the cycloaddition of CO 2 and epoxides. [16] Zinc-based complexes have been extensively used for the cycloaddition of CO 2 and epoxides. [17] How-ever, their use as catalysts for converting aziridines and CO 2 to oxazolidinones is scarcely reported.…”