*2.2. Promotional E*ff*ect of CO2 on Oxidation of Cyclic Olefins*

Park et al. demonstrated the use of CO2 as a promoter for the oxidation of cyclic olefins with mesoporous carbon nitrides (CN) as a metal-free catalyst in the presence of molecular oxygen. Analysis of the surface characteristics of the catalyst after the reaction revealed the presence of carbamate, confirmed by a new band in the FTIR spectrum at 1419 cm−1. This measurement illustrated the incitation of CO2 owing to the accumulation of surface carbamate. This surface carbamate can then react with the cyclic olefins, assisted by the catalyst. After the reaction, the IR spectra showed the presence of extra bands at 2174 and 2115 cm−1, possibly due to a gaseous CO doublet. However, these absorption bands were not present before the reaction. This analysis exposed the production of CO, which is revealed to the increased catalytic activity to credit to carbon dioxide sharing as an 'oxygen atom' onset [31,32]. The production of CO was previously observed in nitrogen including heterocyclic systems [34–36]. The positive impact of CO2 in the oxidation of cyclic olefin was quantified by measuring the catalytic performance using various reactants, cyclopentene (*n* = 1), cyclohexene (*n* = 2), cyclooctene (*n* = 4), and cyclododecene (*n* = 8) (Table 2). The epoxide selectivity was greater in O2/CO2 than O2/N2, suggesting that in the presence of CO2, the mechanism may be altered to improve the conversion and selectivity. The blend of gaseous from the autoclave was studied by IR spectroscopy to better understand the positive impact of CO2. In the reaction with no oxidant and source oxygen, it was presumed that CO2 is reduced to CO and aldehyde is oxidized to carboxylic acid in the same process. The reaction may have occurred via the addition of carbon dioxide to the quickly produced Breslow intermediate A to produce the hydroxy carboxylate B and the tautomer C (Scheme 4) [34]. Possibly, the following intermediate can lose CO and hydroxide to support benzoic acid. Additionally, it was observed that intermediate D is supplicated in the oxidative esterification of aldehydes with CO. Interestingly, phenylglyoxylic acid was revealed to nucleophilic heterocyclic carbenes (NHC) under similar experimental conditions wherein phenylglyoxylic acid was switched to benzoic acid. (Scheme 5). Under mild experimental conditions, CO2 was utilized in an NHC-intermediated conversion of the aldehyde to the carboxylic acid.

**Entry n Gas Conversion of 3 (%) Selectivity (%) 456 ΔC (%)**


Reaction conditions: 20 mg Melamine mesoporous carbon nitride (M-MCN), 10 mL Dimethylformamide (DMF), temperature 373 K, Pressure 80 PSI, gas ratio 0.333, time 10 h; Produced analyzed by GC and GC-MS.

**Scheme 4.** Proposed Mechanism for aldehyde assisted CO2 to carboxylic acid process. (Reprinted from [34]; copyright (2010), American Chemical Society).

**Scheme 5.** Phenylglyoxylic acid to benzoic acid reaction with NHC-intermediate. (Reprinted from [34]; copyright (2010), American Chemical Society).
