Effect of Substituent on Phenanthroline Ligands in Zinc Complex-catalyzed Synthesis of Organic Carbamate from CO2 Using Tetramethyl Orthosilicate

In the synthesis of organic carbamate from CO2 using Si(OMe)4, which is a regenerable reagent, and a Zn(OAc)2/1,10-phenanthroline (phen) catalyst, the effect of substituents on the phen ligands was evaluated to further improve the reaction efficiency and to elucidate linker introduction sites for catalyst immobilization. Therefore, the synthesis of methyl phenylcarbamate was investigated using a Zn(OAc)2 complex with a phen-derivative ligand bearing an electron-donating or electron-withdrawing group, (X-phen)Zn(OAc)2, as the catalyst, and its catalytic activity was compared to that of (phen)Zn(OAc)2. Consequently, catalysts with 2,9-substituted and 4,7-halogenated X-phen ligands (X-phen=2,9-dimethyl-1,10-phenanthroline, 2,9-dibutyl-1,10-phenanthroline, 4,7-dichloro-1,10-phenanthroline, and 4,7-dibromo-1,10-phenanthroline) were demonstrated to be significantly less efficient than (phen)Zn(OAc)2. For the other (X-phen)Zn(OAc)2 catalysts (X-phen = 4,7-bis(dimethylamino)-1,10-phenanthroline, 4,7-dimethoxy-1,10-phenanthroline, 4,7-dimethyl-1,10-phenanthroline, 3,4,7,8-ttramethyl-1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline, and 5-nitro-1,10-phenanthroline), the carbamate yield did not change significantly compared to that obtained with (phen)Zn(OAc)2 regardless of whether the substituent introduced into the phen ligand was an electron-donating or electron-withdrawing group. The above information on the positions of substituents and types of functional groups that do not affect the carbamate synthesis reaction is useful for introducing a linker on the phen ligand to immobilize the catalyst, and will contribute to further improvement of the efficiency of the carbamate synthesis.