Carbon Dioxide As Raw Material, Green Organic Synthesis Research

The development of environmentally friendly process utilizing CO₂,which is the most abundant greenhouse gas and can be also regarded as a typical renewable natural resource,has led to the current interest in synthetic chemistry from the viewpoint of environmental protection and resource utilization.Synthesis of useful compounds viz.cyclic carbonates,oxazolidinones and dimethyl carbonate using CO₂ as starting material is one of the most promising methodologies in view of green chemistry and atom economy.Oxazolidinones are important heterocyclic compounds showing a large application in pharmaceutical or as intermediates and chiral auxiliaries in organic synthesis.The method of insertion of CO₂ into the aziridines moiety utilizing abundant,renewable and nontoxic CO₂ as a feedstock is promising from a green chemistry perspective.In this respect,numerous homogeneous catalysts have been developed including single and dual-component system.However,developing more environmentally benign heterogeneous catalysts for regio-selective synthesis of oxazolidinones will be more desirable.ZrOCl₂·SH₂O has drawn much attention because of its water-stablity,low toxicity,and commercial availablity,especially wide utilization as a catalyst in organic reactions.ZrOCl₂·8H₂O has also been found to be an efficient and recyclable heterogeneous catalyst for the cycloaddition reaction of a wide variety of aziridines to CO₂ under mild conditions without utilization of additional organic solvents or co-catalysts.As a result,5-aryl-2-oxazolidinone has been obtained in high yield with excellent chemo- and regioselectivity.Besides,the catalyst could be recovered by filtration and reused at least five times without significant loss of catalytic activity.Based on the experimental results,a plausible mechanism has been proposed.Dimethyl carbonate(DMC) is an important raw material in organic synthesis, which can be regarded as an environmentally benign solvent and a carbonylation, methoxylation or methylation reagent.Conventional preparation methods for the synthesis of DMC starting from phosgene or CO have been replaced by a "two-step transesterification" process.The first step of the process is the cycloaddition reaction of epoxides and CO₂ to synthesize the intermediate,cyclic carbonates;and the second step is the transesterification reaction of the cyclic carbonates and methanol to produce dimethyl carbonate and glycols.To eliminate the separation process of cyclic carbonate in the two-step manufacture,it is very desirable to integrate the two steps reaction into "one-pot reaction".However,the alcoholysis side-reaction of epoxides and methanol is serious for "one-pot reaction".Therefore,we propose a "sequential transesterification process" for synthesis of dimethyl carbonate.With quaternary ammonium salt functionalized chitosan catalyst as a heterogenous catalyst,which could catalyze both cycloaddition reaction of ethylene oxide and CO₂,and transesterification reaction of ethylene carbonate and methanol,the separation and purification process of the ethylene carbonate intermediate could be omitted.Besides, the catalyst could be recovered by filtration and reused at least five times keeping high catalytic activity,so the "sequential transesterification process" could be much greener.This single component catalyst has obvious advantages comparing with present two independent homogenous catalytic systems.