Hydrogen bond donors promoted organocatalyzedcycloaddition of CO<sub>2</sub> with epoxides

Abstract

Listen

Translate article

Carbon dioxide (CO2) is the greenhouse gas which is generated by the consumption of petrochemicals or fossil resources. CO2 is an abundant and renewable C1 building block. The utilization of CO2 as a C1 raw material in organic synthesis is an environmentally friendly method, which can not only solve the environmental and climate problems, but also facilitates the energy transformation from fossil materials to renewable resources. Thus, the research to transform CO2 into valuable chemicals has attracted attention in the field of green and sustainable chemistry. In the field, cyclic carbonates generated from CO2 and epoxides has been widely investigated. Cyclic carbonate is an important chemical raw material, which is widely applied in high-boiling polar aprotic solvents and electrolytes in batteries. The method of preparing cyclic carbonate with CO2 and epoxides as raw materials is a green synthetic transformation of 100% atomic economy for the utilization of greenhouse gas CO2. There are two kinds of catalysts for cycloaddition, including metal-based catalysts and organocatalysts. Compared with metal catalyst, organic catalysts have the advantages of low cost, simple synthesis steps and no metal contaminants, but harsh reaction conditions and/or higher catalyst loading are often needed. In order to overcome this problem, introduction of hydrogen bond donor to catalytic systems has been prevalent and has provided a simple and efficient protocol for synthesis of cyclic carbonates from CO2 and epoxides. Recent developments in the area of hydrogen bond donor functionalized organocatalysts have demonstrated that the acidity of hydrogen bond donor is crucial for achieving the boosting activity of catalysts in the cycloaddition of CO2 with epoxides. Hydroxyl functionalized organocatalysts are one of the earliest and most widely studied organocatalysts promoted by hydrogen bond donors in the cycloaddition reaction of CO2 with epoxides. The literature results showed that the organocatalysts bearing phenol hydroxyl group showed higher catalytic activity than those with alcohol hydroxyl group, which may be the reason of the stronger acidity of phenol hydroxyl group compared with alcohol hydroxyl group. Increasing the acidity of hydrogen bond donor in certain scale can effectively improve its catalytic activity, but further to increase acidity of hydrogen bond donor may lead to the decrease of catalytic activity. This is because properly increasing the acidity of hydrogen bond donor is beneficial for enhancing the hydrogen bond interaction between hydrogen bond donor and epoxide, thus promoting the ring-opened step of epoxide. However, the excessive enhancement of acidity will lead to too strong hydrogen bond interaction to induce the finally ring-closed step of intermediates. Therefore, the acidity of hydrogen bond donor is very important to the activity of catalyst. In addition to the traditional hydrogen bond donors, some special hydrogen bond donors, such as water and azaphosphatranes, are also employed as hydrogen bond donors in the cycloaddition reaction of CO2 with epoxides. It is particularly noteworthy that water is a kind of abundant and environmentally friendly hydrogen bond donor. All these indicate that the development of a kind of abundant, commercially available, and environmentally friendly hydrogen bond donor is one of the hot topics in the design and synthesis of high efficiency organocatalysts in the future. In this review, the advancement on cycloaddition of CO2 with epoxide promoted by hydroxyl group, carboxyl group, amino group, multi-hydrogen bond donor and special hydrogen bond donor has been summarized over the years. We expected the knowledge from these literatures to provide insight to the rational design of highly active organocatalysts.