Carbon Dioxide Insertion into Group 9 and 10 Metal-Element σ Bonds.

Carbon dioxide (CO2) is an appealing feedstock for the sustainable preparation of a variety of carbon-based commodity chemicals because of its high abundance, low cost, and nontoxicity. The high kinetic and thermodynamic stability of CO2, however, means that there are currently only a limited number of practical catalytic systems for the conversion of CO2 into more valuable chemicals, and continued research in this area is required. One promising approach for the eventual transformation of CO2 is to initially insert the molecule into transition-metal-element σ bonds such as M-H, M-OR, M-NR2, and M-CR3 bonds to form products of the type M-OC(O)E (E = H, OR, NR2, or CR3). CO2 insertion has been demonstrated in numerous stoichiometric reactions involving transition-metal complexes, but in cases where insertion results in the formation of strong M-O bonds, the products are often too stable to undergo further transformations. Group 9 and 10 transition-metal complexes (M = Ni, Pd, Pt, Co, Rh, or Ir) form relatively weak M-O bonds, and as a consequence, a number of group 9 and 10 transition-metal catalysts in which CO2 insertion is proposed as an elementary step in catalysis have been developed. In this Award Article, we summarize group 9 and 10 transition-metal complexes in which CO2 insertion into a metal-element σ bond to form a M-OC(O)E-type product has been observed. Mechanistic similarities and differences are highlighted by comparing CO2 insertion reactions in different types of group 9 and 10 metal-element σ bonds, and a general trend for predicting the rate-determining step of the insertion process is described based on the nucleophilicity of the element in the σ bond. Although we focus on stoichiometric reactivity, the relevance of CO2 insertion to catalytic reactions is also emphasized throughout the paper.