How To Enhance the Efficiency of Breslow Intermediates for SET

Oxidative carbene organocatalysis, which proceeds via single electron transfer (SET) pathways, has been limited by the moderately reducing properties of deprotonated Breslow intermediates BI-s derived from thiazol-2-ylidene 1 and 1,2,4-triazolylidene 2. Using computational methods, we assess the redox potentials of BI-s based on ten different types of known stable carbenes and report our findings concerning the key parameters influencing the steps of the catalytic cycle. From the calculated values of the first oxidation potential of BI-s derived from carbenes 1 to 10, it appears that, apart from the diamidocarbene 7, all the others are more reducing than thiazol-2-ylidene 1 and the 1,2,4-triazolylidene 2. We observed that while the reducing power of BI-s significantly decreases with increasing solvent polarity, the redox potential of the oxidant can increase at a greater rate, thus facilitating the reaction. The cation, associated with the base, also plays an important role when a nonpolar solvent is used; large and weakly coordinating cations such as Cs+ are beneficial. The radical-radical coupling step is probably the most challenging step due to both electronic and steric constraints. Based on our results, we predict that mesoionic carbene 3 and abnormal NHC 4 are the most promising candidates for oxidative carbene organocatalysis.

Automated summary

Using computational methods, the redox potentials of BI-s derived from different types of stable carbenes were assessed, and the key parameters influencing the catalytic cycle were studied. It was found that most carbenes have higher reducing powers than thiazol-2-ylidene 1 and the 1,2,4-triazolylidene 2. The solvent polarity influences the reducing power of BI-s, but the redox potential of the oxidant increases at a greater rate, facilitating the reaction. The cation associated with the base also plays a role, especially in nonpolar solvents, with large and weakly coordinating cations being beneficial. Based on the results, mesoionic carbene 3 and abnormal NHC 4 are suggested as promising candidates for oxidative carbene organocatalysis.