Direct Synthesis of Iron, Cobalt, Nickel, and Copper Complexes of N-Heterocyclic Carbenes by Using Commercially Available Metal Powders

摘要

The development of transition-metal-catalyzed organic transformations based on the first-row transition metals such as Fe, Co, Ni, and Cu is of importance because of their relatively low cost and toxicity relative to precious metals. The strongly σ-donating N-heterocyclic carbenes (NHCs) offer a good opportunity to tune the reactivity and selectivity of transition-metal catalysts. A large number of NHC complexes of precious metals have recently been prepared and characterized, and their catalytic applications in various organic transformations have been widely studied. In contrast, catalytic processes based on NHC complexes of non-noble metals such as manganese, iron, and cobalt have not been well studied. One of the reasons is that NHC complexes of the first-row metals have to be prepared from free carbenes or through reactions of basic metal acetates, alkoxides, or amides with imidazolium salts. The generation of free carbenes often requires special bases, and handling unstable free carbenes requires harsh conditions. The second route is also restricted because metal acetates, alkoxides, and amides are not always available, and such metal salts are moisture-sensitive. The most popular approach to noble-metal-NHC complexes is the transmetalation of NHCs from silver complexes that can be synthesized by treating Ag2O with the imidazolium salts. However, the carbene-transfer reactions of silver-NHC complexes with light metal salts such as Mn, Fe, and Co are not simple. Thus, the development of practical and convenient synthetic approaches to NHC complexes of non-noble metals is highly desired.