N‐Heterocyclic Carbenes with Anionic Functional Groups

Abstract

We have already seen that many functionalised NHC ligands are modelled on previously known functionalised phosphanes. In the case of an oxygen -containing functional group where the oxygen atom can carry a (formal) negative charge, we can expect functionalised NHC ligands that resemble phosphinophenols [ 1 , 2 ], phosphinoalcohols [ 3 -5 ] or phosphino carboxylic acids [ 6 ]. All these functionalised phosphane groups were employed as ligands in catalytic reactions.The rationale behind the concept of NHC ligands carrying anionic oxygen functional groups goes beyond the application in catalysis. NHC ligands do not bond strongly to the early transition metals [ 7 ] or alkaline earth metals [ 8 ]. However, both these groups of metals bind strongly to alkoxides or aryloxides. It is therefore reasonable to design functionalised NHC ligands with a view to an oxygen (nitrogen) based anchor group in addition to the somewhat loosely binding carbene unit. This results in the formation of a hemilabile chelate ligand [ 9 ].Conversely, NHC bind strongly to the late transition metals (group 9 and group 10), metals that have only a low affi nity to oxygen. Here, the oxygen -containing functional group serves as the hemilabile ligand [ 10 ]. We will therefore see a great versatility in the anionic tether and in the range of (transition) metal employed.Thiel and coworkers reacted imidazole with epoxycyclohexane to form the racemic hydroxycyclohexyl imidazole. Attempts to separate the enantiomers by kinetic resolution with lipase B of Candida antarctica and isopropenyl acetate as acylating agent [ 11 , 12 ] failed, but gave the racemic ester in high yields (see Figure 4.1 ). Alkylation was then Functionalised N-Heterocyclic Carbene ComplexesOlaf Kühl