Nucleophilic carbenes: Applications in homogeneous catalysis

Nucleophilic carbenes, typically 1,3-disubstituted imidazol-2-ylidenes are becoming important because they are phosphine mimics. The first isolation of stable nucleophilic carbene was discovered by the Arduengo, group in 1991. In an effort to clarify the importance of steric and electronic effects of this type of carbenes vs phosphines, solution calorimetry of the ruthenium system Cp*Ru(L)Cl (Cp* = pentamethylcyclopentadiene; L = nucleophilic carbenes or phosphines) has been investigated. Quantitative assessment of enthalpy data and structural studies of these complexes enable the determination of their stereoelectronic properties.;Olefin metathesis is one of the most important tools in modern organic synthesis. Catalytic activity, functional group tolerance and thermal stability are crucial to the efficiency of novel catalysts. The exchange of one PCy 3 ligand by bulky nucleophilic carbene ligands in Grubbs' catalyst, Cl2Ru(PCy3)2(CHR) (R = Ph and C=CMe 2), has been studied. RuCl2(L)(p-cymene) (L = nucleophilic carbene; p-cymene = 4-methyl-1-isopropylbenzene) and RuCl(IMes)(p-cymene)(C=C=CPh2) derived from nucleophilic carbene and the ruthenium dimer, [RuCl2(p -cymene)]2 have been found to be easily accessible metathesis catalysts. The resulting complexes were tested for common metathesis reactions such as ring closing metathesis (RCM) and ring opening metathesis polymerization (ROMP). The thermal stability of these complexes has also been studied.;Aryl-C and aryl-N bond formations are undoubtedly some of the most important steps in building organic molecules. Cross-coupling reactions for aryl-C bond formation has been well established in 70--80's, and amination of aryl halides (or equivalents) has been the focus of several research groups since 1994. However, using aryl chlorides as the chemical feedstock is problematic in most cases but would prove beneficial to industrial processes. Bulky nucleophilic carbenes are used as auxiliary ligands in palladium-catalyzed Suzuki, Kumada and amination couplings. These catalytic systems are highly efficient and convenient for aryl chlorides as well as bromides or iodides.