NHCs , applications other reactions

This article presents the principles known so far for the synthesis of metal complexes containing stable carbenes, including the preparation of the relevant carbene precursors. The use of some of these compounds in transition-metal-catalyzed reactions is discussed mainly for ruthenium-catalyzed olefin metathesis and palladium-Znickel-catalyzed coupling reactions of aryl halides, but other reactions will be touched upon as well. Chapters about the properties of metal- carbene complexes, their applications in materials science and medicinal chemistry, and their role in bioinorganic chemistry round the survey off. The focus of this review is on ZV-heterocyclic carbenes, in the following abbreviated as NHC and NHCs, respectively. 

NHC-catalyzed reactions are unique in organic synthesis, and very useful for the construction of carbon-carbon bonds. Great success has been made for the NHC-catalyzed benzoin condensation, Stetter reactions, and a -d Umpolung reactions in the past decades. NHC catalysis has also hnd application in many other reactions, such as umpolung of Michael acceptors, Morita-Baylis-Hilman reaction, Michael additions, redox reaction, and reactions of ketenes. With the rapid development of NHC catalysis, more reactions will surely be found, and the wide applications in organic synthesis could be expected. 

An example of the application of a hydrazide in this general s5mthesis, amongst others 91, 391, 392, 416), is the reaction of isonicotinic acid hydrazide and cyanoguanidine in the presence of acid to yield isonico-tinoylaminobiguanide [C5H4CO NHNH C( NH) NHC NH) NHg] 391, 392, 416). Long-chain fatty acid hydrazides react similarly (655). 

Widely applicable is the last pathway shown in Scheme 4, the transmet-allation reaction . Although other reagents have been proposed [68], the use of Ag20 as introduced by Lin [69,70] is currently the most important pathway for the synthesis of NHC metal complexes. The imidazofiiun halide forms a silverhafide-NHC complex which then transfers the carbene ligand to the metal precursor. Scheme 4 shows a general representation of possible pathways for the synthesis of transition metal NHC complexes. 

One of the novel exiting developments in the field of tin-free radical chain reactions is the development of complexes of borane with NHCs as HAT reagents [6]. Borane (BH3) itself has a BDE of 106.6 kcal mol 1 which is much too high for its use in organic radical chain reactions. The group of Roberts and others have demonstrated that complexes of boranes with amines and phosphines have a reduced BDE and that they can be used in radical chain reactions [7]. However, the reduction is only moderate and too low to make these complexes generally applicable. 

This volume provides the reader with the most important and exiting results pertaining the use of NHC complexes in transition-metal catalysis. Following an introductory chapter, which deals with the properties of NHC compounds and discusses some insightful examples, routes to NHC complexes will be described, a prerequisite for doing catalysis. Chapters on NHC complexes in oxidation chemistry and in metathesis reactions are accompanied by a chapter on palladium-catalyzed reactions and another on catalysis by other metals. Finally, this book would be incomplete without treating applications in asymmetric catalysis, which rounds out this volume. 

Metal complexes of NHCs are extremely important as catalysts in reactions of importance to the field of organic synthesis. Section 10-1-1 has already addressed the electronic and steric properties of NHC ligands that enhance the catalytic activity of NHC-metal complexes, and section 10-2-3 discussed methods of synthesis. Very little chemistry, however, occurs at Ccarbene. Instead, NHCs act mainly as supporting ligands, so that useful chemistry can take place at other positions on the catalyst complex. We will see applications of the use of NHC-metal complexes for synthesis in Chapters 11 and 12. 

During their studies on the application of Pd°-NHC complexes in the Heck reaction, Cavell and co-workers reported the formation of 2-arylimidazolium salts (amongst other organic and inorganic products) upon reaction of Pd -his(NHC) complexes with phenyl iodide. The presence of 2-arylimidazolium salts suggests that the Pd°-bis(NHC) complexes are oxidatively adding the phenyl iodide successfully, and that this process is accompanied hy a reductive elimination process to form a 2-arylimidazolium salt (Fig. 8). 

The remarkable catalytic performances shown by M-NHC complexes (compatibility with many reagents, reaction conditions, other catalysts, and activation modes) have allowed their increasing application in MCRs. In this section, the most representative and recent outcomes developed in this field are presented. With the aim of facilitating the reading, this section has been divided according to the different transition metals used and the specific MCR described. 

Carbon-carbon cross-coupling reactions dominate catalytic applications of NHC-Pd complexes, demonstrating the importance of this class of reaction and the unique suceess of Pd eompounds in these transformations. In the past 10 years, there has been a substantial body of work published on cross-coupling reactions catalysed by NHC-Pd complexes therefore, to provide some degree of structure to this ehapter, reactions have been broadly grouped under Heck, Suzuki and other types of reaction categories. Not surprisingly there is overlap between sections, and many publications report more than one class of reaction, however the layout does provide a coherent structure. 

---