Metal-catalyzed MCRs

Considering the practical importance of isocyanide-free MCRs, metal-catalyzed multicomponent reactions (MC-MCRs) are ideal candidates for polymer synthesis. Among the growing number of MC-MCRs, the three-component reaction between amines, aldehydes, and acetylenes (A -coupling) should be addressed. A -coupling... 

In recent years, many novel MCRs - including Michael addition-initiated three-component domino sequences [10], Knoevenagel/hetero-Diels-Alder-based MCRs [11], radical chain MCRs [12], transition metal-catalyzed Pauson-Khand MCRs [13], as well as Petasis MCRs [14], have been added to the chemisf s armamentarium and successfully applied to all fields of organic synthesis. 

Another approach is the use of multicomponent reactions (MCRs) to rapidly and efficiently construct structurally complex and varied polycyclic natural product-like compounds (Figure 1.5). A number of synthetic transformations played a key role in the rapid assembly of such molecules including isocyanide-based reactions, aza- and non-aza [4+2] cycloadditions, [3+2] cycloadditions, and dansition-metal-catalyzed reactions. Using isocyanide-based MCRs, pyrrolopyridines exemplified by mappicine represent an atttactive library target for their biological activity. Furthermore, the furoquinoline alkaloid tecleabine represents a common quinoline alkaloid core similar to strnctnres fonnd in a polycyclic library. ... 

For representative reviews on metal-catalyzed MCRs, see (a) D. M. D Souza, T. J. J. Muller, Chem. Soc. Rev. 2007, 36, 1095-1108. Multi-component syntheses of heterocycles by transition-metal catalysis, (b) J. A. Varela, C. Saa, Synlett 2008, 2571-2578. Recent advances in the synthesis of pyri-dines by transition-metal-catalyzed [2+2+2] cycloaddition, (c) H. Clavier, H. PeUissier, Adv. Synth. Catal. 2012, 354, 3347-3403. Recent developments in enantioselective metal-catalyzed domino reactions. 

Although most of the established metal-catalyzed MCRs are based on palladium or copper catalysts, the search for new MCR products has resulted in the development of new catalytic systems in the last decade. This chapter has been... 

Other examples of group 11 metal-catalyzed MCRs besides those mentioned in previous sections are listed in the following text. 

GROUP 8 METAL-CATALYZED MCRs 3.6.1 Iron-Catalyzed MCRs... 

Due to the increased interest for the utilization of cheap and environmentally friendly catalysts, during the last years, new variants of previously described metal-catalyzed MCRs have been developed employing iron-based catalysts. 

Metal-Catalyzed MCRs Involving NHCs as Ligands... 

The employment of MCRs is expected to overcome the drawbacks of click reactions because many MCRs proceed with common functional compounds such as aldehydes, ketones, and amines, which are commercially available in most cases. Hence, polymer chemists are currently examining and starting to take advantage of the synthetic utility of MCRs for polymer synthesis [42, 43]. Although the main MCRs used in polymer science are isocyanide-based reactions such as the Passerini and Ugi reactions, this chapter will focus on metal-catalyzed MCRs in the area of polymer chemistry. 

Prior to discussing the current state of MCRs in polymer S3mthesis, the classification of MCRs should be addressed. In spite of the large number of MCRs, these reactions can be divided into three groups (1) isocyanide-based MCRs (IMCRs), (2) non-isocyanide-based MCRs, and (3) metal-catalyzed MCRs (MC-MCRs). The MCR classification and a few examples are listed in Scheme 2. 

The last category is the class of MCRs catalyzed by organometallic species (MC-MCRs). MC-MCRs include the palladium-catalyzed multicomponent reaction of vinyl compounds [52], the three-component Sakurai allylation reaction [53], metal-catalyzed three-component reactions between alkynes, amines, and... 

The advantages of metal-catalyzed MCRs include not only their broad compatibility with various functimiatities and high reliability of reaction yields, but also easy handling of reactions. In addition, MCRs allow chemists to install two or more functionalities into polymer stractures simultaneously. Metal-catalyzed MCRs are in most cases free from isocyanides, which is crucially advantageous for non-experts who wish to employ MCR-based functionalization techniques. This particular advantage of MCRs over classical click reactions should be addressed in the field of polymer chemistiy. Although a number of metal-catalyzed MCRs have... 

Thankfully, numerous experts have kindly agreed to provide up-to-date chapters on several aspects in the area, thereby contributing to a collection of recent developments. In the first chapter, Ryohei Kakuchi discusses the use of metal-catalyzed MCRs for the synthesis of polymers. Special attention is laid to Cu-catalyzed developments. In the second chapter, Rongrong Hu and Ben Zhong Tang summarize developments of multi-component polymerizations of alkynes and... 

With the rich chemical properties of alkyne monomers, polymers with rare and complicated stmctures can be designed and synthesized via alkyne-based polymerizations. For instance, polyamidines are a series of important polymers with potential roles as biomaterials, metallic conductors, and photosensitive semiconductors their syntheses are limited to two-component, step-growth polymerization reactions [37, 38]. An efficient Cu-catalyzed MCR of alkynes, sulfonyl azides, and amines was recently reported by Chang and coworkers to afford a library of small-molecule amidines [3, 5], The reaction undergoes a Cu-catalyzed azide-alkyne... 

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