Pincer-type complexes applications

K. J. Szabo, O. F. Wendt, Pincer and Pincer-Type Complexes Applications in Organic Synthesis and Catalysis, Wiley-VCH, Weinheim, 2014. 

Pincer and Pincer-Type Complexes Applications in Organic Synthesis and Catalysis, First Edition. 

As illustrated by other chapters of this book, a diverse array of pincer and pincer-type ligands are known in the literature and can be incorporated to make new nickel complexes. Their catalytic activity in various cross-coupling reactions is likely to be the focus of future research. Another research direction that deserves more attention is carbon-heteroatom bond-forming reactions very few nickel pincer complexes have been studied for these catalytic applications. 

Even though various C-C cross-coupling reactions have been successfully catalyzed with palladium pincer complexes, often with excellent performances [22], their primarily application nowadays lies in the Heck reaction, of which the role of pincer-type catalysts in this reaction still is unclear and therefore under debate. 

Hahn, F.E., Jahnke, M.C. and Pape, T. (2007) Synthesis of pincer-type bis(benzimidazolin-2-ylidene) palladium complexes and their application in C-C coupling reactions. Organometallics, 26,150-4. 

One of the earliest applications of pincer complexes in catalysis is the ATRA of polyhaloalkanes (mainly CCI4, but also CBt4 or CF3CCI3) to alkenes, also known as the Kharasch reaction (Eq. (2.8)) [33, 87]. Nickel complexes of type 10 (containing... 

The first example of the application of pincer complex catalysts for selective synthesis involves aldol type of reactions involving imines and isocyanoacetates. In aldol [13-27] and Michael addition [13, 28-34] reactions, palladium pincer complexes have usually been employed as Lewis acid catalysts, for the first time by Richards and coworkers [13, 28]. This also means that the palladium(II) atom in these processes is not involved in redox reactions, and therefore reduction to palladium(O) does not happen. The aldol and the Michael reactions are useful C-C bond-forming reactions. In these processes, new stereocenters are generated, and therefore the processes can be used for stereoselective synthesis. A typical example is the reaction of suUbnimines (2) with isocyanoacetate (3) to form imidazohne (4) derivatives. The imidazolines can be easily hydrolyzed to diamino acid derivatives in a one-pot process (Figure 4.2) [24]. 

Mechanistic studies performed with Freeh s pincer catalyst in the Heck reaction excluded catalytic cycles with the involvement of homogeneous palladium(O) species, as indicated by the results obtained from the (recently developed) dibenzyl-test, which is directly applicable under the reactions conditions applied [24aj. Dibenzyl formation was - in contrast to Heck reactions catalyzed by palladium(O) complexes of type [Pd(PR3)2, where Pd /Pd" cycles are operative - not detectable by gas chromatography-mass spectrometry (GC/MS) when reaction mixtures of aryl bromide, olefin, benzyl chloride ( 10 mol% relative to aryl bromide), catalyst, and base were thermally treated. On the other hand, experimental observations, such as quantitative poisoning experiments with metallic mercury and CS2, which were shown to eflfidently inhibit catalysis, as well as analysis of the reaction profiles showed sigmoidal-shaped kinetics with induction periods and hence indicated that palladium nanoparticles are the catalytically active form... 

The particular features of phosphonium salts were exploited for a number of synthetic applications in 2014. Phosphonium chloride salts found applications as chlorine source and as modifiers for homogeneous catalyst systems. As an example, Muller, Rosenthal and co-workers reported the study of a chromium-based catalyst for the selective tri-merization of ethylene. A phosphonium precursor of the type i cyclo-(PR2CH2CH(OH) )2][Br]2) was used for the preparation of iron(n) complexes containing unsymmetrical P-N-P pincer ligands (Scheme 5). The group of Prof. Morris tested these compounds as catalysts for the asymmetric hydrogenation of ketones and imines. ... 

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