NHC-catalysed reaction

Abstract This chapter focuses on carbon monoxide as a reagent in M-NHC catalysed reactions. The most important and popular of these reactions is hydro-formylation. Unfortunately, uncertainty exists as to the identity of the active catalyst and whether the NHC is bound to the catalyst in a number of the reported reactions. Mixed bidentate NHC complexes and cobalt-based complexes provide for better stability of the catalyst. Catalysts used for hydroaminomethylation and carbonyla-tion reactions show promise to rival traditional phosphine-based catalysts. Reports of decarbonylation are scarce, but the potential strength of the M-NHC bond is conducive to the harsh conditions required. This report will highlight, where appropriate, the potential benefits of exchanging traditional phosphorous ligands with iV-heterocyclic carbenes as well as cases where the role of the NHC might need re-evaluation. A review by the author on this topic has recently appeared [1]. 

Ruthenium-NHC complexes exhibit activity in a very wide field of applications. Due to their unique ability to break and reassemble olefin bonds under reaction conditions very favourable to design simple processes, applications in nearly any chemical discipline can be foreseen. This field may span from manufacturing of specialty polymers and rabbers to pharmaceuticals, pharmaceutical intermediates, agrochemicals, fragrances, dyes, specialty chemicals for electronic applications or fine chemicals from natural feedstock and many more. Below are described Ru-NHC catalysed reactions applied from pilot to full commercial scale. 

N-heterocyclic carbene (NHC) catalysed reactions have greatly increased in popularity since 2004, when several groups reported reactions other than the... 

In 2013, Biju and coworkers developed the NHC-catalysed reactions of enolisable aldehydes with a,p-unsaturated acylazoliums generated from 2-bromoenals, affording dihydropyrones 96 in good yields and with excellent enantioselectivities (Scheme 20.42). 

Due to the unique bivalent carbene and diversity of the N-heterocyclic motif, NHCs have been demonstrated to be efficient organocatalysts for various enantioselective reactions. In addition to the traditional thioazolium and imidazolium NHCs, triazolium NHCs have become the most successful organocatalysts. Recently, NHC/Lewis acid cocatalysis and bifunctional NHCs have shown a very promising future. Beyond the classic NHC-catalysed umpolung of aldehydes, the extended umpolung of functionalised aldehydes are extremely successful. A series of NHC-catalysed reactions of ketenes have been developed for the synthesis of various enantioriched heterocycles. Esters, anhydrides, carboxylic acids and even Michael acceptors are useful alternative substrates for NHC-catalysed reactions. With increasing interest and rapid development of NHC catalysis, new structures of the catalysts, new reaction modes, and synthetic applications can be expected in the near future. 

In a related way, c A-enals have been shown/or the first time to undergo NHC-catalysed reactions with a,/ -unsaturated imines to give five-membered cyclic derivatives (137) in an asymmetric manner, interestingly enough, cis- and trans-ensih have been found to exhibit distinct reactivity profiles under the so-established reaction conditions. 

NHC-catalysed reactions involving alkynals have great potential but surprisingly few have been documented, with only two applications being reported in 2012. First, 2 equiv... 

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