Carbene catalysis reactions

The drawback of the CVD method is eliminated in ROMP, which is based on a catalytic (e.g., molybdenum carbene catalyst) reaction, occurring in rather mild conditions (Scheme 2.3). A living ROMP reaction ofp-cyclophanc 3 or bicyclooctadiene 5 results in soluble precursors of PPV, polymers 4 [31] and 6 [32], respectively, with rather low polydispersity. In spite of all cis (for 4) and cis and trans (for 6) configuration, these polymers can be converted into aW-trans PPV by moderate heating under acid-base catalysis. However, the film-forming properties of ROMP precursors are usually rather poor, resulting in poor uniformity of the PPV films. 

Enantiomerically pure copper and rhodium complexes enable enantioselective catalysis of carbene-mediated reactions. Such reactions will be discussed more thoroughly in Section 4.2. Experimental Procedure 4.1.1 describes the preparation of an enantiomerically pure rhodium(II) complex which has proven efficient for catalysis of different types of carbene complex-mediated C-C-bond-forming reactions with high asymmetric induction. 

Keywords Absolute configuration, Amines, Amino acids, Carbenes, Cascade reactions, 2-chloro-2-cyclopropylideneacetates. Combinatorial libraries. Cycloadditions, Cyclobutenes, Cyclopropanes, Diels-Alder reactions. Heterocycles, Michael additions. Nitrones, Nucleophilic substitutions, Peptidomimetics, Palladium catalysis. Polycycles, Solid phase synthesis, Spiro compounds. Thiols... 

Transition-metal catalysis, especially by copper, rhodium, palladium and ruthenium compounds, is another approved method for the decomposition of diazo compounds. It is now generally accepted that short-lived metal-carbene intermediates are or may be involved in many of the associated transformations28. Nevertheless, these catalytic carbene transfer reactions will be fully covered in this chapter because of the close similarity in reaction modes of electrophilic carbenes and the presumed electrophilic metal-carbene complexes. 

Enders D, Niemeier O, Balensiefer T (2006c) Asymmetric intramolecular cros-sed-benzoin reactions by N-heterocyclic carbene catalysis. Angew Chem Int Ed Engl 45 1463... 

Enders D, Kallfass U (2002) An efficient nucleophilic carbene catalyst for the asymmetric benzoin condensation. Angew Chem Int Ed 41 1743-1745 Enders D, Niemeier O (2004) Thiazol-2-ylidene catalysis in intramolecular crossed aldehyde-ketone benzoin reactions. Synlett 2004 2111-2114 Enders D, Niemeier O, Balensiefer T (2006) Asymmetric intramolecular crossed-benzoin reactions by N-heterocyclic carbene catalysis. Angew Chem Int Ed 45 1463-1467... 

With respect to the application of asymmetric carbene catalysis as a tool for enantioselective synthesis, the last decade s major success is based on substantial improvements in catalyst development. Early reports dealt with implementing chirality in thiazolium scaffolds (Sheehan and Hunneman 1966 Sheehan and Hara 1974 Dvorak and Rawal 1998), but their catalytic performance suffered from either low yields or low ee-values. In this regard, the investigation of triazole heterocycles as an alternative core structure (Enders et al. 1995) has played a crucial role to provide heterazolium precatalysts improving both asymmetric benzoin and Stetter reactions. An intramolecular Stetter reaction yielding chromanones upon cyclization of salicylaldehyde-derived substrates is commonly used as a benchmark reaction to compare catalyst efficiency (Scheme 1 Ciganek 1995 Enders et al. 1996 Kerr et al. 2002 Kerr and Rovis 2004). 

Within the field of nucleophilic carbene catalysis, with the exception of umpolung-directed transformations (see below), one of the most important classes in terms of applications already realized are transesterifications (Scheme 5 Grasa et al. 2003 Nyce et al. 2002 Singh et al. 2004) and related reactions (Dove et al. 2006). 

The second major class of non-umpolung nucleophilic carbene catalysis comprises reactions by initial NHC-activation of various silicon compounds. Their proposed common pathway is thought to lead to a hypervalent silicon complex4 and thus provide carbene-catalyzed activation of the corresponding nucleophiles such as TMSCN, TMSCF3 etc. (Kano et al. 2006 Song et al. 2005 2006). It is not only certain carbon-silicon bonds that can be effectively activated, but a comparable activation of Si-O bonds, e.g. of trimethylsily enol ethers etc., allows for mild, NHC-promoted Mukaiyama aldol reactions (Scheme 6 Song et al. 2007). 

Zeitler K (2006) Stereoselective synthesis of (E)-afi—unsaturated esters via carbene-catalyzed redox esterification. Qrg Lett 8 637-640 Zeitler K, Mager I (2007) An efficient and versatile approach for the immobilization of carbene precursors via copper-catalyzed [3+2]-cycloaddition and their catalytic apphcation. Adv Synth Cat 349 1851-1857 Zhao GL, Cordova A (2007) A one-pot combination of amine and heterocyclic carbene catalysis direct asymmetric synthesis of fi-hydroxy and fS-malonale esters from a,fS-unsaturated aldehydes. Tetrahedron Lett 48 5976-5980 Zhou ZZ, Ji FQ, Cao M, Yang GF (2006) An efficient intramolecular Stetter reaction in room temperature ionic liquids promoted by microwave irradiation. Adv Synth Cat 348 1826-1830... 

During the past few decades, a wide variety of molecules with transition metal-carhon mulhple bonds have been studied. The chemistry of doubly bonded species - carbenes - is particularly interesting because it leads to several synthetically important transformations, and for this reason, metal carbenes are the main subject of this chapter. Our discussion begins with a classification of metal-carbene complexes based on electronic structure, which provides a way to understand their reactivity patterns. Next, we summarize the mechanistic highlights of three metal-carbene-mediated reactions carbonyl olefinafion, olefin cyclopropanafion, and olefin metathesis. Throughout the second half of the chapter, we focus mainly on ruthenium-carbene olefin metathesis catalysts, in part because of widespread interest in the applications of these catalysts, and in part because of our expertise in this area. We conclude with some perspectives on the chemistry of metal carbenes and on future developments in catalysis. 

In addition to simple enantioselective two-component tandem multi-organocatalysed reactions, several groups have recently reported multi-component reactions based on the enantioselective sequential multiorganocatalysis concept. For example, Cordova et al. disclosed in 2011 a highly enantioselective one-pot cascade sequence based on the combination of asymmetric amine catalysis and iV-heterocyclic carbene catalysis, which allowed the synthesis of a range of chiral N-tert-butox-ycarbonyl and N-carbobenzyloxy-protected (3-amino acid esters to be... 

Scheme 7.14 Annulation reaction catalysed by chiral JV-heterocyclic carbene catalysis and titanium catalysis.

Scheme 4.41 shows the reaction of Ph2SiH2 with CO2 under imidazolium-carbene catalysis [178-180]. 

Carbene Catalysis Beyond the Benzoin and Stetter Reactions... 

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