Carbenes as catalysts

APPLICATIONS OF NUCLEOPHILIC CARBENES AS CATALYST PRECURSORS IN HOMOGENEOUS CATALYSIS... 

Abstract The use of iV-heterocychc carbenes as catalysts for organic transformations has received increased attention in the past 10 years. A discussion of catalyst development and nucleophilic characteristics precedes a description of recent advancements and new reactions using A-heterocyclic carbenes in catalysis. 

Stetter expanded Umpolung reactivity to include the addition of acyl anion equivalents to a,P-unsaturated acceptors to afford 1,4-dicarbonyls Eq. 5a [57-60]. Utilizing cyanide or thiazolylidene carbenes as catalysts, Stetter showed that a variety of aromatic and aliphatic aldehydes act as competent nucleophilic coupling partners with a wide range of a,p-unsaturated ketones, esters, and nitriles [61]. The ability to bring two different electrophilic partners... 

Connor EE, Nyce GW, Myers M, Mock A, Hedrick JL (2002) First example of N-heterocyclic carbenes as catalysts for living polymerization organocatalytic ring-opening polymerization of cyclic esters. J Am Chem Soc 124 914-915... 

Aryl chlorides are more reluctant to participate in amination than most other aryl halides/pseudohalides. To tackle this problem, Caddick et al. examined the effect of palladium-N-heterocyclic carbenes as catalysts in rapid microwave-promoted reactions [87]. Para-tolyl and -anisyl chloride were reacted with aromatic and aliphatic amines in mostly good yields within 6 minutes of heating at 160 °C. Reactions using anisyl, tolyl or phenyl chlorides and aliphatic amines have also been reported by Maes et al. using a phosphine ligand and a strong base, which creates the desired products after 10 minutes of heating at 110-200 °C [88]. 

Kantchev EAB, O Brien CJ, Organ MG (2007) Palladium complexes of A-heterocyclic carbenes as catalysts for cross-coupling reactions - a synthetic chemist s perspective. Angew Chem Int Ed 46 2768-2813 Kerr MS, Rovis T (2004) Enantio selective synthesis of quaternary stereocenters via a catalytic asymmetric Stetter reaction. J Am Chem Soc 126 8876-8877 Kerr MS, Read de Alaniz J, Rovis T (2002) A highly enantio selective catalytic intramolecular Stetter reaction. J Am Chem Soc 124 10298-1029 Kerr MS, Read de Alaniz J, Rovis T (2005) An efficient synthesis of achiral and chiral 1,2,4-triazolium salts bench stable precursors for A-heterocyclic carbenes. J Org Chem 70 5725-5728... 

Suzuki Y, Yamauchi K, Muramatsu K, Sato M (2004) First example of chiral A-heterocyclic carbenes as catalysts for kinetic resolution. Chem Comm 2004(23) 2770-2771... 

Organocatalytic ring-opening polymerization involving lactones, morpholine-2,6-diones, cyclosiloxanes, and oxadisilacyclohexanes as monomers and/or heterocyclic carbenes as catalysts 07CRV5813. 

Palladium complexes of N-heterocychc carbenes as catalysts for cross-couphng reachons 07AG(E)2768. 

Finally, other important contributions that should be cited as very relevant with regard to the evolution of the organocatalysis field are those related to the use of stable A-heterocyclic carbenes as catalysts. The basis of this chemistry also finds its roots at the very early beginnings of organic synthesis, in particular in the well-known cyanide-catalyzed benzoin condensation by Liebig and Wholer, and... 

Kantchev, E.A.B., O Brien, C.J. and Organ, M.G. (2007) Palladium complexes of N-heterocyclic carbenes as catalysts for cross-coupling reactions - a synthetic chemist s perspective. Angew. Chem. Int. Ed., 46, 2768-813. 

A very interesting catalytic reaction is metathesis, where alkylidenes are exchanged between alkenes. In the classic Chauvin mechanism, an aUgrlidene fragment is cleaved from the alkene substrate and transferred to the metal via the process of equation 24, which requires a metal carbene as catalyst. This has been applied to ring-closing reactions like equation 27 and polymerizations like equation 28. 

Bode demonstrated the effectiveness of A -heterocyclic carbenes as catalysts for inverse electron demand hetero Diels-Alder reactions involving both azadienes and oxodienes. In the oxodiene case, reaction of enones with a-chloroaldehydes affords substituted dihydropyranones. In one example, addition of catalyst 60 to chloroaldehyde 58 followed by elimination of HCl yields the electron-rich dienophile that readily combines with oxodiene 59 to selectively fiimish cycloadduct 61. ... 

C. S. Junker, M. E. Welker, Tetrahedron 2012, 68, 5341-5345. Ruthenium carbenes as catalysts in stereoselective ene-yne metathesis/Diels-Alder and ene-yne metathesis/ Diels-Alder/cross coupling multicomponent reactions. 

Katz and Lee [34] supported this polymerization mechanism by accomplishing acetylene polymerization with metal carbenes as catalysts. Further, the... 

Carbenes as catalysts. The enantioselectivity of the Stetter reaction between 4-Cl-C6H4CH=0 and iV-acylamido acrylate CH2=C(NHAc)C02Me, catalysed by NHC, is known to be dramatically improved by adding Bu OK. This effect has now been studied by DFT(M06-2X) methods and found to stem from binding the K" " ion in the transition state, which lowers the activation energy by 24.2 kcal mol . An intermolecular... 

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