Organocatalysts carbenes

Abstract A-Heterocyclic carbenes (NHCs) have developed into an important class of ligands in transition metal coordination chemistry. They have been employed successfully as spectator ligands in various catalytically active metal complexes and as organocatalysts. In this chapter we present some important synthetic methods for the preparation of various NHCs and their metal complexes. 

Optimized reaction conditions call for the use of Wilkinson s catalyst in conjunction with the organocatalyst 2-amino-3-picoline (60) and a Br0nsted add. Jun and coworkers have demonstrated the effectiveness of this catalyst mixture for a number of reactions induding hydroacylation and C—H bond fundionalization [25]. Whereas, in most cases, the Lewis basic pyridyl nitrogen of the cocatalyst ads to dired the insertion of rhodium into a bond of interest, in this case the opposite is true - the pyridyl nitrogen direds the attack of cocatalyst onto an organorhodium spedes (Scheme 9.11). Hydroamination of the vinylidene complex 61 by 3-amino-2-picoline gives the chelated amino-carbene complex 62, which is in equilibrium with a-bound hydrido-rhodium tautomers 63 and 64. 

The KR of aryl alkyl sec-alcohols using various chiral nucleophilic N-heterocyclic carbenes (NHCs) has also recently been achieved by the groups of Suzuki [125, 126] and Maruolca [127]. These studies build on an emerging body of information showing that achiral NHCs are extremely efficient nucleophilic organocatalysts for transesterification [128]. The levels of selectivity achieved by Maruoka using the Cj-symmetric NHC 29b for the KR of aryl alkyl sec-alcohols (and two allylic alcohols) lie in the range 16 to 80 (Scheme 8.8). 

Abstract. N-Heterocyclic carbenes (NHC) have become an important class of organocatalysts and class of ligands for transition-metal catalysis. In organocat-alyzed umpolung reactions, thiazolium salt-derived NHC have been used successfully for decades. Even so, during recent years there has been an increased interest in NHC-catalyzed transformations and many new reactions have been developed. This article focuses on the use of NHC in the conjugate umpolung of ,f>-unsaturated aldehydes. 

N-Heterocyclic Carbenes Organocatalysts Displaying Diverse Modes of Action... 

Abstract. Within the context of Lewis base catalysis /V-heterocyclic carbenes represent an extremely versatile class of organocatalyst that allows for a great variety of different transformations. Starting from the early investigations on benzoin, and later Stetter reactions, the mechanistic diversity of /V-hctcrocyclic carbenes, depending on their properties, has led to the development of several unprecedented catalytic reactions. This article will provide an overview of the versatile reactivity of A-heterocyclic carbenes. 

In this chapter, an overview of the different modes of action of NHC carbenes and their impact on the catalytic availability of certain classes of reactive intermediates will be provided, prefaced by a short discussion of the current state of the art concerning the preparation of (chiral) heterazolium catalysts including methods for the immobilization of this versatile class of organocatalyst. 

Marion N, Diez-Gonzalez S, Nolan SP (2007) N- Ielcrocyclic carbenes as organocatalysts. Angew Chem Int Ed 46 2988-3000 Matsumoto Y, Tomioka K (2006) C2 symmetric chiral /V-heterocyclic carbene catalyst for asymmetric intramolecular Stetter reaction. Tetrahedron Lett 47 5843-5848... 

Note NHC can be used as organocatalysts in carbene catalysed nucleophilic acylation processes. 

Marion N, Diez-Gonzales S, Nolan SP (2007) N-heterocyclic carbenes as organocatalysts. Angew Chem Int Ed 46 2988-3000... 

Chiral N heterocyclic carbenes (NHCs), as Lewis basic organocatalysts, have been synthesized and applied to enantioselective organocatalytic reactions in recent years. Encouraged by Sheehan and Hunneman s first report of chiral thiazolium salts as NHC precursors for organocatalytic reactions [37], Leeper, Enders, Rovis, Glorius, Herrmann, and others have synthesized series of novel chiral NHCs with mono cyclic, bicyclic, or tricyclic backbones [38]. Recently, a series of bifunctional NHCs were synthesized and applied to aza BMH reaction of cyclopent 2 enone with... 

Recently, the reactivity (base, nucleophile, catalyst) and the role in modem organic synthesis of N-heterocyclic carbenes (cychc carbenes bearing at least one amino substituent) has been set-up by Ender et al. [60,61]. The possible utilization of NHCs as ligands for transition-metal catalyst [62,63] and as organocatalyst has been emphasized [61]. The inversion of the normal reactivity (umpolung) induced via NHCs has been extensively discussed. Classical carbon-carbon-bond-forming reactions (benzoin condensation, Stetter reaction, etc.) have been re-examined using ionic liquids as pre-catalysts in the presence of bases. 

Since the bulk properties of PLA are highly dependent on the stereoregularity or the tacticity of the polymer, i the development of catalysts for rac-lactide (or meso-lactide) polymerization has been focused on achieving stereoselectivity. 2 2 " Numerous organocatalysts such as A-heterocyclic carbenes (NHCs) and phosphine-based compounds " have been investigated for the controlled ROP of lactide. 

Some of the seminal studies of organocatalysts have been described, focusing on chiral Bronsted acid catalysts. Because this review is not comprehensive, there are a number of topics not covered in this chapter, such as carbene catalysts, bifunctional catalysts, and so on. 

N-Heterocyclic carbenes as versatile organocatalysts and reagents 06CJO745. 

A range of metal-free organocatalysts have been used to good effect in the kinetic resolution of alcohols. A number of enantiomerically pure 4-aminopyridine-based catalysts effect kinetic resolution with high S values. These include the conforma-tionallyrestricted2,3-dihydroimidazo[l,2-a]pyridine (12.98) and the atropiso-meric catalyst (12.99). N-heterocyclic carbenes also function as nucleophilic... 

---