A N-heterocyclic

The ring closure of )V-(a-N-heterocyclic)aminomethylenemalonates may lead to the formation of nitrogen bridgehead pyrimidinones when the ring nitrogen is involved in the cyclization. Ring closure may also lead to the formation of condensed pyridines when the ring carbon is involved (Scheme 46). 

The same reaction (RCM) was used as the key step for the formation of a family of potent herbicidal 10-membered lactones. An important aspect from the preparative point of view is the control of stereochemical outcome of the RCM by the choice of catalyst. Thus, the use of the ruthenium indenylidene complex IX always leads to the corresponding ( )-alkenes, whereas the second generation of Grubbs catalyst bearing a N-heterocyclic carbene ligand affords the isomeric (Z)-olefin with good selectivity (Scheme 8.19) [64]. 

One application of this catalytic generation of homoenolate type intermediates is in the stereoselective formation of y-butyrolactones 64 from a,/ -unsaturated aldehydes 62 and their reaction with aldehydes or ketones 63 [60]. (For experimental details see Chapter 14.19.2). Glorius [60a] and Bode [60b] almost simultaneously published their results utilizing a N-heterocyclic carbene generated from a bisar-ylimidazolium salt 65 (IMes). The corresponding disubstituted y-butyrolactones... 

Class III peroxidases isolated from C. roseus are capable of oxidizing ajmalicine (XLVIII) to serpentine (LV) [150]. The kz value of C. roseus peroxidase for this reaction is about 0.0011 pM s at pH 6.0 [74]. This constitutes a unique example in peroxidase-catalyzed metabolic reactions since it involves the aromatization of a N-heterocyclic ring (Scheme XXV). 

An immobilized version of a Heck reaction catalyzed by Pd nanoparticles has been very recently described by Kaiimi and Enders [289]. The nanoparticles were obtained as a result of the covalent anchoring of a N-heterocyclic carbene palladium/ionic liquid matrix on a silica surfece and their nature was confirmed by TEM coupled with EDX analysis. The catalyst showed high thermal stability (up to 280 °C) and could be recycled four times for the reaction of bromobenzene with methylacrylate achieving a total turnover number of 36600. After carrying out a hot filtration process, the authors could not detect any Pd in the filtrate. The filtrate also showed no further reaction progress. Erom these findings the authors concluded that the reaction was, in their case, indeed catalyzed by the heterogeneous Pd particles and not from monomolecular Pd-complexes leached from the sur ce. 

Second-Generation Ruthenium Indenylidene Catalysts Bearing a N-Heterocyclic Carbene or a Chelating NHC-Phosphine Ligand... 

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