Carbenes catalysts type

Although the bulk of this review is concerned with well-defined metal carbene catalysts, it is important to note the contributions made to cross-metathesis chemistry by ill-defined or multicomponent catalysts. A brief discussion of the cross-metathesis reactions of functionalised alkenes using catalysts of this type will therefore be included here [1]. 

Diastereoselective hydrogenations of this type have been reported by Burgess and coworkers [54—59] using chiral-protected and -unprotected allylic and homo allylic alcohols as substrates with their carbene catalyst lr(9). Catalyst control was found to be dominant, but depending on the position and nature of the oxygen substituents, moderate to strong match/ mismatch effects were observed. 

Enyne intramolecular metathesis reactions, of the type shown in equation 61, can be very useful in organic synthesis. A number of such reactions, catalysed by tungsten or chromium carbene complexes, have been reported634,635,737 - 740. The ruthenium carbene catalysts 18-20 (Table 2) are likely to be increasingly used for this purpose because of their stability, ease of handling and good yields, as in the synthesis of various 5-, 6- and 7-membered heterocycles, e.g. equation 67741. 

Coke formation is considered, with just cause, to be a malignant side reaction of normal carbonium ions. However, while chain reactions dominate events occurring on the surface, and produce the majority of products, certain less desirable bimolecular events have a finite chance of involving the same carbonium ions in a bimolecular interaction with one another. Of these reactions, most will produce a paraffin and leave carbene/carboid-type species on the surface. This carbene/carboid-type species can produce other products but the most damaging product will be one which remains on the catalyst surface and cannot be desorbed and results in the formation of coke, or remains in a noncoke form but effectively blocks the active sites of the catalyst. 

A common procedure used to forge carbocyclic systems is represented by the ring-closing metathesis reactions exploiting various Grubbs-type carbene catalysts. Based on this cyclization technology, Skaanderup and Madsen [71], on the way to polyhydroxy-nortropane calystegines, reported the asymmetric synthesis of aminated 6a-carbahexoseptanose derivatives 311. 

Finally, there are also some special NHC-mediated transformations that do not completely fit into the classification, such as triazolylidene-catalyzed hydroacylations (Chan and Scheldt 2006). Aldehydes can serve as hydride donors for activated ketones partly following a standard 1,2-addition of the NHC to the aldehyde, but instead of the usual carbonyl umpolung a hydride ( H-umpolung ) transfer is initiated. A related Cannizzaro-type transformation has been described for indazole-derived carbene catalysts (Schmidt et al. 2007). 

Although the platinum-catalyzed reactions are not as practical as palladium used for the same purpose, several types of unique discoveries still are noteworthy. Besides electrophihc C-H activation, the platinum complexes are efficient catalysts for hydrosilation see Hydrosilation Catalysis) that is important to silicon polymer industry. Platinum carbene catalysts are prepared from Karstedt catalyst (9) and imidazolium salts. The trials of model hydrosilylation reactions show very high yields of regioselective products (<1% impurity) with remarkable TON (see Turnover) (<30ppm catalyst load) (Scheme 81). ... 

In 2003, Stoltz at CalTech described a palladium-catalyzed oxidative Wacker cyclization of o-allylphenols such as 55 in nonpolar organic solvents with molecular oxygen to afford dihydrobenzofurans such as 56.44 Interestingly, when (-)-sparteine was used in place of pyridine, dihydrobenzofuran 56 was produced asymmetrically. The ee reached 90% when Ca(OH)2 was added as an additive. Stoltz considered it a stepping stone to asymmetric aerobic cyclizations. In 2004, Mufiiz carried out aerobic, intramolecular Wacker-type cyclization reactions similar to 55—>56 using palladium-carbene catalysts.45 Hiyashi et al. investigated the stereochemistry at the oxypalladation step in the Wacker-type oxidative cyclization of an o-allylphenol. Like o-allylphenol, o-allylbenzoic acid 57 underwent the Wacker-type oxidative cyclization to afford lactone 58.47... 

Carbenes as Ligands for Metal-Based Polymerization Catalysts (Type 3)... 

In this section, we have concentrated on the use of transition-metal nucleophilic-carbene catalysts to bring about ring-closing alkene metathesis reactions. These same types of compounds can also be used to catalyze a remarkable reaction called ringopening alkene metathesis polymerization (ROMP). A special value of ROMP is that it can be used to prepare highly unsaturated polymers. For a discussion of ROMP techniques, see Section 29.6E. 

Analysis of literature showed tiiat most of information concerning thermal stability, degradation, and deactivation of Ru-carbene catalysts has been obtained for first- and second-generation Grubbs catalysts. There are only solitary reports about decomposition of Hoveyda-type catalysts without details. 

On the other hand type of ligand (P- or N- ligand) at Ru-carbene has strong effect on the stability of methylene species half-life time are 0.6 and 5.5 h for (5) (P)Ru CH2 and (6a) (N)Ru CH2. This fact indicates that ligand L determines the stability of the Ru-carbene catalyst. 

According to the catalyst type, several mechanisms have been invoked to account for the synthesis of the initial carbene. Two different types of catalysts can be envisaged those which require activation with an organo-metallic cocatalyst, and those which are active without cocatalysts. 

Another resin system is worth noting. Dicyclopentadiene (DCPD) ring-opening methathesis polymerization (ROMP) wifli macromer 32 has been carried out using the Grubbs-type mthenium carbene catalyst shown in Figure Macromer... 

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