A -heterocyclic carbine

Figure 7.9 A -Heterocyclic carbine ligands polymerized by the addition of Pt Pd or Ni salts (Boydston et al. 2005, 2006).

The rhodium A-heterocyclic carbine complex [Rh(IMes)(COD)] [IMes = A,A -bis(2,4,6-trimethylphenyl)imidazol-2-ylidine COD = cycloocta-1,5-diene] catalyses the 4 + 2 + 2-carbocyclization of 1,6-enynes (191) to carbocycles [(192) and (193)] (Scheme 54).226 Computational and experimental evidence of a new reaction pathway for the diastereospecific intermolecular rhodium-catalysed 4 + 2 + 2-carbocyclization (g) reactions of 1,6-enynes with r-components has been reported.227... 

Scarborough, C.C., Popp, B.V., Guzei, l.A. and Stahl, S.S. (2005) Development of 7-membered A-heterocyclic carbine ligands for transition metals. Journal of Organometallic Chemistry, 690, 6143-6155. 

The potential importance of many of these types of conformational interconversion may be found in the asymmetric olefin metathesis catalysts recently developed by Grubbs. A key intermediate formed after loss of 3-bromopyridine or CysP would have structure 35. The orientation of the aromatic rings could be influenced by other substituents (R ) on the A -heterocyclic carbine. Those rings would, in turn, select a preference of location of in the... 

Scheme 4.18 Synthesis of benzo-e-lactones by A -heterocyclic carbine-catalyzed [4-1-3]...

Employing A-heterocyclic carbine (NHC) catalysis, in 2010, Sarkar and Studer presented a novel and efficient oxidative azidation of aromatic aldehydes by using 3,3 5,5 -tetra-tert-butyldiphenoquinone as the oxidant (Scheme 6.23) [77]. Reaction works with a low catalyst loading under mild conditions and the acyl azides are obtained in good yields. They further showed that the acyl azides can be rearranged to carbamoyl azides in the same pot. 

A seven-membered amidinium compound can be synthesized from 2,2 -diaminobiphenyl as a seven-membered heterocyclic carbine precursor [94] (Figure 3.10). 

In recent years, catalytic applications of half-sandwich metal complexes have been extensively developed. A series of (p-cymene)Ru(II) complexes containing pyrazole-based iV-heterocyclic carbine (pyrazoUn-3-ylidene) ligands (Fig. 21.13a and b) showed excellent catalytic y3-alkylation of secondary alcohols with primary alcohols and the dimerization of phenylacetylene [30]. 

A facile synthesis of y-ketophosphonates (589) has been realised by intermolecular cross-coupling reaction of aromatic aldehydes (587) with vinylphosphonates (588), catalysed by N-heterocyclic carbine (589) in moderate to good yields (Scheme 171). ... 

Non-metallic homogeneous catalyst systems were also reported for methanol synthesis. Recently, Ashley et al. [49] demonstrated the selective hydrogenation of COj to methanol using a FLP-based nomnetal mediated procedure at low pressures (1-2 atm). N-Heterocyclic carbine (NHC) was found to be an elFective organic catalyst for methanol synthesis from CO2 reduction with silane. Compared to transition metal catalyst, NHC is more efficient at ambient reaction conditions [50,51]. Table 5.1 lists catalytic activities of different heterogeneous catalysts employed for methanol synthesis from CO. It shows that maximum CO conversion of 25.9%, methanol selectivity of 99.5% and methanol yield of378 mg/g-cat h could be achieved. The space velocities were tried between 1800 and 18,000 h and the temperature from 170 to 270 C. 

A recent study of proton transfer from rhenium Fisher-type carbine complexes (13) shows that the reactions lead to the formation of an aromatic product (14), following the same rules as reactions that lead to the formation of products stabilized by simple resonance. The conjugate bases of these carbine complexes represent aromatic heterocycles, i.e., substituted furan, selenophene, and thiophene derivatives, respectively. The aromatic stabilization of these heterocycles is known to follow the order furan < selenophene < thiophene (Scheme 1) [43],... 

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