Iridium carbene

C-C bond formation can be favored over /3-hydride elimination by changing the nature of the catalyst. Hence, cyclizations can be mediated by iridium carbene complexes resulting from a formal intramolecular cross-coupling of the alkene with an. s -C-H bond (Equation (40)). 

Fan et al. also reported DFT calculations on the complete hgand and substrate structures with an iridium carbene-based complex for an Ir(III)-Ir(V) catalytic cycle, which reproduced the correct selectivity order for three different substrates [33]. Calculations on complete ligands and real substrates that reproduce experimentally determined enantioselectivities give plausible credence to these authors computational experiments. 

The catalyst is also effective for the reduction of styrenes, ketones, and aldehydes. Cyclohexenone 16 was reduced to cyclohexanone 11 by transfer hydrogenation, and using a higher catalyst loading, styrene 17 was reduced to ethylbenzene 18. The elaboration of [Ir(cod)Cl]2 into the triazole-derived iridium carbene complex 19 provided a catalyst, which was used to reduce aUcene 20 by transfer hydrogenation [25]. 

Not surprisingly, these rhodium and iridium carbene complexes were tested for their catalytic behaviour in the transfer hydrogenation of benzophenone and acetophenone (M +3), the hydrosilylation of alkynes (M +1) and also the catalytic cyclisation of acetylenic carboxylic acids (M +1). Hydrogenation works better for iridium than rhodium and for aromatic than for aliphatic ketones [40,43,44]. The iridium(I) complex is the first iridium catalyst showing activity for the cyclisation of acetylenic carboxylic acids [40]. The results for the hydrosilylation reactions were very moderate. 

In both cases, no catalytic reactions were performed with the iridium(I) and palladium(II) complexes synthesised with these ligands. However, Peris and coworkers used the synthesis of their iridium carbene complex to show that the formation of the M-NHC bond is an oxidative addition that can be assisted by a weak base. This base does not deprotonate the imidazolium salt, but traps the protons formed in the process [152],... 

IRIDIUM CARBENE AND CARBYNE (See Carbyne Complexes) COMPLEXES ... 

Carbene coupling involving such obtained iridium-carbene species thus explains the observed formation of ethene (see Rg. 37), in analogy with ethene formation upon oxidation of [Re (Cp)Me(NO)(PPh3)] (117). 

A larger range of iridium carbenes exists, predominantly resulting from C—H activation of solvent, as in the activation of a range of ethers, and dimethylaniline, by Tp 4r( Cc4H5Me-3) (238) and Tp " Ir(C6H5)2(N2) (211) " > 65 (Scheme 57), some of which were described above. 

Finally, the iridium carbene complex 580, obtained from the double C-H activation of 2-ethylphenol by Tp Ir(C6H5)2(N2) (211), illustrates the rare incidence of an equilibrium between alkylidene hydride and alkene hydride complexes. The alkylidene forms in admixture with ca. 5% of the alkene hydride isomer 581, illustrating a preference for the a- over hydrogen in the second activation step. However, in isolation 581 is observed to re-establish the same equilibrium mixture (i.e. 20 1 580 581, Scheme 61) a rare example of a metal-alkene converting to a metal-alkylidene, the reverse reaction being more typical. 

The following iridium carbene complex may be reduced by hydrogen as recently described ... 

Valpuesta JEV, Alvarez E, L6pez-Serrano J, Maya C, Carmona E. Reversible double C-H bond activation of linear and cyclic ethers to form iridium carbenes. Chem Eur J. 2012 18 13149-13159. 

In a slow dediazotation reaction of bis(trifluoromethyl)diazomethane with (Ph2PCH3)2lr(CO)Cl, the iridium carbene complex 14 was formed and isolated in 37% yield (reaction 8.32) [63]. 

After the publication of COMC (1995), the chemistry of iridium carbenes has grown considerably and many intriguing applications of these compounds, particularly in the area of C-C bond-forming reactions and homogeneous catalysis, have been reported. 

Sames and coworkers reported that the catalytic oxidative cyclization of amide can be achieved by an iridium carbene complex, which consists of [Ir(coe)2Cl]2,... 

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