NHC-Au complexes

Scheme 31.9 Reaction of bis(NHC)-Au complex 25 with AgBF4 to yield a chiral, alternating Au-Ag polymer 26.

Scheme 31.10 Electrochemical polymerization of bithiophene substituted NHC-Au complex 27 to yield polymer 28.

Veige and coworkers reported in vitro studies of a novel chiral dicarbene dinu-clear NHC-Au complex [11] with ligand-centered chirality. The cytotoxicity of the racemic complex [(p-diNHC)Au ]2[OTF]2 (+/-) and its pure enantiomers (+) or S,S-S,S and (-) or R,R-R,R were tested against human cancer cells (NCI-... 

A highly enantioselective catalyst system for the intermolecular (4 -I- 2) cycloaddition of allenamides and dienes was recently disclosed by Ferndndez and coworkers (Scheme 3.77) [108]. The NHC-Au complex featuring a triazole unit embedded in a rigid axially chiral cyclic frame 121 was used as the catalyst for this transformation. The desired optically active cyclohexene products, which are not easily accessible using other methodologies, were obtained in moderate to good yields (48-88% yields) and excellent enantioselectivities (87 to >99% ee s). 

It is only in recent years, following what is commonly referred to as the Gold Rush in the scientific literature, that NHC-Au complexes have gained popularity and have seen their potential unfold. The first application in catalysis of a NHC-gold species appeared in 2003, but the real surge in publication only started in 2006. Since then, there is hardly a week without a report on the activity of NHC-Au catalysts in organic synthesis is published. The field is dominated by two main topics of crucial importance in modern catalysis, namely cycloisomerization of polyunsaturated substrates and hydrofunctionalization of 7i-bonds. NHC-Au species have been the subject of several reviews... 

Hydroamination of 7i-bonds is one of the most straightforward methods for the construction of C N bonds and, as such, has attracted a lot of attention. NHC- Au catalysts, in line with results obtained in hydration and hydroalkoxylation reactions (vide supra), proved highly efficient in this field and the inter-and intramolecular hydroamination of various alkenes," allenes," and alkynes" were reported with a number of NHC- Au complexes. Among these reports, Widenhoefer published an elegant bis-hydroamination of allenes, leading... 

Figure 11.4 Structures of NHC-Au complexes active in oxidation reactions.

NHC-Au ° complexes have also been shown to catalyze alkyne hydration, see P. de Fremont, R. Singh, E. D. Stevens, J. L. Petersen and S. P. Nolan, Organometallics, 2007, 26, 1376-1385. 

The hydroalkoxylation of alkynes has recently attracted some marked interest. Several groups have developed alkyne alkoxylation/Claisen rearrangement tandem processes using allylic alcohols as nucleophiles and NHC-Au complexes as the catalysts. The group of Nolan developed, in 2013, the hydrophenoxylation of alkynes by using a cooperative NHC-Au catalysis pro-cess. The hydroalkoxylation of allenes could also be performed. This reaction was shown to proceed in a regioselective manner with the alcohol being introduced on the least hindered terminal carbon of the allene. ... 

NHC-Au catalysts, in line with results obtained in hydration and hydroalkox-ylation reactions vide supra], proved to be highly efficient in this field, and the inter- and intramoleeular hydroamination of various alkenes, allenes, and alkynes were reported with a number of NHC-Au complexes. In 2008, Bertrand and co-workers disclosed an extremely efficient alkyne and allene hydroamination catalyzed by a CAAC-Au eomplex (Scheme 11.15). These peculiar NHCs allowed for the challenging use of ammonia and proved useful in a hydroamination/hydroarylation cascade leading to 1,2-dihydroquinolines. The same authors subsequently employed a similar method to perform the addition of hydrazine on alkynes in an intermoleeular manner. ... 

Several other rearrangements leading to the construction of complex scaffolds were developed with NHC-Au complexes as catalysts. More specifically, the gold-catalyzed intramolecular addition of an oxygenated functionality (carbonyl, epoxide, ether) on an alkyne has been largely employed to initiate various types of multi-step rearrangements [Scheme 11.17 and eqn (11.5)]. ... 

The development and use of NHC-Au complexes is intimately linked to the amazing surge in reports on gold homogeneous eatalysis in general. 

Formation of a,p-unsaturated carbonyl compounds from propargylic alcohols was described in 2007 by Chung et al. [119] and from propargylic acetates by Nolan and co-workers (Scheme 8) with [(NHC)Au ] complexes [120]. The presence of water was required for the formation of the desired products. Steric hindrance of the ligand appeared to be crucial for the selectivity of the reaction. The reaction was not... 

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