NHC complexes

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Fig. 2 Ruthenium-NHC complexes active in catalytic olefin metathesis...

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

Palladium(II) complexes provide convenient access into this class of catalysts. Some examples of complexes which have been found to be successful catalysts are shown in Scheme 11. They were able to get reasonable turnover numbers in the Heck reaction of aryl bromides and even aryl chlorides [22,190-195]. Mechanistic studies concentrated on the Heck reaction [195] or separated steps like the oxidative addition and reductive elimination [196-199]. Computational studies by DFT calculations indicated that the mechanism for NHC complexes is most likely the same as that for phosphine ligands [169], but also in this case there is a need for more data before a definitive answer can be given on the mechanism. 

Scheme 15 Two-step route to triarylphosphine-NHC complexes (Grubbs [48])...

For a general account of transition-metal-NHC complexes in homogeneous catalysis, see Yong BS, Nolan SP (2003) Chemtracts 16 205... 

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

The main synthetic routes leading to the formation of NHC complexes are depicted in Scheme 1.2. The methodologies given are shown with imidazolidinylidene and imidazolylidene ligands, however, they are applicable to other NHCs [16, 17]. 

Scheme 1.2 Main synthetic strategies for the formation of NHC-complexes...

More significant is to change the nature of the C and C substituents. Remaining in the class of [IrCl(CO)2(NHC)] complexes, the replacement of the H atoms on the C and C atoms of the IPr complex by Cl atoms resulted in the rather high shift of the average CO stretching frequency from 2023.9 to 2028.3 cm", see Fig. 1.12a [30], Similar effects have been found in the closely related [RhCl(CO)3(NHC)]... 

Abstract The use of A-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation. 

The Rh-NHC complexes, with or without phosphine co-ligands, have been stndied as hydrogenation catalysts of alkenes with molecular hydrogen, with the aim to develop more active, selective (and/or enantioselective) and thermally stable catalysts. 

Fig. 2.6 Chiral functionalised NHC complexes as enantioselective hydrogenation catalysts with dihydrogen...

The transfer hydrogenations of carbonyl compounds to alcohols catalysed by a variety of NHC complexes have been intensively studied. The strong bond... 

Fig. 2.7 Selected NHC complexes that have been studied as transfer hydrogenation catalysts...

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