H FUNCTIONALISATION

Another example of a fragment X attached to a metal centre that attacks a C-H bond is the carbenoid fragment attached to copper or rhodium as 

In heterogeneous metal catalysis alkanes, alkenes, and aromatics adsorbed on the metal surface rapidly exchange hydrogen and deuterium. The multiple adsorption of reactants and intermediates lowers the barriers for such exchange processes. Hydrogenation of unsaturated aliphatics and isomerisation can be accomplished under mild conditions. Catalytic dehydrogenation of alkanes to alkenes requires temperatures 200 °C, but this is because of the thermodynamics of this reaction. 

We are not aware of any industrial application that uses metal activation of C-H bonds to obtain functionalised molecules. We have included this topic, because it is potentially of great importance by providing a short-cut for the conversion of hydrocarbons to its functionalised derivatives. Two extreme cases will be discussed, reactions with electron-rich metal complexes and reactions with electrophilic metal complexes. As always in organometallic chemistry there are cases in between that utilise both bonding interactions. 

For each case we will also present catalytic analogues, namely (1) the activation of methane to form methanol with platinum, the reaction of certain aromatics with palladium to give alkene-substituted aromatics, and (2) the alkylation of aromatics with ruthenium catalysts, and the borylation of alkanes and arenes with a variety of metal complexes. 

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One of the attractions of dirhodium paddelwheel complexes is their ability to catalyse a wide variety of organic transformations such as C-H insertions, cyclopropanations and ylide formation. A review on the application of high symmetry chiral Rh2(II,II) paddlewheel compounds highlights their application as catalysts for asymmetric metal carbenoid and nitrenoid reactions, and as Lewis acids.59 Their impressive performance as catalysts in C-H functionalisation reactions has been exploited in the synthesis of complex natural products and pharmaceutical agents. A recent review on catalytic C-H functionalisation by metal carbenoid and nitrenoid insertion demonstrates the important role of dirhodium species in this field.60... 

C-H functionalisation through singlet chlorocarbenes insertions-MP2 and DFT investigations " ... 

Mahltig, B. Haufe, H. Bottcher, H., Functionalisation of textiles by inorganic sol-gel coatings. Journal of Materials Chemistry 2005, 15, 4385-4398... 

The [Ir(ppy)2bpy] complex photo-catalyses inter- and intramolecular C-H functionalisation reactions of tertiary amines under the visible light irradiation. Oxygen behaves as a chemical switch, triggering different reaction pathways and leading to different products from the same starting material. In anaerobic conditions, the intermolecular addition of iV,iV-dimethyl-anilines to electron-deficient alkenes yields y-amino nitriles. Aerobic conditions, on the other hand, favour a radical addition/ cyclisation reaction, leading to tetrahydroquinoline derivatives. The intramolecular version of the radical addition produces unexpectedly indole-3-carboxaldehyde derivatives. ... 

The success of ortho C-H functionalisation using palladium and rhodium motivated other researchers to the search for a new active and less expensive metal. Initial approaches involved the use of Ru(0). Later the investigations were focused on the use of Ru(n) complexes. Ru(n) species turned out to be very suitable for this kind of transformations due to their easy conversion into the cyclometallated species, their compatibility with common oxidants and their stability in both air and water. 

OrtAo-arylation of arenes has been one of the most studied C-H functionalisation process in the last decade, herein only the more recent examples will be described. ... 

Sterically bulky N-substituents are often incorporated into NHCs to aid in the stabilisation of resulting complexes. The effect of this may also force C-H bonds within close proximity of the metal which can encourage activation. Most often, C-H bond activation of the wingtip substituents of an NHC occurs via C(sp )-H functionalisation of aryl substituted NHC complexes, with an early example documented by Lappert in 1977. Here, the 16-electron complex, [Ru(PPh3)3Cl2], was heated at reflux in the presence of an electron-rich carbene dimer in xylene, to deliver a Ru -NHC complex with a metalated iV-aryl side-arm. Later work suggested that the excess dimer acts as an efficient proton acceptor to promote the C-H activation process. ... 

More recently, Nolan and co-workers have reported an example of C-H activation of the same ligand at Rh upon complexation with the free NHC. Treatment of [Rh(COE)2Cl]2 (COE = cyclooctene) with IMes at room temperature led to the total displacement of the COE ligand with simultaneous splitting of the dimer, affording a bis(NHC) coordinated product [Rh(IMes)(IMes) HCl] ((IMes) = cyclometalated l,3-bis(2,4,6-tri-methylphenyl)imidazol-2-ylidene) with a single C-H functionalised orthomethyl group (Fig. 32). 

Scheme 13.13 Zhang and co-workers allylic C-H functionalisation strategy.

During the current decade, reactions based on direct C-H functionalisation, and in particular Pd-catalysed direct arylation, have witnessed extraordinary progress. The use of NHC-Pd catalysts (especially well-defined complexes) in these transformations is much less documented than ligandless systems or those using commercially available phosphines in situ. 

Silver-mediated annulations of the allg nes (705) by arylphosphinates (706) via C-H/P-H functionalisation has been described as a step-economical route to synthesis of substituted benzo[Z)]phospholes (707) (Scheme 195). ... 

An alternative to this was developed in the Yu laboratory exploiting a weak coordination approach using a perfluorinated aryl amide as a practical auxiliary to direct the C-H functionalisation (Scheme 15.77). ... 

As noted in Scheme 15.108, Yu reported a C-H functionalisation approach that was catalytic in palladium for the trifluoromethylation of aromatic moieties.Similarly, Shi has reported a catalytic palladium system for the amide directed C-H trifluoromethylation. Recently, Sanford has reported a decarbonylative trifluoromethylation catal5dic cycle based on a Pd(0)/Pd(ii) system this observation may open up new opportunities in this important area (Figure 15.9). ... 

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