Oxidative addition C-H bonds

Many related complexes of iridium and rhodium undergo the oxidative addition reaction of alkanes and arenes [1]. Alkane C-H bond oxidative addition and the reverse reaction is supposed to proceed via the intermediacy of c-alkane metal complexes [4], which might involve several bonding modes, as shown in Figure 19.5 (for an arene the favoured bonding mode is r 2 via the K-electrons). 

Dissociation of the carbonyl ligand was initially postulated as the role of the photon (Route A, Scheme 3) [4], Mechanistic investigations by Goldman et al. indicated, however, that C-H bond oxidative addition to the photo-excited sixteen-elec-... 

Bergman RG. A physical organic road to organometallic C-H bond oxidative addition reactions. J Organomet Chem 1990 400 273-282. 

A much wider range of organopalladium compounds is available, however, by means of cyclopalladation reactions. Such reactions involve a Pd(II) salt and an N or P ligand capable of undergoing intramolecular C—H bond oxidative addition. HCl is eliminated as a by-product, and the reactions occur most readily when a five-membered chelate ring is formed " ... 

Few reports have appeared addressing the stereochemistry of either C-H bond oxidative addition or reductive elimination. The most convincing paper was an intramolecular activation examined by Flood in which a chiral 8-ethylquinoline derivative underwent benzylic activation by PdCl42-. The reaction proceeds with the net retention of configuration at carbon (Eq. 10) [49]. 

The search for new reactivity and new reactions is an important target in homogeneous catalysis. A declared goal is the selective activation of C-H bonds under mild conditions. Although there are numerous examples of stoichiometric C-H bond oxidative additions to transition metal centers, successful examples regarding catalytic functionalization of C-H bonds have been made only during the last five years. Notable advances have been achieved by Moore and coworkers who described in 1992 the ortAo-acylation of pyridine with olefins and carbon monoxide. The cluster compound triruthenium dodecacarbonyl has been used as catalyst (Scheme 10). 

A similar reactivity trend has been reported for the oxidative addition of C-H bonds by binuclear iridium complexes. In fact, the Ir2 compoxmd [It2(CO)2(H) (p-l,8-(NH)2naphtha)(P Pr3)2] also undergoes the C-H bond oxidative addition of phenylacetylene to give a p-KC,q -aIkynyl-bridged complex, which, in this case. 

The proposed mechanism (Scheme 8) involves initial coordination of the pyridine to direct sp C-H bond oxidative addition to the Ru(0) centre followed by alkene insertion into a Ru-H bond and reductive elimination as for the Murai reaction involving sp C-H bond alkylation [3]. 

The proposed mechanism (Scheme 17) involves the initial sp C-H bond oxidative addition to ruthenium(O) followed by aryl transmetallation and reductive elimination. It was suggested by Sames [11] that the role of the ketone was to insert into the Ru-H bond to favour traiw-arylation with the phenylboronate ester. Later on it was shown by Maes that pinacolyl alcohol was formed via Ru-catalysed hydrogenation of pinacolone rather than through a Ru-alkoxide intermediate [12]. 

There are two completely different pathways for the cleavage of H-H bonds oxidative addition and heterolytic cleavage. Both pathways have been identified in catalytic hydrogenation and may also be applicable to other types of X-H a bond activation such as C-H cleavage. 

An investigation of the oxidative addition of ChUand CD4[57a], as well as ethane [57b], to a bare palladium atom has demonstrated that quantum tunneling plays a very important role in the process. The barrier of insertion of different transition metal atoms into a C-C bond has been found to be 14-20 kcal mol higher than the barrier for insertion into a C-H bond [57c], Calculations for the activation of the C-H bond in ethylene by second row transition metal atoms showed that the oxidative addition barrier is lowest for the atoms to the right (for rhodium there is no barrier and for palladium the barrier is almost zero) [57d], The activation energy for B2 insertion into methane has been predicted to be 4.1 kcal mol while this value increases to 16.2 kcal mol for insertion of B [58], Two mechanisms have been considered by the SCF CNDO/S method for the oxidative addition of methane to the palladium cluster Pd2 [59a], In the first possible reaction, the C-H bond oxidatively adds to different palladium atoms ... 

This reaction is proposed to involve insertion of the C=C bond into the Ru-H to give a 6-acetoxyethylruthenium(II) intemediate (1,2-addition) followed by the yd-acetate elimination to yield the products. C-0 bond oxidative addition of vinyl acetate to give hydrido(vinyl)ruthenium(IV) followed by reductive elimination of ethylene is also a possible alternative mechanism. 

The hydroarylation of aHc5mes, which has emerged as a powerful alternative to the oxidative arylation of alkene, is not discussed in this chapter as it is not a true Heck alkenylation. Jia, C., Piao, D., Oyamada, J. et al. (2(X)0) Efficient activation of aromatic C—H bonds for addition to C—C multiple bonds. Science, 287,1992-5. 

Surface modifications of polymers is brought about by the introduction of alcohol functionality, e.g., poly(tetrafluoroethylene-co-hexafluoropropylene) on reduction with sodium naphthalide in THF results in an unsaturated modified surface layer, the thickness of which is controlled with reaction time and temperature. The air sensitive surface contains alcohols, ketones, aliphatic C-H bonds in addition to C=C and C C. The more alcoholic groups are introduced by hydroboration-oxidation, but the esterification leads to the formation of ester in lower yield. This reveals that the reactivity of OH group is similar to hindered alcohols. The reactivity of the surface can be enhanced by chain extension of secondary surface alcohols with ethylene oxide to form a surface containing primary alcohols groups separated from the polymer backbone by C-2 spacer. On the other hand, primary alcohols are directly introduced to the surface by reaction of the reduced layer with 9-BBN, followed by carbonylation and reduction [5]. 

---