Alkane metal complexes

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). 

The first reports on c-alkane metal complexes date back to the 1970s, the work of Perutz and Turner on photochemically generated unsaturated metal carbonyls of Group 6 [4], which is well before the C-H oxidative addition studies of alkanes. The enthalpy gain of formation of c-alkane metal complexes... 

Another method found to be useful for the direct observation of alkane metal complexes is fast time-resolved infrared (TRIR) spectroscopy.- - - Upon UV irradiation of a metal-carbonyl complex, such as Cp Rh(CO)2 in liquid rare gases, an intermediate is formed, which, in the presence of an alkane, forms a weakly bound alkane complex [Eq. (6.93)]. Finally, the latter complex rapidly transforms through the transition state 112 into the alkyl hydride [Eq. (6.94)]. 

Metal complexes prepared by reacting less than one mole of an alkan olamine with an excess of a polyhydric alcohol, such as polyethylene glycol 200—400 or glycerol, reportedly impart a greater degree of thixotropy to systems containing protective organic coUoids (501). 

The two main groups of silyl substituted alkane ligands HC(SiMe3)3 and HCH(SiMe3)2 demonstrate significantly different steric demands, and we will discuss the two groups of alkali metal complexes separately. Only select compounds will be discussed. 

Aliphatic ketone, methylene function reduction, 6-phenylhex-l-ene, 131-132 Alkane reduction alcohols, 12-27 allyl alcohols, 24 benzyl alcohols, 18-24 cyclopropylcarbinols, 17-18 metal-complexed alcohols,... 

The proposed mechanism of the bond shift isomerization of neopentane is shown in Scheme I Cl-3). There are now good models for each step in the proposed sequence, but no simple transition metal complex can accomplish all steps since there cannot be sufficient co-ordination sites. The first steps involve a,y-dinstallation of the alkane, for which there are good precedents in both platinum and iridium chemistry (4, 5, 6). The... 

The regioselective borylation of alkanes can be achieved either catalytically or stoichiometrically using a host of metal complexes such as rhenium analogs. Such processes can be photochemically or thermally induced (Equation (19)).30 30a 30c... 

A mechanism has been proposed involving CO dissociation from the metal complex followed by oxidative addition of the diboron analog. This precedes the alkane functionalization process (Scheme 8). 

Thus, a highly reactive species is needed to make this type of bond activation reaction feasible under mild conditions. In addition, to be useful, the C-H bond activation must occur with both high chemo- and regiose-lectivity. Over the past several decades, it has been shown that transition metal complexes are able to carry out alkane activation reactions (1-5). Many of these metal-mediated reactions operate under mild to moderate conditions and exhibit the desirable chemoselectivity and regioselectiv-ity. Thus, using transition metal complexes, alkane activation can be preferred over product activation, and the terminal positions of alkanes, which actually contain the stronger C-H bonds, can be selectively activated. The fact that a hydrocarbon C-H bond has been broken in a... 

Many of these catalysts are derived from metal complexes which, initially, do not contain metal hydride bonds, but can give rise to intermediate MH2 (al-kene) species. These species, after migratory insertion of the hydride to the coordinated alkene and subsequent hydrogenolysis of the metal alkyl species, yield the saturated alkane. At first glance there are two possibilities to reach MH2 (alkene) intermediates which are related to the order of entry of the two reaction partners in the coordination sphere of the metal (Scheme 1.2). 

The selective oxidation and, more generally, the activation of the C-H bond in alkanes is a topic of continuous interest. Most methods are based on the use of strong electrophiles, but photocatalytic methods offer an interesting alternative in view of the mild conditions, which may increase selectivity. These include electron or hydrogen transfer to excited organic sensitizers, such as aryl nitriles or ketones, to metal complexes or POMs. The use of a solid photocatalyst, such as the suspension of a metal oxide, is an attractive possibility in view of the simplified work up. Oxidation of the... 

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. 

These findings have stimulated enormously the search for intermolecular activation of C-H bonds, in particular those of unsubstituted arenes and alkanes. In 1982 Bergman [2] and Graham [3] reported on the reaction of well-defined complexes with alkanes and arenes in a controlled manner. It was realised that the oxidative addition of alkanes to electron-rich metal complexes could be thermodynamically forbidden as the loss of a ligand and rupture of the C-H bond might be as much as 480 kl.mol, and the gain in M-H and M-C... 

There is ample evidence that the reductive elimination of alkanes (and the reverse) is a not single-step process, but involves a o-alkane complex as the intermediate. Thus, looking at the kinetics, reductive elimination and oxidative addition do not correspond to the elementary steps. These terms were introduced at a point in time when o-alkane complexes were unknown, and therefore new terms have been introduced by Jones to describe the mechanism and the kinetics of the reaction [5], The reaction of the o-alkane complex to the hydride-alkyl metal complex is called reductive cleavage and its reverse is called oxidative coupling. The second part of the scheme involves the association of alkane and metal and the dissociation of the o-alkane complex to unsaturated metal and free alkane. The intermediacy of o-alkane complexes can be seen for instance from the intramolecular exchange of isotopes in D-M-CH3 to the more stable H-M-CH2D prior to loss of CH3D. 

Several catalytic processes are known, see below, but it is clear that the compatibility of the above chemistry with functionalisation is limited. Many reagents used to introduce functional groups will react with the reactive intermediates described above, and the alkanes will have no opportunity to react with the catalyst. Below a few catalytic reactions will be described of relatively electron-rich metal complexes. 

The first example of dehydrogenation of alkanes by a transition metal complex was also achieved with iridium. The cationic iridium(III) complex, [IrH2-(acetone)2(PPh3)2]" BF4, with TBE (3,3-dimethyl-l-butene or t-butylethene) as... 

Alkanes can be activated by both hard and soft transition metal complexes. Hard catalysts have been known for a long time, although examples involving alkanes are few in number, and all are from studies of recent years. Soft catalysts have become important and have been much studied during the past decade and are widely used for reactions of unsaturated hydrocarbons. It was the use of such soft catalysts that Halpem had particularly in mind when he spoke of the development to activate alkanes as an important and challenging problem. 

The catalyzed exchange of hydrogen in an organic molecule by deuterium from the solvent or from deuterium gas has been much used to study alkane activation by transition metal complexes it is discussed... 

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