Hydrido-alkyl complex

The activation of methane in solution by an organometallic complex presents some experimental difficulties because any solvent that is likely to be chosen will be more reactive than methane. In addition, insolubility of the complex in liquid methane may preclude reaction with the pure hydrocarbon. These problems were overcome in the case of the reaction of CH4 with the iridium complex of Eq. 15.106 by taking advantage of the fact that the desired hydndo methyl complex Is thermodynamically more stable than other hydrido alkyl complexes. The methyl complex was produced by first creating a hydrido cyclohexyl complex and then allowing it to react with methane, m... 

The mechanism of reductive elimination of a hydrido alkyl complex is therefore often approached in an indirect manner. The hydrido-alkyl complex is made not by oxidative addition of the alkane but by some other route. The decomposition of the hydrido-alkyl complex to give alkane is then studied for mechanistic information. Reductive eliminations of an aldehyde from an acyl-hydrido complex, Reaction 2.7, and acetyl iodide from an iodo-acyl complex,... 

There are quite a number of routes available for the production of iridium(ni) alkyl compounds. In addition to the halide displacement and olefin insertion pathways noted above for iridium(l) compounds, oxidative addition of C-H bonds to iridium(l) to form iridium(in) hydrido alkyl complexes is also a possibihty. This subject will be covered in detail in Section 9 and will not be discussed here. However, there are other oxidative addition routes that lead to the formation of iridium(lll) alkyls. First, oxidative addition of O2 or HCl to some alkyl and aryl iridium(l) complexes can produce iridium(lll) alkyl or aryl compounds. In some cases, HgCl2 can add, but this appears to lead to tractable products only for the very stable pentafluorophenyl complex. Of course, oxidative addition see Oxidative Addition) of alkyl halides such as H3CI will also yield alkyl iridium(lll) compounds. Addition of Mel to Vaska s compound yields a stable iridium(III) complex, but addition of Etl does not produce a stable compound, presumably due to subsequent /J-hydride elimination see fi-Hydride Elimination). A number of mechanistic studies have been done on the oxidative addition of alkyl halides to iridium(l), especially Vaska s complex see Vaska s Complex). 

The addition of Mo and H to these diastereomeric alkenes is stereospecifically cis, based on the observed NMR coupling constants of 10.5 and 5.5 Hz for the threo- and erythro-Mo hydrido(alkyl) complexes formed, respectively. No deuterium scrambling is observed in these reactions, showing that the hydrometalation is not reversible. Protonation of alkenylmolybdenum hydrides by HCl also leads to ff-alkylmolybdenums ... 

Bergman et al. [12] reported one of the first studies of C—H bond activation with a transition metal system capable of intermolecular oxidative addition. The reaction involved the photolysis of [Ir(ri-Cp )(PMe3)(H)J in different hydrocarbon solvents. Figure 25.5 shows that the reaction likely proceeds via loss to form a very reactive IF 16 electron metal intermediate. The C—H activation proceeds via a 3-centered transition state leads to an Ir hydrido-alkyl complex in a high yield at room temperature. The process is well described as an oxidative addition of the alkane. 

Mixed hydrido-alkyl complexes of gallium have been prepared, e.g. reactions (20)—(24), (M = Na or K R = Me or Et). The synthesis and properties of dimethylgallium tetrahydroborate have been examined, ... 

Three years after Green discovered the tetramethyl silane CH bond activation (reaction 5) [14], Bergman et al. [19] and Graham et al. [20] found independently, in 1982, the first examples of oxidative addition reaction of alkane CH bonds with formation of hydrido alkyl complexes (reactions 6 and 7). 

These peculiarities of the rhodium system (greater selectivity and easier reductive elimination of its hydrido alkyl complexes) are in agreement with a lower exothermicity of its CH insertion reaction. Irradiation of the rhenium complex CpLjRe also gave hydrido alkyl compounds CpL2Re(R)H (L = PMe3 R = methyl, -hexyl, cyclopropyl) with the corresponding alkane cyclohexane can be used as an inert solvent [24]. 

The mechanisms of the hydroxycarbonylation and methoxycarbonylation reactions are closely related and both mechanisms can be discussed in parallel (see Section 9.3.6).631 This last reaction has been extensively studied. Two possibilities have been proposed. The first starts the cycle with a hydrido-metal complex.670 In this cycle, an alkene inserts into a Pd—H bond, and then migratory insertion of CO into an alkyl-metal bond produces an acyl-metal complex. Alcoholysis of the acyl-metal species reproduces the palladium hydride and yields the ester. In the second mechanism the crucial intermediate is a carbalkoxymetal complex. Here, the insertion of the alkene into a Pd—C bond of the carbalkoxymetal species is followed by alcoholysis to produce the ester and the alkoxymetal complex. The insertion of CO into the alkoxymetal species reproduces the carbalkoxymetal complex.630 Both proposed cycles have been depicted in Scheme 11. 

Arduengo-type carbenes have also been successfully employed for complexation of group 13 trialkyls (Me3Ga- CNfPflCzMezNfPr1) 13091) and mixed hydrido/alkyls (Cp 2(H)Ga- CN(Me)C2Me2N(Me) 13192). Adduct 131... 

The reductive dehalogenation of haloalkanes has also been achieved in high yield using polymer supported hydridoiron tetracarbonyl anion (Table 11.15). In reactions where the structure of the alkyl group is such that anionic cleavage is not favoured, carbonylation of the intermediate alkyl(hydrido)iron complex produces an aldehyde (see Chapter 8) [3]. 

The products of oxidative addition of acyl chlorides and alkyl halides to various tertiary phosphine complexes of rhodium(I) and iridium(I) are discussed. Features of interest include (1) an equilibrium between a five-coordinate acetylrhodium(III) cation and its six-coordinate methyl(carbonyl) isomer which is established at an intermediate rate on the NMR time scale at room temperature, and (2) a solvent-dependent secondary- to normal-alkyl-group isomerization in octahedral al-kyliridium(III) complexes. The chemistry of monomeric, tertiary phosphine-stabilized hydroxoplatinum(II) complexes is reviewed, with emphasis on their conversion into hydrido -alkyl or -aryl complexes. Evidence for an electronic cis-PtP bond-weakening influence is presented. 

Preparing trans-hydrido-alkyls or -aryls of platinum(II) does not require prior isolation of a hydroxo complex. The main routes we have used are shown in Equations 9-11, and selected spectroscopic data, including some from the literature, are in Table IV. 

Thus reaction with alkyl halides such as allyl bromide or pro-pargyl bromide allow for the introduction of oleflnlc2. . or acetylenic side groups onto the phosphazene ring VI, while alcohol leads to the formation of hydrido-phosphazene complexes VII. The hydrogen in these compounds can be replaced with halogen to yield the first series of iodor-phosphazene compounds VIII. 

Additives are also used to improve the solubility of halide donors [382, 383]. Metal(II) halides such as magnesium chloride, calcium chloride, barium chloride, manganese chloride, zinc chloride and copper chloride etc. are used as halide sources. In order to increase the solubility of the halides they are reacted with electron donors which have been previously described for the increase of solubility of Nd-components [338,339]. The number of catalyst components is further increased if two Al-compounds (alumoxane + aluminum (hydrido) alkyl) are used. In addition, a small amount of diene can also be present during the preformation of the different catalyst components as described by JSR. In some catalyst systems the total number of components reaches up to eight [338,339]. Such complex catalyst systems are also referred to in other JSR patents [384,385] (Sect. 2.2.6). 

Binuclear [RuX2(arene)]2 (1) and mononuclear RuX2(L) (arene) (3) derivatives have been shown to be useful precursors for access to alkyl-or hydrido(arene)ruthenium complexes. The latter are key compounds for the formation of arene ruthenium(O) intermediates capable of C—H bond activation leading to new hydrido and cyclometallated ruthenium arene derivatives. Arene ruthenium carboxylates appear to be useful derivatives of alkyl-ruthenium as precursors of hydrido-ruthenium complexes their access is examined first. 

The bisacetato ruthenium complex 28, on heating in 2-propanol, leads to the bridged hydrido dinuclear complexes 73 and 74. The bistrifluoroacetato complex 28 also leads to complex 73. The Tj2-acetato complex 39 was transformed in hot 2-propanol to another bridged hydrido derivative (75, arene = durene, mesitylene, p-cymene, hexamethylbenzene 60-70%). The introduction of alkyl substituents on the benzene ring is reflected by a shift of the p FI resonance toward high field (14,53). 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

Both in situ infrared and multinuclear NMR under less severe conditions have been used to gain mechanistic insights. For the hydroformylation of 3,3-dimethyl but-l-ene, the formation and hydrogenolysis of the acylrhodium species Rh(C()R)(C())4( R=CH2CH2Bur) can be clearly seen by IR. NMR spectroscopy has also been very useful in the characterization of species that are very similar to the proposed catalytic intermediates. We have already seen (Section 2.3.3, Fig. 2.7) NMR evidence for equilibrium between a rhodium alkyl and the corresponding hydrido-alkene complex. There are many other similar examples. Conversion of 5.3 to 5.4 is therefore well precedented. In the absence of dihydrogen allowing CO and alkene to react with 5.1, CO adducts of species like 5.6 can be seen by NMR. Structures 5.11 and 5.12 are two examples where the alkenes used are 1-octene and styrene, respectively. 

---