Intermediate metal alkyl

Electrophilic C-H activations can also be effected in water. At first glance, water would appear to be particularly unpromising as a solvent for such reactions. Because of their extremely poor coordinating ability alkanes should not be able to compete with water for coordination sites. Moreover, the intermediate metal-alkyl species would be prone to hydrolytic decomposition. In one respect, however, water is an almost ideal medium for C-H functionalization the O-H bond energy exceeds the corresponding C-H bond energy of even methane. Indeed, the selective oxidation of methane to methanol is carried out by methane monooxygenase in aqueous medium. 

The dimerization of ethylene to form a mixture of butene isomers is not particularly useful in the field of commodity chemicals at this time because this mixture of butenes is usually cheaper than ethylene. Selective dimerization of ethylene to 1-butene using a titanium catalyst is practiced, but this chemistry occurs through metallacycles and is described in the next section. The dimerization of propylene by migratory insertion chemistry typically produces the mixture of isomeric olefins shown in Equation 22.32. Four skeletal isomers of the intermediate metal alkyl can arise from the two different directions of M-H insertion, followed by two different inodes of M-R insertion. The dimerization of ethylene is particularly fast when catalyzed by the combination of NiBr(-r) -C3H5)(PCy3) and EtAlCl this dimerization in chlorobenzene at 25 °C occurs witii turnover frequencies up to 60,000 per second. The more selective dimerization of propene to 2,3-dimethylbutene is conducted on an industrial scale with titanium catalysts, again via metallac clic intermediates described in the next section. 

Cations such as Zn " " or Ga+ behave as soft Lewis acids in the reactions discussed above with the formation of intermediate metal-alkyl or metal—hydride species. This implies an electron transfer between the ligand and cation. To illustrate ftirther the Lewis acid nature of Zn " ", we analyze the mechanism for the hydrolysis of CH3CN in which there is no change in formal valency of Zn +, and compare the energetics for this ion-exchanged Zn " " reaction with that for the zeolitic proton / The overall reaction scheme... 

The other commercially important routes to alkyltin chloride intermediates utilize an indirect method having a tetraalkjitin intermediate. Tetraalkyltins are made by transmetaHation of stannic chloride with a metal alkyl where the metal is typicaHy magnesium or aluminum. Subsequent redistribution reactions with additional stannic chloride yield the desired mixture of monoalkyl tin trichloride and dialkyltin dichloride. Both / -butjitin and / -octjitin intermediates are manufactured by one of these schemes. 

In principle, polymers equivalent to those obtained from vinyl and divinyl monomers may be synthesized by this method. The product in the above example possesses the same chain structure as polyethylene. The polymerization process, notwithstanding the likelihood of a metal alkyl intermediate, should conform satisfactorily to stepwise condensation. However, the product, and those obtained by Friedel-Crafts condensation as well, lack the recurrent functional groups which generally characterize condensation polymers. 

The reactions of oxygen with metal-alkyl derivatives typically involve radical intermediates (73, 74), and a likely mechanism for the reactions of [TpBut]MgR with 02 is illustrated in Scheme 6. Specific... 

Wilke s allyl compounds were found to be very poor catalysts indeed, e.g., Ti(2 Me-allyl)4, was found only to have an activity equal to 0.5 gm/m.M Ti/atm/C2H4/hr. For this reason there has been considerable dispute that transition metal alkyls can be the intermediates in Ziegler polymerization. 

If coordination were not important, the polymerization process would presumably resemble those initiated by alkali metal alkyls. The latter are very effective initiators for the polymerization of such monomers as styrene, butadiene, etc., and it is generally considered that propagation proceeds through intermediates of the type (43)... 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

The initially expected (75) cis-hydrometallation or olefin-insertion step with fumarate (R = C02Me) yields the threo isomer 8, which then undergoes the k2 step with retention to give racemic 1,2-dideuterosuccinate. Such retention is necessary to give the usually observed (7, p. 407) overall cis addition of H2 to olefinic bonds, but this study provided the first direct experimental proof, the difficulty being the scarcity of stable metal alkyl-hydride intermediates. The Cp2MoH2 complex also catalyzes hydrogenation of 1,3- or 1,4-dienes to monoenes (197). 

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). 

Tandem procedures under hydroformylation conditions cannot only make use of the intrinsic reactivity of the aldehyde carbonyl group and its acidic a-position but they also include conversions of the metal alkyl and metal acyl systems which are intermediates in the catalytic cycle of hydroformylation. Metal alkyls can undergo -elimination leading to olefin isomerization, or couplings, respectively, insertion of unsaturated units enlarging the carbon skeleton. Similarly, metal acyls can be trapped by addition of nucleophiles or undergo insertion of unsaturated units to form synthetically useful ketones (Scheme 1). 

As indicated in Chapter 8, the production of alkanes, as by-products, frequently accompanies the two-phase metal carbonyl promoted carbonylation of haloalkanes. In the case of the cobalt carbonyl mediated reactions, it has been assumed that both the reductive dehalogenation reactions and the carbonylation reactions proceed via a common initial nucleophilic substitution reaction and that a base-catalysed anionic (or radical) cleavage of the metal-alkyl bond is in competition with the carbonylation step [l]. Although such a mechanism is not entirely satisfactory, there is no evidence for any other intermediate metal carbonyl species. 

In the last decade an enormous revival of late transition catalysts for the polymerisation of alkenes has taken place [45] (remember that the first discovery of Ziegler for ethene polymerisation also concerned nickel and not titanium). The development of these catalysts is due to Brookhart in collaboration with DuPont (Figure 10.28) [46], Detailed low-temperature NMR studies have revealed the mechanism of the reaction [47], Interestingly, the resting state of the catalyst is the ethene-metal-alkyl complex and not the metal-alkyl complex as is the case for the ETM catalysts. For ETM catalysts the alkene complex intermediates are never observed. Thus, the migratory insertion is the rate-determining step (the turnover limiting step , in Brookhart s words) and the reaction rate is independent of the ethene concentration. 

Non-heteroatom-substituted carbene complexes can also be generated by treatment of electrophilic transition metal complexes with ylides (e.g. diazoalkanes, phosphorus ylides, nucleophilic carbene complexes, etc. Section 3.1.3). Alkyl complexes with a leaving group in the a-position are formed as intermediates. These alkyl complexes can undergo spontaneous release of the leaving group to yield a carbene complex (Figure 3.2). 

Some of these intermediates are analogous to those proposed by Chauvin in olefin metathesis ( Chauvin s mechanism ) [36]. They can be transformed into new olefins and new carbene-hydrides. The subsequent step of the catalytic cycle is then hydride reinsertion into the carbene as well as olefin hydrogenation. The final alkane liberation proceeds via a cleavage of the Ta-alkyl compounds by hydrogen, a process already observed in the hydrogenolysis [10] or possibly via a displacement by the entering alkane by o-bond metathesis [11]. Notably, the catalyst has a triple functionality (i) C-H bond activation to produce a metallo-carbene and an olefin, (ii) olefin metathesis and (iii) hydrogenolysis of the metal-alkyl. 

Breslow suggested a mechanistic scheme (Scheme 8.1) involving a zwitterionic metal alkyl species, with a tetrahedral geometry of the titanium center. However, it was not possible at the time to provide direct structural evidence for these species. Alternative suggestions, e.g. by Olive et al. [11], included octahedral intermediates (Scheme 8.2), while an early proposal by Shilov et al. [12], based on electrochemical studies in dichloromethane, that the active species was likely to be... 

The first step in the peroxide-induced reaction is the decomposition of the peroxide to form a free radical. The oxygen-induced reaction may involve the intermediate formation of a peroxide or a free radical olefin-oxygen addition product. (In the case of thermal and photochemical reactions, the free radical may be formed by the opening up of the double bond or, more probably, by dissociation of a carbon-hydrogen bond in metal alkyl-induced reactions, decomposition of the metal alkyl yields alkyl radicals.)... 

There would appear to be two distinct modes of reactivity of early transition metal alkyls with O2. When the metal is not in its highest oxidation state, an O2 complex of variable stability may form, and its subsequent reactivity may or may not involve the metal-carbon bond. The formation of remarkable stable 0x0 alkyls is an example of this pathway. In contrast, d°-alkyls react with O2 by a radical chain mechanism that invariable leads to formation of alkoxide complexes labile alkylperoxo ligands are clearly imphcated as intermediates in these reactions. 

Leaving aside the autoxidations of d°-alkyls, which only formally yield O2 insertion products, there are a very few examples of reactions where migratory insertion of a coordinated O2 into a metal alkyl bond seems indicated. Thus, heating of Cp 2Ta( -02)Me (Sect. 2.1) in solution in the absence of any base results in its transformation into Cp 2Ta(0)0Me [1]. Lewis acids were noted to catalyze the reaction. While there is no direct evidence for the formation of an alkylperoxo intermediate, the final product could easily be rationaUzed as resulting from an a-alkoxide eUmination (Scheme 10). 

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