Metal alkyl bond

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

An example of a complex that reacts slowly with ethylene in a manner consistent with insertion of ethylene into the metal alkyl bond is Co(ti5-C5H5)(PPh3)Me2 Evitt, E.R. Bergman,... 

Depending on the nature of surface chain growth species, on the other hand, one is confronted mainly with the alkyl mechanism,6 based on the insertion of a methylene species C CHj) into the metal-alkyl bond, or with the alkenyl mechanism,2 wherein a surface vinyl species ( CH=CH2) reacts with a surface methylene ( CII2) to form an allyl species ( CH2CH=CH2). 

BH3 acts both as a reducing agent for the acyl carbonyl and as a promoting agent for subsequent CO insertion into the metal-alkyl bond. As yet the process has been carried as far as C Hg, with Mn(CO)5(CH3), CO, and I B THF as reactants. 

E) Sigma-bond metathesis. Dihydrogen is observed to react with transition-metal-alkyl bonds even when the metal lacks lone pairs. In this case the reaction cannot be explained in terms of the oxidative-addition or reductive-elimination motif. Instead, we can view this reaction as a special type of insertion reaction whereby the ctmr bond pair takes the donor role of the metal lone pair and donates into the cthh antibond. When the M—R bonds are highly polarized as M+R, the process could also be described as a concerted electrophilic H2 activation in which R acts as the base accepting H+. 

Insertion reactions of C02 into the metal-hydride and metal-alkyl bonds are of considerable importance, since these reactions are involved not only in the catalytic cycle of the hydrogenation of C02 into formic acid but also in the catalytic cycle of co-polymerization of C02 and epoxide. In this regard, insertions of C02 into various metal-hydride, metal-alkyl, and similar bonds have been the subject of intense experimental investigation. For instance, C02 insertions into Cu(I)-CH3, Cu(I)-OR, Cu(I)-alkyl [26-28], Ru(II)-H [29], Cr(0)-H, Mo(0)-H, W(0)-H [30], Ni(II)-H and Ni(II)-CH3 bonds [31, 32] have been so far reported. 

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. 

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). 

LANGER (11) has postulated the presence of a 1 1 chelate and attributed its reactivity in polymerizing ethylene and butadiene to the absence of association of the metal alkyl and the increased polarization of the metal alkyl bond. 

CO into a metal-hydrogen bond, apparently analogous to the common insertion of CO into a metal-alkyl bond (6). Step (c) is the reductive elimination of an acyl group and a hydride, observed in catalytic decarbonylation of aldehydes (7,8). Steps (d-f) correspond to catalytic hydrogenation of an organic carbonyl compound to an alcohol that can be achieved by several mononuclear complexes (9JO). Schemes similar to this one have been proposed for the mechanism of CO reduction by heterogeneous catalysts, the latter considered to consist of effectively separate, one-metal atom centers (11,12). As noted earlier, however, this may not be a reasonable model. 

A thermally robust dihapto C02 complex was easily obtained according to equation (62).631 Complex (46) is the first authentic (X-ray diffraction) C02 complex of a hard oxophilic early transition element. The C02 (C—O bond lengths 1.283(8) and 1.216(8) A) lies close to the alkyl group, but its orientation is such that its insertion into the metal-alkyl bond would produce the unfavored metallacarboxylate ester instead of a carboxylato complex. 

At present, however, most experimental evidence supports the mechanism in which propagation takes place at the transition-metal-alkyl bond. Of the different interpretations, the one proposed by Cossee and Arlman303-306 is the most widely accepted.125 254 294... 

The strongest evidence in favor of propagation at the transition metal-alkyl bond is the existence of one-component, that is, metal-alkyl-free polymerization catalysts. Of these systems the Phillips catalyst was studied most thoroughly because of its commercial importance. Originally it was believed that Cr(VI) ions stabilized in the form of surface chromate and perhaps dichromate resulting from the interaction of Cr03 with surface hydroxyl groups above 400°C are the active species in polymerization 286,294... 

The possibility of insertion of C02 into metal-amide bonds via a mechanism not unlike S02 insertion into metal-alkyl bonds remains a viable possibility in C02 chemistry although it was not realized in these studies by Chisholm and co-workers. In this mechanism, C02 acting as an electrophile, attacks an amide nitrogen, with the intermediate then rearranging to the metal carbamate, (68). 

The migratory insertion of alkyl and aryl isocyanides into metal alkyl bonds produces an iminoacyl function. The early transition metal iminoacyl, like its oxygen counterpart, has been shown to bond in a dihapto fashion.62 It has recently been shown that it is possible to couple either an acyl and art iminoacyl or two iminoacyls on the metals titanium or zirconium to produce enamidolate and enediamide ligands, respectively (equation 34).62,63... 

Insertion of carbon monoxide, which is so common for metal-alkyl bonds, is very rare for metal amides but has been documented for amides of the actinides (equations 81 and 82),225... 

Carbanionic metal alkyls and hydridic metal hydrides will react with alcohols or phenols to give alkoxides and phenoxides, typically in excellent yields. The reaction is also important as it forms the basis for the calorimetric measurement of a large number of metal-alkyl bond dissociation energies.93,94 This synthetic method tends to be very convenient due to the volatility of the generated alkane or hydrogen side products. Monoalkyl alkoxides of Be,95 Mg96 and Zn97 can be obtained in this way (equation 26). 

The final step in the catalytic cycle is the cleavage of the metal-alkyl bond with acid, which must take place faster in the hydrocarboxylation of alkenes than -elimination. 

Fig. 10. Proposed intermediate for C02 insertion into metal alkyl bond in the presence of lithium counterion.

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