Metal—carbon bonding insertion

Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiCU (169) and the catalyst Cr -CjHsU/SiCU (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ir-allyl complexes (ir-allyl ligands being removed from the active center) (140) C14 of the...

A. Insertion Reactions into Metal-Carbon Bonds... 

The subjects of structure and bonding in metal isocyanide complexes have been discussed before 90, 156) and will not be treated extensively here. A brief discussion of this subject is presented in Section II of course, special emphasis is given to the more recent information which has appeared. Several areas of current study in the field of transition metal-isocyanide complexes have become particularly important and are discussed in this review in Section III. These include the additions of protonic compounds to coordinated isocyanides, probably the subject most actively being studied at this time insertion reactions into metal-carbon bonded species nucleophilic reactions with metal isocyanide complexes and the metal-catalyzed a-addition reactions. Concurrent with these new developments, there has been a general expansion of descriptive chemistry of isocyanide-metal complexes, and further study of the physical properties of selected species. These developments are summarized in Section IV. 

Insertion Reactions of Transition Metal-Carbon -Bonded Compounds I Carbon Monoxide Insertion... 

Carbon monoxide insertion is not restricted to transition metal-carbon bonds, although M—C is by far the most common substrate involved. Reactions have also been reported which lead to insertion of CO into M—O (114) and M—N (199) bonds. 1,1-Additions of M—H (27, 114) and M—M (104) linkages to CO have been postulated, too. However, direct replacement of CO, without rupture of the W—H bond, is indicated for the reaction between CpW(CO)3H (or -D) and PPhj (5) ... 

The insertion of unsaturated molecules into metal-carbon bonds is a critically important step in many transition-metal catalyzed organic transformations. The difference in insertion propensity of carbon-carbon and carbon-nitrogen multiple bonds can be attributed to the coordination characteristics of the respective molecules. The difficulty in achieving a to it isomerization may be the reason for the paucity of imine insertions. The synthesis of amides by the insertion of imines into palladium(II)-acyl bonds is the first direct observation of the insertion of imines into bonds between transition metals and carbon (see Scheme 7). The alternating copolymerization of imines with carbon monoxide (in which the insertion of the imine into palladium-acyl bonds would be the key step in the chain growth sequence), if successful, should constitute a new procedure for the synthesis of polypeptides (see Scheme 7).348... 

A detailed study of the mechanism of the insertion reaction of monomer between the metal-carbon bond requires quantitative information on the kinetics of the process. For this information to be meaningful, studies should be carried out on a homogeneous system. Whereas olefins and compounds such as Zr(benzyl)4 and Cr(2-Me-allyl)3, etc. are very soluble in hydrocarbon solvents, the polymers formed are crystalline and therefore insoluble below the melting temperature of the polyolefine formed. It is therefore not possible to use olefins for kinetic studies. Two completely homogeneous systems have been identified that can be used to study the polymerization quantitatively. These are the polymerization of styrene by Zr(benzyl)4 in toluene (16, 25) and the polymerization of methyl methacrylate by Cr(allyl)3 and Cr(2-Me-allyl)3 (12)- The latter system is unusual since esters normally react with transition metal allyl compounds (10) but a-methyl esters such as methyl methacrylate do not (p. 270) and the only product of reaction is polymethylmethacrylate. Also it has been shown with both systems that polymerization occurs without a change in the oxidation state of the metal. 

It is premature to discuss the mechanism of the insertion reaction in the propagation step. More information is required on the effect of various ligands on the equilibrium and character of the transition metal-carbon bond before this can sensibly be done. 

The transition metal-catalyzed polymerization of olefins yields high molecular weight polymers as the result of the successive insertion of monomer into the metal-carbon bond of the growing polymer chain. This chain growth is... 

In the propagation process of Ziegler-Natta polymerization, the insertion of olefin into a metal-carbon bond is the most important basic step, but many questions concerning to this process remained unanswered for a long time. 

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]. 

These reactions are examples of 1,2-addition. In addition to insertion in the M-H bond as shown earlier, this type of reaction also occurs with metal-halogen and metal-carbon bonds. The following equation shows an example of a reaction in which insertion occurs between a metal and a halogen ... 

The free dithiocarboxylic acids can be isolated, but their salts are preferred. In some cases their metal complexes can be prepared directly by insertion of carbon disulfide into metal-carbon bonds. Thus, the reaction of Grignard reagents, RMgX, with CS2, followed by acid treatment gives the dithiocarboxylic acids RCSSH and metal complexes in good yields.311... 

The molecular weight (M 10,200,000) represents the highest molecular weight known to date for a linear, synthetic copolymer. DFT calculations suggest that steric congestion, derived from the triethylsilyl group and the amine moiety, near the polymerization reaction center diminishes the rates of chain termination or transfer processes yet permits the monomer access to the active site and the monomer s insertion into the metal-carbon bond (Fig. 21). 

Polymerization occurs by repeated migratory insertion of olefin into the (Tv-oriented metal-carbon bond by the generally accepted Cossee mechanism [5, 60]. This mechanism is believed to be shared by all transition metal coordination polymerization... 

Each step includes elementary acts that require different properties of the metal, for example, sufficiently low ionization potential to favor oxidative addition, sufficiently weak metal-carbon bonds, tendency to form square-planar complexes and to reach pentacoordination to allow insertion, a sufficiently high electron affinity to allow reductive elimination, and so on. Some properties are conflicting and a compromise has to be reached. 

The basic assumptions common to most mechanism studies relative to transition metal catalyzed polymerizations are as follows (i) The mechanism is essentially monometallic and the active center is a transition metal-carbon bond.13-15,18,19 (ii) The mechanism is in two stages coordination of the olefin to the catalytic site followed by insertion into the metal-carbon bond through a cis opening of the olefin double bond.13,20,21... 

Since the 1960s the syndiospecific chain-end controlled polymerization of propene in the presence of homogeneous vanadium-based catalytic systems has been known. For these systems, it has been well established by the work of Zambelli and co-workers that the polymerization is poorly regioselective and the stereoselective (and possibly syndiospecific) step is propene insertion into the metal secondary carbon bond with formation of a new secondary metal-carbon bond.133134... 

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