Carbon into metal alkyls

The study of stoichiometric CO insertions into transition metal complexes is of great importance because this reaction is the first step m the catalytic conversion of carbon dioxide. Hence, these investigations can lead to the possibility of introducing carbon dioxide into transition metal-catalyzed synthetic processes. Analogies with carbon monoxide chemistry may be drawn, for instance. from the CO insertion into metal alkyl bonds leading to such important industrial processes as hydroformylation and carbonylalion. 

The low reactivity of many transition metal alkyls toward CO2 insertion may be attributable to the less polar nature of the M-R bond or to steric hindrance from ancillary ligands. The poor coordinating ability of CO2 may also be a factor via-a-vis CO. Carbon monoxide insertions into metal-alkyl bonds are common for transition metal systems. Knowledge about CO2 insertion into metal alkyls may lead to catalytic systems for carboxylation of organic substrates, an initial step for producing organic compounds from carbon dioxide. 

Because of the importance of the CO insertion into a transihon metal-carbon a-bond in relation to the transition metal-catalyzed carbonylation, the mechanism of the CO insertion has attracted considerable attenhon [1,2]. Through fundamental studies on model complexes, the route of CO insertion has been established for most transition metal complexes to be the one through alkyl migration mechanism [3-8]. While CO insertion can occur with various metal-heteroatom o-bonds, only insertion into metal alkyls (hydrocarbyls) will be discussed in this chapter. 

Carbon Disulphide. This can insert into metal-alkyl and into metal-hydrogen bonds a metal-carbon disulphide 7r-complex is a possible intermediate. ... 

Like the insertion of CO into metal-alkyl bonds, the insertion of olefins into metal-alkyl bonds occurs by a concerted migratory insertion pathway (Equation 9.59). The alkyl group migrates with retention of configuration at carbon. Syn addition of the metal and the alkyl group occurs across the olefin. 

Based on time-resolved spectroscopic techniques, the existence of an intermediate having an -carbonyl ligand was proposed to occur during the carbon monoxide migratory insertion into metal-alkyl bonds (Scheme 38a). 

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... 

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]. 

Certain metal alkyl compounds from p- and d-block elements react under very mild conditions with 1 under insertion into the element-carbon bond. Some examples are shown in Scheme 9. 

Many transition metal alkyls react with carbon monoxide to give acyl compounds. In all these cases the acyl derivatives can be detected at least by infrared methods and in most cases isolated. Molybdenum, manganese, rhenium, iron, cobalt, rhodium, nickel, palladium, and platinum alkyls, Grignard reagents, and boranes, all react with carbon monoxide, and one can explain the products from these on the basis of carbon monoxide inserting into the metal alkyl. 

In the process of olefin insertion, also known as carbometalation, the 1,2 migratory insertion of the coordinated carbon-carbon multiple bond into the metal-carbon bond results in the formation of a metal-alkyl or metal-alkenyl complex. The reaction, in which the bond order of the inserted C-C bond is decreased by one unit, proceeds stereoselectively ( -addition) and usually also regioselectively (the more bulky metal is preferentially attached to the less substituted carbon atom. The willingness of alkenes and alkynes to undergo carbometalation is usually in correlation with the ease of their coordination to the metal centre. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

The insertion of ligated CO into metal-carbon -bonds (or rather the migration of an alkyl group to a coordinated CO) is a key step in a variety of synthetic and catalytic important processes, e.g., in hydroformylation (145), the Fischer-Tropsch reaction (146) and the synthesis of acetic acid from methanol (147). 

The insertion of CO into metal-carbon a bonds has been reviewed.585-590 Carbonylation of alkyl platinum(II) complexes usually requires elevated temperatures, although at higher temperatures the reaction is reversible (equation 211).591 With PtMe2(dppe) insertion occurs into only one of the Pt—Me bonds. For complexes PtX(Ar)L2, carbonylation follows pseudo first-order kinetics. Rates are decreased by addition of L to a maximum value where the carbonylation rate is independent of L. The pathway involves formation of a five-coordinate intermediate PtX(Ar)(CO)L2, followed by dissociation to form PtX(Ar)(CO)L. The migratory step to yield PtX(COAr)L is unaffected by added L. This pathway is outlined in Scheme 6.502... 

SO2 inserts into metal-carbon a bonds18). The reaction is stereospecific with respect to the configuration at the a-carbon atom of the alkyl group19) and proceeds with inversion of configuration at the a-carbon atom, as shown on threo-erythro isomeric derivatives of C5H5Fe(CO)2CHDCHDC(CH3)3 by H NMR spectroscopy20 70). 

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