Metal-carbon bonds, insertion reactions

It has been proposed that poisons, such as CO and COz, insert selectively into the propagative transition metal-carbon bond. The reactions involved are believed to proceed in the following sequence (for CO as the poison) ... 

The electrophile ftat modifies a coordinated ligand can be a proton, a strong Lewis acid, or an unsaturated electrophile. Examples of Lewis acids include tiityl cahon or per-fluoroarylboranes, and examples of unsaturated electrophiles include CO, SO, isocyanates, aldehydes, ketones, and related compoimds. Reactions of these electrophiles can lead to the formation of catioruc metal complexes by abstraction of a hydride or hydrocarbyl group by the Lewis acid, or they can lead to products from insertion of the unsaturated electrophile into the metal-carbon bond. These reactions are sho-wn generically in Equations 12.1-12.3. 

Carbon-sulfur bond breaking was observed while treating thiophene with Cp"Co(C2H4)2 at 70 °C in benzene. A cobalt metal center had inserted into the C-S bond and a second Cp Co unit had complexed to the cobalt, sulfur, and C-C double bond 330. Further reaction of the resulting compound with H2S produced a new butadiene dithiolate compound, in which a sulfur atom has been inserted into the metal-carbon bond 331. Reaction with dibenzothiophene produced an analogous dinuclear C-S insertion-reaction adduct (Equation (54)). ... 

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) ... 

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. 

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). 

The polymerization of conjugated dienes with transition metal catalytic systems is an insertion polymerization, as is that of monoalkenes with the same systems. Moreover, it is nearly generally accepted that for diene polymerization the monomer insertion reaction occurs in the same two steps established for olefin polymerization by transition metal catalytic systems (i) coordination of the monomer to the metal and (ii) monomer insertion into a metal-carbon bond. However, polymerization of dienes presents several peculiar aspects mainly related to the nature of the bond between the transition metal of the catalytic system and the growing chain, which is of o type for the monoalkene polymerizations, while it is of the allylic type in the conjugated diene polymerizations.174-183... 

The most efficient catalysts for the homo Diels-Alder reactions of norbornadiene were found to be cobalt327 and nickel328 complexes. The general mechanistic pathway that has been proposed for these reactions has been depicted in equation 161329. According to this mechanism, co-ordination of norbornadiene and the olefin or acetylene to the metal center gives 557, which is in equilibrium with metallocyclopentane complex 558. Then, insertion of the olefin or acetylene in the metal-carbon bond takes place to form 559. Reductive elimination finally liberates the deltacyclane species. 

Insertion of carbon monoxide and alkenes into metal-carbon bonds is one of the most important reaction steps in homogeneous catalysis. It has been found for insertion processes of platinum [16] that the relative positions of the hydrocarbyl group and the unsaturated fragment must be cis in the reacting complex [17], The second issue concerns the stereochemical course of the reaction, insertion versus migration as discussed in Chapter 2.2. 

In general, carbonylation proceeds via activation of a C-H or a C-X bond in the olefins and halides or alcohols, respectively, followed by CO-insertion into the metal-carbon bond. In order to form the final product there is a need for a nucleophile, Nu". Reaction of an R-X compound leads to production of equivalent amounts of X", the accumulation of which can be a serious problem in case of halides. In many cases the catalyst is based on palladium but cobalt, nickel, rhodium and mthenium complexes are also widely used. 

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