Alkylmetal insertion

Belonging to group (i) are alkylmetal carbonyls and cyclopentadienylmetal alkyl carbonyls of formula RMn(CO)5, CpFe(CO)2R, and CpMo(CO)3R. Solvent dependence of the reaction of MeMn(CO)5 with CjHi,NH2 is illustrated also in Table I. The rate varies markedly with the dielectric constant and with the nucleophilic power of the solvent. For example, on going from dimethylformamide to mesitylene, the rate of insertion is reduced by 10. Similarly, the sequence MeCN > MejCO > THF > CHCI3 > CjHj was reported for the reaction of MeMn(CO)5 with P(0CH2)3CR (R = Me and Et) in various solvents (97). Analogous trends were observed for the insertion reactions of CpFe(CO)2R and CpMo(CO)3R (48, 80, 98). 

Addition of an alkene to a compound containing a metal-H bond usually results in insertion, and it does in this case, too, to give the stabler 1° alkylmetal. Addition of CuBr to this complex might result in transmetallation, to give a C2-Cu bond. Addition of the copper compound to the unsaturated imide gives conjugate addition, perhaps by coordination of the C3=C4 tz bond and insertion into the C2-Cu bond. Workup gives the observed product. 

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

The insertion of alkene to metal hydride (hydrometallation of alkene) affords the alkylmetal complex 34, and insertion of alkyne to an M—R (R = alkyl) bond forms the vinyl metal complex 35. The reaction can be understood as the cis carbometallation of alkenes and alkynes. 

Aromatic carboxylic acids, a,/f-unsaturated carboxylic acids, their esters, amides, aldehydes and ketones, are prepared by the carbonylation of aryl halides and alkenyl halides. Pd, Rh, Fe, Ni and Co catalysts are used under different conditions. Among them, the Pd-catalysed carbonylations proceed conveniently under mild conditions in the presence of bases such as K2CO3 and Et3N. The extremely high toxicity of Ni(CO)4 almost prohibits the use of Ni catalysts in laboratories. The Pd-catalysed carbonylations are summarized in Scheme 3.9 [215], The reaction is explained by the oxidative addition of halides, and insertion of CO to form acylpalladium halides 440. Acids, esters, and amides are formed by the nucleophilic attack of water, alcohols and amines to 440. Transmetallation with hydrides and reductive elimination afford aldehydes 441. Ketones 442 are produced by transmetallation with alkylmetal reagents and reductive elimination. 

In a broad class of reactions of CH bonds with low-valent transition metals, however, the metal inserts into a CH bond of the alkane to give an alkylmetal hydride in which there is often a preference for formation of the least substituted alkyl. In any subsequent fimctionalization, the linear (or least branched) product is obtained, for example, nPrX and not iPrX. Since the branched product can be obtained... 

By the nature of its molecular mechanism, the carbonyl-insertion reaction represents a typical reaction mode of o alkyltransition metal complexes. Formation of the new C—C cr-bond takes place during a 1,2-alkyl-migration step, transforming an alkylmetal carbonyl moiety [cts-M(CO)R] into an acylmetal unit (M—COR) (89). In general, (s-cir-diene)-zirconocene complexes 5 appear to exhibit a substantial alkylmetal character (90). Therefore, it is not too surprising that some members of this class of compounds [in contrast to most other dienetransition metal complexes (97)] react with carbon monoxide with C—C bond formation (45). However, as demonstrated by X-ray structural data for 5 (Tables V... 

The trialkyls R3M (M=A1, Ga, In) undergo insertion reactions with COS to form alkylmetal thiocarboxylates (RCOS)xMR3 x (M=A1, Ga, In x=l-3), which are difficult to separate [327]. Treatment of trimethylaluminum with thioacetic acid, however, leads to a good yield of dimethylaluminum thioacetate 75 as yellow crystals that sublime (Scheme 6) [327]. On the basis of infrared and NMR spectra, the structure of 78 has been deduced to be a dimer [327]. Syntheses of Group 13 metal derivatives of selenocarboxylic acids have not been reported, although the formation of RCOSeAl(CH3)3 has been described as a short-lived intermediate in the reaction of RCOOR with [(CH3)2Al]2Se to form diacyl diselenide [281]. 

In the second type of process the metal acts as a carbenoid and inserts into the C—H bond, a process generally termed oxidative addition in organometallic chemistry (equation 1 b). This reaction is believed to go via the same sort of alkane complex as in the Shilov system, but, instead of losing a proton, it goes instead to an alkylmetal hydride. This may be stable, in which case it is observed as the final product, or it may react further. 

All of the metal atoms of the first transition series undergo photoinsertion into the C—H bond of methane in matrices (equation 38) . In some cases (e.g. Fe, Co), a precursor to the insertion product, identified as an alkane complex, was also detected. Subsequent irradiation at a longer wavelength generally caused reductive elimination (e.g. Fe, Co). In other cases (e.g. Cu), the alkylmetal hydride fragments to an alkyl radical and an M—H group. Longer-chain alkanes also react with Fe . 

Early-transition-metal-alkoxo complexes are also stable toward other typical organome-tallic processes, such as reductive elimination. Hydridometal-alkoxo, alkylmetal-alkoxo, and arylmetal-alkoxo complexes of the early metals are known, but none of them undergoes reductive elimination to form a carbon-oxygen bond. Early-metal-alkoxo complexes also do not tend to undergo migratory insertion processes. In fact, this lack of insertion was exploited by Jordan to illustrate coordination of alkenes to d metal centers. " ... 

The transformation of an alkylmetal carbonyl complex into a metal-acyl complex is one of the most common types of migratory insertion reactions (Equation 9.3). Examples of CO insertion into metal-alkyl complexes are known for all of the transition elements. This reaction class has been the subject of review articles. These reactions occur by a family of diverse, delicately balanced reaction pathways the dominant mechanism depends on the reaction conditions, especially the solvent. Although these pathways are now imderstood in considerable detail, the precise identities of the intermediates in some of these reaction pathways are unknown. 

Reactions of mtemal olefins are more complex than reactions of terminal olefins (Scheme 16.3). As mentioned previously, terminal nitriles are often formed from reactions of internal olefins. The formation of terminal nitriles results from insertion of the internal olefin to form a branched alkylmetal intermediate (A in Scheme 16.3) that undergoes isomerization to the terminal alkyl intermediate B prior to reductive elimination of the final linear nitrile faster than it undergoes reductive elimination to form the branched nitrile. Internal olefins react more slowly than terminal olefins, and this relative rate can be traced to the slower insertion of internal olefins into metal hydrides. Lewis acids, such as ZnClj and AlClj, promote these reactions of isolated alkenes. 

Equilibrium between Acyl- and Alkylcobalt Carbonyls. The insertion of carbon monoxide into the carbon-metal bond of alkylmetal carbonyls to form the corresponding acyl derivatives is one of the most important and most investigated organometallic reactions (169-173). In this reaction, a new carbon-carbon... 

Another way to trap the alkylmetal complex is to fill the vacant site that opens up on the metal in the insertion with another ligand ... 

Examples of inserting species are carbenes, olefins, acetylenes, and epoxides. Reaction of diazomethane with metal hydrides or halides has given several alkylmetal complexes, for example. 

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