Carbonyl insertion reactions mechanism

An allenylaldehyde can be transformed efficiently into an a-methylene-y-butyro-lactone by a ruthenium-catalyzed carbonylative cycloaddition process (Scheme 16.34) [37]. The reaction mechanism may involve a metallacyclopentene, which undergoes insertion of CO and reductive elimination leading to the product. 

Jhe reaction mechanism of the various metal complexes clearly have much in common. The recently recognized insertion reaction appears to be a particularly good example of a reaction which is general among the metal compounds. In the following discussion I intend to point out the generality of the insertion reaction with examples from the literature and from our own wrork. The most complete series of substantiated insertion reactions involves the organocobalt carbonyl complexes, and these reactions will form the nucleus of the discussion. 

The five-coordinate complex then reacts rapidly with external CO to give (CO)5MnCOCH3. Thermodynamically, the progress of the reaction to completion requires a ligand (not necessarily CO PPh3 will do) which can react quickly with the five-coordinate acyl complex to remove it from the system, so that more will be formed by rearrangement of Mn(CO)5CH3. The mechanism for the intramolecular carbonyl insertion is presumably a concerted process ... 

Imhof et al. [22] studied the reaction mechanism of the [2+2+1] cycloaddition reactions of diimines, CO, and ethylene catalyzed by iron carbonyl complexes on the basis of density functional theory (Scheme 4). The catalytic reaction does not start when CO dissociates from 10 followed by the addition of ethylene, but instead the associative pathway to 11 is proposed. In addition, it can be concluded that the insertion of CO in 11 takes place into a C-Fe bond but not... 

The mechanisms of the two key steps are worth discussion. Hydrometallation occurs by initial n-complex formation followed by addition of the metal to one end of the alkene and hydrogen to the other. Both of these regioisomers are formed. The carbonyl insertion reaction is another migration from the metal to the carbon atom of a CO ligand. 

The possible reaction mechanism for the formation of 31 is shown in Scheme 15. Insertion of alkyne 14 into silazirconacyclopropane 3 gives silazirconacyclopentene 22. Then, insertion of carbon monoxide into the carbon-zirconium bond in silazirconacyclopentene 22 gives silazirconacyclohexenone 34, whose carbonyl oxygen would coordinate to zirconium metal. Then the zirconium carbon bond migrates onto silicon to afford oxazirconacyclohexene 36 via 35 [26]. Deuterolysis of 36 would afford 31-D2, which has two deuteriums. 

Under pressure of CO and H2, the cobalt catalyst precursor is transformed into cobalt carbonyl hydride, HCo(CO)4. The main steps of the reaction mechanism, first elucidated by D. S. Breslow and R. F. Heck, involve (a) /3-hydrogen transfer to the coordinated olefin, (b) the insertion of CO to form an acyl intermediate, and (c) the hydrogenolysis of the acyl, with formation of the aldehyde product ... 

The proposed reaction mechanism (see Scheme 4) is similar to that which is well established for olefin reduction, with the insertion of a meial-hydride bond into the carbonyl group coordinated to (he metal center. A mechanism in which reduction occurs by a simple nucIcopliiUc attack of the hydrogen atom of the transition metal hydride (which behaves like Blit or AlHa ) into the carbonyl group of the aldehyde, cannot be ignored ... 

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... 

Detailed mechanisms for the amidocarbonylation reaction have been proposed by both Pino [2] and Magnus [4], wherein the first step is the formation of a hemi-amidal, followed by the nucleophilic substitution of a hydroxyl group by cobalt tetracarbonyl hydride and carbonyl insertion to an (ct-amidoalkanoyl) cobalt intermediate. This intermediate then provides the desired 7V-acyl-a-amino acid by direct hydrolysis, or via an lactame intermediate, followed by hydrolysis. 

Alkenyl substituents attached to aziridine ring carbons allow a palladium-catalyzed carbonyl-insertion reaction to occur, producing a /1-lactam (Scheme 46) <91SL91>. The proposed mechanism <93BMC2415> is based on an analogous reaction of vinyl epoxides <85JA6123>. Initial attack of palladium(O) on the optically active /ra .s-vinylaziridine (67) leads to the zr-allyl palladium complex... 

In many cycloaddition reactions brought about with metal carbonyls, insertion of CO is observed. The probable mechanism of such reactions has already been depicted with the transformation (27) - -(28) (see section HI). 

The hydride [MoH(CO)3Cp] forms a monocation which rapidly yields [ Mo(CO)3Cp 2] by proton dissociation (39). Analogous alkyl cations are intermediates in the oxidatively induced carbonyl insertion reactions of [MR(CO)3Cp] (377). Thus, [M(CH2-p-F-C6H4)(CO)3Cp](M = MoorW), LiCl, and Ce(IV) in methanol give methyl-p-fluorophenyl acetate (Scheme 23) the mechanism is discussed more fully in Section VII,E. 

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