Carbonyl Insertion Alkyl Migration

Carbonyl insertion, the reaction of CO with an alkyl complex to give an acyl [—C(=0)R] product, has been well-studied. The reaction of CHsMnfCOls with CO is an excellent example  

The insertion of CO into a metal-carbon bond in alkyl complexes is of particular interest for its potential applications to organic synthesis and catalysis (Section 14.3), and its mechanism deserves careful consideration. 

From the net equation, we might expect that the CO inserts directly into the Mn—CH3 bond. However, other mechanisms are possible that would give the overall reaction stoichiometry and involve steps other than the insertion of an incoming CO. Three plausible mechanisms have been suggested  

Direct insertion of CO into a metal-carbon bond. 

Migration of a CO ligand to give inhamolecular CO insertion. This would yield a 5-coordinate intermediate, with a vacant site available for attachment of an incoming CO. 

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Carbon Monoxide.—Some examples of carbon monoxide insertion (alkyl migrations) have been mentioned in the discussion of carbonylation and hydroformylation reactions in the previous chapter (Part IV, Chapter 3, Section 10). The remaining reports will be treated here in Periodic Table order. 

Evidence was shown for migration of an alkyl group in carbonyl insertion, and deinsertion steps between the methyl carbonyl rhodium complex [ r/Vi/ -Indenyl-l -(CH2)3PPh2 Rh(CO)-Me](BF4) and the acetyl rhodium complex [ r/5 r/l-(Indenyl-l -(Cl I2)3PPh2) RhI(COMe)] by crystallography as well as by 1H NMR spectroscopy.28... 

Upon the addition of Lewis (i.e. All ) and protonic non-compSexing. acids (i.e. HPF5) promoters to [Ru(00)3X3] catalysts, improvements in selectivity to valuable products (acetic acid + ethyl acetate) and in reaction rate are observed. This is believed to be due to an acceleration by acids of the alkyl migration-carbonyl insertion step of the process, as discussed in detail in a recent paper (10). 

CO is a representative species for n.o-insertion its insertion into C—Pd bonds affords acylpalladium complexes such as 15. Mechanistically, the CO insertion is 1.2-alkyl migration to coordinated CO. This is an important step in carbonylation. SO , isonitriles, and carbenes are other species which undergo a.a-insertion. 

A chiral metal center, as is found in a pseudotetrahedral iron complex with cyclopentudienyl. carbonyl, triphenylphosphine, and ethyl ligands, hus also beer used to address the question of alkyl migration versus carbonyl insertion. Inversion of... 

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... 

Monsanto acetic acid synthesis 4), and the hydroformylation or 0X0 reaction (5). A key mechanistic step in catalytic carbonylation reactions is the migration of an alkyl group onto an adjacent carbonyl ligand. This reaction involves the formation of a new carbon-carbon bond and has been termed a carbonyl insertion reaction since a CO ligand has been formally inserted into the transition metal-carbon (r-bond. Because of the industrial and commercial importance of these catalytic reactions, the search for stoichiometric systems in which this step can be observed directly has been, and still is, one of great endeavor. 

When octahedral alkylpentacarbonyl derivatives of manganese(I) undergo carbon monoxide insertion assisted by a nucleophile L different from carbon monoxide (L = tertiary phosphines, amines, CO, etc.), the cis product is initially formed this observation, however, is not sufficient to permit one to distinguish between the alkyl migration mechanism [reaction (a)] and the insertion of a precoordinated carbonyl group into the manganese-carbon bond [reaction (e)]. In both cases mutually cis positions are involved. 

Carbonyl insertion in MeFe( -C5H5)(PPh3)(CO) is promoted by BF3, with high stereospecificity, by means of alkyl migration. For the CO insertion in MeMn(CO)5, rate enhancements up to one order of magnitude were found in the presence of protonic acids the reaction was, however, complicated by the cleavage of the Mn-Me bond to give methane,... 

This section focuses on the second important means of C-C bond construction migratory insertion. As discussed in Chapter 8, transition metal-catalyzed 1,1-carbonyl insertion (more properly called alkyl migration to a carbonyl ligand)... 

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