Assisted carbonyl insertion reactions

The use of CO containing 3% H2 in the hydroesteriflcation reaction is standard, suggesting that a cobalt hydrocarbonyl is the active catalyst species. The reaction sequence involves olefin insertion into the Co—H bond, (carbonyl insertion) to give an acyl complex and cleavage with alcohol assisted by the pyridine promoter ... 

In Scheme 1.2, all of the types of carbonylations that are discussed in the book are depicted. Alcohols, amines, ethers, carboxylic acids and halides can be converted to acids, amides, esters, ketones, alkynones, alkenones, anhydrides and acid halides with the assistance of transition metal catalysts in the presence of a CO source. The CO sources used can be carbon monoxide gas, Mo(CO)6, Co(CO>6, formic acid, aldehyde, etc. If the starting material is alcohols or amines, some additives for activation are needed, such as BuONO, TsCl, AcCl. If the substrate is (Hetero)ArH, additional oxidants will be necessary this is a so-caUed oxidative carbonylation. If an unsaturated compound is to be carbonylated, a nucleophile NuH that carries an acidic hydrogen has to be present. In the case of insertion reactions, this is not necessary. 

However, iron complexes may also display a beneficial effect. Thus, a promoting effect on hydroformylation was observed with Si02-supported Rh-Fe " bimetallic carbonyl clusters (Scheme 1.61) [12]. Based on Mossbauer spectroscopy, it was proposed that iron assists during the insertion reaction of CO into the Rh-C bond. Likewise, the hydrogenation of the intermediary alkoxy rhodium species to produce the alcohols may benefit from this bi-site interaction. 

It is not easy to distinguish between these alternatives since they give rise to such a similar kinetic pattern. It is possible that each may occur under appropriate conditions analogies for both possibilities exist in metal carbonyl chemistry (96). Subsequent steps such as the insertion of olefin to give the alkylcobalt carbonyl are also susceptible to SN1 or SN2 interpretations. Carbon monoxide insertion does occur in the absence of an atmosphere of carbon monoxide but the reaction could be assisted here by the presence of olefin (73). 

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. 

This base-induced CO insertion is more complex from a kinetic viewpoint than the simple carbonylation of CH3Mn(CO)s. Although in nonpolar solvents such as hexane and mesitylene, the rate of carbonylation is dependent on both CH3Mn(CO)5 and base [C6H11NH2, PhaP, (PhO)3P] concentrations, the rate depends only on CH3Mn(CO)s concentration in polar solvents (757). This result suggests the operation of two competitive mechanisms (1) a bimolecular reaction like that proposed for the CO reaction, and (2) a solvent-assisted sequence such as... 

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