Formylation of Organometallic Compounds

Aryl- and heteroaryl halides can be converted to organolithium reagents which on reaction with iron pentacarbonyl to give adducts [ArCOFe(CO)4]. These generate aldehydes in moderate yields on acidification of the reaction mixture (Eq 1.31). °  

In the presence of palladium-phosphine complexes such as Pd(PPh3)2X2 (X=CI, Br or I), aryl, heteroaryl and vinylic halides are catalytically converted to aldehydes on treatment with synthesis gas (CO/H2). Although a stoichiometric amount of tertiary amine is necessary to remove the hydrogen halide formed in the reaction, the yields are generally good, given the somewhat forced conditions (50-100bar at 100-150 C) (Eq 1.32). °  

Interestingly the formylation of o-dibromobenzene yields only benzaldehyde due to reductive removal of the second bromine group. The reactivity of the iodides and bromides are comparable. 

With iodo compounds yields are particularly high as they carbonylate more rapidly compared to the chloride and bromide derivatives. This can be ascribed to the greater ease of oxidative addition of the C-l bond to the zero-valent palladium complex. Activation to oxidative addition of the C-CI bond in chloroarene may be achieved by the formation of the corresponding arenetricarbonyl derivative, after which palladium-catalyzed formylation of the complex proceeds in fair yield at 30 bar CO/H2 pressure at 130°C (Eq 1.33). °  

The results obtained for various chloroarenes show that coordination of the tricarbonylchromium group to the chloroarenes enables their carbonylation even in the presence of moderately electron donating groups. The reaction fails in the absence of the tricarbonylchromium group. 

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