Preparation of Aldehydes Hydroformylation and Alcohols

Formation of aldehydes by the reaction of alkene, CO and H2 catalysed by Co2(CO)8 was discovered by Rolen in 1938 [25]. This is the 1,2-addition of H and CHO to alkenes, and hence called hydroformylation or the oxo reaction. Production of butanal, (33) from propylene as a main product is an important industrial process. Aldol condensation of butanal, followed by hydrogenation affords 2-ethyl-1-hexanol (34), which is converted to phthalate, and used as a plasticizer of poly(vinyl chloride). 

Hydrogenolysis of Co2(CO)8 generates hydrocobalt carbonyl 35 as a catalyst precursor. The hydroformylation is explained by the insertion of propylene to hydrocobalt carbonyl 36 to give propylcobalt 37. Then CO insertion generates the 

Rh complexes show higher catalytic activity than Co2(CO)8, and at present the Rh 

Regioselective formation of linear aldehydes is important in industrial process. The ligand BIPHEPHOS (L), developed by Union Carbide, gives the highest ratio of butanal from propylene. This ligand is useful for regioselective formation of linear aldehydes from various functionalized 1-alkenes under mild conditions. The linear aldehyde 40 was obtained from 39 and converted to the indolizidine alkaloid 41 [27]. 

Several interesting synthetic applications of hydroformylation are known. As one example, hydroformylation of enol ethers to afford -hydroxy aldehydes can be extended to catalytic aldol synthesis. The Rh-catalysed hydroformylation of the cyclic enol ether 42 affords the protected. vy -3,5-dihydroxyaldchydc 43 without forming the anti product 44. The regioselectivity of attack on the terminal carbon is also very high [28], 

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