Reactivity of the Original Catalyst

Relative rates and n i ratios for reaction of substituted alkenes. 

Many studies have been conducted on the mechanism of hydroformylation catalyzed by HRh(CO)2(PPh3)2. Some of these studies have focused on identifying the complexes in the catalytic system, and others have focused on obtaining kinetic data. Together, these data have provided mechanistic insight. However, hydroformylation involves many different steps, the formation of at least two isomeric products, and the intermediacy of species that can exist in several isomeric forms. Thus, this catalytic system is very complex, and only broad conclusions are drawn here. 

The basic steps of the mechanism of hydroformylation catalyzed by the combination of Rh and PPhj are similar to those of hydroformylation catalyzed by HCo(CO). However, the number of possible intermediates is much larger because many combinations of the number of phosphines and geometric orientation are possible in each intermediate complex. For example, phosphines in a five-coordinate complex can occupy the apical or equatorial positions, and these complexes are typically stereochemically nonrigid. Similarly, two phosphines in a four-coordinate intermediate can be located cis or trans to each other. A five-coordinate hydridorhodium-alkene complex containing diequatorial phosphines might be expected to convert to a four-coordinate Rh alkyl complex containing trans phosphines, but it could also lead to an alkyl complex containing cis phosphines. 

In addition, many different combinations of steps can control the n i ratio. The regiochemistry of aldehyde might be controlled by irreversible addition of the Rli-H unit across the alkene, irreversible insertion of CO into an equilibrating mixture of n-alkyl- and i-alkylrhodium complexes, or by irreversible hydrogenolysis of an equilibrating mixture 

A serious problem encountered when practicing industrial hydroformylation is separation and recycling of the valuable rhodium catalyst. This problem is particularly severe when forming aldehydes that require temperatures for distillation that lead to 

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