Hydroformylation reversed regioselectivity

A catalyst used for the u-regioselective hydroformylation of internal olefins has to combine a set of properties, which include high olefin isomerization activity, see reaction b in Scheme 1 outlined for 4-octene. Thus the olefin migratory insertion step into the rhodium hydride bond must be highly reversible, a feature which is undesired in the hydroformylation of 1-alkenes. Additionally, p-hydride elimination should be favoured over migratory insertion of carbon monoxide of the secondary alkyl rhodium, otherwise Ao-aldehydes are formed (reactions a, c). Then, the fast regioselective terminal hydroformylation of the 1-olefin present in a low equilibrium concentration only, will lead to enhanced formation of n-aldehyde (reaction d) as result of a dynamic kinetic control. 

It is generally accepted that in rhodium-catalyzed hydroformylation the formation of the most stable linear and branched alkyl rhodium complexes is assumed to be the step that determines the regioselectivity when there are no P-elimination processes (Figure 1), (see chapter 2). But the reverse process, the dissociation of the metal-alkyl via P hydride elimination, plays an important role in the regioselectivity of the reaction, because it helps to equilibrate the alkyl species and can produce isomerization. In this case the regioselectivity is determined by the relative migratory ability of both metal-alkyl intermediates. 

Related to this work, Seller and Geissler [109] advocated the use of bimetallic catalysts, one for the isomerization and the other for the hydroformylation. Indeed, with a catalytic system comprising a rhodium complex based on a chelating phosphine-phosphite ligand and Ru3(CO)j2 (0.1-0.5 mol%), almost a reversal of the regioselectivity in the reaction with -2-butene in comparison to the monometallic rhodium catalyst l/b = 42 58 TOP = 700 h ) was achieved. 

Halide anions affect the rate of hydroformylation of internal olefins as well as the regioselective properties of the catalyst [136]. The rate of hydroformylation of thermally equilibrated internal higher alkenes increased by a factor of 6-7 by adding substoichiometric amounts (with respect to palladium) of Cl" or Br" and by a factor of 3-4 with I" [137]. When a thermally equilibrated mixture of internal Cg-Cj g olefins was subjected to isomerization-hydroformylation, a reverse effect on regioselectivity was observed [136e]. Thus, the formation of the linear aldehyde increased in the following order iodide > bromide > chloride. 

In the HAM of internal olefins with rhodium Xantphos-type ligands, the same steric dependency on the bite angle concerning the formation of the linear amine as in hydroformylation was observed. An increase in the bite angle up to 125°, which was realized with Nixantphenoxaphos (1), resulted in an increase in n-regioselectivity, whereas with wider angles a reversed tendency was... 

---