Rhodium thermodynamic control

Alcohols will serve as hydrogen donors for the reduction of ketones and imi-nium salts, but not imines. Isopropanol is frequently used, and during the process is oxidized into acetone. The reaction is reversible and the products are in equilibrium with the starting materials. To enhance formation of the product, isopropanol is used in large excess and conveniently becomes the solvent. Initially, the reaction is controlled kinetically and the selectivity is high. As the concentration of the product and acetone increase, the rate of the reverse reaction also increases, and the ratio of enantiomers comes under thermodynamic control, with the result that the optical purity of the product falls. The rhodium and iridium CATHy catalysts are more active than the ruthenium arenes not only in the forward transfer hydrogenation but also in the reverse dehydrogenation. As a consequence, the optical purity of the product can fall faster with the... 

With substrates bearing two different substituents at the 3-position, such as 3-ary 1-3-melhyl-1.4-pentadienes, tm/rs-products are favored under thermodynamic control (transjeis 4 1). Preferential formation of c/s-products (up to transicis 1 2) can be achieved under kinetic control using lower reaction temperatures, shorter reaction times or variations in the catalytic system. However, epimerization of stereogenic carbon centers adjacent to the carbonyl group is observed under the reaction conditions15. Preliminary experiments u ith chirally modified rhodium catalysts show only low asymmetric induction in the cyclopentanone products19. 

However, considerable amounts of 2,3-dihydrofuran 50 and tetrahydro-furan-2-carbaldehyde 53 were present because of an isomerization process. The isomerization takes place simultaneously with the hydroformylation reaction. When the 2,5-dihydrofuran 46 reacts with the rhodium hydride complex, the 3-alkyl intermediate 48 is formed. This can evolve to the 2,3-dihydrofuran 50 via /3-hydride elimination reaction. This new substrate can also give both 2- and 3-alkyl intermediates 52 and 48, respectively. Although the formation of the 3-alkyl intermediate 48 is thermodynamically favored, the acylation occurs faster in the 2-alkyl intermediates 52. Regio-selectivity is therefore dominated by the rate of formation of the acyl complexes. The modification of the phosphorus ligand and the conditions of the reaction make it possible to control the regioselectivity and prepare the 2- or 3-substituted aldehyde as the major product [78]. As far as we know, only two... 

In this case a thermodynamic and/or a kinetic factor can control the overall difference in the activation energy and hence the diastereomeric composition of the alkyl-rhodium complexes and the asymmetric induction. The thermodynamic factor is the energy difference AG°C between the two conformers (XII a) and (XII b) the kinetic factor is the difference between the free energy of activation AAG C of the reaction leading from each conformer to the corresponding alkyl-rhodium complex. 

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