Chiral metal complexes hydroformylation

Experimental evidence of the —S03" H0Si— interaction have been obtained from IR, Rh K-edge EXAFS, and CP MAS 3 IP NMR studies. These supported catalysts have been tested for the hydrogenation and hydroformylation of alkenes. No Rh leaching was observed.128-130 An extension to the immobilization of chiral metal complexes for asymmetric hydrogenation is reported below. 

Highly enantioselective hydroformylation catalyzed by chiral metal complexes has been obtained with only a few catalytic systems. Many chiral phosphorus ligands have been used in Pt(II) and Rh(I) systems in the asymmetric hydroformylation of styrene. The first highly enantioselective examples of the asymmetric hydroformylation of styrene were reported by Consiglio et al. in 1991 and used Pt-Sn systems. ligand 1 achieved an ee of 86% (Fig. 1) [10-12],... 

We were particularly interested in complex 32 reported by Boerner [79], an early-late transition metal complex that was used in hydroformylation, aiming at effects that the second metal (titanium) might bring about. In this instance the chiral titanium fragment may induce chirality, but only the racemic product was obtained. The activity of the catalyst was rather moderate and, perhaps, the rhodium center was... 

Asymmetric hydroformylations (22a-c) and a variety of other chiral reactions (23a-d) have also been observed with metal complexes made from chiral phosphines (Table III). 

Especially noteworthy is the field of asymmetric catalysis. Asymmetric catalytic reactions with transition metal complexes are used advantageously for hydrogenation, cyclization, codimerization, alkylation, epoxidation, hydroformylation, hydroesterification, hydrosilylation, hydrocyanation, and isomerization. In many cases, even higher regio- and stereoselectivities are required. Fundamental investigations of the mechanism of chirality transfer are also of interest. New chiral ligands that are suitable for catalytic processes are needed. 

The use of asymmetric catalysts in chiral syntheses is taking on increasing importance. Asymmetric ligands or asymmetric metal complexes used in these transformations are quite expensive and need to be efficiently separated from reaction mixtures and recycled. Scheme 16 shows the preparation of a polymer-anchored dibenzophosphole-DIOP platinum-tin catalysts system. The asymmetric ligand places the Pt-SnClj system in a chiral environment. This catalyst has given the highest enantiometric excesses ever observed in catalytic hydroformylation. The initially achieved 70-83% e.e. values were improved to >95% by the use of triethylorthoformate (TEOF) as the solvent. ... 

The first example of application of a chiral early-late heterobimetallic complex in asymmetric catalysis was reported by Bomer in 1999 and deals with hydroformylation of activated olefins (Scheme 39) [122]. The chiral bimetallic complex 66 was generated in situ by reacting the (R,R)-Diph-salenophos-Ti(0 Pr)2 ligand (67) with [Rh(acac)(CO)2]. This complex gives rise to a diminished conversion in aldehyde (21 % vs 99%) as well as a lower selectivity (i n = 77/23 vs 99/1) with respect to the monometallic salenophos-Rh complex generated in situ from the free-metal ligand 68 and [Rh(acac)(CO)2] but affords the branched aldehyde with 30% ee. 

In contrast to the normal-scXcctwc hydroformylation mainly developed in industry, asymmetric hydroformylation, which requires /i o-aldehydes ( branched aldehydes) to be formed from I-alkenes, was first examined in the early 1970s by four groups independently, using Rh(i) complexes of chiral phosphines as catalysts. " Since then, a number of chiral ligands have been employed for asymmetric hydroformylation and used in combination with transition metal ions, especially Pt(ii) and Rh(i). Asymmetric hydroformylation of I-alkenes is most extensively studied. 

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