Rh/tangphos

Itaconic acids are reduced in very high enantiomeric excesses (>97%) with Rh-TangPhos catalysts 658 Itaconic acid is reduced in 99.5% ee with the sugar-derived ferrocenyl phosphine 123.659... 

Recent results have shown that a number of other commercially available ligands can be expected to have industrial potential for the hydrogenation of dehydro a-amino acid derivatives. However, it must be pointed out that in most cases model substrates and not industrially relevant targets have been investigated until now. Chiral Quest has shown that Rh-TangPhos as well as Rh-f-Ke-talPhos (for structures, see Fig. 37.9) were able to hydrogenate a variety of a-de-hydro amino acid derivatives with ee-values of 98->99%, TONs of up to 10000... 

Tang W, Liu D, Zhang X (2003) Asymmetric hydrogenation of itaconic acid and enol acetate derivatives with the Rh-TangPhos catalyst. Org Lett 5 205-207... 

Similar analysis was made for the catalytic cycle of asymmetric hydrogenation of 4 with the Rh-TangPhos catalyst (Figure 1.10). In the case of formation of the chelate cycle in the less hindered quadrant, the... 

Noteworthy, this mechanism of enantioselection explains well the absence of the temperature effect on the optical yield in the asymmetric hydrogenation of 4 catalyzed by the Rh-TangPhos catalyst. Indeed, in the case when the ee is determined by the difference in the free energies of the diastereomeric transition states, significant dependence of the optical yields on the temperature is usually observed due to the natural temperature variation of the rate constants. On the other hand, the structure of the catalyst implying facile formation of the chelate cycle in a less hindered quadrant and impossibility of similar transformation with the chelate cycle forming in hindered quadrant, remains the same at any temperature. Hence, the almost perfect enantioselectivity can be observed either in catalytic or low-temperature stoichiometric reaction (>99% ee in both cases). 

A highly regio- and enantio-selective hydroformylation of alkenes, such as PhCH= CH2, CH2=CHCH2CN, and CH2=CHOAc, catalysed by ruthenium complexes with (g) 2,5-disubstituted phospholane ligands has been reported. With (83) as the ligand, the turnover rates over 4000 h-1 at 80 °C, have been attained.108 (Acac)Rh(CO)2-TangPhos [Tangphos = (84)] has been developed as a new enantioselective catalyst for asymmetric hydroformylation of norbornene and other [2.2.1]-bicyclic alkenes (55-92% ee).109... 

Since this seminal work, a considerable amount of research has focused on the AHF reaction" For instance, Huang, Bunel and coworkers have reported that the combination of Rh(acac)(CO)2 (5) and TangPhos (20) is an active catalyst for the AHF of norbornylene (18) under mild conditions to give 19 with ee up to 93% (equation 4) . 

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