Rhodium different chiral ligands

The comparison of hydrogen consumption in the rhodium-catalyzed enantiomeric hydrogenation of a yS-dehydroamino acid using Et-Duphos (Et-Du-PHOS = l,2-bis(2,5-diethyl-phospholanyl)benzene)) as the chiral ligand shows the huge differences in rate, depending on the manner in which the catalyst was prepared (Fig. 44.1) [10b,c]. 

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

If no asymmetric induction takes place in the diastereomeric alkyls formation, the chiral aldehyde resulting from the three olefins must have the same chirality and the same optical purity. The experiments indicate (Table IV) the opposite result. Using the same chiral ligand [( — )-DIOP] the aldehyde obtained from 1-butene has prevailing [(R)] chirality while the same aldehyde arising from the two 2-butenes has prevailing [ (S) ] chirality. Furthermore, the two aldehydes obtained from cis-butene and trans-butene under the same reaction conditions have different optical purity (8.1 and 3.2% respectively). These results imply that the diastereomeric composition of the mixture IX(S) + IX,R) depends on the type of the starting C4 olefin and that for at least two of the olefins used the asymmetric induction occurs, at least in part, in the alkyl-rhodium complex formation. 

In Table 6 the results concerning the asymmetric hydroformylation of 1-butene and of styrene with different catalytic systems are reported. When rhodium-containing catalytic systems are used in the presence of several diphosphine ligands, the face of the prochiral unsaturated carbon atom which is preferentially formylated is the same in both substrates for each chiral ligand. 

Only very low catalyst concentrations down to 5 x 10-5 kmol/m3 are consumed that keeps also the catalyst inventory very small [266], Only 0.08 mg of Rh and about 0.2 mg-13 pg of the very expensive chiral ligands (about 300-1000 /g), depending on their molecular weight, are consumed. Finally, a performance comparison for three different reactors was made for the substrate methylacetamidocinnamate and the two rhodium diphosphine complexes Rh/Josiphos and Rh/Diop (see Figure 4.57). The first reactor was a commercial Caroussel reactor (Radleys... 

Chiral rhodium(II) oxazolidinones 5-7 were not as effective as Rh2(MEPY)4 for enantioseleetive intramolecular cyclopropanation, even though the sterie bulk of their chiral ligand attachments (COOMe versus /-Pr or C Ph) are similar. Significantly lower yields and lower enantiomeric excesses resulted from the decomposition of 11 catalyzed by either Rh2(4S-IPOX)4, Rh2(4S-BNOX)4, or Rh2(4R-BNOX)4 (Table 3). In addition, butenolide 12, the product from carbenium ion addition of the rhodium-stabilized carbenoid to the double bond followed by 1,2-hydrogen migration and dissociation of RI12L4 (Scheme II), was of considerable importance in reactions performed with 5-7 but was only a minor constituent ( 1%) from reactions catalyzed by Rh2(5S-MEPY)4. This difference can be attributed to the ability of the carboxylate substituents to stabilize the earboeation form of the intermediate metal carbene. 

These catalytic reactions of dihydrosilanes make possible the use of asymmetric catalysts to produce chiral silicon compounds. Introduction of a chiral ligand L on the rhodium complex will not change the validity of the kinetic Scheme 12. However, in this case complexes 56 and 57 will be diastereomeric and their equilibrium concentrations will be different. The ratio of the substituted silanes will be close to k, [56] k2 [57]. 

A novel chiral ligand type for asymmetric hydroformylation with rhodium and platinum complexes11 125,1 54,156 is the hydroxyproline derived (2S,4>S)-l-(/err-butoxycarbonyl)-4-(diphenylphosphino)-2-[(diphenylphosphino)methyl]pyrrolidine [(-)-BPPM]2-6. As with other diphosphane systems, modifications of BPPM by exchange of either one or both of the different diphenylphosphane groups with dibenzophosphole (DBP) units [BPPM-2DBP, BPPM-4DBP and BPPM-(DBP)2] have been studied158. 

The same ligand can also give opposite results when used with different metals (platinum or rhodium). In some cases 1,1-disubstituted ethylenes and styrene converted in the presence of platinum and rhodium complexes modified with the same chiral ligand give opposite results, while with other alkenes [( /Z)-2-butenes] this is not observed. Thus, at least in some cases, the overall metal center geometry, and not solely the ligand configuration, is relevant for the direction of asymmetric induction. 

These catalysts are likely to have different stabilities and be formed in unequal amounts. Both the homochiral complexes (Lx )2Rh and (Ly )2Rh are accessible by combining a chiral ligand with the rhodium precursor. The heterochiral complex, however, is new. If the heterochiral complex exhibits greater activity and enantioselectivity than the homochiral complexes, not only will the resulting combination lead to improved... 

The influence of the concentration of hydrogen in [BMIMjlPFe] and [BMIM][BF4] on the asymmetric hydrogenation of a-acetamidodnnamic add catalyzed by rhodium complexes bearing a chiral ligand has been investigated. Hydrogen vras found to be four times more soluble in the [BF4] -based salt than in the [PFe]" -based one, under the same pressure. This difference in molecular hydrogen concentration in the ionic phase (rather than pressure in the gas phase) has been correlated with the remarkable effect on the conversion and enantioselectivity of the reaction [38]. 

Rhodium precatalysts with phosphites are mostly prepared by the reaction of Rh(acac)(CO)2 (acac = acetylacetonate) with the phosphite under syngas (Scheme 2.102). The precatalysts formed, such as HRh(CO) P , (with w + w = 4), usually cannot be isolated. They are observable only by spectroscopic methods [114,115]. The progress ofthe precatalyst formation can be investigated by means of UV-vis spectroscopy or, in case of chiral ligands, with UV-vis CD (circular dichroism) spectroscopy [116]. Different high-pressure NMR techniques coupled with IR provide valuable information about the structure of the precatalyst that is formed [92b, 114,117]. 

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