Rhodium complexes diastereomeric

Ligand 73 was prepared directly from a single enantiomer of the corresponding naphthol of QUINAP 60, an early intermediate in the original synthesis, and both enantiomers of BINOL. Application in hydroboration found that, in practice, only one of the cationic rhodium complexes of the diastereomeric pair proved effective, (aA, A)-73. While (aA, A)-73 gave 68% ee for the hydroboration of styrene (70% yield), the diastereomer (aA, R)-73 afforded the product alcohol after oxidation with an attenuated 2% ee (55% yield) and the same trend was apparent in the hydroboration of electron-poor vinylarenes. Indeed, even with (aA, A)-73, the asymmetries induced were very modest (31-51% ee). The hydroboration pre-catalyst was examined in the presence of catecholborane 1 at low temperatures and binuclear reactive intermediates were identified. However, when similar experiments were conducted with QUINAP 60, no intermediates of the same structural type were found.100... 

The oxidative addition can take place from the top of the molecule (as shown), but it can also take place from the bottom, giving another diastereomeric intermediate that probably does not undergo migration. The two oxidative additions require rotations in opposite directions of the substrate with respect to the rhodium phosphine complex. The rotation required also depends on the geometrical isomer of the rhodium complex to be formed (alkene/amide trans or cis to phosphine here we have chosen an amide cis to both phosphorus atoms). Both the major and the minor diastereomeric substrate complex require such a rotation upon oxidative addition. 

To obtain information about the steps in which the asymmetric induction actually takes place, 1-butene, cis-butene, and trans-butene were hydroformylated using asymmetric rhodium catalyst. According to the Wilkinson mechanism, all three olefins yield a common intermediate, the sec-butyl-rhodium complex, which, if the asymmetric ligand contains one asymmetric center, must exist in the two diastereomeric forms, IX(S) and IX(R),... 

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. 

The results do not prove that in the reaction conditions used the alkyl formation is not reversible, but only that, if it is reversible, the carbon monoxide insertion on both diastereomeric rhodium-alkyls must be much faster than the rhodium-alkyls decomposition. Restricting this analysis of the asymmetric induction phenomena to the rhodium-alkyl complexes formation, two 7r-olefin complexes are possible for each diastereomer of the catalytic rhodium complex (see Scheme 11). The induction can take place in the 7r-olefin complexes formation (I — II(S) or I — II(R)) or in the equilibrium between the diastereomeric 7r-olefin complexes (II(r) and... 

II(S)) and/or to a different reaction rate of the two diastereomeric 7r-olefin complexes to the corresponding diastereomeric alkyl-rhodium complexes (VI(s) and VI(R)). For diastereomeric cis- or trans-[a-methylbenzyl]-[vinyl olefin] -dichloroplatinum( II) complexes, the diastereomeric equilibrium is very rapidly achieved in the presence of an excess of olefin even at room temperature (40). Therefore, it seems probable that asymmetric induction in 7r-olefin complexes formation (I — II) cannot play a relevant role in determining the optical purity of the reaction products. On the other hand, both the free energy difference between the two 7r-olefin complexes (AG°II(S) — AG°n(R) = AG°) and the difference between the two free energies of activation for the isomerization of 7r-com-plexes II(S) and II(R) to the corresponding alkyl-rhodium complexes VI(s) and VI(R) (AG II(R) — AG n(S) = AAG ) can control the overall difference in activation energy for the formation of the diastereomeric rhodium-alkyl complexes and hence the sign and extent of asymmetric induction. 

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. 

This mechanism consists of several steps (1) oxidative addition of hydrosilane to the rhodium(I) complex (2) and (3) coordination and insertion of the ketone into the rhodium-silicon bond to form a diastereomeric a-silyloxyalkylrhodium intermediate (4) reductive elimination of alkoxysilane as a primary product and (5) hydrolysis of the alkoxysilane yielding an optically active alcohol. Hydrosilylation of prochiral ketones by prochirally disubstituted silanes leads to asymmetry on the silicon atom as well as on the carbon atom and, in the presence of chiral rhodium complexes, results in optically active monohydrosilanes (eq. (5)) [2] ... 

Rhodium complex 1 and its stereoisomer 2 (ratio of 1 2 = 85 15) were treated with AgBF4 to give diastereomeric products 3 and 4 (ratio of 3 4 = 84 16).104... 

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]. 

The addition of prochiral substrates, such as esters of a-acetamidocin-namic acids, to a solution of the solvate of a chiral diphosphine rhodium complex allowed the observation of at least one of the two diastereomeric complexes. Equation 49. 

Two sets of data showed that the minor diastereomeric olefin complex generated the major enantiomer of the product. First, the structure of the major diastereomer of [Rh(S,S-CHIRAPHOS)(EAC)] was determined by single-crystal X-ray diffraction. The complex in this structure would form N-acetyl-(S)-phenylalanine ethyl ester, but tfie major enantiomer of the product was the (R)-isomer. Second, both diastereomers of the olefin complex were observed during the hydrogenation of MAC by the rhodium complex of (R,R)-DIPAMP. At temperatures low enough that these diastereomers do not interconvert, NMR studies showed that the minor diastereomer reacted much more rapidly with to form the reduced product than the major diastereomer. 

Hamada s group has developed the rhodium-catalysed asymmetric hydrogenation of a-amino-)S-ester hydrochlorides via DKR. The reaction proceeded with the catalyst derived from a rhodium complex and a chiral ferrocenylphosphine under hydrogen in the presence of sodium acetate in acetic acid to alford the corresponding awti-jS-hydroxy-a-amino esters with 58-83% ee in a diastereomeric ratio of 92 8 to 97 3 (Scheme 2.83). 

Rhodium-Diphosphine Catalysts. The mechanism of rhodium-catalyzed asymmetric hydrogenation is one of the most intensively investigated and best understood. Reaction pathways have been accurately studied both experimentally and theoretically (138,162,213-221). In early studies, Halpern (222) and Brown (214) established that the hydrogenation proceeds according to the reaction sequence presented in Figure 51 for the hydrogenation of a dehydroamino acid with a chiral diphosphine-rhodium complex. Many variants on both catalyst and reactant have been described. Stereoselectivity takes place via the difference in reactivity of the involved diastereomeric square-planar... 

Among others, this concept has been demonstrated for rhodium promoted ene-type cycloisomerizations of 1,6-enynes [42]. In these studies, Mikami et al. combined the tropos Xylyl-BIPHEP ligand with the enantiomerically pure (/ )- ,I -diaminobinaphthyle (DABN) to produce the cationic rhodium complex C2 shown in Fig. 10.28. At 80 °C, the initially formed diastereomeric mixture was converted into a single diastereomer which proved to be configurationally stable at lower temperature. Moreover, removal of the chiral amine at 5 or below by addition of triflic acid, afforded a configurationally stable Xylyl-BIPHEP rhodium complex. The amine-free, enantiopure Xylyl-BIPHEP complex could be used as... 

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