Rhodium complexes trans

However, TRIR has also been applied to more classical coordination compounds. Ford and co-workers have used a combination of ns-TRIR and time-resolved UV/vis spectroscopy to investigate the mechanism of hydrocarbon C—H bond activation with the rhodium complex, trans-RhCl(CO)(PR3)2 (R = Ph, />-tolyl, or Me). Upon photoexdtation, each of these species was found to undergo CO dissociation to form the transient solvated complex, tra 5-RhCl(Sol)(PR3)2 (Sol = solvent). The solvated complexes reacted with added CO to regenerate the parent complex, and also underwent competitive unimolecular C H activation to form the Rh products of hydrocarbon oxidative addition. These were identified from the step scan FTIR spectra, which showed a positive shift in /(CO) relative to the parent complex, which is consistent with oxidation of the metal center. 

Rhodium complexes with chelating bis(oxazoline) ligands have been described to a lesser extent for the cyclopropanation of olefins. For example, Bergman, Tilley et al. [32] have prepared a family of bis(oxazoline) complexes of coordinatively unsaturated monomeric rhodium(II) (see 20 in Scheme 13). Interestingly, the use of complex 20 in the cyclopropanation reaction of styrene afforded mainly the cis cyclopropane cis/trans = 63137), with 74% ee and not the thermodynamically favored trans isomer. No mechanistic suggestions are proposed by the authors to explain this unusual selectivity. 

Study of the mechanism of the rhodium-catalyzed hydroamination of ethylene with secondary amines indicated that the piperidine complex trans-RhCl(C2H4)(piperidine)2 can serve as a catalyst precursor [109, 110]. 

Scheme 8 Rhodium complexes of trans-chelating diphosphine ligands...

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. 

Extending the speculations presented in Section 8.2 for PPh3 and its rhodium complexes one expected that BISBI would coordinate in a bis-equatorial fashion (14) in RhH(L-L)(CO)2, thus leading to 3t only when dissociation of a CO molecule takes place (due to strain in the backbone 3t might not be completely trans). NMR and IR spectroscopy proved [57] that the structure of complexes 11 indeed contained the two phosphorus atoms in the equatorial plane and hydrogen in one of the apical positions (14, Figure 8.9). 

The axial alignment of Rh2(5R-MEPY)4 leads to probable structures for the carbene intermediate as shown in Figure 17.15. Approach of styrene will occur with the phenyl group pointing away from the rhodium complex, and also in a trans (anti) fashion with respect to the ester group of the carbene moiety. The 2-phenylcyclopropane-l-carboxylic ester resulting from this is indeed the 1R,2R (1R-trans) diastereomer. 

Quite remarkably partial hydrogenation of alkynes to trans alkenes is also possible with homogeneous rhodium complexes 168169... 

Anionic polymerization of phenylacetylene to a trans-cisoid polymer in the presence of crown ether phase-transfer catalysts initiated by sodium amide has been reported.425 In contrast, the zwitterionic rhodium complex Rh+(COD)BPhJ yields a ds-transoid product in the presence of Et3SiH.426... 

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. 

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. 

The studies of Wilkinson et al. included IR and H-l NMR spectroscopy of the intermediate species of this catalyst system (7). This led to recognizing tris(triphenylphosphine)rhodium(I) carbonyl hydride (D) as the key stable rhodium complex. The reactive trans-bis-(triphenylphosphine)rhodium(I) carbonyl hydride (E) resulting via the dissociation of this complex... 

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