Bisphosphine rhodium complexes

Fig. 2. Drawing showing the bisphosphine- rhodium complex viewed along the plane of the five-membered ring (diene omitted), and the Z-a-benzamido cinnamic acid substrate in a linear and flat conformation.

A monophosphine complex is formed when 3 is mixed with three equivalents of a zinc(ii)salphen complex and half an equivalent of Rh(acac)(CO)2 (acac = acetyl acetonate), whereas the assembly based on template 4 and the zinc(n)salphen complexes forms a bis-phosphine rhodium species. In the latter case, the bisphosphine rhodium complex is completely encapsulated by six salphen building blocks. This difference in mono- versus diphosphine ligation to the Rh -center and, to a lesser extent, the difference in electronic features (and thus donating properties of the phosphine) between template ligands 3 and 4, can be used to induce a different catalytic behavior. 

P-31 NMR Studies of Equilibria and Ligand Exchange in Triphenylphosphine Rhodium Complex and Related Chelated Bisphosphine Rhodium Complex Hydroformylation Catalyst Systems... 

Achiwa K. Asymmetric hydrogenation with new chiral functionalized bisphosphine-rhodium complexes. J. Am. Chem. Soc. 1976 98(25) 8265-8266. 

A variety of rhodium complexes, including [Rh(CO)2Cl]2 and [Rh(COD)Cl]2 when used in combination with a variety of bisphosphine ligands, will catalyze the ring opening of vinyl epoxides in the presence of aniline nucleophiles [19, 20]. These reactions occur under very mild and neutral conditions (at room temperature or with mild heating) and are highly regio- and stereoselective. In all cases, nucleophilic attack occurs at the allylic epoxide carbon atom and proceeds with inversion of stereochemistry (Scheme 9.11). 

Even though the outlined approach allowed the successful rationalisation of many experimentally observed shift/structure and shift/reactivity correlations, Leitner et al. have pointed out that such relations cannot be expected to be universally valid and require that structural variations are modest and avoid large simultaneous changes in parameters that may have opposite effects on metal chemical shifts.61 To overcome these drawbacks and establish a more rational interpretation of chemical shift trends, they used a combination of experimental and computational efforts to assess the importance of different electronic and structural factors on the metal chemical shifts of a series of rhodium complexes with bidentate chelating bisphosphine ligands. The basis of their approach is first the validation of experimentally observed metal shifts by... 

The chiral phosphine 31 or 32-rhodium complex catalyzed the addition of arystannanes 30 to N-sulfonylimines 29 to give diarylmethylamines 33 with high enantioselectivity (75-96% ee) [21]. The choice of the chiral monoden-tate phosphine ligand is essential for their catalytic asymmetric arylation. With chelating bisphosphine ligands the arylation was very slow. The authors hypoth-... 

P-31 NMR studies also were carried out in a similar manner on rhodium complexes of two chelating bisphosphines—bis-l,3-diphenyl-phosphinopropane and bis-l,2-diphenylphosphinoethane. These complexes were generated in solution via ligand displacement from tris(tri-phenylphosphine)rhodium carbonyl hydride. For example, one of the possible displacement products of bis-l,3-diphenylphosphinopropane (F) is a cis-chelate (G) that can undergo dissociation to yield a chelating bisphosphine complex (H) ... 

Minami, T., Okada, Y., Nomura, R., Hirota, S., Nagahara, Y., Mid Fukuyama, K. Synthesis and resolution of a new type of chiral bisphosphine ligand, trans-bis-l,2-(diphenylphosphino)cyclobutane. and asymmetric hydrogenation using its rhodium complex, Chem. Lett. 1986, 613-616. 

This chiral bisphosphine in combination with rhodium complexes also effects efficient asymmetric reduction of suitably substituted carbonyl compounds such as a-aminoacetophenones.23... 

Knowles [1] and Homer [2] independently discovered homogeneous asymmetric catalysts based on rhodium complexes bearing a chiral monodentate tertiary phosphine. Continued efforts in this field have produced hundreds of asymmetric catalysts with a plethora of chiral ligands [7], dominated by chelating bisphosphines, that are highly active and enantioselective. These catalysts are beginning to rival biocatalysis in organic synthesis. The evolution of these catalysts has been chronicled in several reviews [8 13]. 

In a series of studies performed by different groups hydroformylation of higher alkenes in microemulsions resulted in high turn-over frequencies (TOFs) of up to 5000 per hour for 1 -octene [46] and 1000 per hour for 1-decene [44]. Cationic surfactants such as cetyltrimethyl ammoniumbromide (CTAB) were used in the first studies of hydroformylation of various alkenes with the Rh-TPPTS catalyst. Both high activity and selectivity were observed in the hydroformylation of 1-octene and 1-decene using a sulphonated bisphosphine modified rhodium complex in the presence of ionic surfactants or methanol. 

Ruthenium complexes analogous to the preceeding rhodium complexes also gave higher n b aldehyde rados from 1-pentene when the catalyst had a high ratio, in support of the bisphosphine complex in Equation 20 as the species that favors the normal aldehyde (92). 

Enantiomerically pure bisphosphine (BisP ) 161 [83] and tris-phosphine (MT-Siliphos) 162 [21] ligands were obtained in high yields and used for preparation of various complexes of transition metals (Pd, Pt, Cu, Rh, and Ru) (Scheme 49). Cationic rhodium complexes 162 of bis-phosphines 161 were used as catalysts in asymmetric hydrogenation of (acylamino)acrylates with enantioselectivities up to 99.9% ee. 

Three classes of catalysts have been studied for the asymmetric hydrogenation of imines. One class of catalyst is generated from late transition metal precursors and bisphosphines. These catalysts have typically been generated from rhodium and iridium precursors. A second class of catalyst is based on the chiral titanocene and zirconocene systems presented in the previous section on the asymmetric hydrogenation of unfunctionalized olefins. The third class of catalyst is used for the transfer hydrogenation of imines and consists of ruthenium or rhodium complexes containing diamine, amino tosylamide, or amino alcohol ligands. " ... 

Because decarbonylation of the acyl intermediate competes with olefin insertion into the same species, and the carbonyl complexes are inactive as catalysts, the catalyst is poisoned by the competing decarbonylation. The use of a cationic rhodium complex containing a chelating ligand suppresses poisoning of the catalyst by decarbonylation, and reactions of aIk-4-en-l-als catalyzed by rhodium complexes of bisphosphines formed the desired cyclo-pentanones faster than reactions catalyzed by neutral rhodium complexes. "... 

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