Asymmetric hydrogenation DIOP complexes

The bis-DIOP complex HRh[(+)-DIOP]2 has been used under mild conditions for catalytic asymmetric hydrogenation of several prochiral olefinic carboxylic acids (273-275). Optical yields for reduction of N-acetamidoacrylic acid (56% ee) and atropic acid (37% ee) are much lower than those obtained using the mono-DIOP catalysts (10, II, 225). The rates in the bis-DIOP systems, however, are much slower, and the hydrogenations are complicated by slow formation of the cationic complex Rh(DIOP)2+ (271, 273, 274) through reaction of the starting hydride with protons from the substrate under H2 the cationic dihydride is maintained [cf. Eq. (25)] ... 

The cluster complex H4Ru4(CO)8(DIOP)2 has been used at 150°C with 100 atm H2 for asymmetric hydrogenation of acetophenone and methyl-... 

Aromatic Ketones The DIOP-Rh [116] and DBPP-Rh [117] complexes, in conjunction with a tertiary amine, have been employed in the asymmetric hydrogenation of acetophenone, albeit with moderate enantioselectivity (80 and 82% respectively Tab. 1.10). The asymmetric hydrogenation of aromatic ketones was significantly improved by using the Me-PennPhos-Rh complex, with which enantioselectivities of up to 96% ee were achieved [36]. Interestingly, the additives 2,6-lutidine and potassium bromide were again found to be crucial for optimum selectivity, although their specific role has not been determined. 

The diop system is the most effective of the Ru(II) chiral phosphine complexes that we have found for asymmetric hydrogenation (25, 26). The hydrogenation rates are about V50 as large as those using HRuCl(PPh3)3 under corresponding conditions (32) but are reasonably efficient nevertheless. For example, 1M solutions of atropic acid are converted quantitatively to 2-phen-ylpropionic acid (40% enantiomeric excess (ee)) in one day with 10 2 M catalyst at 1 atm H2. 

The activity and enantioselectivity of chiral Ir catalysts have been tested by using 2,3,3-trimethylindolenine as a model substrate. Hydrogenation of the cyclic imine with [Ir(bdpp)Hl2 2 gives the corresponding chiral amine with 80% ce (Scheme 1.99) [350]. The stereoselectivity is somewhat better than that with acyclic substrates (see Scheme 1.94). A neutral BCPM-Ir complex with Bil3 effects asymmetric hydrogenation in 91% optical yield [354], A complex of MCCPM shows similar enantioselection [354], These complexes are not applicable to the reaction of other acyclic and six-membered cyclic imines. An MOD-DIOP-Ir complex is also usable with the aid of ( -C4H9)4NI [355], An Ir complex of BICP with phthalimide effectively... 

The catalyst precursor complex [ Rh(COD) Diop] BF4 has been used for the screening of five substrates containing prochiral C=C double bond (COD = 1,5-cyclooctadiene) [266]. These were methylacetamidoacrylate (SI), Z-a-methylacetamidocinnamate (S2), dimethylitaconate (S3), methone (S4) and rac-a-pinene (S5) (see Figure 4.56). Activated C=C bonds such as those in the two acetamido derivatives were more reactive. The most reactive molecule is the less sterically hindered substrate methylacetamidoacrylate. Reaction was less pronounced for unsubstituted and sterically hindered substrates such as methone. The reduction of C=0 bond in a-pinene is more difficult. These results are in agreement with the general trends reported for asymmetric hydrogenations. 

Asymmetric hydrogenation of itaconic acids.1 Japanese chemists have prepared a new bisphosphine ligand (2), which is more efficient than DIOP for asymmetric hydrogenation of itaconic acids when complexed with rhodium. It is available in four steps from 4-bromo-2,6-dimethylphenol. 

The homogeneous catalyst has been prepared in alcoholic media and is a cation formed by loss of chloride. The procedure is described here for production of the neutral hydrido species HRh[( + )-diop]2, which is a slower catalyst than the in situ species for asymmetric hydrogenation but is equally effective in terms of optical yields. The method follows that of Levison and Robinson6 for synthesis of hydrido(triphenylphosphine)rhodium complexes. 

Introduction. DIOP was the first example of a C2 chelating diphosphine for transition metal complexes to be used in asymmetric catalysis. It was also one of the first examples of a useful C2 chiral auxiliary. DIOP can be considered as an example of the first generation of chelating diphosphine ligands with a chiral carbon skeleton, which were followed over the next 20 years by many examples of chelating diphosphines, one of the most efficient of which is BINAP [2,2 -Bis(diphenylphosphino)-l,l -binaphthyl] The ready availability of DIOP has stimulated research in asymmetric catalysis beyond the area of asymmetric hydrogenation. 

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