Rhodium phosphan complexes

Homogeneous asymmetric hydrogenation reactions have been studied intensively with amino acid precursors in aqueous micellar solutions. In early work only stabilising effects of added amphiphiles were observed [53, 54]. However, for the hydrogenation of Z-a-acetamidocinnamic acid methyl ester (Fig. 5.10) with an optically active rhodium-phosphane complex (Fig. 5.11) in the presence of micelles a significant increase in activity and enantioselectivity was found in comparison to reaction in pure water [55]. 

There arc two main classes of catalysts neutral chlororhodium(I) diphosphane complexes and cationic rhodium(I) complexes having the general structure [Rh(diphosphane)(olefin)2]+. The cationic complexes are often more active and more selective than the corresponding chloro complexes60. In contrast to heterogeneous hydrogenation catalysts rhodium phosphane complexes are not pyrophoric. However, they are sensitive to oxygen and should be stored and handled under an inert atmosphere. 

In addition to rhodium phosphane complexes, ruthenium phosphane complexes have also been successfully applied as catalysis for enantioseleetive hydrogenation of 2-acylamino-2-alkenoic acids and esters1 71,72b 3, enol acetates 18 (R = i-Pr E = COOEt X = OCOCH3 98% ee with BINAP)137, and itaeonic acid138. The absolute configuration of the products from the ruthenium-catalyzed reactions shown below is opposite to that obtained with the corresponding rhodium catalysts. 

Other catalysts such as rhodium phosphane complexes are much less effective in the hydrogenation of unfunctionalized olefins. The best results have been obtained with 2-phenyl-l-butene using a polymeric phosphinite ligand derived from cellulose113, (S,S)-1,2-bis-(diphenylphosphinoxy)cyclopentane114, or BDPP24 (50-60% ee). 

Nagel, U. and Kinzel, E., Enantioselective catalytic hydrogenation of z-(acetamido)cinnamic acid with a rhodium phosphane complex in aqueous solution, Chem. Bet, 1986,119, 1731. 

Hydroaminomethylation of 1 -dodecene with dimethylamine using a water-soluble rhodium-phosphane complex, RhCl(CO)(TPPTS)2 in an aqueous-organic two-phase system in the presence of cetyltrimethylammonium bromide was investigated. High reactivity and selectivity for tertiary amine were achieved at 130 °C and 30 bar pressure [163]. 

Fell B, Leckel D, Schobben C (1995) Micellar two phase-hydroformylation of multiple unsaturated fatty substances with water soluble rhodium carbonyl/tertiary phosphane complex catalyst systems. Fett Wiss Technol 97 219-228... 

In recent years the synthesis of chiral and achiral tripodal phosphines and their application in homogeneous catalysis has been studied in more detail [2]. Enantiomerically pure tripodal ligands were synthesized from the corresponding trichloro compounds and chiral, cyclic lithio-phosphanes, e.g. 17, (Scheme 6) [21,22], Using a rhodium(I) complex of ligand 18, an enantiomeric excess of 89 % was obtained in the asymmetric hydrogenation reaction of methyl acetami-docinnamate (19). 

The first homogeneously catalysed example was demonstrated by Inoue et. al. in 1976 . They used Rhodium(I) phosphane complexes such as Wilkinson s catalyst... 

The ethylene spacer is necessary as a shield from the strong electron-withdrawing effect of the perfluoroalkyl chain, which would decrease the donor properties of the phosphane. This phosphorous-ligand is extremely soluble in per-fluorinated solvents such as perfluoromethylcyclohexane (CFgCsFn) and only trace amounts of it can be extracted with organic solvents. The in situ prepared rhodium(l) complex 14 is a useful catalyst for the hydroformylation of terminal alkenes under FBS-conditions. The aldehydes 16 and 17 were formed in 85% yield by hydroformylation of 1-octene (15) with an linear to iso ratio of nearly 3 1 Eq. (8). 

Pracejus was fascinated by the idea of functionalizing cellulose as the cheapest chiral material and to use it in this form as a carrier for monovalent rhodium for asymmetric hydrogenation. Rh was shown by Wilkinson to be useful as a catalyti-cally active central metal in phosphane complexes. However, the catalytic activities of the new cellulose immobilized complexes in the hydrogenation of unsaturated amino acid precursors were low and the enantioselectivities did not exceed 35%... 

I) and -rhodium(III) complexes with a pendant phosphane ligand known. Poilblanc prepared chelate 366 in 70% yield by treatment of [RhCl(CO)2]2 with anionic ligand 5 (Chart 20). In addition to 366 it was... 

H. Brunner, S. Rosenboem, Enantioselective catalyses CXXXV. Stereoselective hydrogenation of folic acid and 2-methylquinoxaline with optically active rhodium(I)-phosphane complexes, Chem. Monthly 2000,131, 1371. 

The pro-ligands 16-R (Scheme 5.10) led to (carbene)rhodium(I) complexes upon deprotonation in the presence of potassium ferf-butoxide and [RhCl(COD)]2 in THF (Scheme 5.12), followed by chloride abstraction, which could also be carried out with AgBF4/CH2Cl2 without oxidization of the ferrocene unit. While the selectivity for 20-Mes was almost total, complex 20-Me was obtained along with another carbene/phosphane complex in a 85 15 ratio, the minor species... 

Another attempt to apply the oxo reaction to the synthesis of fine chemicals was made by the hydroformylation of styrene. The chiral catalyst, a rhodium complex of a surfactant phosphane 12, gave no optical induction (76). This result agrees with the known poor enantioselection ability of other complexes with monodentate chiral phosphanes. 

Complexes of cationic rhodium compounds with asymmetric phosphane ligands catalyze the hydroboration of prostereogenic alkenes with catecholborane (see Section D.2.5.2.1.4.). The product alcohols are of 7-96% cc (Table 3). Enantioselectivity is excellent for the hydroboration/oxidation of styrenes but low for stilbenes. The small number of examples studied to date precludes generalizations, however, compared to the uncatalyzed reaction, opposite regioselec-tivity is observed for the addition to styrenes. 

---