Asymmetric cationic rhodium complex

Catalytic Asymmetric Hydroboration. The hydroboration of olefins with catecholborane (an achiral hydroborating agent) is cataly2ed by cationic rhodium complexes with enantiomericaHy pure phosphines, eg, [Rh(cod)2]BE4BINAP, where cod is 1,5-cyclooctadiene and BINAP is... 

Compared with the Osborn-type cationic rhodium complexes (Section III,A,3), the iridium analogs are much less active for asymmetric hydrogenation of ketones (280). 

Asymmetric hydrogenations have also been carried out in scCOi. a-Enamides have been hydrogenated by using a cationic rhodium complex [19]. Enantiomeric... 

The dynamic behavior of the model intermediate rhodium-phosphine 99, for the asymmetric hydrogenation of dimethyl itaconate by cationic rhodium complexes, has been studied by variable temperature NMR LSA [167]. The line shape analysis provides rates of exchange and activation parameters in favor of an intermo-lecular process, in agreement with the mechanism already described for bis(pho-sphinite) chelates by Brown and coworkers [168], These authors describe a dynamic behavior where two diastereoisomeric enamide complexes exchange via olefin dissociation, subsequent rotation about the N-C(olefinic) bond and recoordination. These studies provide insight into the electronic and steric factors that affect the activity and stereoselectivity for the asymmetric hydrogenation of amino acid precursors. 

Carbonyl groups are not reduced with classical Wilkinson catalysts. However, some cationic rhodium complexes show catalytic activity 52K There are only a few examples of asymmetric hydrogenation of ketones. Addition of base to a neutral rhodium complex is also a way to produce a catalyst for ketone reduction 44). Acetophenone... 

Studies in Asymmetric Synthesis, Preparation, Reactivity, and Solid-State Structures of Cationic Rhodium Complexes Containing a Chiral Amino-phosphine or Aminoarsine Ligand... 

The cationic rhodium complexes such as [Rh(cod)(MeCN)2]BF4/dppb were the most efficient catalysts for addition of arylboronic acids or Ph4BNa to both aromatic and aliphatic A-sulfonyl imines.993 994 The asymmetric version giving optically active amines was achieved by amidomonophosphine catalysts (Equation (225)).995... 

Catalytic asymmetric hydrogenations have also been performed in supercritical carbon dioxide [79-81]. For example, a-enamides were hydrogenated in high enantioselectivities comparable to those observed in conventional solvents, using a cationic rhodium complex of the EtDuPHOS ligand (Fig. 7.24) [79]. More recently, catalytic asymmetric hydrogenations have been performed in scC02 with... 

Asymmetric Hydrogenation. The diene-free cationic rhodium complex of (R)-BINAP catalyzes the enantioselective hydrogenation of dehydroamino acids. a-(Benzoylamino)acrylic acid is hydrogenated at rt to afford (S)-lV-benzoylphenylalanine in 100% ee (eq 1). To obtain maximal stereoselectivity the reaction should be carried out under a low concentration of substrate (100% in 0.013 M vs. 62% in 0.15 M) and low initial hydrogen pressure (100% at 1 atm, but 71% at 50 atm). 

Asymmetric Hydroboration. Rhodium complexes are known to catalyze hydroboration of alkenes with unreactive borane derivatives, e.g. catecholborane and oxaborolidine. This reaction proceeds enantioselectively by use of BINAP as a ligand for neutral " or cationic rhodium complexes. Reaction of styrene with catecholborane followed by oxidation affords (R)-1-phenylethanol in 96% ee in the presence of (R)-BINAP and [Rh(cod)2]Bp4 (eq 5). ... 

Asymmetric hydrogenation of enamides CO2 Cationic rhodium complex... 

The isomerization of allylamines could be more selective compared to their oxygen homologues due to the higher coordination property of nitrogen. This assumption was clearly exemphfied by the discovery of the cobalt-catalyzed isomerization of allylamines to enamines. With the introduction of cationic rhodium complexes of BINAP, the reaction has became one of the most successful asymmetric reactions [32]. 

Since catalytic asymmetric hydrogenation can generate either or R configuration at the position of the dehydroamino acid residue, this method could be potentially useful for the specific labeling of certain amino acid residues in a polypeptide. The regiospecific and stereoselective labeling of an amino acid residue is difficult to achieve with the conventional stepwise peptide synthesis. We carried out the dideuteration of Ac-APhe-( )Ala-OMe with the use of the cationic rhodium complexes with (-)BPPM and (+)BPPM (Scheme 1), which gave Ac- (R,R)Phe (d ) - (S)Ala-OMe [(R,Rf )/... 

From benzophenone (560) and phenylsilane (559), mono- (561) and bis(diphenylmethyl)phenylsilane (562) can be obtained (equation 282)315. The asymmetric dimethyl(2,2-dimethyl-l-phenylprop-l-yloxy)phenylsilane (563) can be synthesized in the presence of a cationic rhodium complex (equation 283)316. An alkoxy replacement can be used for the synthesis of alkoxysilanes with non-identical alkoxy moieties, e.g. for diethoxymethoxysilane (565) (equation 284)317. 

Scheme 4.7-3 Asymmetric hydrogenation of dehydroamino acids in SCCO2 using cationic rhodium complexes. Representative results are listed in Table 4.7-2.

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