Amino acids rhodium complexes

Oxidative cleavage of P-aminoacyl complexes can yield P-amino acid derivatives (320,321). The rhodium(I)-catalyzed carbonylation of substituted aziridines leads to P-lactams, presumably also via a P-aminoacyl—metal acycHc compound as intermediate. The substituent in the aziridine must have 7T or electrons for coordination with the rhodium (322,323). 

The most effective catalysts for enantioselective amino acid synthesis are coordination complexes of rhodium(I) with 1,5-cyclooctadiene (COD) and a chiral diphosphine such as (JR,jR)-l,2-bis(o-anisylphenylphosphino)ethane, the so-called DiPAMP ligand. The complex owes its chirality to the presence of the trisubstituted phosphorus atoms (Section 9.12). 

Boehm et al.100 have synthesised and studied a series of half-sandwich rhodium (III) and iridium (III) complexes, derivatives of salicylaldehyde and L-amino acid esters. The diastereoselectivity has shown strong dependence on the type of metal as well as amino acid residue. The labile configuration of the metal atoms was suggested because of changes in the diastereomers ratio with increasing temperature. Fast epimerisation at the metal atom was suggested for some S-phenylalanine complexes. 

Oheme and co-workers investigated335 in an aqueous micellar system the asymmetric hydrogenation of a-amino acid precursors using optically active rhodium-phosphine complexes. Surfactants of different types significantly enhance both activity and enantioselectivity provided that the concentration of the surfactants is above the critical micelle concentration. The application of amphiphilized polymers and polymerized micelles as surfactants facilitates the phase separation after the reaction. Table 2 shows selected hydrogenation results with and without amphiphiles and with amphiphilized polymers for the reaction in Scheme 61.335... 

A complex naturally occurring amino acid 5-hydroxypiperazic acid (5HyPip) 100 was prepared by a multistep procedure that included Diels-Alder addition of 2,4-pentadienoic acid to phthalazinedione 83a as a first step (Scheme 24). Adduct 97 was esterified and oxidized with mercuric acetate to 98, which on hydrogenation over rhodium on alumina and subsequent hydrolysis provided a mixture of enantiomers from which the required enantiomer 99 was obtained by resolution with quinine. Its hydrazinolysis provided 100 [71JCS(C)514 77H119],... 

In situ rhodium(I) complexes containing both triphenylphosphine and optically active amines are said to effect asymmetric hydrogenation of the amino acid precursors (265). 

The rhodium complexes of the ferrocene derivatives 39 have shown useful characteristics for the reduction of itaconates as well as dehydroamino acid derivatives [15, 167-170]. These compounds are hybrids between ferrocene-based ligands and the various other types. The P-chiral compounds, which in some ways are DIPAMP hybrids, showed tolerance for the reduction of N-methyl en-amides to produce N-methyl-a-amino acid derivatives [169-171]. 

This strategy also gives access to a variety of non-natural a-amino acids. Furthermore, rhodium-DuPHOS complexes catalyse the asymmetric reduction of enol esters of the type PhCH = CH — C(OCOCH3) = CH2 to give (R)-2-acetoxy-4-phenylbut-3-ene (94% ee)[64]. 

The rhodium acetate complex catalyzed the intramolecular C-H insertion of (/ )-diazo-fR)-(phenylsulfonyl)acet-amides 359 derived from (f )-amino acids to afford in high yield the 6-benzenesulfonyl-3,3-dimethyl-7-phenyl-tetrahydro-pyrrolo[l,2-c]oxazol-5-one 360 (Equation 63) <2002JOC6582, 2005TL143>. 

A major advantage that nonenzymic chiral catalysts might have over enzymes, then, is their potential ability to accept substrates of different structures by contrast, an enzyme will select only its substrate from a mixture. Striking examples are the chiral phosphine-rhodium catalysts, which catalyze die hydrogenation of double bonds to produce chiral amino acids (10-12), and the titanium isopropoxide-tartrate complex of Sharpless (11,13,14), which catalyzes the epoxidation of numerous allylic alcohols. Since the enantiomeric purities of the products from these reactions are exceedingly high (>90%), we might conclude... 

Asymmetric C-H insertion using chiral rhodium catalysts has proven rather elusive (Scheme 17.30). Dimeric complexes derived from functionalized amino acids 90 and 91 efficiently promote oxidative cychzation of suifamate 88, but the resulting asymmetric induction is modest at best ( 50% ee with 90). Reactions conducted using Doyle s asymmetric carboxamide systems 92 and 93 give disappointing product yields ( 5-10%) and negligible enantiomeric excesses. In general, the electron-rich carboxamide rhodium dimers are poor catalysts for C-H amination. Low turnover numbers with these systems are ascribed to catalyst oxidation under the reaction conditions. 

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. 

An especially important case is the enantioselective hydrogenation of a-amidoacrylic acids, which leads to a-amino acids.14 A particularly detailed study has been carried out on the mechanism of reduction of methyl Z-a-acetamidocinnamate by a rhodium catalyst with a chiral disphosphine ligand.15 It has been concluded that the reactant can bind reversibly to the catalysts to give either of two complexes. Addition of hydrogen at rhodium then leads to a reactive rhodium hybride and eventually to product. Interestingly, the addition of hydrogen occurs most rapidly in the minor isomeric complex, and the enantioselectivity is... 

Chiral catalysis was introduced in industrial synthesis in the mid-1970 s. The standard example is the catalytic hydrogenation of dehydroamino acid derivatives such as (Z)-2-acetyl-amino-3-phenylpropenoic acid with chiral rhodium complexes to give /V-acetylphenylalanine in high optical purity1. 

Since Wakamatsu serendipitously discovered amidocarbonylation while performing the cobalt-catalyzed hydroformyla-tion of olefins in 1971, this unique carbonylation reaction, affording a-amino acids directly from aldehydes, has been extensively studied.More recently, palladium-catalyzed processes have been developed to expand the scope of this reaction.The Pd-catalyzed amidocarbonylation has been applied to aldehydes,aryl halides, and imines. As a related reaction, lactamization " of aryl halides catalyzed by a rhodium complex has also been developed. 

The tetrasulfonated chiraphos reacts in water with [Rh(COD)Cl]2 in a ratio of 2 1 to give a rhodium complex [Rh(COD)(ChiraphosTS)]Cl1 [31P(DzO) S ppm 57.4 (d, JRh P = 148 Hz)] which catalyzes the enantioselec-tive reduction of amino acid precursors in a two-phase system of water-ethyl acetate with ee in the range 81-88%. 

Asymmetric Hydrogenation. Asymmetric hydrogenation with good enantio-selectivity of unfunctionalized prochiral alkenes is difficult to achieve.144 145 Chiral rhodium complexes, which are excellent catalysts in the hydrogenation of activated multiple bonds (first, in the synthesis of a-amino acids by the reduction of ol-N-acylamino-a-acrylic acids), give products only with low optical yields.144 146-149 The best results ( 60% ee) were achieved in the reduction of a-ethylstyrene by a rhodium catalyst with a diphosphinite ligand.150 Metallocene complexes of titanium,151-155 zirconium,155-157 and lanthanides158 were used in recent studies to reduce the disubstituted C—C double bond with medium enantioselectivity. 

Rate-determining step, hydroformylation, 163 Reactivity, enantiomers, 286 Recognition, enantiomers, 278 Reduction and oxidation, 5 Reductive coupling, dissolving metal, 288 Reductive elimination, 5, 111 Resolution. See Kinetic resolution Rhenium-carbene complexes, 288 Rhodium-catalyzed hydrogenation, 17, 352 amino acid synthesis, 18, 352 BINAP, 20... 

T7fficient catalytic asymmetric hydrogenations have been achieved using an optically active phosphine complexed with rhodium (I, 2,3, 4, 5, 6, 7, 8). Through this process it is now possible to prepare a number of optically active a-amino acids from the corresponding unsaturated precursor without the usual resolution step by the following sequence. 

When the catalyst was used for simple olefin systems, it was not as active as with the amino acid precursors. Table III shows the relative rates for a variety of substrates, special care being taken in each case to purge oxygen. The slow rate of a-phenylacrylic acid was unexpected, but, it may be the result of a stable olefin-rhodium complex similar to the one Wilkinson (15) experienced with ethylene. Such a contention is consistent with the increased speed of hydrogenation with increased pressure. 

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