Structural requirements in chiral diphosphine-rhodium complexes. II. N.M.R. determination of E, Z-geometry in prochiral substrates used in asymmetric hydrogenation reactions: α-acetamidocinnamic acids, esters, and parent azlactones.

doi:10.1016/S0040-4039(00)78022-1

Tetrahedron Letters

Volume 17, Issue 15, April 1976, Pages 1219-1222

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Abstract

Using n.m.r. the configuration of the stable 4-benzylidene-2-methyl-2-oxazolin-5-one, substituted derivatives, and the corresponding acids and esters derived from the parent stable azlactones were all found to be of Z-stereochemistry.

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4-(4′-Acetoxybenzylidine)-2-methyl-5-oxazolone (Z-isomer) was synthesized, which upon basic hydrolysis yielded the unexpected 2-acetoxy-3-(p-hydroxyphenyl)-propenoic acid and treatment with acetic acid formed 2-acetoxy-3-(p-acetoxyphenyl)-propenoic acid. The structures of the three compounds were assigned by NMR spectroscopy. An X-ray crystallographic study of the starting azlactone confirmed its structure and supported the fact that the Z-configuration was retained during the synthesis of the phenylpropenoic acid derivatives.
A simple synthetic approach to homochiral 6- and 6′-substituted 1,1′-binaphthyl derivatives
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Various homochiral binaphthyl derivatives having functional groups at the 6-position are important key intermediates for the immobilization of binaphthyl compounds on various solid-supports and have been prepared from commercially available 1,1′-bi-2-naphthol via controlled monopivalation of the 2-hydroxyl group and electrophilic aromatic substitution at the 6-position. (S)-2,2′-Bis-((S)-4-alkyloxazol-2-yl)-6-(2-methoxycarbonyl)ethyl-1,1′-binaphthyls (6-functionalized (S,S)-boxax)) were prepared and immobilized on various polymer supports including PS–PEG, PS, PEGA and MeO–PEG resin.
Asymmetric synthesis of l-[3-<sup>11</sup>C]phenylalanine using chiral hydrogenation catalysts
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The seven-step synthesis of L-[3-¹¹C]phenylalanine using chiral diphosphines as ligands in rhodium catalysts is reported. [¹¹C]Benzaldehyde, prepared in a three-step reaction from [¹¹C]carbon dioxide, as reported elsewhere, ⁽¹⁾ was reacted with 2-phenyl-5-oxazolone or 2-(4-chloro)phenyl-5-oxazolone in the presence of the tertiary amine diazabicyclooctane (DABCO). The resultant [α-¹¹C]-4-arylene-2-aryl-5-oxazolones were hydrogenated after ring opening, using the chiral rhodium complex of (R)-1,2-bis(diphenylphosphino)propane [(R)-PROPHOS] or (+)-2,3-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane [(+)-DIOP]. After removal of the amino protecting group, the labelled amino acid was obtained on purification by preparative LC in 10–15% radiochemical yield, and radiochemical purity higher than 95%, from [¹¹C]carbon dioxide within 60 min. The optical purity of the products determined by the tRNA method and capillary GC, was 80 and 60% e.e., respectively (i.e. $L/D = 9010 and 8020$ ).
Synthesis of dehydrophenylalanine derivatives by palladium catalyzed arylation of 2-acetamidoacrylic acid
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This chapter focuses on the hydrogenation reactions catalyzed by transition metal complexes, with the aim of developing catalysts for selective hydrogenation under mild conditions. Olefin hydrogenations—for example—are important industrial processes, and selectivity is critical to the success of such processes. Greater product selectivity has an important impact on energy and resource utilization in terms of reduced process energy requirements for product separation and purification, and in terms of low-value byproducts. The advances in asymmetric hydrogenation—a stereospecific selectivity—have been notable, and an understanding of the detailed pathways is just beginning to emerge, although much remains to be done, and matching of substrates with the most suitable chiral catalyst still remains an empirical art. The chapter demonstrates the degree of understanding that can be attained for a homogeneous hydrogenation catalyst at the molecular level. Enantiomeric products are used widely in the pharmaceutical industry and as food additives, and the production of either the natural or nonnatural amino acids is at least one advantage shown by the organometallic catalysts compared to enzyme systems. The incorporation of an effective rhodium catalyst into a protein begins to bring closer together analogies between the two areas. Interest is growing in chiral catalysts based on less expensive metals, such as cobalt, and a wider range of chiral ligands–including naturally occurring ones, is being exploited.
Structural requirements in chiral diphosphine-rhodium complexes-XIII<sup>1</sup> 1 Part XII. R. Glaser, S. Geresh, M. Twaik and N.L. Benoiton, Tetrahedron 34, 3617 (1978). Steric and electronic factors in the asymmetric homogeneous hydrogenation of Z-α-acylaminocinnamic acids and esters catalyzed by rhodium(I) complexes
1979, Tetrahedron
Z-α-acylaminocinnamic acids and esters were hydrogenated with rhodium(I) complexes containing (4R,5R) - trans - 4,5 - bis(diphenylphosphinomethyl) - 2,2 - dimethyl - 1,3 - dioxolan (DIOP). Increasing the steric bulk of the acyl function (NHCOR, where R is an alkyl moiety) resulted in a lowered reduction of the si-re prochiral face to yield a decreasing excess of the (R)-amino acid derivatives. In the series of N-acylphenylalanine free acids (resulting from hydrogenation of Z-α-acylaminocinnamic acids) the optical purity decreased from 82% ee-(R) [Me]; 57% ee-(R) [i-Pr]; 52% ee-(R) [t-Bu]; to 46% ee-(R) [1-adamantyl]. Theα-benzamido, α-formamido and α-trifluoroacetamido substrates gave hydrogenation products having 68% ee-(R) [Ph]; 60% ee-(R) [H]; and 16% ee-(R)[CF₃]. In the corresponding methyl esters, increasing the steric bulk of the acyl function (NHCOR) resulted in a markedly greater decrease in enantioface differentiation. In the series of N-acylphenylalanine methyl ester products (resulting from hydrogenation of Z-methyl α-acylaminocinnamates) the optical purity decreased from 69% ee-(R)[Me]; 15% ee-(R) [i-Pr]; to 0% ee[t-Bu and 1-adamantyl]. The α-benzamido, α-formamido, and α-trifluoroacetamido substrates gave hydrogenation products having 36% ee-(R) [Ph]; 58% ee-(R) [H]; and 22% ee-(S) [CF₃]
In the series of N-acetylphenylalanine alkyl ester products (resulting from hydrogenation of Z-alkyl α-acetamidocinnamate esters) trifluoro substitution in the alkyl alcohol moiety resulted in a decrease in optical purity to 52% ee-(R) [CH₂CF₃] compared to 72, 76 and 77% ee-(R) [Et, i-Pr and t-Bu, respectively].

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^☆: For part I see: R. Glaser, Tetrahedron Lett., 2127 (1975).

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Structural requirements in chiral diphosphine-rhodium complexes. II. N.M.R. determination of E, Z-geometry in prochiral substrates used in asymmetric hydrogenation reactions: α-acetamidocinnamic acids, esters, and parent azlactones.☆

Abstract

J. Organometal. Chem.

J. Biol. Chem.

Adv. Heterocyclic Chem.

J. Amer. Chem. Soc.

J.C.S. Chem. Commun.

Chem. Commun.

Chem. Commun.