Optische Aktivität an asymmetrischen Übergangsmetallatomen

Brunner, Henri

doi:10.1002/ange.19710830803

Cited by 43 publications

(3 citation statements)

References 61 publications

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“…In nonpolar solvents the half-lives τ 1/2 are in the range of minutes to hours at ambient temperatures. ,,– The concentration and solvent dependence of the racemization and epimerization have been investigated including Hammett correlations and the influence of added triphenylphosphine. ,– These studies as well as exchange reactions with deuterated triphenylphosphine and substitution reactions with other phosphines, which occurred with partial retention of configuration, ,, are in accord with Scheme . Reviews summarizing these mechanistic studies are available. ,– …”

Section: Introductionmentioning

confidence: 99%

Pyramidal Stability of Chiral-at-Metal Half-Sandwich 16-Electron Fragments [CpRu(P−P′)]

et al. 2008

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The chiral-at-metal diastereomers (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)Hal] (P−P′ = (R)-Prophos and (R,R)-Norphos, Hal = Cl, Br, and I) were synthesized, separated, and characterized by X-ray crystallography. Halide exchange and epimerization reactions were studied in 9:1 and 1:1 chloroform/methanol mixtures proceeding at room temperature or slightly above according to first-order. The rate-determining step in the Hal exchange reactions was the dissociation of the Ru−Hal bond, forming the pyramidal 16-electron intermediates (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)]+, which maintain the metal configuration. These intermediates can invert their metal configuration or react with nucleophiles with retention of the metal configuration. The measured competition ratios showed that the inversion of the intermediates was slow compared to quenching with nucleophiles, indicating a high pyramidal stability of the 16-electron fragments (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)]+ toward inversion in agreement with a basilica-type energy profile. Stereochemically this implies that substitution reactions in (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)Hal] occur with predominant retention of configuration, however, accompanied by a well-defined share of inversion, a point overlooked when (R Ru,R C)- and (S Ru,R C)-[CpRu(Prophos)Cl] were extensively used as starting materials in the synthesis of a variety of organometallic derivatives. The rates of the approach to the epimerization equilibrium were much smaller than those of the Hal exchange reactions, because in the basilica-type energy profile the intermediates (R Ru,R C)-/(S Ru,R C)-[CpRu(P−P′)]+, formed in the cleavage of the Ru−Cl bond, have to cross another barrier of appreciable height for inversion. Increasing the methanol content of the solvent increased the rates of the Hal exchange and epimerization reactions. Obviously, the pyramidality of the fragments [CpRu(P−P′)]+ is enforced by the small P−Ru−P angles (83° in the Prophos derivatives and 86° in the Norphos derivatives of (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)Hal]). Due to these small angles, intermediates (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)]+ resist planarization and thus inversion of the metal configuration.

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Section: Introductionmentioning

confidence: 99%

Pyramidal Stability of Chiral-at-Metal Half-Sandwich 16-Electron Fragments [CpRu(P−P′)]

et al. 2008

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“…C 53,27 H 3,97 (38.73 N 10,35% Gef. ,,53,2 ,,3,9 ,,8,s ,, C2,Hl,CoN8NaO12.C3H,0,20) Ber. C 44,06 H 3,06 Co 7,45 N 14,18;/,Gcf.…”

Section: Experimenteller Teilmentioning

confidence: 98%

Untersuchungen der Symmetrieverhältnisse in äquatorial koordinierten 2:1‐Arylazo‐Co^III‐Komplexen mit Hilfe der Protonenresonanz

Schetty

Steiner

1972

Helvetica Chimica Acta

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The magnetic equivalence of ligand protons was examined with aid of NMR. spectroscopy for a large number of 1 :2 CoIII-complexes of tridentated azo dyestuffs forming a representative cross section of the whole class of substances. It was established by this investigation that ligands with a moderately polarised azo group have a particular inclination to form asymmetrical complexes in which the protons of the two ligands are not magnetically equivalent. On the other hand, ligands having a non-polarised or a strongly polarised azo group tend to form symmetrical complexes. In three cases both symmetrical and asymmetrical complexes were isolated. As in previous investigations [I], up to a total of 5 complexes were again observed, which indicates a t least two causes governing the symmetry conditions. The phenomena described here could be explained with an alternative coordination of the two azo nitrogen atoms. On the basis of the NMR. results, however, preference is given to an exchange of the azo with the hydrazone form.

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“…But it might take place simply by inter action between substrate and chiral transition metal atoms without direct interaction between substrate and ligand. Asymmetric transition metal atoms are known to exist in the absence of chiral ligands, and they have been isolated when no rapid ligand exchange takes place (33). If chiral ligands are present in the complexes, and transition metal atoms (M) are chirality centers (23), for example in pentacoordinated complexes of the type a or b (Scheme 5), the complexes must exist in the most simple case ( L* contains only one chirality center and is optically pure) as two diastereomeric forms a, a' and b, b'.…”

Section: General Aspects Of Asymmetric Hydroformylationmentioning

confidence: 99%

Asymmetric Hydroformylation. Hydroformylation of Olefins in the Presence of Chiral Rhodium and Cobalt Catalysts

Pino

Consiglio

Botteghi

et al. 1974

Advances in Chemistry

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Optical yields up to 17% and 25%, respectively, have been reached in the styrene hydroformylation in the presence of cobalt or rhodium catalysts using N-alkylsalicylaldimine or phosphines as asymmetric ligands. Furthermore the hydro formylation of aliphatic and internal olefins have been achieved using rhodium catalysts in the presence of opti cally active phosphines. With the same catalysts, cis -butene surprisingly undergoes asymmetric hydroformulation with optical yields up to 27%. On the basis of the results obtained for cis-butene and the asymmetric induction phenomena in dichlor(olefin)(amine)platinum(II) com plexes, a simple model is proposed for the catalytic complex. The correlation among the prevailing chirality obtained in the asymmetric hydroformylation of vinyl, vinylidene, and internal olefins is discussed.

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Optische Aktivität an asymmetrischen Übergangsmetallatomen

Cited by 43 publications

References 61 publications

Pyramidal Stability of Chiral-at-Metal Half-Sandwich 16-Electron Fragments [CpRu(P−P′)]

Pyramidal Stability of Chiral-at-Metal Half-Sandwich 16-Electron Fragments [CpRu(P−P′)]

Untersuchungen der Symmetrieverhältnisse in äquatorial koordinierten 2:1‐Arylazo‐Co^III‐Komplexen mit Hilfe der Protonenresonanz

Asymmetric Hydroformylation. Hydroformylation of Olefins in the Presence of Chiral Rhodium and Cobalt Catalysts

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Optische Aktivität an asymmetrischen Übergangsmetallatomen

Cited by 43 publications

References 61 publications

Pyramidal Stability of Chiral-at-Metal Half-Sandwich 16-Electron Fragments [CpRu(P−P′)]

Pyramidal Stability of Chiral-at-Metal Half-Sandwich 16-Electron Fragments [CpRu(P−P′)]

Untersuchungen der Symmetrieverhältnisse in äquatorial koordinierten 2:1‐Arylazo‐CoIII‐Komplexen mit Hilfe der Protonenresonanz

Asymmetric Hydroformylation. Hydroformylation of Olefins in the Presence of Chiral Rhodium and Cobalt Catalysts

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Untersuchungen der Symmetrieverhältnisse in äquatorial koordinierten 2:1‐Arylazo‐Co^III‐Komplexen mit Hilfe der Protonenresonanz