Konformation und Struktur von Organometallkomplexen im Festkörper: zwei für das Verständnis chemischer Reaktionen wichtige Studien

Abstract

Professor Jack Dunitz gewidmetIn dieser Ubersicht werden zwei Klassen von Organometallkomplexen unter Benutzung der Cambridge Structural Database (CSD) auf ihre bevorzugten Konformationen hin untersucht. Die Komplexe von Lewis-Sauren mit Carbonylverbindungen zeigen je nach den elektronischen und sterischen Anspriichen der Carbonylkomponente und der Art der Lewis-Saure eine Vielfalt von Koordinationsgeometrien. AuDerdem werden die Festkorper-Konformationen von Acyl-, Nitrosyl-, Acetylen-und Imidoliganden in Komplexen, die x-bindende Liganden enthalten, diskutiert. Die dabei gewonnenen Erkenntnisse regten die Entwicklung eines Konformationsmodells an, das auf den Konzepten der ,,x-Bindungshybridisierung" und der Grenzorbitaltheorie beruht. Die Befunde sind fur ein Verstandnis der Mechanismen und obergangszustande vieler praparativ wichtiger Reaktionen von Bedeutung. Ein auf genauen Strukturdaten beruhendes tieferes Verstandnis der konformativen Eigenschaften von Organometallkomplexen wird wahrscheinlich das Entwerfen und Optimieren von Prozessen fordern, in denen Metalle eine Rolle spielen.

Catalytic Carbophilic Activation: Catalysis by Platinum and Gold π Acids

“…However, we believe that it is useful and appropriate to define any metal fragment that binds to a carbon–carbon multiple bond, and thereby deprives it of part of its electron density, as a “π acid” 40. Coordination to such electron sinks induces partial positive charge on the ligand, just as complexation of classical Lewis acids such as TiCl 4 or ZnCl 2 does to a carbonyl group,41 and therefore renders the π system electrophilic in character. It is the partial slippage away from the symmetrical η 2 coordination in the transition state which engenders productive overlap of the orbitals of the nucleophile with the distorted π system.…”

Coordination of the Ester Group – Hydrido‐Rhodium(III) and Iridium(III) Complexes of Orthometallated Diphenylmethylene Glycine Esters [1]

“…E and Z isomers of the tungsten and rhenium complexes (or conformers, concerning metal atom and acyl alkyl group) andtheir interconversion were observed by Faller 15 and Gladysz 16. The structures of conformers of the TiCl 4 complexeswith carboxylic acid esters were discussed 18.…”

Possible Intervention of Prototropic and Silatropic Ene‐type Mechanisms in Asymmetric Catalytic Mukaiyama Aldol Reactions

“…The complexation mode of aldehydes and Lewis acids (MX n ) and X-ray analyses thereof have been reviewed in view of the asymmetric catalysis of Mukaiyama aldol reactions in particular. [16] The aldehyde lone pair has been reported to interact with the M À X antibonding orbital s*M À X. [17] Furthermore, the aldehyde C À H has been shown to bind to heteroatom ligands on metal catalysts.…”

“…The formyl C À H hydrogen bonding with the oxygen lone pair of BINOL has also been proposed. [16] The coordination of glyoxylate with the BINOL-Ti complex was thus set to monodentate (nonchelation) mode [15b] with the formyl CÀH group hydrogen bonding with the BINOL ligand. [16] BINOL-Ti IV Catalysis of the Mukaiyama Aldol Reaction BINOL-Ti IV complexes serve as efficient catalysts for the Mukaiyama-type aldol reaction of ketone ESEs, with good control of both absolute and relative stereochemistry.…”

“…[16] The coordination of glyoxylate with the BINOL-Ti complex was thus set to monodentate (nonchelation) mode [15b] with the formyl CÀH group hydrogen bonding with the BINOL ligand. [16] BINOL-Ti IV Catalysis of the Mukaiyama Aldol Reaction BINOL-Ti IV complexes serve as efficient catalysts for the Mukaiyama-type aldol reaction of ketone ESEs, with good control of both absolute and relative stereochemistry. [14,15,28] Remarkably, the aldol products are sometimes isolated as the ESE (prototropic ene) form with high syn-diastereoselectivity starting from either geometrical isomer of the starting ketone ESEs.…”