Asymmetrische Synthesen mit Ketenen, III. Über den Lösungsmitteleinfluß auf den sterischen Ablauf zweier asymmetrischer Amidsynthesen

Abstract

Asymmetrische Synthesen mit Ketenen, 1113) uber den LiisungsmitteleinfiuD auf den sterischen Ablauf zweier asymmetrischer Amidsynthesen Die Temperaturabhangigkeit der Stereospezifitat bei der Addition von a-Phenylathylamin an Phenyl-methyl-keten bzw. Phenyl-a.o-trimethylen-keten wurde in I 1 verschiedenen Lasungsmitteln untersucht. Fur die a-Phenyl-propionsaure-[a-phenyl-athylamidl-Bildung ergaben sich Ubersichtliche Beziehungen zwischen den die Spezifitat kontrollierenden Aktivierungsenthalpie-und -entropie-Differenzen und der ,,Baskitat'' der Lasungsmittel, die im vorliegenden Fall ein geeignetes MaB fUr die ,,Solvatationskraft" darstellt. Eine Deutung der beobachteten Phlnomene scheint auf einer recht allgemeinen Grundlage mbglich zu sein. GETAG und G . LEHMANNS), von WALBORSKY und Mitarbb.6) sowie von 0. &RVINKA7) Emah-1) Auszug aus der Habilitationsschrift, Univ. Halle/Saale 1962. 2) Diplomarb. Univ. Halle/Saale 1961.

Temperature and solvent effects in facial diastereoselectivity of nucleophilic addition: entropic and enthalpic contribution

“…All these arguments support the hypothesis that the main factor in diastereofacial control is a stereospecific solvation of the p-system. 25 Our interpretation simply constitutes an attractive proposal and an intriguing challenge for forthcoming research on stereoselectivity.…”

Solvent effects on stereoselectivity: more than just an environment

“…This phenomenon was thoroughly reviewed in 1991 by Scharf et al, 12 and it has been observed quite frequently for different selective processes. Pracejus and Tille first described a non-linear trend of enantioselectivity in an asymmetric addition of alcohols to a ketene catalyzed by chiral amines, 13 and non-linear Eyring plots have been observed in a large number of completely different reactions: catalytic hydrogenations, photochemical reactions like the Paterno`-Bu¨chi reaction, the Sharpless dihydroxylation, Diels-Alder cycloadditions and several addition reactions. 14 Very recently, interesting examples of non-linear Eyring plots in an enantioselective cycloaddition have been reported by Buono et al 15 Fig.…”

“…To support our interpretative model of inversion temperature, we considered spectroscopic techniques. We started with 13 C NMR spectroscopy because chemical shifts are affected by intermolecular interactions with the solvent. We recorded 13 C NMR spectra of some aldehydes in d 8 -THF or d 14 -n-hexane at several temperature values and noted an evolution of the CQO chemical shift(s) with temperature.…”

“…We started with 13 C NMR spectroscopy because chemical shifts are affected by intermolecular interactions with the solvent. We recorded 13 C NMR spectra of some aldehydes in d 8 -THF or d 14 -n-hexane at several temperature values and noted an evolution of the CQO chemical shift(s) with temperature. 32 After full assignment, all spectra showed a unique set of signals in the temperature range explored, which ensured the presence of a population-weighted average of rapidly interconverting conformers.…”

“…33 UV-vis absorption and CD are two techniques also affected by solute-solvent interactions, CD being generally the more sensitive. In collaboration with Spada et al, 33 we evaluated the UV and CD spectra of a-chiral aldehydes in n-decane and n-dodecane solution upon changing the temperature, and compared the results with 13 C NMR variable temperature analyses and the observed T inv values of diastereoselective reactions. As an example, in Fig.…”