Three closely related previously synthesized monoaza[5]helicenes have been resolved into their enantiomers via enantioselective HPLC using a cellulose-derivative Chiralcel OD column. Circular dichroism (CD) spectra of the enantiomerically enriched samples have been recorded and assigned. The spectra were analyzed as a function of time, and different rate constants were found in the kinetics of racemization for the three molecules. Ab initio DFT calculations for the ground electronic states were employed to determine minima and saddle point structures and to understand the racemization process. The theoretical geometries compared well with those from X-ray structures. CD spectra were calculated by TD-DFT ab initio methods, and compared with experimental data
Obvious but unknown in asymmetric catalysis were chiral six‐membered‐ring phosphanes and secondary phosphanes. As first examples of such ligands, oxaphosphinanes were now prepared and examined in asymmetric hydrogenation. With the monodentate oxaphosphinane 1, for example, 96 % ee was achieved with itaconic acid as the substrate and 97.5 % ee was achieved with the chelate ligand 2 and 2‐acetamidoacrylic acid as the substrate.
Professor Dieter Hoppe zum 60. Geburtstag gewidmet Modular aufgebaute chirale Phosphane sind als Liganden f¸r die asymmetrische Katalyse mit ‹bergangsmetallkomplexen wichtig. Zu den effektivsten Liganden, besonders bew‰hrt in enantioselektiven Hydrierungen, gehˆren die von Burk et al. eingef¸hrten Ethylen-verbr¸ckten BPE-Liganden 2 und 1,2-Phenylen-verbr¸ckten DuPHOS-Liganden [1] sowie terschied zu Indigo werden die Akashine allerdings nicht von roten Isomeren begleitet. ExperimentellesDer terrestrische Streptomycet GW 48/1497 stammt aus der Sammlung von bioLeads, Heidelberg. Er wurde unter¸blichen Bedingungen [20] auf Soja-Mannit-Medium (Sch¸ttelkultur, 15 L verteilt auf 75 1 L-Schikane-Kolben) 3 Tage bei 28 8C, 110 Upm) fermentiert und aufgearbeitet.Nach Vortrennungen an Kieselgel wurden die blauen Pigmentfraktionen durch pr‰parative D¸nnschichtchromatographie (PDC) (CHCl 3 /15 % MeOH/0.1 % AcOH) nachgereinigt. Aus der Zone mit einem R f -Wert von 0.15 (R f 0.11, CH 2 Cl 2 /10 % MeOH) fielen beim Einengen des Eluats 35.6 mg 3 a als ein in Methanol schwerlˆslicher blauer Feststoff aus. Zwei weitere Fraktionen ergaben bei der Nachtrennung (Sephadex LH-20, 3 Â 70 cm, CHCl 3 /40 % MeOH) 6 mg 3 b (R f 0.48, CHCl 3 /17 % MeOH/0.2 % AcOH; R f 0.22, CH 2 Cl 2 /10 % MeOH) und (Sephadex LH-20 und PDC 20 Â 20 cm, CHCl 3 /10 % MeOH) 5 mg 3 c (R f 0.49 (CHCl 3 /17 % MeOH/ 0.2 % AcOH; R f 0.52 (CH 2 Cl 2 /10 % MeOH) als dunkelblaue Feststoffe.
A variety of new chiral phosphanes were prepared by highly diastereoselective additions of phosphanes to α,β-unsaturated carbonyl compounds and related acceptor-substituted olefins derived from myrtenal as ex chiral pool source. Monophosphanes with astereogenic as well as stereogenic phos-[a] Organisch-Jansen and Feringa who employed this approach for a synthesis of the ligand chiraphos using as key steps diastereoselective addition of lithium diphenylphosphide to a γ-alkoxybutenolide and trapping of the resulting enolate with PPh 2 Cl (Scheme 2). [6] Scheme 2. Asymmetric synthesis of chiraphos via conjugate addition.Afterwards, the concept of the diastereoselective conjugate addition of phosphides as a simple route to novel ligands was further elaborated in our group. In contrast to Jansen and Feringa we directly used the addition products as ligands and selected the chiral starting materials from ex chiral pool sources in order to assure enantiomeric purity and relative configuration of the new phosphane ligands. In this way it was possible to obtain enantiomerically pure trans-3-(diphenylphosphanyl)myrtanic acid [7] in only two steps: addition of lithium diphenylphosphide to tert-butyl myrtenate which is easily accessible from commercial, cheap myrtenal followed by cleavage of the ester (Scheme 3). [8] In the meantime related diastereoselective conjugate additions to EWG-substituted olefins to give enantiomerically pure P,N-ligands were described by Knochels group. [9] Further-New Chiral Phosphane Ligands FULL PAPER more, enantioselective additions of phosphanes under control of asymmetric catalysts have been achieved by Togni et al. [10] Scheme 3. Synthesis of trans-3-(diphenylphosphanyl)myrtanic acid, an excellent ligand for Pd-catalyzed allylic substitutions, by diastereroselective conjugate addition.trans-3-(Diphenylphosphanyl)myrtanic acid is an excellent ligand for Pd-catalyzed allylic substitutions. [11] Minami et al. described an alternative approach to enantiomerically pure β-phosphanylcarboxylic acids via a multi-step synthesis of a racemate followed by enantiomer resolution by crystallization of diastereomeric salts. [12] Both the butenolide-derived as well as the myrtane-based system impressively demonstrate that phosphane additions on such rigid, cyclic cores proceed with a high degree of diastereoselectivity and at a high reaction rate. The almost unlimited availability of a large variety of enantiomerically pure enones, enoates and enals, therefore, offers the possibility to generate libraries of structurally diverse ligands. This consideration and the high efficiency of trans-3-(diphenylphosphanyl)myrtanic acid as ligand in Pd-catalyzed allylic alkylation reactions [8] prompted us to examine the conjugate addition more thoroughly. To begin with, the following aspects were most relevant:1. The torsion angle between the phosphanyl and the carboxyl group of phosphanylcarboxylic acids can be systematically varied by choosing appropriate conjugated acceptors. As shown in Scheme 4 phosphanylcarbo...
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