Browsing by Author "Schöllkopf, Klaus"

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    Crystal and molecular structures of novel (arene)chromium tricarbonyl complexes (Aminobenzenes ; 18)
    (1985) Schöllkopf, Klaus; Stezowski, John J.; Effenberger, Franz
    Several new (1,3,5-tris(dialkylamino)benzene)chromium tricarbonyl (chromium tricarbonyl = TCC) complexes (2) were prepared and crystal structures for two of them, 2a ((1,3,5-tripyrrolidinobenzene)TCC) and 2b ((1,3,5-tripyrrolidino-2-methylbenzene)TCC), were determined to high precision with X-ray data collected from cooled crystals (T ≈ 120 K). Complex 2a crystallizes in space group symmetry P1 with a = 8.7670 (5) A, b = 10.6023 (8) A, c = 11.6337 (8) A, α = 66.724 (6)°, β = 72.560 (6)°, γ = 79.651 (7)°, and Z = 2. For 2b the space group is P2 l/c with a = 12.323 (1) A, b = 8.2651 (8) A, c = 23.902 (2) A, β = 123.114 (7)°, and Z = 4. The model for 2a was refined with 13 110 contributing reflections to give Rw = 0.053; for 2b refinement with 9154 reflections gave Rw = 0.058. A crystal structure was also determined for the [(phenyl(trimethylsiloxy)methyl)benzene]TCC complex (3) for which the space groups is P2 1/c with a = 6.282 (1) A, b = 11.319 (4) A, c = 26.962 (12) A, β = 93.82 (2)°, and Z = 4 (room temperature). All three complexes display a close approximation to the syn-e conformation for the (arene)TCC moiety. The rotational barriers for compounds 2 could not be established because of their low solubility. The application of conclusions from crystal structures to reaction behavior in solution was proved in the case of the [(phenyl)trimethylsiloxy)methyl)benzene]TCC complex (3). The experimental results are in agreement with the reactivity pattern calculated for the crystal structure determined for 3.
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    Nucleophil katalysierte Reaktionen von Tricarbonyl(trimethylsilylaren)chrom-Komplexen mit Elektrophilen
    (1981) Effenberger, Franz; Schöllkopf, Klaus
    Elektrophile Substitutionen donorsubstituierter Aryltrimethylsilane (z.B. Alkylphenyl- oder Aminophenyltrimethylsilane) lassen sich nicht durch Nucleophile katalysieren. Gerade diese Verbindungen reagieren aber mit Elektrophilen in Gegenwart eines Friedel-Crafts-Katalysators besonders gut; hierbei ist jedoch die dirigierende Wirkung der Substituenten oft stärker als die Neigung zur ipso-Substitution der Trimethylsilylgruppe, so daß häufig "normale" H-Substitution stattfindet. Der elektronenziehende und damit anionenstabilisierende Einfluß der Tricarbonylchrom-Gruppe in Aren(tricarbony1)chrom-Komplexen sowie deren präparative Verwendung ist beschrieben.
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    Structure and reactivity of aromatic σ-complexes (cyclohexadienylium ions) : a correlated experimental and theoretical study
    (1987) Effenberger, Franz; Reisinger, Friedrich; Schönwälder, Karl-Heinz; Bäuerle, Peter; Stezowski, John J.; Jogun, Kurt H.; Schöllkopf, Klaus; Stohrer, Wolf-Dieter
    Steric and electronic properties for a series of σ-complexes have been examined by experimental and theoretical techniques. Crystal structures for three 2,4,6-tripyrrolidinocyclohexadienylium salts 3a, 3b, and 4a and for l-methyl-2,4,6-tripyrrolidinobenzene 5 are reported. With respect to the cyclohexadienylium ring, the H,H σ-complexes, 3, display a planar conformation whereas σ-complexes 4 and 6 are bent. Steric interactions force the larger substituents of the tetrahedral carbon atom in 4 and 6 into the pseudoaxial position. The results of EH, MNDO, and 3-21G calculations are in agreement with the crystal structure determinations for σ-complexes 3,4, and 6. Calculations performed for other σ-complexes indicate that the ring conformation for σ-complexes with small substituents in the 2- and 6-positions should be planar even in cases with different substituents on the tetrahedral carbon atom; with larger substituents, a bent conformation is favored with the larger substituent at C1 axial. The activation energy for planarization of the bent structure is fairly high, and it is even higher for inversion. For stereoelectronic reasons, only the ligand in the axial position of bent σ-complexes can dissociate during rearomatization. Thus the stability and reactivity of σ-complexes are strongly dependent on their conformation. The large differences in pKa values of planar and nonplanar σ-complexes, as well as the unexpectedly high stability of phloroglucinophane σ-complexes, can be explained by the high inversion energy for the formation of the σ-complexes with the proton as the leaving group in the axial position.
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    Tricarbonylchrom-Komplexe aromatischer Aldehyde und Ketone
    (1985) Effenberger, Franz; Schöllkopf, Klaus
    Tricarbonylchrom (TCC)-Komplexe 2, 3 aromatischer Aldehyde und Ketone werden einmal durch basekatalysierte Carbodesilylierung von TCC-Aryltrimethylsilan-Komplexen 1 mit Dime-thylformamid bzw. Benzoylfluorid, zum anderen durch selektive Oxidation von TCC-Phenyl-methanol-Komplexen 5 mit speziell präpariertem Mangandioxid in Ether dargestellt.
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    Tricarbonylchrom-Komplexe von Aryltrimethylsilanen - Darstellung und Reaktionen
    (1985) Effenberger, Franz; Schöllkopf, Klaus
    Tricarbonylchrom(TCC)-Komplexe 3, 12 trimethylsilylierter Benzole werden aus Aryltrimethylsilanen 2 bzw. Phenylenbis(trimethylsilanen) 11 und Hexacarbonylchrom (1) dargestellt. Sie reagieren - bedingt durch die Acceptorwirkung der TCC-Gruppe - basekatalysiert mit Aldehyden und Ketonen unter Carbodesilylierung zu den TCC-Alkyl(trimethyl-silyl)ether-Komplexen 4,9,13. Durch Hydrolyse der TCC-Komplexe 4, 13, werden die TCC-Komplexe 5 substituierter Benzylalkohole erhalten. Die oxidative Abspaltung der TCC-Gruppe wird beispielhaft an 4a mit Iod bzw. Natriumdichromat beschrieben, wobei Benz-hydrol (7) bzw. Benzophenon (8f) entsteht. An die Aromaten der TCC-Komplexe 4,9,13 können nach bekannten Methoden Nucleophile addiert werden; damit eignen sich die TCC-Komplexe 3, 12, für die gezielte Synthese mehrfach substituierter Aromaten durch Kombination der elektrophilen Carbodesilylierung und Addition von Nucleophilen.