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  • 1
    ISSN: 0009-2940
    Keywords: C—C Bond cleavage, kinetics of ; Heats of formation ; Radicals, stability of ; Capto-dative effect ; Geminal substituents, energetic interaction of ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Substituent Effects on the Strength of C—C Bonds, 14[1].  -  Kinetic and Thermodynamic Stability of 2,3-Bis(dialkylamino)-1,4-diketones  -  Energy of Stabilization of α-Dialkylamino α-Carbonylalkyl Radicals with Capto-dative SubstituentsProf. Dr. H.-G. Viehe zum 65. Geburtstag gewidmet.The equilibrium constants and rate constants for the dissociation of the 2,3-bis(dialkylamino)-1,4-diketone diastereomers meso- and DL-7a and 7b were measured over a temperature range of 40°C. From the enthalpies of dissociation ΔHDiss and enthalpies of activation ΔH≠ and the strain enthalpies of 7 the bond dissociation enthalpies BDE(C—C) of 7 were determined. By comparison with the dissociation enthalpies of Ct-Ct alkanes[21] the change of these BDEs(C—C) by the cap-to-dative substitution was determined to be 85.4 kJ mol-1 (20.4 kcal mol-1). The heats of formation ΔHof (g) of a series of amino ketones 8 were determined from their heats of combustion and their heats of evaporation. From the ΔHof(g) values in combination with MM2 calculations of their strain enthalpies strain-free increments CHn[N, CO, C2 - n] with n With n = 0, 1, 2 were derived and geminal interaction enthalpies in the ground states were obtained thereof. The radical stabilization enthalpy RSE of 6 was deduced from the ΔBDE(C—C) values and the ground state effect to be 73.6 kJ mol-1 (17.6 kcal mol-1). From these data and the radical stabilization enthalpies RSE of α-aminoalkyl radicals (4.2 kJ mol-1) and α-carbonyl radicals (28.9 kJ mol-1) a synergetic radical stabilization enthalpy of 40.5 kJ mol-1 (9.7 kcal mol-1) is deduced. This number combines “extra” resonance stabilization and general inductive or anomeric geminal substituent interaction in the radicals. The crystal structure of meso-7a has been determined by X-ray diffraction methods.
    Additional Material: 2 Ill.
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  • 2
    ISSN: 0009-2940
    Keywords: syn1-Periplanar bisazo skeletons ; Carbonylmetal complexes ; Iron complexes ; Ruthenium complexes ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Preparatively useful synthesis for the novel, relatively mobile polycyclic syn1-periplanar bisazo polycycles 4 and 5 are based on hydrazine condensation with readily available dienedione 6 and acid-catalyzed rearrangement of bisurazole 12, respectively. In the preferred “closed” and “open” conformations of 4 and 5 the N=N bonds are 314 and 496 pm, respectively, apart (MMX). From the reactions of 4 and 5 with carbonyliron and -ruthenium compounds, complexes are derived which are typical of monoazo ligands [4 · Fe(CO)4 ≡ 20, 4 · Fe2(CO)7≡21, 4 · Fe3(CO)9≡22, 4≡[Fe2(CO)6]2≡23, 4 · Ru3(CO)9 · 24, 5 · [Fe2(CO)6]2 ≡ 25, 5 · Fe2(CO)6 · Fe3(CO)9 ≡ 26]. For 21, 23 and 26 crystal structure analyses were performed, confirming i.a. in 26 the unique presence of a Fe2(CO)6 and a Fe3(CO)9 fragment attached to two azo units of the same molecule.
    Additional Material: 3 Ill.
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  • 3
    ISSN: 0009-2940
    Keywords: Uncharged auxiliary bases ; Triaminoiminophosphorane bases ; Deprotonation ; Hindered bases ; Polymeric bases ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis and properties of a number of very strong iminophosphorane bases up to an extremely high level of steric hindrance are described. They cover a range of ca. 4 pK units in basicity and a range of more than 11 orders of magnitude in their rates of methylation with methyl iodide. Most of the systems are readily prepared in up to molar quantities, conveniently recovered from their salts and are of high chemical and thermal stability. Crystal structures were determined in order to parametrize a force field, which is utilized in molecular modeling studies offering a rationalization of the observed differences in steric hindrance and basicity. Depending on the degree of steric protection of the basic center, these novel bases are proposed as unprecedented, versatile auxiliary bases in E2 eliminations and in reactions involving deprotonation in the presence of more or less strong electrophiles.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
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