Oxalic acid forms in superacidic solutions HF/MF5 (M = As, Sb) its corresponding mono‐ and diprotonated salts [HOOCC(OH)2][MF6] and [(HO)2CC(OH)2][MF6]2 (M = As, Sb). The number of protonations is strongly dependent on the stoichiometric ratio of the Lewis acid with regard to oxalic acid. Mono‐ and diprotonated salts were characterized by vibrational spectroscopy and in the case of [HOOCC(OH)2][AsF6] (1) and [(HO)2CC(OH)2][SbF6]2 (4) by a single‐crystal X‐ray structure analysis. The salts crystallize in the monoclinic space groups P21/c and P21/n with eight, respectively four, formula units per unit cell. The vibrational spectra were compared to quantum chemical calculations of the cations [HOOCC(OH)2]+ and [(OH)2CC(OH)2]2+⋅4HF. In addition to this, an MEP analysis together with the NPA charges of [(OH)2CC(OH)2]2+, [(OH)2CC(OH)2]2+⋅6HF and oxalic acid were calculated to locate the positive charge. The protonation of oxalic acid does not lead to a remarkable change of the C‐C bond length, which is discussed for the entire series of the oxalic skeleton, starting with the dianion and ending with the tetrahydroxy dication.
Described is the in situ formation of triorganocerium reagents and their application in catalyst‐free Zweifel olefinations. These unique cerium species were generated through novel exchange reactions of organohalides with n‐Bu3Ce reagents. The adequate electronegativity of cerium allowed for compensating the disadvantages of both usually functional‐group‐sensitive organolithium species and less reactive organomagnesium reagents. Exchange reactions were performed on aryl and alkenyl bromides, enabling enantiospecific transformations of chiral boron pinacol esters. Finally, these new organocerium species were engaged in selective 1,2‐additions onto enolisable and sterically hindered ketones.
Croconic acid reacts in superacidic solutions HF/MF5 (M = As, Sb) to yield its corresponding salts [H3O5C5][MF6] and [(H3O5C5)H(H3O5C5)][MF6]3·2HF (M = As, Sb). The degree of protonation is strongly dependent on the stoichiometric ratio of the Lewis acid regarding croconic acid. Monoprotonated salts were characterized by vibrational spectroscopy and in the case of [H3O5C5][AsF6] (1) by a single‐crystal X‐ray structure analysis. [H3O5C5][AsF6] crystallizes in the monoclinic space group P21/c with four formula units per unit cell. The sesquiprotonated species of croconic acid [(H3O5C5)H(H3O5C5)][SbF6]3·2HF (4) was also characterized by single‐crystal X‐ray structure analysis. It crystallizes in the triclinic space group P1 with one formula unit per unit cell. The vibrational spectra of the monoprotonated salts were compared to quantum chemical calculations of the [H3O5C5]+·3HF cation and experimental data reported for croconic acid.
Guanidinium chloride reacts with the superacidic solutions HF/MF (M=As, Sb) at a molar ratio of 1:2 under formation of the diprotonated guanidinium salts [C(NH ) (NH )][AsF ] and [C(NH ) (NH )][SbF ] . The compounds were characterized by using infrared and Raman spectroscopy. Furthermore, single-crystal X-ray structure analysis of the guanidinium(2+) salts [C(NH ) (NH )][SbF ] ⋅HF, [C(NH ) (NH )] [Ge F ]⋅HF, and [C(NH ) (NH )] [Ge F ]⋅2 HF and the guanidinium(1+) salt [C(NH ) ][SbF ] is reported. The discussion of the experimental data is supported by quantum-chemical calculations of the [C(NH ) (NH )] and [C(NH ) ] ions to investigate the modification of the resonance stabilization during the protonation process at the PBE1PBE/6-311G++(3df,3pd) level of theory. The planar CN skeleton of the guanidinium(2+) ion has two carbon-nitrogen bonds in the range 1.286(4)-1.293(4) Å and one carbon-nitrogen bond of 1.453(4) Å, which can be explained with a decreased resonance stabilization relative to the guanidinium(1+) ion.
Wirb eschreiben die In-situ-Synthese von Triorganocer-Reagentien und deren Anwendung in katalysatorfreien Zweifel-Olefinierungen. Diese einzigartigen Cer-Spezies wurden durch neuartige Austauschreaktionen von Organohalogeniden mit n-Bu 3 Ce-Reagentien erzeugt. Durch die geeignete Elektronegativitätv on Cer konnten sowohl die Empfindlichkeit von Organolithium-Verbindungen fürfunktionelle Gruppen als auch die geringere Reaktivitätv on Organomagnesium-Verbindungen ausgeglichen werden. Austauschreaktionen wurden an Aryl-und Alkenylbromiden durchgeführt, die mit chiralen Borpinakolestern enantiospezifische Transformationen eingingen. Abschließend wurden diese neuen Organocer-Spezies eingesetzt, um selektive 1,2-Additionen an enolisierbaren und sterischg ehinderten Ketonen durchzuführen.
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