Hans Schmickler

Applications of carbon-13 resonance spectroscopy. V. “Fingerprints” for the assignment of carbon-13 resonance signals☆

Abstract It is shown that the splitting pattern observed for the 13 C NMR signals in the 1 H-undecoupled spectra of symmetrically ortho-disubstituted benzenes differs characteristically for carbons in the position α and β to the substituted carbons, thus, giving rise to typical “fingerprints.” These may be used in turn to assign 13 C resonances for these compounds and other unsaturated systems containing a HC-CH-CH-HC fragment.

Results of carbon-13 n.m.r. spectroscopy in the field of cyclic conjugated π-electron systems

ABSTRACT The theoretical basis for the interpretation of 13C chemical shifts is reviewed briefly. Possible ring current contributions to the shielding of the 13C nucleus are discussed in detail with examples from the field of polycyclic nonbenzenoid π-systems and bridged annulenes. The fingerprint nature of 13C n.m.r. data is stressed and the significance of σ-bond structure is demonstrated for bridged [14]annulenes and related compounds. Pyrene and the two isopyrenes dicyclopenta[ef,kl]heptalene and dicyclohepta[cd,gh]pentalene serve to illustrate typical features of 13C n.m.r. spectra of alternating and non-alternating π-systems. Finally, the sensitivity of 13C n.m.r. for the detection of valence isomerizations is exemplified with results obtained for 11,11-disubstituted 1,6-methano-[10]annulenes.

ISOPORPHYCENE : THE FOURTH CONSTITUTIONAL ISOMER OF PORPHYRIN WITH AN N4 CORE : OCCURRENCE OF E/Z ISOMERISM

Liberation of the ligand from the nickel complex 1 obtained by template synthesis yielded isoporphycene (as the octaethyl derivative 2), the first constitutional isomer of porphyrin with an N(4) core for which E/Z isomerism is involved: Compound 2 is present as the E isomer, which is in rapid, presumably acid-catalyzed equilibrium with a small amount (2 %) of the Z isomer. The remaining unknown constitutional isomers of porphyrin are considerably higher in energy than the already rather labile isoporphycene, so that the latter should mark the border of existence for this type of structural variant of porphyrin.

Effect of the location of hydrogen abstraction on the fragmentation of diuretics in negative electrospray ionization mass spectrometry.

The diuretic agents bumetanide, xipamide, indapamide, and related compounds were investigated in order to determine the effect of different ionization sites on their collisionally activated dissociation and the corresponding fragmentation pathways. Therefore, analytes were selectively alkylated, and structural analogues as well as deuterium labeled compounds synthesized, which contain a reduced number of ionizable hydrogen atoms. Thus, specific hydrogen abstractions and their correlated dissociation routes of the negatively charged molecules were eliminated, providing evidence for the influence of the location of ionization on product ion spectra. Fragment ions such as m/z 78 indicate ionization at the commonly present sulfamoyl residue of diuretics but does not exclude additional ionization sites. Product ion spectra of the investigated diuretic agents proved to be composed by fragmentations initiated from different hydrogen abstractions. Moreover, the generation of radical anions by collision-activated dissociation of even-electron precursor ions was observed, the generation of which is discussed by proposed fragmentation pathways.

Regio- and stereoselective 1,6-photocyclization of aspartic acid-derived chiral γ-ketoamides☆

Abstract Chiral α-acylamino γ-ketoamides 2 were irradiated and regio-isomeri 1,6-cyclization products 3 and 4 obtained in high yields. Symmetrically N , N -disubstituted substrates 2a,b reacted diastereoselectively to give the all-cis products 3a,b in high yields. High 1,3-asymmetric induction was also observed for the unsymmetrically N , N -disubstituted 2d and 2e . The stereoselectivity of these reactions is discussed in terms of SOC-controlled spin inversion geometries.

Doubly charged ions of bridged [4n] annulenes. An evaluation of diatropic ring current effects.

Abstract [4n]Annulenes are transformed into doubly charged ions (dianion, dication) and characterized NMR spectroscopically. The diatropic character of the ionic (4n+2)π-systems is studied as a function of the number of π-electrons.

Isoporphycen, das vierte Porphyrin‐Konstitutionsisomer mit N4‐Kern – Auftreten von E/Z‐Isomerie

FreisetzungdesLiganden aus dem durch Templatsynthese gewonnenen Nickelkomplex 1 lieferte Isoporphycen (als Octaethylderivat 2), das erste Porphyrin-Konstitutionsisomer mit einem N4-Kern, bei dem E/Z-Isomerie ins Spiel kommt: 2 liegt als E-Isomer vor, das mit einer gerade noch nachweisbaren Menge (2 %) an Z-Isomer in sehr raschem, vermutlich saurekatalysiertem Gleichgewicht steht. Die verbleibenden unbekannten Porphyrin-Konstitutionsisomere sind noch erheblich energiereicher als das bereits labile Isoporphycen, so das mit diesem die Grenze der Existenzfahigkeit von Porphyrin-Strukturvarianten dieses Typs erreicht sein durfte.

Structure and fluxional behaviour of heptaleneirontricarbonyl and heptalenebis(irontricarbonyl)

Abstract The synthesis of heptaleneirontricarbonyl ( 4 ) is described. The structures of 4 and of the closely related heptalenebis(irontricarbonyl) ( 3 ) are elucidated by NMR spectroscopy ( 1H and 13C) and by X-ray crystallography. Compounds 3 and 4 are shown by dynamic NMR to undergo an isodynamic 1 ,2-migration of the Fe(CO3)-groups as well as a carbonyl scrambling. The relevant kinetic data allow for a mechanistic discussion of the dynamics and a comparison with other irontricarbonyl complexes.

1,6:9,14-bismethano[16]annulene - a new bridged [4n]annulene

Abstract The molecular structure and dynamic behaviour of the bridged [4n] annulene 1,6:9,14-bismethano-[16]annulene is unravelled by NMR and X-ray investigations, combined with force field calculations.

13C nmr spectroscopy of carbonyl compounds : 2. Conformational amd configurational dependence of the (z)-epoxyenone/furan isomerization

Abstract The furan derivative 4 which has been isolated upon photolysis of the epoxyenone compound (E)- 1 results from a thermal isomerization of the intermediate (Z)-isomer (Z)- 1 The latter reaction can best be understood as a multi-centre process implying a nucleophilic attack of the carbonyl oxygen upon the oxirane ring. Inspection of related epoxyenones (e.g. 2 and 3 ) reveals that the ease of the above isomerization sensitively depends upon the conformation with respect to the C-6/C-7 single bond. The conformational properties of a series of epoxyenones as well as of related dienones and epoxydienes have been studied by dynamic 13C NMR spectroscopy. The chemical and spectroscopic information is combined to describe the stereochemical course of the epoxyenone/furan isomerization. As a consequence we can control the thermal reactivity of the title compounds by creating an appropriate conformational behaviour.