[18]Annulene

The geometry of [18]annulene has been fully optimized within the D_6h point group (la) utilizing ab initio MO theory at the STO-3G level to give carbon-carbon bond lengths of 1.391 and 1.400 Å. This structure is found to be 35.7 kcal/mol less stable than a D_3h structure (1b) with alternating bond lengths of 1.330 and 1.479 Å. The ionization potentials are calculated to be in the ranges. 6.2–7.1 (1a) and 7.4–8.1 eV (1b)     

Tetrakisdehydro[16]annuleno[18]annulene

Tetrakisdehydro[16]annuleno consisting of trisdehydro[16]annulene and bisdehydro[18] annulene has been synthesized. Induction of para- and diamagnetic ring currents in 16- and 18-membered rings, respectively, was clearly recognized by the ¹H NMR spectroscopy. A marked suppresion of the diatropicity in the 18π moiety was observed in the same trend as observed in tetrakisdehydro[14]annuleno[16]annulene.     

Tetrakisdehydro[18]annuleno[20]annulene

Tetrakisdehydro[18]annuleno[20]annulene has been synthesized. The ¹H NMR spectra clearly indicate the induction of dia- and paramagnetic ring currents in 18- and 20-rings, respectively. A marked suppression of the diatropicity in the 18π moiety was observed being in the same trend in other tetrakisdehydro[4n]annuleno[4n′+2]annulenes. The ¹H NMR spectroscopic behavior of the annulenoannulenes is consistent with theoretical conclusion.     

Subphthalocyanine-dehydro[18]annulenes

A subphthalocyanine trimer built around a dehydro[18]annulene core was prepared. The synthesis was achieved through direct homocoupling of an ortho-diethynyl-functionalized subphthalocyanine, obtained by palladium-catalyzed cross-coupling of the corresponding diiodo-subphthalocyanine with an ethynyl derivative. The lower analogue dehydro[12]annulene did not form in these homocoupling conditions. The trimers were fully characterized and their electrochemical properties investigated.     

[18F]CuCF3: A [18F]Trifluoromethylating Agent for Arylboronic Acids and Aryl Iodides

Positron emission tomography has emerged as the leading method for medical imaging with fluorine‐18 as the most widely used radioactive isotope. Here we report a semi‐automated method for the preparation of valuable [¹⁸F]trifluoromethylcopper, as well as its use for the radiosynthesis of [¹⁸F]trifluoromethylarenes and heteroarenes. Mild conditions of [¹⁸F]trifluoromethylation make this method particularly useful for the radiosynthesis of pharmacologically relevant [¹⁸F]trifluoromethylarenes and heteroarenes.     

The Photoelectron Spectrum of [18]Annulene

The PE. spectrum of [18]annulene has been measured and correlated with MO-calculations. The experimental ionization energies can only be explained by computing the electronic states of the cation, that is by taking into account the electron correlation and reorganization in the ionic states. The results allow a discussion of the structure of the neutral molecule; they are consistent with a D_6hpoint group of symmetry.     

Concerning the Structure of [18]Annulene

A recent computational study of Schleyer and co‐workers [1] is reviewed, which led these authors to the firm conclusion that [18]annulene has a localized structure with alternating single and double C,C bonds, contrary to earlier crystallographic analyses of X‐ray‐diffraction data favoring a delocalized non‐alternating form. It is pointed out i) that deceptive orientational disorder phenomena in the crystal might be subject to experimental resolution in this case, and ii) that, in contrast to gas and solution phases, [18]annulene might possibly assume the non‐alternating structure in the crystalline solid state.     

[Mn18]2+ and [Mn21]4+ single-molecule magnets

The synthesis and structural characterization of the two new Mn complexes [Mn₁₈O₁₄(O₂CMe)₁₈(hep)₄(hepH)₂(H₂O)₂](ClO₄)₂ (1) and [Mn₂₁O₁₆(O₂CMe)₁₆(hmp)₆(hmpH)₂(pic)₂(py)(H₂O)](ClO₄)₄ (3) are presented, together with a detailed study of their magnetic properties. Complex 1 possesses a ground-state spin of S=13, and the ground-state spin for 3 is estimated to be S=17/2 or 19/2. Both complexes 1 and 3 are new examples of single-molecule magnets (SMMs), displaying frequency-dependent out-of-phase AC signals, as well as magnetization vs. DC field hysteresis at temperatures below 1 K. Complex 1 straddles the classical/quantum interface by also displaying quantum tunneling of the magnetization (QTM).     

Electrochemical transfer of [18F]fluoride from [18O]water into organic solvents ready for labeling reactions

For labeling reactions [¹⁸F]fluoride has to be separated from [¹⁸O]water and transferred into an organic solvent suitable for nucleophilic substitutions. An electrolytical method is described for depositing [¹⁸F]fluoride on a vitreous carbon electrode and releasing it directly into CH₃CN or DMSO. In the presence of Et₃N×3HF, [¹⁸F]fluoride is almost quantitatively released into acetonitrile. When using n.c.a conditions, i.e., Et₃N^.HCl, desorption of the ¹⁸F activity is almost 70% and 60% in acetonitrile and DMSO, respectively, already within 5 minutes.     

Enantiospecific synthesis of 2-[18F]fluoro-L-phenylalanine and 2-[18F]fluoro-L-tyrosine by isotopic exchange

2-[(18)F]Fluoro-L-phenylalanine and 2-[(18)F]fluoro-L-tyrosine have been developed as promising radiopharmaceuticals for molecular imaging using positron emission tomography (PET). However, the lack of a convenient radiosynthetic pathway has limited their practical use. In this work a new three-step nucleophilic synthesis of these compounds starting from [(18)F]fluoride is described. Corresponding precursors (1a and 1b) were (18)F-fluorinated by isotopic exchange, followed by the removal of an activating formyl group with Rh(PPh(3))(3)Cl and subsequent hydrolysis of protecting groups in acidic medium. All reactions were carried out using both conventional and microwave heating. Conventional heated reactions yielded the desired products 2-[(18)F]Fphe and 2-[(18)F]Ftyr in 43% and 49% whereas radiochemical yields of 34% and 43%, respectively, were obtained when they were heated by microwaves. Under optimized conditions the enantiomeric purity was ≥94% for both radiopharmaceuticals.     

Magnetic susceptibilities and resonance energies of annelated [14] - and [18] annulenes

Calculations of magnetic susceptibilities and topological resonance energies of annelated [14] - and [18] annulenes are reported. For several compounds the magnetic susceptibilities are also measured. In these cases a good agreement between calculated and experimental values of magnetic susceptibilities is obtained. All compounds studied are found to be diamagnetic. According to the magnetic exaltation criterion all compounds studied are predicted to be aromatic. This result is supported in the case of annelated annulenes by the TRE criterion. However, the parent structures, [14] annulene and [18] annulene, are found by the TRE to be non-aromatic. Finally, it is found that the aromatic stability does not parallel inert reactivity, i.e., some highly aromatic annulenoids are predicted by the low HOMO—LUMO separation to be rather reactive and some low aromatic structures are predicted by the high HOMO—LUMO separation to exhibit a considerable reactive inertness.