Doris Kunz

Electrosynthesis of Imidazolium Carboxylates

Synthesis of imidazolium carboxylate compounds was efficiently achieved by electrochemical reduction of imidazolium precursors under very mild conditions.

First X-Ray Structure Analyses of Rhodium(III) η1-Allyl Complexes and a Mechanism for Allylic Isomerization Reactions†

Metal-to-metal allyl transfer: Using the first structurally characterized rhodium eta(1)-allyl complexes it is shown that the sigma-bound allyl substituent can be transferred from the Rh(III) complex to a Rh(I) complex in a fast equilibrium. This process may account for the decrease in regioselectivity observed in allylic alkylation reactions in which complex 1 is used as a catalyst.

Synthesis of copper(II) and gold(III) bis(NHC)-pincer complexes

Abstract CuII and AuIII chlorido complexes bearing the bis(NHC) carbazolide pincer ligand (bimca) were synthesized by transmetallation from the respective lithium complex [Li(bimca)] (NHC=N-heterocyclic carbene). In the case of copper, two different molecular structures were obtained depending on the copper source. With Cu(II) chloride the paramagnetic mononuclear [Cu(bimca)Cl] complex is formed and has been characterized by EPR spectroscopy and X-ray structure analysis, while copper(I) chloride leads under oxidation to a dinuclear structure in which two cationic [CuII(bimca)] moieties are bridged by one chlorido ligand. The positive charge is compensated by the [CuCl2]− counter ion, as proven by X-ray structure analysis. Transmetallation of [Li(bimca)] with AuCl3 leads to the [Au(bimca)Cl]+ complex with a tetrachloridoaurate counter ion.

Reduction of Dipyrido Ureas via 6-Alkyloxydipyrido(1,2-c;2 ,1 -e)imidazolium Salts

Dipyrido uronium salts can readily be synthesized by alkylation of dipyrido ureas with Meerwein’s reagent. Compared to the corresponding ureas, the uronium salts are more reactive towards basic or reducing agents like metal hydrides. Reactivity studies show that the uronium salts can react as alkylating agents towards DMSO, DBU and NaOEt along with release of the respective dipyrido ureas. In contrast, reduction of the dipyrido uronium salts with sodium borohydride or sodium trimethoxyborohydride in dry and degassed acetonitrile leads to the imidazolium salts 7a and 7b in moderate yields. Analysis of the by-products reveals an in situ carbene formation which can be reversed by using degassed but wet acetonitrile as solvent. The yield of 7b was increased significantly by these means. Graphical Abstract Reduction of Dipyrido Ureas via 6-Alkyloxydipyrido[1,2-c;2´,1´-e]imidazolium Salts

Structural Features of Primary 6‐Aminopentafulvenes and Some of Their Derivatives

Starting from 6-amino-6-methylpentafulvene (2) “ortho”-C-acylation or N-acylation can be achieved selectively. Treatment of 2 with phthaloyl-protected amino acid chlorides derived from alanine (3a) or valine (3b) leads exclusively to the formation of the “ortho”-C-acylated derivatives 4, isolated as racemates. Optically active N-acylated products 7 were prepared by coupling 5, obtained by NH deprotonation of 2, with the 1-hydroxybenzotriazole (HOBt) active-esters of alanine (6a), valine (6b), or leucine (6c) bearing benzylprotected amino groups. Complexes 2, 4a, 4b, and 7a were characterized by X-ray diffraction. Electron-withdrawing groups attached at the “ortho” ring position lead to a shortening of the external C6−N bond, whereas electron-withdrawing groups bound to the nitrogen atom lead to an elongation of the C6−N bond, compared to the parent system 2.

Consequences of the One‐Electron Reduction and Photoexcitation of Unsymmetric Bis‐imidazolium Salts

Coupling of uronium salts with in situ generated N-heterocyclic carbenes provides straightforward access to symmetrical [4](2+) and unsymmetrical bis-imidazolium salts [6](2+) and [9](2+) . As indicated by cyclic and square-wave voltammetry, [6](2+) and [9](2+) can be (irreversibly) reduced by one electron. The initially formed radicals [6](.+) and [9](.+) undergo further reactions, which were probed by EPR spectroscopy and density functional calculations. The final products of the two-electron reduction are the two carbenes. Upon irradiation with UV light both [6](2+) and [9](2+) emit at room temperature in solution but with dramatically different characteristics. The different fluorescence behavior is analyzed by emission spectroscopy and interpreted by using time-dependent density functional calculations as largely due to different excited-state dynamics of [6](2+) and [9](2+) . The geometries of both radicals [6](.+) and [9](.+) and excited states {[6](2+) }* and {[9](2+) }* are substantially different from those of the parent ground-state molecules.

Optimised synthesis of monoanionic bis(NHC)-pincer ligand precursors and their Li-complexes

Herein we report the optimised synthesis of a versatile bis(imidazole)carbazole framework 4, a precursor to our previously reported C–N–C bis(NHC) pincer ligand, bimca. We have also used 4 as a basis for constructing a library of fully characterised bis(imidazolium) salt pre-ligands which vary the steric and electronic parameters of the subsequently formed NHC moieties. Lithium bis(NHC)carbazolide complexes Li(bimcaR) were generated from their parent bis(imidazolium)carbazole salts, and their behavior in solution and the solid state is discussed.

Synthesis of a lithium–cyclopentadienide complex by addition of LiNTf2 to a zwitterionic fulvalene

The synthesis of a η5-coordinated LiCp complex by simple addition of a Li-salt in benzene is presented. A strongly zwitterionic fulvalene serves as the Cp-precursor. Evidence for the coordination of Li+ was obtained by the characterisitic 7Li NMR chemical shifts, variable temperature experiments in solution and by X-ray structure analysis in the solid state.