Gabriele Kociok-Köhn

Ruthenium-catalyzed meta sulfonation of 2-phenylpyridines

A selective catalytic meta sulfonation of 2-phenylpyridines was found to occur in the presence of (arene)ruthenium(II) complexes upon reaction with sulfonyl chlorides. The 2-pyridyl group facilitates the formation of a stable Ru-C(aryl) σ bond that induces a strong para-directing effect. Electrophilic aromatic substitution proceeds with the sulfonyl chloride to furnish a sulfone at the position meta to the chelating group. This new catalytic process offers access to atypical regioselectivity for reactions involving chelation-assisted cyclometalation.

Magnesium-catalysed hydroboration of aldehydes and ketones

The heteroleptic magnesium alkyl complex [CH{C(Me)NAr}(2)Mg(n)Bu] (Ar = 2,6-(i)Pr(2)C(6)H(3)) is reported as a highly efficient pre-catalyst for the hydroboration of aldehydes and ketones with pinacolborane.

Triazenide complexes of the heavier alkaline earths: Synthesis, characterization, and suitability for hydroamination catalysis

A series of triazenide complexes of the heavier alkaline earths, Ca, Sr and Ba, have been synthesized by either protonolysis or salt metathesis routes. Although complexes of the form [{Ar 2N 3}M{N(SiMe 3) 2}(THF) n ] (M = Ca, n = 2; M = Sr, n = 3; Ar = 2,6-diisopropylphenyl) and [{Ar 2N 3}Ca(I)(THF) 2] 2 could be isolated and characterized by X-ray crystallography, solution studies revealed the propensity of these species to undergo Schlenk-like redistribution with the formation of [{Ar 2N 3} 2M(THF) n ] (M = Ca, n = 1; M = Sr, n = 2). The latter compounds have been synthesized independently. In the case of the large barium dication, attempts to synthesize the heaviest analogue of the series, [{Ar 2N 3} 2Ba(THF) n ], failed and led instead to the isolation of the potassium barate complex [K{Ar 2N 3}Ba{N(SiMe 3) 2} 2(THF) 4]. Single crystal X-ray diffraction studies demonstrated that, although in all the aforementioned cases the triazenide ligand binds to the electrophilic group 2 metal centers via symmetrical kappa (2)- N, N-chelates, in the latter compound an unprecedented bridging mode is observed in which the triazenide ligand coordinates through both terminal and internal nitrogen centers. A series of density-functional theory computational experiments have been undertaken to assist in our understanding of this phenomenon. In further experiments, the calcium and strontium amide derivatives [{Ar 2N 3}M{N(SiMe 3) 2}(THF) n ] (M = Ca, n = 2; M = Sr, n = 3) proved to be catalytically active for the intramolecular hydroamination of 1-amino-2,2-diphenylpent-4-ene to form 2-methyl-4,4-diphenylpyrrolidine, with the calcium species demonstrating a higher turnover number than the strontium analogue ( 2a, TOF = 500 h (-1); 2b, TOF = 75 h (-1)). In these instances, because of ambiguities in the structural charcterization of the precatalyst in solution, such quantification holds little value and detailed catalytic studies have not been conducted.

Synthesis, structure and light-harvesting properties of some new transition-metal dithiocarbamates involving ferrocene.

Nine new transition-metal dithiocarbamates involving ferrocene (Fc), namely, [M(FcCH(2)Bzdtc)(2)] (M=Ni(II) (1), Cu(II) (2), Cd(II) (3), Hg(II) (4), Pd(II) (5), Pt(II) (6) and Pb(II) (7); Bzdtc=N-benzyl dithiocarbamate) and [M(FcCH(2)Bzdtc)(3)] (M=Co(II) (8) and UO(2) (VI) (9)), have been synthesised and characterised by micro analyses, IR spectroscopy, (1)H and (13)C NMR spectroscopy, and in three cases by single-crystal X-ray analysis. The peak broadening in the (1)H spectrum of the copper complex indicates the paramagnetic behaviour of this compound. A square-planar geometry around the nickel and copper complexes and distorted linear geometry around the mercury complex have been found. The latter geometry is attributed to the bulkiness of the methylferrocenyl and benzyl groups. The observed single quasi-reversible cyclic voltammograms for complexes 2, 8 and 9 indicate the stabilisation of a metal centre other than Fe in their characteristic oxidation state. These complexes have been used as a photosensitiser in dye-sensitised solar cells.

Beryllium-Induced CN Bond Activation and Ring Opening of an N-Heterocyclic Carbene†

Berylliant! Interaction of a well-defined adduct of MeBeH and an N-heterocyclic carbene (NHC) with PhSiH 3 results in complete rupture of the heterocycle, and activation of the NHC through effective BeH 2 insertion into a C-N bond of the heterocycle (see scheme; Ar=2,6- diisopropylphenyl, IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene).

Magnesium catalysis of imine hydroboration.

The β-diketiminato magnesium alkyl complex [LMgnBu] (L=CH{CMe(NDipp)}(2), Dipp=diisopropylphenyl) is shown to be a highly effective precatalyst for the hydroboration of alkyl and aryl substituted aldimines and ketimines with pinacol borane (HBpin). Catalysis is proposed to occur through a sequence of Mg-N/B-H metathesis and rate-determining Mg-H/N=C insertion steps, a proposal strongly supported by stoichiometric studies and kinetic analysis. The reactions are observed to proceed through the intermediacy of well-defined magnesium amides, two examples of which have been isolated and structurally characterized. Mechanistic investigations suggest that the catalytic rate-determining process occurs at an isolated magnesium center and requires the presence of two molecules of the imine substrate for effective turnover. This latter observation is rationalized as a requirement for the secondary substrate molecule to displace HBpin from the coordination sphere of the catalytic magnesium center.

Heavier group-2-element catalyzed hydroamination of carbodiimides

Abstract The heteroleptic calcium amide [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe3)2}(THF)] (Ar = 2,6‐diisopropylphenyl) and the homoleptic heavier alkaline earth amides, [M{N(SiMe3)2}2(THF)2] (M = Ca, Sr and Ba) are reported as competent pre‐catalysts for the hydroamination of 1,3‐carbodiimides. Whilst the reaction scope is currently limited to reactions of aromatic amines with 1,3‐dialkylcarbodiimides, in most cases preparations in hydrocarbon solvents proceed rapidly at room temperature with catalyst loadings as low as 0.2 mol‐% and the guanidine reaction products crystallize directly from the reaction mixture. Initial studies are consistent with the intermediacy of heavier group‐2 guanidinate complexes.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Group 3-centred dehydrocoupling of Me2NH·BH3

Stoichiometric and catalytic reactions of group 3 (Sc, Y) amides with Me(2)NH·BH(3) result in formation of the corresponding amidoborane derivatives and dehydrogenative coupling of two amine borane fragments under mild conditions.

Novel Ti(IV) and Zr(IV) complexes and their application in the ring-opening polymerisation of cyclic esters

A series of group 4 amine tris(phenolate) complexes have been prepared and characterised by single crystal X-ray diffraction and multinuclear NMR spectroscopy. It was found that the ligands afforded monomeric titanium complexes and dimeric zirconium structures in the solid-state. The complexes have been tested for the ring-opening polymerisation of rac-lactide under both solution and melt conditions, which showed varying degrees of selectivity and control, with PDIs in the range of 1.09-2.07. The initiators were also tested for the production of copolymers containing rac-lactide and isosorbide. From NMR spectroscopic analysis and MALDI-TOF mass spectrometry the isosorbide is incorporated into the polymer. The complexes were also screened for the ring-opening polymerisation of 1,3-dioxan-2-one to produce a polycarbonate with good conversions (24-99%).

Photooxygenation of a microbial arene oxidation product and regioselective Kornblum-DeLaMare rearrangement: total synthesis of zeylenols and zeylenones.

We report the enantioselective total syntheses of zeylenol (+)-1, as well as its congeners (-)-7 and 16, and of 3-O-debenzoylzeylenone 28. To this end, a new variant of the Kornblum-DeLaMare rearrangement, which utilises neighbouring-group participation to impart regioselectivity, has been developed. The approach employs photooxygenation of building blocks derived from a microbial arene oxidation product.