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Roland Fröhlich

Publications and source records attributed to Roland Fröhlich.

At least 19 recordsLinked to original sources
PubMed2007

Helical complexes containing diamide-bridged benzene-o-dithiolato/catecholato ligands.

The benzene-o-dithiol/catechol ligands H4-2 and H4-3 react with [TiO(acac)2] to give the dinuclear, double-stranded anionic complexes [Ti2(L)2(mu-OCH3)2](2-) ([22](2-), L=2(4-); [23](2-), L=3(4-)). NMR spectroscopic investigations reveal that the complex anion [Ti2(2)2(mu-OCH3)(2)](2-) is formed as a mixture of three of four possible isomers/pairs of enantiomers, whereas only one isomer of the complex anion [Ti2(3)2(mu-OCH3)(2)](2-) is obtained. The crystal structure analysis of (PNP)2[Ti2(3)2(mu-OCH3)2] shows a parallel orientation of the ligand strands, whereas the structure determination for (AsPh4)2[Ti2(2)2(mu-OCH3)2] does not yield conclusive results about the orientation of the ligand strands due the presence of different isomers in solution, the possible co-crystallisation of different isomers and severe disorder in the crystal. NMR spectroscopy shows that ligand H4-3 reacts at elevated temperature with [TiO(acac)2] to give the triple-stranded helicate (PNP)4[Ti2(3)3] ((PNP)4[24]) as a mixture of two isomers, one with a parallel orientation of the ligand strands and one with an antiparallel orientation. Exclusively the triple-stranded helicates [Ti2(L)(3)](4-) ([25](4-), L=1(4-); [26](2-), L=4(4-)) are formed in the reaction of ligands H4-1 and H4-4 with [TiO(acac)2]. The molecular structures of Na(PNP)3[Ti2(1)3]CH(3)OHH(2)OEt(2)O (Na(PNP)3[25]CH(3)OHH(2)OEt(2)O) and Na(1.5)(PNP)(6.5)[Ti2(4)3]2.3 DMF (Na(1.5)(PNP)(6.5)[26]2.3 DMF) reveal a parallel orientation of the ligand strands in both complexes, which is retained in solution. The sodium cations present in the crystal structures lead to two different kinds of aggregation in the solid state. Na-[25]-Na-[25]-Na polymeric chains are formed from compound Na(PNP)3[25], with the sodium cations coordinated by the carbonyl groups of two ligand strands from two different [Ti2(1)3](4-) ions in addition to solvent molecules. In contrast to this, two [Ti2(4)3](4-) ions are connected by a sodium cation that is coordinated by the three meta oxygen atoms of the catecholato groups of each complex tetraanion to form a central {NaO6} octahedron in the anionic pentanuclear complex {[26]-Na-[26]}(7-).

Journal Article↗
PubMed2007

Mechanistic study on rearrangement cascades starting from annulated 2-aza-hepta-2,4-dienyl-6-ynyl anions: formation of aniline and azepine derivatives.

Deprotonation of benzothiophene-derived alkynyl imine 11 with lithium diisopropylamide (LDA) and subsequent transmetalation with ZnCl2 etherate furnished azepine 12 upon aqueous workup. Similarly, alkynyl benzaldimine 1a gave a mixture of benzazepine 13 and naphthylamine 14. Allylic benzonitriles 15 a,b reacted to produce naphthylamine 16 upon deprotonation with LDA at room temperature. In an analogous manner, imino benzonitrile 17 may be converted into 4-amino isoquinoline 18 by means of an intramolecular nucleophilic attack on the nitrile function upon treatment with LDA. The allylic benzonitriles 19 a,b were prepared by LDA treatment of alkynyl imine 11. They were further converted to amino dibenzothiophene 20 by LDA deprotonation and aqueous workup. These various transformations represent the key steps of a multistep reaction cascade, which was previously postulated on the basis of quantum chemical calculations. Thus, all features of this complex rearrangement mechanism could now be confirmed experimentally. DFT calculations support the lower reactivity of zinc species in the ring-opening step compared to the lithium intermediates. All new compounds were completely characterized by spectroscopic data, including X-ray diffraction studies for the key compounds 12, 19 a, and 20.

Journal Article↗
PubMed2006

Ansa-metallocene polymerization catalysts derived from [2+2]cycloaddition reactions of bis(1-methylethenyl-cyclopentadienyl)zirconium systems.

Bis(1-methylethenyl-cyclopentadienyl)zirconium dichloride (7a) was prepared by a fulvene route. Photolysis at 0 degrees C with Pyrex-filtered UV light resulted in a rapid and complete intramolecular [2+2]cycloaddition reaction to yield the corresponding cyclobutylene-bridged ansa-zirconocene dichloride isomer (8a). This is one of the rare examples of an organic functional group chemistry that leads to carbon-carbon coupling at the framework of an intact sensitive group 4 bent metallocene complex. More sterically hindered open metallocenes that bear bulky isopropyl or tert-butyl substituents at their Cp rings in addition to the active 1-methylethenyl functional group undergo the photochemical ansa-metallocene ring closure reaction equally facile. The metallocene systems used and obtained in this study have served as transition metal components for the generation of active metallocene propene polymerization catalysts.

Journal Article↗
PubMed2006

Novel template-directed anodic phenol-coupling reaction.

Substituted phenols were anodically coupled to the corresponding 2,2'-biphenols via tetraphenoxy borate derivatives. This electrochemical method is particularly useful for methyl-substituted substrates, such as 2,4-dimethyl phenol. The selective ortho-coupling reaction can be easily performed on a multikilogram scale.

Borates↗
PubMed2006

Synthesis and fluorescent properties of a new class of heterocycles of isoindole fused imidazoles with phenolic subunits.

A new class of heterocycles of isoindole fused imidazoles with phenolic subunits has been readily synthesized by a two-step one-pot reaction. In aprotic solvent they show high fluorescent properties (Phi(F) up to 0.93), but in protic polar solvent fluorescent intensity decreases. They show green fluorescence in weak acidic medium such as acetic acid but lack emission in basic medium. The compounds can also stain human squamous epithelium cells.

Crystallography, X-Ray↗
PubMed2006

First mixed hydrazide/hydroxylamide metal aggregates.

The first organometallic clusters of mixed hydrazide/hydroxylamide clusters of zinc, [Zn(MeZn)(4)(HNNMe(2))(2)(ONEt(2))(4)] and {Zn(EtZn)(4)[HNN(CH(2))(5)](2)(ONEt(2))(4)} were synthesized in one-pot synthesis protocols from dialkylzinc solutions, substituted hydrazines and N,N-diethylhydroxylamine; competing for the Zn atoms, the different binding properties of hydrazide and hydroxylamide ligands in these heteroleptic clusters are discussed.

Journal Article↗
PubMed2006

Dicatechol cis-dioxomolybdenum(VI): a building block for a lithium cation templated monomer-dimer equilibrium.

Dinuclear helicate-type complexes form from 3-carbonyl catechol derivatives and MoO2 units. The two monomeric subunits are non-covalently bound through templating Li+ ions. The monomeric and dimeric complexes as well as a hydrolysis product have been investigated in the solid state by X-ray crystallography, in solution by NMR experiments, and in the gas phase by ESI mass spectrometry. Dimerization constants can easily be extracted from NMR experiments. A particular property of these complexes is the mutual homochiral recognition between the two halves of the dimers which only occurs between homochiral monomers.

Journal Article↗
PubMed2006

Asymmetric gamma-deprotonation and homoaldol reaction of 1,3-dien-2-yl carbamates: stereo- and regiochemistry.

[reaction: see text] Lithium compounds 7 are configurationally stable intermediates obtained by deprotonation of 1,3-dien-2-yl carbamates 6 with n-butyllithium/(-)-sparteine with a high degree of enantiotopic differentiation at the gamma-position. They react with electrophiles regioselectively giving highly enantioenriched products. Starting with different isomers or changing the double-bond geometries in 6 leads to either of the enantiomers.

Journal Article↗
PubMed2006

3,3'-oxazolidinyl-substituted 2,2'-biphenyldiols: novel tropos ligands with a large induction on the chiral axis.

A novel type of bisphenol ligand is presented which provides high modularity. Chiral information is introduced in the 3,3'-positions as N-arenesulfonyl-1,3-oxazolidines inducing an unexpectedly high diastereoselectivity of up to 98:2 onto the chiral axis. The impact of each part of the molecule on the diastereomeric ratio is evaluated with extended variable-temperature (VT) NMR studies. A first example of application in asymmetric catalysis is given. [structure: see text]

Journal Article↗
PubMed2006

Carbohydrate-metallocene conjugates: selective formation of a zirconadioxacyclopentane-type dimer from the reaction of a bis(enolate)ZrCp2 reagent with a glucofuranoside derivative.

The zirconocene enolate complex bis(2-propenolato)ZrCp2 (1) reacts with two molar equivalents of the 1,2,3,4-O-tetramethyl-alpha-D-glucopyranoside (2) with liberation of two equivalents of acetone to yield cleanly the bis(carbohydrate)zirconcene complex (3). Alternatively 1 and the "bifunctional" glucose derivative 3-O-benzyl-1,2-O-isopropylidene-glucofuranoside (4) react to the corresponding zirconadioxacyclopentane-type metallacyclic product that was isolated as the respective dimer (5) featuring a sequence of linearly anellated five-, four-, five-membered metallacycles. Both carbohydrate zirconocene complexes 3 and 5 were characterized by NMR experiments as well as by X-ray diffraction.

Carbohydrate Conformation↗
PubMed2006

N,N'-carbonyldiimidazole-mediated cyclization of amino alcohols to substituted azetidines and other N-heterocycles.

Amino alcohols are important synthons for N-heterocycles. We have developed an efficient method to activate hydroxyl groups, which avoids the use of toxic reagents and tolerates a wide variety of functional groups. Our strategy has been applied to the synthesis of functionalized p-methoxyphenyl-protected azetidines, pyrrolidines, and piperidines. The required amino alcohols were synthesized according to an optimized proline-catalyzed Mannich protocol. An azetidine analogue of ezetimibe was synthesized to demonstrate the potential for the synthesis of drug-like molecules.

Amino Alcohols↗