Jörg-M. Neudörfl

Characterization of the Key Intermediates of Carbene‐Catalyzed Umpolung by NMR Spectroscopy and X‐Ray Diffraction: Breslow Intermediates, Homoenolates, and Azolium Enolates

Caught in the act: Diamino enols, diamino dienols, azolium enolates, and azolium enols are postulated intermediates of the N-heterocyclic carbene catalyzed umpolung of aldehydes and enals. Several of these elusive reaction intermediates were generated with the saturated imidazolidin-2-ylidene SIPr (R=2,6-bis(2-propyl)phenyl) and characterized by NMR spectroscopy and X-ray crystallography.

Titanium salalen catalysts based on cis-1,2-diaminocyclohexane: enantioselective epoxidation of terminal non-conjugated olefins with H2O2.

Titanium Salalen Catalysts Based on cis1,2-Diaminocyclohexane: Enantioselective Epoxidation of Terminal Non-Conjugated Olefins with H2O2 Help for the neglected : Terminal, nonconjugated olefins, such as 1-octene, are difficult to epoxidize asymmetrically. Ti salalen complexes based on cis-1,2-diaminocyclohexane catalyze this demanding reaction giving high yields and enantioselectivities (up to 95 % ee), with H2O2 as the oxidant. The X-ray structures of the m-oxo and peroxo complexes shed light on the coordination behavior of this novel class of ligands. Angewandte Chemie

Anion‐Binding Catalysis by Electron‐Deficient Pyridinium Cations

A new activation principle in organocatalysis is presented: halide binding through Coulombic interactions. This mode of catalysis was realized by using 3,5-di(carbomethoxy)pyridinium ions that carry an additional electron-withdrawing substituent on the nitrogen atom, for example, pentafluorobenzyl or cyanomethyl. For the N-pentafluorobenzyl derivative, Coulombic interaction with the pyridinium moiety is complemented in the solid state by anion-π interactions with the perfluorophenyl ring. Bromide and chloride are bound by these cations in a 1:1 stoichiometry. Catalysis of the C-C coupling between 1-chloroisochroman (and related electrophiles) with silyl ketene acetals occurs at -78 °C and at low catalyst loading (2 mol%).

Enantioselective self-sorting on planar, π-acidic surfaces of chiral anion-π transporters

Self-sorting at interfaces is one of the big challenges we face to prepare the functional organic materials of the future. As a first and decisive step to self-sorting into π-stacks or bundles, we here elaborate self-sorting of π-dimers in solution. Design, synthesis and study of planar naphthalenediimides (NDIs) with one shielded and one free chiral π-surface to direct self-assembly into dimers are described. Stereoisomers are isolated by chiral, preparative HPLC and characterized by X-ray crystallography. NMR studies show that racemates with almost planar, nearly identical π-surfaces prefer uniform self-sorting into homodimers at large differences in π-acidity and alternate self-sorting into heterodimers at small differences in π-acidity. In contrast, enantiomers self-sort “narcissistically” into heterodimers and diastereomers show moderate preference for homodimers. Whereas the lessons learned from dimerization are directly applicable to self-sorting of π-stacks on surfaces, anion transport in lipid bilayers is shown to require a more subtle, somewhat inverse interpretation, with diastereomeric transporters differing dramatically in activity but the least visible supramolecule being confirmed as the best performer.

Organocatalytic Glycosylation by Using Electron‐Deficient Pyridinium Salts

A new organocatalytic glycosylation method based on electron-deficient pyridinium salts is reported. At ambient temperature and catalyst loadings as low as 1 mol %, 2-deoxyglycosides were formed from benzyl- and silyl-protected glycals and primary or secondary glycosyl acceptors, with excellent yields and anomeric selectivity. Mechanistic investigations point to alcohol-pyridinium conjugates (1,2-addition products) as key intermediates in the catalytic cycle.

On the quantitative recycling of Raney–Nickel catalysts on a lab-scale

An efficient protocol for the selective hydrogenation of heteroarenes and olefins has been developed exhibiting quantitative and hazard-free recycling of the Raney–Nickel catalyst.

The Chiral Pool as Valuable Natural Source: New Chiral Mesogens Made From Lactic Acid

Twenty new enantiopure chiral materials have been synthesized, showing liquid crystalline phases of smectic (SmA*), twisted grain boundary (TGB) and cholesteric (N*) type. The central ester-linkage as well as the length of the attached alkyl-chains were varied in a systematic way and the effects of these structural changes on the liquid crystalline properties have been studied. The liquid crystalline phases were characterized by means of differential scanning calorimetry, polarisation microscopy as well as small angle X-ray diffraction (SAXS).

Titanium cis-1,2-diaminocyclohexane (cis-DACH) salalen catalysts for the asymmetric epoxidation of terminal non-conjugated olefins with hydrogen peroxide.

Chiral Ti salalen complexes catalyze the asymmetric epoxidation of terminal non-conjugated olefins with hydrogen peroxide. Modular ligands based on cis-1,2-diamino-cyclohexane (cis-DACH) were developed, giving high yields and enantiomeric excesses (ee, up to 96 %) at catalyst loadings as low as 0.1-0.5 mol %, and even under solvent-free conditions.

Planar and distorted indigo as the core motif in novel chromophoric liquid crystals

Symmetrically bis-substituted indigo derivatives with long peripheral alkyl chains were synthesised by the reductive condensation of corresponding isatin derivatives. Their thermotropic mesomorphism was investigated with respect to different substitution patterns, which include the position and lateral modifications of the substituents. A systematic investigation of structure–property relationships revealed that only substitution at the 6 and 6′ positions affords the calamitic shape necessary to form smectic or nematic liquid crystalline phases. This finding is rationalised on the basis of a structural analysis of N,N′-diacetyl indigo and the consequences for 5,5′- and 6,6′-bis-substituted derivatives are discussed. Some of the liquid crystalline substances exhibit dichroism, which is especially pronounced in highly ordered phases. In addition, the 6,6′ substitution leads to significantly enhanced activity with respect to the photochemical trans–cis isomerization of N,N′-diacetylated indigo derivatives.

Hydrogen-bonding cyclodiphosphazanes: superior effects of 3,5-(CF3)2-substitution in anion-recognition and counter-ion catalysis

New HB-cyclodiphosph(V)azanes with a variety of structural modifications, e.g. unsymmetrical substitution of phosphorus atoms with sulfur and oxygen atoms as well as either phenyl- (O(P)/S(P)-13) or 3,5-(CF3)2-C6H3-substitution (O(P)/S(P)-14) and 3,5-F2C6H3-substituted cyclodiphosph(V)azanes with either oxygen (O(P)-15) or sulfur (S(P)-16) substitution at the phosphorus atoms, are synthesized. These new systems are employed together with sulfur substituted cyclodiphosph(V)azanes with phenyl- (11) and 3,5-(CF3)2-C6H3-substitution (12) in recognitions of chloride and acetate anions. These HB-systems are compared to the previously established reference systems, i.e. cyclodiphosph(V)azanes (4, 5), thiourea (20) and squaramides (21, 22). Modifications of the chalcogen atom in the cyclodiphosph(V)azane moieties from oxygen (O(P)-5) to sulfur (O(P)/S(P)-14, S(P)-12) reveal a decrease in anion binding capabilities. 3,5-(CF3)2-C6H3 substituted O(P)-cyclodiphosph(V)azane 5 exhibits the strongest anion binding effect (chloride: log[K] 5.91, log[K] acetate: 6.06) in acetonitrile, surpassing even the established thiourea 20 (chloride: log[K] 4.30, log[K] acetate: 5.47) as well as squaramides 21 (chloride: log[K] 4.92, acetate: log[K] 4.24) and 22 (chloride: log[K] 5.13, acetate: log[K] 5.37). Computational studies confirm 3,5-(CF3)2-C6H3 substituted 5 to be the strongest here studied anion-binding cyclodiphosph(V)azane with computed binding energies ΔGin–out·Cl of −21.1 kcal mol−1 and ΔGin–out·OAc of −14.3 kcal mol−1, surpassing thiourea 20 (ΔGin–out·Cl = −19.10 kcal mol−1, ΔGin–out·OAc = −13.81 kcal mol−1). The catalytic efficiency of 3,5-(CF3)2-C6H3 substituted cyclodiphosph(V)azane 5 is examined in a N-acyl-Mannich reaction, showing a significantly higher reactivity (up to 45% yield) compared to the alternative hydrogen-bonding catalyst di(1-naphthyl)silanediol 28. In all these applications, the superiority of the 3,5-(CF3)2-C6H3 substitution pattern in combination with O(P)-groups in the cyclodiphosph(V)azane scaffold is apparent.