Johann Lex

Figure Eight Cyclooctapyrroles: Enantiomeric Separation and Determination of the Absolute Configuration of a Binuclear Metal Complex.

The hypothesized racemate separation of cyclooctapyrroles with chiral figure eight conformations-and of metal complexes derived from these ligands-has been realized. The cyclooctapyrrole 1 (as hexadecaethyl derivative), which according to NMR analysis exhibits restricted mobility, and its binuclear palladium and copper complexes could be separated into stable enantiomers by preparative chromatography on a chiral phase. In the case of the palladium complex and the free ligand 1 it was also possible to determine the absolute configuration of the enantiomers.

Diastereo- and Enantioselective Synthesis of Pyrrolo[1,4]benzodiazepines through Decarboxylative Photocyclization

Memory of chirality in triplet 1,7-diradicals was detected for the first time and to a remarkably high degree in the decarboxylative photocyclization of the proline derivative of N-phthaloyl anthranilic acid 1 into 2 (X=H: 45 % yield, 86 % ee; X=Cl: 50 % yield, 79 % ee).

Electrochemical and Spectral Characterization of Iron Corroles in High and Low Oxidation States: First Structural Characterization of an Iron(IV) Tetrapyrrole π Cation Radical

The electrochemistry and spectroscopic properties of three iron corroles were examined in benzonitrile, dichloromethane, and pyridine containing 0.1 M tetra-n-butylammonium perchlorate or tetra-n-ethylammonium hexafluorophosphate as supporting electrolyte. The investigated compounds are represented as (OEC)FeIV(C6H5), (OEC)FeIVCl, and (OEC)FeIII(py), where OEC is the trianion of 2,3,7,8,12,13,17,18-octaethylcorrole. Each iron(IV) corrole undergoes two one-electron reductions and two or three one-electron oxidations depending upon the solvent. Under the same solution conditions, the iron(III) corrole undergoes a single one-electron reduction and one or two one-electron oxidations. Each singly oxidized and singly reduced product was characterized by UV-vis and/or EPR spectroscopy. The data indicate a conversion of (OEC)FeIV(C6H5) and (OEC)FeIVCl to their iron(III) forms upon a one-electron reduction and to iron(IV) corrole π cation radicals upon a one-electron oxidation. The metal center in [(OEC)FeIII(C6H5)]- is low spin (S = 1/2) as compared to electrogenerated [(OEC)FeIIICl]-, which contains an intermediate-spin (S = 3/2) iron(III). (OEC)FeIII(py) also contains an intermediate-spin-state iron(III) and, unlike previously characterized (OEC)FeIII(NO), is converted to an iron(IV) corrole upon oxidation rather than to an iron(III) π cation radical. Singly oxidized [(OEC)FeIV(C6H5)]•+ is the first iron(IV) tetrapyrrole π cation radical to be isolated and was structurally characterized as a perchlorate salt. It crystallizes in the triclinic space group P1̄ with a = 10.783(3) Å, b = 13.826(3) Å, c = 14.151(3) Å, α = 78.95(2)°, β = 89.59(2)°, and γ = 72.98(2)° at 293 K with Z = 2. Refinement of 8400 reflections and 670 parameters against F o2 yields R1 = 0.0864 and wR2 = 0.2293. The complex contains a five-coordinated iron with average Fe-N bond lengths of 1.871(3) Å. The formulation of the electron distribution in this compound was confirmed by Mössbauer, X-ray crystallographic, and magnetic susceptibility data as well as by EPR spectroscopy, which gives evidence for strong antiferromagnetic coupling between the iron(IV) center and the singly oxidized corrole macrocycle.

Molecular and Electronic Structure of (2,2′‐Bidipyrrinato)nickel(II) Complexes

Nickel complexes of eight differently substituted 2,2′-bidipyrrins have been prepared and fully characterized. The X-ray analyses of three of these complexes revealed helical chiral molecules. Despite the tetrahedral deviation from the square-planar coordination geometry at the metal centres, all compounds were found to be diamagnetic in nature. For (3,3′,4,4′,8,8′,9,9′-octaethyl-10,10′-dimethyl-6,6′-diphenyl2,2′-bidipyrrinato)nickel, a separation into the enantiomers by chiral MPLC could be achieved, and the first CD spectra of enantiomerically pure tetrapyrrole helicates are reported. An electrochemical study of the new complexes allowed a first insight into the electronic structure of (2,2′-bidipyrrinato)nickel(II), disclosing a rather high energy HOMO and metal−ligand interaction similar to that observed in metalloporphyrins.

Synthesis of Sulfur-Containing Tricyclic Ring Systems by Means of Photoinduced Decarboxylative Cyclizations

The intramolecular and intermolecular photoinduced electron transfer reactions of a series of mercaptoacetic acid and mercaptopropionic acid derivatives were investigated. In the intermolecular series, the phthalimidoalkylsulfanylalkylcarboxylates 1a-j and 2 were transformed into the tricyclic ring systems 3a-j and 4, respectively, with high regioselectivities. The mercaptoacetic acid and 2-mercaptopropionic acid derived substrates 1a-g and 2 readily cyclized in good to excellent yields (60-98%) but with low diastereoselectivities (except for 1d), whereas the corresponding 3-mercaptopropionic acid derived substrates 1h-j gave the corresponding tricyclic products 3h-j after prolonged irradiation, but with poor yields (11-20%). The intermolecular version - i.e., photodecarboxylative addition to N-methylphthalimide (5) as electron acceptor - was successful with mercaptoacetic acid, and 2-mercaptopropionic acid substrates 6a-c and the addition products 7a-c were obtained in high yields (57-90%). No ad-dition, however, was observed with 3-(methylsulfanyl)propionic acid (6d). The regioselectivity of decarboxylation proceeded in a controlled manner for the mercaptosuccinic acid derivatives in both the intramolecular (with 8a-c) and the intermolecular (with 9) versions. Comparison between sulfuractivated and nonactivated species (13, 15) or irradiation of 1a under nonactivating conditions showed that the carboxylate anion in the position alpha to the electron-donating sulfur atom acts as a superior leaving group. This efficiency is drastically reduced for carboxylate anions in the alpha position. With the former substrates, the photochemical cyclization proceeds with high product yields. Quantum yield measurements for decomposition (as a measure for cyclization) supported these observations. CV measurements indicated preorientation prior to electron transfer in the intramolecular pathway.

The photodecarboxylative addition of carboxylates to phthalimides: scope and limitations

Intermolecular photoinduced decarboxylative additions of a series of alkylcarboxylates to N-substituted phthalimides gave the corresponding hydroxy-phthalimidines in moderate to high yields of 39-89%. The potassium salt of 1- adamantanecarboxylic acid predominately underwent simple decarboxylation when irradiated in the presence of N-methylphthalimide. In case of phthalimides carrying suitable leaving groups within the N-side chain, decarboxylation, retro-Aldol cleavage or decarbonylation preceded the intermolecular addition step.

ISOPORPHYCENE : THE FOURTH CONSTITUTIONAL ISOMER OF PORPHYRIN WITH AN N4 CORE : OCCURRENCE OF E/Z ISOMERISM

Liberation of the ligand from the nickel complex 1 obtained by template synthesis yielded isoporphycene (as the octaethyl derivative 2), the first constitutional isomer of porphyrin with an N(4) core for which E/Z isomerism is involved: Compound 2 is present as the E isomer, which is in rapid, presumably acid-catalyzed equilibrium with a small amount (2 %) of the Z isomer. The remaining unknown constitutional isomers of porphyrin are considerably higher in energy than the already rather labile isoporphycene, so that the latter should mark the border of existence for this type of structural variant of porphyrin.

Stereoselective generation of vicinal stereogenic quaternary centers by photocycloaddition of 5-methoxy oxazoles to α-keto esters: synthesis of erythroβ-hydroxy dimethyl aspartates

The photocycloaddition of methyl pyruvate and methyl phenylglyoxylate, respectively, to 5-methoxy oxazoles bearing additional substituents at C-2 and C-4 leads to bicyclic oxetanes 2 and 3 with high to moderate (exo) diastereoselectivity that can be easily ring-opened to give bis-quaternary aspartic acid diester derivatives 4 and 5.

Enantiomerically Pure β-Amino Acids: A Convenient Access to Both Enantiomers of trans-2-Aminocyclohexanecarboxylic Acid

Enantiomerically pure trans-2-aminocyclohexanecarboxylic acid is an important building block for helical β-peptides. We report here that this amino acid can be obtained from trans-cyclohexane-1,2-dicarboxylic acid in good yield by a simple one-pot procedure comprising cyclization to the anhydride, amide formation with ammonia, and a subsequent Hofmann-type degradation with phenyliodine(III) bis(trifluoroacetate) (PIFA) as the oxidant. The N-Fmoc- and N-BOC-protected derivatives were obtained by treatment of the amino acid with Fmoc-OSu and BOC2O, respectively. The N-BOC derivative could be prepared in even better overall yield by a one-pot procedure leading directly from trans-cyclohexane-1,2-dicarboxylic acid to the N-BOC-protected amino acid. Both enantiomers of the starting trans-1,2-cyclohexanedicarboxylic acid can be obtained easily and in large quantities by separating commercially available racemic trans-1,2-cyclohexanedicarboxylic acid using either (R)- or (S)-1-phenethylamine. X-ray crystallography of the diastereomerically pure salt obtained from (R)-1-phenethylamine revealed that the configuration of the diacid component is (1R,2R), and not (1S,2S) as reported in the literature. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Manganese(III) and manganese(IV) corroles: synthesis, spectroscopic, electrochemical and X-ray structural characterization

The synthesis, spectroscopic characterization and electrochemistry of four Mn(III) and Mn(IV) octaethylcorroles are reported and the potentials of the Mn(III)/Mn(IV) and Mn(IV)/Mn(III) processes examined as a function of the axial ligand. The investigated compounds are represented as (OEC)Mn, (OEC)MnCl, (OEC)Mn(py) and (OEC)Mn(C6H5) where OEC is the trianion of octaethylcorrole. The first one-electron oxidation of (OEC)MnIII and (OEC)MnIII(py) in PhCN or pyridine containing 0.1 M TBAP leads to the facile formation of a Mn(IV) species while the first one-electron reduction of (OEC)MnIVCl and (OEC)MnIV(C6H5) in the same two solvents leads to the Mn(III) corrole. All other redox reactions occur at the corrole macrocycle to give π-cation radicals or π-anion radicals and there is no evidence for electrogeneration of a compound with a Mn(II) oxidation state as is the case for manganese(III) porphyrins which are all easily reduced to the Mn(II) state in nonaqueous media. The products of each Mn(III)/Mn(IV) redox reaction were characterized by UV-visible and/or ESR spectroscopy and the structures of (OEC)MnCl, (OEC)Mn(py) and (OEC)Mn(C6H5) were determined by single-crystal X-ray diffraction.