Showing papers by "Eberhardt Herdtweck published in 2009"

Total Synthesis of Meloscine by a [2+2]‐Photocycloaddition/Ring‐Expansion Route

Abstract: The stereogenic centers at C3 and C12 of meloscine (3) can be established in the photochemical key step 1 → 2. 1,2-retro-Benzilic acid rearrangement to a five-membered ring, reductive amination, Claisen rearrangement, and ring-closing metathesis are further key steps in the transformation of cyclobutane 2 into the target molecule 3 (14 steps, 9 % overall yield). Enantioselective access to (+)-meloscine was possible when the [2+2]-photocycloaddition was conducted in the presence of a chiral template. The unusual monoterpenoid indole alkaloid meloscine was synthesized starting from a protected aminoethylquinolone in 15 steps and an overall yield of 9 %, employing a [2+2]-photocycloaddition as the stereochemistry defining key step. After the initial plan of a Wagner–Meerwein type rearrangement of a [4.2.0]- into a [3.3.0]-bicyclic substructure could not be realized, the required ring enlargement of a cyclobutane was eventually achieved by a retro-benzilic acid rearrangement. Generation of the central pyrrolidine ring was possible by a three-step reductive amination domino sequence. The final ring was built up by a ring-closing metathesis after the last quaternary stereocenter had been constructed by a Johnson–Claisen rearrangement. The synthesis was concluded by a selenylation–elimination sequence to build up the exocyclic vinyl group of meloscine. Using our methodology for enantioselective [2+2]-photocycloaddition mediated by a chiral complexation agent, the experimentally very simple synthesis could be performed in an enantioselective fashion (7 % overall yield). The enantioselective synthesis of (+)-meloscine represents the first example of a natural product synthesis employing an enantioselective [2+2]-photocycloaddition as its key step, and illustrates nicely the synthetic potential of photochemical transformations for the construction of complex heterocyclic structures.

N-heterocyclic carbenes of iridium(I): ligand effects on the catalytic activity in transfer hydrogenation

Abstract: New Ir-NHC complexes based on different heterocyclic moieties like imidazole, benzimidazole and imidazolidine are presented and tested in transfer hydrogenation catalysis. A broad range of steric and electronic properties of NHC ligands is covered to give an idea for catalyst design from the experimental point of view.

Bidentate Lewis Base Adducts of Methyltrioxidorhenium(VII): Ligand Influence on Catalytic Performance and Stability

Abstract: Methyltrioxidorhenium (MTO) forms 1:1 adducts of the general formula CH(3)ReO(3)center dot L(2) with bidentate Lewis bases (L(2) = 5,5'-dimethyl-2,2'-bipyridine, 5,5'-diamino-2,2'-bipyridine, 4,4'-dibromo-2,2'-bipyridine, 5,5'-dibromo-2,2'-bipyridine, diethyl 2,2'-bipyridine-5,5'-dicarboxylate, 1,10-phenanthroline-5,6-dione, 3,6-di(2-pyridyl)pyridazine). Due to the steric demands of the ligands, the complexes display a distorted octahedral geometry as confirmed by solid state X-ray crystallography. The rhenium center is disordered in all examined crystal structures. The complexes synthesized are thermally stable but sensitive to light and moisture. The 2,2'-bipyridine derived complexes exhibit good catalytic activities for cyclooctene epoxidation in a biphasic H(2)O(2)/organic solvent catalytic system using hydrogen peroxide as oxidizing agent. The functional groups on the bipyridine rings play an important role with respect to the differences in formation, stability and activity of the complexes. Their influence depends largely on their electron donor capabilities. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Palladium N(CH2CH2PiPr2)2-Dialkylamides: Synthesis, Structural Characterization, and Reactivity

Abstract: Palladium(II) aminodiphosphine PNP pincer complexes [PdR(PNPH)]PF6 (1R; R = Cl Me, Ph; PNPH = HN(CH2CH2PiPr2)2) were prepared. Deprotonation with KOtBu affords dialkylamides [PdR(PNP)] (2R; R = Cl Me, Ph; PNP = (NCH2CH2PiPr2)2) in high yield which are stable toward β-H elimination. While AgPF6 oxidizes the amides, cationic amido complexes [PdL(PNP)]PF6 (3L; L = CNtBu, PMe3) were obtained upon chloride abstraction from 1Cl with TlPF6. The reaction of amide 2Cl with MeOTf results in N-methylation yielding [PdCl(PNPMe)]OTf (5) quantitatively. N−H acidities of the amino complexes 1Me (pKa = 24.2(1)) and 1Ph (pKa = 23.2(1)) were determined in dmso. Complexes 1Cl, 1Me, 2Cl 2Me, 3CNtBu, and 5 were structurally characterized by single crystal X-ray diffraction. The amido complexes feature pyramidal nitrogen atoms in the solid state. The molecular structures, high N-basicity, and reactivity of the amido complexes can be explained with Pd−Namido bonding that is characterized by strong N→Pd σ-donation and repulsive ...

N-Salicylidene)aniline Derived Schiff Base Complexes of Methyltrioxorhenium(VII): Ligand Influence and Catalytic Performance

Abstract: It′s a fine line: Methyltrioxorhenium(VII) (MTO) readily forms 1:1 adducts with several N-(salicylidene)aniline derived Schiff bases. Steric and electronic effects of the ligands play a major role when it comes to stability and catalytic activity. Good donors on the Schiff base ligands, in general, lead to shorter ReO(Schiff base) bridges and lower catalytic activity, while it is the opposite for acceptor ligands on the Schiff bases. Methyltrioxorhenium(VII) (MTO) readily forms 1:1 adducts with several N-(salicylidene)aniline derived Schiff bases. If the aromatic rings of the N-(salicylidene)aniline ligands display non-donating or electron withdrawing substituent groups, the resulting MTO adducts show good activities in olefin epoxidations. However, steric effects seem to play a major role, leading often to instable o- and m-Schiff base–MTO adducts, while p-substituted Schiff bases usually lead to more stable adducts. In catalysis, electron-withdrawing substituents on the aniline moiety lead to better catalysts than electron donating ones. The gap between good catalysts and instable or non-existing compounds, however, is small. The general tendency, however, that good donors on the Schiff base ligands lead to shorter ReO(Schiff base) bridges and lower catalytic activity, while the opposite is true with acceptor ligands on the Schiff bases, seems to be quite clear.

Synthesis and Characterization of Two New fac‐Tricarbonylrhenium(I) Biscarbene Complexes

Abstract: fac-Bromidotricarbonylbis(1,3-diisopropylimidazoline-2-ylidene)rhenium(I) (4a) and fac-bromidotricarbonylbis(1,3-dicyclohexylimidazoline-2-ylidene)rhenium(I) (4b) with two coordinated 1,3-dialkylsubstituted imidazoline-2-ylidenes were obtained by deprotonation of the corresponding imidazolium chloride salts. In this reaction two coordinated bromides of the metal precursor [NEt4]2[ReBr3(CO)3] were substituted by 1,3-di-R-imidazoline-2-ylidene (R = iPr, Cy) ligands to generate two stable rhenium(I) biscarbene complexes that have similar structures, as proven by X-ray diffraction and DFT calculations.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Studies on bis(halogeno) dioxomolybdenum(VI)-bipyridine complexes: synthesis and catalytic activity.

Abstract: Dioxomolybdenum(VI) complexes with the general formula [MoO2Cl2L2] (L2=3,3′-dimethyl-2,2′-bipyridine, 5,5′-dimethyl-2,2′-bipyridine, 6,6′-dimethyl-2,2′-bipyridine, 4,4′-dibromo-2,2′-bipyridine, 5,5′-dibromo-2,2′-bipyridine, 5,5′-diamino-2,2′-bipyridine; 5,5′-dinitro-2,2′-bipyridine; 5,5′-di-ethoxycarbonyl-2,2′-bipyridine; 6-phenyl-2,2′-bipyridine; 2,2′:6′,2″-terpyridine) have been prepared and characterised. [MoO2Cl2(5,5′-di-ethoxycarbonyl-2,2′-bipyridine)] has been examined by single crystal X-ray analysis. The complexes were applied as homogenous catalysts for the epoxidation of cyclooctene with tert-butyl hydroperoxide (TBHP) as oxidising agent. The new compounds show an overall high activity and are highly selective catalysts in the epoxidation of cyclooctene. The stability of the complexes and differences in the catalytic activity can be clearly attributed to electronic contributions of the functional groups on bipyridine ligands and to steric restrictions. DFT calculations have assisted in a better understanding of the stability of the complexes and are in agreement with experiment. The influence of the terminal oxo ligands and the Lewis base ligands on the Mo center keep the compounds on quite a stable level of electron density.

Multifrequency ESR in ET2 MnCu[N(CN)(2)](4): A radical cation salt with quasi-two-dimensional magnetic layers in a three-dimensional polymeric structure

Abstract: The radical cation salt, ET2MnCu[N(CN)(2)](4), [ET=bis(ethylenedithio)tetrathiafulvalene] with an unusual three-dimensional anionic polymeric network is studied by x-ray diffraction, static susceptibility measurements, and electron spin resonance (ESR) at frequencies between 9 and 420 GHz. The magnetic properties are determined by the alternating two-dimensional layers of the Mn2+ ions of the network and the partially charged ET molecules. At ambient temperature the overlap between Mn2+ ions and ET molecules is weak and an exchange integral vertical bar J(Mn-ET)vertical bar approximate to 4.10(-2) K is estimated from their resolved ESR lines. At lower temperatures, ET2MnCu[N(CN)(2)](4) is not a simple system of weakly interacting paramagnetic ions in spite of the isotropic, Curie-like static susceptibility. There are first-order phase transitions at 292 K and in the range of 120-180 K. One of the lattice constants shows anomalous temperature dependence below 292 K. Anisotropic ESR shifts appear below 150 K, which we explain by demagnetizing fields of the platelike crystals and an exchange-narrowed fine structure. The latter contributes significantly to the shift when the populations of Zeeman levels are altered in high magnetic fields at low temperatures. We estimated the exchange coupling between Mn2+ ions within a layer, J(Mn-Mn)approximate to-48 K and determined the fine structure parameters below 150 K, showing a distortion in the plane of the Mn2+ ions.

A novel Δ-thiolactone scaffold by a versatile intramolecular multicomponent reaction

Abstract: The three component reaction of α-aminoacids, mercaptoacetaldehyde and an isocyanide smoothly and stereoselectively yields the novel scaffold 1,2-disubstituted N-alkyl(aryl)-6-oxo thiomorpholine-3-carboxamide. In this communication we present our preliminary results on six compounds derived from this unprecedented reaction.

Total Synthesis of Meloscine by a [2 + 2]-Photocycloaddition/Ring-Expansion Route.

Abstract: The stereogenic centers at C3 and C12 of meloscine (3) can be established in the photochemical key step 1 → 2. 1,2-retro-Benzilic acid rearrangement to a five-membered ring, reductive amination, Claisen rearrangement, and ring-closing metathesis are further key steps in the transformation of cyclobutane 2 into the target molecule 3 (14 steps, 9 % overall yield). Enantioselective access to (+)-meloscine was possible when the [2+2]-photocycloaddition was conducted in the presence of a chiral template. The unusual monoterpenoid indole alkaloid meloscine was synthesized starting from a protected aminoethylquinolone in 15 steps and an overall yield of 9 %, employing a [2+2]-photocycloaddition as the stereochemistry defining key step. After the initial plan of a Wagner–Meerwein type rearrangement of a [4.2.0]- into a [3.3.0]-bicyclic substructure could not be realized, the required ring enlargement of a cyclobutane was eventually achieved by a retro-benzilic acid rearrangement. Generation of the central pyrrolidine ring was possible by a three-step reductive amination domino sequence. The final ring was built up by a ring-closing metathesis after the last quaternary stereocenter had been constructed by a Johnson–Claisen rearrangement. The synthesis was concluded by a selenylation–elimination sequence to build up the exocyclic vinyl group of meloscine. Using our methodology for enantioselective [2+2]-photocycloaddition mediated by a chiral complexation agent, the experimentally very simple synthesis could be performed in an enantioselective fashion (7 % overall yield). The enantioselective synthesis of (+)-meloscine represents the first example of a natural product synthesis employing an enantioselective [2+2]-photocycloaddition as its key step, and illustrates nicely the synthetic potential of photochemical transformations for the construction of complex heterocyclic structures.