Pavel Fiedler

Rapid Access to Dibenzohelicenes and their Functionalized Derivatives

Spiraling up: Easy access to dibenzo[5]-, dibenzo[6]-, and dibenzo[7]helicenes as well as their functionalized derivatives includes Sonogashira and Suzuki-Miyaura couplings, desilylation, and [2+2+2] alkyne cycloisomerization. The simplicity of this non-photochemical approach combined with the potential for helicity control favors dibenzohelicenes over the parent helicenes for practical applications.

Analysis of the ortho effect: acidity of 2-substituted benzoic acids

The classical term ortho effect was quantitatively analyzed and decomposed into its real or supposed components, using the acidity of eleven 2-substituted benzoic acids as a sample. The substituent effects were evaluated by calculations at the B3LYP/6-311+G(d,p) level by means of isodesmic reactions, separately for the acid molecules and for their anions. An intramolecular hydrogen bond, affecting the acidity moderately, was found only in the case of 2-methoxy- and 2-dimethylaminobenzoic acids. For the other acids, the often-employed interpretation invoking a hydrogen bond is wrong: the pertinent form was not revealed by calculations or by infrared spectroscopy. Another widespread explanation relying on the steric inhibition of resonance of the COOH group is also not correct in most cases: only some acid molecules are nonplanar (with substituents tert-Bu, CH2Cl, CF3, NO2, Cl) and the steric inhibition exists also in the anions, thus partly cancelling this effect. The main part of the ortho effect consists of polar effects (inductive and resonance) transmitted through the ring and of purely steric effects. Their tentative separation was based on comparison with the 4-substituted isomers. Polar effects are most important for the acidity and are not very different in the ortho and para positions (rough estimate 0.8∶1). Steric effects are strong but mostly of similar magnitude in the acid as in the anion; hence they are of restricted importance for the acidity. They may be better described as van der Waals interactions rather than as short-range field effects. The whole analysis leads to the conclusion that the acidity of ortho-substituted benzoic acids cannot be interpreted by a simple universal theory.

COUPLING REACTIONS OF ORTHO-SUBSTITUTED ARYL HALIDES WITH ALKYNES. THE SYNTHESIS OF FUNCTIONALIZED 1-NAPHTHYL-, 1-(1-NAPHTHYL)-2-PHENYL-, AND 1,2-BIS( 1-NAPHTHYL))ACETYLENES

Abstract Coupling of 2-functionalized 1-naphthyl halides with gaseous acetylene, (trialkylsilyl)acetylenes, and aryl acetylenes under Pd(PPh3)4 or Pd(PPh 3 ) 4 CuI catalysis has been investigated to prepare 1-naphthyl-, 1-(1-naphthyl)-2-phenyl-, and 1,2-bis(1-naphthyl)acetylenes with various ortho substituents, i.e., the -CH3, -CH2OH, -CO2Me, and -CH2OCH2CCCH3 groups. The reaction conditions have been optimized (yields up to 96 %) by changing halogen atom in aryl halides, solvent, alkyl in (trialkylsilyl)acetylenes, and catalyst (Pd(O) vs. Pd(O) Cu(I) ). In case of 1-naphthyl iodide with a tethered alkyne unit, coupling has been observed to compete with a cascade of intramolecular Heck-type reactions. The mechanism of β-elimination of a hydridopalladium species has been discussed. 1-Naphthyl bromide bearing the -CO2Me group has been found to be susceptible to nucleophilic aromatic substitution with a solvent. The successful synthesis of an unsymmetrical 1-(1-naphthyl)-2-phenylacetylene derivative has been shown to depend critically on combination of aryl halide/aryl acetylene.

Acidity of ortho-substituted benzoic acids: an infrared and theoretical study of the intramolecular hydrogen bonds.

The structures of ortho-substituted benzoic acids with substituents bearing hydrogen atoms (OH, NH2, COOH and SO2NH2) were investigated by means of IR spectroscopy and of density functional theory at the B3LYP/6-311 + G(d,p) level. All possible conformations, hydrogen bonds, tautomeric forms and zwitterions were taken into consideration and particular attention was given to intramolecular H-bonds and their effect on acidity. Strong H-bonds in the anions of all four acids, were revealed by calculations. In three cases they were confirmed by the IR spectra of the tetrabutylammonium salts in tetrachloromethane solution, while the salt of 1,2-benzenedicarboxylic acid was not sufficiently soluble. The H-bonds are of different strengths but in all cases they are the main cause of the strengthened acidity of these acids in the gas phase and also in solution, although their effect is opposed by weaker H-bonds present in the undissociated acid molecules. The substituent effect on the acidity was evaluated in terms of isodesmic reactions, separately in the acid molecules and in the anions. While the acidity of the 2-OH and 2-NH2 acids is determined essentially by the H-bonds, that of the 2-COOH and 2-SO2NH2 acids is strengthened by the polar effect operating in the undissociated molecule in addition to the H-bond in the anion. The steric inhibition of resonance (SIR), estimated from model conformations with fixed torsional angles, is of little importance. This analysis goes significantly beyond the classical explanation obtained from the acidities in solution but essentially conforms with it.

Transition metal control in the reaction of alkyne-substituted phenyl iodides with terminal alkynes: Sonogashira coupling vs cyclic carbopalladation

Abstract Reaction between terminal alkynes and phenyl iodides bearing a tethered alkyne unit can be effectively controlled by the nature of a transition metal catalyst. Whereas the use of Pd(0)/Cu(I) promotes the expected Sonogashira coupling to give phenyl alkynes, the absence of the copper co-catalyst triggers a palladium-mediated cyclisation providing 1,2-dihydroacenaphthylene, 1H,3H-benzo[de]isochromene, (1Z)-1-(2-propynylidene)-2,3-dihydro-1H-indene and (4E)-4-(2-propynylidene)-3,4-dihydro-1H-isochromene derivatives. In the latter Pd-catalysed two-component process, the product distribution depends on the structure of alkyne-substituted phenyl iodides, terminal alkynes and secondary amines used as solvents. The proposed reaction mechanism reflects a competitive formation of intermediary σ-(acetylide) vs π-alkyne Pd(II) complexes.

[2+2+2] Cycloisomerisation of Aromatic Cyanodiynes in the Synthesis of Pyridohelicenes and Their Analogues

We have developed a methodology for the synthesis of pyridohelicenes and their analogues based on the Ni(0) -, Co(I) - or Rh(I) -mediated intramolecular [2+2+2] cycloisomerisation of cyanodiynes. It allows for folding the linear precursors into the corresponding helical backbones comprising the newly formed pyridine unit in their central part. Along with racemic pyrido[n]helicenes (n=5,6,7) and their derivatives, both enantio- and diastereomerically pure pyrido[n]helicene-like molecules (n=5,6) were prepared by employing the chiral substrate-controlled cyclisation of the corresponding enantiopure cyanodiynes.

STERIC EFFECTS AND STERIC HINDRANCE TO RESONANCE IN TERT-BUTYLBENZOIC ACIDS IN THE GAS PHASE AND IN SOLUTION

Substituent effects of the tert-butyl group in isomeric tert-butylbenzoic acids have been investigated on the basis of enthalpies of formation, gas-phase acidities, acidities in methanol and in dimethyl sulfoxide, and the IR spectra in tetrachloromethane. In contrast to 2-methylbenzoic acid, 2-tert-butylbenzoic acid must adopt a non-planar conformation, as is confirmed by its IR spectrum. Enhanced acidity in the gas phase has been found for all isomers and attributed to a pole-induced dipole interaction in the anion (polarizability effect); in the ortho isomer the steric inhibition of resonance is only responsible for a minor part of the observed effect. In solution, the electrostatic interaction is attenuated, but remains strong in the case of the ortho isomer.

Nonsaponifiable lipid components of the pollen of elder (Sambucus nigra L.)

Pollen of the elder (Sambucus nigra L.) was extracted with chloroform-methanol. The extract was separated by column chromatography into the following groups of compounds: hydrocarbons (8.7%). polycyclic aromatic hydrocarbons (0.2%), complex esters (5.2%), triglycerides (18.7%), hydroxy esters (27.9%), free fatty acids and alcohols (16.8%), free sterols (6.8%), and triterpenic alcohols (4.0%). The nonsaponifiable components (hydrocarbons, fatty acids, alcohols, and sterols) were examined in detail using spectroscopic and chromatographic methods (IR spectroscopy, GC, and GC-MS). The identified compounds were characterized by their mass spectra and Kováts retention indices. The double bond positions and their configurations in unsaturated compounds are also reported.

IR laser thermolysis of tetramethylgermane for CVD of germanium

Abstract The CO 2 laser photosensitized (SF 6 ) decomposition of tetramethylgermane (TMG) yields C 1 and C 2 hydrocarbons along with a germanium deposit which contains some amounts of carbon and oxygen. The presence of these elements is judged to be caused by the adsorption of TMG on the surface of the deposit and by the sensitivity of the deposit to air.

Synthesis of α-carboxyphosphinopeptides derived from norleucine

In the present study, we describe in detail the synthesis of a relatively rare class of phosphorus compounds, α-carboxyphosphinopeptides. We prepared several norleucine-derived α-carboxyphosphinic pseudopeptides of the general formula Nle-Ψ[PO(OH)]-Gly. These compounds could have important applications as transition state-mimicking inhibitors for methionine or leucine aminopeptidases or other enzymes. For the preparation of the key α-carboxyphosphinate protected precursors, we investigated, compared and improved two different synthetic methods described in literature: the Arbuzov reaction of a silylated N-protected phosphinic acid with a bromoacetate ester and the nucleophilic addition of a mixed O-methyl S-phenyl N-protected phosphonic acid or a methyl N-protected phosphonochloridate with tert-butyl lithioacetate. We also prepared two N-Fmoc protected synthons, Fmoc-Nle-Ψ[PO(OH)]-Gly-COOH and Fmoc-Nle-Ψ[PO(OAd)]-Gly-COOH, and demonstrated that these precursors are suitable building blocks for the solid-phase synthesis of α-carboxyphosphinopeptides.