Impact of Heterocycle Annulation on NIR Absorbance in Quinoid Thioacene Derivatives.

The synthesis and characterisation of a homologous series of quinoid sulfur-containing imidazolyl-substituted heteroacenes is described. The optoelectronic and magnetic properties were investigated by UV/vis, fluorescence and EPR spectroscopy as well as quantum-chemical calculations, and were compared to those of the corresponding benzo congener. The room-temperature and atmospherically stable quinoids display strong absorption in the NIR region between 678 and 819 nm.

Tailoring the Gibbs Free Energy of the Nitroxide Exchange Reaction: Substituent and Solvent Effects.

A quantum chemical study of the nitroxide exchange reaction is presented. Inspired by the recent use of this reaction in the synthesis of dynamic covalent polymer networks, we studied the influence of substituents and solvents on the Gibbs free energy, which plays a crucial role for both the reversibility of the reaction and the extent of product formation. We provide accurate benchmark values based on CCSD(T) and COSMO-RS theory for a series of structural modifications and make suggestions for improving the molecular building blocks used so far.

Two-photon absorption in a series of 2,6-disubstituted BODIPY dyes.

We report on the two-photon absorption spectra of a series of 2,6-disubstituted BODIPY dyes. Depending on the substituents, we observe increasing two-photon absorption cross sections with values up to 350 GM compared to 70 GM in the unsubstituted dye. Quantum chemical calculations are performed to assign the absorption bands and to understand the factors controlling the size of the two-photon absorption cross section.

Aluminium Diphosphamethanides: Hidden Frustrated Lewis Pairs.

The synthesis and characterisation of two aluminium diphosphamethanide complexes, [Al(tBu)2 {κ(2) P,P'-Mes*PCHPMes*}] (3) and [Al(C6 F5 )2 {κ(2) P,P'-Mes*PCHPMes*}] (4), and the silylated analogue, Mes*PCHP(SiMe3 )Mes* (5), are reported. The aluminium complexes feature four-membered PCPAl core structures consisting of diphosphaallyl ligands. The silylated phosphine 5 was found to be a valuable precursor for the synthesis of 4 as it cleanly reacts with the diaryl aluminium chloride [(C6 F5 )2 AlCl]2 .

Modeling the Histidine-Phenylalanine Interaction: The NH···π Hydrogen Bond of Imidazole·Benzene.

NH···π hydrogen bonds occur frequently between the amino acid side groups in proteins and peptides. Data-mining studies of protein crystals find that ∼80% of the T-shaped histidine···aromatic contacts are CH···π, and only ∼20% are NH···π interactions. We investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet-cooled imidazole·benzene (Im·Bz) complex as a model for the NH···π interaction between histidine and phenylalanine.

Activation enthalpies and entropies of the atropisomerization of substituted butyl-bridged biphenyls.

A combined quantum chemical and experimental study of the atropisomerization energies of di-para-substituted butyl-bridged biphenyl cyclophanes is presented. We studied the influence of electron donor and electron acceptor substituents on the height of the enantiomerization barrier and examined the enthalpic and entropic contributions. The reaction pathway with minimum and transition state structures was established using density functional theory calculations.

Unprecedented pseudo-ortho and ortho metallation of [2.2]paracyclophanes--a methyl group matters.

The first example of a pseudo-ortho metallation on [2.2]paracyclophane is presented, giving easy access to enantiopure compounds of this type. By slight modification of the directing group, metallation of the ortho- and C2-positions is possible.

Oriented circular dichroism analysis of chiral surface-anchored metal-organic frameworks grown by liquid-phase epitaxy and upon loading with chiral guest compounds.

Oriented circular dichroism (OCD) is explored and successfully applied to investigate chiral surface-anchored metal-organic frameworks (SURMOFs) based on camphoric acid (D- and Lcam) with the composition [Cu2(Dcam)(2x)(Lcam)(2-2x)(dabco)]n (dabco = 1,4-diazabicyclo-[2.2.2]-octane).

Asymmetric organocatalytic synthesis of 4,6-bis(1H-indole-3-yl)-piperidine-2 carboxylates.

We developed an asymmetric organocatalytic synthesis of 4,6-bis(1H-indole-3-yl)-piperidine-2-carboxylates using 10 mol% of a chiral phosphoric acid. The products, which are novel bisindole-piperidine-amino acid hybrids, can be obtained in one step from 3-vinyl indoles with imino esters in dichloromethane at room temperature after 1 h of reaction time. A variety of these compounds could be synthesized in up to 70% yield and 99% ee, and they were experimentally and computationally analyzed regarding their relative and absolute stereochemistry.

Tethering for selective synthesis of 2,2'-biphenols: the acetal method.

2,2'-Biphenols are a large and diverse group of compounds with exceptional properties both as ligands and bioactive agents. Traditional methods for their synthesis by oxidative dimerisation are often problematic and lead to mixtures of ortho- and para-connected regioisomers. To compound these issues, an intermolecular dimerisation strategy is often inappropriate for the synthesis of heterodimers.

ortho-Perfluoroalkylation and ethoxycarbonyldifluoromethylation of aromatic triazenes.

A robust protocol for perfluoroalkylation and ethoxycarbonyldifluoromethylation of functionalized aromatic triazenes is described. Using silver(I)-fluoride and different fluorinated (trimethyl)silyl substituted species, it was possible to synthesize various ortho-fluorinated triazenes in good yields via simple CH-substitution. Initial reactions under solvent-free (neat) conditions indicate a stabilizing interaction between "AgRf" and the triazene moiety, which may be responsible for the good yields and regioselectivity.

Atropisomerization of di-para-substituted propyl-bridged biphenyl cyclophanes.

The influence of electron donors and electron acceptors of variable strength in the 4 and 4' position of 2 and 2' propyl-bridged axial chiral biphenyl cyclophanes on their atropisomerization process was studied. Estimated free energies ΔG(‡)(T) of the rotation around the central biphenyl bond which were obtained from (1)H-NMR coalescence measurements were correlated to the Hammett parameters σ(p) as a measure for electron donor and acceptor strength. It is demonstrated that the resulting nice linear correlation is mainly based on the influence of the different substituents on the π-system of the biphenyl cyclophanes.

The plakotenins: biomimetic Diels-Alder reactions, total synthesis, structural investigations, and chemical biology.

The total synthesis of plakotenin, a cytotoxic marine natural product, using a biomimetic Diels-Alder reaction is described in detail. Two approaches were used, whereby the Diels-Alder reaction occurs at different stages of the synthesis. Homo- and nor-plakotenin, related natural products, were also prepared, as well as iso-plakotenin, a diastereoisomer of plakotenin.

Structure revision of plakotenin based on computational investigation of transition states and spectroscopic properties.

We show that the previously [Tetrahedron Lett.1992, 33, 2579] proposed structure of natural plakotenin must be revised. Recently, the total synthesis of plakotenin was achieved via an intramolecular Diels-Alder reaction from a (E,E,Z,E)-tetraene as linear precursor.

Derivatives and dimers of C50-D5h and C50-D3: a comparison of two closely related but quite differently behaving fullerenes.

A density functional theory study on the reactivity of the energetically most stable C(50) isomers, C(50)-D(5h) and C(50)-D(3), is presented. We explore the reactivity of both fullerenes towards the addition of hydrogen and halogen atoms and towards the formation of dimers. The addition patterns of the derivatives C(50)X(2n) (X = H, F, Cl) that are preferably formed after the saturation of the most reactive sites are investigated as well.

Hydrides and dimers of C(58) fullerenes: structures and stabilities.

A density functional theory (DFT) study of fullerene hydrides C(58)H(2x) (2x = 2,4,...

Non-IPR C60 solids.

Films comprising predominantly novel isomers of C(60) [=C(60)(nIPR)] have been generated by low energy ion beam deposition of vibronically excited C(60)(+) onto graphite followed by selective sublimation of C(60)(I(h)) from the deposited isomer mixture. The incident ions were generated by electron impact ionization/fragmentation of sublimed C(70). Images of the C(60)(nIPR) films obtained by applying atomic force microscopy show aggregates, which we attribute to covalently interlinked C(60)(nIPR) units.

Low energy hydrogenation products of extended pi systems CnH2x: a density functional theory search strategy, benchmarked against CCSD(T), and applied to C60.

An approach for the systematic determination of particularly stable hydride compositions C(n)H(2x) of a fullerene C(n) is presented. The study is divided into three parts. First, a CCSD(T) benchmark study on benzene and naphthalene hydrogenation is carried out.

Nucleophilic additions to alkylidene bis(sulfoxides): stereoelectronic effects in vinyl sulfoxides.

Conjugate additions of nucleophiles (e.g. enolates, amines and malonate anions) to bis(p-tolylsulfinyl)alkenes, alkylidene-1,3-dithiane-1,3-dioxides and alkylidene-1,3-dithiolane-1,3-dioxides have recently been published.

DFT study of fullerene dimers.

Several [2+2] dimerization products of a fullerene Cn (n=50, 52, 54, 56, 58, 60) in its most stable form are presented and discussed. In total, 34 dimers are studied. Only symmetric Cn-Cn dimers (no mixed products) are reported, and only dimers of the fullerenes C50 to C58 with bonds between C atoms in adjacent pentagons are considered.

Solid C58 films.

A new solid material has been created in ultra high vacuum by utilizing the aggregation process of C58 molecules deposited onto highly oriented pyrolytic graphite from a mass selected low-energy ion beam comprising C58+. Cluster fluxes of up to 3x10(11) ions s-1 cm-2 with impinging kinetic energies of 6+/-0.5 eV were typically applied.

From weakly coordinating to non-coordinating anions? A simple preparation of the silver salt of the least coordinating anion and its application to determine the ground state structure of the Ag(eta2-P4)2+ cation.

The unexpected but facile preparation of the silver salt of the least coordinating [(RO)3Al-F-Al(OR)3]- anion (R=C(CF3)3) by reaction of Ag[Al(OR)4] with one equivalent of PCl3 is described. The mechanism of the formation of Ag[(RO)3Al-F-Al(OR)3] is explained based on the available experimental data as well as on quantum chemical calculations with the inclusion of entropy and COSMO solvation enthalpies. The crystal structures of (RO)3Al<--OC4H8, Cs+[(RO)2(Me)Al-F-Al(Me)(OR)2]-, Ag(CH2Cl2)3+[(RO)3Al-F-Al(OR)3]- and Ag(eta2-P4)2+[(RO)3Al-F-Al(OR)3]- are described.

Ti2: accurate determination of the dissociation energy from matrix resonance Raman spectra and chemical interaction with noble gases.

UV-visible and resonance Raman spectra of Ti(2) isolated in Ar, Kr, and Xe matrices at temperatures of 10 K were measured by using the 514 nm line of an Ar ion laser. The data show that the Ti(2) molecule interacts strongly with Xe, leading to a significant weakening of the Ti[bond]Ti bond strength. The f(Ti[bond]Ti) force constant decreases in the series Ar>Kr>Xe, from 232.