Synthesis of 2-substituted piperazines via direct α-lithiation

A novel efficient synthetic route towards the pharmaceutically relevant 2-substituted piperazine class is described. The key step involves α-lithiation of N-Boc piperazines, followed by reaction with several electrophiles. To obtain high yields, in some cases transmetallation to copper after the lithiation step is required.     

Pion absorption in the deuteron

Pion absorption in the deuteron is mainly sensitive to the absorption form factor, the propagator of the rescattered pion and the off-shell behaviour of the pion-nucleon interaction. Current ideas about them are difficult to reconcile with existing experimental data.Presented at the symposium Mesons and Light Nuclei, Bechyn, Czechoslovakia, May 27–June 1, 1985.     

Synthesis and Characterization of the Zinc Amides: [EtZnL]2 and [ZnL2] (L = N,N‐dimethyl(N′‐trimethylsilyl)ethane‐1,2‐diaminate)

The ligand N,N‐dimethyl(N′‐trimethylsilyl)ethane‐1,2‐diamine (HL) was treated with ZnEt₂ in varying stoichiometric ratios to synthesize [EtZnL]₂ and [ZnL₂] complexes. Crystal data: [EtZnL]₂, monoclinic, P2₁/n, a = 10.0149(5) Å, b = 8.0296(3) Å, c = 16.1689(8) Å, β = 91.715(2)°. [ZnL₂], monoclinic, P2₁/n, a = 8.8457(3) Å, b = 15.4249(6) Å, c = 16.0121(7) Å, β = 92.656(1)°. The former complex is an amido nitrogen bridged dimer with distorted tetrahedral stereochemistry of the zinc atom and the latter is a distorted tetrahedral monomer based on amide/amine chelation.     

Enantioselective synthesis of chiral porphyrin macrocyclic hosts and kinetic enantiorecognition of viologen guests

The construction of macromolecular hosts that are able to thread chiral guests in a stereoselective fashion is a big challenge. We herein describe the asymmetric synthesis of two enantiomeric C₂-symmetric porphyrin macrocyclic hosts that thread and bind different viologen guests. Time-resolved fluorescence studies show that these hosts display a factor 3 kinetic preference (ΔΔG^‡_on = 3 kJ mol⁻¹) for threading onto the different enantiomers of a viologen guest appended with bulky chiral 1-phenylethoxy termini. A smaller kinetic selectivity (ΔΔG^‡_on = 1 kJ mol⁻¹) is observed for viologens equipped with small chiral sec-butoxy termini. Kinetic selectivity is absent when the C₂-symmetric hosts are threaded onto chiral viologens appended with chiral tails in which the chiral moieties are located in the centers of the chains, rather than at the chain termini. The reason is that the termini of the latter guests, which engage in the initial stages of the threading process (entron effect), cannot discriminate because they are achiral, in contrast to the chiral termini of the former guests. Finally, our experiments show that the threading and de-threading rates are balanced in such a way that the observed binding constants are highly similar for all the investigated host–guest complexes, i.e. there is no thermodynamic selectivity.

Chiral guests display kinetic stereoselective threading through chiral porphyrin cages if their chirality is located at the chain ends and not in the centers, supporting the previously reported entron effect of threading.     

Synthesis and Structures of Two Thiocyanato-Lanthanoid Cages

Abstract  

The cages [{La₂(NCS)₅(NCMe)₆}₂O]·MeCN and [{Eu₂(NCS)₅(NCMe)₅}₂O]·MeCN have been prepared by redox transmetallation between the lanthanoid metals and mercuric thiocyanate in acetonitrile. The structure of the former has a core of four La atoms bonded to a central oxygen atom. Two La atoms are nine coordinate being bonded to oxygen, four terminal MeCN ligands and four N-bridged thiocyanato ligands, whereas the other two are eight coordinate being bonded to oxygen, the four bridging thiocyanato ligands, two terminal acetonitrile molecules, and a terminal N-bonded thiocyanate ion. In the Eu complex, there is a similar core of four Ln atoms bound to a central oxygen atom, but all Eu atoms are eight coordinate, and have four N-bridged thiocyanate ligands. Two have three MeCN ligands, whilst the other two have one terminal NCS⁻ and two acetonitrile ligands.     

Effect of Confinement on Proton‐Transfer Reactions in Water Nanopools

Proton transfer from the photoacid 8‐hydroxy‐1,3,6‐pyrenetrisulfonic acid (HPTS) to water is studied in reverse micelles with ionic (AOT=sodium dioctyl sulfosuccinate) and non‐ionic (BRIJ‐30=polyoxyethylene(4)lauryl ether) surfactants. The dynamics are studied by probing the transient electronic absorption and transient vibrational absorption, both with sub‐picosecond resolution. The reverse micelle sizes range from approximately 1.6 to 5.5 nm in diameter. For both surfactants it is found that the rate of proton transfer decreases with decreasing reverse micelle size, regardless of surfactant. In addition, for AOT reverse micelles, a fraction of the photoacid molecules exhibit non‐radiative decay, preventing proton transfer.