Orthoamides and Iminium Salts LXXII: N,N,N′,N′,N′′,N′′,N′′′,N′′′‐Octamethyl‐(but‐2‐yne)‐bis(amidinium) Tetrafluoroborate: The First Bis‐amidinium Salt of Acetylenedicarboxyclic Acid. A New Superelectrophile?

A method for the preparation of the first acetylenedicarboxamidinium salt from a bis‐orthoamide derivative of acetylenedicarboxyclic acid has been established. The salt reacted with cyclopentadiene and furan at room temperature to give bicyclic [4+2]‐cycloaddition products. The solid compounds were characterized by solution NMR spectroscopy and by single‐crystal X‐ray diffraction. Quantum‐chemical calculations of the isolated N,N,N′,N′,N′′,N′′,N′′′,N′′′‐octamethyl‐acetylene‐bis(carboxamidinium) ion showed very good agreement with the spectroscopic and diffraction data.     

Cadmium(II) chloride,bromide and iodide complexes with 4,4′‐bipyridazine: when are diazine and halide bridges (in)compatible?

In poly[di‐μ‐chlorido‐μ‐(4,4′‐bipyridazine)‐κ²N¹:N^1′‐cadmium(II)], [CdCl₂(C₈H₆N₄)]_n, (I), and its isomorphous bromide analogue, [CdBr₂(C₈H₆N₄)]_n, (II), the halide atom lies on a mirror plane and the Cd^II ion resides at the intersection of two perpendicular mirror planes with m2m site symmetry. The pyridazine rings of the ligand lie in a mirror plane and are related to each other by a second mirror plane perpendicular to the first. The compounds adopt the characteristic structure of the [M^IIX₂(bipy)] type (bipy is bipyridine) based on crosslinking of [Cd(μ‐X)₂]_n chains [Cd—Cl = 2.5955 (9) and 2.6688 (9) Å; Cd—Br = 2.7089 (4) and 2.8041 (3) Å] by bitopic rod‐like organic ligands [Cd—N = 2.368 (3)–2.380 (3) Å]. This feature is discussed in terms of supramolecular stabilization, implying that the periodicity of the inorganic chain [Cd...Cd = 3.7802 (4) Å in (I) and 3.9432 (3) Å in (II)] is favourable for extensive parallel π–π stacking of monodentate pyridazine rings, with centroid–centroid distances of 3.7751 (4) Å in (I) and 3.9359 (4) Å in (II). This is not the case for the longer iodide bridges, which cannot stabilize such a pattern. In poly[tetra‐μ‐iodido‐μ₄‐(4,4′‐bipyridazine)‐κ⁴N¹:N²:N^1′:N^2′‐dicadmium(II)], [Cd₂I₄(C₈H₆N₄)]_n, (III), the ligands are situated across a centre of inversion; they are tetradentate [Cd—N = 2.488 (2) and 2.516 (2) Å] and link successive [Cd(μ‐I)₂]_n chains [Cd—I = 2.8816 (3)–3.0069 (4) Å] into corrugated layers.     

Warum pentose-und nicht hexose-nucleinsäuren? Teil III. Oligo(2′,3′-dideoxy-β-D-glucopyranosyl) nucleotide (‘homo-DNS’): Paarungesigenschaften

Why Pentose-And Not Hexose-Nucleic Acids? Part III. Oligo(2′,3′-dideoxy-β-D -glucopyranosyl)nucleotides. (‘Homo-DNA’): Base-Pairing Properties

1 Summary in collaboration with Prof. Dr. C. E. Wintner, Haverford College, Haverford, PA 19041-1392.

The paper presents results of a comprehensive investigation on the pairing properties of homo-DNA oligonucleotides, the preparation of which has been described in Part II of this series [2]. The investigation was carried out by using established methods described in the literature for the characterization of oligonucleotides in the natural series, such as determination of melting temperatures of oligonucleotide duplexes by temperature-dependent of melting temperatures, determination of pairing stoichiometry by ratio-dependent UV spectroscopy of binary mixtures of pairing partners, temperature-dependent CD spectroscopy, gel electrophoresis under non-denaturing conditions, and – in selected cases – ¹H – and³¹P-NMR spectroscopy. The systematic comparison of the paring properties of homo-DNA oligonucleotides with corresponding DNA nucleotides (up to dodecamers) indicates that homo-DNA is a highly efficient, autonomous, artificial pairing system with a pairing behavior that is in part similar to, but also, in part, strikingly different from, the pairing behavior of DNA. The pairing properties established so far are listed below in a manner that reflects the sequence of subtitles in Chapt.2 of the text; they were determined under the conditions: H²O, 0.15M NaCl, 0.01M Tris-HCl buffer, pH 7, oligonucleotide concentrations in the μM range, 1:1 ratio of single strands in the case of non-selfcompementary sequences.     

Stereoselective total syntheses of (±)-1,14-herbertenediol and (±)-tochuinyl acetate and facile total syntheses of (±)-α-herbertenol, (±)-β-herbertenol and (±)-1,4-cuparenediol

Stereoselective total syntheses of (±)-1,14-herbertenediol (7) and (±)-tochuinyl acetate (10) and facile total syntheses of (±)-α-herbertenol (2), (±)-β-herbertenol (3) and (±)-1,4-cuparenediol (8) have been successfully accomplished involving intramolecular cyclisation of 3-aryl-3-methyl-6-bromohexanoates and in situ methylation of the resulting cyclopentanecarboxylates as the key reactions.     

Science and Arts,Philosophy and Science: Why after All? Why Not?

This perspective summarizes some interdisciplinary aspects of science and the relation to philosophy, also including the basic motivations and aims as they might be discussed with young scientists starting their careers and presented also in the form of a commencement speech. The contents of this speech were repeatedly discussed also with Jack Dunitz, who showed great interest in it, given his broad interests. The speech also referred to an earlier commencement speech by Jack Dunitz in 1989. In the introduction of our essay, we mention the early common history of science and humanities under the name of philosophy. This early history can be traced back to ancient Greek philosophy and the ‘academy’ of Platon in Athens with a history of more than 1000 years until closure in 529 AD, in modern times revived as the National Greek Academy in Athens in the 19th and 20th centuries. Other ‘academies’ in Europe started in the 17th century and had publications under various names involving ‘philosophy’ with a focus on what we call science (natural science) today. After about 1800 there was increasing fragmentation of the various fields of knowledge and philosophy was considered to be part of the modern ‘humanities’ quite separate from science, and the natural sciences were fragmented into physics, chemistry, biology etc., and even finer subdivisions. The essay also describes an effort at ETH Zurich, reintegrating the various subfields of science and also stressing an education of scientists and engineers in the humanities. The essay concludes with a discussion of several global risks for mankind and a scientific imperative to maintain life on Earth. The common aspects and the foundations of all sciences as fields of knowledge aiming for an understanding of the world around us and of human beings as part of it are discussed from various perspectives.     

How Strong Are Hydrogen Bonds in Metalla‐β‐diketones?

The energies of the kinetically inert, electronically saturated Lukehart-type metalla-beta-diketone [Re{(COMe)2H}(CO)4] (9 a) and of the kinetically labile, electronically unsaturated platina-beta-diketones [Pt{(COMe)2H}Cl2]- (10 a), [Pt2{(COMe)2H}2(micro-Cl)2] (11 a), and [Pt{(COMe)2H}(bpy)]+ (12 a) have been calculated by DFT at the B3LYP/6-311++G(d,p) level using effective core potentials with consideration of relativistic effects for the transition metals. Analogously, energies of the requisite open (non-hydrogen-bonded) equilibrium conformers (9 b, 10 c, 11 b, 12 b) and energies which were obtained from the hydrogen-bonded conformers by rigid rotation of the OH group around the C--O bond by 180 degrees followed by relaxation of all bond lengths and angles (9 c, 10 d, 11 c, 12 d) have been calculated. These energies were found to be higher by 14.7/27.2 (9 b/9 c), 20.7/27.2 (10 c/10 d), 19.2/25.7 (11 b/11 c), and 9.4/19.6 kcal mol(-1) (12 b/12 d) than those of the intramolecularly O--HO hydrogen-bonded metalla-beta-diketones 9 a, 10 a, 11 a, and 12 a, respectively. In acetylacetone (Hacac), the generic organic analogue of metalla-beta-diketones, the energies of the most stable non-hydrogen-bonded enol isomer (6 b) and of the conformer derived from the H-bonded form by rigid rotation of the OH group by 180 degrees followed by subsequent relaxation of all bond lengths and angles (6 k) were found to be 10.9/16.1 kcal mol(-1) (6 b/6 k) higher compared to the intramolecularly O--HO bonded isomer 6 a. Thus, the hydrogen bonds in metalla-beta- diketones must be regarded as strong and were found to be up to twice as strong as that in acetylacetone. A linear relationship was found between the hydrogen-bond energies based on the rigidly rotated structures and the OO separation in the hydrogen-bonded structures. Furthermore, these energies were also found to be correlated with the electron densities at the OH bond critical points (rhobcp) in the O--HO bonds of metalla-beta-diketones 9 a, 10 a, 11 a, and 12 a (calculated using the AIM theory). The comparison of the energies of the doubly intermolecularly hydrogen-bonded dinuclear platina-beta-diketone [{Pt{(COMe)2H}(bpy)}2]2+ (14) with that of the mononuclear intramolecularly hydrogen-bonded cation [Pt{(COMe)2H}(bpy)]+ (12 a) showed that the intermolecular hydrogen bonds in 14 are weaker than the intramolecular hydrogen bond in 12.     

Are Sc—C and Sc—P Bonds Reactive in Scandium Phosphinoalkylidene Complex? Insights on a Versatile Reactivity

The peculiar electronic structure of scandium phosphinoalkylidene complex [LSc{C(SiMe₃)PPh₂}THF] (L=[MeC(NDIPP)CHC(NDIPP)Me]⁻), DIPP=2,6‐(ⁱPr)₂C₆H₃) leads to an interesting versatile reactivity, which is demonstrated both experimentally and computationally. The complex undergoes [2+2] cycloaddition reactions with alkynes, and easily activates various X—O bonds such as C—O of propylene oxide, N—O of 3,5‐dimethylisoxazole, B—O of pinacolborane and Si—O of triethoxysilane. These reactions occur on the Sc—C bond of the phosphinoalkylidene complex. Interestingly, the Sc—P bond can also be activated as the presence of a Sc—C—P three center π interaction in the complex allows performing C—F activation of 2,6‐difluoropyridine and 1,2 addition with imine or ketone. The complex also reacts with metal complexes, [(COD)RhCl]₂ and (Ph₃P)AuCl, to form structural intriguing heterobimetallic complexes.     

PIXE and charge-induced X-rays (CHIX): Are they complementary or competitive?

Enhanced yields from CHIX studies on non-conducting samples have demonstrated conclusively that such yields are exceptionally high when compared to PIXE at low incident particle energies. This was confirmed with low energy¹⁴N⁺,¹⁶O⁺ and²⁰Ne⁺ ions whose X-ray production crosssections are negligibly small for PIXE yields. In particular, a combination of low incident energies and high X-ray yields could be useful for XSQR investigations and elemental studies with low energy accelerators. Furthermore, extended studies on pure metal targets revealed that the PIXE yield could be improved by insulating the targets thus making them suitable to produce the CHIX yield under identical experimental conditions. This paper discusses the complementary and competitive features of PIXE and CHIX.     

Why Does Pivalaldehyde (Trimethylacetaldehyde) Unexpectedly Seem More Basic Than 1‐Adamantanecarbaldehyde in the Gas Phase? FT‐ICR and High‐Level Ab Initio Studies

Fourier transform ion cyclotron resonance spectroscopy (FT ICR) techniques, including collision-induced dissociation (CID) methodology, were applied to the study of the gas-phase protonation of pivalaldehyde (1) and 1-adamantanecarbaldehyde (2). A new synthetic method for 2 was developed. The experiments, together with a thorough computational study involving ab initio and density functional theory (DFT) calculations of high level, conclusively show that upon monoprotonation in the gas phase, compound 1 yields monoprotonated methyl isopropyl ketone 3. The mechanism of this gas-phase acid-catalyzed isomerization is different from that reported by Olah and Suryah Prakash for the reaction in solution. In the latter case, isomerization takes place through the diprotonation of 1.     

Acid?Base Properties of Adenosine 5′‐Monophosphate,Guanosine 5′‐Monophosphate,and Inosine 5′‐Monophosphate in Aqueous Solutions of Methanol

The protonation constants of adenosine 5′‐monophosphate, guanosine 5′‐monophosphate, and inosine 5′‐monophosphate were determined in binary mixtures of H₂O containing 0, 10, 15, 20, 25, 30, 35, 40, 45, and 50% MeOH, using a combination of potentiometric and spectrophotometric methods at a constant temperature (25°) and constant ionic strength (0.1 mol?dm^?3 NaClO₄). The protonation constants were analyzed using the normalized polarity parameter (E ), and Kamlet, Abboud, and Taft (KAT) parameters. A linear correlation of log K vs. the normalized polarity parameter was obtained. Dual‐parameter correlation of log K vs. π* (dipolarity/polarizability) and α (H‐bond‐donor acidity), as well as π* and β (H‐bond‐acceptor basicity) also gives good results in various aqueous organic solvent mixtures. Finally, the results are discussed in terms of the effect of solvent on the protonation equilibria.     

Design and Function of Pre‐organised Outer‐Sphere Amidopyridyl Extractants for Zinc(II) and Cobalt(II) Chlorometallates: The Role of C?H Hydrogen Bonds

Four new sterically hindered pyridines, L¹–L⁴‐containing amido substituents at the 2‐position act as efficient solvent extractants for [CoCl₄]^2? or [ZnCl₄]^2? from acidic chloride solutions through protonation of the pyridino N‐centre to form the neutral outer‐sphere complexes [(LH)₂MCl₄]. These ionophores show very high selectivity for chlorometallate anions over chloride ion and are readily stripped to liberate the free‐metal chlorides without the formation of inner‐sphere complexes [ML₂Cl₂]. Single‐crystal X‐ray structure determinations of [(L²H)₂CoCl₄] and [(L²H)₂ZnCl₄] (L²=2‐(4,6‐di‐tert‐butylpyridin‐2‐yl)‐N,N′‐dihexylmalonamide) coupled with ¹H NMR spectroscopy and DFT calculations on L²H⁺ and other complexes of [ZnCl₄]^2? confirm that the pyridinium NH group does not address the outer co‐ordination sphere of the metallanion, but rather forms a hydrogen bond to the pendant amide groups and thus pre‐organizes the ligand to present both C? H and amido N? H hydrogen‐bond donors to the [MCl₄]^2? ions. The selectivity for chlorometallates over chloride ions shown by this class of extractants arises from their ability to present several polarized C? H units towards the charge‐diffuse ions [MCl₄]^2?, whereas the smaller, “harder” chloride anion prefers to be associated with the amido N? H hydrogen‐bond donors.