Machines in art and chemistry

The present paper is intended to expose an example of similarity between chemistry and art not based on shape or appearance, but rather on the common origins of the concepts involved. Specifically, the subject of this text is about machines and machinery and makes a parallel between the construction of multi-machines in art and chemistry.     

A colourful azulene-based protecting group for carboxylic acids

An intensely blue-coloured protecting group for carboxylic acids has been developed. The protecting group is introduced through a Steglich esterification that couples 6-(2-hydroxyethyl)azulene (AzulE) and the carboxylic acid substrate. Deprotection is effected under basic conditions by the addition of the amidine base DBU, whereupon cleavage occurs, accompanied by a colour change. A two-step deprotection methodology comprising activation with oxalyl chloride and deprotection with a very mild base was developed for use with base-sensitive substrates. The AzulE esters were found to be compatible with other commonly employed protecting groups – silyl ethers, MOM acetals – by studying their orthogonal and concomitant deprotections. The stability of the new protecting group towards various synthetic processes – oxidation, reduction, cross-coupling, olefination and treatment with base – provided the basis of a versatility profile. This indicated that AzulE esters are sensitive to strongly oxidising and basic agents while being compatible with reducing conditions and selected other reactions. The convenience of a highly coloured protecting group for tracking material (and avoiding loss of compound) through laboratory processes warrants further investigation of this and/or related species.     

Iterative oxime bond chemistry leads to protease inhibitors

[structure: see text] In the present paper, we have looked at iterative coupling as a strategy to form new druglike molecules. We have developed an iterative coupling chemistry based on oxime bond formation between hydroxyaromatic aldehyde building blocks to form linear oxime oligomers. The strategy is validated by the discovery of micromolar protease inhibitors.     

Reactivity of the metal-silicon bond in organometallic chemistry

The reactivity of molecules containing a metal-silicon bond is currently attracting considerable attention owing to its relevance to a number of important stoichiometric and catalytic transformations. The focus of this review concerns those stoichiometric reactions involving the metal-silicon bond which result in isolable Si-containing metal complex(es). Catalytic reactions are therefore only considered when strong evidence for M---Si intermediate complexes is provided. Reactions in which the silicon atom leaves the metal complex are not examined. For convenience, the reactions will be classified into insertion and migration reactions, although this terminology does not necessarily have a mechanistic implication and is somewhat arbitrary since many reaction products could belong to one or the other section.     

Amide bond isosteres: Imidazolines in pseudopeptide chemistry

The 2-imidazoline ring has been incorporated as an amide bondreplacement into pseudodipeptides, a pseudotripeptide, andpseudopentapeptide enkephalin analogues.     

From molecular chemistry to supramolecular chemistry to superdupermolecular chemistry. Controlling covalent bond formation through non-covalent and magnetic interactions

The reactions of carbon centered radical pairs often involve diffusion controlled combination and/or disproportionation reactions which are non-selective. A triplet geminate pair of radicals is produced by the photolysis of suitable ketones. The reactions of such geminate pairs can be controlled though the application of supramolecular concepts which emphasize non-covalent interaction to "steer" the geminate pair toward a selected pathway. In addition, "superdupermolecular" concepts, which emphasize the control of radical pair reactions through the orientation of electron spins, can be employed to further control the course of geminate pair reactions. Examples of control of a range of the selectivity of geminate radical combinations, which form strong covalent bonds, through supramolecular and superdupermolecular effects will be presented for the photolysis of ketones adsorbed in the supercages of zeolites.     

Applying the principles of green chemistry in art: design of a cross-disciplinary course about ‘art in the Anthropocene: greener art through greener chemistry’

ABSTRACT

This cross-disciplinary course bridging the disciplines of art and chemistry provides an exceptional environment in which first-year students and beyond are engaged and challenged in new ways. This dual lab/studio-based course, titled “Art in the Anthropocene: Greener Art through Greener Chemistry,” enables students to use their imagination, creativity, and innovation to respond to environmental issues and concerns through art. Students are asked to reflect on the nature and implications of the actual physical materials that artists use and to achieve a renewed sense of social and ethical responsibility through the content of their artwork. The curriculum is designed so that teachers guide students on how chemical processes are used to make art materials in an environmentally friendly way. The overall goal is to apply green chemistry principles in the making of artworks that can be crafted with reclaimed, recycled, and naturally available materials, non-toxic solvents and paints, and using sustainable forms of energy while keeping ethical values in mind.     

Electrophilic terminal phosphinidene complex-Lewis base adducts: chemistry between carbon-halide bond activation and weak Lewis base adduct formation

Comparative studies on the reactivity of a transiently formed terminal phosphinidene complex towards various organobromide derivatives show that carbon-bromine bond insertion is preferred with benzyl bromide, whereas formal HBr-insertion resulted with 2-bromopyridine and a surprising selectivity enhancement (of the phosphinidene complex) was observed with bromobenzene; all new products were established by elemental analyses, NMR spectroscopy, mass spectrometry and single crystal X-ray diffraction studies.     

14C dating of ancient rock art: A new application of plasma chemistry

We have developed a new statically operated oxygen plasma method that allows direct¹⁴C dates to be obtained from ancient rock paintings. The method is applicable even to paintings on limestone (CaCO₃) walls. A sample of a pictograph which had naturally spalled offa shelter wall in the Lower Pecos region of Texas was subjected to a low temperature ( 150°C) oxygen plasma to selectively remove the organic carbon-containing material used in the paint as CO₂, without contamination from the limestone substrate. The Zürich EHT accelerator mass spectrometer was then used to determine the radiocarbon age of this prehistoric rock painting. It was successfully dated at 3865 ± 100 years BP, in good accord with the archaeological context which has set the onset of this Pecos River style of pictograph between 4100 and 3200 years BP. The method appears feasible and is applicable to rock art in which organic materials were used in the paint.     

P-C bond activation chemistry: evidence for 1,1-carboboration reactions proceeding with phosphorus-carbon bond cleavage

A series of diarylphosphinyl-substituted acetylenes of the type (aryl)(2)P-C≡C-R (aryl = phenyl or mesityl, R = Ph or n-propyl) react with the strongly Lewis acid reagent B(C(6)F(5))(3) in toluene at elevated temperatures (70-105 °C) to give the 1,1-carboboration products 4. Treatment of bis(diphenylphosphinyl)acetylene with B(C(6)F(5))(3) under analogous conditions proceeded with phosphinyl migration to yield the 1,1-carboboration product 4d, bearing a geminal pair of Ph(2)P substituents at one former acetylene carbon atom and a C(6)F(5) substituent and the remaining -B(C(6)F(5))(2) group at the other. Prolonged thermolysis of 4d resulted in an intramolecular aromatic substitution reaction by means of Ph(2)P attack on the adjacent C(6)F(5) ring to yield the zwitterionic phospha-indene derivative 7. The compounds 4a, 4c, 4d, and 7 were characterized by X-ray diffraction.     

A new approach to the relationship between bond energy and electronegativity

The familiar equation whereby Pauling related heteronuclear bond energies D_A–B to the electronegativity difference Δχ (=∣χ_A − χ_B∣) and the homonuclear bond energies D_A–A and D_B–B has been the subject of critical scrutiny for at least half a century. A modification to this equation that combines the concepts of electronegativity and hardness/softness can be rewritten in terms of two quantities x and y, both having absolute significance. Both homo- and heteronuclear bond energies can be rationalised from these new equations. The quantities x are linearly related to Pauling electronegativities, while y appears to be a measure of an atom’s intrinsic bonding potential, related to size and availability of valence orbitals.     

Beyond the metal-metal triple bond in binuclear cyclopentadienylchromium carbonyl chemistry

The cyclopentadienylchromium carbonyls Cp(2)Cr(2)(CO)(n) and Cp*(2)Cr(2)(CO)(n) (Cp = eta(5)-C(5)H(5) and Cp* = eta(5)-Me(5)C(5); n = 3, 2) have been studied by density functional theory using the B3LYP and BP86 functionals. Triplet and singlet structures are found for Cp(2)Cr(2)(CO)(3), with the triplet isomer having an apparent Cr[triple bond, length as m-dash]Cr triple bond (2.295 A by BP86) and predicted to have a lower energy than the singlet isomer having an apparent Cr[quadruple bond, length as m-dash]Cr quadruple bond (2.191 A by BP86). Quintet, septet, and singlet structures, as well as a highly spin contaminated triplet structure, were found for the dicarbonyl Cp(2)Cr(2)(CO)(2). In all of the Cp(2)Cr(2)(CO)(n) (n = 3, 2) structures the carbonyls are asymmetric semi-bridging groups, typically with differences of 0.3-0.5 A between the shortest and longest M-C distances. Very little difference was found between the structures and energetics of the corresponding Cp and Cp* derivatives. These DFT studies suggest that the reported unstable photolytic decarbonylation product of Cp*(2)Cr(2)(CO)(4), characterized only by its infrared nu(CO) frequencies, is the singlet isomer of the tricarbonyl Cp*(2)Cr(2)(CO)(3).     

Understanding organofluorine chemistry. An introduction to the C-F bond

Fluorine is the most electronegative element in the periodic table. When bound to carbon it forms the strongest bonds in organic chemistry and this makes fluorine substitution attractive for the development of pharmaceuticals and a wide range of speciality materials. Although highly polarised, the C-F bond gains stability from the resultant electrostatic attraction between the polarised C delta+ and F delta- atoms. This polarity suppresses lone pair donation from fluorine and in general fluorine is a weak coordinator. However, the C-F bond has interesting properties which can be understood either in terms of electrostatic/dipole interactions or by considering stereoelectronic interactions with neighbouring bonds or lone pairs. In this tutorial review these fundamental aspects of the C-F bond are explored to rationalise the geometry, conformation and reactivity of individual organofluorine compounds.     

New relation between ionic radii,bond length,and bond strength

Based on the table of the effective ionic radii of R. D. Shannon (Acta Crystallogr. Sect. A32, 751, 1976), the linear relation of the cationic radii on the coordination numberk has been observed:

$r_k=r_0+dk?\frac{0.0236\mathit{k}}{z},$

wherer₀ is the radius of free cation, z its valence, andd = 0.1177 - 0.0081 z - 0.0347 r₀ - 0.0050 zr₀. The analogous relation for O^2? anion has been found to ber′_k′ = r′₀ + a′k′ = 1.328 + 0.0118 k′. By addingr_k andr′_k′ corresponding to the same bond strength new dependence between bond lengthR and bond strengths has been established for cation-oxygen bonds:

$s=\frac{d\mathit{z}}{R?R_0},$

whereR₀ = r₀ + r′₀. The necessity to choose a standard state for ionic radii and for bond strength is pointed out and argued. Structures of simple oxides at room temperature and at normal pressure are chosen as standard state at which the sum of the strengths of bonds around the cation is assumed to be exactly equal to itsz.Standard radii of free ions r₀ are determined for about 230 ions and are listed. As in Shannon's table some ionic radii were found to be negative. Physical sense can be hardly attributed to this finding. To avoid the negative radii a new scale ofabsolute ionic radiiρ₀ is proposed, based on assumption thatρ₀(H¹⁺) = 0 instead ofr₀(H¹⁺) = ?h = ?0.499 (the size of the proton is known to be of the order of 10^?5A?, i.e., much less than the accuracy of determination of ionic radii). Consequentlyρ₀ for all cations is assumed to beρ₀ = r₀ + h and for anionsρ′₀ = r′₀ ? h (e.g.,ρ′₀(O^2?) = 0.829A?). Bond-length-bond-strength relationship expressed by the equation indicated above is rationalized in terms of the new proposedelectrostatic hover model of crystal structure. In this model ions are of constant size (r₀ orρ₀), they do not touch each other, but they are maintained at distancesL = R - R₀ by the electrostatic forces. This remains in agreement with the suggested (J. Zio´?kowski,J. Catal.84, 317, (1983)) linear relation between bond strength and bond energyE = Js which will be proved in the forthcoming paper (J. Zio´?kowski and L. Dziembaj,J. Solid State Chem.57, 291 (1985)).