83 research outputs found
Chiral chemistry of single molecules.
No full textIf a single chiral molecule stands alone in a chemical system
(sample) it provides necessarily 100 percent e.e. (enantiomeric
excess). This rule is an axiom of stereochemistry, which has
not been studied earlier. The present paper discusses conditions
when a single chiral molecule can appear in a chemical
sample and analyses consequences of such situations. One
of the most important consequences is, that the origin of
biological chirality can be traced back to such systems in
combination with asymmetric autocatalysis
Single-molecule chirality
No full textIf a chiral substance is prepared from achiral precursors, the first chiral molecule represents obligatorily 100 percent enantiomeric excess. Similar cases can be identified at the degradation of a racemate, for the last chiral molecule and the one-molecule excess obligatorily formed if a racemate contains an odd number of molecules. These cases have a particular significance in speculations regarding the origins of biological chirality and the molecular-level events in chiral autocatalysis. The paper gives a short summary of these problem
Diastereoselection through chiral conformations
No full textChiral conformations of flexible molecules may develop in a concerted manner if the molecule is crowded enough to assure sufficient level of through-the-space contacts. Higher number (>4) of groups connected to the same atom, as in many coordination compounds, can be advantageous in this resp ect. The case study of R,S-[(sec-butoxycarbonyl)methyl]cobalt tricarbonyl triphenylphosphine is presented here. X-ray diffraction shows that the possible number of enantiomeric and diastereomeric conformations is reduced by 75% (from 8 to 2) by concerted development of the molecular conformations in crystalline phase
Surface organometallic chemistry : facile mu2-carbene to mu3-carbyne transformation of organocobalt carbonyls on silica surface
No full textmu2-[ROC(O)C(R)]-mu2-(CO)Co2(CO)5 (R = Me, i-Pr, t-Bu, Ph) bridging carbene-type dinuclear Co carbonyls (1) undergo a facile surface-mediated clusterification while chromatographed on silica gel, yielding [mu3-RC(O)OC]Co3(CO)9 (2a) complexes. These latter are then transformed to the corresponding (mu3-RC)Co3(CO)9 (2b) derivatives by silica-mediated decarboxylation. The x-ray diffraction structure of 2a (R = t-Bu) was determined and that of 2b (R = Ph) was confirmed
Astrobiology and Biological Chirality
No full textThe emerging discipline of astrobiology could gain valuable support from research dealing with the problems of biological chirality. The most profitable fields of common interest are: (a) living organisms under extraterrestrial conditions, (b) extraterrestrial signatures of life and (c) origin(s) of biological chirality. These areas of complementary and overlapping fields are analysed on the basis of selected references
Synthesis and X-ray powder diffraction characterization of (OC)2RhCl2Rh(cod) (cod = cycloocta-1,4-diene)
No full textIn order to elucidate the nature and the structure of the elusive (OC)2Rh(Ph3SiO)2Rh(COd) (cod) = cycloocta-1,5-diene) complex, an important model compound for surface catalysis, (OC)2RhCl2Rh(cod) has been synthesized, and structurally characterized by ab initio X-ray powder diffraction. Crystals of (OC)2RhCl2Rh(cod) are monoclinic, space group P21/c, a = 6.659(1), b = 12.274(1) and c = 16.096(1) Å, β= 92.176(5)°, Z = 4, ρcalc = 2.209 g cm-3. The structure has been solved, from powder diffraction data only, by Patterson and Fourier-difference methods and has been ultimately refined, by the Rietveld method, down to Rp = 0.116 and Rwp = 0.154 for 4050 data points collected in the 12-93° (2θ) range. The molecule contains two square-planar rhodium atoms, one bearing two terminal carbonyls and the other bound to the chelating cod fragment, and two chlorine atoms bridging the Rh⋯Rh vector. The Rh2Cl2 core is markedly non-planar, the dihedral angle about the Cl⋯Cl hinge being 135.4(6)°
First Molecules, Biological Chirality, Origin(s) of Life
No full textOrigin(s) of biological chirality appear(s) to be intimately connected to origin(s) of life. Prebiotic evolution toward these important turning points can be traced back to single chiral molecules. These can be small (monomeric) units as amino acids or monosaccharides or oligomers as oligo-RNA type molecules. Earlier speculations about these two kinds of entries to biological chirality are critically reviewe
Synthesis and X-ray powder diffraction characterization of (OC)(2)RhCl2Rh(cod) (cod = cycloocta-1,4-diene)
No full textIn order to elucidate the nature and the structure of the elusive (OC)(2)Rh(Ph3SiO)(2)Rh(cod) (cod)=cycloocta-1,5-diene) complex, an important model compound for surface catalysis, (OC)(2)RhCl2Rh(cod) has been synthesized, and structurally characterized by ab initio X-ray powder diffraction. Crystals of (OC)(2)RhCl2Rh(cod) are monoclinic, space group P2(1)/c, a = 6.659(1), b = 12.274(1) and c = 16.096(1) Angstrom, beta = 92.176(5)degrees, Z = 4, rho(calc),,,, = 2.209 g cm(-3). The structure has been solved, from powder diffraction data only, by Patterson and Fourier-difference methods and has been ultimately refined, by the Rietveld method, down to R-p, = 0.116 and R-wp, = 0.154 for 4050 data points collected in the 12-93 degrees (2 theta) range. The molecule contains two square-planar rhodium atoms, one bearing two terminal carbonyls and the other bound to the chelating cod fragment, and two chlorine atoms bridging the Rh ... Rh vector. The Rh2Cl2, core is markedly non-planar, the dihedral angle about the Cl ... Cl hinge being 135.4(6)degrees
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