Showing papers on "Steric effects published in 1982"

The Unambiguous Specification of the Steric Course of Asymmetric Syntheses

Abstract: uous specification of the steric course and the product configuration of diastereoselective reactions. The reflection-invariant relative topicity of approach of reactants is defined as like {Ik) and unlike (ul) if the corresponding descriptor pairs are Re*,Re*. or R*,Re*, and Re*,Si* or R*,Si*, respectively. The descriptor pair notations (Ik and uc) of reactants disclose related steric courses of reactions more often than do the relative configurations of their products, for which the configurational notation I= R*,R* and u= R*,S* is proposed. The advantage of specifying the relative topicity is demonstrated by means of a series of recent examples of importance to the synthetic organic chemist taken from the literature and from our own work. 1. Introduction-The Problem Complex natural products such as macrolides, ionophores, and related compounds have recently become target molecules for synthetic organic chemists[’]. The often formidable array of asymmetric carbon atoms in the structures of these compounds[21 has stimulated the development of highly stereoselective asymmetric syntheses. Foremost amongst these are the ald~l[~-~], nitr~aldoF“~], and Michael additions[81. In all these reactions two planar trigonal carbon atoms are linked together to create a pair of asymmetric tetrahedral carbon atoms [route (a) in Scheme 11. Stereoselective additions to 01efins[~] and to trigonal carbon atoms a to an asymmetric carbon [routes (b) and (c), respectively, in Scheme 11, give the same type of products 1L7.9-121

The methylene bridge: a challenge to synthetic, mechanistic and structural organometallic chemistry

Abstract: Within the past few years transition metal complexes containing bridging alkylidene functions (Fig. 1) have stimulated wide—spread interest in organometallic chemistry, due at least in part to the apparent signifi— cance of methylene intermediates in the metal—surface catalyzed reduction of carbon monoxide. Following the preparation of the first unsubstituted t—methylene complex in 1974, we have continued our investigations into the synthesis, structures, and reactivity of dimetallacyclopropane—type compounds. An overview of these results will be presented. The most versa— , tile synthetic method for the construction of ii—alkylidene R R systems is carbene transfer from easily accessible, highly reactive diazoalkane precursors to either mononuclear, / \ , coordinatively unsaturated substrates, or complexes with M M metal—metal multiple bonds. The structural features and the reactivity pattern of the resulting dimetallacyclopropanes Figure 1 are governed by both steric and electronic effects arising from the nature of the metals, the methylene bridges as well as the peripheral ligands. Emphasis will also be placed on the spectro— scopic characteristics and the bonding situation of simple methylene bridges.

Gas-phase reactions of rhodium(+) ion with alkanes

Abstract: The gas-phase reactions of laser-desorbed Rh/sup +/ with various alkanes are examined using Fourier transform mass spectrometry (FTMS). The secondary reactions of these product ions are also reported. In contrast to the first-row group 8 transition metal ions whose reactions with alkanes show extensive cleavage of C-C bonds, Rh/sup +/ shows few products resulting from C-C cleavage, and dehydrogenation is the major process in all reactions. Evidence is given that suggests the existence of a ..pi..-allylic intermediate, and some dehydrocyclization is indicated in our results. Ligands with molecular formulas corresponding to trienes bound to rhodium show no further reaction which may be caused by either steric or electronic effects.

17O NMR investigation of p,π‐interactions in α,β‐unsaturated and aromatic ethers

Abstract: The 17O chemical shifts have been measured for 51 α,β-unsaturated and aromatic ethers. A good linear relationship is found between the 17O chemical shifts in a series of dialkyl and the corresponding alkyl vinyl ethers. Hence, the extent of p,π-interaction, between the oxygen atom and the vinyl group in the latter series does not, apparently, depend upon branching at the α-carbon atom in the alkyl moiety of these ethers. The PhOBut ether, however, as compared to the other alkyl phenyl ethers, shows significantly weakened p,π-interaction, which is apparently related to the steric hindrance of this interaction. The effects of two unsaturated groups upon the 17O chemical shifts in the corresponding ethers are non-additive. This is undoubtedly a result of ‘rivalry’ between these groups for conjugation with the lone electron pairs on the ethereal oxygen. The 17O chemical shift ranges of substituted methyl and vinyl phenyl ethers are nearly equal (≈30 ppm). An analysis of the 17O shielding for cyclopropyl ethers shows no observable p,σ-conjugation in these compounds. Excellent correlation (r>0.99) between the values of 17O chemical shifts and the calculated (MO LCAO SCF, CNDO/2) π-electron charges on the corresponding oxygen atoms look promising for experimental estimations of π-electron densities on the ethereal oxygen.

The amino group and steric factors in thiamin catalysis

Abstract: Pyruvate decarboxylase (PDC) belongs to those enzymes, that can be isolated easily from yeasts. Its mechanism therefore has been discussed and analyzed since Langenbeck made his first model experiments on the nonenzymatic decarboxylation of 2-0x0 acids by primary amines in 1930.' Its molecular weight has been estimated by Ullrich to 240,0002 .and the molecule obviously consists of two different types of subunits as shown first by Hopmann' in the case of the brewers yeast enzyme. FIGURE 1 shows a disc gel electropherogram of a preparation with an activity of about 80 U/mg, as is now used in our lab.4 The enzyme shows an interesting architecture, characterized by two diagonally crossed sausages, as observed in electron micrographs' and confirmed recently by X-ray small-angle scattering6 PDC needs thiamin pyrophosphate (TPP) and Mg2+ ions in order to show catalytic activity. Nature prefers Mg in the active centers of TPP enzymes, but this metal ion can be replaced with no reduction in activity by other bivalent ions such as Mn, Co, Zn, or Ca.' It is one of the open questions in the PDC mechanism so far, whether the metal ion is only engaged in the cofactor binding or whether it also takes part in the catalytic mechanism in any way. The kinetics of cofactor binding by the protein component has been studied in our lab very carefully by Hubner.' He succeeded in finding a mechanism in which the two low molecular components interact-each separately-with the associated protein molecule via reversible mechanisms (Kboth). Only in the presence of both species is an irreversible binding of the cofactors realized by the protein component. This process, showing first-order kinetics, is rather slowly (k = 0.45 min-') and directly dependent on the complex-forming rate of the different metal ions. I n accordance with TABLE 1, no dependence of the recombination rate on the protein concentration is observed. But in the presence of the substrate (pyruvate) the recombination is enhanced by about one order of magnitude. The native enzyme, which is obtained by taking TPP and Mg as cofactors, is rather stable a t optimum pH conditions (pH 6.2). If it is passed through a Sephadex (G 10) column for separation of the cofactors, there is no loss of activity-contrary to other TPP enzymes such as pyruvate dehydrogenease complex (PDH). On the other hand, labile PDC enzymes showing nevertheless a high catalytic activity can be obtained by replacing the native cofactors by other metal ions (such as CaZ+) or certain TPP analogues (such as the N,-pyridyl-analogue). The catalytic mechanism of TPP enzymes is mainly realized by the cofactor TPP (FIGURE 2). This kind of highly specialized cyanide ion obtains its typical behavior 51

Beiträge zur Chemie des Phosphors, 116 Kernresonanzspektren und Kristallstruktur von Tri-tert-butylcyclotriphosphan / Contributions to the Chemistry of Phosphorus, 116 NMR Spectra and Crystal Structure of Tri-tert-butylcyclotriphosphane

Abstract: A complete analysis of the ³¹P, ¹³C and ¹H NMR spectra of tri-tert-butyl-cyclotriphosphane (1) was carried out and the resulting NMR parameters were correlated with the X-ray data of the molecule. It was found, that increasing P-P-C-bond angles correspond to a downfield shift and an increase of the absolute values of the negative ¹J(PP) and ¹J(PC) coupling constants. Generally, the chemical shifts of cyclotriphosphanes as well as of larger cyclic phosphanes depend on the endo- and exocyclic bond angles, the dihedral angle between the electron lone pairs of adjacent P-atoms, and the β-effect. On the basis of the X-ray and chemical shift data of 1, the bond angles of other cyclotriphosphanes can be deduced from their δ(³¹P) values. 1 crystallizes triclinically in the space group P1 with Z = 4 formula units. The X-ray analysis confirms the NMR-spectroscopically determined three-membered ring structure with the tert-butyl substituents on either side of the ring plane. Due to steric hindrance between the two tert-butyl groups in cis position, the corresponding P-P-C-bond angles show the largest values so far observed for organylcyclophosphanes.

Multipole Model of the Molecular Asymmetry in Thermotropic and Lyotropic Liquid Crystals. Volume and Surface Effects

Abstract: The molecular asymmetry of mesogenic molecules is defined as a deviation of the corresponding molecular property from spherical and axial symmetry. Three types of asymmetry-electrical, steric and biphilic, are considered using a multipole representation. Two basic molecular shapes are included-rod-like and plate-like. The first three multipoles-monopole, dipole, and quadrupole, are involved. In the steric case a monopole does not exist. The effective shape of a molecule includes not only steric, but also electrical intermolecular repulsive interactions. The biphilic asymmetry is determined by the distribution of the hydrophilic and hydrophobic groups along the molecule. Three types of generalized fields—electrical, deformational, and biphilic, are also defined and their volume and surface sources are discussed. The effects of the molecular asymmetry on the intermolecular interactions in thermotropic (rod-like and disc-like) and lyotropic liquid crystals are qualitatively discussed in an analogy t...

Mesogenic Polymers. 5. 1Thermotropic Polyamide Liquid Crystals and Analogous Small Molecule Diamides

Abstract: The use of substituents (X # H) ortho to the amide nitrogen in both aromatic polyamides and analogous diamides renders these compounds to be enantiotropic mesogens by lowering the solid → mesophase temperature. At the molecular level this is understood as a reduction in interchain hydrogen bonding through either a steric blocking effect and/or through formation of intramolecular hydrogen bonds (when X = C1, OCH3). DSC, optical microscopy and infrared data are presented for the eight compounds reported.

Inductive, steric, and environmental effects in the nonaqueous electrochemistry of hexakis(aryl isocyanide)chromium complexes

Abstract: The electrochemical behavior of a series of hexakis(aryl isocyanide)chromium(I) complexes has been studied by cyclic voltammetry. The inductive effects of para substituents on E/sup 0/' for the (0 reverse arrow..-->.. 1+),(1+ reverse arrow..-->.. 2+), and (2+ reverse arrow..-->.. 3+) couples have been found to give linear correlations with the Hammett sigma/sub p/ parameter. Solvent and supporting-electrolyte effects have also been investigated and are discussed in terms of donor/acceptor theory and ion-pairing effects, respectively. The introduction of bulky ortho substituents on the aryl isocyanide ring was found to dramatically influence both the E/sup 0/'/sub 3+///sub 2+/ and the substitutional lability of the Cr(CNR)/sub 6//sup 3 +/ complexes. These effects are attributed to significant weakening of the Cr/sup III/-C bonds in the ortho-substituted complexes relative to the para-substituted complexes due to steric interactions among the ortho substituents.

Structure and conformation of dihydroaromatic compounds. 1-Alkyl-1,4-dihydrobenzenes, 1-alkyl-1,4-dihydronaphthalenes, and 9-alkyl-9,10-dihydroanthracenes

Abstract: The molecular geometries of 1-alkyl-1,4-dihydrobenzenes, 1-alkyl-1,4-dihydronaphthalenes, and 9-alkyl-9,10-dihydroanthracenes have been evaluated by empirical force-field calculations. Unlike the parent dihydroaromatics, the alkyl-substituted hydrocarbons all exhibit optimum geometries that are nonplanar, and the distortion from planarity increases with the steric bulk of the substituent. In contrast to earlier suggestions of a boat-to-boat equilibrium, all of the 1,4-dihydrobenzenes studied have a single energy minimum in which the substituent occupies a pseudoaxial position. Several of the dihydronaphthalene and dihydroanthracene derivatives exhibit two distinct nonpolar energy minima, but the pseudoaxial conformation is much more stable than the pseudoequatorial form in each case. Consequently, even these compounds can be considered to exist in a single conformation.