Showing papers on "Steric effects published in 1973"

Steric inhibition of resonance studied by molecular photoelectron spectroscopy. Part 3.—Anilines, phenols and related compounds

Abstract: Photoelectron spectroscopic data for amino- and oxy-benzenes are discussed in terms of a simple p-π interaction scheme. Steric hindrance is shown only to be important in ortho substituted NN-dialkylanilines and in 2,6-dimethylanisole. In the nitrogen series there is indication that the N atom is not planar.

Steric inhibition of resonance studied by molecular photoelectron spectroscopy. Part 2.—Phenylethylenes

Abstract: Some aspects of the HeI photoelectron spectroscopy of substituted phenylethylenes and related molecules are presented. The interpretation of the variation of the photoelectron π ionization energies with substituents allows the separation of steric from electronic effects. Interaction of semi-localised π molecular orbitals incorporated within a Huckel type theory provides a means of estimation of the dihedral angles.

Electrochemical oxidation of organometallic complexes. Carbene and Lewis base complexes of chromium, molybdenum, and tungsten carbonyls

Abstract: Tha oxidative one-electron transfer reactions of a wide variety of Group VI metal carbonyl derivatives have been detected by voltammetry. The compounds studied are of the type M(CO)6 –nLn or M(CO)6 – 2n(LL)n(M = Cr, Mo, W; L = monodentate Lewis base, n= 1 or 2; LL = bidentate Lewis base, n= 1 or 2) and carbene complexes of the type M(CO)5C(X)Y. The value of the potential, E½, is influenced by each of the variables M, L(or LL), n, X, and Y. The order of E½-values follows closely the apparent π-acceptor/σ-donor ratio of the ligand, but comparison with the results of molecular orbital calculations suggests that the influence of L (or of {C(X)Y}) upon the redox orbital is indirect. Steric effects are shown to influence the value of E½, especially where bidentate ligands are present. The oxidation potentials are related to synthetic chemistry in these systems.

Steric effects in 31P NMR spectra: ‘Gamma’ shielding in aliphatic phosphorus compounds

Abstract: An examination of published 31P NMR spectral data for aliphatic phosphorus compounds has revealed that chain lengthening and branching effects on the chemical shift can be interpreted in terms similar to those used for 13C and 15N shifts. For six families of phosphorus compounds, a β-carbon substituent was shown to deshield phosphorus, while a γ-carbon caused shielding. The effects are additive, and good agreement was obtained between 31P shifts calculated with the appropriate constants and the experimental values. Shielding by γ-carbon is indicative of the operation of a steric influence on 31P chemical shifts, not heretofore articulated. The γ-effect is also useful in explaining the unusually large shielding found in six-membered cyclic phosphines.

Stereochemical and Steric Effects in Nucleophilic Substitution of α-Halo Ketones

Abstract: Bimolecular substitution reactions of α-halo ketones with acetate and azide ions are remarkably insensitive to steric hindrance. 1-Bromo-4,4-dimethyl-2-pentanone (bromomethyl neopentyl ketone) is o...

Isoxazoline derivatives. Part VI. Regioselectivity in the 1,3-dipolar cycloaddition of nitrile oxides to αβ-unsaturated ketones

Abstract: 1,3-Cycloadditions of nitrile oxides to αβ-unsaturated ketones have been studied. In most cases a mixture of the two regioisomeric 3-substituted 4- and 5-acyl-Δ2-isoxazolines was obtained. The individual compounds were characterised by chemical and/or spectroscopic methods. Effects of steric and electronic factors on the cycloadditions are discussed.

Carbon-13 chemical shifts of substituted nortricyclenes

Abstract: 13C NMR spectra of a series of substituted nortricyclene derivatives have been measured, and the 13C chemical shifts interpreted in terms of α, β, γ and δ-effects. The nature of these substiuent shifts is discussed together with some analytical possibilities. The substituent shifts provide valuable data about the steric effects in strained molecules and can be used as increments for structural analysis, particularly for the determination of orientations of substituent groups.

Optically active complexes of Schiff bases. Part II. Complexes of cobalt(II) with tetradentate Schiff bases and their reactivity with oxygen

Abstract: Complexes of cobalt(II) with Schiff bases obtained by condensation of salicylaldehyde with substituted ethylenediamines have been synthesised together with five-co-ordinated pyridine adducts. E.s.r., electronic, and c.d. spectra and magnetic susceptibilities have been reported. Energy level schemes have been calculated by a ligand field approach in order to fit the magnetic properties and allow an assignment of the observed absorption bands. The reaction with oxygen has been studied and discussed in terms of both the electron configuration of the cobalt atom and the steric effects of substituents on the carbon atoms of the substituted ethylenediamines.

Conformational studies on pyrimidine 5'-monophosphates and 3',5'-diphosphates. Effect of the phosphate groups on the backbone conformation of polynucleotides

Abstract: In continuation of our studies on the effect of the base and the phosphate groups on the glycosyl and the sugar-phosphate backbone conformation, we have carried out semi-empirical potential energy calculations on the common 5'- and 3'5'-ribopyrimidine mono- and diphosphates by considering simultaneous rotations about the glycosyl (χ ) and the C(4')-C(5') (ψ ) bonds. This calculation provides an assessment of the nature and orientation of the base on the sugar-phosphate backbone conformation of nucleotides and polynucleotides. It is found that the attractive inetractions between the 5'-phosphate group and the base mutually stabilize the antiand the gauche-gauche (gg) conformations about χ and ψ , respectively, in 5'-ribopyrimidine nucleotides. The introduction of the 3'-phosphate group as in 3',5'-ribopyrimidine diphosphates, still leaves the anti-gg as the most favored conformation with the important difference that the probability of occurrence of the anti, gauche-trans (gt) is how substantially increased. This is dependent to a large extent on the sugar conformation and to a lesser extent on the base. Uracil and thymine show a greater probability for the anti-gt than cytosine. The syn conformation is considerably less likely and its occurrence is also dependent on the base type, cytosine showing a lesser tendency than uracil and thymine. For the syn base, the most favourec conformation for ψ is gt, since gg is sterically disallowed and tg is destabilized by electrostatic repulsive interactions between the 3' and 5'-phosphate groups. Thus, there is a striking correlation between the glycoysl and the backbone C(4')-C(5') bond conformations. The rest of the bonds of the backbone are considerable less dependent on the glycosyl conformation. These studies reveal that in poly-ribopyrimidine nucletides the majority of the nucleotide residues are expected to occur in the anti-gg conformation.

Organo-transition metal chemistry without phosphines: the palladium catalysed cyclotrimerization of acetylenes

Abstract: The argument is presented that organo-transition metal complexes also containing phosphines and similar ligands are often too kinetically inert to act as good models for catalytic processes. A new generation of ‘lightly stabilized’ complexes are needed to probe these reactions more deeply. Some novel palladium complexes of this type, derived from the reactions of acetylenes with either Pd 0 or Pd 11 , are described and the mechanisms of acetylene oligomerization are discussed. For Pd 0 electronic factors seem to be of greatest importance while steric factors play a greater role with Pd 11 . The structure and properties of a new complex, (Ph 3 P) 2 Pd(Br)[(CCOOMe) 4 H], derived from dimethyl acetyl-enedicarboxylate, are reported as well as that of [C 5 Me 5 CHPhCH 2 PdCl] 2 , derived from but-2-yne, PdCl 2 , and diphenylmercury. The latter is a dihapto-cyclopentadiene complex which shows stereochemical non-rigidity in solution. Since this complex also possesses a Pd-alkyl bond it is an ideal ‘lightly-stabilized’ model and its reactions with acid, base, hydrogen, CO, and triphenylphosphine have been investigsted. This work sheds important new light on the chemistry of the Pd—C σ-bond.

Influence of Lewis Acids on the Diels–Alder Reaction. V. The Reaction of Furan with Dimethyl Acetylenedicarboxylate

Abstract: The Diels–Alder addition of furan to dimethyl acetylenedicarboxylate (DMAD) affords a monoadduct 1. The position and steric course of further addition of furan to the latter are temperature depende...

Crystal and molecular structure of chloro(2,2′,2″-terpyridine)palladium(II)chloride dihydrate, C15H15Cl2N3O2Pd

Abstract: The crystal and molecular structure of chloro(2,2′,2″-terpyridine)palladium(II)-chloride dihydrate, C15H15Cl2N3O2Pd, has been determined by single crystal X-ray diffraction techniques. The crystals are orthorhombic: space groupPna21,a = 17·217(9),b = 14·026(8),c = 6·801(5) A,Z = 4. Least-squares refinement of 2064 non-zero manually gathered diffractometer reflexions was carried out to a conventional unweightedR of 0·069. The molecule consists of a basically planar 4-coordinate [Pd(terpy)Cl]+ cation and a Cl− anion. The molecules are packed in a layer structure in which the average separation is about 3·4 A, the closest approach being 3·13 A between a palladium atom and one of the nitrogen atoms of a neighboring molecule. Bond distances are within acceptable limits of predicted values. Deviations from planarity in the coordinated terpyridine ligand are presumably due to its steric limitations and are similar to those found in other terpyridine complexes.