Showing papers on "Steric effects published in 1975"

Ternary Cu2+ Complexes: Stability, Structure, and Reactivity

Abstract: The driving forces leading to the formation of ternary Cu2+ complexes are outlined; they are (i) statistical reasons, (ii) neutralization of charge in the mixed-ligand complexes, (iii) steric factors (bulky groups, ring size of chelates), and (iv) formation of π bonds. The last point is important for achieving a large stability, as well as for the observation of discriminating qualities. An additional stability increase may possibly be achieved through a direct interaction between the two ligands bound to the same metal ion, i.e. Schiff base formation, hydrogen bonding or a charge-transfer relation. The latter leads to distinct structures, i.e. metal ion-bridged charge-transfer complexes. The relation between stability and structure on the reactivity of mixed-ligand complexes is emphasized. The relevance of the results for mixed-ligand complexes containing metal ions other than Cu2+ is briefly outlined.

Vicinal C,H spin coupling in substituted alkenes. Stereochemical significance and structural effects

Abstract: Vicinal C,H spin coupling (3JC,H) in substituted alkenes has been investigated systematically. Emphasis is laid on the stereochemical significance (Jtrans/Jcis) and on the various structural factors which influence 3JC,H, such as π-bond order, torsional angle ϕ, bond angle θ, electronegativity of substituents and steric effects. A new type of γ-effect is observed in 3JtransC,H which appears to have the same origin as the γ-shift effect. By comparison of 3J and 3JH,H, it was found that the relation 3JC,H ≈ 0·6 3JH,H holds for both trans and cis coupling constants. Finally, it is concluded that 3JC,H constitutes a valuable criterion to distinguish E- and Z-isomers, particularly in trisubstituted alkenes. Applications to natural products are presented.

Stereochemistry of the Bucherer–Bergs and Strecker Reactions of 4-tert-Butylcyclohexanone

Abstract: The spirohydantoin (α-isomer) obtained from 4-tert-butylcyclohexanone by the Bucherer–Bergs reaction has been shown by n.m.r., u.v., and considerations of steric hindrance to have the tert-butyl gr...

Pyrolysis of aryl azides. III. Steric and electronic effects upon reaction rate

Abstract: First-order rate constants have been measured for the pyrolysis of 15 phenyl azides in decalin solution. The rate for phenyl azide is increased only slightly by all para and many ortho substituents; in these cases Eact and ΔSact values are related linearly. The very large rate increases when the ortho substituent is phenylazo, nitro, acetyl or benzoyl cannot be from steric or normal electronic effects and therefore identify a specific involvement of these groups in the transition state. This rate enhancement is reduced to scarcely significant levels by a 6-chloro or 6-methyl group in 2- nitrophenyl azide, but not by a 6-nitro group. These results raise doubts about recent claims1 to establish mechanism by measuring polar effects on rates of pyrolysis of azides in which steric effects might also operate.

Steric hindrance in potassium atom-oriented molecule reactions. Methyl iodide and tert-butyl iodide

Abstract: The reaction of K atoms with oriented CHJ or t-CqH9I molecules has been studied via the crossed molecular beam method. Oriented molecules are produced by passing a molecular beam through an inhomogeneous electric field which rejects unwanted orientations. The remaining molecules are oriented with respect to a weak electric field and can be reversed in the laboratory by changing the direction of the applied field, The reaction is studied for impact a t the two ends of the molecule and for both reactions the iodine end is most reactive. A simple model is used to interpret the results and suggests that the hindering size which can be ascribed to the R groups is only roughly compatible with van der Waals radii. Very few gas phase bimolecular chemical reactions proceed on every gas kinetic-collision. Most reactions have an activation energy which (presumably) restricts reaction to those collisions with energy greater than the activation energy, E,. But counting only those collisions with energy greater than E , still gives a rate faster than the rate of almost any chemical reaction. To account for this discrepancy between theory and fact the notion was advanced that only certain orientations of the reagents were effective in promoting reaction, and the “steric factor”, p , was introduced as the fraction of gas-kinetic collisions which had the right orientation to react. Journal of the American Chemical Society / 97:7 / April 2, 1975

Restricted Rotation Involving the Tetrahedral Carbon. XIV. Conformational Equilibria and Attractive Interactions in Substituted 9-Benzyltriptycenes

Abstract: Several 9-benzyltriptycene derivatives were prepared by addition of benzynes to 9-benzylanthracenes. Rotation about the C9–Cbenzyl bond of compounds with a substituent at a peri-position to the benzyl group was found to be frozen at low temperatures on the NMR time scale but not at room temperature. In contrast, compounds with two substituents at two peri-positions to the benzyl group showed a frozen rotation about the C9–Cbenzyl bond at room temperature. Distribution of the conformers, as judged from the PMR intensities, showed preference to the dl-isomers in spite of the fact that they are the ones which are highly disfavored by steric effects. Interpretation of the phenomenon was based on the fact that attractive interactions, mainly charge-transfer type, exist between the substituted benzo group in the triptycene skeleton and the benzene ring in the benzyl group.

Cyclic peptides of sarcosine. Syntheses and conformation.

Abstract: A series of cyclic peptides of sarcosine with the general formula c-Sar-n, n=2-8, has been synthesized and conformational studies carried out both in solution and in the solid. The rings are conformationally very homogeneous and contain both cis and trans amide bonds. Their barriers to ring inversion are high; in the smaller rings this is attributed to steric hindrance, caused by the N-methyl-groups, whilst in the larger rings the folding of the chain in helical segments plays an important role.

Stereoselectivity in the Reactions of Nitrosopiperidine Carbanions. Steric vs. Stereoelectronic Control

Abstract: The anion of N-nitroso-4-phenylpiperidine reacts with methyl iodide, carbon dioxide, and benzophenone to give solely the axial substitution product. A second methylation of the monomethyl derivative gave the 2,6-diaxial derivative. None of the isomer having trans methyl groups could be detected. This trans isomer dominates over cis by 75:25 at equilibrium. From a consideration of the steric interactions present in these nitrosamines, it can be concluded that they alone cannot explain the observed high stereoselectivity. It is proposed that stereoelectronic control contributes significantly to the exclusive axial attack.

Hindered rotation in thioureas: Steric effects and conformations

Abstract: The temperature dependent spectra of several mono-, di- and trialkylthioureas have been recorded. Free energy barriers to internal rotation about the CN bonds have been calculated. In thioureas that were unsymmetrically substituted, free energy barriers were found to be different for each CN bond with the more substituted amino group exhibiting the higher barrier. The monosubstituted thioureas showed different rotational barriers for the NH2 groups of the cis and trans isomers, respectively. The free energy barriers for the trans isomers were found to be substituent dependent and substantiate the reassignment of the high and low field substituent signals to the trans and cis isomers, respectively. The spectrum of 1-methyl-3-t-butylthiourea indicates restricted rotation of the t-butyl group at temperatures below 200 K.

Pyridine derivatives as complexing agents x. thermodynamics of complex formation of n, n'-bis-(2-pyridylmethyl)-ethylenediamine and of two higher homologues

Abstract: The complex formation of Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ and Ag+ with three amines of the type with n values from 2 to 4, has been investigated. The stability constants and the heats evolved by formation of the 1:1 complexes have been determined. The thermodynamic data are discussed in relation of the preferred stereochemistry of the cation and of the steric requirements of the ligands using also visible spectra of solutions, i.r. and reflection spectra of solid complex salts.

On the Synthesis of Arylacetones by the SRN 1 Arylation of Acetone Enolate Ion

Abstract: Numerous aryl bromides and iodides react with acetone enolate ion in liquid ammonia under irradiation to form arylacetones in high yield. This synthesis is successful with bromo-or iodobenzene derivatives carrying alkoxy, alkyl, phenyl, halogen, and carboxylate substituents, and with halogen derivatives of polynuclear aromatic hydrocarbons. The method is remarkably insensitive to steric hindrance ; for example, 2, 4, 6-triethylbromobenzene reacts quite well. With greater steric hindrance, as in 2, 4, 6-triisopropyliodobenzene, reactivity falls and a side reaction of dehalogenation becomes appreciable, for reasons which are suggested. The synthesis was unsuccessful with the diethylamino, nitro and ionized hydroxy (-O-) substituents. Potassium metal-stimulated reactions of a few aryl diethyl phosphates with acetone enolate ion give generally lower yields of arylation and larger yields of dephosphation (hydrocarbon) products, compared even to potassium-stimulated reactions with aryl bromides. It is postulated that the lesser formation of hydrocarbon products from the aryl bromides is related to transport effects and solution inhomogeneity.

Effects of 3-substituents upon orientation in the 1,3-dipolar cyclo-addition reaction between 3-substituted pyridine N-imides and ethyl propiolate: syntheses of ethyl 4- and 6-substituted pyrazolo[1,5-a]-pyridine-3-carboxylates

Abstract: Quantitative data are presented relating to the effects of a series of 3-substituents upon orientation in the formation of ethyl pyrazolo[1,5-a]pyridine-3-carboxylates by a 1,3-dipolar cycloaddition reaction between 3-substituted pyridine N-imides and ethyl propiolate. The observed regioselectivity is discussed in terms of electronic and steric factors as well as hydrogen-bond formation.

Steric effects in singlet and triplet electronic energy transfer to azo compounds

Abstract: Rate constants have been determined for the solution-phase quenching of singlet and triplet excited states of a vari- ety of sensitizers (aromatic hydrocarbons, aryl and alkyl ketones, a-diketones) by a series of azo compounds with varying steric properties. Both singlet and triplet quenching are attributed to electronic energy transfer by the collisional, electron- exchange mechanism. Steric hindrance to exothermic triplet energy transfer is significant, with azo-n-butane a better accep- tor than azo-tert-butane by a factor of 3.6-10.7. Steric hindrance to singlet energy transfer is less pronounced (comparable steric factors are 1.7-2.9), apparently because diffusion rather than energy transfer is rate limiting in solution. For a given donor-acceptor pair, singlet energy transfer is two to three times faster than triplet energy transfer. The triplet energy level of the azobutanes is estimated to be 53 f 1 kcal/mol.