Showing papers on "Steric effects published in 1970"

Nuclear magnetic resonance spectroscopy. Carbon-13 chemical shifts in norbornyl derivatives

Abstract: The ^(13)C chemical shifts for a number of norbornyl derivatives have been obtained in natural abundance. The compounds studied were the series of methylnorbornanes, methylnorbornenes, methyl-2-norbornanones, methyl-2,2-difluoronorbornanes, and a variety of exo- and endo-2-substituted norbornanes. The observed shifts have been interpreted in terms of inductive, bond length, and steric effects. The ɑ- and β-substituent effects are shown to be dependent on the extent of substitution at both the α and β carbon atoms and changes in bond lengths have been used to rationalize the shift changes. Steric interactions between the endo-2 and endo-6 positions, and the exo-2 and syn-7 positions have been shown to generate important chemical- shift changes and these changes can be used as a semiquantitative measure of steric interactions in norbornyl derivatives. Studies with electron-withdrawing groups have revealed a new long-range cmr y effect produced by exo-2 substituents on C-6. These shifts appear to provide a sensitive and quantitative measure of the electronic demands of a neighboring group as a function of stereochemistry. The substituent shifts obtained from the monosubstituted derivatives have been used to predict the shifts of each of the 1- and 3-methyl-2-norbornanols. The agreement between observed and calculated values is generally good and supports the general approach. Several ^(13)C-^(13)C coupling constants for norbornane, nortricyclene, and quadricyclene are reported and interpreted in terms of the s character of the C-C bonds.

Relations Between Structure and Reactivity in Free‐Radical Chemistry

Abstract: An analysis of the relations between structure and reactivity in free-radical chemistry has shown that the usual interpretation of reactivities by means of the stability of the radicals involved is greatly simplified and often incorrect. The CX bond energies of the alkanes and simple alkyl derivatives can be explained qualitatively by strain effects in the ground state on the basis of the VSEPR theory and nonbonding interactions. To be able to explain reactivities in free-radical chemistry, it is necessary to deduce information about the geometry of the transition states during free-radical formation from experimental measurements. The relations between structure and reactivity in free-radical chemistry are interpreted in terms of bond dissociation energies, as well as polar and steric effects.

Location of proline derivatives in conformational space. I. Conformational calculations; optical activity and NMR experiments.

Abstract: In order to develop methods of analysis applicable to the determination of the conformation of biological polymers in solution, a series of proline derivatives was studied. The steric constraints of the pyrrolidine ring limit these compounds to a relatively small set of conformations. This set was further reduced by eliminating conformations with large computed conformational energy. Computations revealed that the conformational energy of the proline derivatives fits into one of three classes, depending on the bulk and the polarity of the C-terminal group. Three analogous classes of optical activity were observed. The optical activity data were analyzed in terms of conformations computed to be of low energy. In some cases qualitative theoretical considerations enabled molecular groups to be located. For example, solvent-dependent isomerization of the carboxyl hydrogen of N-acetyl-L-proline was detected. Nuclear magnetic resonance provided an experimental measure of the fraction of molecules which had cis unsymmetrically-substituted tertiary amide groups. This information aided and confirmed the other measures of molecular conformation.

Nuclear Magnetic Resonance Spectroscopy. Carbon-13 Spectra of Some Inositols and Their O-Methylated Derivatives

Abstract: The ^(13)C nmr spectra of some inositols and their partially O-methylated derivatives are tabulated and discussed. Methods of assignment are detailed. A system of empirical constants which can be used to estimate the spectra of the carbons of these ring systems is derived. The substituent effects are noted to conform generally to the notion that steric or proximity effects are very important sources of chemical-shift differences in these systems.

Additions to bicyclic olefins. III. Stereochemistry of the epoxidation of norbornene, 7,7-dimethylnorbornene, and related bicyclic olefins. Steric effects in the 7,7-dimethylnorbornyl system

Abstract: A systematic study of addition reactions of norbornene, 7,7-dimethylnorbornene, and related bicyclic olefins has been undertaken in order to define more precisely the role of steric effects in controlling the stereo- chemistry of the additions and the nature of the intermediates in such addition reactions. The addition of borane to norbornene proceeds almost exclusively exo (99.573, whereas the corresponding addition to 7,7-dimethylnor- bornene proceeds preferentially endu (78%). Similarly, hydroboration of 2methylenenorbornane gives pref- erentially exo (85%), whereas 2-methylene-7,7-dimethylnorbornane gives preferentially (85%) endo product. Similar results were realized with 1-methylnorbornene, 2-methylnorbornene, bornene, and 2,7,7-trimethylnorbor- nene. Consequently, hydroboration of norbornene, 1- and 2-methylnorbornene, and 2-methylenenorbornane goes predominantly exo, evidently reflecting the greater steric availability of the exu position in this bicyclic system. However, the presence of 7,7-dimethyl substituents causes the addition to proceed preferentially from the endu direction. Consequently, in hydroboration the 7,7-dimethyl substituents alter the normal direction of addition to olefins of the norbornane structure, irrespective of whether the double bond is endocyclic, directly under the 7,7- substituents, or exocyclic, located to the side of the substituents. major argument for the u-bridged norbornyl cation A is the almost exclusive exo substitution realized in the solvolysis of 7,7-dimethylnorbornyl derivatives. 334

Reactions of Nitrones with Free Radicals. II. Formation of Nitroxides

Abstract: Reactions of N-t-butylaldonitrones with free radicals gave stable nitroxides, one of which was isolated in pure state. The stability of the nitroxides was explained by the steric effect around the nitrogen and α-carbon atoms. In the reaction of 5,5-dimethyl-Δ1-pyrroline N-oxide with AIBN, nitroxide was detected by ESR technique, and 1,3-adduct and ketonitrone were isolated from the reaction mixture. From the results, it was confirmed that nitroxide is the intermediate of the 1,3-adduct formation and that a radical first adds to the carbon atom and then adds to the oxygen atom of the nitrones.

A uniform site theory of ziegler catalysis

Abstract: Polypropylene produced by the TiCl3–diethylaluminum chloride catalyst system was separated into two kinds of polymers, a minor atactic fraction with little or no steric order and a major fraction with a high degree of steric order. Each fraction had a wide molecular weight distribution. The relative amounts, the molecular weights and the molecular weight distributions of the fractions responded differently to changing reaction conditions. This indicates that the fractions are produced by separate catalysts and by different reaction mechanisms. Polymer/catalyst mole ratios from polymerizations at various conditions showed that a least one-half of the TiCl3 content of a catalyst may become directly involved in the polymerization process. A theory is developed which states that (1) the (unactivated) stereospecific fraction of the catalyst consists of molecules containing eight TiCl3 units which are separated from each other by AlCl3 in solid solution; (2) two chlorine atoms in this molecule are replaced with alkyl groups in the first step of activation; (3) the molecule contains two active sites, one d- and the other l-orienting; (4) the molecules are crystallized in an open structure which has equally reactive sites uniformly spaced; and (5) the AlCl3 component is not an inherent part of the stereospecific catalytic structure but it contributes to high activity by opening the crystal structure.

Crystal structure and stereochemistry of the solvated complex dichlorodipyridine-[o-(di-o-tolylphosphino)benzyl]rhodium(III)–0·61 chloroform: the trans-influence of ligands in octahedral complexes

Abstract: The complex [RhCl2(C6H5N)2{P(o-C6H4Me)2(o-C6H4CH2)}] crystallises from chloroform–ethanol as monoclinic crystals containing a monstoicheiometric amount of chloroform. The cell dimensions are a= 18·43, b= 10·22, c= 19·15, β= 112·4 ± 0·1°; the space group is P21/c with one molecule of metal complex per asymmetric unit.A three-dimensional X-ray analysis, based on 2430 independent diffractometer-measured reflections, has converged to a final weighted R of 0·077. The value of σ for the Rh–Cl and Rh–P bond distances is 0·004 A, and for Rh–C and Rh–N is 0·013–0·016A; within the pyridine and tolyl groups the mean σ for bond distances is 0·027 A.The steric requirements of the five-membered chelate ring and the methyl groups of the nonchelating tolyl groups give rise to several significant deviations from the C2v symmetry of the carbon skeleton of free toluene. The trans-influence exerted by a saturated carbon ligand is evidenced by the significant difference between the Rh–Cl bond lengths, one chlorine being trans to a pyridine (Rh–Cl 2·339 A), the other trans to the benzyl carbon (Rh–Cl 2·531 A). A general discussion of the trans-influence in octahedral complexes is provided.The molar ratio, within the crystal, complex:chloroform, determined from a least squares refinement of the chloroform group population parameter, is 1 :0·61 ± 0·02. Complete disorder of the solvents chloroform appears to be prevented by a weak hydrogen bond between the proton of the chloroform group and one of the chlorine atoms co-ordinated to the metal.

A ligand exclusion theory of allosteric effects.

Abstract: Cooperative and allosteric effects are explained by a theory involving not conformational changes but steric exclusion through interactions between multifunctional ligands which bind to common and topologically distinct subsites.

Intramolecular platinum–carbon bond formation promoted by steric hindrance

Abstract: Bulkyl substituents on tertiary phosphine ligands co-ordinated to platinum(II) are shown to promote internal platinum–carbon bond formation.

NN′-ethylenebis-(o-aminobenzylideneiminato)nickel(II) and its derivatives

Abstract: The title compound and several of its derivatives have been prepared; their properties, particularly with respect to geometry and magnetism, have been investigated. The compounds are found to be planar even when steric factors are unfavourable. The ligands are shown to exert a high ligand field, as large as if not larger than their salicylideneiminato-analogues. The preparation and properties of the parent diamino-diimino-compounds are described in the Appendix.

Kinetics of substitution of co-ordinated carbanions in cobalt(III) corrinoids

Abstract: Kinetic studies on the AgI- and HgII-assisted substitution of co-ordinated carbanions in CoIII corrinoids show that (i) AgI assists the removal of CN– while HgII assists the removal of organo-ligands, (ii) steric hindrance stabilises Et, Prn, and Pri, relative to Me, (iii) steric compression and/or electronic changes destabilise Pri relative to Prn, and (iv) 5,6-dimethylbenzimidazole (bzm) shows a trans-labilising effect relative to H2O.

Mesomeric Vinyl Cations. Part II. Solvolysis of 2‐bromo‐1, 3‐butadienes

Abstract: In contrast to 2-bromo-1, 3-butadiene (5), the di-, tri- and tetramethyl derivatives 6–11 show remarkable solvolytic reactivity. First order rate constants and products of these bromodienes in 80% ethanol have been determined. Rates are very sensitive to the ionising power of the solvent but are not affected by triethylamine. Three types of products are formed, i.e. alkenynes, α,β-unsaturated ketones and 4-ethoxyallenes. One methyl group on C-4 of 5 increases the reaction rate by a factor of ca. 102; two methyl groups increase the rate by ca. 104. Methyl substituents at C-1 have only a slight influence, apparently as a result of a rate-decreasing polar effect and a compensating rate-increasing steric effect. The results of this study are consistent with a unimolecular ionization mechanism involving a mesomeric vinyl cation 12a 12b as an intermediate.

Pyrrole studies. Part XV. Vilsmeier–Haack formylation of 1-substituted pyrroles

Abstract: The ratio of α- to β-formylated products obtained by the Vilsmeier–Haack reactions with 1-alkyl- and 1-aryl-pyrroles is controlled mainly by steric factors. Electronic effects of 1-(p-substituted aryl) substituents upon the position of formylation of the pyrrole ring are small and are essentially inductive.