Steric effects

About: Steric effects is a research topic. Over the lifetime, 16112 publications have been published within this topic receiving 319615 citations. The topic is also known as: steric hindrance.

Mechanism of homogeneous Ir(III) catalyzed regioselective arylation of olefins.

Abstract: The mechanism of hydroarylation of olefins by a homogeneous Ph-Ir(acac)_2(L) catalyst is elucidated by first principles quantum mechanical methods (DFT), with particular emphasis on activation of the catalyst, catalytic cycle, and interpretation of experimental observations. On the basis of this mechanism, we suggest new catalysts expected to have improved activity. Initiation of the catalyst from the inert trans-form into the active cis-form occurs through a dissociative pathway with a calculated ΔH(0 K)^⧧ = 35.1 kcal/mol and ΔG(298 K)^⧧ = 26.1 kcal/mol. The catalytic cycle features two key steps, 1,2-olefin insertion and C−H activation via a novel mechanism, oxidative hydrogen migration. The olefin insertion is found to be rate determining, with a calculated ΔH(0 K)^⧧ = 27.0 kcal/mol and ΔG(298 K)^⧧ = 29.3 kcal/mol. The activation energy increases with increased electron density on the coordinating olefin, as well as increased electron-donating character in the ligand system. The regioselectivity is shown to depend on the electronic and steric characteristics of the olefin, with steric bulk and electron withdrawing character favoring linear product formation. Activation of the C−H bond occurs in a concerted fashion through a novel transition structure best described as an oxidative hydrogen migration. The character of the transition structure is seven coordinate Ir^V, with a full bond formed between the migrating hydrogen and iridium. Several experimental observations are investigated and explained: (a) The nature of L influences the rate of the reaction through a ground-state effect. (b) The lack of β-hydride products is due to kinetic factors, although β-hydride elimination is calculated to be facile, all further reactions are kinetically inaccessible. (c) Inhibition by excess olefin is caused by competitive binding of olefin and aryl starting materials during the catalytic cycle in a statistical fashion. On the basis of this insertion-oxidative hydrogen transfer mechanism we suggest that electron-withdrawing substituents on the acac ligands, such as trifluoromethyl groups, are good modifications for catalysts with higher activity.

Progress in the stereospecific polymerization

Abstract: Results of new research work in the field of polymers with an ordered steric structure, obtained by stereospecific polymerization, are reported Some of the results concern the relationship between steric structure and crystallinity; from these it results that a regular steric structure is a necessary, but not a sufficient condition for the crystallizability of the polymers In the isotactic polymers of CH2=CHR type monomers, the shape and size of the R group not only determine the different conformation of isotactic helices (with a number of monomeric units per pitch varying form 3 to 4) but in some particular cases are responsible for the lack of crystallinity It is possible, through chemical processes which modify the shape and size of the R substituent, to obtain crystalline polymers from isotactic, amorphous, not crystallizable polymers The crystalline structure of syndiotactic polypropylene is described for the first time and is compared with the structure of other syndiotactic polymers Furthermore the structure of crystalline polymers of “di-isotactic type”, obtained polymerizing RCH=CHR1 type monomers is discussed; and a nomenclature for the different possible stereoisomers is suggested Cases of polymorphism, isomorphism and di-isomorphism in the macromolecular field are examined and a new type of isomorphism among different monomeric units is discussd Crystalline copolymers of isomorphous monomeric unit is discussed Crystalline copolymers of isomorphous monomeric units have a melting point which is intermediate between the ones of the corresponding pure homopolymers The effects which influence the steric purity of polymers having an ordered structure are discussd and the influence of steric purity of the physical properties and mainly on the mechanical properties of the polymers (especially for the case of polybutadiene obtained at the Polytechnic of Milan with a percentage of cis 1,4 units higher than 99%) is pointed out

Rules governing the crystal packing of mono- and dialcohols.

Abstract: A new program, Mercury, has been used to survey 144 monoalcohol (CmHnOH) and 101 dialcohol [CmHn(OH)2] crystal structures. Results show that their hydrogen-bonding patterns are strongly correlated with steric effects. Primary monoalcohols have a strong preference to form infinite ⋯OH⋯OH⋯ chains. Secondary monoalcohols form chains and rings of hydrogen bonds with about equal facility. Tertiary monoalcohols very often form isolated OH⋯O hydrogen bonds or structures containing no OH⋯O hydrogen bonds at all. In the latter case, however, the structures almost invariably contain CH⋯O and/or OH⋯π interactions. Substitution on the β-carbon(s) of monoalcohols has a profound effect on packing patterns, with increased substitution disfavouring chains and rings. Dialcohols show a much stronger preference for chains of hydrogen bonds, compared with monoalcohols. This is particularly so when at least one of the hydroxyl groups is primary, in which case chains are overwhelmingly preferred. Once again, substitution on the β-C atoms is influential, heavy substitution tending to lead to packing arrangements that involve isolated or intramolecular OH⋯O hydrogen bonds. Dialcohols almost never crystallize without at least some OH⋯O hydrogen-bond formation. In both monoalcohols and dialcohols, chains show a stronger preference to be helical (usually threefold helices) as steric hindrance increases. Hydrogen-bonded rings usually contain four OH⋯O hydrogen bonds. It is possible that empirical observations such as these may aid crystal-structure prediction.

Systematic Variation of Bidentate Ligands Used in Aryl Halide Amination. Unexpected Effects of Steric, Electronic, and Geometric Perturbations

Abstract: This paper presents effects of varying bidentate phosphine steric properties, electronic properties, and bite angle on product ratios in the amination of aryl bromides. Comparisons of the ratios of...