Showing papers on "Steric effects published in 1980"

Some guidelines for radical reactions

Abstract: Some generalisations of predictive utility are presented concerning the influence of steric and stereoelectronic effects on radical reactions.

Chiral Metal Atoms in Optically Active Organo-Transition-Metal Compounds

Abstract: Publisher Summary This chapter discusses chiral metal atoms in optically active organo-transition-metal compounds. An important approach to stereochemical problems is to make use of the concept of chirality. The chiral center most frequently encountered is the asymmetric carbon atom––a tetrahedral C atom––bonded to four different substituents. Chiral molecules may be studied by a great many techniques. Occasionally, optically active ligands have been employed in organo-transition-metal chemistry to demonstrate the stereospecificity of reactions at metal–ligand bonds with respect to the α-carbon atom of the ligand. The chapter focuses on organo-transition-metal compounds having chiral metal atoms whose optical activity have been demonstrated. The principle of introducing a diastereoisomer relationship into a pair of mirror-image isomers is the basis for each optical resolution. After the preparation of diastereoisomers by the introduction of an optically active resolving agent, the next problem in an optical resolution is to separate the diastereoisomers. The optical purity of enantiomers has been determined by nuclear magnetic resonance (NMR) spectroscopy with the help of optically active shift-reagents. Thus, for racemization, the steric effect seems to be most important. Metal–alkyl bonds can be carbonylated, and metal–acyl bonds can be decarbonylated. Optically active organo-transition-metal compounds exhibit extremely large specific rotations, usually exceeding the specific rotations encountered in organic chemistry by a factor of one or two powers of ten.

The influence of chain-branching on the steric outcome of some olefin-forming reactions

Abstract: When applied to the construction of internal disubstituted double bonds in compounds such as pentadec-7-ene and 2,6,11,15-tetramethylhexadeca-2,6,8,10,14-pentaene (5), the reductive elimination of vicinal benzoyloxy-sulphones gives products with a trans : cis ratio of ca. 4 : 1. When applied to the construction of disubstituted olefins in which branching occurs adjacent to the newly formed double bond, much higher trans-stereoselectivity is observed, e.g, for 5-ethyl-2-methylhept-3-ene (21) and 3,6-dimethylocta-2,4,6-triene (25) it is at least 98%. A similar effect of chain-branching is also observed in some Wittig and Homer reactions using primary allylic phosphorus derivatives. When a Z-allylic sulphone is used as the starting material in the benzoyloxy-sulphone method, the original Z-geometry is preserved in the product; (Z)-2-methylbut-2-enyl p-tolyl sulphone and tiglic aldehyde give almost exclusively (2E,4E,6Z)-3,6-dimethylocta-2,4,6-triene (27).

The role of electron transfer and charge transfer in organometallic chemistry

Abstract: The principal objective of this lecture is to describe the efficacy of organometals as electron donors, and to show how this property dominates many aspects of their chemistry. First, the structural effects of alkyl ligands (B) on the highest occupied molecular orbital (HOMO) of the neutral alkylmetals, R4Sn, R4Pb and R2Hg, are probed by photoelectron spectroscopy. The chemical properties of the resultant paramagnetic cation is then discussed. Electron transfer from alkylmetals to iron(III) complexes is shown to proceed via an outer-sphere mechanism, whereas that to iridate(IV) and tetracyanoethylene are inner-sphere processes. The difference lies in their response to steric effects in the alkylmetals. Steric effects are quantitatively evaluated with the aid of charge transfer transition energies in the absorption spectra of alkylmetal-tetracyanoethylene complexes. After correction for the steric effect, the rates of electron transfer from alkylmetal to both hexachloroiridate(IV) and tetracyanoethylene follow a linear free energy relationship with a Bronsted slope = 1, predicted by Marcus theory for inner-sphere mechanisms. The generalized concept of charge transfer is applied to a variety of organonickel systems, including (1) the oxidative addition of aryl halides to triethylphosphinenickel(0) complexes, (2) biaryl synthesis from the induced decomposition of arylnickel(II) halides, (3) oxygen atom transfer from organic nitro compounds to coordinated phosphines in nickel(0) complexes and finally even to (4) n-ligand substitution of benzophenones into phosphinenickel(0) complexes.

Carbon‐13 NMR studies of nitrogen compounds. I—substituent effects of amino, acetamido, diacetamido, ammonium and trimethylammonium groups

Abstract: 13C NMR chemical shifts are reported for several alkyl or aryl amines, acetamides, diacetamides and ammonium and trimethylammonium salts. Substituent effects, determined with respect to chemical shifts of amino derivatives, are rationalized in terms of γ effects, steric interactions and for benzene derivatives, in terms of competition between inductive (σ1) and resonance (σR) effects. These perturbations can be used to assign chemical shifts of complex structures and to study certain chemical processes.

Application of 15N spectroscopy and dynamic NMR to the study of ureas, thioureas and their lewis acid adducts

Abstract: Rotational barriers and 15N chemical shifts have been measured in a number of ureas and thioureas. As anticipated on the basis of the 15N shifts, several previously unobserved rotational barriers could be detected by using lanthanide reagents or a high field spectrometer. Nearly constant effects on both the rotational activation energy and the 15N shift are produced on going from ureas to the corresponding thioureas, and correlations are found between the ΔG‡ and δ15N values. The results are discussed in terms of lone pair delocalization, and anomalies with respect to the general behaviour are tentatively explained in the light of the effect of steric torsion in crowded structures on the 15N shifts and rotation barriers.

Vibrational spectra and conformations of (chloromethyl)cyclopropane and epichlorohydrin

Abstract: The IR and Raman spectra (4000–50 cm−1) of (chloromethyl)cyclopropane and epichlorohydrin have been recorded in all three physical states. It has been concluded from the spectroscopic data and from variable temperature studies that (chloromethyl)cyclopropane exists as an equilibrium mixture of gauche (95%) and cis (5%) conformers in the liquid state, while only the gauche form persists in the solid phase. The enthalpy difference between these two rotamers was found to be 1.40±0.10 kcal mol−1. Similarly, epichlorohydrin has also been shown to exist as an equilibrium mixture of gauche-1 (70%), gauche-2 (21%) and cis (9%) conformers in the liquid state, with only the gauche-1 form remaining in the solid phase. Enthalpy differences among these conformers were determined to be 1.2±0.2 kcal mol−1 (gauche-1 vs. cis) and 0.70±0.10 kcal mol−1 (gauche-1 vs gauche-2). Apparently steric repulsions between the chlorine atom and β ring-hydrogens destabilize the cis conformation in each molecule. For each molecule, a vibrational assignment has also been proposed.

Protonenresonanz‐Untersuchungen zur inneren Rotation, 10. Konformation und Stereodynamik dipolarer cyclischer Enamine und Hydrazone

Abstract: 2-Aminomethylene and 2-hydrazono derivatives 1–6 of carbocyclic and heterocyclic 1,3-dicarbonyl compounds which may form up to four rotational diastereoisomers were investigated by 1H and 13C NMR spectroscopy. The isomeric compositions of the crystalline compounds can be deduced from the 1H signals of equilibrating solutions. Free enthalpies of activation for rotation about the CN and CC bonds of the enamines and about the NN and CN bonds in the hydrazones were determined by line broadening measurements (Tables 4, 5, and 6). These findings were compared with the results (Table 4) of equilibrations of CN rotamers of enamines which were obtained by separation or enrichment at a preparative scale. Electronic and steric contributions to the rotational barriers are discussed.

The chemistry of vitamin B12. Part 19. Labilisation of the cobalt–carbon bond in organocobalamins by steric distortions; neopentyl-cobalamin as a model for labilisation of the vitamin B12 coenzymes

Abstract: Neopentylcobalamin in neutral solution at 25 °C is stable under nitrogen but is decomposed by O2 and by imidazole to give a cob(III)alamin and by hydrogen-atom donors such as thiols and PriOH to give the cob(II) alamin and neopentane. The reaction is ascribed to reversible homolytic fission of the Co–C bond to give a low steady-state concentration of the cob(II)alamin and neopentyl radicals, which can then form neopentane by the abstraction of a hydrogen atom. Neopentylcobinamide is also decomposed by high concentrations of imidazole, while the purely five-co-ordinate neopentylcobalamin in acid and neopentylcobinamide in neutral solution are stable both in the presence and absence of O2. Labilisation of the Co–C bond is ascribed to steric distortion around the co-ordinated C atom in the six-co-ordinate neopentyl complexes. It is suggested that neopentylcobalamin provides a model for the labilisation of the Co–C bond in the vitamin B12 coenzymes, and possible mechanisms for the isomerase reactions are discussed.

Stereochemistry of Four-coordinate Chelate Compounds of Schiff Bases and Their Analogues

Abstract: The problem of the configuration of the chelate group and the metal ring in chelates of transition metals with Schiff bases and their analogues is discussed, and the influence of various factors analysed, in particular the nature of the central metal atom and the steric and electronic effects of the ligands Various types of conformational changes in conjugated metal rings in bicyclic and tricyclic chelates are surveyed, and factors influencing these changes are noted The conformation of the non-conjugated metal rings in tricyclic chelates is discussed A list of 188 references is included

The chemistry of vitamin B12. Part 17. The effect of steric distortion of the cobalt–carbon bond on the pK values and spectra of organocobalamins

Abstract: The spectra of the neutral and acid forms and the apparent pK values for protonation and displacement of the heterocyclic base have been determined for organocobalamins with various alkyl and cycloalkyl ligands possessing no functional group. Increasing the degree of substitution or distortion on either Cα(in the series methyl < ethyl < isopropyl and cyclo-propyl < -butyl < -pentyl ∼-hexyl) or Cβ(ethyl ∼ n-propyl < isobutyl < neopentyl) leads to parallel changes in the spectra and in the pK values with no fundamental difference between primary and secondary groups (methyl cyclopropyl, ethyl cyclobutyl, isopropyl cyclo-pentyl and -hexyl) or between substitution on Cα and Cβ(neopentyl cyclopentyl). It is shown that these effects must be steric, rather than electronic, in origin and are probably due to repulsion between the bonding electron pairs on Cα, on the one hand, and the lone pairs on Co and the equatorial N atoms on the other. It is suggested that the main variable, as seen by the Co ion and the rest of the complex, is the Co–Cα bond length.