Showing papers on "Substituent published in 1976"

Substituent Effect on the Oxidation of Phenols and Aromatic Amines by Horseradish Peroxidase Compound I

Abstract: A stopped-flow kinetic study shows that the reduction rate of horseradish peroxidase compound I by phenols and aromatic amines is greatly dependent upon the substituent effect on the benzene ring. Moreover it has been possible to relate the reduction rate constants with the ionization constants of monosubstitutcd substrates by a linear free-energy relationship (Hammett equation). The correlation of log (rate constants) with σ values (Hammett equation) and the absence of correlation with σ+ values (Okatnoto-Brown equation) can be explained by a mechanism of aromatic substrate oxidations, in which the substrate gives an electron to the enzyme compound I and simultaneously loses a proton. The analogy which has been made with oxidation potentials of phenols or anilines strengthens the view that the reaction is only dependent on the relative ease of oxidation of the substrate. The rate constant obtained for p-aminophenol indicates that a value of 2.3 × 108 M−1 s−1probably approaches the diffusion-controlled limit for a bimolecular reaction involving compound I and an aromatic substrate.

On the Way to Carbene and Carbyne Complexes

Abstract: Publisher Summary The chapter focuses on carbene andcarbyne complexes. If one of the hydrogen atoms in an alkane hydrocarbon, such as ethane, is formally replaced by a metal atom, an organometallic compound in which the organic entity is bonded to the metal by a σ-bond is obtained. If a system with 2 carbon atoms bonded to each other by a double bond, i.e., an alkene molecule, is considered, a number of separate paths leading to organometallic derivatives are recognized. One path involves the replacement of a substituent by a metal atom and leads to σ compounds that are exemplified by vinyl-lithium derivatives. If molecules with a carbon-carbon triple bond of the type that exists in alkynes are considered, then it is realized that there also are three possible paths to metal-containing derivatives. The chapter discusses various properties of transition metal-carbene complexes such as preparation of the first carbene complexes, bonding concepts, and spectroscopic findings. It also presents various properties of transition metal-carbyne complexes such as preparation of the first carbyne complexes and x-ray structural analyses.

Thermal Cis-to-Trans Isomerization of Substituted Azobenzenes II. Substituent and Solvent Effects

Abstract: The thermal cis-to-trans isomerization rate of various azobenzenes was followed by means of spectrophotometric and flash photolysis techniques. For para-donor/para′-acceptor-substituted azobenzenes such as 4-nitro-4′-dimethylaminoazobenzene, the rate was distinctly accelerated, the activation energy decreasing with the increase in the polarity of solvents. Introduction of substituents in para positions with respect to azo group increased the rate irrespective of substituent. The effect is additive and a Hammett-type equation holds. For 4-dimethylamino-and 4-nitroazobenzenes, while the 2-methyl group accelerated the rate, the 2′-methyl group did not. The results suggest that the isomerization proceeds via inversion mechanism and the rate is controlled mainly by the resonance stabilization in the coplanar transition state. The inversion center for asymmetric azobenzenes is discussed.

Lubricants containing amino phenol-detergent/dispersant combinations

Abstract: Disclosed are combinations of amino phenols, wherein said phenols contain a substantially saturated hydrocarbon substituent of at least 10 aliphatic carbon atoms, and one or more detergent/dispersants selected from the group consisting of (I) neutral or basic metal salts of an organic sulfur acid, phenol or carboxylic acid; (II) hydrocarbylsubstituted amines wherein the hydrocarbyl substituent is substantially aliphatic and contains at least 12 carbon atoms; (III) acylated nitrogen-containing compounds having a substituent of at least 10 aliphatic carbon atoms; and (IV) nitrogen-containing condensates of a phenol, aldehyde and amino compound. Fuels and lubricants containing such combinations as additives are particularly useful in two-cycle (two-stroke) engines.

Structure-activity relationships of adenylate cyclase-coupled beta adrenergic receptors: determination by direct binding studies.

Abstract: Recently developed techniques for directly studying ligand binding to beta adrenergic receptors with (-)-[3H]alprenolol have been used to delineate in detail the binding specificity of the adenylate cyclase-coupled beta adrenergic receptors in a model system, the frog erythrocyte membrane. The abilities of 60 beta adrenergic agents to compete for the binding sites and to interact with the adenylate cyclase (as agonists or antagonists) were quantitated and compared. The specificity of the receptors determined by direct binding studies or by adenylate cyclase studies was comparable. The KD values of the agents as determined by inhibition of (-)-[3H]alprenolol binding correlated well ( r = 0.95) with their apparent dissociation constants determined by enzyme studies. The latter were determined as the concentrations of agonists necessary to cause 50% maximal enzyme stimulation, or the concentrations of antagonists necessary to produce a 2-fold rightward shift in the (-)-isoproterenol dose-response curve. Agonists and antagonists appeared to compete for the same set of receptor binding sites. Structure-activity relationships determined by the direct binding studies were in excellent agreement with those previously determined in more intact tissue preparations. For agonists the structural features which determined receptor affinity (assessed by direct binding studies) were distinct from those which determined intrinsic activity (maximum ability to stimulate adenylate cyclase). The affinity of agonists was increased by increasing the size of the substituent on the amino nitrogen, by a (-) configuration of the hydroxyl on the β-carbon, and by the presence of a catechol moiety. Methyl or ethyl substitution on the α-carbon had only a slight (generally inhibitory) effect on affinity. Intrinsic activity of agonists was determined primarily by the nature of the substituents on the phenyl ring. Full intrinsic activity requires the presence of hydroxyl groups on the ring at positions 3 and 4 as well as the β-carbon hydroxyl in the (-) configuration. Deletion of the β-carbon hydroxyl, as in compounds such as dopamine, dobutamine, and related agents, leads to substantial loss of intrinsic activity and affinity even in the presence of large amino nitrogen substituents. A methanesulfonamide group substituted for the hydroxyl in position 3 on the ring results in reduced intrinsic activity. Deletion of the ring hydroxyl at either position 3 or 4 or substitution by chlorine produces competitive antagonists. Structure-activity relationships of antagonists were similar to those of agonists, except that the catechol moiety was replaced by a single or double aromatic ring structure. Separation of this moiety from the ethanolamine side chain by an ether function significantly increased affinity. When a phenyl group was present, a single substituent at the para position was associated with reduced affinity. ACKNOWLEDGMENT We gratefully acknowledge the expert assistance of the analytical chemistry department of New England Nuclear Corporation in developing chromatographic procedures for (-)-alprenolol.

The kinetics of formation of horseradish peroxidase compound I by reaction with peroxobenzoic acids. pH and peroxo acid substituent effects.

Abstract: 1. The kinetics of formation of horseradish peroxidase Compound I were studied by using peroxobenzoic acid and ten substituted peroxobenzoic acids as substrates. Kinetic data for the formation of Compound I with H2O2 and for the reaction of deuteroferrihaem with H2O2 and peroxobenzoic acids, to form a peroxidatically active intermediate, are included for comparison. 2. The observed second-order rate constants for the formation of Compound I with peroxobenzoic acids decrease with increasing pH, in the range pH 5-10, in contrast with pH-independence of the reaction with H2O2. The results imply that the formation of Compound I involves a reaction between the enzyme and un-ionized hydroperoxide molecules. 3. The maximal rate constants for Compound I formation with unhindered peroxobenzoic acids exceed that for H2O2. Peroxobenzoic acids with bulky ortho substituents show marked adverse steric effects. The pattern of substituent effects does not agree with expectations for an electrophilic oxidation of the enzyme by peroxoacid molecules in aqueous solution, but is in agreement with that expected for a reaction involving nucleophilic attack by peroxo anions. 4. Possible reaction mechanisms are considered by which the apparent conflict between the pH-effect and substituent-effect data may be resolved. A model in which it is postulated that a negatively charged 'electrostatic gate' controls access of substrate to the active site and may also activate substrate within the active site, provides the most satisfactory explanation for both the present results and data from the literature.

Carbon-13 nuclear magnetic resonance spectra of organic sulfur compounds.' Comparison of chemical shifts for carbonyl and thiocarbonyl compounds in the pyrone, thiopyrone, and pyridone series

Abstract: 13C nmr data have been obtained for a series of 2- and 4-pyrones and pyridones, and their sulfur-containing analogues. Correlations have been observed between the nature of the ring hetero-atom and the chemical shift difference (Δδ) for the Cα and Cβ carbons in these conjugated systems. No significant correlation, however, appears to exist between the chemical shifts of the C=O and C=S groups. Substituent chemical shift (s.c.s.) effects for various simple substituents are compared with those in related series of compounds.

Stable Cis and Trans Rotational Isomers of 1,8-Di-o-tolylnaphthalene

Abstract: Studies of CPK (Corey-Pauling-Koltun) or similar space-filling models suggest that there should be a very substantial barrier to a 180° rotation about the phenyl-naphthy bonds in 1,8-diphenylnaphthalene derivatives. This leads to the expectation that such compounds with a substituent at one meta position of each phenyl ring should be expected to exist as stable cis and trans isomer pairs. However, House and co-workers have shown that several such derivatives, including 1,8-bis(3-chlorophenyl)naphthalene and 1,8-bis(3-methylcarboxyphenyl)naphthalene cannot be resolved into stable configurational isomers. Further, proton NMR studies of these derivatives indicate that ΔG^≠ for rotation is 15-16 kcal/mol, which corresponds to rather rapid rotation in solution at room temperatures.

Determination of the site of protonation of substituted benzenes in water chemical ionization mass spectrometry

Abstract: The site of protonation of a substituted benzene may be determined using chemical onization mass spectrometry with D2O as a reagent gas. The observation of extensive exchange of the ring hydrogens for deuteriums is linked to protonation on the benzene ring. The lack of this exchange coupled with the formation of cluster ions (the association of the protonated species with one or more D2O molecules) is evidence of protonation on the substituent rather than the ring. Aniline, benzaldehyde and nitrobenzene are observed to protonate at the substituent while toluene, bromobenzene, biphenyl and iodobenzene protonate on the ring. The dimethylbenzenes protonate on the ring while the diaminobenzenes protonate at one of the substituents. The dihydroxybenzenes, as well as a number of other compounds in which an oxygen is attached directly to the ring, protonate predominantly at the substituent although a small amount of exchange of one ring hydrogen is observed.

13C NMR of organosulphur compounds: I—the effects of sulphur substituents on the 13C chemical shifts of alkyl chains and of S-heterocycles

Abstract: The 13C NMR spectra have been determined of: (i) aliphatic compounds having at one end a functionalized sulphur atom (SH, S−, SMe, S(O)Me, SO2Me and S+Me2) and (ii) saturated sulphur heterocycles variously substituted at the S-atom . The results are discussed in terms of the familiar deshielding effects for α- and β-carbons and shielding effects for γ-carbons, exerted by the sulphur atom itself and/or by the atoms or groups of which the sulphur function is made up. The γ-effect of the S-atom appears to be nearly independent of the nature of the S-function and of comparable magnitude to that of an aliphatic carbon (−2·5 + −3·0 ppm). Surprisingly, however, a SCH3 group shields the carbon in γ position with respect to CH3 by an amount (−5·4 ppm) which is more than twice that (−2·5 ppm) exerted by the aliphatic γ-carbon on the S-CH3 carbon itself. As to the cyclic compounds, the shieldings of the α- and β-carbons can be rationalized in terms of the conformational orientation of the substituent at sulphur, and the equilibrium distribution of the conformers. The results confirm the great value of 13C NMR for configurational and conformational assignment of S-heterocycles.

Metallierung einer nichtaktivierten Alkyl‐Gruppe im Nickelkomplex

Abstract: Aus dem Diazadien (DAD) Glyoxalbis (diisopropylmethylimin) (1) und wasserfreiem Nickelbromid wird (DAD)NiBr2 (2) erhalten, dessen Reaktion mit Hauptgruppenmetall-alkylen zu wenig stabilen Zwischenprodukten fuhrt. Mit dem sperrigen O-Tolylmagnesiumbromid wird unter formaler HBr-Abspaltung die Metallierung des Diisopropylmethyl-Rests von 2 in γ-Stellung zum nachsten aktivierenden Zentrum (N) unter Bildung eines NC-Chelatrings erreicht. Eigenschaften und Struktur dieses Produkts (3) einer neuartigen Alkanaktivierung werden diskutiert. Metalation of a Non-activated Alkyl Group in Nickel Complexes The diazadiene (DAD)glyoxalbis(diisopropylmethylimine) (1) reacts with anhydrous nickel bromide to yield (DAD)NiBr2 (2). Its reactions with main group metal alkyls lead to rather unstable intermediates. With the bulky O-tolylmagnesium bromide -under formal HBr abstraction- metalation of the diisopropylmethyl substituent occurs in γ-position to the next activating center (N) with formation of an NC chelate. Properties and structure of this product 3 from this novel alkane activation reaction are discussed.