Significant Substituent Effect Research Articles

Identification of multiple transition structures on the potential energy hypersurface for [2+2] electrocyclization of the pentadienyl cation bearing a phosphorus in the 3-position

Ab initio molecular orbital investigations of the [2+2] electrocyclization of 3-substituted pentadienyl cations, where the substituents are BH 2, AlH 2, OH, SH, NH 2, PH 2, or H, show significant substituent effects on the ground state conformations, transition structures, and energies of activation. Locating the transition structure for the cyclization of the PH 2-substituted system was particularly trouble-some due to the tendency of the phosphorus group to undergo inversion. The AVS Chemistry Viewer, a data flow environment-based application for the analysis, visualization, and animation of molecular modeling results, proved useful in mapping the potential energy hypersurface and understanding the transition structures for this system.

Photo‐oxidation and photoreduction of bis(dithiobenzil)nickels

Photo-oxidation and photoreduction of bis(dithiobenzil)nickels have been investigated. The quantum yields for photo-oxidation of anionic bis(dithiobenzil)nickels by u.v. irradiation (313 nm) were in the range 0.07–0.14 in dichloromethane, and this photo-oxidation on poly(methylmethacrylate) was also observed. Some of the neutral bis(dithiobenzil)nickels in acetonitrile/benzene mixture were photoreduced in the presence of methylene blue as a sensitiser by irradiation in visible light (>390 nm), and a significant substituent effect was observed.

Electron-transfer photoreactions of small-ring compounds: nature and role of cation-radical intermediates

Abstract

Mass spectrometric study on 2‐amino‐as‐triazino[6,5‐c]quinoline and some of its derivatives

AbstractElectron impact induced fragmentations of 2‐amino‐as‐triazino[6,5‐c]quinoline and its 2‐methylamino, 2‐dimethylamino and 2‐benzylamino analogues have been investigated. The main primary decomposition route of both the singly and the doubly charged molecular ions is the N2 loss. For the singly charged ions the critical energy of this reaction is 110±10 kJ mol−1 and the kinetic energy release is 61±4 kJ mol−1. For the doubly charged ions these values are 90±10 kJ mol−1 and 5±2 kJ mol−1, respectively, indicating a significantly different reaction profile. The further fragmentation of [MN2]+˙ ions consists of radical eliminations from the 2‐amino group with cleavages of the α‐ and β‐bonds. Here a significant substituent effect is eliminations found suggesting an intramolecular cyclization reaction with a substituent migration. D and 15N labelling experiments have shown a minor extent of randomization of the labelled atoms and the occurrence of other hidden skeletal rearrangements during the fragmentation.

Asymmetric Hydrogenation of Schiff Base Prepared from α-Keto Ester with Aliphatic Amino Acid Ester

Abstract Catalytic hydrogenation of Schiff bases prepared from α-keto esters with amino acid esters was carried out under various conditions. A significant substituent effect of the alkyl group of the ester moiety of amino acid on the stereoselectivity was observed in the hydrogenation of the Schiff bases prepared from methyl pyruvate with (S)-alanine ester. Higher asymmetric yields were obtained as the ester residue of amino acid ester became bulkier. A clear temperature effect on the asymmetric hydrogenation was also observed. These results could be explained by the chelation mechanism.

Basicity of carbonfunctional organosilicon compounds and mechanism of the α effect

On the basis of decomposition of the protonation energy resulting from Longuet-Higgins theory of proton affinity, the significant stereospecific effect of silyl substituents that manifests itself in basicity trends in a series of carbonfunctional organosilicon compounds has been analysed.

Association between polar molecules. Part 1.—Nuclear magnetic resonance study of the dipole association of hexamethylphosphoramide with p-substituted nitrobenzenes

Dipole association between hexamethylphosphoramide (HMPA) and p-substituted nitrobenzenes was investigated in non-polar solvents. N,N-dimethyl, O-methyl, methyl and chloro groups were considered as substituents, and carbon tetrachloride and methylcyclohexane were used as solvents. The data are analysed assuming the formation of a 1:1 associate between HMPA and nitrobenzenes. A significant substituent effect on both the equilibrium constant and the induced shift of association is confirmed, and these results are discussed in relation to the dipole moment or the linear combination of the substituent constants σi and σ+π.

Kinetics of radical polymerization—XXVIII. Investigation of radical reactivity of aromatic aldonitrones in the polymerization of styrene

A study was made of the kinetics of polymerization of styrene initiated by azoisobutyronitrile at 50 in the presence of ξ-aryl- N-phenyl-nitrones meta- and para-substituted. Aromatic aldonitrones were found to be effective inhibitors of radical polymerization. The reactivities of the inhibitors show significant substituent effect obeying the Hammett-Taft equation, and ρ = +0.86. The rate constant of the side-reaction (dipolar cycloaddition) between the monomer and inhibitor was determined. The substituent effect can be described by the Hammett equation, with ρ = +0.88.

13C-NMR-Spektren von γ-substituierten pentamethincyanin-farbstoffen mit indolenin-endgruppen

The 13C-resonances of fourteen γ-substituted pentamethincyanine dyes 1 with indolenine nuclei have been measured and assigned. The effect of the γ-substituents R on the 13C chemical shifts is compared with the corresponding effect of substituents R of monosubstituted benzenes and 1-substituted trans-butadienes. The chemical shifts of the C 2-methincarbons, which are separated by four bonds from R, correlate with Hammett's substituent parameters σ para. A significant substituent effect through nine covalent bonds has been observed.