Substituent

About: Substituent is a research topic. Over the lifetime, 42877 publications have been published within this topic receiving 516716 citations. The topic is also known as: side chain & side group.

Thermodynamic significance of .rho.+ and .rho.- from substituent effects on the redox potentials of arylmethyl radicals

Abstract: The electrochemical oxidation and reduction potentials of substituted benzyl, cumyl, and diphenylmethyl radicals are found to give linear correlations with σ + and σ − . Although the redox potentials reflect the thermochemical properties of both the radicals and the product ions substituent effects on the ion dominate leading to a simple relationship between the oxidation potential and pK R + and the reduction potential and pK a (R−H)

4.5 – Additions to N-Acyliminium Ions

Abstract: The addition of strongly polarized carbon-carbon double bonds (enols) to simple iminium ions, known as the Mannich reaction (equation 1), is a fundamentally important route to amines, especially in biosynthetic processes.1,2 This reaction type is less synthetically useful if weakly polarized or unpolarized carbon-carbon π-bonds are used as nucleophiles (equation 2).3 Because of the low electrophilic reactivity of the iminium moiety, the reverse reaction, known as the Grob fragmentation (equation 2), is often the more important process.4 However, if the iminium moiety has a carbonyl substituent on nitrogen, its electrophilicity is strongly enhanced, and reactions with CC bonds are irreversible and synthetically useful (equation 3).

Theoretical studies of stereoselectivities of intramolecular aldol cyclizations catalyzed by amino acids.

Abstract: The effects of different amino acid catalysts and substrate substituents on the stereoselectivity of the title reactions have been studied with the aid of density functional theory methods. Experimental data available in the literature have been compiled. B3LYP/6-31G(d) calculations match the general experimental trends and provide useful insights into the origins of the variations in stereoselectivities. Acyclic primary amino acids allow a greater conformational flexibility in the aldol transition states compared with proline. This makes them poorer enantioselective catalysts with triketone substrates with a methyl ketone side chain. The steric repulsion upon substitution at the terminal methyl group increases the energy difference between anti- and syn-chairs with primary amino acid catalysts and, consequently, the stereoselectivities. Proline, in contrast, is a poor catalyst for the latter reactions because the substituent's steric bulkiness raises the activation energy of the favored C−C bond-forming ...

Silver halide photographic material

Abstract: PURPOSE:To obtain a coupler capable of controlling in a wide range and independently of a coupling reaction rate and liberating rate of a photographically useful group from a timing group by using a specified coupler. CONSTITUTION:In the coupler having a cleavable group by the reaction with an oxidized product of a developing agent, the cleavable group is expressed by the formula, wherein L is -X-CR1R2-, or -O-CO-; X is O, S, or tert. amino; R1 and R2 express H or a substituent, or R1 may form a ring by combining with R2; L may be bonded to TIME with a carbon atom. Preferred substituent for R1 and R2 is such as halogen, alkyl, alkenyl, aralkyl, aralkenyl, aryl, or heterocyclic residue. TIME is a timing group, and PUG is a photographically useful group.