Electromeric effect

Abstract: The hydrodenitrogenation of several aliphatic and heterocyclic amines was investigated using unsupported and silica-supported platinum catalysts. With the amines used as reactants it was found that the rate of C-N bond scission was hardly affected by substitution at the nitrogen atom, but was strongly dependent on the substitution at the carbon atom. The rate decreases in the series: primary C-N > secondary C-N > tertiary C-N. However, the bond between nitrogen and a methyl group is an exception since the corresponding C-N bond is obviously stabilized, presumably due to an electromeric effect. The hydrogenolytic ring opening of heterocyclic amines proceeds faster on pyrrolidine rings than on piperidine rings. This behaviour parallels the results of hydrogenolysis of cyclopentane and cyclo-hexane. The turnover frequency of platinum black was considerably lower than that of supported platinum, i.e. the presence of the support seems important for the catalytic activity. Therefore we assume that the hydrodenitrogenation on platinum demands the presence of a support. Most likely ionic sites prevailing on the support combined with the metallic sites form catalytic ensembles for this reaction.

Abstract: Five 3-Hydroxychromones (3HCs), namely, 2-(furan-2-yl)-3-hydroxy-4H-chromen-4-one (FHC) and its four derivatives by substitution of -CH3, -OH, -NO2 and -Cl at 6th position were synthesized from their corresponding 2’-hydroxyacetophenone and furan-2-carboxaldehyde. Various spectral transitions of all these 3-hydroxychromones (3-HCs) have been assigned by interpreting their absorption spectra in cyclohexane, acetonitrile and methanol. It has been shown that the electromeric effects of substitution at 2nd and 6th positions on the 2–3 double bond in ‘C’ ring are similar but the effect on the double bond of 4-carbonyl group is opposite. It has been found that the substitution at 2nd position changes mainly the electron density directly at the 4-carbonyl group and substitution at 6th position changes the electron density of the ‘C’ ring, changing the overall dipole moment of the molecule, which in turn changes the electron density at the 4-carbonyl group. Emission spectral studies showed that the increase and decrease in dipole moment by substitution at 6th position with electron withdrawing group like NO2 and electron donating group like -CH3 and -OH, stabilizes and destabilizes the N∗ state in the polar solvents respectively.

Abstract: 1. Resorcinol and trihydroxybenzenes react with hexafluoroacetone and methyl trifluoropyruvate in the absence of a catalyst in nonpolar media to give nearly quantitative yields of the mono-C-alkylation products. The reaction is regiospecific, at the site of concerted ortho-para orientation by the OH groups. 2. Of the mono-hydroxybenzenes, only those compounds with meta-substituents in which the substituent has a large electromeric effect undergo C-alkylation with polyfluorocarbonyl compounds.