Synthesis of stable phosphorus ylides from 3, 5-dimethylpyrazole and kinetic investigation of the reactions by UV spectrophotometry

Abstract

Stable crystalline phosphorus ylides were obtained in excellent yields from the 1:1:1 addition reactions between triphenylphosphine and dialkyl acetylenedicarboxylates, in the presence of a NH-acid, such as 3,5-dimethylpyrazole. These stable ylides exist in solution as a mixture of two geometrical isomers as a result of restricted rotation around the carbon-carbon partial double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group. To determine the kinetic parameters of the reactions, they were monitored by UV spectrophotometery. The second order fits were drawn and the values of the second order rate constant (k 2 ) were calculated automatically using standard equations within the program. At the temperature range studied, the dependence of the second order rate constant (Ln k 2 ) on reciprocal temperature was in good agreement with the Arrhenius equation. This provided the relevant plots to calculate the activation energy of all reactions. Furthermore, useful information was obtained from studies of the effect of solvent, structure of reactants (different alkyl groups within the dialkyl acetylenedicarboxylates) and also concentration of reactants on the rate of reactions. The proposed mechanism was confirmed according to the obtained results and steady state approximation and first step (k 2 ) of reaction was recognized as a rate-determining step on the basis of experimental data.

Mutual Placement of Isocyanide and Phosphine Ligands in Platinum(II) Complexes [PtHal2L1L2] (Hal = Cl, Br, I; L1,L2 = CNCy, PPh3) Leads to Highly-Efficient Photocatalysts for Hydrosilylation of Alkynes

“…The prepared complexes are colourless (1, 4, 7), yellowish (2, 5, 8), or yellow (3,6,9) solids. The UV-vis spectra of 1-9 exhibit strong absorption bands below 300 nm [ε = (0.20-1.94) × 10 4 M −1 cm −1 ] that can be assigned to ligand-centred transitions of phosphine and/or isocyanide ligands [32]. The wide weak absorption bands in the 300-360 nm range with molar extinction coefficients [ε = (0.03-0.17) × 10 4 M −1 cm −1 ] can be assigned the d → d, π → π*, n → π* and changetransfer transition arising from π electron interactions between the metal and ligands, which involve either a metal-to-ligand or ligand-to-metal electron transfer [33,34].…”

Effect of Reactivity on Kinetics and a Mechanistic Investigation of the Reaction between Dimethyl Acetylenedicarboxylate and 1,3-Dicarbonyl Compounds in the Presence of a Catalyst: A Spectrophotometric Approach

“…Often, it is sufficient to record continuously the absorbance of a reaction mixture in a thermostated cuvette at a fixed wavelength. The required instrumentation is www.prkm.co.uk readily accessible and numerous kinetic investigations of organic reactions have been reported using the UV-Vis technique [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The first catalytic reaction of arsonium ylides in 1989 reported that this property was vital in the synthesis of cyclopropanes with high diasteroselectivity [16][17][18][19][20].…”

“…A) Eyring plot (ln k obs /T versus 1/T) according to Eqn(9) for the reaction between DMAD, TPA and 1,3-dicarbonyl compounds in methanol. (B) A linearised form of the Eyring equation [Eqn(10), T ln k obs /T against T] for the reaction between DMAD, TPA and 1,3-dicarbonyl compounds in methanol.…”