Dynamic ¹H NMR investigation along with a theoretical study around the C–C and C = C bonds in a particular phosphorus ylide

Abstract

In the present work, the dynamic 1 H NMR effects were investigated at variable temperatures within a particular phosphorus ylide involving a 2-benzoxazolinone around the carbon-carbon single bond and also partial carbon-carbon double bond in two Zand E-rotational isomers. Activation and kinetic parameters including ΔH 6 ¼ , ΔG 6 ¼ , ΔS 6 ¼ and E a were determined in accord with the dynamic 1 H NMR data for three rotational processes. In addition, theoretical studies based upon rotation around the same bonds were investigated using ab initio and DFT methods at the HF/6-31G(d,p) and B3LYP/6-31G(d,p) levels of theory. Theoretical activation and kinetic parameters including ΔH 6 ¼ , ΔG 6 ¼ , ΔS 6 ¼ and E a were calculated at 298 K and experimental temperatures for five rotational processes. These results (experimental and theoretical), taken together, indicate that the rotational energy barrier around the C = C double bond was considerably high and the observation of the two rotational isomers was impossible (seen as a single isomer) at the high temperatures, in this case rotation around the C = C bond was too fast on the NMR time scale. When the temperature was relatively low, the rate of rotation was sufficiently slow; therefore, observation of the two Zand E-isomers was possible. In addition, calculations at the HF/6-31G(d,p) level of theory showed very favorable results in agreement with the experimental data on rotation around the C = C bond. While, B3LYP level using the 6-31G(d,p) basis set was provided the reasonable data for the restricted rotations around the C-C and C-N single bond.

Preparation of New Phosphoranes by Chemoselective Reaction of Acetylenic Esters with Compounds Containing Two Nitrogen Atoms

“…The probable mechanistic pathway that best explains the synthesis of these compounds, is depicted in Scheme 2. On the basis of the property of trivalent phosphorus nucleophiles, it is generally accepted [32][33][34][35][36][37][38][39] that the products (3a-b) are produced from the initial attack of triphenylphosphine to the acetylenic ester and concomitant protonation of the adduct I by NH group which is placed between two carbonyl groups. Then, the positively charged ion is attacked by the corresponding conjugated base to form phosphorane (3).…”

Mechanistic Study of the Formation of a Phosphonate Ester Containing 2-Fluoroaniline: Theoretical Assignment of the Resulting Isomer

“…This equation which was obtained from a mechanistic scheme (shown in Figure 7) is compatible with the experimental results obtained [Eqn (23)]. With respect to Eqn (23), that is the overall reaction rate, the activation parameters involving ∆G # , ∆S # and ∆H # can be now calculated for the first step (RDS, k 2 ), as an elementary reaction, on the basis of the Eyring equation.…”

“…Also this equation indicates that the overall order of reaction is two and it depends on concentration of compounds 1 and 2 and is independent of the concentration of compound 3. In addition, according to this equation, the order of reaction with respect to TOPP, DMAD (23) and N -H is 1, 1 and zero, respectively.…”

“…With respect to Eqn (23), that is the overall reaction rate, the activation parameters involving ∆G # , ∆S # and ∆H # can be now calculated for the first step (RDS, k 2 ), as an elementary reaction, on the basis of the Eyring equation. The results are reported in Table 8.…”

The Hindered Internal Rotations in a Particular Phosphorane Involving a 6-aza Thiothymine: An Experimental Dynamic 1H NMR Study with Three Methods of Determining Activation Parameters

“…Some of phosphorus ylides exhibit dynamic 1 H nuclear magnetic resonance (NMR) effects that afford good information regarding the interchangeable process of rotational isomers that provide important kinetic data, and also it is a useful tool when discussing the barriers separating two states that are observable by NMR spectroscopy [7][8][9][10].…”