Search

Examination of alcoholic solutions of dimethyl acetylphosphonate (1) and dimethyl benzoylphosphonate (2) by 31P NMR spectroscopy reveals the presence of considerable amounts of hemiketals. Because of the great difference between the 31P chemical shifts of acylphosphonates (ca. 0 ppm) and their hemiketals (17–21 ppm), 31P NMR spectroscopy is a uniquely suitable method for studying the rates and equilibria of hemiketal formation of acylphosphonates with different alcohols. The equilibrium constants Kf, K′f (K′f = Kf [ROH]), pseudo-first-order rate constants k′f, the second order rate constants, kf for hemiketal formation from dimethyl acetylphosphonate with various alcohols, as well as the reverse reaction rate constants, kr to starting materials, were determined. The kinetic isotope effect of 2.8 for the forward reaction kf (EtOH addition) and the backward reaction kr indicates a general catalysis pathway. On the other hand, the calculated values of the enthalpies of activation ΔH‡ = 10.37 kcal mol-1 (forward), ΔH‡ = 13.66 kcal mol-1 (backward) and the entropies of activation, ΔS‡ = -17.25 cal mol-1 K-1 (forward), ΔS‡ = -9.82 cal mol-1 K-1 (backward) are not in accord with high molecularity of the reaction (1 cal = 4.184 J). Our analysis led to the conclusion that this is probably due to the fact that the transition state is mainly reactant-like with the development of only limited extent of bond formation. Various plausible reaction pathways for hemiketal formation are discussed. In addition, we have calculated the value of 2.65 σ* for the P(O)(OMe)2 group based on proton affinity obtained from heats of formation (ΔHf) of applying the MNDO techniques. The following linear correlation between pKa values and PA values of hemiketals of the form (Me)(R)C(OH)(OCH2X) was developed: pKa = PA - 356.58 + 9.18 [σ*(Me) + σ*(R) 0.2σ*(X)].

Geometries, torsional barriers, and dipole moments have been computed for mono-, di-, tri-, and tetra-substituted five-membered cyclic carbonates and for some substituted cyclopentanones, oxolanes, 4-butyrolactones, and 1,3-dioxolanes. With respect to cyclic carbonates it was deduced that the potential curve increases moderately with T3456(torsion angle) and for most compounds a 1 kcal difference between 4T5 or 5T4 conformers and planar form corresponds to a torsional angle of ±14–16°. The minimum is, in general, flattened and close to planar. Additional modes of ring, puckering such as 5E, E5, 4E, E4 and 2E, E2 were also examined. Between the enantiomeric structures with opposite distortions, the quasi equatorial conformation of the alkyl group is preferred. endo and exo anomeric effects in carbonate systems have been studied by introducing strong electron-withdrawing substituents, in the carbonate 4-position. The exo anomeric effect was observed with many of the polar substituents, but the role of the anomeric effect in ring puckering (endo effect) was found to be small.For other five-membered ring systems MNDO predicts that the energy barrier to interconversion of planar ↔ twist is in the following order: carbonates > 1,3-dioxolanes > butyrolactones > oxolanes > cyclopentanones.N.m.r data are also included as well as the single-crystal X-ray diffraction study of 4-chlorophenyloxymethyl-1,3-dioxolan-2-one.

Analysis of the pH–rate profiles for the hydrolysis of 4-methyl-2-(N-phenylimino)-1,3-dioxolane (1) and 2-[N-(p-fluorophenyl) imino]-4-phenyl-1,3-dioxolane (2) together with product distribution of compounds (1)–(5) and pK values estimated for the various ionic forms T0, T+, T±, and T– of the tetrahedral intermediates, enables the determination of the reactivity of the tetrahedral intermediates along their reaction pathways, At pH 11 the only product produced is the hydroxycarbamate (C–O fission)via its anionic intermediate T–. Mono- and gemdimethyl substituted iminocarbonates T– undergo two modes of ring-opening. Employing MNDO/H calculations the relative stability (ΔHF) of the intermediates and their formed products was predicted and correlated with actual experimental product distribution.

The appearance of intermediates and the formation of isomeric products resulting from interactions of ten highly branched ethylene (Ib–k) and seven trimethylene carbonates (IIa–g) with nucleophiles such as hydroxide, n-butylamine, ethylenediamine, glycine, and imidazole, were monitored and quantitatively estimated by the n.m.r. technique. The nucleophilic ring-opening of the 1-monosubstituted (Ib, If–k, IIc–g) and 1,1-disubstituted series (Id, IIe–f) gave rise to isomeric hydroxyalkyl carbonates and/or hydroxyalkyl carbamates, the ratio of which varies with substituents and pH. In the 1-monosubstituted carbonates the preferred mode of O–CO bond cleavage is that which leads to the more acidic alcoholic species, whereas in the 1,1-disubstituted series the mode of ring-opening which leads to the more basic alcohol species is the preferred one. The product ratio in the 1-monosubstituted series is linearly dependent on the σ* value of the substituent on the ring, and also yields a linear Bronsted correlation. The two modes of ring-opening in 1-monosubstituted carbonates is governed by the difference in the electronic effects of the leaving oxygens. In the case of 1,1-disubstituted carbonates the polar effect of the leaving group appears to be obliterated by steric factors (conformational and torsional effects) which become mode-determining for the breakdown of the tetrahedral intermediate.