Transition state oxalates

The oxidation by Mn(lII) chloride involves three complexes and the kinetic data of Taube " are summarised in Table 15. The greater thermal stability of the /m-complex is considered to result from the lowering of the free energy relative to the transition state as compared with bis- and mono-complexes. The study of MnC204 was based on the Mn(III)-catalysed chlorine oxidation of oxalic acid. ... 

Following earlier studies of the oxidation of formic and oxalic acids by pyridinium fluoro-, chloro-, and bromo-chromates, Banerji and co-workers have smdied the kinetics of oxidation of these acids by 2, 2Tbipyridinium chlorochromate (BPCC) to C02. The formation constant of the initially formed BPCC-formic acid complex shows little dependence on the solvent, whilst a more variable rate constant for its decomposition to products correlates well with the cation-solvating power. This indicates the formation of an electron-deficient carbon centre in the transition state, possibly due to hydride transfer in an anhydride intermediate HCOO—Cr(=0)(0H)(Cl)—O—bpyH. A cyclic intermediate complex, in which oxalic acid acts as a bidentate ligand, is proposed to account for the unfavourable entropy term observed in the oxidation of this acid. 

Rate constants and Arrhenius parameters for the reaction of Et3Si radicals with various carbonyl compounds are available. Some data are collected in Table 5.2 [49]. The ease of addition of EtsSi radicals was found to decrease in the order 1,4-benzoquinone > cyclic diaryl ketones, benzaldehyde, benzil, perfluoro propionic anhydride > benzophenone alkyl aryl ketone, alkyl aldehyde > oxalate > benzoate, trifluoroacetate, anhydride > cyclic dialkyl ketone > acyclic dialkyl ketone > formate > acetate [49,50]. This order of reactivity was rationalized in terms of bond energy differences, stabilization of the radical formed, polar effects, and steric factors. Thus, a phenyl or acyl group adjacent to the carbonyl will stabilize the radical adduct whereas a perfluoroalkyl or acyloxy group next to the carbonyl moiety will enhance the contribution given by the canonical structure with a charge separation to the transition state (Equation 5.24). 

A theoretical study of the thermal isomerization and decomposition of oxalic acid has attempted to account for the predominant formation of C02 and HCOOH from the vapour at 400-430 K.41 Transition-state theory calculations indicate that a bimolecular hydrogen migration from oxygen to carbon of intermediate dihydroxycarbene (formed along with C02) achieved through a hydrogen exchange with a second oxalic acid... 

Oxalic acid dinitrate ester, 02N—02C—C02—N02 is metastable with respect to decomposition into C02 and N02. The reaction barrier (transition state) for the monomolecular dissociation was calculated to be 37 kcal mol-1 at CBS-4M level of theory. 

The rates of alkaline hydrolysis of the half-esters, potassium ethyl oxalate, malonate, adipate, and sebacate were studied in the presence of potassium, sodium, lithium. thallium(I), calcium(II), barium(II), and hexamminecobalt(III) ions (106). On the basis of the results obtained, chelate formation between the metal ions and the transition state of the substrate was postulated. In these chelate structures (structures XXXVIII), formally similar to those postulated in the hydrolysis of a-amino esters (26), the metal ion facilitates the attack by the hydroxide ion by positioning it in a suitable manner. The rate of hydrolysis of the oxalate half-ester is greater than that of the malonate, which in turn is greater than that of the adipate. This is in the expected order of the stability of the metal chelates. The order for the rate of hydrolysis of the ethyl oxalate and ethyl malonate is Ca2+ Ba2+ > [Co(NH3)6]3+ > T1+. The hexamminecobalt(III) ion seems to be less effective than expected, since it is too large to satisfy the steric requirements of the chelate structures. The alkali metals were found to have marked negative specific salt effects on the rates of reaction of the adipate and sebacate, but only a small negative salt effect on the hydrolysis of potassium ethyl malonate. 

The following sequence of dipositive metal ions shows a decreasing effect on the rate of decarboxylation of oxaloacetic acid Cu(II), Zn(II), Co(II), Ni(II), Mn(II), Cu(II) (91). The rate constants for these decarboxylations approximately parallel the formation constants of the corresponding metal oxalates. A similar result was found in the decarboxylation of acetonedicarboxylic acid in the presence of certain transition metal ions the decarboxylation rates paralleled the formation constants of the metal malonates (170). These parallelisms indicate that the effectiveness of a metal ion in these decarboxylation reactions depends on its ability to chelate with the oxalate ion and the malonate ion, which resemble the transition states of the oxaloacetic and acetonedicarboxylic acids, respectively. 

The formation constants Kma = [MA]/[M ][A ] do not correlate with the Rma values but there is a quite good linear free energy relationship between log Icma and the formation constants of the corresponding oxalato complexes. Fig. 7. It is argued that the transition state for decarboxylation should more closely resemble the enolic intermediate which is "oxalate-like" in character. 

A study of the rates of isomerization of cyclohex-3-enone and cyclopent-3-enone with a variety of general bases has been made differences in effectiveness of the catalysts were attributed to electrostatic interactions in the transition state. As a result of studies on the effect of a-, and y-substituents on the equilibration of aj8- and )Sy-unsaturated bicycloalkanones, it now seems erroneous to assume that the resonance-stabilized a)3-unsaturated isomer will always predominate. Methyl ketones may be converted into diketolactones (42) by successive base-catalysed reactions with ethyl oxalate and an aldehyde (R CHO). These diketolactones lose the elements of carbon monoxide and carbon dioxide when pyrolysed in a quartz tube at 620 C to give aj8-unsaturated ketones. ... 

Scheme 13 The transition state proposed by Pizer was considered associative with the intermediate complex being tetrahedral. In this transition state it was suggested that proton transfer from the entering ligand (here, the oxalic acid on the right) to the leaving hydroxyl (the proton acceptor) could be ratedetermining (for clarity the mobile proton is indicated in bold red). One caveat for the proposed transition state was that the interconversion between trigonal and tetrahedral geometries should be rapid, as in the addition of OH to B(OH) 3, which had been previously assumed to be diffusion controlled ...

Fig. IIA-D. Modeled reaction sequences of the oxalate-silica complex. A Oriented hydrogen-bonded complex (probably the most common species). B Transition state to the monodentate complex via Sis[2-type ligand exchange. Geometry is a penta-coordinated triganol bipyramid. C Monodentate complex proceeding into the transition state to the bidentate via the same reaction as B. D The bidentate silica-oxalate 1 1 complex...

Scuseria, G.E., and H.F. Schaefer III (1989), The unimolecular triple dissociation of gly-oxal Transition-state structures optimized by configuration interaction and coupled cluster methods, J. Am. Chem. Soc., Ill, 7761-7765. 

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