Oximate ions

It is accepted that the acmal nucleophile in the reactions of oximes with OPs is the oximate anion, Pyr+-CH=N-0 , and the availability of the unshared electrons on the a-N neighboring atom enhances reactions that involve nucleophilic displacements at tetravalent OP compounds (known also as the a-effect). In view of the fact that the concentration of the oximate ion depends on the oxime s pATa and on the reaction pH, and since the pKs also reflects the affinity of the oximate ion for the electrophile, such as tetra valent OP, the theoretical relationship between the pATa and the nucleophilicity parameter was analyzed by Wilson and Froede . They proposed that for each type of OP, at a given pH, there is an optimum pK value of an oxime nucleophile that will provide a maximal reaction rate. The dissociation constants of potent reactivators, such as 38-43 (with pA a values of 7.0-8.5), are close to this optimum pK, and can be calculated, at pH = 7.4, from pKg = — log[l//3 — 1] -h 7.4, where is the OP electrophile susceptibility factor, known as the Brpnsted coefficient. If the above relationship holds also for the reactivation kinetics of the tetravalent OP-AChE conjugate (see equation 20), it would be important to estimate the magnitude of the effect of changes in oxime pX a on the rate of reactivation, and to address two questions (a) How do changes in the dissociation constants of oximes affect the rate of reactivation (b) What is the impact of the /3 value, that ranges from 0.1 to 0.9 for the various OPs, on the relationship between the pKg, and the rate of reactivation To this end, Table 3 summarizes some theoretical calculations for the pK. ... 

Rubidium chloride even slows the reaction, this is especially well seen within a time span of 1-3 hr after the start of the process (Fig. 2, curve 2). In this case the normal salt effect is likely to prevail over the effect of oximate ion pair separation due to substitution of the potassium cation by the rubidium cation. The addition of cesium carbonate during the first 1.5 hr does not much affect the rate of the formation of 2-phenylpyrrole. The accelerating effect of these additives becomes evident only 2 hr after the beginning of the reaction and gradually increases (5 hr later the yield gain of pyrrole is 7% as compared with a standard run, Fig. 2, curve 4) which seems to result from a slow rate of heterophase exchange process ... 

In reactions that resemble the pinacol rearrangement, thiopyranoside sulphonate esters undergo ring contraction and expansion reactions via the intermediacy of sulphonium ions, e.g. equations 37 and 38102. Similarly, cyclic sulphonates are able to trap oxime anions at the carbon atom (equation 39)103. Such carbon alkylation of oxime ions is rare, and by suitable choice of substrate both ring-contraction and ring-expansion reactions are possible103. 

Crown ether/cryptand effects were also revealed by Buncel et al. in the oximate-promoted 1,2-elimination from (3-phenylmercaptoethyl p-nitrophenolate in tetra-glpme (Eq. The addition of cryptand [2.2.2], 9, in fact, greatly enhanced (654 times) the oximate reactivity in comparison with the potassium oximate ion-paired, whereas the effect was much lower with crown ether DCH18C6, 5 (only 11 times) (see equation below)... 

It has been shown that 5y -p-nitrophenyl phenacyl methylphosphonate oxime undergoes rapid hydrolysis (64) in which the oximate ion acts as an internal base. 

Sodium 9-fluorenone oximate, as dissociated ions (114), is alkylated to the extent of 65% at O and 35% at N. When existing as ion pairs the oximate reacts to give 30% 0-methyl product. In the presence of a crown ether the oximate ion undergoes almost exclusive (95-99%) 0-methylation with MeOTs and 0/N-methylation (65 35) with Mel (115). Addition of NaBPh4 to suppress dissociation of the sodium oximate reduces the alkylation with both MeOTs and Mel to nearly the same rate and gives essentially an identical proportion (ca. 43 57) of 0/N-alkylation. Under such conditions, the two heteroatoms are of approximately the same hardness. 

The changes observed in the and F NMR spectra result from averaging a number of chemical shifts and coupling constants. This can be rationalized by the occurrence, at low temperatures, of two enantiomers, which are intercon-verted at elevated temperatures due to the internal rotation of the oximate ion ... 

If the above inference is correct, the difference between the barriers to ligand rotation in compounds with the anti and syn conformation of the oximate ion should considerably increase with an increase in the difference between the... 

Theoretical studies of the hydrolysis of methyl phosphate anion have found that the dissociative mechanism involving the formation of metaphosphate is favoured over the associative mechanism involving a pentacoordinated intermediate.Phytic acid, myo-inositol hexakis(dihydrogen phosphate) (127), was completely hydrolysed to inositol in an aqueous polybasic alcohol at 150 °C a reaction mechanism was proposed. The reactions of p-nitrophenyl diethyl phosphate (128) and ethyl p-niuophenyl ethylphosphonate (129) with a wide range of oximate ions (p/(a = 7-13) in 10% aqueous ethanol at 25 °C have been studied. For oximate ions with p a 9.0, the reactivity of the oximes tends towards that of alcoholate ions and their Q -effect disappears (as had been previously shown with / -niuophenyl acetate). The reason for this is the unfavourable solvation effects of the solvent. A review (253 references) has appeared on the synthesis and reaction of Q -aminophosphonates. ° ... 

Extremely strong solvational imbalances in the transition state for deprotonation of a trinitrobenzylic carbon acid (96) by oximate bases in 1 1 (v/v) H20-Me2S0 accounts for the occurrence of rapid Marcus curvature in the corresponding Brpnsted plots and the consequent levelling of oximate reactivity in the proton transfer process. " The desolvation of the oximate ion pair prior to actual proton transfer ensures that the intrinsic barrier becomes dominated by the work term for formation of the encounter complex. 

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