Relative rates deprotonation

Kinetic enolate- deprotonation of the most accessable proton (relative rates of deprotonation). Reaction done under essentially irreversible conditions. 

Structural effects on the rates of deprotonation of ketones have also been studied using veiy strong bases under conditions where complete conversion to the enolate occurs. In solvents such as THF or DME, bases such as lithium di-/-propylamide (LDA) and potassium hexamethyldisilylamide (KHMDS) give solutions of the enolates in relative proportions that reflect the relative rates of removal of the different protons in the carbonyl compound (kinetic control). The least hindered proton is removed most rapidly under these... 

Chelating ligands for magnesium were expected to influence the relative rate of deprotonation and addition [19], TMEDA gave a modest improvement in the reaction affording a 92 8 ratio of 3 61 under the best conditions. N,N -Dimethylimidazolidinone was not as effective in suppressing the formation of 61. No reaction was observed in DME, even on heating to 50°C. 

Streitwieser and Boerth studied the kinetic acidities of cycloalkenes with lithium cyclo-hexylamide (LiCHA) in cyclohexylamine for comparison with those of benzene and toluene66. The relative rates of deprotonation and the corresponding equilibrium pK values are tabulated in Table 12. These proton transfer transition states are stabilized by conjugation of the reacting C—H bond with the double bond. 

TABLE 12. Relative rates of deprotonation at 50 °C in cyclohexy-lamine, dihedral angle (C=C—C—H) as determined from force field calculations, and deduced equilibrium pAT sCHA values for several carbon acids0... 

Product 34 predominates in the polar aprotic solvent (acetonitrile), while in the polar protic solvent (methanol) products 35 are formed preferentially. The different products are caused by the relative rate of deprotonation against desilylation of the aminium radical, that is in turn governed by the action of enone anion radical in acetonitrile as opposed to that of nucleophilic attack by methanol. In an aprotic, less silophilic solvent (acetonitrile), where the enone anion radical should be a strong base, the proton transfer is favoured and leads to the formation of product 34. In aprotic solvents or when a lithium cation is present, the enone anion radical basicity is reduced by hydrogen bonding or coordination by lithium cation, and the major product is the desilylated 35 (Scheme 4). 

Results of a theoretical study of 1,3-prototropic rearrangement of 1-methylindene, catalysed by ammonia and MesN in water and in cyclohexane, have confirmed earlier predictions that the proton moves freely over the indene ring once it has been abstracted by the base. The relative rates of deprotonation, ion-pair collapse and ion-pair rearrangement have been estimated and discussed in each case. 

Base-catalyzed deprotonation of pjn idine 1-oxides should occur much more readily than in the pjrridines themselves and this prediction has been borne out by several workers. These compounds exhibit the same relative positional reactivity as do the pyridinium ions. For example, for 3-bromopyridine 1-oxide in 0.1 N NaOD-DjO the order is 2>6>4>>5, as would be expected on the basis of the net (it -t- a) electron densities at the various nuclear positions. For pyridine 1-oxide the relative rates of exchange in methanolic methoxide solutions at 50° are... 

The simplest case in which the relative rates of deprotonation determine the enantiomeric ratio of product is only given when the intermediate carbanionic species are... 

Bertini-Gross and Beak demonstrated in a broad study that conformationally restricted bicyclic carbamates undergo rapid diastereoselective deprotonation with s-BuLi/TMEDA". The proton closest to the carbonyl group is preferentially removed. Competition experiments gave information about the relative rates of deprotonation. Scheme 3 summarizes some second-order competitive efficiencies. 

The relative rates of reaction of the nucleic acid bases with heavy transition metal ions at neutral pH are in the same order as the relative nucleophilicites of the bases, that is G > A > C > U or T. This order parallels the relative rates of reactions for cA-[(NH3)2Pt(OH2)2] (see Figure 9), while the equilibrium constants for the same reactions are very close in magnitude. In contrast, HsCHgOH, which is more labile to substitution, nndergoes more favorable binding with deprotonation at N-3 of thymine residues in nucleic acids. Thus the relative facilities of individual reactions can lead to differences in initial product formation (kinetic control). Subsequent changes in the metal-nucleic acid complexes can be nnder kinetic or thermodynamic control. 

Table 7. Relative rates of deprotonation of the a-substituted tertiary amine cation radical from product distribution and laser spectroscopic studies. ...

A rough correlation between the relative rates of deprotonation and the Jq h for the bridgehead protons in the series 7, 8 and 9, was observed by Gloss and Larrabee. Taking the C-H coupling constant as a measure of the acidity of hydrocarbons, the acidity of the proton in bicyclobutane is expected to be between that of acetylene and ethylene (Jq-h values for acetylene, ethylene and bicyclobutane are 248,156 and 205 Hz respectively). 

The chemistry involved in the formation of mesoporous silica thin films is qualitatively well understood. However, specific reaction mechanisms of the individual steps are still debated. In addition, owing to the complexity of the sol-gel reaction pathways and cooperative self-assembly, full kinetic models have not been developed. From the time of mixing, hydrolysis reactions, condensation reactions, protonation and deprotonation, dynamic exchange with solution nucleophiles, complexation with solution ions and surfactants, and self-assembly, all occur in parallel and are discussed here. Although the sol-gel reactions involved may be acid or base catalyzed, mesoporous silica film formation is carried out under acidic conditions, as silica species are metastable and the relative rates of hydrolysis and condensation reactions lead to interconnected structures as opposed to the stable sols produced at higher pH. Silicon alkoxides are the primary silica source (tetramethyl orthosilicate, tetraethyl orthosilicate, tetrapropyl orthosilicate, etc.) and are abbreviated TMOS, TEOS, and TPOS, respectively. Starting from the alkoxide, Si(OR)4, in ROH and H2O solution, some of the general reactions are ... 

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