Kinetic isotope effects fluorination

A detailed kinetic study using UV-vis, FTIR (Fourier-transform infrared), and NMR spectroscopy a Hammett plot with /0 = -h1.98 using para-substituted styrene oxides an inverse solvent kinetic isotope effect (KIE) (fcnHp/ HHP-d2) = 0.86 and nonlinear effects studies" have all shown that the (salen)Co- (J) and amidine-co-catalysed enantioselective ring opening of terminal- and mew-epoxides by fluoride ion (forming trans- -ttuoTO alcohols in a 42-89% yield with 84-99% ee) occurs by the mechanism shown in Scheme 5. /-BuOOH oxidizes the Co(H) to Co(III) in the (salen)Co(III) catalyst. PhCOF provides the fluorine for the reaction. 

CFjCl, CFjCl CFaCl, and CFs CFaCT and CFCl, >-i have been made, and Arrhenius parameters for the rate-determining step F + RQ - R + CIF determined the heat of this reaction was estimated for each substrate, and the values were used to calculate a large number of C—Cl, C—F, and C—C bond strengths [e.g. Z)°(CC1 F—Cl) = 300 8 kJ mol ]. Kinetic isotope effects for H-atom abstraction reactions of fluorine atom with chloroform, methane, and hydrogen have been measured the results are those expected from the differences in zero-point energies for the relevant vibrations, so H-atom transfer by quantum mechanical tunnelling is apparently unimportant. ... 

Since oxygen is not monoisotopic, isotope ratios of the product oxide ions are complicated by the superimposition of oxygen s isotope ratios. To avoid this complication, Vanhaecke et al. used monoisotopic F-atom transfer to selectively fluorinate Sr" " in reactions with CH3F, in order to overcome isobaric overlap of Rb on Sr. The Sr isotope ratios were then measured as SrP" " ratios. The Sr/Rb age of magmatic rocks determined using this approach was in close correlation with TIMS data. Kinetic isotope effects (discussed in Section 8.1.8 of Part 8.1 of Chapter 8) and mass bias were compensated for by using matrix-matched external isotope standard. 

The influence of steric effects on the SNAr mechanism has been studied by means of leaving group fluorine kinetic isotope effects. The reaction of DNFB with o-and -toluidines shows a change in the rate-determining step depending on the steric hindrance of the amine employed. 

The double-isotopic fractionation method was employed in this study. This procedure consists of the use of deuterium substitution to selectively slow down the rate of one step in a reaction and observing the changes in a second kinetic isotope effect. In this study flie aim is to obtain information from the secondary D KIE at C4 and the fluorine isotope effects, respectively. For that reason, substrates on which the deuterium has been placed at C3 to slow down the step in which the C3-H bond is being broken, have been employed. Then, the extra labels were deuterium at C4 (compound 4) and labeled fluorine (compound 6) (atoms colored red in Fig. 37.1). Compounds 3 and 4 were used to determine the effect of the deuterium at C3 on the secondary D KIE values at C4 (k lk ) (Fig. 37.1). Similarly, compounds 5 and 6 were used to study the influence of the deuterium label at C3 on the leaving group F KIEs... 

Stereochemistry, substituent effects and activation parameters of most ketene reactions are consistent with a one-step cycloaddition polar effects of substituents and solvents, as well as the isotope effect, often require, however, that a fair amount of charge separation (that is, unequal bond formation) characterises the transition state. It has been kinetically proved that cycloadditions of enamines to ketenes can also proceed through a dipolar intermediate this is so for the reaction between dimethylketene and N-isobutenylpyrrolidine . In the latter case, the rate coefficient for the formation of the intermediate strongly depends on solvent polarity itacetonuriie/ cyclohexane = 560. Use of the Same criteria used for ketenes (as far as experimental data allow it) in the case of the 1,2-cycloadditions of fluorinated olefins results, instead, in the conclusion that a two-step biradical mechanism is operating. Results for 1,2-cycloaddition of sulfonyl isocyanates to olefins, cases (g) and (h) in Table 17, give indications of dipolar intermediates during the course of these reactions. 

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