Bronsted bases, enolate formation

The only a-alkyl substrate (R = n-Pr, X = S) reacted with poor yield (15%) and enantioselectivity (11% ee). The unique combination of the catalytic components was specifically chosen to offer a dual activation of the substrate and reagent the formation of the activated nickel-enolate complex was assisted by the presence of the noncoordinating Bronsted base, while the Lewis acidic EtsSiOTf activated NFSI to become a stronger nucleophile, without interfering with the formation of the enolate. C-F bond formation resulted from the reaction between the activated substrate and the activated electrophile, prior to product release. 

The postulated catalytic cycle of the asymmetric epoxidation reaction is shown in Figure 13.10. A lanthanide metal alkoxide moiety changes to a rare earth metal-peroxide through proton exchange (I). In this step, lanthanide metal alkoxide moiety functions as a Bronsted base. The rare earth metal-BINOL complex also functions as a Lewis acid to activate electron-deficient olefins through monoden-tate coordination (II). Enantioselective 1,4-addition of rare earth metal-peroxide gives intermediate enolate (III), followed by epoxide formation to regenerate the catalyst (IV). 

Step of a general acid-catalysed reaction is general base catalysed. There is a simple relationship between values of the Bronsted coefficients a and p for the forward and reverse reactions respectively, and this may be derived using the example of enolisation of acetone (Equation 36). The equilibrium constant for the formation of enolate ion is given by the equation (= so that the Bronsted relationships... 

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