Reactivity of Alkaline-Earth Metal Alkoxides

MeO N Me -catalyzcd raethanolysis of phenyl acetate is accelerated by the addition of alkaline-earth metal salts. The kinetics are consistent with a reaction scheme [Eq. (1)] involving preassociation of MeO ion with the metal cation (Km) and independent contributions to the overall rate from free (ko) and cation-paired (Icm) MeO [3,4]. 

Treatment of rate data according to a standard binding isotherm [4] gave K r = 60 Kbs = 44 M = 4.7, Ba/fco = 3-7- Larger rate enhancements were obser- 

It is unknown whether and to what an extent the metal-ethoxide species contains a bromide counterion, but its kinetic behavior is that of a single species. Quite remarkably, cleavage of phenyl acetate is 62 and 45 times faster with EtOSrBr and EtOBaBr, respectively, than with EtONMe4 [6]. The corresponding figures in the cleavage of p-nitrophenyl acetate are 8.0 and 7.0, respectively [5]. 

In view ofthe widespread notion that ion pairs are less reactive than free anions [7], the very finding that metal-bound alkoxides cleave esters more rapidly than free alkoxides do came as a surprise. It seems likely that transition state stabilization takes place via chelate structures having the form of a four-membered contact ion pair I, or of the kinetically equivalent six-membered solvent shared ion pairs II and III. 

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