"New" Catalysts for the Ester-Interchange Reaction: The Role of Alkali-Metal Alkoxide Clusters in Achieving Unprecedented Reaction Rates

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• 作者：Matthew G. Stanton ; Cara B. Allen ; Rebecca M. Kissling ; Alice L. Lincoln ; and Michel R. Gagné
• 刊名：Journal of the American Chemical Society
• 出版年：1998
• 出版时间：June 24, 1998
• 年：1998
• 卷：120
• 期：24
• 页码：5981 - 5989
• 全文大小：378K
• 年卷期：v.120,no.24(June 24, 1998)
• ISSN：1520-5126

文摘

The catalytic effect of alkali-metal tert-butoxideclusters on the rate of ester interchange for severalpairs of esters has been determined in nonpolar and weakly polarsolvents. Reactivities increase in the order(Li⁺ < Na⁺ < K⁺ <Rb⁺ < Cs⁺) with the fastest rates reaching10⁷ catalytic turnovers per hour (TO/h).Ester interchange rates were sensitive to the size of both thetransferring OR groups and the ester substituent.Phenyl esters did not exchange with aliphatic esters due tononstatistical breakdown patterns in the tetrahedralintermediate. A first-order equilibration analysis on theinterchange between tert-butyl acetate (tBuAc)andmethyl benzoate (MeBz) (5 mol % NaOtBu) indicated enhancedreaction rates as the reaction proceeded.Isolation and quenching (DCl/D₂O) of precipitatedcatalyst points to a mechanism whereby sequential methoxyincorporation into the catalyst cluster increases activity, buteventually precipitates out of solution as a 3:1OMe:OtBu cluster. The rate law was determined to bek_obs[MeBz]¹[tBuAc]⁰[NaOtBu]^x,where x = 1.2(1),1.4(1), and 0.85(1) in hexane, ether, and THF, respectively,under conditions where tetrameric catalyst aggregatesare expected. Reaction rates were generally observed to be higherin nonpolar solvents (hexane &gt; toluene,ether &gt; THF). Eyring analysis over a 40 ges/entities/deg.gif">C range yieldedges/gifchars/Delta.gif" BORDER=0 >H^{ges/entities/Dagger.gif">} = 10.0(1) kcalmol^-1 and ges/gifchars/Delta.gif" BORDER=0 >S^{ges/entities/Dagger.gif">} =-32(3) eu.A Hammett (ges/gifchars/sigma.gif" BORDER=0 >) plot generated with para-substituted methylbenzoates gave ges/gifchars/rho.gif" BORDER=0 > = +2.35 (R = 0.996).Theseresults are interpreted in terms of a catalytic cycle composed of twocoupled transesterification reactions witha turnover-limiting addition of a tert-butoxy-containingcluster (tetramer) to methyl benzoate. Catalystrelativereactivities (Cs⁺ &gt; Rb⁺ &gt;K⁺ &gt; Na⁺ &gt; Li⁺) areinterpreted in terms of competitive electrostaticinteractionsbetween the alkali-metal and ground-state and transition-state anions.This analysis predicts the observedlinear dependence between log(k_obs) and1/r_ionic.