NEW CATALYSTS FOR THE ESTER-INTERCHANGE REACTION - THE ROLE OF ALKALI-METAL ALKOXIDE CLUSTERS IN ACHIEVING UNPRECEDENTED REACTION-RATES

The catalytic effect of alkali-metal tert-butoxide clusters on the rat e of ester interchange for several pairs of esters has been determined in nonpolar and weakly polar solvents. Reactivities increase in the o rder (Li+ < Na+ < K+ < Rb+ < Cs+) with the fastest rates reaching 10(7 ) catalytic turnovers per hour (TO/h). Ester interchange rates were se nsitive to the size of both the transferring OR groups and the ester s ubstituent. Phenyl esters did not exchange with aliphatic esters due t o nonstatistical breakdown patterns in the tetrahedral intermediate. A first-order equilibration analysis on the interchange between tert-bu tyl acetate (tBuAc) and methyl benzoate (MeBz) (5 mol % NaOtBu) indica ted enhanced reaction rates as the reaction proceeded. Isolation and q uenching (DCl/D2O) of precipitated catalyst points to a mechanism wher eby sequential methoxy incorporation into the catalyst cluster increas es activity, but eventually precipitates out of solution as a 3:1 OMe: OtBu cluster. The rate law was determined to be k(obs)[MeBz](1) [tBuAc ](0)[NaOtBu](x), where x = 1.2(1), 1.4(1), and 0.85(1) in hexane, ethe r, and THF, respectively, under conditions when tetrameric catalyst ag gregates are expected. Reaction rates were generally observed to be hi gher in nonpolar solvents (hexane > toluene, ether > THF). Eyring anal ysis over a 40 degrees C range yielded Delta H-double dagger = 10.0(1) kcal mol(-1) and Delta S-double dagger = -32(3) eu. A Hammett (sigma) plot generated with para-substituted methyl benzoates gave rho = +2.3 5 (R = 0.996). These results are interpreted in terms of a catalytic c ycle composed of two coupled transesterification reactions with a turn over-limiting addition of a tert-butoxy-containing cluster (tetramer) to methyl benzoate. Catalyst relative reactivities (Cs+ > Rb+ > K+ > N a+ > Li+) are interpreted in terms of competitive electrostatic intera ctions between the alkali-metal and ground-state and transition-state anions. This analysis predicts the observed linear dependence between log(k(obs)) and 1/r(ionic).