FINGERPRINTING A TRANSITION-STRUCTURE GUEST BY A BUILDING-BLOCK APPROACH WITH AN INCREMENTAL SERIES OF CATALYTIC HOSTS - STRUCTURAL REQUIREMENTS FOR GLYME AND ALPHA,OMEGA-DIMETHOXYALKANE CATALYZES IN N-METHYLBUTYLAMINOLYSIS AND BUTYLAMINOLYSIS OF 4-NITROPHENYL ACETATE IN CHLOROBENZENE

Glymes, H-(CH2OCH2)n-H, GLM(n), catalyze butylaminolysis of 4-nitrophenyl acetate in chlorobenzene. Values of k(cat)/Oxy, where Oxy is the number of oxygens in the catalyst, increase with oligomer length up to triglyme, GLM(4), and then plateau. Optimal catalysis on a per oxygen basis requires a -(CH2OCH2)4-fragment, which suggests a four-point recognition of the secondary ammonium ion of the zwitterionic tetrahedral intermediate (TI) (J. Org. Chem. 1991, 56, 2821-2826). Dissection of individual structural components and reassembly to the same structure of the complex verifies this model. The following kinetic studies of 4-nitrophenyl acetate in chlorobenzene have accomplished the task: (a) methylbutylaminolysis catalyzed by GLM(n), n = 2-4; (b) methylbutylaminolysis catalyzed by alpha,omega-dimethoxyalkanes, CH3O-(CH2)n-OCH3, DME(n), n = 2-10 and 12; and (c) butylaminolysis catalyzed by DME(n), n = 2-10 and 12. Experiment a has revealed that k(cat)/Oxy is the same for GLM(2)-GLM(4). Optimal catalysis for breakdown of a zwitterionic TI with one ammonium proton only requires a -(CH2OCH2)2- fragment. Experiment b has shown that k(cat)/Oxy is largest for DME(2) with the values for the remaining DMEs 2-2.5-fold lower. A -CH2CH2- is the best spacer between the two oxygens. Thus, bifurcated hydrogen-bond formation between the two oxygens and the one ammonium proton enhances catalysis. Experiment c has revealed that k(cat)/Oxy for DME(2) exceeds the remaining DMEs by 3-3.6-fold, except for DME(8) and DME(10), which have values of k(cat)/Oxy only 1.7-fold slower. DME(8), the carba analogue of GLM(4), likely binds to the two ammonium protons individually with the two oxygens. DME(10) behaves similarly. GLM(4) catalysis of butylaminolysis identifies -(CH2OCH2)4- as an optimal size. DME(8) catalysis confirms this size, although the two catalysts stabilize the two-proton ammonium ion differently. GLM(4) catalyzes butylaminolysis by forming two bifurcated hydrogen bonds. This suggested structure defines the size of the ammonium ion, which agrees with X-ray structural studies of polyether-ammonium complexes. Mechanistic proposals of butylaminolysis of aryl esters require such an ion. The results of this study confirm the structure of the ion in the rate-limiting step. This building-block approach is a method for ''fingerprinting'' ammonium ions in transition structures of ionogenic reactions.