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General Acid Catalysis If Acetal Hydrolysis GENERAL ACID CATALYSIS IF ACETAL HYDROLYSIS by ICSITH NB3!0. A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY OF THE UNIVERSITY OF GLASGOW* September, 1971 Chemistry Departnen University of Glasy ProQuest Number: 11011971 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 11011971 Published by ProQuest LLC(2018). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 To my wife - Isobel. Ac know! e d; ;c;.:e nts I should like to .record my sincere gratitude to Dr* 3* Capon, my supervisor, for his constant enthusia and the many informative and stimulating discussions I had with him. I am indebted also to the technical staff of this department for the many services they so willingly pro­ vided, and to Hiss A. Ltatheson who typed this thesis. Finally, I should like to acknowledge the finane:Uil support given by the Science Research Council for the undertaking* of this work. TCo Fimmo, Chemistry Departi:lent, September, 1971. University of Glstsgow. Abstract Recear advances in the study of acetal hydrolysis and the use of acetals in model systems for the investigation of enzymically catalysed hydrolysis are reviewed. The kinetics of the hydrolysis of benzaldehyde sub­ stituted (X) - phenyl methyl acetals and substituted (Y) - benzaldehyde phenyl methyl acetals were measured in pivalate, acetate, d-|_-deutero-acetate, 6 -cliloropropionate, formate and chloroacetate buffers, and in hydrochloric and deutero- chloric acids. The hydrolysis rates measured in buffer solutions, of the twelve acetals sin died were found to be dependent on the concentration of undissooiated acid and shorn to be authentic examples of general acid catalysis, with reference to current opinions that such observations might be manifestations of solvent, electrolyte or buffer effects. The d-g-acetic acid catalysed dy-methanolysis of benzf- aldehyde phenyl methyl acetal was followed in a high resolu­ tion 1J.M.R. spectrometer and the initial bond fission found to occur between carbon and the phenolic oxygen. It seemed not unreasonable to assume that the same process occurs also in the hydrolysis reaction. Breasted and Hamnett linear free energy relatronsnips were found to correlate m e ca calycic constants obrained from the ir^drolvsis of both (X) and (Y) substituted-acetal serves. Tne catalytic constants of the substituted (X) ~ phenyl acetals increased as the e le ctron-withdrawing power ox (X) increased,' and tlie positive p value increased con­ sistently as the catalytic power of the buffer decreased, vis« 0.43 in chloroaeetate to 1.25 in. pivalate. The P value for the hydronium ion catalysed reaction, was -0 .4 5 . Similarly, the negative p values obtained from the sub­ stituted (X) - benzaldehyde acetals increased in magnitude from. -2.04 in chloroaeetate to -2.34 in pivalate, with a value of -1 , 9 4 for the hydroniuni ion catalysed reaction. As the electron - withdrawing power of (X) increased, the Br-^nsteda value decreased consistently from 0.96 for X = p-IIeO to 0.49 forX = m - '102. Conversely as the electron - withdrawing power of (Y) increased, the a value increased from 0.63 for f = n - HeO to 1.05 for m - FO^. Solvent isotope effects k(IT^0+)/k(D^0+ ), indicated not only a consistency of mechanism in. the hydrolysis of all the substrates studied, but the trend in. values, from 0.62 for X = E-IIeO to 1.1 for X = n - H02, and 0.35 for T - n-HeO to 0 . 6 6 for f = m - F, correlated with the « and p values in determining the relative amount of bond making and brak- • ing in the transition states. The results obtained were all indicative of an A-SB 2 mechanism in the general acid catalysed hydrolysis of aromatic ar^l methvi acetals, with concerted proton transfer to phenolic oxygen and carbon-phenolic oxygon bond fission in •tne rate limiting step and where incipient carbonium ion stability and leaving group ability, rather than a high relative degree of proton, transfer in the transition state, were commensurate to faster hydrolysis rates. The com­ parison of this data with that obtained in. other reaction series indicates that incipient carbonium ion stability and leaving group ability are prerequisites for the obser­ vation. of general acid catalysis in acetal hydrolysis, although relief of steric strain through carbonium ion ' formation is also relevant. The hydrolysis of benzaldehyde methyl acetyl acylal, the possible intermediate of acetate anion nucleophilic attack, on benzaldehyde phenyl methyl acetal, was found to be general acid catalysed, and is believed to be the first reported example of buffer catalysis in acylal hydrolysis. The hydrolysis of r-methylbenzaldehyde metnyl S—phenyl thioacetal was carried out in weak—acid Duffers, and the results suggested, although not conclusively, that this substrate might also be catalysed by undissociateo. acid. General: Enzymic Reactions; Model System: General Acid Catalysis; Model System Acetal Hydrolysis: A-l: Transition-State Theories: Electrolyte and Solvent Effects; 3r/nsted Correlation:- Isotope Effects: Hammett Correlation: Discussion/ C 0 IT T B :i rp 3 (cont'd) Paye lo, Discussion 1 Buffer Catalysis: (Solvent-Electrolyte Effects) 62 2 Site-of Bond fission 78 3 Kinetic Hate Law 85 4 Mechanism 93 5 Br^nsted Correlation 95 6 Hamnett Congelation 107 7 Solvent Isotope Effect 117 Results: 125 Experimental: Kinetic: 229 Preparative: 233 References: 218 I H f H O D U C 0? 1.0 IT IITTR ODD C TI PIT' ,TIt is not generally appreciated how little is under­ stood about the mechanism by which enzymes bring about their extraordinary and specific rate accelerations". W.P. Jeneks.1 (1969) The mechanism of the enzymically catalysed reaction has been studied over many years, and the recent statement by Jeneks indicates the complexity of this important cata­ lytic system. A more specific example of the problem is that the elucidation of the amino acid sequence and ultim- 2 ately the three dimensional structure of the glycosidase lysozyne led to an intensive chemical and biophysical investigation of that system which has not yet revealed the exact mechanistic principles involved. Workers in this field have sought model systems in an attempt to explain the mechanistic and rate differences observed between gly­ coside hydrolysis in the enzymic and non-enzymic systems. The significance of the "non-general" behaviour propounded in more recent work must be interpreted with caution, although it is hoped that it will lend itself to a closer insight of the enzyme problem. The aim of this introduction will be to outline the most significant of the mechanisms suggested to account for the "extraordinary and specific rate accelerations" of the enzymically catalysed hydrolysis of glycosides, and the model systems used to explore these hypotheses. A briej. review of the generally accepted mechanism of acetal hydrolysis will be followed by a more detailed discussion of the requirements necessary for the observation of general acid catalysis and the physical parameters incor­ porated in such an investigation. The enzyme lysozyme will frequently be referred to because of the more detailed knowledge surrounding it, although it must be remembered that "\7e do not understand the mechanism of action of any glycosidase". B. Capon.5 (1971) The existence of enzyme-substrate complexes as inter­ mediates in enzymically catalysed reactions has been shown either by isolation and chemical characterisation of the complex, by accommodating observed rates with the formation and breakdown of such intermediates, or showing that a common intermediate is formed with specific substrates in the enzymatic reaction (see Ref* 1)0 The general enzy­ matically catalysed reaction is then described, on this basis, by equation (1) in which ES represents the inter­ mediate complex. k-> k’z E+S = = £ BS — ^ E + P ............. (1) k2 The formation and nature of this intermediate complex is critical to the observed rate enhancements brought about by enzyme tion (tej) of products (P) will depend on the ease of the chemical breakdown of the substrate within this environment0 G-lycosidase catalysed hydrolysis resemble the acid catalysed hydrolysis of alkyl and anyl glycosides in that bond fission in the substrate usually occurs between carbon 1 and the anomeric oxygen. Pig. 1. This has been demonstrated for several glycosidase 5 catalysed hydrolysis, including the lysozyme catalysed 18 hydrolysis of tri-lT-acetylchitotriose in 0 -enriched water, where the products, di-IT-acetyl chitobiose and N-acetylglucosamine contain the label at position 1 only.^ Further, since these reactions are formally nucleo- philic substitutions of the saturated carbon 1, the hydro­ lysis by the enzymes proceed either with inversion or retention of configuration (Fig. 2) Fig. 2. Retention Inversion Several processes have "been forwarded to explain the facilitation of glycosidic oxygen bond fission after forma­ tion of the enzyme substrate complex, and thus the enhanced rate of enzymic catalysis. Six such processes are des­ cribed below, although it is unlikely that one
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