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I would like to thank Dr. Stephen A. Glover for encouraging me to persevere with this project. His direction, skill and insight into fundamental chemical processes enabled this work to progress along paths that would not necessarily have been obvious nor solvable to the author without consistent and pertinent advice.
Many thanks must go to Dr. A. Bonin for performing the mutagenicity testing.
Financial assistance was obtained from an "Australian Postgraduate Research Award" federal government scholarship. A small grant from the "Run For Your Life" research funds from the University of New England also provided important financial support for this work.
I would also like to thank my wife, Veronica for the sacrifice she made in enabling me to return and complete the project after a long delay. Without her support the project would not have been completed. Thanks must also go to our two families who helped and encouraged me throughout this project.
Lastly, I would like to thank my Nan.
Preface:
Parts of this work have appeared in three publications to
date.
Evidence for the formation of nitrenium ions in the acid-catalysed solvolysis of mutagenic N-acetoxy-N-alkoxybenzamides.
Campbell, J. J.; Glover, S. A.; Hammond, G. P.; Rowbottom, C. A.
J. Chem. Soc., Perkin Trans. 2. 1991, 2067.
Reactive intermediates from the solvolysis of mutagenic O-alkyl N-acetoxybenzohydroxamates.
Bonin, A. M.; Glover, S. A.; Hammond, G. P. J. Chem. Soc.,
Perkin Trans. 2. 1994, 1175.
Decomposition of N-N'-diacyl-N-N'-dialkylhydrazines revisited.
Buccigross, J. M.; Glover, S. A.; Hammond, G. P. Aust. J. Chem.
1995, 48, 353.
A new class of relatively stable compounds, alkyl N-acyloxybenzohydroxamates, were synthesised and found to be excellent sources of alkoxy stabilised nitrenium ions under acidic conditions. Under basic conditions the alkyl N-acyloxybenzohydroxamates are rapidly consumed in bimolecular reactions. All precursors were determined to be mutagenic.
In aqueous acetonitrile, butyl N-acetoxybenzohydroxamate decomposes via an AAl1 mechanism to an N-acyl-N-butoxynitrenium ion and the progress of the reaction was monitored by 1H NMR across the temperature range 298-338K. The acid-catalysed solvolysis was repeated with a number of electron-donating and electron-withdrawing para substituents and the Arrhenius data was obtained, which determined that nitrenium ion formation proceeds with positive entropy of activation and negative heat of enthalpy. Analysis of the Hammett plot revealed an excellent s+ relationship with moderate slope that reflected the separation between the ring and the developing positive charge of the nitrenium ion.
The extent of alkoxy stabilisation on the formation of nitrenium ion during acid-catalysed solvolysis was measured by synthesis of a series of para-substituted benzyl N-acetoxybenzohydroxamates. Hammett and Arrhenius relationships have been presented which indicate that nitrenium ion formation is modified by electronic effects exerted by the benzyloxy side chain. Positive mesomeric groups resulted in a change of mechanism to an acid catalysed E1 process which results in direct formation of para-substituted benzyl cations.
The formation of the nitrenium ion was also investigated as a function of the leaving group potential by synthesis and study of a series of benzyl N-(para-substituted benzoyloxy)benzohydroxamates. Increased electron-withdrawing ability of the para substituent lowered the energy required for separation of the leaving group from the precursor and favoured nitrenium ion formation.
Analysis of the solvolysis products revealed a number of complex solvolysis pathways which emanated from the acid-catalysed decomposition of the transient intermediate, alkyl N-hydroxybenzohydroxamate.
Alkyl N-acyloxybenzohydroxamates react with hydroxide ion via a fast BAl2 mechanism, and attack at nitrogen rather than the carbonyl was confirmed by studying rates of reaction with benzyl N-(para-substituted benzoyloxy)benzohydroxamates. The reaction products were very different from those obtained under acidic conditions and were characterised by HERON reactions which lead to ester formation.
The mutagenicity of alkyl N-acyloxybenzohydroxamates were
determined by the Ames test using TA100 salmonella typhimurium
bacteria. No clear and conclusive relationship between electronic
effects and mutagenicity levels was detected in the case of the butyl
N-acetoxy-para-substituted benzohydroxamate and
para-substituted benzyl N-acetoxybenzohydroxamate
series but the benzyl N-(para-substituted benzoyloxy)
benzohydroxamate series exhibited an inverse dependence upon
reactivity. An increase in the potency as a function of the number of
aromatic rings was evident suggesting that hydrophobicity is also a
factor promoting biological activity.
Gerard Hammond
January 1997