Mok, Pearl Ling Ho; (1993) Polarity reversal catalysis and enantioselective hydrogen-atom transfer. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is divided into two sections: Section A Polarity reversal catalysis (PRC) is a method by which sluggish abstraction of electron deficient hydrogen by electrophilic radicals, such as t-butoxyl radical (But0.), can be accelerated. In this section of the thesis, the effect of PRC by amine-alkylborane complexes on hydrogen-atom transfer from ketones was studied. ESR spectroscopy was used to monitor the radical reaction products. t-Butyl methyl ether was found to be 30 times more reactive than acetone towards hydrogen-atom abstraction by But0. at 190 K, and this difference is attributed entirely to polar effects. In contrast, the highly-nucleophilic amine-boryl radical Me3N→BHThx abstracts hydrogen very much more rapidly from acetone than from the ether. Competitive abstraction from the two different types of α-C-H groups in methyl isopropyl ketone MeC(0)CHMe2 by amine-alkylboryl radicals which have differing steric demands has also been quantified. Specific alkyl radicals can be generated for ESR studies at low temperature by UV irradiation of a solution containing di-t-butyl peroxide, trimethylamine-butylborane, and the corresponding alkyl chloride. This procedure complements the "triethylsilane method" of Hudson and Jackson for alkyl radical generation from bromoalkanes. Section B This section describes the application of PRC to bring about enantioselective α-hydrogen-atom abstraction from esters of the type R1R2CHCO2R 3 using optically active amine-alkylboranes to catalyse the reaction of t-butoxyl radicals. This procedure has been used to bring about kinetic resolution of such esters. The bis(isopinocampheylborane) complex of N,N,N1,N1-tetramethyl- ethylenediamine and some of its derivatives were investigated as optically active polarity reversal catalysts. After partial consumption of the substrate, the amount remaining and its enantiomeric excess (ee) have been used to derive enantioselectivity constants (kR/S/kS/R) for α-hydrogen-atom abstraction from a variety of esters. Enantioselectivity is sensitive to the substituent bulk at the 2-position of the pinene moiety of the catalyst, and varies considerably with the structure of the substrate. The highest selectivity was observed for hydrogen abstraction from dimethyl 2,2-dimethyl-1,3-dioxolane-trans-dicarboxylate, when after 75% consumption of initially-racemic ester at 183 K, the residual substrate showed an ee of 97%. A transition state model is proposed to account for the observed enantioselectivities. In addition, ESR spectroscopy has been used, for the first time, to measure the relative rates of the elementary enantioselective hydrogen-atom abstraction reactions of optically active amine-boryl radicals.

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