Diart, Valerie; (1995) The control of selectivity in free radical reactions: A mechanistic and synthetic approach. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is divided into three sections. SECTION A: ESR spectroscopic studies of the addition of α-(alkoxycarbonyl)alkyl radicals to terminal alkenes, furan and isocyanides were carried out in cyclopropane solution at low temperatures. The radical addenda were generated by UV photolysis of di-t-butyl peroxide in the presence of the corresponding ester and trimethylamine-butylborane as a polarity reversal catalyst. For all acceptors, addition of each of the electrophilic α-(alkoxycarbonyl)alkyl radicals is faster than the corresponding addition of simple (nucleophilic) alkyl radicals, a result which is attributed to the importance of polar effects in the transition states. The radical H2C=CH(CH2)3C(CO2Et)2 undergoes 5-exocyclisation more rapidly than the unsubstituted hex-5-enyl radical; this is attributed to the electrophilic nature of the radical centre in the former species. SECTION B: A variety of optically active amine-borane complexes have been used as polarity reversal catalysts for the kinetic resolution of racemic carbonyl-containing compounds at -74 °C in oxirane solvent. The key step involves enantioselective abstraction of hydrogen from a C-H bond α to the carbonyl function by optically active amine-boryl radicals. The more reactive substrate enantiomer can generally be predicted by consideration of the steric interactions between the substituents attached to the reacting centres in the transition states. However, hydrogen-bonding, dipole-dipole interactions, together with stereoelectronic effects, may also play a part in determining enantioselectivity particularly when there is not marked steric asymmetry around the reacting centres. The molecular structures of optically active quinuclidine-isopinocampheylborane and of the polycyclic amine-borane formed by cyclisation of N-(1 R)-nopylpyrrolidine-borane have been determined by X-ray crystallography. The steric demands of a variety of amine-boryl radicals in H-atom transfer reactions have been assessed by determining the relative rates of abstraction from the α-C-H bonds in H2C(CO2Et)2 and Me(H)C(CO2Et)2 . SECTION C: The polycyclic amine-borane formed by cyclisation of N-(1 R)-nopylpiperidine-borane was used, in the presence of BF3.OEt2, for the asymmetric hydroboration of 1-methylcyclohexene and for the asymmetric reduction of acetophenone, in a variety of solvents at 25 °C and -78 °C. Hydroboration of 1-methylcyclohexene proceeds in good yield at 25 °C and frans-2-methylcyclohexanol with an e.e. of up to 52% was obtained after oxidation of the organoborane. Reduction of acetophenone was found to proceed rapidly, even at -78 °C, to give the alcohol with an e.e. of 15%.

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