The synthesis and application of heteroatom boralanes containing nitrogen, oxygen and sulfur.
Hadebe, Khethukuthula Nozipho.
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There is a constant need for robust and highly reactive hydroborating reagents which will be able to yield stable organoborane compounds upon their hydroboration with olefins. These stable organoborane compounds can be used as a starting material in a number of cross-coupling reactions. The objectives of this project were to synthesize heterocyclic borolanes (with mixed donor atoms) and to evaluate the stability and reactivity of such borolanes towards the hydroboration reaction. The second part of the objective was to evaluate the application of arylbenzo-1,3,2-diazaborolane compounds as potential arylating reagent in the copper(II)acetate catalyzed N-arylation of imidazole using the Chan-Lam coupling reaction. Three heterocyclic borolanes were successfully synthesized from the reaction of borane-dimethyl sulfide complex with the corresponding chelating group in yield ranging from 45-96 %. These borolane showed good stability towards atmospheric oxidation and disproportionation due to elevated temperatures. A density functional calculation conducted on these borolanes showed that there was an decrease in the gap energy in the order of benzo-1,3,2-dioxaborolane > benzo-1,3,2-oxothiaborolane > benzo-1,3,2-thiazaborolane > benzo-1,3,2-dithiaborolane > benzo-1,3,2-diazaborolane. The use of benzo-1,3,2-thiazaborolane as a hydroborating reagent showed that this compound was prone to disproportionation. The condensation reaction of boronic acid with 1,2-diaminophenyl, 1,2-dihydroxybenzene and o-aminophenyl mercaptan, resulted in the synthesis of eleven arylbenzo-1,3,2-boronate esters in yields between 66-99 %. The investigation on the use of arylbenzo-1,3,2-diazaboronate ester as N-arylating reagents in the Chan-Lam coupling showed that these compounds were unsuitable arylating reagents and that the boronic acid proved to be better arylating reagent.