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Title: Borenium cations for the direct electrophilic borylation of arenes
Author: Del Grosso, Alessandro
ISNI:       0000 0004 2737 5507
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2013
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A catalytic (in Brønsted superacid) and a stoichiometric process were developed to synthesise aryl boronic esters with boron cations via electrophilic arene borylation. The treatment of CatBX (Cat = catecholate; X = Cl, Br) with the triethyl salt [Et3Si][closo-CB11H6Br6] in arene solvent gave a transient boron electrophile that reacted as a synthetic equivalent of [CatB]+ in intermolecular electrophilic aromatic borylation at 25 °C. The by-product of the reaction was a strong Brønsted acid that was able to catalyse arene borylation using CatBH at high temperature. This catalytic process furnished aryl boronic esters in high yield with H2 as the only by-product. The use of the robust and weakly coordinating anion [closo-CB11H6Br6]- and the electrophile-resistant catecholborane were crucial for the catalytic process. The reaction mixture of R2BCl (R2 = Cat, Cl4Cat, Cl2), aprotic amine and AlCl3 mainly gave a borenium salt [R2B(amine)][AlCl4] which was in equilibrium with neutral species as revealed by NMR spectroscopy and reactivity studies. This reaction mixture was effective for the regioselective borylation, by electrophilic aromatic substitution, of a range of N-heterocycles, thiophenes and anilines at room temperature. The transterification in situ provided the synthetically useful and more stable pinacol boronate esters in excellent isolated yield. This process displayed remarkable functional-group tolerance for a boron based strong Lewis acid with weak bases (for example -NMe2), ether, and halogen groups all compatible. This process represents a new and inexpensive one-pot direct arene borylation methodology for producing pinacol boronate esters.
Supervisor: Ingleson, Michael Sponsor: University of Manchester
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: borenium ; borocation ; borylation ; boronic ester ; boronate ester ; electrophilic borylation