Boron trichloride, a cheap and commercially available Lewis acid, has been demonstrated to activate alkynes possessing appropriate nucleophiles, facilitating borylative cyclisation. This reaction furnishes polycyclic compounds possessing a new C(sp2)-B bond externally to the newly formed ring (through concomitant C-C and C-B bond formation). The RBCl2 intermediates generated from cyclisation were esterified with pinacol to furnish air/moisture stable boronic esters. This methodology has been applied to the following classes of starting materials: 1,4-disubstituted but-1-ynes (including N- and O- linked analogues), 2-alkynylanisoles, 2-alkynylthioanisoles and 1,2-bis(alkynyl)benzenes. Thus, borylated scaffolds such as dihydronaphthalenes, dihydroquinolines, 2H-chromenes, benzofurans, benzothiophenes, dibenzopentalenes and benzofulvenes have been synthesised. A variety of functionalities (e.g. amines, esters, nitriles) were tolerated by the reaction, with a number of substrates cyclised on either a gram scale, or under ambient conditions, demonstrating the robust nature of this methodology. An oxidation reaction with [Ph3C][BF4] was carried out on some of the borylated dihydronaphthalene compounds to obtain borylated naphthalenes. Suzuki-Miyaura cross-coupling reactions were carried out on certain borylated cycles to furnish new C-C bonds and generate analogues of established pharmaceuticals such as Nafoxidine or Raloxifene, demonstrating the synthetic value of these borylated cycles. Additionally, a one-pot borylative cyclisation/Suzuki-Miyaura cross-coupling reaction was also developed. Throughout this investigation, alternative reactivity has been observed when using BCl3 to activate certain alkynes, including intermolecular 1,2-trans-carboboration and a rare example of N- and O-directed 1,2-trans-haloboration. Additionally, multiple borylative cyclisations have been carried out on an appropriate alkyne to obtain a B-doped polyaromatic hydrocarbon (PAH), which has potential material-based applications.

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.