Construction of 1,2,3-Benzodiazaborole by Electrophilic Borylation of Azobenzene and Nucleophilic Dialkylative Cyclization

Metal-Free ortho C-H Borylation of Thiobenzamides

A BBr3-mediated S-directed ortho C-H borylation of thiobenzamides was developed. A variety of ortho-borylated thiobenzamides were obtained in moderate to good yields with a wide functional group tolerance under simple and metal-free conditions. This transformation provided a convenient and practical route to important functionalized thiobenzamides.

Transition Metal-Free Aromatic C-H, C-N, C-S and C-O Borylation

Aromatic organoboron compounds are highly valuable building blocks in organic chemistry. They were mainly synthesized through aromatic C-H and C-Het borylation, in which transition metal-catalysis dominate. In the past decade, with increasing attention to sustainable chemistry, numerous transition metal-free C-H and C-Het borylation transformations have been developed and emerged as efficient methods towards the synthesis of aromatic organoboron compounds. This account mainly focuses on recent advances in transition metal-free aromatic C-H, C-N, C-S, and C-O borylation transformations and provides insights to where further developments are required.

Boron-Mediated Regioselective Aromatic C-H Functionalization via an Aryl BF2 Complex

An efficient regioselective functionalization of 2-aryl-heteroarenes and aryl aldehydes via an azaaryl BF₂ complex has been developed. Mechanistically the reaction comprises fluoride to bromide ligand exchange on an aryl boron species and consecutive C-B bond cleavage to deliver a broad range of functionalized products. The reaction is high yielding, has a broad substrate scope where several different heteroarenes can be functionalized with chloro, bromo, iodo, hydroxyl, amine and BF₂ in a highly regioselective fashion. The method can be applied for late-stage functionalization or for rapid skeleton remodeling with for instance cross-couplings.

Two Directing Groups Used for Metal Catalysed Meta-C-H Functionalisation Only Effect Ortho Electrophilic C-H Borylation

Two templates used in meta-directed C-H functionalisation under metal catalysis do not direct meta-C-H borylation under electrophilic borylation conditions. Using BCl3 only Lewis adduct formation with Lewis basic sites in the template is observed. While combining BBr3 and the template containing an amide linker only led to amide directed ortho C-H borylation, with no pyridyl directed meta borylation. The amide directed borylation is selective for the ortho borylation of the aniline derived unit in the template, with no ortho borylation of the phenylacetyl ring - which would also form a six membered boracycle - observed. In the absence of other aromatics amide directed ortho borylation on to phenylacetyl rings can be achieved. The absence of meta-borylation using two templates indicates a higher barrier to pyridyl directed meta borylation relative to amide directed ortho borylation and suggests that bespoke templates for enabling meta-directed electrophilic borylation may be required.

Palladium-Catalyzed Borylative Cyclizations of α-(2-Bromoaryl) Ketones to Form 1,2-Benzoxaborinines

Herein, we report that palladium catalyzes the borylative cyclization of α-(2-bromoaryl) ketones to afford 1,2-benzoxaborinines. The developed system is compatible with a variety of functionalities (Me, t-Bu, OMe, NMe₂, F, Cl, CN, CF₃, CO₂Me, and heteroaryl groups) and is applicable to the synthesis of B-O-containing tri- and tetracyclic fused-ring compounds.

Electrochemical formal [3 + 2] cycloaddition of azobenzenes with hexahydro-1,3,5-triazines

A catalyst-free electrochemical [3 + 2] cycloaddition of azobenzenes with hexahydro-1,3,5-triazines without an external oxidant has been developed for constructing the 1,2,4-triazolidine skeleton.

Origin of the Enhanced Reactivity in the ortho C-H Borylation of Benzaldehydes with BBr3

The metal-free ortho C-H borylation of benzaldehyde derivatives using a transient imine directing group was recently developed by our group, providing an efficient strategy for the synthesis of organoboron reagents. Herein, we report on an extensive investigation of the reaction mechanism using density functional theory (DFT) calculations. Computations for the reaction pathway with various imine substrates, as well as the effect of an added base were examined, and the experimentally observed reactivity enhancement is proposed to originate from the tunability of the destabilizing strain energies that results in a reversible complexation process with BBr₃.