Acyl‐Directed ortho ‐Borylation of Anilines and C7 Borylation of Indoles using just BBr 3

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.

Borylation Directed Borylation of Indoles Using Pyrazabole Electrophiles: A One-Pot Route to C7-Borylated-Indolines

Pyrazabole (1) is a readily accessible diboron compound that can be transformed into ditopic electrophiles. In 1 (and derivatives), the B⋅⋅⋅B separation is ca. 3 Å, appropriate for one boron centre bonding to N and one to the C7 of indoles and indolines. This suitable B⋅⋅⋅B separation enables double E-H (E=N/C) functionalisation of indoles and indolines. Specifically, the activation of 1 with HNTf₂ generates an electrophile that transforms N-H indoles and indolines into N/C7-diborylated indolines, with N-H borylation directing subsequent C7-H borylation. Indole reduction to indoline occurs before C-H borylation and our studies indicate this proceeds via hydroboration-C3-protodeboronation to produce an intermediate that then undergoes C7 borylation. The borylated products can be converted in situ into C7-BPin-N-H-indolines. Overall, this represents a transient directed C-H borylation to form useful C7-BPin-indolines.

Metal-Free Phosphorus-Directed Borylation of C(sp2 )-H Bonds

Spectacular progress has recently been achieved in transition metal-catalyzed C-H borylation of phosphines as well as directed electrophilic C-H borylation. As shown here, P-directed electrophilic borylation provides a new, straightforward, and efficient access to phosphine-boranes. It operates under metal-free conditions and leverages simple, readily available substrates. It is applicable to a broad range of backbones (naphthyl, biphenyl, N-phenylpyrrole, binaphthyl, benzyl, naphthylmethyl) and gives facile access to various substitution patterns at boron (by varying the boron electrophile or post-derivatizing the borane moiety). NMR monitoring supports the involvement of P-stabilized borenium cations as key intermediates. DFT calculations reveal the existence and stabilizing effect of π-arene/boron interactions in the (biphenyl)(i-Pr)₂ P→BBr₂ ⁺ species.

BBr3 -Mediated P(III)-Directed C-H Borylation of Phosphines

Transition-metal-catalyzed C-H borylation has been widely used in the preparation of organoboron compounds. Here, we developed a general protocol on metal-free P(III)-directed C-H borylation of phosphines mediated by BBr₃ , resulting in the formation of products bearing both phosphorus and boron. The development of the metal-free strategy to mimic previous metallic processes has shown low cost, superior practicality, and environmental friendliness. Density functional theory (DFT) calculations demonstrate the preferred pathway for this metal-free directed C-H borylation process.

Site-Fixed Hydroboration of Terminal and Internal Alkenes using BX3 /i Pr2 NEt*

An unprecedented and general hydroboration of alkenes with BX₃ (X=Br, Cl) as the boration reagent in the presence of ⁱ Pr₂ NEt is reported. The addition of ⁱ Pr₂ NEt not only suppresses alkene polymerization and haloboration side reactions but also provides an "H" source for hydroboration. More importantly, the site-fixed installation of a boryl group at the original position of the internal double bond is readily achieved in contrast to conventional transition-metal-catalyzed hydroboration processes. Further application to the synthesis of 1,n-diborylalkanes (n=3-10) is also demonstrated. Preliminary mechanistic studies reveal a major reaction pathway that involves radical species and operates through a frustrated Lewis pair type single-electron-transfer mechanism.

Metal-Free Directed C-H Borylation of Pyrroles

Robust strategies to enable the rapid construction of complex organoboronates in selective, practical, low-cost, and environmentally friendly modes remain conspicuously underdeveloped. Here, we develop a general strategy for the site-selective C-H borylation of pyrroles by using only BBr₃ directed by pivaloyl groups, avoiding the use of any metal. The site-selectivity is generally dominated by chelation and electronic effects, thus forming diverse C2-borylated pyrroles against the steric effect. The formed products can readily engage in downstream transformations, enabling a step-economic process to access drugs such as Lipitor. DFT calculations (wB97X-D) demonstrate the preferred positional selectivity of this reaction.

Metal-Free C-H Borylation of N-Heteroarenes by Boron Trifluoride

Organoboron compounds are essential reagents in modern C-C coupling reactions. Their synthesis via catalytic C-H borylation by main group elements is emerging as a powerful tool alternative to transition metal based catalysis. Herein, a straightforward metal-free synthesis of aryldifluoroboranes from BF3 and heteroarenes is reported. The reaction is assisted by sterically hindered amines and catalytic amounts of thioureas. According to computational studies the reaction proceeds via frustrated Lewis pair (FLP) mechanism. The obtained aryldifluoroboranes are further stabilized against destructive protodeborylation by converting them to the corresponding air stable tetramethylammonium organotrifluoroborates.