Steric Shielding Effects Induced by Intramolecular C-H⋯O Hydrogen Bonding: Remote Borylation Directed by Bpin Groups

Balancing Reactivity, Regioselectivity, and Product Stability in Ir-Catalyzed Ortho-C-H Borylations of Anilines by Modulating the Diboron Partner

Ir-catalyzed arene C-H borylations (CHB) of anilines can be highly ortho selective by using a small B2eg2 (eg = ethane-1,2-diol) as the borylating reagent. Unfortunately, the products are prone to decomposition, and transesterification with pinacol is required prior to isolation. This work offers a solution by adjusting the size of the diboron reagent. Based on our evaluation, we conclude that B2bg2 (bg = butane-1,2-diol) achieves an optimal balance between CHB regioselectivity and stability for the borylated products.

A Hydrazone Ligand for Iridium-Catalyzed C-H Borylation: Enhanced Reactivity and Selectivity for Fluorinated Arenes

Ir-catalyzed C-H borylations of fluorinated and cyanated arenes with high meta-to-F/CN are described. Use of a dipyridyl hydrazone framework as the ancillary ligand and pinacolborane (HBpin) as the functionalizing reagent generates catalysts that are significantly more active and selective than 4,4'-di-tert-butyl-2,2'-bipyridine (dtbpy) for both electron-deficient and electron-rich substrates. Investigation of the ligand framework resulted in the observation of formal N-borylation of the hydrazone by HBpin, as evidenced by NMR spectroscopy and X-ray crystallography. Subsequent stoichiometric reactions of this adduct with an iridium precatalyst revealed the formation of an unusual IrI hydrazido. Isolation and use of this hydrazido reproduce the selectivity of in situ generated catalysts, suggesting that it leads to formation of the active species.

Synthetic progress of organic thermally activated delayed fluorescence emitters via C-H activation and functionalization

Thermally activated delayed fluorescence (TADF) emitters have become increasingly prominent due to their promising applications across various fields, prompting a continuous demand for developing reliable synthetic methods to access them. This review aims to highlight the progress made in the last decade in synthesizing organic TADF compounds through C-H bond activation and functionalization. The review begins with a brief introduction to the basic features and design principles of TADF emitters. It then provides an overview of the advantages and concise development of C-H bond transformations in constructing TADF emitters. Subsequently, it summarizes both transition-metal-catalyzed and non-transition-metal-promoted C-H bond transformations used for the synthesis of TADF emitters. Finally, the review gives an outlook on further challenges and potential directions in this field.

Trialkoxysilane-Induced Iridium-Catalyzed para-Selective C-H Bond Borylation of Arenes

An ideal approach for the construction of aryl boron compounds is to selectively replace a C-H bond in arenes with a C-B bond, and controlling regioselectivity is one of the most challenging aspects of these transformations. Herein, we report an iridium-catalyzed trialkoxysilane protecting group-assisted regioselective C-H borylation of arenes, including derivatives of benzaldehydes, acetophenones, benzoic acids, benzyl alcohols, phenols, aryl silanes, benzyl silanes, and multi-functionalized aromatic rings are all well tolerated and gave the para -selective C-H borylation products in a short time without the requirement of inert gases atmosphere. The site-selective C-H borylation can be adjustable by installing the developed trialkoxysilane protecting group on different functional groups on one aromatic ring. Importantly, the preparation process of the trialkoxychlorosilane is efficient and scalable. Mechanistic and computational studies reveal that the steric hindrance of the trialkoxysilane protecting group plays a key role in dictating the para-selectivity.

Regioselective Iridium-Catalyzed C8-H Borylation of 4-Quinolones via Transient O-Borylated Quinolines

The quinolone-quinoline tautomerization is harnessed to effect the regioselective C8-borylation of biologically important 4-quinolones by using [Ir(OMe)(cod)]2 as the catalyst precursor, the silica-supported monodentate phosphine Si-SMAP as the ligand, and B2 pin2 as the boron source. Initially, O-borylation of the quinoline tautomer takes place. Critically, the newly formed 4-(pinBO)-quinolines then undergo N-directed selective Ir-catalyzed borylation at C8. Hydrolysis of the OBpin moiety on workup returns the system to the quinolone tautomer. The C8-borylated quinolines were converted to their corresponding potassium trifluoroborate (BF3 K) salts and to their C8-chlorinated quinolone derivatives. The two-step C-H borylation-chlorination reaction sequence resulted in various C8-Cl quinolones in good yields. Conversion to C8-OH-, C8-NH2 -, and C8-Ar-substituted quinolones was also feasible by using this methodology.

Transition metal-catalyzed remote C─H borylation: An emerging synthetic tool

Transition metal-catalyzed C─H bond activation and borylation is a powerful synthetic method that offers versatile synthetic transformation from organoboron compounds to virtually all other functional groups. Compared to the ortho-borylation, remote borylation remains more challenging owing to the inaccessibility of these C─H bonds. Enforcing the metal catalyst toward the remote C─H bonds needs well-judged catalyst design through proper ligand development. This review article aims to summarize the recent discoveries for the remote C─H borylation by the employment of new catalyst/ligand design with the help of steric of the ligand, noncovalent interactions. It has been found that C─H borylation now takes part in the total synthesis of natural products in a shorter route. Whereas, Ir-catalyzed C─H borylation is predominant, cobalt catalyst has also started to affect this field for sustainable and cost-effective development.

Borenium-Ion-Catalyzed C-H Borylation of Arenes

Non-metal-catalyzed C-H borylation of arenes represents a sustainable and environment-friendly approach for the functionalization of arenes. Despite its promise as an alternative to traditional transition-metal systems, its substrate scope is generally limited to electron-rich arenes, thus hindering its application in organic synthesis. Herein, we report the development of a borenium-ion catalyst which can borylate unactivated arenes under ambient conditions with 4-chlorocatecholborane (HBcat^Cl) as borylation reagent. This metal-free catalytic system is suitable for the borylation of C-H bonds in sterically encumbered positions, which has been a challenging task for transition-metal systems. Additionally, this catalytic system allows para-selective one-pot borylation of phenols, which has not been achieved by using transition-metal systems. Our mechanistic investigations and computational studies support a synergistic activation of the H-Bcat^Cl bond by the arene substrate and the borenium-ion catalyst. This generates a Wheland intermediate and a neutral hydroborane species and is followed by deprotonation of the Wheland intermediate with the hydroborane species. The latter step of C-H bond cleavage is likely the rate-limiting step.

Relative rates of alkylation for B-substituted triarylphosphines: an ortho-Boron group enhances reactivity on phosphorus

Advancements in main-group catalysis are contingent on our ability to quantify effects that enhance reactivity in these systems. Herein we report the rates of alkylation for several substituted phosphines. We report that by incorporating a single pinacol boronic ester group in the ortho-position on triphenylphosphine, the rate of substitution with benzyl bromide is approximately 4.7 times faster than the parent compound as measured by initial rates. The corresponding meta- and para-isomers are only 1.3 and 1.5 times as fast, respectively. Using X-ray crystallographic data and quantum chemical calculations, we propose this rate acceleration occurs from an O to P electrostatic interaction that stabilizes the transition state.

Metal-catalysed C-H bond activation and borylation

Transition metal-catalysed direct borylation of hydrocarbons via C-H bond activation has received a remarkable level of attention as a popular reaction in the synthesis of organoboron compounds owing to their synthetic versatility. While controlling the site-selectivity was one of the most challenging issues in these C-H borylation reactions, enormous efforts of several research groups proved instrumental in dealing with selectivity issues that presently reached an impressive level for both proximal and distal C-H bond borylation reactions. For example, in the case of ortho C-H bond borylation reactions, innovative methodologies have been developed either by the modification of the directing groups attached with the substrates or by creating new catalytic systems via the design of new ligand frameworks. Whereas meta and para selective C-H borylations remained a formidable challenge, numerous innovative concepts have been developed within a very short period of time by the development of new catalytic systems with the employment of various noncovalent interactions. Moreover, significant advancements have occurred for aliphatic C(sp³)-H borylations as well as enantioselective borylations. In this review article, we aim to discuss and summarize the different approaches and findings related to the development of directed proximal ortho, distal meta/para, aliphatic (racemic and enantioselective) borylation reactions since 2014. Additionally, considering the C-H borylation reaction as one of the most important mainstream reactions, various applications of this C-H borylation reaction toward the synthesis of natural products, therapeutics, and applications in materials chemistry will be summarized in the last part of this review article.

Nickel-catalyzed para-selective carboxylation of phenols with CBr4/MeOH

1,10-Phenanthroline-assisted, Ni(ii)-catalyzed para-carboxylation of phenol derivatives is reported.