1. Introduction

As a very important class of substances, organoboron compounds are widely used in organic synthesis, materials science, medicinal chemistry, fine chemicals, chemical sensing, and so on [1-4]. With the development of boron chemistry, more and more boron-containing drugs have been developed and benefited mankind, such as bortezomib, ixazomib, tavaborole, crisaborole and vaborbactam (Fig. 1) [5-9]. In addition, organoboron compounds, especially organoboronate esters, are indispensable synthetic reagents in the construction of carbon-carbon bond and carbon-heteroatom bond reactions due to their high stability, appropriate reactivity, low toxicity and easy processing [10,11].

In the past few decades, transition metal-catalyzed C–H borylation has been developed rapidly because of their unique selectivity and reactivity [12-15]. However, harsh reaction conditions, complex ligands, noble transition metals, especially the difficulty to remove toxic trace metals from pharmaceuticals limit the application of these methods. In sharp contrast, photoinduced borylation, electrocatalytic borylation, and metal-free borylation have therefore attracted considerable attention in recent years due to the mild, efficient and economical reaction conditions [16-18]. Recently, haloboranes (BX₃) have been attractive borylation agents in the field of metal-free C–H borylation, as they are commercially available in grams to kilograms and are cheaper than most common boron reagents [19]. In this review, we present a systematic and comprehensive overview of the literature on C–H borylation using BX₃ as boron source with different directing group auxiliary since 2010. The methods of borylation processes as well as the substance scopes, limits, and mechanisms of these routes are also discussed.

2. C–H borylation using BX₃ with different directing group auxiliary 2.1. Nitrogen directed C–H borylation

The pioneering work on N-directed electrophilic C–H borylation of arene was conducted by the Dewar’s group. They synthesized a series of polycyclic aromatic hydrocarbons through N-directed C–H borylation using BCl₃ and PhBCl₂ with AlCl₃ as catalyst from 1958 [20-23]. However, these reactions usually need to be completed under forcing conditions. In 2009, Vedejs’s group reported an important study on C–H borylation via N→BH₃ intermediate using the trityl salt of the robust weakly coordinating anion [B(C₆F₅)₄]⁻ to abstract hydride from N,N-dimethylbenzylamine borane [24]. However, this reaction still required high temperatures. After Vedejs’ seminal work, N-directing groups, such as pyridine, pyrimidine, pyrazole, imidazole directed C–H borylation has been achieved using BX₃.

2.1.1. Pyridine directed C–H borylation

As a strongly coordinating group, pyridine is usually used as a directing group or ligand in C–H bond activation [25-27]. In 2010, Murakami’s group reported the pioneering work on electrophilic aromatic borylation of 2-arylpyridines with BBr₃ to synthesize pyridine-borane complexes under metal-free conditions (Scheme 1a) [28]. This simple method was useful for the synthesis of aza-π-conjugated materials with boron-nitrogen coordination. The plausible reaction mechanism was presented as follows: the Lewis basic nitrogen atom in pyridine formed a coordination with the Lewis acidic boron center, thereby generating complex 4. Another BBr₃ extracted bromide anion from complex 4 to deliver boron cations 5 [29]. Subsequently, the boron cation underwent an electrophilic attack on the adjacent aromatic group [30], and the following rearomatization process leaded to the formation of the pyridine-dibromoborane complex.

After that, many researches about BBr₃-mediated pyridine-directed borylation by similar method for preparing boron-containing organic materials had been reported (Scheme 1b). For example, Yam and co-workers designed and synthesized a series of diarylethene-containing N,C-chelate thienylpyridine-bis(alkynyl)borane complexes [31,32]. In 2013, Tang’s team synthesized two novel N,C-chelate four-coordinate organoborons, which exhibited good thermal stability and could be used as efficient solid-state emitters [33]. In 2016, Jäkle’s group obtained luminescent organoboron ladder compounds and Wang et al. generated a series of new N,C-chelate organoboron compounds with donor-functionalized aryl groups to investigate the effect of enhanced charge-transfer character [34,35]. In 2018, Hatakeyama and co-workers offered benzoaceanthrylene analogs with tetracoordinate boron at the ring junction through tandem electrophilic C–H borylation using BI₃ and the derivative was promised to be a viable n-type semiconducting material [36]. In 2022, Saito and co-workers obtained chiral aza-boraspirobifluorenes and evaluated their molecular structures as well as their photophysical properties, which provided important insights into the stereochemistry of chiral tetrahedral boranes [37]. Up to now, it is still an extremely hot topic to construct organic materials via Murakami’s C–H borylation.

In the fields of organic synthesis, Murakami’s C–H borylation has also been further developed and expanded. In 2012, Fu’s group developed an efficient route to metal-free ortho-C–H borylation of 2-phenoxypyridines (Scheme 2a) [38]. The corresponding aryl boronates 16 were obtained in good yields and the pyridine group could be deprotected to offered 2-phenylphenol derivatives. However, the condition of removing the directing group was harsh (MeOTf/pyridine, then Na/MeOH). In 2021, Ingleson finished pyridine directed C2 borylation of indoles using BCl₃ (Scheme 2b) [39]. Subsequently, Ji’s group reported a novel method for ortho-selective C–H borylation of 2-phenylthiopyridines using BBr₃ (Scheme 2c) [40]. The synthesized aryl boronates 21 were freely transformed into various useful products, which would be helpful to generate structurally diversified phenylthioethers. Then, they efficiently realized C–H borylation of 2-(N-methylanilino)-5-fluoropyridines and 2-benzyl-5-fluoropyridines (Scheme 2d) [41]. The corresponding compounds 23 contained both fluorine and boron elements, which could provide a practical tool for the discovery of new fluorine-containing compounds.

The catalytic borylation of Csp³–H substrate presents unique challenges because of the sterically demanding geometry of Csp³–H compared to planar Csp²–H. In 2023, Fontaine’s group developed site-selective Csp³–H borylation of 2-dialkylaminopyridines (Scheme 2e) [42]. Interestingly, piperidine borate derivatives 25 and 26 at different positions were obtained when the reaction conditions were different. The mechanism studies suggested that an enamine intermediate was produced in the borylation followed by subsequent Csp³–H insertion or electrophilic borylation.

2.1.2. Pyrazine directed C–H borylation

The studies of pyrazine directed C–H borylation using BX₃ was mainly focused on the preparation of boron-containing organic materials. In 2016, Feng and co-workers created low band gap coplanar organic materials 28 via pyrazine directed borylation on donor-acceptor-donor process (Scheme 3a) [43]. Then, Zhan’s group synthesized four B–N embedded indacenodithiophene structures 29–32 and obtained a new electron-deficient moiety containing B–N bonds using BBr₃ 33 (Scheme 3b) [44]. Similarly, Huang’s group disclosed two embedded units 34 and 35, which exhibited good stability and strong electron-affinity in 2019 (Scheme 3b) [45]. Meanwhile, Gelfand and co-workers synthesized a series of boron-nitrogen doped dihydroindeno[1,2-b]fluorenes 38 which had a wide spectrum of visible to near-IR light absorption profiles (Scheme 3c) [46]. Pyrazine directed C–H borylation is worthy of further exploration in organic synthesis.

2.1.3. Pyrazole directed C–H borylation

Pyrazole directed transition-metal catalyzed C–H borylation has been made significant progress in the past few decades [47,48]. However, as a common unit in organic compound, the studies of pyrazole directed C–H borylation under metal-free conditions were rarely reported, especially in the field of organic synthesis. In 2019–2022, Venkatasubbaiah’s group synthesized a series of triaryl/tetraaryl pyrazoles B–N coordinated boron compounds 39–43 through electrophilic aromatic borylation strategy (Scheme 4a) [49-51]. All of these compounds exhibited good optical properties which were expected to be applied to luminescent materials. In 2022, Ji’s group disclosed a novel method of boron-mediated C–H hydroxylation of N-benzyl-3,4,5-tribromopyrazoles (Scheme 4b) [52]. The corresponding phenols 46 could be obtained in moderate to excellent yields by borylation/oxidation using BBr₃ and NaBO₃·4H₂O. Mechanism studies revealed that a six-membered B–N cyclic intermediate was formed in this reaction. Very recently, Huang and co-workers realized boron-mediated selective C–H hydroxylation of 1-phenyl-1H-pyrazoles (Scheme 4c) [53]. Compared to previous work of photo-induced C–H borylation/oxidation of 1-phenyl-1H-pyrazoles, this approach was more convenient, economical and practical. However, pyrazole directed C–H borylation using BX₃ should be further developed.

2.1.4. Pyrimidine directed C–H borylation

In 2019, Kumagai, Shibasaki and co-workers reported a reliable route to generate a series of Pym-DATBs (DATB = 1,3-dioxa-5-aza-2,4,6-triborinane) 52 from inexpensive materials through pyrimidine directed C–H borylation (Scheme 5a) [54]. And these obtained derivatives could be used as effective catalysts for dehydrative amidation. In 2021, Ingleson’s group disclosed the site-selective C–H borylation of N-pyrimidine-carbazole 53 and N-pyrimidine-indole 56 by forming six membered boracycles using BCl₃ (Scheme 5b) [55]. The reason for site selectivity was due to blocking of the C2 position of indole. Then, Chatani’s group presented an efficient directed ortho-C–H borylation of 2-pyrimidylanilines 59 under metal-free conditions (Scheme 5c) [56]. This protocol exhibited broad scope of substrates and high functional group tolerance. Various boronic esters and four-coordinated triarylborane derivatives 61 were obtained in good yields. In addition, the reaction could be conducted well even there were some external impurities. It might be due to the reaction proceeding rapidly. Mechanism studies suggested a Wheland intermediate was formed in this reaction.

2.1.5. Imidazole/imidazolinone directed C–H borylation

As early as 1963, MacLean’s group achieved the borylation of 2-phenylbenzimidazoles using BCl₃ at 300 ℃ and subsequently obtained phenolic products (Scheme 6a) [57]. Until now, imidazole or imidazolinone directed metal-free C–H borylation in the field of organic synthesis had not made much progress, but there were certain applications in the field of organic materials. In 2012–2016, Solntsev and co-workers exploited a series of new fluorescent dyes 65–67 directed by imidazolinone using BBr₃ (Scheme 6b) [58,59]. Then, in 2018, Venkatasubbaiah’s group obtained several phenanthroimidazole-based four coordinate organoboron compounds 68, which possessed moderate to good solution state quantum yields (Scheme 6b) [60].

2.1.6. Benzothiadiazole/benzoselenadiazole/benzotriazole directed C–H borylation

In 2015–2019, Ingleson, Turner and co-workers had been working on the construction of organic materials using benzothiadiazole/benzoselenadiazole/benzotriazole directed C–H electrophilic borylation (Scheme 7) [61-66]. As a conclusion, benzothiadiazole directed C–H borylations with BBr₃ or BCl₃ at room temperature were more rapidly than pyridyl, even in the absence of exogenous bases. This might be due to benzothiadiazole being more effective in promoting deprotonation. In addition, despite there were two nitrogen Lewis basic sites of benzothiadiazole, only a single electrophilic C–H borylation could be directed because of the lower Lewis basicity of benzothiadiazole. Furthermore, the steric hindrance effect was obvious in this type of electrophilic borylations, which usually only occurred at the less steric hindrance position (compound 69). As expected, C–H borylation was more rapid with increasing arene nucleophilicity and the C–H borylation mainly occurred on the thiophene ring with a higher electron cloud density when using the unsymmetrically substituted derivatives (compound 70).

2.1.7. Diazene/hydrazone directed C–H borylation

The studies of diazene directed C–H bond activation are not surprising on transition-metal-catalyzed. However, there are few reports on C–H borylation under metal-free conditions with diazene auxiliary. In 2021, Shigeno and co-workers disclosed a new route to construct 1,2,3-benzodiazaboroles 75 via C–H electrophilic borylation and additional nucleophilic dialkylative cyclization (Scheme 8a) [67]. The method exhibited good functional group tolerance with high yield, especially the yield of electrophilic borylation step could reach 100%. Mechanistically, the reaction proceeded via nucleophilic addition of a Grignard reagent to the azo-unit, followed by intramolecular cyclization, and nucleophilic substitution of the B–Br bond.

In 2015, Cui’s group produced a series of novel B–N analogues of benzopentalene 77 by the one-pot multiple borylation through the C–H bond cleavage of PhBBr₂ mediated by triethylamine (Scheme 8b) [68]. In 2023, Ji's group achieved a metal-free C–H borylation of benzophenones using hydrazone as the traceless directing group (Scheme 8c) [69]. Similar to Shigeno’s work, the dibromoboron intermediates could be generated in excellent yields and site selectivity. The arylboronic esters 80 could also be obtained in moderate to excellent yields and possessed a variety of transformations, which providing a practical tool to construct structurally diversified benzophenones.

2.1.8. Imine directed C–H borylation

In 2015, Cui’s group fabricated a series of 1,2-borazaronaphthalenes 83 from benzylic imines through base-facilitated borylation (Scheme 9a) [70]. In this reaction pathway, enamidyl dibromoborane was first formed, and then base-facilitated aromatic C–H bond borylation was performed to produce the polycyclic aromatic hydrocarbons with moderate yield.

In recent years, transient directing group assisted C–H bond functionalization has arisen. This strategy is step economical because the directing group does not need to be preinstalled and then removed after the activation. In 2021, Chatani and co-workers developed a metal-free ortho–C–H borylation of electron-deficient benzaldehydes using imine as a transient directing group (Scheme 9b) [71]. The method exhibited good functional group tolerance and site selectivity. Interestingly, the reaction could be gone well even in the presence of external impurities. Mechanism studies suggested a dibromo borane intermediate was formed in this reaction. Based on this, very recently, Chen, Feng and co-workers disclosed a boron-mediated ortho-C–H hydroxylation of benzaldehydes using imine as a transient directing group in a one-pot (Scheme 9c) [72]. The corresponding salicylaldehydes 89 could be obtained in moderate to good yields via borylation and oxidation using BBr₃ and NaBO₃^·4H₂O.

2.1.9. Arylamine directed C–H borylation

In the past few years, the purpose of arylamine directed C–H borylation is mainly to construct B–N analogues of polycyclic aromatic hydrocarbons. As early as 1964, Dewar and Peosche developed aniline directed highly selective electrophilic C–H borylation using BCl₃ and AlCl₃ (Scheme 10a) [73]. Subsequently, Pei, Wang and co-workers performed a lot of in-depth researches in this filed. They synthesized a series of novel BN-substituted tetrathienonaphthalene derivatives 95 via electrophilic borylation in a one-pot (Scheme 10b) [74]. These works opened the door to the rapidly developing application of azaborine chemistry in electronic science. In 2016, they established an efficient route to generate BN-fused polycyclic aromatic hydrocarbons 97 with multiple functionalized sites via electrophilic borylation and Suzuki cross-coupling (Scheme 10b) [75].

In 2016–2020, Zhang, Feng and co-workers obtained a range of structurally diverse polycyclic aromatic hydrocarbons 99 through arylamine directed C–H borylation (Scheme 10c) [76-78]. These works offered practical tools to fabricate tailor-made complex architectures such as dendrimers, conjugated polymers and organic framework materials. In addition, Hatakeyama’s group synthesized several benzo[fg]tetracenes and benzo[hi]hexacenes 101 using the strategy of demethylative direct borylation (Scheme 10d) [79]. The results of the optimization of reaction conditions indicated that the key to successful synthesis was to select the appropriate boron source and Brønsted base. In 2017, Bettinger’s group obtained a new compound benzo[fg]tetracene 104 which displayed blue fluorescence and had considerable antioxidant and reductive capabilities (Scheme 10e) [80]. Then, Zhou’s group generated a range of polycyclic aromatic hydrocarbons 106 containing two pairs of BN units via electrophilic C–H borylation (Scheme 10f) [81]. In 2024, Song’s group disclosed a practical method for producing ¹⁰B-enriched 2,1-borazaronaphthalenes 108 from o-aminostyrenes and ¹⁰BF₃ in the presence of chlorosilane, which might provide new lead compounds for boron neutron capture therapy. The mechanistic studies indicated that this ¹⁰B transformation might involve the formation of boron chloride species and subsequent cyclization with o-amino-styrene (Scheme 10g) [82].

2.1.10. Indole directed C–H borylation

In 2024, Yu, Liu and co-workers developed an efficient method to generate BN-embedded polycyclic aromatic hydrocarbons via indole N-directed electrophilic borylation (Scheme 11) [83]. Notably, the types of products were dependent on the boron source. When using BBr₃ as the boron source, dimers of BN-polycyclic aromatic hydrocarbons 110 were obtained, while monomers of BN-PAHs 111 were generated when BCl₃ was used. Additionally, the dimers exhibited better photo-physical properties than monomers. This discovery opened new possibilities for expanding BN-doped polycyclic aromatic hydrocarbons by simply switching boron source.

2.2. Oxygen directed C–H borylation

Similar to N-directed electrophilic C–H borylation of arene, Dewar and co-workers also reported O-directed C–H borylation of 2-phenylphenol using BCl₃ and AlCl₃ in 1959 [84]. Unfortunately, the development of O-directed C–H borylation had been slow. In 1993, Nicholson’s group developed the first example of a carbonyl directed electrophilic C–H borylation using BBr₃ [85]. Satisfactorily, in recent years, following the pioneering work of the Shi’s and Ingleson’s work, the acyl-directed borylation under metal-free conditions has developed rapidly.

2.2.1. Methoxyl/hydroxyl directed C–H borylation

Most of methoxyl directed C–H borylation undergoes the cleavage of ArylO-Me with BBr₃ to form ArylOBBr₂ units that then effect intramolecular C–H borylation. In 2016, Hatakeyama and co-workers disclosed a demethylative direct borylation to synthesize benzo[fg]tetracenes 114 using BBr₃ as a boron source and 2,6,6-tetra-methylpiperidine as a Brønsted base (Scheme 12a) [79]. Also, they successfully obtained the π-extended product 10,11-dioxa-10a-borabenzo[hi]hexacene in 76% yield, which further verified the versatility of the direct borylation. In 2024, Chen’s group reported a Fe(OTf)₃-promoted, directing group-controlled, regioselective C–H borylation of 2-arylphenolics 115 at room temperature using BBr₃ (Scheme 12b) [86]. In this reaction system, the anion ligand of Lewis acid played a crucial role and the diversity diaryloxaborin could be generated in moderate to excellent yields [87].

In the past few years, the reports of hydroxyl directed C–H borylation using BX₃ was rare. In recent patent literature, Liu’s group developed a hydroxyl directed electrophilic C–H borylation to access benzoxaboroles 120 (Scheme 12c) [88]. Under low temperature conditions, benzyl alcohol was reacted with BCl₃ to form the RO-BCl₂ species, which underwent the C–H borylation after refluxing in toluene. Of note, the commercialized tavaborole could be obtained in moderate yield in this reaction system.

2.2.2. Acyl directed C–H borylation

In 2019, Shi, Houk, and co-workers as well as the Ingleson group developed a pivaloyl group directed C4-, C7-borylation of indoles and ortho-borylation of anilines by just using BBr₃ (Scheme 13a) [89,90]. The method exhibited good functional group tolerance, offering the corresponding arylboronates 122, 123 in moderate to excellent yields under simple, mild, and efficient conditions. Mechanistic studies indicated that the BBr₃ acting as both a catalyst and a reagent. Based on this, Shi’s group reported a novel protocol for prepare diverse phenols 124, 125 via BBr₃-mediated directed aromatic C–H hydroxylation (Scheme 13a) [91]. The practicability of this method has been confirmed by synthesizing some phenol intermediates of bioactive molecules. Subsequently, using the same directing group, they achieved the C2-selective C–H borylation of pyrroles under metal-free conditions (Scheme 13b) [92]. And they discovered that the site-selectivity hinged on chelation and electronic effects. In 2023, they disclosed a pivaloyl group directed stereoconvergent C–H borylation of enamides for constructing diastereomerically pure β-borylenamides 131 in the presence of 4-dimethylaminopyridine (DMAP) and N,N-diisopropylethylamine (DIPEA) (Scheme 13c) [93]. The mechanism studies revealed the key roles played by DMAP and DIPEA, the reactive boron species, and the phenomenon of stereoconvergence. In 2024, they realized the synthesis of stereochemically precise γ-borylenamides 134 via σ-C–C bond eliminative borylation using BCl₃ and 4-cyclohexylmorpholine (CHMP) under the directing of N-acyl group (Scheme 13d) [94]. This mild strategy exhibited excellent functional group compatibility and high efficiency. Mechanism studies indicated that BCl₃ was unlikely to form a borenium species through proportionation as BBr₃ did. The practicability of this method has been further confirmed by Reddy’s group (Scheme 13e) [95]. They described an efficient method for the C7-selective C–H borylation of tryptophan using BBr₃ and this strategy enabled peptide extension and late-stage borylation of peptides, natural product Brevianamide F and drug Oglufanide.

Inspired by Shi’s work, Ji and co-workers developed a method for C–H borylation of diphenylamines 137 through adamantane-1-carbonyl auxiliary using BBr₃ under metal-free conditions in 2020 (Scheme 13f) [96]. This strategy exhibited excellent site exclusivity and most of the borylation occurred at the more electron-rich aromatic ring. Furthermore, the method showed good functional group compatibility and application prospect. Using the same directing group, they achieved the ortho-C–H borylation and hydroxylation of benzenethiols 139, providing a useful tool for the construction of structurally diverse thiophenol (Scheme 13g) [97].

In 2023, Ingleson’s group realized amide directed electrophilic borylation-reduction of phenyl-acetylamides, mono and diamino-arenes and carbazole using greater than or equal to 2 equiv. of BBr₃ and hydrosilanes (Scheme 13h) [98]. This approach was a useful addition to the metal-free borylation toolbox for obtaining valuable arylboron compounds and novel B,N-polycyclic aromatic hydrocarbons. Given the importance of (2-hydroxyaryl)boronic acid (2-HAB) in organic synthesis and medicinal chemistry, Chen’s group conquered the competing Fries rearrangement and realized the ortho-C–H borylation of phenolic esters and carbamates, generating 2-HABs 147 in good yields (Scheme 13i) [86]. However, the substrate scope was relatively narrow, especially for the substrate with strong withdrawing group. It might due to the lower nucleophilicity of aryl ring. Besides, only two examples of substrates bearing fluorine or chlorine at meta-position were reported. The tolerance of substrates with electron-withdrawing groups at ortho- and para-positions need to be explored.

It is worth noting that the previously reported metal-free directed C–H borylation mainly focused on electron-rich arene systems, such as phenols, thiophenols, indoles and anilines. Achieving electrophilic borylation of electron-defect arenes is difficult because of the low electron cloud density in the aromatic ring. In 2024, Our group presented a regioselective C–H hydroxylation and borylation of N-phenylbenzamides 148 using BBr₃ (Scheme 13j) [99]. The method delivered the corresponding phenols and arylboronic esters 149 in moderate to excellent yields under metal-free conditions. Density functional theory calculations indicated that the preferred pathway for this C–H hydroxylation/borylation was to form a five-membered boracycle. Subsequently, Maiti’s group achieved the BBr₃-mediated selective C–H borylation of α–naphthamides and phenylacetic acid 150 (Scheme 13k) [100]. This strategy appeared to be economical and cost-effective and the borylation of drug molecules such as ibuprofen and indoprofen could be performed efficiently. Compared to our work, this method was more likely to form a six-membered boracycle. Very recently, Zhang and co-workers reported a BBr₃-mediated ortho-C–H borylation of benzamides under metal-free conditions (Scheme 13l) [101]. Although this method could provide borylated benzamides 155 in moderate to good yields, it needed to be carried out at high temperature.

2.3. Sulfur directed C–H borylation

The first example of sulfur directed C–H borylation was also reported by Dewar’s group in 1968 [102]. They found that the sulfur congener underwent C–H borylation most readily presumably as poorer π donation to the boron center (compared to O and N congeners) generates the most reactive boron electrophile of the series. However, sulfur directed metal-free C–H borylation is a direction that urgently needs to be developed. In 2016, Ingleson’s group established a novel strategy to form the C3-borylated benzothiophenes 159 via BCl₃-induced annulative thioboration and the activation process was revealed through ¹¹B NMR spectroscopy (Scheme 14a) [103]. Hatakeyama and co-workers reported a route to synthesize benzo[fg]tetracene 161 using thioester as the substrate via demethylative direct borylation (Scheme 14b) [79]. However, only 21% yield of the desired compound could be obtained even at 200 ℃. In addition, the target compound was air-stable, but immediately decomposed in protic solvents. In 2024, Zhang and co-workers disclosed a BBr₃-mediated S-directed ortho C–H borylation of thiobenzamides (Scheme 14c) [104]. The method exhibited wide functional group tolerance, affording the corresponding borylated thiobenzamides 164 in moderate to good yields. Nevertheless, when using meta-substituted thiobenzamide as the substrates, the site selectivity was poor and a mixture of regioisomers was delivered. The reaction mechanism was basically the same as that of the acyl-directed borylation, with the difference being that a strong S–B bond was not formed in the borylated product.

2.4. Phosphorus directed C–H borylation

Over the past years, boron hydride phosphide has been attracted considerable attention and significant progress has been made in the C–H borylation of phosphines catalyzed by transition metals [105]. However, the studies about metal-free phosphorus-directed C–H borylation are rare. In 2022, Bourissou’s group developed a novel and straightforward method to obtain phosphine-borane 166 via phosphorus-directed C(sp²)–H borylation under metal-free conditions (Scheme 15a) [106]. The method was applicable to various types of backbones and could conveniently realize the conversion of various substitution patterns at boron. The key intermediates P-stabilized borenium cations was verified by NMR and the existence and stabilizing effect of π-arene/boron interactions in the (biphenyl)(i-Pr)₂P→BBr₂⁺ species was demonstrated via DFT. Subsequently, Shi and co-workers reported a protocol on P(Ⅲ)-directed metal-free C–H borylation of phosphines 168 mediated by BBr₃ (Scheme 15b) [107]. Diverse phosphine boronate esters 169 could be obtained under the easily handleable, low cost, and environmental friendliness conditions.

3. Conclusions and outlook

In conclusion, the recent advanced in C–H borylation using BX₃ with different directing group auxiliary was summarized. In terms of directing groups, these reactions mainly used functional groups containing elements such as nitrogen, oxygen, sulfur and phosphorus as directing groups, among which pyridine and acyl that have emerged in recent years were the most common. In terms of boron sources, BBr₃ with strong activity and low cost was the main choice. Although significant progress has been made in methods and applications for these reactions, the following challenges still exist: (1) these methods are electrophilic borylation, which have poor adaptability to electron-deficient compounds. The currently developed methods still have drawbacks such as harsh reaction conditions and narrow substrate applicability; (2) these methods are mainly suitable for Csp²–H borylation but not to the Csp³–H bond; (3) more efficient, concise, practical directing groups and traceless directing group or without directing group directed electrophilic borylation need be further developed; (4) in the field of organic boron materials, on the one hand, more types of directing groups other than nitrogen-containing groups should be further developed (such as double oxygen-containing functional groups [108]); on the other hand, more mild reaction conditions need to be explored. We believe that with the joint efforts of chemists, the above challenges will be overcome one by one, promoting the vigorous development of organoboron chemistry.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Gaorong Wu: Writing – original draft, Methodology, Funding acquisition. Tao Ma: Writing – review & editing, Funding acquisition.

Acknowledgments

We are grateful to Early-Career Young Scientists and Technologists Project of Jiangxi Province (No. 20244BCE52224), Jiangxi Provincial Natural Science Foundation (No. 20252BAC200240), the Start-up Funds of Gannan Medical University (No. QD202406) and National Key R&D Program of China (No. 2023YFC3504100) for financial support.