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Synthetic chemistry is built around the formation of carbon–carbon bonds. However, the development of methods for selective carbon–carbon bond cleavage is a largely unmet challenge1–6. Such methods will have promising applications in synthesis, coal liquefaction, petroleum cracking, polymer degradation and biomass conversion. For example, aromatic rings are ubiquitous skeletal features in inert chemical feedstocks, but are inert to many reaction conditions owing to their aromaticity and low polarity. Over the past century, only a few methods under harsh conditions have achieved direct arene-ring modifications involving the cleavage of inert aromatic carbon–carbon bonds7,8, and arene-ring-cleavage reactions using stoichiometric transition-metal complexes or enzymes in bacteria are still limited9–11. Here we report a copper-catalysed selective arene-ring-opening reaction strategy. Our aerobic oxidative copper catalyst converts anilines, arylboronic acids, aryl azides, aryl halides, aryl triflates, aryl trimethylsiloxanes, aryl hydroxamic acids and aryl diazonium salts into alkenyl nitriles through selective carbon–carbon bond cleavage of arene rings. This chemistry was applied to the modification of polycyclic aromatics and the preparation of industrially important hexamethylenediamine and adipic acid derivatives. Several examples of the late-stage modification of complex molecules and fused ring compounds further support the potential broad utility of this methodology. Common aromatic rings, such as anilines, arylboronic acids and aryl halides, can be opened up and converted to alkenyl nitriles through carbon–carbon bond cleavage using a copper catalyst.

Electrophilic halogenation reactions have been a reliable approach to accessing organohalides. During the past decades, various catalytic systems have been developed for the activation of haleniums. However, there is still a short of effective catalysts, which could cover various halogenation reactions and broad scope of unsaturated compounds. Herein, TEMPO (2,2,6,6-tetramethylpiperidine nitroxide) and its derivatives are disclosed as active catalysts for electrophilic halogenation of olefins, alkynes, and aromatics. These catalysts are stable, readily available, and reactive enough to activate haleniums including Br+, I+ and even Cl+ reagents. This catalytic system is applicable to various halogenations including haloarylation of olefins or dibromination of alkynes, which were rarely realized in previous Lewis base catalysis or Lewis acid catalysis. The high catalytic ability is attributed to a synergistic activation model of electrophilic halogenating reagents, where the carbonyl group and the halogen atom are both activated by present TEMPO catalysis., Organohalides are widely used as synthetic precursors and target products, but for various halogenation reactions there is a need for effective catalysts to activate commercially available haleniums. Here, the authors report that TEMPO and its derivatives are active catalysts for electrophilic halogenation of olefins, alkynes and aromatics, under mild reaction conditions and with good functional group tolerance.

Nickel has been found in some metalloproteins that execute catalytic aerobic oxygenation reactions in which nickel–dioxygen species are proposed as the key catalytic species. Thus, various nickel–dioxygen complexes have been prepared for biomimetic oxygenation reactions over the past 50 years, but they have often been used stoichiometrically rather than catalytically. The typical inertness of Ni(ii) complexes toward O2 poses a considerable challenge for their application in catalytic aerobic oxygenation. Here, we report a strategy to enable catalytic oxygenation with a simple Ni(ii)–bipyridine complex in which electrochemistry is employed to drive the cascade-activation process for the generation of both active Ni(ii)–peroxo species and O2 for the subsequent oxygenation reaction. We anticipate that this strategy will inspire the development of efficient nickel-catalysed aerobic oxygenations and promote the exploration of transition-metal-based oxidation chemistry in combination with electrochemistry. As Ni(ii) complexes are generally considered inert toward O2, their utilization for catalytic oxygenation reactions is extremely rare. Here, the authors introduce an electrocatalytic method for the oxidation of sulfides based on bipyridine complexes of simple Ni(ii) salts using water as the oxygen source.

Molecular oxygen as a green, non-toxic and inexpensive oxidant has displayed lots of advantages compared with other oxidants towards more selective, sustainable, and environmentally benign organic transformations. The oxygenation reactions which employ molecular oxygen or ambient air as both an oxidant and an oxygen source provide an efficient route to the synthesis of oxygen-containing compounds, and have been demonstrated in practical applications such as pharmaceutical synthesis and late-stage functionalization of complex molecules. This review article introduces the recent advances of radical processes in molecular oxygen-mediated oxygenation reactions. Reaction scopes, limitations and mechanisms are discussed based on reaction types and catalytic systems. Conclusions and perspectives are also given in the end.

Oxygenation reactions with molecular oxygen (O2 ) as the oxygen source provides a green and straightforward strategy for the construction of O-containing compounds. Demonstrated here is a novel N-heterocyclic carbene (NHC) catalyzed oxidative transformation of simple and readily available organic halides into valuable esters through the incorporation of O-atoms from O2 . Mechanistic studies prove that the deoxy Breslow intermediate generated in situ is oxidized to a Breslow intermediate for further transformation by this oxidative protocol. This method broadens the field of NHC catalysis and promotes oxygenation reactions with O2 .

Selective C-C bond cleavage under mild conditions can serve as a valuable tool for organic syntheses and macromolecular degradation. However, the conventional chemical methods have largely involved the use of noble transition-metal catalysts as well as the stoichiometric and perhaps environmentally unfriendly oxidants, compromising the overall sustainable nature of C-C transformation chemistry. In this regard, electrochemical C-C bond cleavage has been identified as a sustainable and scalable strategy that employs electricity to replace byproduct-generating chemical reagents. To date, the progress made in this area has mainly relied on Kolbe electrolysis and related processes. Encouragingly, more and more examples of the cleavage of C-C bonds via other maneuvers have recently been developed. This review provides an overview on the most recent and significant developments in electrochemically oxidative selective C-C bond cleavage, with an emphasis on both synthetic outcomes and reaction mechanisms, and it showcases the innate advantages and exciting potentials of electrochemical synthesis.

Control of selectivity is one of the central topics in organic chemistry. Although unprecedented alkoxyl-radical-induced transformations have drawn a lot of attention, compared to selective C-H activation, selective radical O-H activation remains less explored. Herein, we report a novel selective radical O-H activation strategy of diols by combining spatial effects with proton-coupled electron transfer (PCET). It was found that DMSO is an essential reagent that enables the regioselective transformation of diols. Mechanistic studies indicated the existence of the alkoxyl radical and the selective interaction between DMSO and hydroxyl groups. Moreover, the distal C-C cleavage was realized by this selective alkoxyl-radical-initiation protocol.

The chlorination of a bioactive compound can change its physiological properties and improve its pharmacokinetic and pharmacological profiles. It therefore has been an important strategy for drug discovery and development. However, the direct aromatic chlorination of complex bioactive molecules is too difficult to be practical. In fact, many functional groups such as hydroxyls, amines, amides or carboxylic acids may strongly restrain the reactivity of Cl+ by forming a halogen bond. Here we report a highly efficient aromatic chlorination of arenes that is catalysed by dimethyl sulfoxide with N-chlorosuccinimide as the chloro source. The mild conditions, easy-availability and stability of the catalyst and reagents, as well as good functional-group tolerance, showed the approach to be a versatile protocol for the late-stage aromatic chlorination of complex natural products, drugs and peptides. The multi-gram experiment and low-cost of N-chlorosuccinimide and dimethyl sulfoxide shows great potential for drug discovery and development in industrial applications. Late-stage aromatic chlorination of active pharmaceutical ingredients has enormous potential in drug discovery yet still features limited applicability due to issues of functional-group tolerance. Now, dimethyl sulfoxide is reported as catalyst for the chlorination of a diverse family of bioactive molecules in combination with N-chlorosuccinimide.

A novel activation of acetonitrile for the construction of cyclobutenones by [2+2] cyclization was developed. Acetonitrile is utilized for the first time as two-carbon (C2) cyclization building block. The present protocol successfully inhibits the competitive cycloaddition with the C≡N bond of acetonitrile, but enables the in situ formation of an unsaturated carbon-carbon bond and the subsequent cycloaddition as a C2 unit. This chemistry features simple reaction conditions, high chemoselectivities, wide substrate scope, and offers a new and practical approach to cyclobutenones and cyclobuteneimines.

Although the application of 1,2-dichloroethane (DCE) as a chlorinating reagent in organic synthesis with the concomitant release of vinyl chloride as a useful byproduct is a fantastic idea, it still presents a tremendous challenge and has not yet been achieved because of the harsh dehydrochlorination conditions and the sluggish C-H chlorination process. Here we report a bifunctional electrocatalysis strategy for the catalytic dehydrochlorination of DCE at the cathode simultaneously with anodic oxidative aromatic chlorination using the released HCl as the chloride source for the efficient synthesis of value-added (hetero)aryl chlorides. The mildness and practicality of the protocol was further demonstrated by the efficient late-stage chlorination of bioactive molecules.

An efficient M2CO3-catalyzed (M = K or Cs) aerobic cross dehydrogenative coupling reaction of thiols was described. This reaction provided an efficient approach to unsymmetrical disulfides which are ubiquitous structures of pharmaceuticals and pesticides. This is the first aerobic oxidative CDC of thiols to unsymmetrical disulfides. The high atom-economy, easy accessibility of catalyst, O2 as the ideal green oxidant, mild reaction conditions, and broad substrate scope demonstrate that the present methodology provides a green, attractive, and practical approach to disulfides.

Described here is a Cu-catalyzed [2 + 2 + 1] modular synthesis of full-substituted β-pyrrolinones from simple amines, alkynes, and α-diazo-β-ketoesters. This approach involving the regioselective C-nucleophilic attack of enamines, uncommon C-nucleophilic addition to ketenes, and umpolung of imines enables the direct synthesis of full-substituted β-pyrrolinones, which were hardly constructed by traditional synthetic strategies.

β-Halohydrins bearing transformable halo- and hydroxyl groups, are easily converted into various valuable blocks in organic and pharmaceutical synthesis. A diastereoselective β-halogenation of benzylic alcohols was achieved under simple and low-cost conditions, which provided a direct synthesis of β-halohydrins. The simple reaction conditions, easily available reagents, high diastereoselectivities, and additional oxidant-free make this reaction very attractive and practical.

Summary The cycloaddition reactions have been explored extensively and provided an efficient strategy for the synthesis of cyclic compounds. Traditionally, the reaction partners were in extenso incorporated into the cyclic products without fragmentation. From a different perspective, if certain fragmentations via chemical-bond cleavage are involved in this cycloaddition reaction, it would change the assembly sequence and enable more product diversity. Here, we report a chemoselective nitrosylation of anilines and alkynes through fragmentary or complete NO radical incorporation. The formation of multiple C–N bonds, an unexpected C–N bond, and N=O bond cleavage make this fragmentary cycloaddition reaction an efficient approach to 2,5-dihydrooxazoles, 1 H -1,2,3-triazole 2-oxides or quinoxaline N -oxides. Facile operation in open-air, metal-free, and mild conditions renders this protocol particularly practical and attractive. A series of mechanistic studies and density functional theory calculations were also conducted, which help to explain the fragmentary or complete NO incorporation processes, broadening the field of new reaction discovery.

A novel and efficient oxygenation approach to trisubstituted oxazoles via a copper-catalyzed aerobic oxidative dehydrogenative annulation of amines, alkynes, and O2 has been developed. This transformation combines dioxygen activation and oxidative C–H bond functionalization and provides a practical protocol for the preparation of oxazole derivatives, which are privileged units found in various bioactive compounds or other natural products. 18O-labeling experiments were conducted to reveal that oxygenation was involved in this chemistry.

A novel and practical Cu-catalyzed aerobic oxidative synthesis of thiazoles was developed. This chemistry for the first time achieved thiazole construction from simple aldehydes, amines, and element sulfur through multiple Csp3–H bond cleavage processes. Molecular oxygen was used as a green oxidant in this oxidative protocol. The substrate scope is broad with the tolerance of aliphatic amines. The mechanistic study might promote the reaction design for a new sulfuration/annulation reaction with readily available element sulfur.

Oxygenases-catalyzed reductive activation of molecular oxygen and incorporation of O atoms into an organic molecule is undoubtedly one of the most attractive research areas. Typically, these oxygenation reactions proceed with high selectivity and reactivity, which is seldom found in its "biomimetic" chemocatalytic counterparts. Furthermore, enzymatic oxygenation can avoid undesired overoxidation, which is frequently observed in (industrial) chemical transformation. Therefore, it is not surprising that tremendous attention has been paid to enzymatic oxygenation. Their application in organic synthesis has been steadily growing over the years. The goal of the present Review is to provide a handy reference for chemists interested in using homogeneous oxygenase catalysis and those interested in discovering new types of biomimetic oxidations and oxygenations with dioxygen. In this Review, we will review the recent advances in in homogeneous oxygenase catalysis to reveal the great achievements and potentials in this field.

Aerobic epoxidation of tertiary allylic alcohols remains a significant challenge. Reported here is an efficient and highly chemoselective copper-catalyzed epoxidation and semipinacol rearrangement reaction of tertiary allylic alcohols with molecular oxygen. The solvent 1,4-dioxane activates dioxygen, thereby precluding the addition of a sacrificial reductant.

An unexpected metal-free C≡C triple bond cleavage, dioxygen activation, and reassembly into tryptophol derivatives has been developed. This chemistry provides a novel, simple, and efficient approach to highly valuable tryptophol derivatives from simple substrates under mild conditions. The mechanistic studies may promote the discovery of new methodologies through C-C bond cleavage and dioxygen activation.

A novel Rh(III)-catalyzed C-H bond amination with the simultaneous release of a formyl group at distal positions is realized employing anthranil as a new type of C-H amination reagent. This chemistry provides an efficient protocol for the synthesis of 2-acyl diarylamines, which are important structural motifs in many bioactive compounds. This new type of C-H amination reagent possesses the advantages of high atom economy, avoids the use of external oxidants, and enables further transformation of the amination products.

Silver, known and utilized since ancient times, is a coinage metal, which has been widely used for various organic transformations in the past few decades. Currently, the silver-catalyzed reaction is one of the frontier areas in organic chemistry, and the progress of research in this field is very rapid. Compared with other transition metals, silver has long been believed to have low catalytic efficiency, and most commonly, it is used as either a cocatalyst or a Lewis acid. Interestingly, the discovery of Ag-catalysis has been significantly improved in recent years. Especially, Ag(i) has been demonstrated as an important and versatile catalyst for a variety of organic transformations. However, so far, there has been no systematic review on Ag-catalyzed C-H/C-C bond functionalization. In this review, we will focus on the development of Ag-catalyzed C-H/C-C bond functionalization and the corresponding mechanism.

Anilines are fundamental motifs in various chemical contexts, and are widely used in the industrial production of fine chemicals, polymers, agrochemicals and pharmaceuticals. A recent development for the synthesis of anilines uses the primary amination of C-H bonds in electron-rich arenes. However, there are limitations to this strategy: the amination of electron-deficient arenes remains a challenging task and the amination of electron-rich arenes has a limited control over regioselectivity-the formation of meta-aminated products is especially difficult. Here we report a site-directed C-C bond primary amination of simple and readily available alkylarenes or benzyl alcohols for the direct and efficient preparation of anilines. This chemistry involves a novel C-C bond transformation and offers a versatile protocol for the synthesis of substituted anilines. The use of O2 as an environmentally benign oxidant is demonstrated, and studies on model compounds suggest that this method may also be used for the depolymerization of lignin.

A gold-catalyzed nitrogenation of alkynes for the synthesis of carbamides and amino tetrazoles through C-C and C≡C bond cleavages is described. A diverse set of functionalized carbamide and amino tetrazole derivatives were selectively constructed under mild conditions. The chemoselectivity can be easily switched by the selection of the acid additives. The reaction is characterized by its broad substrate scope, direct construction of high value products, easy operation under air, and mild conditions at room temperature. This chemistry provides a way to transform alkynes by splitting the substrate into three parts.

A cationic cobalt(III)-catalyzed direct C-H amidation of unactivated (hetero)arenes and alkenes by using 1,4,2-dioxazol-5-ones as the amidating reagent has been developed. This transformation proceeds efficiently under external oxidant-free conditions with a broad substrate scope. Moreover, 6-arylpurine compounds, which often exhibit high potency in antimycobacterial, cytostatic, and anti-HCV activities, can be smoothly amidated, thus offering a mild protocol for their late stage functionalization.

An efficient Pd-catalyzed oxime ether directed ortho C–H hydroxylation of arenes under neutral conditions has been developed. The efficiency of this hydroxylation is significantly improved by a ligand. Oxone, an inexpensive, readily available, and safe reagent, was employed as terminal oxidant and oxygen source. The challenging electron-deficient substrates could also be monohydroxylated in high efficiency. Drug modification with this protocol was also successfully demonstrated.

A novel iron-catalyzed transformation of benzyl azides to give the corresponding amides via C–C bond cleavage under mild reaction conditions is developed. This method provides a new synthetic tool for the construction of amides and the opportunity to accomplish C–C functionalization under mild conditions.

A novel and efficient Rh-catalyzed carbonylation and annulation of simple anilines with CO and alkynes through N-H and C-H bond activation for the direct synthesis of quinolin-2(1H)-ones has been developed. Simple anilines without preactivation, broad substrate scope with hetero/polycycles, and high-value products make this protocol very practical and attractive. A key rhodacycle complex was isolated and well-characterized.

A unique copper-catalyzed aerobic oxidative C-C bond cleavage of simple unstrained ketones with air and amines has been developed. In this chemistry, amides and oxo amides are easily synthesized through the selective C-C bond cleavage of simple ketones or unstrained cycloketones. The broad substrate scopes and use of an inexpensive copper catalyst and green molecular oxygen as an oxidant as well as an O-source make this protocol very attractive for potential synthetic applications. The control experiments reveal that the present copper-catalyzed oxidative C-C bond cleavage of simple ketones proceeds in a novel catalytic pathway rather than through the cleavage of a dioxetane intermediate.

A simple and efficient approach towards α-oxyaminated ketones has been developed through CAN-catalyzed C–O bond formation with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). The environmentally friendly transformation is practical due to the use of commercial available catalyst, easy operating procedures, the broad substrate scope, and short reaction time.

Copper-catalyzed oxoazidation and alkoxyazidation of indoles has been developed. The dearomatization reaction which leads to versatile 3-azido indolenine and oxindole derivatives in moderate to good yields could be used in a further transformation.

A mild procedure of Cu-catalyzed decarboxylative cross-coupling of aryl- and alkynyl-boronic acids for construction of unsymmetrical substituted alkynes has been developed. The usage of inexpensive copper chloride as catalyst, and employing stable alkynl carboxylic acids and boronic acids as the substrates under oxidative conditions for sp–sp 2 coupling, make this method very easy to operate.

Nitrogen-containing heterocyclic compounds are ubiquitous in numerous natural and synthetic bioactive molecules. Nitrogenation strategy is one of the emerging strategies that is currently attracting tremendous attention with the aim to provide alternative highly efficient and concise ways for the construction of N-heterocyclic compounds especially from simple hydrocarbon substrates through C–H/C–C bond cleavage. In this chapter, the recent developments of facile synthesis of N-heterocyclic compounds via nitrogenation strategy are summarized.

In this chapter, the recent developments of facile nitrile synthesis via N atom incorporation are summarized. Simple hydrocarbons, such as alkenes, and alkynes, have been converted to nitriles through C–H/C–C bond cleavage. Mechanistic studies and investigations of substrate scope in this nitrogenation strategy exhibit its broad application in chemistry community.

A novel Pd-catalyzed intermolecular dehydrogenative annulation of aryl iodides and aryl carbamic chlorides for the efficient synthesis of phenanthridinone derivatives was developed. Simple aryl iodides and carbamic chlorides readily made from various anilines, a broad substrate scope with hetero/polycycles, as well as high-value products, make this direct dehydrogenative annulation approach very practical and attractive.

A novel Rh(III)-catalyzed C–H bond amination with the simultaneous release of a formyl group at distal positions is realized employing anthranil as a new type of C–H amination reagent. This chemistry provides an efficient protocol for the synthesis of 2-acyl diarylamines, which are important structural motifs in many bioactive compounds. This new type of C–H amination reagent possesses the advantages of high atom economy, avoids the use of external oxidants, and enables further transformation of the amination products.

Silver, known and utilized since ancient times, is a coinage metal, which has been widely used for various organic transformations in the past few decades. Currently, the silver-catalyzed reaction is one of the frontier areas in organic chemistry, and the progress of research in this field is very rapid. Compared with other transition metals, silver has long been believed to have low catalytic efficiency, and most commonly, it is used as either a cocatalyst or a Lewis acid. Interestingly, the discovery of Ag-catalysis has been significantly improved in recent years. Especially, Ag(I) has been demonstrated as an important and versatile catalyst for a variety of organic transformations. However, so far, there has been no systematic review on Ag-catalyzed C–H/C–C bond functionalization. In this review, we will focus on the development of Ag-catalyzed C–H/C–C bond functionalization and the corresponding mechanism.

A novel and efficient rhodium- and copper-co-catalyzed C–H bond activation and annulation for the construction of bioactively important quinazolines has been developed. This [4 + 2] annulation strategy utilizing alkyl azides as the carbon–heteroatom synthons shows high efficiency in the synthesis of six-membered benzoheterocycles containing two heteroatoms. This aerobic oxidative protocol provides a useful application of simple alkyl azides in N-heterocycle synthesis with N2 and H2O as byproducts.

The NHPI and palladium cocatalyzed radical oxidative acylation of arenes with aldehydes and alcohols as acyl equivalents via selective C–H functionalization has been described. Molecular oxygen, the most environmentally friendly oxidant, was used as the terminal oxidant in this catalytic cycle.

A novel and efficient Cu-catalyzed nitrogenation of alkynes and alkenes for the direct synthesis of sulfur-containing triazoles is described. Simple and readily available sulfoxides and azides are employed as the S- and N-source to prepare highly value N-,S-containing compounds from simple alkynes and alkenes.

A novel and efficient Rh-catalyzed carbonylation and annulation of simple anilines with CO and alkynes through N–H and C–H bond activation for the direct synthesis of quinolin-2(1H)-ones has been developed. Simple anilines without preactivation, broad substrate scope with hetero/polycycles, and high-value products make this protocol very practical and attractive. A key rhodacycle complex was isolated and well-characterized.

A novel iron-catalyzed transformation of benzyl azides to give the corresponding amides via C–C bond cleavage under mild reaction conditions is developed. This method provides a new synthetic tool for the construction of amides and the opportunity to accomplish C–C functionalization under mild conditions.

A dual catalytic approach enlisting gold and iron synergy is described. This method offers readily access to substituted heterocycle aldehydes via oxygen radical addition to vinyl–gold intermediates under Fe catalyst assistance. This system shows good functional group compatibility for the generation of substituted oxazole, indole, and benzofuran aldehydes. Mechanistic evidence greatly supports selective radical addition to an activated vinyl–Au double bond over alkene. This unique discovery offers a new avenue with great potential to further extend the synthetic power and versatility of gold catalysis.

A novel and efficient copper-catalyzed azidation reaction of anilines via C-H activation has been developed. This method, in which the primary amine acts as a directing group by coordinating to the metal center, provides ortho azidation products regioselectively under mild conditions. This effective route for the synthesis of aryl azides is of great significance in view of the versatile reactivity of the azide products.

The formation of carbon–heteroatom bonds is one of themost attractive and fundamental transformations in organic synthesis. For example, the construction of C O bonds plays an important role in producing alcohols, ethers, and esters in the synthesis of drugs, materials, and natural products. In general, great progress has been made in C O bond formation from carbon–halide bonds, although prefunctionalization of the substrates was often required and halidecontaining by-products were formed during the process (Scheme 1a). Thus, the alternative strategy involving direct

A rhodium(III)-catalyzed direct dehydrogenative annulation of benzamides with alkynes through chelating-assisted CH activation has been developed. Naphthylamide and isoquinolinone derivatives can be chemoselectively obtained by this protocol.

A novel Et 3 N-catalyzed oxidative dehydrogenative coupling of α-unsubstituted carbonyl compounds with aryl diamines leading to quinoxaline derivatives using molecular oxygen as oxidant has been developed. Six hydrogen dissociations involving 2 sp 3 C–H bonds activation are realized under mild conditions in this approach. Plausible mechanism is proposed for this novel Et 3 N-catalyzed transformation on the basis of the aboratively designed mechanistic studies including the radical detection by EPR. © 2012 Elsevier Science. All rights reserved.

A novel and efficient approach to alkenyl nitriles from allylesters has been developed. A tandem Pd-catalyzed substitution andthe subsequent oxidative rearrangement are involved in this transformation.The method provides an important supplement for the synthesis ofalkenyl nitriles from allyl esters.

A novel and efficient transition metal-free C–H bond halogenation of indole derivatives has been developed. 3-Halogenated (3-Br, 3-I) indoles are highly regioselectively produced by this protocol. Simple and readily available halide salts (TBAB, KI) are employed as the halogen source. The transition metal-free and the mild conditions make this protocol very easy to handle and practical.