HETEROCYCLES

Review | Regular issue | Vol. 85, No. 8, 2012, pp. 1795-1819
Received, 28th February, 2012, Accepted, 10th April, 2012, Published online, 16th April, 2012.
DOI: 10.3987/REV-12-736

■ Transition-Metal-Catalyzed Borylation of Organic Halides with Hydroboranes

Miki Murata*

Department of Materials Science and Engineering, Kitami Institute of Technology, Koen-cho 165, Kitami, Hokkaido 090-8507, Japan

Abstract

Metal-catalyzed cross-coupling reaction of B–H compounds provides a direct method for conversion of C–X bonds to C–B bonds. Dialkoxyboranes are excellent boron sources in the borylation of aryl, 1-alkenyl, allyl or benzyl halides or pseudohalides. These reactions tolerate large number of functional groups and proceeds with excellent yields, thereby allowing the simple, convenient, and elegant preparation of functionalized C–B compounds.

CONTENTS
1. Introduction
2. Borylation of Aryl Electrophiles
2-1. Reaction Conditions
2-1-1. Electrophiles and Catalysts
2-1-1-1. Aryl Iodides and Perfluoroalkanesulfonates
2-1-1-2. Electron-Rich Aryl Bromides
2-1-1-3. Electron-Deficient Aryl Bromides and Aryl Chlorides
2-1-1-4. Aryl Arenesulfonates
2-1-1-5. Other Catalyst Systems
2-1-2. Hydroboranes
2-1-2-1. 4,4,6-Trimethyl-1,3,2-dioxaborinane
2-1-2-2. 5,5-Dimethyl-1,3,2-dioxaborinane
2-2. Catalytic Cycle
2-3. Synthesis of Arylboronates
3. Borylation of Other Electrophiles
3-1. Vinyl Halides and Triflates
3-2. Benzyl Halides
3-3. Allyl Halides
4. Conclusion

1. INTRODUCTION
Organoboron compounds are an important class of organometallics, which can be widely used as versatile building blocks in modern organic chemistry.1-3 Among many methods available to prepare this class of compounds, representative synthetic route to organoboron compounds from organic electrophiles is illustrated in Scheme 1.

The most common method for the preparation of boronic acids and esters is based on the reaction of boron halides or alkoxides with carbon nucleophiles including organolithiums or Grignard reagents derived from the corresponding organic halides R–X.1 Although this traditional transmetalation protocol is still common for large scale synthesis of simple boronates, the use of such reactive organometallics R–M is inefficient in terms of step-economy because of the protection of electrophilic or protic functional groups. Since boronic acid and their esters are playing the crucial role for the construction of complex molecules, carbon–boron bond-forming reaction is demanded to tolerate the presence of a diverse and adequate number of functionalities. Another approach is based on direct construction of C–B bonds from organic electrophiles R–X with anionic boron species such as B–Li4 and B–Cu.5
While a palladium-catalyzed coupling reaction of organic electrophiles with mild boron nucleophiles should be an alternative route to a wide range of functionalized boronates, the lack of boron nucleophiles has nevertheless limited the catalytic carbon–boron bond-forming reaction. However, at the end of 20th century, two reports on boron nucleophiles for palladium-catalyzed coupling opened the door to the catalytic borylation of organic electrophiles R–X (Scheme 2). On one hand, Ishiyama and Miyaura reported on the palladium-catalyzed borylation with bis(pinacolato)diboron.6 This remarkable discovery is now called “Miyaura borylation”.7 On the other, we demonstrated pinacolborane (1) acts as a boron source in the coupling with organic electrophiles in the presence of a palladium catalyst and a tertiary amine.8 Transition metal-catalyzed cross-coupling reaction of tetra(alkoxo)diborons or dialkoxyboranes provides a simple, elegant and direct method for preparation of functionalized boronates from organic electrophiles. The advantage of pinacolborane (1) over bis(pinacolato)diboron is more economically profitable. Additionally, the borylation using dialkoxyborane is an atom-economical method; thus, the reaction is recommended for large-scale preparation of organoboronates. In this review, we will focus on the catalytic borylation of organic electrophiles with dialkoxyboranes. Miyaura borylation has been reviewed elsewhere.7

Another method of the catalytic C–B bond formation is the borylation of C–H bonds of hydrocarbons.9 During the past decade, much attention has been focused on the C–H borylation using bis(pinacolato)diboron or pinacolborane (1) as a boron source, and numerous catalyst systems were reported in the literature for these transformations. The borylation of ubiquitous C–H bonds is the most straightforward and atom-economical route; however, this is still a little problematic with regard to the control of regioselectivity. The catalytic C–H borylation has been reviewed elsewhere,9 and will not be covered by the present review.

2. BORYLATION OF ARYL ELECTROPHILES
In 1997, we reported the preparation of pinacol arylboronates from the corresponding aryl halides by palladium-catalyzed cross-coupling with pinacolborane (1) in the presence of Et3N (Table 1).8 To the best of our knowledge, this is the first example of the catalytic borylation of C–X bonds using B–H compounds. Dialkoxyboranes are now recognized as atom-economical and cost-effective alternatives to the catalytic borylation of aryl electrophiles.

2-1 REACTION CONDITIONS
As shown in Table 1, 1-iodonaphthalene 2 undergo the borylation with 1 in the presence of 3 mol% of PdCl2(dppf) and 3 equiv of tertiary amines.8 The reaction often suffers from arene byproduct 4 by partially reduction of C–X bonds. The choice of base is critical for the selective borylation, that is, Et3N plays a crucial role in not only preventing the formation of undesired dehalogenated arenes but also facilitating the borylation. Hunig’s base promoted an increase in dehalogenation. In the presence of other bases, such as pyridine, DBU, and KOAc, the formation of byproduct 4 predominated over that of desired boronate 3. Although the use of DMF caused both a low yield and selectivity, the yields are not significantly influenced by the choice of solvent. In 2003, Zaidlewicz has described that the use of ionic liquids as the reaction medium for the borylation of aryl iodides with 1 is advantageous over conventional solvents.10

2-1-1. ELECTROPHILES AND CATALYSTS
2-1-1-1. Aryl Iodides and Perfluoroalkanesulfonates In our first report, PdCl2(dppf) (dppf = 1,1'-bis(diphenylphosphino)ferrocene, 5) and PdCl2(PPh3)2 were recommended for the reaction of aryl iodides and triflates.8 Representative results are shown in Table 2. Generally, aryl iodides exhibited higher reactivity than aryl bromides or fluoroalkanesulfonates in the present reaction. Even at elevated temperatures, pinacolborane (1) is inert to much reducible functionality due to low reducing ability.9 Consequently, the present reaction tolerates various functional groups such as carbonyl, cyano, and nitro groups. However, the dehalogenation of C–X bonds was not completely suppressed, and small amount of arene byproducts (ca. 10%) was observed in most instances. The differences in the yields and on the selectivity among aryl iodides possessing electron-donating or -withdrawing groups were not particularly large under these conditions. Although higher reaction temperature was required, the conditions for aryl iodides using PdCl2(dppf) in dioxane facilitate the borylation of aryl fluoroalkanesulfonates, such as aryl triflates (X = OSO2CF3, OTf) and nonaflates (X = OSO2(CF2)3CF3, ONf), bearing either electron-donating or -withdrawing groups.

Under these conditions, the reactivity of aryl bromides was not favorable when compared with that of aryl iodides or triflates. The borylation of less-hindered electron-rich aryl bromides proceeded at 100 °C by using the PdCl2(PPh3)2 catalyst; however, the presence of electron-withdrawing substituents remarkably decreased the yield and the product selectivity of arylboronates.

2-1-1-2. Electron-Rich Aryl Bromides. Under the conditions using PdCl2(dppf) catalyst, ortho-substituted aryl bromides were also problematic. In 2000, Baudoin described that the borylation of ortho-substituted aryl iodides and bromides can be readily accomplished by using a catalyst system based on CyJohnPhos (6).11 In 2004, Colobert reported that DPEphos (7) as a supporting ligand can be employed for the borylation of para- and ortho-substituted aryl bromides.12 Table 3 compares three catalyst systems. Whereas the scope of the aryl halides in the palladium-catalyzed borylation has been improved, these catalyst systems still have some drawbacks; e.g., the reactions of electron-deficient or ortho,ortho’-disubstituted bromides produced only poor yields due to the formation of considerable amount of dehalogenated arene byproducts.

2-1-1-3. Electron-Deficient Aryl Bromides and Aryl Chlorides. In 2006, we found that the use of tBuDPEphos (10)13 or dtbpf (12) as a ligand has overcome many limitations of the above catalyst systems.14 That is, the selective borylation of electron-deficient or ortho,ortho’-disubstituted bromides and that of electron-rich chlorides have been achieved by using the combination of Pd(dba)2 and 10. As shown in Table 4, the use of electron-rich and sterically-hindered bidentate phosphine 10 or 12 effectively suppressed the formation of dehalogenated arene byproduct 9.
The substrate scope was significantly broad, and a variety of functional groups, such as ester and ketone carbonyl, cyano, and nitro groups, were tolerated under these conditions (Table 5). However, the reaction of electron-deficient chlorides has remained a challenging task; i.e., the presence of electron-withdrawing substituents remarkably increases the yield of dehalogenated arene byproducts. In 2010, we found that electron-deficient chlorides undergo the selective borylation using a catalyst system based on 10 with the aid of 2 equiv of Bu4NI.15 The addition of an iodide anion source improved the product selectivity on the borylation of aryl chlorides; although we still have no definitive explanation for the role of halide ion. Bu4NBr or Bu4NCl was less effective. To the best of our knowledge, this catalyst system based on 10 has the widest substrate scope of any reported to date.

Baudoin first introduced Buchwald's biarylphosphine ligands16 into the borylation with pinacolborane (1), as already mentioned.11 In 2008, the positive role of biarylphosphines in the catalytic borylation was described again by Buchwald.17 The use of S-Phos (15) allows high efficiency and relatively low catalyst loadings for the borylation of aryl bromides and electron-rich chlorides, provided that Et3N was used as solvent for the reaction of aryl chlorides (Scheme 3). However, they provided no example of the borylation of aryl chlorides possessing electron-withdrawing groups on the aromatic rings.

2-1-1-4. Aryl Arenesulfonates. Aryl arenesulfonates are relatively unreactive compared to the corresponding aryl halide and triflate counterparts; however, they undergo the borylation with 1 in the presence of 5 mol% of Pd(dba)2 and dtbpf (12) and 1.1 equiv of Bu4NI.18 Unlike the reaction of aryl halides, tBuDPEphos (10) did not promote the selective borylation with 1. As shown in Table 6, electron-deficient aryl benzenesulfonates were efficiently converted to the corresponding boronates. In contrast to the electron-deficient substrates, an electron-neutral aryl sulfonate, phenyl benzenesulfonate, did not have enough reactivity. We found, however, phenyl 4-trifluoromethylbenzenesulfonate participated in the present palladium-catalyzed borylation.

2-1-1-5. Other Catalyst Systems. Floch reported that palladium complexes with phosphorus heterocycle ligands 16–18 exhibit remarkable potential as catalysts for the borylation with pinacolborane (1).19 TONs exceed classical catalysts by a few orders of magnitude (3000–100000) and impressively underline the catalytic activity of these catalysts.

Additionally, it has been reported that NiCl2(dppp)20 and CuI21 catalyze the cross-coupling of aryl halides with 1. It is interesting to note that CuI allowed the substitutional borylation of aryl iodides to proceed at room temperature, although it required more reactive sodium hydride as a base. Furthermore, there are several reports focusing on an improved catalyst system for specific targets. Christophersen reported that Pd(PtBu3)2 catalyzes the borylation of 2,3-substituted bromothiophenes under the mild conditions, which tolerates even formyl group.22 Westcott recommended PdCl2(tBu2PCl)2 for the borylation of iodoaniline derivatives.23 The Miller group described that palladium black was the ideal catalyst for the large scale preparation of functionalized arylboronates in their drug synthesis.24

2-1-2. HYDROBORANES
Our and other groups successfully introduced several cyclic dialkoxyboranes, such as catecholborane (19),8 20,25 4,4,6-trimethyl-1,3,2-dioxaborinane (21),15,18,26-28 and 5,5-dimethyl-1,3,2-dioxaborinane (neopentylglycolborane, 22),29,30,31 to participate in the catalytic borylation of aryl halides. (Diisopropyl)aminoborane (23) is also an efficient boron source in the palladium-catalyzed borylation affording aryl(dialkylamino)boranes.32,33 Among the hydroboranes, pinacolborane (1) has become one of the most popular borylation reagents, predominantly because of the high stability, availability from

commercially source, and functional group compatibility.34 While the arylboronates synthesized by dialkoxyboranes 1, 21, or 22 are generally stable to air, water and chromatography, they can be directly employed in the Suzuki–Miyaura cross-coupling reaction.1-3 Additionally, they can be transformed into the corresponding boronic acids by several methods, but deprotection protocols have not always been trouble-free.25,35 On the other hand, monomeric aminoborane 23 has been recommended for the synthesis of arylboronic acids via the borylation followed by simple hydrolysis procedure (Scheme 4).33

20 has been introduced for similar propose by Morin (Scheme 5).25 The borylation with 20 has been achieved by using catalytic amounts of Pd(PPh3)4 and CuI.

2-1-2-1. 4,4,6-Trimethyl-1,3,2-dioxaborinane. In 2007, we demonstrated that 4,4,6-trimethyl-1,3,2- dioxaborinane (21), which is easily accessible, much less expensive, and more conveniently prepared, is an alternative for the borylation of aryl halides (Table 7).26 It is remarkable that 21 also tolerates various functional groups such as ester, ketone and nitrile functionalities. The palladium complex coordinated with tBuDPEphos (10) can catalyze the reaction of aryl iodides even in the presence of the electron-withdrawing or ortho substituent to afford the corresponding hexylene glycol boronates in high yields.26 The Chavant group has achieved the borylation of aryl electrophiles by using PPh327 or CyJohnPhos (6) ligand28 after our intervention, but these conditions result in somewhat low yields when aryl halides possesses electron-withdrawing groups. We have also achieved the coupling of electron-deficient aryl chlorides15 and that of aryl benzensulfonates18 with the aid of tetrabutylammonium halides. NiCl2(dppp) has been recommended for electron-deficient aryl chlorides, but the Pd(dba)2/10 catalyst system was not suitable.

2-1-2-2. 5,5-Dimethyl-1,3,2-dioxaborinane. Borylation of aryl electrophiles 5,5-dimethyl-1,3,2- dioxaborinane (22) with has been energetically developed by the group of Percec. 5,5-Dimethyl- 1,3,2-dioxaborinane (22), which can be conveniently prepared in situ from much less expensive neopentyl glycol and borane dimethylsulfide complex, is found to be an effective boron source in the nickel-catalyzed coupling of aryl halides. The use of PdCl2(dppf) with in situ-prepared 22 was ineffective due to the palladium catalyst poisoning in the presence of dimethylsulfide from borane dimethylsulfide complex. The combination of NiCl2(dppp) and additional dppp ligand was originally used as a catalyst system, but this catalyst has a limited substrate scope as only less hindered aryl iodides and bromides were efficiently converted to the corresponding boronates.29 Percec has found that the mixed-ligand catalyst system NiCl2(dppp)/dppf30 and the addition of Zn powder as a reductant31 greatly improved the reactivity and the substrate scope (Table 8).

2-2 CATALYTIC CYCLE
The accepted mechanism of the palladium-catalyzed borylation is illustrated in Figure 1. This catalytic cycle was theoretically proposed using DFT methods by Marder and Lin.36 First, oxidative addition of an aryl halide to Pd(0) species 24 generates an arylpalladium(II) halide 25. After displacement of the halide ligand on 25 with Et3N to form a cationic arylpalladium(II) species 26, σ-bond metathesis between the Pd–aryl bond of 26 and the B–H bond of 1 through a transition state 27, in which the boron atom is pointing toward the aryl ligand, affords the desired borylation product and a palladium(II) hydride 28. The energy profiles suggest that the B–C bond-forming metathesis is rate-determining step. This cationic pathway is favored over the neutral one which should involve ligand dissociation. Finally, deprotonation of 28 regenerates the Pd(0) active species 24 with the aid of Et3N. Thus, Et3N plays a dual role, assisting the formation of the cationic palladium(II) 26 as well as reducing 28 to Pd(0). A transition state 29, in which the hydride is pointing toward the aryl ligand, would account for the undesirable dehalogenation, but the activation barrier is higher than the formation of transition state 27.
This B–C bond-forming reaction using 1 differs from many cross-coupling reactions in that electron-donating substituents on the aryl halide substrates promote their catalytic borylations. Our experiments showed that the neutral arylpalladium(II) halide 25 underwent predominantly the aryl–H bond formation with pinacolborane (1),8 suggesting that in this B–C bond-forming reaction the formation of cationic arylpalladium(II) species 26 should be a key step. The electron-withdrawing substituents would prevent the formation of 26. The use of electron-rich and bulky ligands, such as tBuDPEphos (10), dtbpf (12), and Buchwald's biarylphosphine ligands (6 and 15), would assist the dissociation of the halide anion on arylpalladium(II) halide 25, and the cationic species 26 would be stabilized. The addition of Bu4NI would prevent arylpalladium(II) halide 25 from reacting with hydroboranes 1, and thus the product selectivity would increase. The order of an inhibiting effect on the transmetalation between Pd(II) and boron reagents by additional halide ions has been reported to be as follows: I- > Br- > Cl-.37

2-3 SYNTHESIS OF ARYLBORONATES
Arylboronates have become a useful building block in the construction of complex aromatic compounds through Suzuki–Miyaura cross-coupling.2 Therefore, the application of the catalytic borylation began to appear rapidly, and a large number of functionalized aryl- and heteroarylboronates have been synthesized via the palladium-catalyzed borylation with pinacolborane (1). Representative examples are selected in Figure 2. In most cases, arylboronates thus obtained has been used for the synthesis of functionalized biaryls including natural compounds and biologically active molecules via Suzuki–Miyaura coupling.

As shown in Figure 2, the reaction with 1 tolerates various functional groups; e.g., aldehyde (3022) and ketone carbonyl (3138 and 3239), nitrile (3340), amino acid (34,41 3542 and 3643) and sulfonamide functionalities (3724). Interestingly, Spino has described that 32 could be obtained without racemization. Substituents at the ortho positions (38,44 39,45,46 4047 and 4148) are also tolerated. These conditions also offer the synthesis of functionalized heteroarylboronates containing sulphur, nitrogen (42,49 43,50 44,51 4552 and 4653) and/or oxygen atoms (4754 and 4855). The borylation of polymer-supported aryl halides has been achieved (4939). The substrate scope of the palladium-catalyzed borylation with 1 seems to be similar to that of the Miyaura borylation using B–B reagents,7 except that the examples of substrates possessing pyridine moieties or aldehyde carbonyl group (e.g., 43 and 30) are still rare. Although the difference in the substrate scope between two borylation methods is undefined, in some cases shown in Figure 2 (e.g., 38, 42 and 46), 1 was a much more efficient borylation agent as compared to bis(pinacolato)diboron.44,49,53
The reaction also offers an easy access to building blocks for the construction of teraryl frameworks in functional molecules. Figure 3 presents selected examples of the palladium-catalyzed double-borylation of diharoarenes. With an excess of 1, arylene-bridged bis-boronates (50,56 51,57 52,58 53,59 54,45,60 5561 and 5662) were obtained in moderate to high yields. As shown in Figure 3, the borylation tolerates various arylene cores including porphyrin (50), carbazole (51), pyrrole (52), phenothiazine (53) and thiophen rings (55).

The palladium-catalyzed, stepwise, one-pot borylation/Suzuki coupling (BSC) procedure was pioneered by the work of Baudoin in 2000,10 and it has been applied to the concise synthesis of functionalized biaryls. In most of the reports about the BSC procedure using 1, the first catalyst for the borylation works on the next Suzuki coupling. For examples, Pd(OAc)2/ CyJohnPhos (6) catalyzes both the borylation of 2-bromoaniline derivatives and Suzuki coupling with ortho-substituted aryl halides.11,63,64 In 2002, Baudoin reported the synthesis of paullone (57) (Scheme 6).64

Another application of BSC procedure is the preparation of heterobiaryls. Queiroz showed that tri-t-butylphosphonium tetrafluoroborate (tBu3PHBF4), a tBu3P ligand precursor,65 was the efficient supporting ligand in the synthesis of 6-(thien-2-yl)thieno[3,2-b]pyridine (58) through the one-pot, two-step procedure (Scheme 7).66

In 2011, Müller reported the synthesis of various 3-(hetero)aryl substituted indoles, 7-azaindoles, and pyrroles.67 The N-Boc protected azaindolyl iodide 59 was borylated with 1 and a catalytic amount of Pd(PPh3)4, and then the coupling with 4-chloropyrimidin-2-amine took place without further addition of palladium complexes to afford meriolin 1 (60) (Scheme 8).

3. BORYLATION OF OTHER ELECTROPHILES
The catalytic borylation with pinacolborane (1) has been extended to other organic electrophiles, such as 1-alkenyl, benzyl, and allyl halides. In either case, tertiary amines play a crucial role in the selective borylation.

3-1. VINYL HALIDES AND TRIFLATES
Hydroboration of 1-alkynes is a method of choice for the syntheses of 1-alkenylboronates.1,3 However, the protocol is inapplicable to the synthesis of cyclic 1-alkenylboronates. Such boronates can be conveniently synthesized from the corresponding 1-alkenyl electrophiles by the palladium-catalyzed coupling with boron nucleophiles. In 2000, we reported that the use of a mixed-ligand catalyst system

PdCl2(dppf)/AsPh3 achieved the borylation of cyclic 1-alkenyl iodides and triflates 61 with pinacolborane (1) (Table 9).68 The absence of AsPh3 caused the catalyst decomposition during the reaction, resulting in low conversion. In addition, the use of excess phosphine ligand, such as dppf and PPh3, in place of AsPh3 rather retarded the reaction. But then, Occhiato described that the presence of AsPh3 as an additional ligand was unnecessary in the borylation of lactam-derived 1-alkenyl triflates 64 (Scheme 9).69 1 showed a greater reactivity than bis(pinacolato)diboron toward 64.

Under these conditions, some cyclic 1-alkenyl halides partly undergo positional isomerization of C–C double bonds to afford an inseparable mixture of the desired 1-alkenylboronates 67 and allylic byproducts 68 (Table 10).68 Interestingly, the borylation of 66 including an allyl ether moiety afforded the allyl boronate 68 as a major product due to the n–π conjugation.

As shown in Scheme 10, heterocyclic 1-alkenyl triflate 66 (R = O, X = OTf) can be product-selectively and asymmetrically transformed to allylboronate 69 by treatment with 5 mol% of Pd(OAc)2, 10 mol% of TANIAPHOS (71), and 1.1 equiv of PhNMe2.70 The borylation and subsequent allylboration gave moderate yield of homoallylalcohol 70 with high enantiomeric excess.

The borylation of acyclic 1-alkenyl halides with 1 suffers from low yields and isomerization of C–C double bonds, resulting in a complex mixture of several byproducts. In the case of acyclic (Z)-alkenyl halides with 1 or (diisopropylamino)borane (23), the complete isomerization of the starting geometry occurred to give (E)-alkenylboron compounds (Scheme 11).68,71

3-2 BENZYL HALIDES
Benzyl chlorides and bromides can be coupled with pinacolborane (1) in the presence of a catalytic

amount of PdCl2(PPh3)2 to afford the corresponding benzylboronates in high yields (Scheme 12).72 iPr2NEt was more effective base for the palladium-catalyzed borylation of benzyl halides than Et3N.
In 2010, Duñach described that a catalytic amount of Mg(0) permitted the reductive coupling between benzyl bromides and 1.73 The borylation takes place via an organomagnesium species generated in situ. The DFT study showed that 1 acts both as an electrophile and as a reducing agent to regenerate the Grignard reagents.

3-3 ALLYL HALIDES
Unlike the coupling between most organic electrophiles and pinacolborane (1), palladium catalysts did not promote the borylation of allylic halides. In 2000, we reported that the use of a platinum catalyst Pt(dba)2/AsPh3 achieved the selective borylation of allylic halides with 1 (Table 11).74 The presence of Et3N plays a key role in facilitating the B–C bond formation. Although the starting crotyl chloride was a mixture of (E)- and (Z)-isomers, the borylation of such aliphatic allyl electrophiles proceeded regio- and stereoselectively to give (E)-crotylboronates exclusively. Also, 3-chloro-1-butene afforded the same product, suggesting a process via π-allyl platinum species.

4. CONCLUSION
As illustrated in this review, dialkoxyboranes has offered the transition metal-catalyzed borylation of organic electrophiles, such as aryl, 1-alkenyl, benzyl and allyl (pseudo)halides. Pinacolborane, 4,4,6-trimethyl-1,3,2-dioxaborinane, 5,5-dimethyl-1,3,2-dioxaborinane, etc., were introduced to use in the catalytic borylation, and dialkoxyboranes are now recognized as atom-economical and cost-effective boron sources. While numerous catalyst systems and solvents have been reported in the literature for the selective borylation, the use of tertiary amines is essential in either case. The substrate scope of the catalytic borylation is significantly broad and includes organic electrophiles possessing diverse functional groups.

ACKNOWLEDGEMENTS
The author sincerely thanks all co-workers involved in our own studies referred to in this review for their invaluable contributions. Our work has been supported in part by Grants-in-Aid from MEXT and JSPS. BASF Corporation is gratefully acknowledged for a generous donation of pinacolborane.

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