Recent Progress on Synthesis of N,N′-Chelate Organoboron Derivatives

N,N′-chelate organoboron compounds have been successfully applied in bioimaging, organic light-emitting diodes (OLEDs), functional polymer, photocatalyst, electroluminescent (EL) devices, and other science and technology areas. However, the concise and efficient synthetic methods become more and more significant for material science, biomedical research, or other practical science. Here, we summarized the organoboron-N,N′-chelate derivatives and showed the different routes of their syntheses. Traditional methods to synthesize N,N′-chelate organoboron compounds were mainly using bidentate ligand containing nitrogen reacting with trivalent boron reagents. In this review, we described a series of bidentate ligands, such as bipyridine, 2-(pyridin-2-yl)-1H-indole, 2-(5-methyl-1H-pyrrol-2-yl)quinoline, N-(quinolin-8-yl)acetamide, 1,10-phenanthroline, and diketopyrrolopyrrole (DPP).

In recent years, the research on their syntheses falls into the following categories. At first, bipyridine derivatives could be used as bidentate ligand reacting with triphenylboron to form the desired products, as shown in Figure 2. The second, an indole connecting with pyridine derivatives (or other nitrogen heterocyclic molecule) reacted with triphenylboron to obtain the corresponding compounds, as shown in Figure 2. The third, pyridine attaching on pyrrole derivatives (or other nitrogen heterocyclic molecule) and trivalent boron could produce the fluorescent organoboron compounds, as shown in Figure 2. The fourth, a quinoline linking with pyrrole derivatives (or other nitrogen heterocyclic molecule) has been reported as a bidentate ligand to react with boron reagents. The different fluorescent compounds could be synthesized by N-(quinolin-8-yl)acetamide derivatives and boron reagents, as shown in Figure 2. The fifth, other bidentate ligands including 1,10-phenanthroline and diketopyrrolopyrrole (DPP) derivatives could also react with trivalent boron to obtain tetracoordinated organoboron compounds, as shown in Figure 2. ing with pyridine derivatives (or other nitrogen heterocyclic molecule) reacted with triphenylboron to obtain the corresponding compounds, as shown in Figure 2. The third, pyridine attaching on pyrrole derivatives (or other nitrogen heterocyclic molecule) and trivalent boron could produce the fluorescent organoboron compounds, as shown in Figure 2. The fourth, a quinoline linking with pyrrole derivatives (or other nitrogen heterocyclic molecule) has been reported as a bidentate ligand to react with boron reagents. The different fluorescent compounds could be synthesized by N-(quinolin-8-yl)acetamide derivatives and boron reagents, as shown in Figure 2. The fifth, other bidentate ligands including 1,10-phenanthroline and diketopyrrolopyrrole (DPP) derivatives could also react with trivalent boron to obtain tetracoordinated organoboron compounds, as shown in Fig [30][31][32][33][34][35][36].
Molecules 2021, 26, x FOR PEER REVIEW 2 of 21 ing with pyridine derivatives (or other nitrogen heterocyclic molecule) reacted with triphenylboron to obtain the corresponding compounds, as shown in Figure 2. The third, pyridine attaching on pyrrole derivatives (or other nitrogen heterocyclic molecule) and trivalent boron could produce the fluorescent organoboron compounds, as shown in Figure 2. The fourth, a quinoline linking with pyrrole derivatives (or other nitrogen heterocyclic molecule) has been reported as a bidentate ligand to react with boron reagents. The different fluorescent compounds could be synthesized by N-(quinolin-8-yl)acetamide derivatives and boron reagents, as shown in Figure 2. The fifth, other bidentate ligands including 1,10-phenanthroline and diketopyrrolopyrrole (DPP) derivatives could also react with trivalent boron to obtain tetracoordinated organoboron compounds, as shown in Fig [30][31][32][33][34][35][36].

Bipyridine-Based Derivatives as Bidentate Ligand
In this section, different traditional methods for the formation of N,N -chelate organoboron derivatives will be displayed in detail from the following aspects.

Bipyridine as Bidentate Ligand
In 1985, Heinrich Noeth and co-workers reported that dibutyl(((trifluoromethyl) sulfonyl)oxy)borane (1) reacted with bipyridine (2) to complete desired product. The solution of the diorganylborane should be cooled to −78 • C in this reaction. Bipyridinedibutylboronium(1+) triflate (3) was confirmed by 11 B NMR in their lab [37], as shown in Scheme 1. This protocol realized the synthesis of bipyridine coordinated organoboron complex at low temperature. It played a certain role in promoting the deep study of tetracoordinated organoboron compounds.

Bipyridine-Based Derivatives as Bidentate Ligand
In this section, different traditional methods for the formation of N,N'-chelate organoboron derivatives will be displayed in detail from the following aspects.

Bipyridine as Bidentate Ligand
In 1985, Heinrich Noeth and co-workers reported that dibutyl(((trifluoromethyl)sulfonyl)oxy)borane (1) reacted with bipyridine (2) to complete desired product. The solution of the diorganylborane should be cooled to −78 C in this reaction. Bipyridine-dibutylboronium(1+) triflate (3) was confirmed by 11 B NMR in their lab [37], as shown in Scheme 1. This protocol realized the synthesis of bipyridine coordinated organoboron complex at low temperature. It played a certain role in promoting the deep study of tetracoordinated organoboron compounds.

Bipyridine-Based Derivatives as Bidentate Ligand
In this section, different traditional methods for the formation of N,N'-chelate organoboron derivatives will be displayed in detail from the following aspects.

Bipyridine as Bidentate Ligand
In 1985, Heinrich Noeth and co-workers reported that dibutyl(((trifluoromethyl)sulfonyl)oxy)borane (1) reacted with bipyridine (2) to complete desired product. The solution of the diorganylborane should be cooled to −78 C in this reaction. Bipyridine-dibutylboronium(1+) triflate (3) was confirmed by 11 B NMR in their lab [37], as shown in Scheme 1. This protocol realized the synthesis of bipyridine coordinated organoboron complex at low temperature. It played a certain role in promoting the deep study of tetracoordinated organoboron compounds.

Bipyridine for the Formation of N,N′-chelate Organoboron and Ferrocene Derivatives
In 2010, Matthias Wagner's group reported a more powerful synthetic route of ferrocene complexes [43]. They used bromide (26) to obtain FcBBr polymers (27,28) and continued to form the bipyridine organoboron polymers with main chain charge-transfer structure (30, 32, 33, 80%, 65%, 76%), as shown in Scheme 6. This method provided a new opportunity for the synthesis of multifunctional organoboron polymers.

Bipyridine for the Formation of N,N′-chelate Organoboron and Ferrocene Derivatives
In 2010, Matthias Wagner's group reported a more powerful synthetic route of ferrocene complexes [43]. They used bromide (26) to obtain FcBBr polymers (27, 28) and continued to form the bipyridine organoboron polymers with main chain charge-transfer structure (30, 32, 33, 80%, 65%, 76%), as shown in Scheme 6. This method provided a new opportunity for the synthesis of multifunctional organoboron polymers.  In 2009, Warren E. Piers's lab prepared a series of neutral radicals, which had significant spin density on boron [42]. The scaffold of 2,2 -bipyridyl-stabilized boronium ions was interesting and demonstrated bipyridine adducts persistent neutral radical. They added AgBF 4 in this reaction and offered moderate yields (20-23, yield 53-77%), as shown in Scheme 5. It was proved that this protocol could easily get the target 2,2 -bipyridyl boronium ions and neutral radicals.

Bipyridine for the Formation of N,N -Chelate Organoboron and Ferrocene Derivatives
In 2010, Matthias Wagner's group reported a more powerful synthetic route of ferrocene complexes [43]. They used bromide (26) to obtain FcBBr polymers (27,28) and continued to form the bipyridine organoboron polymers with main chain charge-transfer structure (30, 32, 33, 80%, 65%, 76%), as shown in Scheme 6. This method provided a new opportunity for the synthesis of multifunctional organoboron polymers.

Bipyridine Reacting with 5-bromo-10-mesityl-5,10-Dihydroboranthrene
In 2011, Matthias Wagner's group has been committed to the development of more diversified N,N′-chelate organoboron chemistry for many years [44]. It always showed wonderful results in bipyridine organoboron adducts. They realized cleavage of the B-O-B bridge in this reaction and got the desired product with moderate yield (36, 51%), as shown in Scheme 7. In this protocol, they prepared 5-bromo-10-mesityl-5,10-dihydroboranthrene (35) as a boron reagent. This building block revealed a novel synthetic route for us. Scheme 7. Bipyridine reacting with 5-bromo-10-mesityl-5,10-dihydroboranthrene [44].  In 2011, Matthias Wagner's group has been committed to the development of more diversified N,N -chelate organoboron chemistry for many years [44]. It always showed wonderful results in bipyridine organoboron adducts. They realized cleavage of the B-O-B bridge in this reaction and got the desired product with moderate yield (36, 51%), as shown in Scheme 7. In this protocol, they prepared 5-bromo-10-mesityl-5,10-dihydroboranthrene (35) as a boron reagent. This building block revealed a novel synthetic route for us.

Bipyridine Reacting with 5-bromo-10-mesityl-5,10-Dihydroboranthrene
In 2011, Matthias Wagner's group has been committed to the development of more diversified N,N′-chelate organoboron chemistry for many years [44]. It always showed wonderful results in bipyridine organoboron adducts. They realized cleavage of the B-O-B bridge in this reaction and got the desired product with moderate yield (36, 51%), as shown in Scheme 7. In this protocol, they prepared 5-bromo-10-mesityl-5,10-dihydroboranthrene (35) as a boron reagent. This building block revealed a novel synthetic route for us.

2-
In 2016, David Curiel's group described a reaction of 2- (62) to generate organoboron complexes (63-65) with tuned emission and cell bioimaging utility [32], as shown in Scheme 12. To synthesize these diphenylboron complexes, they choose triphenylboron as boron reagent. In this paper, they succeeded in introducing more heteroatoms (S, O, N) in this building block. Molecules 2021, 26, x FOR PEER REVIEW 8 of 21

Pyridine-Based Derivatives as Bidentate Ligand
In 2003, Yun Chi's group reported a reaction of 2-(1H-pyrazol-5-yl)pyridine derivatives (67) and triphenylboron to produce pyridyl pyrazolate boron complexes (68-73, 48-90%) [47], as shown in Scheme 13, which exhibited the ability of remarkable dual fluorescence through the photoinduced electron transfer reaction. In this system, it was more efficient to synthesize the N,N -chelate organoboron adducts in tetrahydrofuran (THF) solution for one hour. They found that electron-deficient B(C 6 F 5 ) 3 reagent could be also feasible in this reaction.
It continued to react with alkynes to obtain the final polymers (115-117, 40-51%). This paper showed that the π-conjugated linker unit had a great influence on the fluorescent quantum efficiencies of the synthesized polymers. Besides, they also observed an obvious energy transfer.

N-(quinolin-8-yl)acetamide as Bidentate Ligand
Two years later, in 2012, Yoshiki Chujo's group designed and synthesized 8-aminoquin olate-based organoboron polymers with rigid structure, which had electron-donating and electron-accepting structure in the basic skeleton [57]. In order to get the corresponding starting material, they added tin compound (128) and boron tribromide to give the intermediate (130) in two steps, as shown in Scheme 25. Next, it continued to react with N-(quinolin-8-yl)acetamide to afford adduct monomer in acceptable yield (131, 34%). Final 8-aminoquinolate-based organoboron products were prepared by classical Sonogashira-Hagihara cross-coupling reaction (132, 133, 84%, 84%). The quantum yield of polymer (132) was up to 0.53. It was certain that this scaffold with 8-aminoquinolate-based could achieve more efficient luminescent properties by introducing the -C 8 F 17 group.

2,2,2-Trifluoro-N-(quinolin-8-yl)acetamide Derivatives as Bidentate Ligand
In 2020, our group almost at the same time reported the reaction of 2,2,2-trifluoro-N-(quinolin-8-yl)acetamide derivatives (145) and sodium tetraarylborate, resulting in aminoquinolate-based organoboron complexes (146-168) with high yields using iodine as catalyst [31], as depicted in Scheme 28. We made a modification from 2,2,2-trifluoro-N-(quinolin-8-yl)acetamide with various functional groups. In our protocol, we synthesized successfully a series of cheap and air-stable sodium tetraarylborates. Only a catalytic amount of iodine was enough to drive this reaction. In addition, we showed that adding external acids in this system formed the novel N,N′-chelate organoboron aminoquinolate with moderate yields (159-168, 45-89%). All the complexes were fully characterized and we found that the quantum yield of organoboron compound (146) reached 0.79 when dissolved in dichloromethane.  In 2013, Douglas W. Stephan's group treated 1,10-phenanthroline with the same amount of the boron reagent B(C 6 F 5 ) 3 to afford the desired salt (170, 73%) under H 2 (4 atm) condition [59], as shown in Scheme 29. The authors tried to develop an organoboron catalyst, which has the ability of frustrated Lewis pairs (FLPs) catalyst in practical applications. In this paper, they developed a new synthetic route of tetracoordinated organoboron complexes by using 1,10-phenanthroline.

Diketopyrrolopyrrole (DPP) Derivatives as Bitentate Ligand
In 2014, Takaki Kanbara and co-workers disclosed a route for the generation of diketopyrrolopyrrole (DPP)-based diphenylboron complexes with moderate yields (184-186, 21-71%) [62], as shown in Scheme 32. In this case, diketopyrrolopyrrole derivatives were synthesized by using 5-substituted-picolinonitrile (179) and dibutyl succinate (180). Next, triphenylboron reacted with them to form the diphenylboron complexes (184-186). At last, the authors studied the corresponding theoretical calculation in their work, which indicated that the introduction of the coordinating boron element could lower the band gap.

Diketopyrrolopyrrole (DPP) Derivatives as Bitentate Ligand
In 2014, Takaki Kanbara and co-workers disclosed a route for the generation of diketopyrrolopyrrole (DPP)-based diphenylboron complexes with moderate yields (184-186, 21-71%) [62], as shown in Scheme 32. In this case, diketopyrrolopyrrole derivatives were synthesized by using 5-substituted-picolinonitrile (179) and dibutyl succinate (180). Next, triphenylboron reacted with them to form the diphenylboron complexes (184-186). At last, the authors studied the corresponding theoretical calculation in their work, which indicated that the introduction of the coordinating boron element could lower the band gap.

Diketopyrrolopyrrole (DPP) Derivatives as Bitentate Ligand
In 2014, Takaki Kanbara and co-workers disclosed a route for the generation of diketopyrrolopyrrole (DPP)-based diphenylboron complexes with moderate yields (184-186, 21-71%) [62], as shown in Scheme 32. In this case, diketopyrrolopyrrole derivatives were synthesized by using 5-substituted-picolinonitrile (179) and dibutyl succinate (180). Next, triphenylboron reacted with them to form the diphenylboron complexes (184-186). At last, the authors studied the corresponding theoretical calculation in their work, which indicated that the introduction of the coordinating boron element could lower the band gap.

Diketopyrrolopyrrole (DPP) Derivatives as Bitentate Ligand
In 2014, Takaki Kanbara and co-workers continued to improve this scaffold by introducing phenylacetylene derivatives [63], by palladium-catalyzed Sonogashira coupling. Synthesized diketopyrrolopyrrole-based diphenylboron complexes (187-188) showed red-shift of the location of wavelengths to a near-infrared region, as shown in Scheme 33. This was a good strategy to extend π-conjugation of diketopyrrolopyrrole-based diphenylboron complexes by linking with more rigid phenylethynyl groups, which reached red-shift in near-infrared region and achieved high quantum yields.

Diketopyrrolopyrrole (DPP) Derivatives as Bitentate Ligand
In 2014, Takaki Kanbara and co-workers continued to improve this scaffold by introducing phenylacetylene derivatives [63], by palladium-catalyzed Sonogashira coupling. Synthesized diketopyrrolopyrrole-based diphenylboron complexes (187-188) showed redshift of the location of wavelengths to a near-infrared region, as shown in Scheme 33. This was a good strategy to extend π-conjugation of diketopyrrolopyrrole-based diphenylboron complexes by linking with more rigid phenylethynyl groups, which reached red-shift in near-infrared region and achieved high quantum yields.

Conclusions and Outlook
In summary, tetracoordinated organoboron compounds have been widely applied in cell imaging, organic light-emitting diodes (OLEDs), functional polymer, photocatalyst, electroluminescent (EL) devices, and other science and technology areas. N,N -chelate diarylboron complexes have been successfully synthesized by using a series of bidentate ligands and diversified boron reagents. Bidentate ligands mainly consist of two parts by using bipyridine, indole, pyrrole, quinoline, isoquinoline, pyridine, 1H-benzo[d]imidazole, and others. The boron reagents mainly include triphenylboron, boron trichloride, boron tribromide, B(C 6 F 5 ) 3 , HB(C 6 F 5 ) 2 , hydroxydiphenylborane (Ph 2 BOH), phenylboronic acids, sodium tetraaylborate, and FcMeBr in this review. However, there is main focus including exploring wider substrates as bidentate ligand, more diversified boron reagents, simpler and greener reaction condition, and higher yields. Besides, this N,N -chelate organoboron framework should be developed in these fields such as molecular probes, cell imaging, fluorescent dyes, OLEDs, and so on. We expect with continuous effort more efficient and greener methods for exploiting more powerful diphenylboron complexes.