Transition metal-free B(dan)-installing reaction (dan: naphthalene-1,8-diaminato): H-B(dan) as a B(dan) electrophile

NBN- and BNB-Phenalenyls: the Yin and Yang of Heteroatom-doped π Systems

NBN- and BNB-doped phenalenyls are isoelectronic to phenalenyl anions and cations, respectively. They represent a pair of complementary molecules that have essentially identical structures but opposite properties as electron donors and acceptors. The NBN-phenalenyls 1-4 considered here were prepared from N,N'-dimethyl-1,8-diaminonaphthalene and readily available boron-containing building blocks (i. e., BH3⋅SMe2 (1), p-CF3-C6H4B(OH)2 (2), C6H5B(OH)2 (3), or MesBCl2/iPr2NEt (4)). Treatment of 1 with 4-Me2N-2,6-Me2-C6H2Li gave the corresponding NBN derivative 5. The BNB-phenalenyl 6 was synthesized from 1,8-naphthalenediyl-bridged diborane(6), PhNH2, and MesMgBr. A computational study reveals that the photoemission of 1, 4, and 5 originates from locally excited (LE) states at the NBN-phenalenyl fragments, while that of 2 is dominated by charge transfer (CT) from the NBN-phenalenyl to the p-CF3-C6H4 fragment. Depending on the dihedral angle θ between its Ph and NBN planes, compound 3 emits mainly from a less polar LE (θ >55°) or more polar CT state (θ <55°). In turn, the energetic preference for either state is governed by the polarity of the solvent used. An equimolar aggregate of the NBN- and BNB-phenalenyls 3 and 6 (in THF/H2O) shows a distinct red-shifted emission compared to that of the individual components, which originates from an intermolecular CT state.

Ethynyl-B(dan) in [3+2] Cycloaddition and Larock Indole Synthesis: Synthesis of Stable Boron-Containing Heteroaromatic Compounds

The cycloaddition of nitrile oxides with ethynyl-B(dan) (dan=naphthalene-1,8-diaminato) allowed the facile preparation of diverse isoxazolyl-B(dan) compounds, all of which displayed excellent protodeborylation-resistant properties. The dan-installation on the boron center proves vital to the high stability of the products as well as the perfect regioselectivity arising from hydrogen bond-directed orientation in the cycloaddition. The diminished boron-Lewis acidity of ethynyl-B(dan) also renders it amenable to azide-alkyne cycloaddition, Larock indole synthesis and related heteroannulations. The obtained boron-containing triazole, indoles, benzofuran and indenone exhibit sufficient resistance toward protodeborylation. Despite the commonly accepted transmetalation-inactive property derived from the diminished Lewis acidity, the synthesized heteroaryl-B(dan) compound was still found to be convertible to the oligoarene via sequential Suzuki-Miyaura coupling.

Efficient one-pot synthesis of dan-substituted organo- and silyl-boron compounds

Direct, transition metal-free B(dan)-installation into organic frameworks has been developed. Heteroaryl-H bonds were transformable into the respective heteroaryl-B(dan) bonds through deprotonation. The resulting heteroaryl-B(dan) compounds, which are otherwise difficult to access, can undergo the direct Suzuki-Miyaura coupling. The method was demonstrated to apply to a silicon nucleophile, giving Lewis acidity-diminished stable silyl-B(dan) and -B(aam) in one pot.

Copper-Catalyzed Borylation of Styrenes by 1,8-Diaminonaphthalene-Protected Diboronic Acid

We report a concise synthesis of 1,8-diaminonaphthalene-protected diboronic acid (B₂(dan)₂), which is a promising borylating agent. B₂(dan)₂ is a bench-stable compound, and it could be utilized for Cu-catalyzed borylation of styrene derivatives. The reaction proceeded in a highly selective manner, and the products were isolated in up to 97% yields. Mechanistic studies revealed that a borate species would be a key intermediate for the borylation reaction.

A stable silylborane with diminished boron Lewis acidity

A new dimethyl(phenyl)silylborane having a naphthalene-1,8-diaminato (dan) substituent on the boron center, PhMe₂Si-B(dan), was synthesized. Owing to the diminished boron Lewis acidity, it is highly stable toward air. Synthetic application of the silylborane to catalytic silylboration and silylation of alkynes is also described.

Borylation and Stannylation Reactions with Tuning of Lewis Acidity

A B(dan) moiety (dan=naphthalene-1,8-diaminato) of diminished boron-Lewis acidity has efficiently been installed into organic frameworks by three-component carboboration of alkenes under copper catalysis, where a Cu-B(dan) species, generated by chemoselective σ-bond metathesis between a copper catalyst and an unsymmetrical diboron [(pin)B-B(dan)], acts as a key intermediate. The Cu-B(dan) species has also turned out to serve as a B(dan) nucleophile to afford various dan-substituted organoboranes via borylative substitution of carbon electrophiles. Furthermore, borylation reactions with another Lewis acidity-diminished boron unit, B(aam) (aam=anthranilamidato), have become feasible by use of (pin)B-B(aam) or H-B(aam). The resulting dan/aam-substituted organoboranes have been demonstrated to undergo direct cross-coupling without prior acidic deprotection, regardless of their diminished boron-Lewis acidity. Synthesis of diverse organostannanes based upon copper-catalyzed carbostannylation and borylstannylation, in which Lewis acidity increment of a tin center facilitates the progress in some cases, have also been described.

The 2-Pyridyl Problem: Challenging Nucleophiles in Cross-Coupling Arylations

Azine-containing biaryls are ubiquitous scaffolds in many areas of chemistry, and efficient methods for their synthesis are continually desired. Pyridine rings are prominent amongst these motifs. Transition-metal-catalysed cross-coupling reactions have been widely used for their synthesis and functionalisation as they often provide a swift and tuneable route to related biaryl scaffolds. However, 2-pyridine organometallics are capricious coupling partners and 2-pyridyl boron reagents in particular are notorious for their instability and poor reactivity in Suzuki-Miyaura cross-coupling reactions. The synthesis of pyridine-containing biaryls is therefore limited, and methods for the formation of unsymmetrical 2,2'-bis-pyridines are scarce. This Review focuses on the methods developed for the challenging coupling of 2-pyridine nucleophiles with (hetero)aryl electrophiles, and ranges from traditional cross-coupling processes to alternative nucleophilic reagents and novel main group approaches.