Synthesis and Properties of B4 N4 -Heteropentalenes Fused with Polycyclic Hydrocarbons.

Hybrid molecules of π-conjugated carbon rings and BN-heterocyclic rings (h-CBNs) fused with each other have been a rare class of compounds due to the limited availability of their synthetic methods. Here we report the synthesis of new h-CBNs featuring a B4 N4 -heteropentalene core and polycyclic aromatic hydrocarbon wings. Using 1,2-azaborinine derivatives as a building block, we developed a rational synthetic protocol that allows the formation of a B4 N4 ring in a stepwise manner, resulting in the fully fused ABA-type triblock molecules. Thus, three derivatives of 1 bearing naphthalene (1Naph ), anthracene (1Anth ), or phenanthrene (1Phen ) wings fused with the B4 N4 core were synthesized and characterized. Among them, 1Phen , which displays the highest triplet-state energy, was found to serve a host material for phosphorescent OLED devices, for which a maximum external quantum efficiency of 13.7 % was recorded. These findings may promote the synthesis of various types of h-CBNs aiming at new properties arising from the synergy of two different π-electronic systems.

Bringing out the potential of organoboron compounds by designing the chemical bonds and spaces around boron.

Since the structures, reactivity and properties of organoboron compounds stem from the electron deficiency and low electronegativity of boron, the design of the chemical bonds attached to boron as well as the space provided by substituents around boron leads to particular reactivity and functions of organoboron compounds. Based on the concept of vacant boron p-orbital engineering in combination with peripheral space design for boron, we have developed new organoboron compounds, which exhibit exotic reactivity and properties and boron-mediated organic transformations capable of constructing large π-conjugated systems. Here we describe our recent study by focusing on three topics: (i) the synthesis and its derivatization, small-molecule activation and application to materials science of a two-coordinate boron cation (borinium ion), (ii) a 9-borafluorene-mediated benzannulation reaction of alkyne derivatives and (iii) BN-substituted molecules featuring a 4nπ heterocycle exhibiting unique electronic and emission properties. The molecules and reactions appearing in this article are unprecedented, yet feature very simple systems, and therefore clearly demonstrate intrinsic and new potential of organoboron compounds.

An Air- and Water-Stable B4 N4 -Heteropentalene Serving as a Host Material for a Phosphorescent OLED.

Replacement of the carbon-carbon bonds of antiaromatic compounds with polar boron-nitrogen bonds often provides isoelectronic BN compounds with excellent thermodynamic stability and interesting photophysical properties. By this element-substitution strategy, we synthesized a new B4 N4 -heteropentalene derivative, 1, which is fully substituted with mesityl groups. Owing to kinetic protection by the sterically bulky substituents, 1 is remarkably stable toward air and even water. Single-crystal X-ray analysis of 1 revealed the bonding characteristics of the B4 N4 -heteropentalene structure. In a glassy matrix, 1 emitted short-wavelength phosphorescence with an onset at 350 nm, indicating that the triplet energy is substantially high. DFT calculations reasonably explained the ground- and excited-state electronic structures of 1 as well as its emission properties. Motivated by the high-energy triplet state of 1, we used it as a host material to fabricate a phosphorescent organic light-emitting diode with an external quantum efficiency of 15 %.

Observation of Borane-Olefin Proximity Interaction Governing the Structure and Reactivity of Boron-Containing Macrocycles.

While attractive interactions between borane and olefin have been postulated to trigger various boron-mediated organic transformations, proximity structures of these functional groups, other than the formation of weak van der Waals complexes, have never been directly observed. Here we show that a close intramolecular borane-olefin interaction operates in macrocyclic systems containing borane and olefinic groups obtained by multi-step 1,2-carboboration between a strained alkyne and 9-borafluorene derivatives. Depending on Lewis acidity of the borane moiety and the size of the macrocycles, the magnitude of interaction changes, resulting in different reaction modes. The whole picture of the multi-step reactions has been revealed experimentally with theoretical supports. The present finding may not only provide a deeper understanding of the fundamental boron-mediated interaction but also lead to the development of new organic transformations involving molecular activation by boranes.

Synthesis and Reactivity of Cyclic Borane-Amidine Conjugated Molecules Formed by Direct 1,2-Carboboration of Carbodiimides with 9-Borafluorenes.

Efficient 1,2-carboboration reactions to the C=N bond of carbodiimides with 9-borafluorenes, which give rise to cyclic borane-amidine conjugates with a seven-membered BNC5 ring, are reported. The resulting cyclic borane-amidine conjugates can be hydrolyzed into an acyclic bifunctional biaryl compound carrying both boronic acid and amidine groups, rendering the utility of the two-step protocol for the synthesis of multi-functionalized molecular systems with a potential as a supramolecular building block. Furthermore, the conjugated structure of the cyclic boron-amidine compounds can be changed upon alkylation of the boron atom that increases the coordination number of boron. The combination of Lewis acid (borane) and conjugated base (amidine) provides rich structural diversity of heteroatom-containing π-conjugated systems.