A 1,3-dihydro-1,3-azaborine debuts

We present the first synthesis and characterization of a 1,3-dihydro-1,3-azaborine, a long-sought BN isostere of benzene. 1,3-Dihydro-1,3-azaborine is a stable structural motif with considerable aromatic character as evidenced by structural analysis and its reaction chemistry. Single crystal X-ray analysis indicates bonding consistent with significant electron delocalization. 1,3-Dihydro-1,3-azaborines also undergo nucleophilic substitutions at boron and electrophilic aromatic substitution reactions. In view of the versatility and impact of aromatic compounds in the biomedical field and in materials science, the present study further expands the available chemical space of arenes via BN/CC isosterism.     

Crystal clear structural evidence for electron delocalization in 1,2-dihydro-1,2-azaborines

The first examples of "pre-aromatic" 1,2-dihydro-1,2-azaborine heterocycles have been structurally characterized, enabling the direct comparison of delocalized bonds of 1,2-dihydro-1,2-azaborines to their corresponding formal double and single bonds in nonaromatic systems. The crystallographic data provide an unprecedented look into the structural changes that occur in six-membered BN-heterocycles on their road to aromaticity, and they establish with little ambiguity that 1,2-dihydro-1,2-azaborines possess delocalized structures consistent with aromaticity.     

Functionalization of Heterocycles through 1,2-Metallate Rearrangement of Boronate Complexes

Over the last decade, 1,2-metallate rearrangement of boronate complex has been dominating the literature of organoboron chemistry for the construction of very important C−C and C−boron bonds. Owing to the coordinative unsaturated nature of the boron atom, a nucleophile can attack on boron center for the formation of a boronate complex, which triggers 1,2-migration under electrophilic activation at the α-carbon. Apart from using stochiometric electrophilic activating reagents, several catalytic methods using transition metals in the presence or absence of light have been reported. The 1,2-migration of boronate complexes allows synthesis of many different classes of racemic and chiral compounds including a wide range of substituted heterocycles. Synthesis of chiral and achiral substituted heterocycles by using 1,2-metallate rearrangement of boronate complexes has been extensively reported by several groups owing to its prevalence in medicinal chemistry. This minireview highlights the methods known to date for the synthesis of heteroaryls by using 1,2-migration of boronate complexes, organized in a chronological manner.     

Novel buron heterocycles. I. 2,3-dihydro-1,3,2-benzodiazaborin-4(III)-ones and 1,2-dihydro- 1,3,2-benzodiazaborines

A series of 2,3-dihydro-1,3,2-benzodiazaborin-4(IH)-ones have been prepared and their ir spectra compared with those of their carbon isosteres. Solvolysis of these boron compounds in ethanol has been followed by uv and reveals a relationship between structure and rates of ethanol-ysis. Unexpectedly, these boron heterocycles, in contrast to their carbon isosteres, dissolve in aqueous alkali to form stable anions and the significance of this is discussed. The 1 :1 adducts of 1 and phosphorus oxychloride has been utilized to prepare two derivatives of the very stable 1,2-dihydro-2-phenyl-1,3,2-benzodiazaborin heterocycle.     

BN-embedded aromatics for optoelectronic applications

The knowledge of azaborine chemistry is growing as an important branch in organic semiconductor materials. Specifically, BN-embedded aromatic compounds have attracted great attention due to their fascinating properties resulted from the replacement of CC unit with isoelectronic BN unit in aromatics. Though great insights have been provided into the synthetic chemistry and photophysical properties of BN-embedded aromatics, their applications in optoelectronic areas are still at a young stage. This short review summarizes the recent progress of BN-embedded aromatics with optoelectronic applications in organic field-effect transistors, organic light-emitting diodes, organic photovoltaics, stimuli-responsive luminescent devices, and chemical sensors.     

1,2-BN cyclohexane: synthesis, structure, dynamics, and reactivity

BN/CC isosterism has emerged as a viable strategy to increase the structural diversity of carbon-based compounds. We present the first synthesis and characterization of the parent 1,2-BN cyclohexane, the BN-isostere of cyclohexane. 1,2-BN cyclohexane is an air- and water-stable compound that cleanly forms a trimer with release of dihydrogen when thermally activated. We also demonstrate that 1,2-BN cyclohexane has a lower activation barrier for ring inversion than cyclohexane due to BN/CC isosterism.     

Diversity through isosterism: the case of boron-substituted 1,2-dihydro-1,2-azaborines

The first general synthesis of boron-substituted 1,2-dihydro-1,2-azaborines is described. The versatile 1,2-dihydro-1,2-azaborine precursor 4 is synthesized through a ring-closing metathesis-oxidation sequence. Treatment of 4 with a wide range of anionic nucleophiles furnishes the desired adducts 5 in good yields. The scope includes hydrogen- and a variety of carbon- and heteroatom-based nucleophiles. Furthermore, the boron-containing isostere (7) of the potent hypolipidemic agent, methyl 2-ethylphenoxyacetate (8), is readily prepared through our method.     

Heteroelement Analogues of Benzoxaborole and Related Ring Expanded Systems

The review covers the chemistry of organoboron heterocycles structurally related to benzoxaboroles where one of the carbon atoms in a boracycle or a fused benzene ring is replaced by a heteroelement such as boron, silicon, tin, nitrogen, phosphorus, or iodine. Related ring expanded systems including those based on naphthalene and biphenyl cores are also described. The information on synthetic methodology as well as the basic structural and physicochemical characteristics of these emerging heterocycles is complemented by a presentation of their potential applications in organic synthesis and medicinal chemistry, the latter aspect being mostly focused on the promising antimicrobial activity of selected compounds.     

Nucleophilic aromatic substitution reactions of 1,2-dihydro-1,2-azaborine

Could go either way: The addition of nucleophiles to the parent 1,2-dihydro-1,2-azaborine and subsequent quenching with an electrophile generates novel substituted 1,2-azaborine derivatives. Mechanistic studies are consistent with two distinct nucleophilic aromatic substitution pathways depending on the nature of the nucleophile.     

Expanding the functional group tolerance of cross-coupling in 1,2-dihydro-1,2-azaborines: Installation of alkyl,alkenyl, aryl,and heteroaryl substituents while maintaining a B−H bond

Palladium-catalyzed Negishi cross-coupling of 3-bromo-1-(tert-butyldimethylsilyl)-1,2-dihydro-1,2-azaborine while maintaining the B?H functionality has been demonstrated. 17 examples, including dialkylzinc, alkyl-, alkenyl-, aryl-, as well as nitrogen-, sulfur-, and oxygen-containing heteroaryl-zinc halide reagents have been coupled to generate new C(3) substituted 1,2-azaborines in moderate to excellent yields.     

Poly(p‐phenylene iminoborane): A Boron–Nitrogen Analogue of Poly(p‐phenylene vinylene)

Substitution of selected CC units in π‐conjugated organic frameworks by their isoelectronic and isosteric BN units (BN/CC isosterism) has proven to be a successful concept for the development of BN‐doped polycyclic aromatic hydrocarbons (PAHs) with intriguing properties and functions. The first examples have just demonstrated the applicability of this approach to polymer chemistry. Herein, we present the synthesis and comprehensive characterization of the first poly(p‐phenylene iminoborane). This novel inorganic–organic hybrid polymer can be regarded as a BN analogue of the well‐known poly(p‐phenylene vinylene) (PPV). Photophysical investigations on the polymer and a series of model oligomers provide clear evidence of some π‐conjugation across the B=N bonds and extension of the conjugation path with increasing chain length. TD‐DFT calculations provide deeper insight into the electronic structure of the new materials.     

In Situ Solid‐State Generation of (BN)2‐Pyrenes and Electroluminescent Devices

New BN‐heterocyclic compounds have been found to undergo double arene photoelimination, forming rare yellow fluorescent BN‐pyrenes that contain two B? N units. Most significant is the discovery that the double arene elimination can also be driven by excitons generated electrically within electroluminescent (EL) devices, enabling the in situ solid‐state conversion of BN‐heterocycles to BN‐pyrenes and the use of BN‐pyrenes as emitters for EL devices. The in situ exciton‐driven elimination (EDE) phenomenon has also been observed for other BN‐heterocycles.     

B=N Units as Part of Extended π‐Conjugated Oligomers and Polymers

The replacement of C=C units by their isoelectronic and isosteric B=N units (BN/CC isosterism) in π‐conjugated organic compounds, as a strategy to produce novel organic–inorganic hybrid materials, has recently been successfully transferred to π‐conjugated polymers. This Concept provides an overview of the recent advances in this quickly evolving field, with a focus on synthesis, photophysical and electrochemical properties of the new polymers and related oligomers, as well as possible future applications in organic electronics and optoelectronics.     

Microwave-assisted copper-catalyzed stereoselective ring expansion of three-membered heterocycles with α-diazo-β-dicarbonyl compounds

Microwave-assisted copper-catalyzed ring expansions of three-membered heterocycles with α-diazo-β-dicarbonyl compounds were investigated. Thiiranes generated 3-acyl-5,6-dihydro-1,4-oxathiines in the presence of copper sulfate and trans-3-acyl-5,6-dihydro-1,4-oxathiines as stereospecific products for 1,2-disubstituted cis-thiiranes through an intramolecular S_N2 process. Oxiranes gave rise to 2-acyl-5,6-dihydro-1,4-dioxines under the catalysis of copper hexafluoroacetylacetonate and cis-3-acyl-5,6-dihydro-1,4-dioxines as stereospecific products for 1,2-disubstituted cis-oxiranes via an intimate ion-pair mechanism. The current method provides a direct and simple strategy in efficient preparation of 3-acyl-5,6-dihydro-1,4-oxathiines and 2-acyl-5,6-dihydro-1,4-dioxines, important agents in medicinal and agricultural chemistry, from readily available thiiranes and oxiranes, respectively.     

Electrophilic aromatic substitution reactions of 1,2-dihydro-1,2-azaborines

The aromatic boron-nitrogen heterocycle 1,2-dihydro-1,2-azaborine undergoes classical electrophilic substitution. These reactions allow easy functionalization to provide a variety of 3- and 5-substituted derivatives. [reaction: see text].     

Isocyanides in the synthesis of nitrogen heterocycles

Isocyanides have long proved themselves to be irreplaceable building blocks in modern organic chemistry. The unique features of the isocyano group make isocyanides particularly useful for the synthesis of a number of important classes of nitrogen heterocycles, such as pyrroles, indoles, and quinolines. Several cocyclizations of isocyanides via zwitterions and radical intermediates as well as transition-metal-catalyzed syntheses of different types of heterocycles have recently been developed. Methods starting from isocyanides often have distinct advantages over alternative approaches to the same heterocycles because of their enhanced convergence, the great simplicity of most of the operations with them, and the great variety of isocyanides readily available for use. Isocyanides have also been used in some enantioselective syntheses of chiral heterocyclic compounds, including natural products as well as precursors thereof.     

硼氮杂稠环化合物的合成与电子学应用

有机半导体材料的开发为有机电子学的发展提供了材料基础。杂原子的引入进一步丰富了材料的种类和数量。作为CC单元的等电子体,BN单元对有机半导体材料的性能调节受到了科学家们的关注。本文主要介绍了有机共轭体系中BN单键的构筑方法,以及这类硼氮杂稠环分子在有机电子学领域的应用。     

Strained heterocycles in radical chemistry

Over the last few decades the use of radicals in synthesis has witnessed an explosive growth through introduction of efficient chain and electron-transfer reactions. Strained heterocycles, in particular, have emerged as a highly versatile and readily available class of radical precursors. The generation of carbinyl radicals of heterocycles has resulted in many elegant applications of heteroatom-centered radicals, such as beta fragmentations, cyclizations, and intramolecular hydrogen atom abstractions. Direct electron transfer to strained heterocycles has been realized through the use of arene radical anions. The method combines the virtues of radical and organometallic chemistry to yield useful functionalized organolithium compounds. Epoxides have been opened with high regioselectivity by titanocene(III) reagents in either stoichiometric or catalytic quantities to yield beta-titanoxy radicals. This development has resulted in many new applications in natural product synthesis.     

Asymmetric Catalytic Approaches Employing α,β-Unsaturated Imines

Nitrogen-containing heterocyclic compounds have consistently been a noticeable center of attention due to their significant utilization in the domain of synthetic organic chemistry, agrochemicals, and pharmaceuticals. α,β-Unsaturated imines or conjugated imines provide a lot of significant cyclic as well as acyclic products through reacting with a versatile family of compounds. This review summarizes the recent advances in enantioselective reactions of α,β-unsaturated imines by using synthetic methodologies for synthesizing various nitrogen-containing heterocycles that contain four to six-membered rings. The synthesis of rarely found seven-, eight- and nine-membered nitrogen-containing heterocycles have also been reported.