Iminoboranes With Parent B=NH Entity: Imino Group Metathesis, Nucleophilic Reactivity and N-N Coupling.

While R2 C=N-R double bonds in organic imines are well established, the related iminoboranes R-B=N-R are either prone to oligomerization or can only be stabilized at sufficient steric congestion. In particular, the examples of unsubstituted parent B=N-H entity are rare. We demonstrate that the amino imidazoline-2-imine ligand system (HAmIm) not only gives rise to the isolation of a parent (AmIm)B=N-H iminoborane, but also to species of type (AmIm)B=N-SiMe3 with concomitant stabilization by lithium bromide. The double bond character in these systems is unambiguously corroborated by DFT calculations. The steric accessibility of the (AmIm)B=NH unit allows facile reactivity including metathesis reactions with C=O and C=S bonds, nucleophilic addition toward organic and organometallic carbonyl compounds, but also oxidative N-N coupling within a dimeric unit. Thus, the chemical behavior of the (AmIm)B=N-H and (AmIm)B=N-SiMe3 is distinctly different from that of organic imines.

Reactions of a Four-Membered Borete with Carbon, Silicon, and Gallium Donor Ligands: Fused and Spiro-Type Boracycles.

Donor-acceptor cyclopropanes or cyclobutanes are dipolar reagents, which are widely used in the synthesis of complex organic (hetero)cycles in ring expansion reactions. Applying this concept to boron containing heterocycles, the four-membered borete cyclo-iPr2 N-BC10 H6 reacted with the carbon donor ligands 2,6-xylylisonitrile and the carbene IMes :C(NMesCH)2 with ring expansion and ring fusion, respectively. In particular, the tetracyclic structure formed with IMes displays zwitterionic character and absorption in the visible region. In contrast to the carbene IMes, the heavier carbenoids :Si(NDippCH)2 and :Ga(AmIm) with a two-coordinate donor atom afford spiro-type bicyclic compounds, which display four-coordinate geometry at silicon or gallium. (TD-)DFT calculations provide deeper insight into the mechanism of formation and the absorption properties of these new compounds.

Superseding ß-Diketiminato Ligands: An Amido Imidazoline-2-Imine Ligand Stabilizes the Exhaustive Series of B=X Boranes (X=O, S, Se, Te).

Boron reluctantly forms B=X (X=O, S, Se, Te) moieties, which has stimulated the quest for such species in the past few years. Based on the N,N'-chelating ß-diketiminato ligand (HNacNac), a new amido imidazoline-2-imine ligand system (HAmIm) is presented, giving rise to the isolation of an exhaustive series of Lewis acid free, monomeric chalcogen B=X boranes with documented π-bond character between boron and the chalcogen. The chalcogenoboranes are isoelectronic and isolobal to the respective ketones. The chemical behavior of the oxoborane (B=O) strongly resembles the classical carbonyl reactivity in C=O bonds. The improved stability provided by HAmIm arises from the formation of more-stable five-membered boron chelates versus the six-membered NacNac analogues and from the imidazoline-2-imine moiety providing enhanced σ- and π-donation. The HAmIm ligand class may supersede the widely employed NacNac system in certain applications.

Two π-Electrons Make the Difference: From BODIPY to BODIIM Switchable Fluorescent Dyes.

(aza-)BODIPY dyes (boron dipyrromethene dyes) are well-established fluorophores due to their large quantum yields, stability, and diversity, which led to promising applications including imaging techniques, sensors, organic (opto)electronic materials, or biomedical applications. Although the control of the optical properties in (aza-)BODIPY dyes by peripheral functional groups is well studied, we herein present a novel approach to modify the 12 π-electron core of the dipyrromethene scaffold. The replacement of two carbon atoms in the ß-position of a BODIPY dye by two nitrogen atoms afforded a 14 π-electron system, which was termed BODIIM (boron diimidazolylmethene) in systematic analogy to the BODIPY dyes. Remarkably, the BODIIM dye was obtained with a BH2 -rigidifying entity, which is currently elusive and highly sought after for the BODIPY dye class. DFT-Calculations confirm the [12+2] π-electron relationship between BODIPY and BODIIM and reveal a strong shape correlation between LUMO in the BODIPY and the HOMO of the BODIIM. The modification of the π-system leads to a dramatic shift of the optical properties, of which the fluorescent emission is most noteworthy and occurs at much larger Stokes shift, that is, ≈500 cm-1 in BODIPY versus >4170 cm-1 in BODIIM system in all solvents investigated. Nucleophilic reactivity was found at the meso-carbon atom in the formation of stable borane adducts with a significant shift of the fluorescent emission, and this behavior contrasts the reactivity of conventional BODIPY systems. In addition, the reverse decomplexation of the borane adducts was demonstrated in reactions with a representative N-heterocyclic carbene to retain the strongly fluorescent BODIIM compound, which suggests applications as fully reversible fluorescent switch.

Synthesis and Antimicrobial Evaluation of 4H-Pyrans and Schiff Bases Fused 4H-Pyran Derivatives as Inhibitors of Mycobacterium bovis (BCG).

The focus of our study is the synthesis and biological activity evaluation of a series of 4H-Pyran compounds and schiff bases fused 4H-Pyran derivatives which are known to possess a wide variety of biological activities. In this paper at first a simple and efficient one-pot synthesis of 4H-Pyran s from the three-component reaction between malononitrile, aldehydes, and active methylene compounds in the presence of N-methylmorpholine (NMM) as catalyst at room temperature is reported, the reaction between these synthesized products and trimethylorthoformateor triethylorthoformateto produce schiff base compounds were also considered. The key advantages of synthesis of 4H-Pyran derivatives are short reaction time, high yield, and simple work-up. Then, these compounds were evaluated for anti-Mycobacterium activity against Mycobacterium bovis (Bacillus Calmette-Guerin). The preliminary results indicated that most of the tested compounds showed relatively good activity against the test organism. Moreover, antifungal activities of these compounds were evaluated. Finally, their effect was more noticeable on Mycobacterium bovis (BCG).

Synthesis and Evaluation of the Antimicrobial Activity of Spiro-4H-pyran Derivatives on Some Gram Positive and Gram Negative Bacteria.

Infections are one of the most important causes of death, disability and inappropriate conditions for millions of people around the world. Therefore, the development in prognosis, prevention and treatment of infectious diseases made a considerable progress in designing and synthesis of new antimicrobial drugs. Nowadays, due to the increase in microbial resistance, discovery of new compounds with broad spectrum effects is granted. 4H-pyran derivatives and spiro compounds are the most important fragments in some effective drugs with antimicrobial activity. Therefore, in this study, 6 compounds with spiro-4H-pyran core were synthesized and evaluated for their antimicrobial activity against four different bacterial species using microbroth dilution and disk diffusion methods. Minimum inhibitory concentration (MIC) has been measured for each compound and also for the standard antibiotic, gentamicin, and they were all compared together. According to our result, one of the spiropyran derivative (5d) containing both the indole and the cytosine ring, has been shown good antibacterial effects against standard and clinical isolates of Staphylococcus aureus and Streptococcus pyogenes.