No abstract

The synthesis and physicochemical properties of benzosiloxaboroles, the silicon analogues of an important class of heterocyclic compoundsbenzoxaborolesis presented. They were prepared by halogen−lithium exchange reactions of (2-bromophenyl)boronates with n-BuLi followed by the silylation or boronation of (2-lithiophenyl)dimethylsilanes. The cyclization of the resulting 2-(dimethylsilyl)phenylboronates apparently occurs through intramolecular dehydrogenative cyclization reaction in the presence of water. Unlike the case for benzosiloxaborole, the formation of its analogue containing a thiophene ring is thermodynamically unfavorable, which was confirmed by theoretical calculations. The presence of a B−O−Si linkage results in increased Lewis acidity with respect to the analogous benzoxaboroles. The acidity is strongly enhanced by fluorination or introduction of phenyl groups at the silicon atom. Selected compounds show good antifungal activity, and thus they are potential small-molecule therapeutic agents. They can also serve as effective receptors for biologically relevant diols under neutral pH conditions.

The results of X-ray crystallographic and computational studies of a series of fluorinated 1,4-phenylenediboronic acids (i.e., fluoro-1,4-phenylenediboronic acid, 2,6-difluoro-1,4-phenylenediboronic acid, 2,3-difluoro-1,4-phenylenediboronic acid, 2,5-difluoro-1,4-phenylenediboronic acid, and tetrafluoro-1,4-phenylenediboronic acid) are reported. The effect of fluorine substitution on crystal organization in the presence of strong and directional hydrogen bonds was studied. Comparison with the two previously reported forms of the unsubstituted 1,4-phenylenediboronic acid revealed a strong relation between a supramolecular network and the number of water molecules present in the crystal lattice. As indicated by the theoretical calculations performed in the CRYSTAL and PIXEL programs, the structures with greater amount of water are better stabilized (from about −170 kJ·mol–1 for anhydrous forms to about −420 kJ·mol–1 for tetrahydrate). The energy of hydrogen bonded dimers vary from −40 kJ·mol–1 to −50 kJ·mol–1. Contacts with fluorine atoms play rather a secondary role in the crystal packing. Fluorine substituents tend to interact with the electropositive boron atom. Furthermore, intramolecular interactions significantly affect the torsion angle of the B­(OH)2 group. The constrained energy scan revealed that stronger interactions with substituents stabilize the planar conformation and hamper the rotation of the boronic group. This in turn has a further impact on the interactions within selected crystal motifs and supposedly rules the proton disorder within boronic fragments. Besides the interactions with the fluorine atoms, other weak contacts such as C­(π)···B and O···B also influence the molecular organization. The energy of the corresponding dimers varies from −15 kJ·mol–1 to −25 kJ·mol–1.

The metalation of selected oligobromobenzenes with lithium diisopropylamide (LDA) was investigated. 1,3-Dibromo-substituted benzenes were metalated without special precautions since the resultant 2,6-dibromophenyllithium intermediates are relatively stable under reaction conditions: corresponding benzaldehydes were obtained in good or moderate yields after subsequent quench with N,N-dimethylformamide (DMF). Aryllithium compounds derived from 1,4- and 1,2-dibromobenzene are much less stable, but they could be trapped by the in situ use of chlorotrimethylsilane. The one-pot metalation/disilylation of 1,4-dibromo- and 1,2-dibromobenzene afforded 1,4-dibromo-2,5-bis(trimethylsilyl)benzene and 2,3-dibromo-1,4-bis(trimethylsilyl)benzene, respectively.

Ten bis(boranils) differently substituted at the boron atom and iminophenyl groups were synthesized from 1,5-dihydroxynaphthalene-2,6-dicarboxaldehyde using a simple one-pot protocol. Their photophysical properties can be easily tuned in a wide range by the variation of substituents. Their absorption and emission spectral bands are significantly red-shifted (λ = 495-590 nm, λ = 533-683 nm) when compared with simple boranils, whereas fluorescence quantum yields are strongly improved to reach 83%. The attachment of pendant NO and NEt groups at the opposite positions of the π-conjugated bis(boranil) scaffold resulted in the formation of an unprecedented system featuring push-pull architecture.