Pentaphenylborole and a ferrocenylborole were synthesized and structurally characterized. Both experimental and theoretical data reveal that rather weak intermolecular interactions are able to significantly alter the bond lengths in the borole ring of pentaphenylborole (see picture). Moreover, the high Lewis acidity of boroles is demonstrated by a significant Fe⋅⋅⋅B interaction in the ferrocenylborole in the solid state.

First isolated in 1969, most progress in the chemistry of the antiaromatic 4π electron borole system has been made during the 1980s. However, besides the fundamental aspects of the electronic structure and reactivity, boroles have not encountered serious research efforts for a rather long timeframe. This is somewhat surprising given the fact that boroles feature a unique combination of antiaromaticity, strong electrophilicity and unusual electronic properties. It was not until 2008 that interest was resparked. Since then, tremendous progress has been achieved in this area, particularly with respect to synthetic access, structural characterization and reactivity. Various differently substituted borole derivatives have been successfully isolated and characterized both in solution and in the solid state, which provided a more thorough understanding of the structure/reactivity relationship. This feature article is intended to provide a general overview on the electronic structure and the consequences of antiaromaticity on the inherent properties of these highly reactive species. The different synthetic methodologies to generate boroles and their divergent reactivity patterns will be described in great detail, which will emphasize their high potential and relevance in modern chemistry.

A new dimeric naphthylisoquinoline alkaloid, jozimine A(2) (2), was isolated from the root bark of an Ancistrocladus species from the Democratic Republic of Congo. Its absolute stereostructure was determined by chemical, spectroscopic, and chiroptical methods, and confirmed by X-ray crystallography. Jozimine A(2) (2) is one of the as yet very rare naphthylisoquinoline dimers whose central biaryl axis is rotationally hindered. Moreover, it is the first natural dimer of a Dioncophyllaceae-type alkaloid, that is, lacking oxygen functions at C6, and bearing R configurations at C3 in its two isoquinoline portions. Despite this decreased steric hindrance, the outer biaryl axes are chiral, too, so that jozimine A(2) (2) has three consecutive stereogenic axes and, together with the four stereogenic centers, seven elements of chirality and is C(2)-symmetric. The new dimer exhibits excellent, and specific, antiplasmodial activity. To further confirm its stereostructure and for likewise testing the bioactivities of its (unnatural) atropo-diastereomer, compound 2 was prepared by semi-synthesis from the co-occurring (and likewise synthetically available) dioncophylline A (5), along with its atropo-diastereomer, 3'-epi-2.

(B)olefin complexes: Reductive coupling of designed monoborane precursors (see scheme; Dur=2,3,5,6-tetramethylphenyl) gives convenient access to N-heterocyclic carbene stabilized diborenes. The presence of B-B multiple bonds in the dark red diborenes is shown experimentally and theoretically. Reaction with AgCl afforded a Ag(I) species with an unprecedented, olefin-like η(2) coordination mode.

The realization of a phosphine-stabilized diborene, Et3P⋅(Mes)B=B(Mes)⋅PEt3 (4), by KC8 reduction of Et3P⋅B2Mes2Br2 in benzene enabled the evaluation and comparison of its electronic structure to the previously described NHC-stabilized diborene IMe⋅(Dur)B=B(Dur)⋅IMe (1). Importantly, both species feature unusual electron-rich boron centers. However, cyclic voltammetry, UV/Vis spectroscopy, and DFT calculations revealed a significant influence of the Lewis base on the reduction potential and absorption behavior of the BB double bond system. Thus, the stronger σ-donor strength and larger electronegativity of the NHC ligand results in an energetically higher-lying HOMO, making 1 a stronger neutral reductant as 4 (1: E(1/2)=-1.55 V; 4: -1.05 V), and a smaller HOMO-LUMO gap of 1 accompanied by a noticeable red-shift of its lowest-energy absorption band with respect to 4. Owing to the highly negative reduction potentials, 1 and 4 were easily oxidized to afford rare boron-centered radical cations (5 and 6).