Multifaceted behavior of a doubly reduced arylborane in B-H-bond activation and hydroboration catalysis.

Alkali-metal salts of 9,10-dimethyl-9,10-dihydro-9,10-diboraanthrancene (M2[DBA-Me2]; M+ = Li+, Na+, K+) activate the H-B bond of pinacolborane (HBpin) in THF already at room temperature. For M+ = Na+, K+, the addition products M2[4] are formed, which contain one new H-B and one new B-Bpin bond; for M+ = Li+, the H- ion is instantaneously transferred from the DBA-Me2 unit to another equivalent of HBpin to afford Li[5]. Although Li[5] might commonly be considered a [Bpin]- adduct of neutral DBA-Me2, it donates a [Bpin]+ cation to Li[SiPh3], generating the silyl borane Ph3Si-Bpin; Li2[DBA-Me2] with an aromatic central B2C4 ring acts as the leaving group. Furthermore, Li2[DBA-Me2] catalyzes the hydroboration of various unsaturated substrates with HBpin in THF. Quantum-chemical calculations complemented by in situ NMR spectroscopy revealed two different mechanistic scenarios that are governed by the steric demand of the substrate used: in the case of the bulky Ph(H)C[double bond, length as m-dash]NtBu, the reaction requires elevated temperatures of 100 °C, starts with H-Bpin activation which subsequently generates Li[BH4], so that the mechanism eventually turns into "hidden borohydride catalysis". Ph(H)C[double bond, length as m-dash]NPh, Ph2C[double bond, length as m-dash]O, Ph2C[double bond, length as m-dash]CH2, and iPrN[double bond, length as m-dash]C[double bond, length as m-dash]NiPr undergo hydroboration already at room temperature. Here, the active hydroboration catalyst is the [4 + 2] cycloadduct between the respective substrate and Li2[DBA-Me2]: in the key step, attack of HBpin on the bridging unit opens the bicyclo[2.2.2]octadiene scaffold and gives the activated HBpin adduct of the Lewis-basic moiety that was previously coordinated to the DBA-B atom.

High-Yielding Water-Soluble Asymmetric Cyanine Dyes for Labeling Applications.

A simple and efficient microwave-assisted synthesis of asymmetric pentamethine cyanine dyes with various functional groups was developed, which allows high-yielding results. The synthesized dyes are modifiable and suitable for single-molecule imaging in biological and medical sciences by application of click chemistry or classic esterification and amidation.

Bond-Strengthening Backdonation in Aminoborylene-Stabilized Aminoborylenes: At the Intersection of Borylenes and Diborenes.

Singly NHC-coordinated (aminoboryl)aminoborenium salts react with Na2 [Fe(CO)4 ] to yield stable coordination complexes of aminoborylene-stabilized aminoborylenes, which exhibit exceptional σ-donor properties. Upon photolytic CO extrusion from the metal center, the diboron ligand adopts a novel η3 -BBN coordination mode, where bond-strengthening backdonation from the metal center into the vacant B-B π-orbital is observed. This bonding situation can be alternatively described as a Fe-diaminodiborene complex. In a related reduction of CAAC-stabilized (aminoboryl)aminoborenium with KC8 , the reduced species can be captured with nucleophiles to form three-coordinate (diaminoboryl)borylenes, where both amino groups have migrated to the distal boron atom. Collectively, these reactions illustrate the isomeric flexibility imparted by amino groups on this reduced diboron system, thus opening multiple avenues of novel reactivity.