The
reactions of organic azides with diaryl(dihalo)diboranes(4)
were explored, resulting in the observation of a number of surprising
reactivity patterns. The reaction of phenyl azide with 1,2-diaryl-1,2-dihalodiboranes(4)
resulted in the formation of five-membered rings comprising diboryl-triazenes
with retention of the boron–boron bond, while the reaction
of the peculiar 1,1-di(9-anthryl)-2,2-difluorodiborane(4) with phenyl
azide yielded a six-membered ring bearing a diboryl-triazene, whereby
the B–B bond was ruptured by the insertion of an arylnitrene-like
reactive intermediate. Both types of heterocycles feature unprecedented
connectivity patterns and are very rare examples of boryl triazenes
beyond the more common 1,2,3-triazolatoboranes. They are also the
product of a unique type of aryl migration from a boron center to
the phenyl azide γ-nitrogen center. Lastly, the substitution
of 1,2-diaryl-1,2-dihalodiboranes(4) with azide groups, using trimethylsilyl
azide as the transfer reagent, yielded boryl-tetrazaboroles and diboryldiazadiboretidines
(as side-products), invoking the intermediacy of the first N-boryl-substituted
iminoboranes, which are BN isosteres of monoborylated alkynes. The
synthetic results are complemented with mechanistic proposals derived
from quantum-chemical calculations.
We synthesized new pyrene derivatives with strong bis(para‐methoxyphenyl)amine donors at the 2,7‐positions and n‐azaacene acceptors at the K‐region of pyrene. The compounds possess a strong intramolecular charge transfer, leading to unusual properties such as emission in the red to NIR region (700 nm), which has not been reported before for monomeric pyrenes. Detailed photophysical studies reveal very long intrinsic lifetimes of >100 ns for the new compounds, which is typical for 2,7‐substituted pyrenes but not for K‐region substituted pyrenes. The incorporation of strong donors and acceptors leads to very low reduction and oxidation potentials, and spectroelectrochemical studies show that the compounds are on the borderline between localized Robin‐Day class‐II and delocalized Robin‐Day class‐III species.
The
stepwise reduction of a doubly cyclic alkyl(amino)carbene (CAAC)-stabilized
2,3-bis(dibromoboryl)naphthalene enables the isolation of the corresponding
mono- and bis(boryl) radicals (one- and two-electron reduction), a
2π-aromatic 1,2-diborete (four-electron reduction), which shows
biradical character in the solid-state EPR spectrum, and its cyclic
bis(alkylidene)diboron dianion (six-electron reduction). The X-ray
crystallographic analysis of the diborete shows a highly strained
and twisted four-membered ring with a formal cis-diborene
motif featuring a very elongated B–B double bond. Calculations
based on DFT and multireference approaches reveal that the diborete
possesses an open-shell singlet biradicaloid ground state, which is
slightly energetically preferred to its EPR-active triplet-state congener.
The addition of CO to the diborete resulted in B–B bond splitting
and the formation of the corresponding closed-shell singlet, doubly
Lewis base-stabilized bis(borylene), whereas a twofold γ insertion
of phenyl azide generates a 1,3-bis(diazenyl)-1,3,2,4-diazadiboretidine.
The γ-nitrogen insertion of arylazides into the B–B bond of electron-rich cyclic μ-hydridodiboranes yields unsymmetrical polyheterocyclic 1,1-diboryltriazenes, which may undergo further NHC ring expansion/fusion and thermally induced loss of N2.
The oxidation of doubly cyclic alkyl(amino)carbene-stabilised closed-shell 1,4-diborabenzene with sulphur or selenium yields S4/S5- or Se4-bridged hexa-1,4-dienes, respectively, whereas that of the related open-shell singlet biradical 9,10-diboraanthracene with O2, sulphur...
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