Diborylated magnesium anthracene as precursor for B2H5(-)-bridged 9,10-dihydroanthracene

Arylboronic Acid Pinacol Esters as Stable Boron Sources for Dihydrodibenzoborepin Derivatives and a Dibenzoborole

The general synthesis of boron-containing cyclic compounds (boracycles) necessitates toxic organotin precursors or highly reactive boron halides. Here, we report the synthesis of seven- and five-membered boracycles utilizing arylboronic acid pinacol esters (ArBpins) as stable boron sources. Grignard reagents generated from 2,2'-dibromodibenzyl or 2,2'-dibromobiphenyl reacted with ArBpins, where Ar = 9-anthryl (Anth), 2,4,6-trimethylphenyl (Mes), or 2,4,6-triisopropylphenyl (Tip), to give 10,11-dihydro-5H-dibenzo[b,f]borepins or dibenzoborole derivatives. This Bpin-based method was successfully applied to a one-shot double boracycle formation, providing a dihydrodibenzoborepin-anthracene-dihydrodibenzoborepin triad molecule in a good yield. The dihydrodibenzoborepin bearing the Anth group was directly converted to the unsaturated borepin by NBS/AIBN. All products were characterized by NMR, HRMS, and in some cases, single-crystal X-ray diffraction analysis. Additionally, the photophysical properties of the products are also reported.

Crystal structure of di-μ-tri-hydro-(penta-fluoro-phenyl)-borato-tetra-kis-(tetra-hydro-furan)-disodium

The title compound, [Na(μ-C6F5BH3)(C4H8O)2]2, represents a dimeric structure of sodium and organoborohydride, located about a centre of inversion. The Na⋯B distances of 2.7845 (19) and 2.7494 (18) Å were apparently longer than the Li⋯B distances (2.403-2.537 Å) of the lithium organotri-hydro-borates in the previous reports. Moreover, an inter-action between the sodium atom and one fluorine atom on the 2-position of the benzene ring is observed [Na-F = 2.6373 (12) Å]. In the crystal, the dimeric mol-ecules are stacked along the b-axis via a π-π inter-action between the benzene rings.

Reductive α-borylation of α,β-unsaturated esters using NHC-BH3 activated by I2 as a metal-free route to α-boryl esters

Useful α-boryl esters can be synthesized in one step from α,β-unsaturated esters using just a simple to access NHC-BH₃ (NHC = N-heterocyclic carbene) and catalytic I₂. The scope of this reductive α-borylation methodology is excellent and includes a range of alkyl, aryl substituted and cyclic and acyclic α,β-unsaturated esters. Mechanistic studies involving reductive borylation of a cyclic α,β-unsaturated ester with NHC-BD₃/I₂ indicated that concerted hydroboration of the alkene moiety in the α,β-unsaturated ester proceeds instead of a stepwise process involving initial 1,4-hydroboration; this is in contrast to the recently reported reductive α-silylation. The BH₂(NHC) unit can be transformed into electrophilic BX₂(NHC) moieties (X = halide) and the ester moiety can be reduced to the alcohol with the borane unit remaining intact to form β-boryl alcohols. The use of a chiral auxiliary, 8-phenylmenthyl ester, also enables effective stereo-control of the newly formed C-B bond. Combined two step ester reduction/borane oxidation forms diols, including excellent e.e. (97%) for the formation of S-3-phenylpropane-1,2-diol. This work represents a simple transition metal free route to form bench stable α-boryl esters from inexpensive starting materials.